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An Overview of Catalytic Carbonylative Double Cyclization Reactions

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Abstract
This short review is aimed at giving an overview of catalytic carbonylative double cyclization reactions, which are processes in which suitable organic substrates and carbon monoxide are sequentially activated by a promoting catalyst to afford the formation of two new cycles with concomitant incorporation of carbon monoxide as a carbonyl function into the final product. Paradigmatic examples of this powerful synthetic methodology, which allows the one-step synthesis of complex molecular architectures from simple building blocks using the simplest and readily available C-1 unit (CO) are illustrated and discussed. The review is divided into five sections: 1) Introduction; 2) Functionalized Olefinic Substrates; 3) Functionalized Acetylenic Substrates; 4) Functionalized Halides; 5) Conclusions and Future Perspectives.
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Subject: Chemistry and Materials Science  -   Organic Chemistry

1. Introduction

The importance of the use of carbon monoxide as a C-1 unit in organic synthesis can hardly be overemphasized [1]. Carbon monoxide is a readily available feedstock, which can be easily obtained by steam reforming of light hydrocarbons (including natural gas), partial oxidation of petroleum hydrocarbons, or gasification of coal to give syngas (CO and H2) [2] and can be installed into an organic substrate, usually under catalytic conditions, with the direct formation of high value added carbonylated compounds with 100% atom economy (carbonylation reactions) [1]. It should also be considered that recent progress in the chemical utilization of carbon dioxide has led to the implementation of efficient methods for the reduction of CO2 to CO [3]. Therefore, carbonylation reactions may also represent a very important indirect method for the conversion of carbon dioxide (the main waste currently produced by human activities, and principal responsible for the greenhouse effect [4]) into useful chemicals and materials.
Since their discovery at the beginning of the 19th century, carbonylation reactions have acquired a steadily increasing importance both at industrial and academic level, and nowadays a huge number of examples of these important processes has been reported in the scientific as well as patent literature [1]. In particular, the development of more efficient and selective catalytic systems, associated with the use of suitably functionalized starting materials, has opened the way to the achievement of sophisticated synthetic processes, with formation of complex carbonylated molecular architectures, with potential applications in many fields of science (including drug discovery and material science) in one step. Among processes like these, carbonylative double cyclization processes represent a particularly important methodology, as they allows obtaining the construction of two new cycles in one synthetic procedure with formation of carbonylated polycyclic structures starting from readily available and suitably functionalized substrates.
The present short review is aimed at offering a description of paradigmatic synthetic methodologies based on catalytic carbonylative double cyclization reactions.

2. Functionalized Olefinic Substrates

It is well-know that palladium(II)-based catalysts are able to activate unsaturated carbon-carbon bonds towards the nucleophilic attack of a variety of nucleophilic groups (mainly oxygen- or nitrogen-based). The intramolecular version of this reactivity is of particular importance, as it allows the construction of heterocyclic derivatives in a straightforward manner and under mild reaction conditions [Pd(II)-catalyzed heterocyclization reactions] [5]. On the other hand, it is also very well known that Pd(II) catalysts are able to promote many important kinds of carbonylation processes, particularly under oxidative conditions, including cyclization processes in which carbon monoxide is inserted as a carbonyl function inside the newly formed ring (cyclocarbonylation reactions) [6]. It is therefore not surprising that several important methods have been developed in which a single Pd(II)-based catalytic system is able to promote, in one synthetic step, the sequential heterocyclization ‒ cyclocarbonylation of suitably functionalized olefinic substrates, bearing two nucleophilic moieties placed in appropriate positions for undergoing the double cyclization process.
Pioneering studies on this kind of reactivity were conducted by the Semmelhack and Yoshida research groups during the 1980ties. Thus, in 1984 Semmelhack and coworkers reported the Pd(II)-promoted stereoselective carbonylative double cyclization of 1-(2-(hydroxymethyl)phenyl)prop-2-en-1-ols to give 3,3a,5,9b-tetrahydro-2H-furo[3,2-c]isochromen-2-ones with a cis junction between the newly formed rings using a stoichiometric amount of Pd(OAc)2 [7]. The process started with the intramolecular 6-exo-trig nucleophilic attack of the benzylic hydroxyl group to the double bond activated by coordination to the Pd(II) center, with formation of a cis-type alkylpalladium intermediate stabilized by chelation of the second hydroxyl group. The final bicyclic product was then formed through CO migratory insertion followed by intramolecular nucleophilic displacement by the hydroxyl (possibly, through the formation of a palladacycle followed by reductive elimination) (Scheme 1).
Interestingly, when a classical reoxidant for Pd(0) such as CuCl2 was employed to make the process catalytic, the reaction led to the formation of (E)-(2-(3-chloroprop-1-en-1-yl)phenyl)methanol from allylic chlorination (74% yield) [7]. Later on, however, suitable conditions were elaborated by the Yoshida group for performing the carbonylative double cyclization of 3-hydroxy-4-pentenoic acids to stereoselectively give tetrahydrofuro[3,2-b]furan-2,5-diones with a cis junction between the rings under Pd(II) catalysis [10 mol% PdCl2 in the presence of 3 equiv of CuCl2 and 3 equiv of AcONa, in glacial acetic acid as the solvent, at room temperature and under 1 atm of CO] (Scheme 2) [8]. The process took place through 5-exo-trig cyclization (by intramolecular nucleophilic attack of the carboxylic group to the double bond coordinated to the metal center, stabilized by chelation of the hydroxyl) to give a cis-type alkylpalladium complex, followed by CO insertion and intramolecular nucleophilic displacement (possibly, via the formation of a palladacycle followed by reductive elimination) (Scheme 2) [8].
The same research group then published the carbonylative double cyclization of 4-en-1,3-diols under similar reaction conditions, to obtain tetrahydrofuro[3,2-b]furan-2(3H)-ones (Scheme 3) [9].
Considering that the bicyclic tetrahydrofuro[3,2-b]furan-2(3H)-one substructure is largely found in natural and biologically active molecules, the methods disclosed by Semmelhack and Yoshida for the construction of this important core by carbonylative double cyclization of enediol derivatives have been largely employed as the key step in the semi- or total synthesis of natural products and bioactive compounds. Representative examples are shown in Table 1.
Interestingly, using the appropriate enantiopure ligand, kinetic resolution of (±)-pent-4-ene-1,3-diols is possible, with formation of the corresponding bicyclic lactone in noracemic form. This is exemplified by the Pd(OAc)2-catalyzed carbonylation of (±)-pent-4-ene-1,3-diol performed in the presence of an enantiopure bis(oxazoline) ligand and p-benzoquinone as external oxidant, to give (3aR,6aR)-tetrahydrofuro[3,2-b]furan-2(3H)-one in 29% yield and 62% ee (Scheme 4) [32].
More recently, the kinetic resolution of (±)-pent-4-ene-1,3-diols to give nonracemic tetrahydrofuro[3,2-b]furan-2(3H)-ones [2-(S,S) up to 80% ee, 2-(R,R) up to 57% ee] has been realized under similar conditions [4 mol% of Pd(OAc)2, 12 mol% of 2,6-bis[(4R)-4-phenyl-2-oxazolinyl]pyridine as enantiopure ligand, 0.5 equiv of p-benzoquinone, and 10 equiv AcOH] using an ionic liquid as the solvent (such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [Bmim][NTf2], 10 equiv), as exemplified in Scheme 5 [33].
Interestingly, the group of Gracza has reported that the use of iron pentacarbonyl as in situ liquid CO source may lead to improved results (significantly shorter reaction times, in particular) in the Pd(II)-catalyzed carboylative double cyclization of enediols with a terminal double bond, as exemplified in Scheme 6 [29,34,35].
The same research group recently reported their reaction under flow conditions using a continuous microflow system, as shown in Scheme 7 [31,36].
The carbonylative double cyclization process of enediols has also been reported to occur with 4-ene-1,2-diol derivatives. In this case, after the initial 5-exo-trig O-cyclization, in the cyclocarbonylation it is the free hydroxyl at C-2 that acts as internal nuclophile, with formation of a 6-membered ring. This is exemplified by the formation of 8-((tert-butyldimethylsilyl)oxy)-2,6-dioxabicyclo[3.2.1]octan-3-one from 3-((tert-butyldimethylsilyl)oxy)pent-4-ene-1,2-diol, as shown in Scheme 8 [37].
The nucleophilic group undergoing the initial heterocyclization process can also be nitrogen-based. Thus, as early as 1985, the Tamaru and Yoshida group disclosed the Pd(II)-catalyzed carbonylative double cyclization of the N-protected 5-aminopent-1-en-3-ols to yield N-protected 6-hydroxyhexahydro-2H-furo[3,2-b]pyrrol-2-ones, using the same conditions employed for 4-penten-1,3-diols [38] Among the protective groups tested, the carbamoyl group (P = CONHMe) turned out as the most suitable, as exemplified in Scheme 9 [39].
As predictable owing to the higher degrees of freedom of the alkyl chain, N-protected 6-aminohex-1-en-3-ols were significantly less reactive, and relatively good results were usually observed with P = CONHPh, as exemplified in Scheme 10 [39].
Later on, Jäger and coworkers reported the carbonylation of benzyl ((2R,3S)-2,3-dihydroxypent-4-en-1-yl)carbamate (Scheme 11) as a key step in the synthesis of novel 1,4-iminoglycitol derivatives as potential glycosidase inhibitors [40].
On the other hand, PdCl2-catalyzed carbonylation of benzyl ((2S,3S)-2,3-dihydroxypent-4-en-1-yl)carbamate afforded benzyl (3aR,6S,6aS)-6-hydroxy-2-oxohexahydro-4H-furo[3,2-b]pyrrole-4-carboxylate in 23% yield (Scheme 12) [41].
Kinetic resolution of N-protected 5-aminopent-1-en-3-ols in the Pd(II)-catalyzed carbonylative double cyclization has been reported by Gracza and coworkers. Thus, using enantiopure bisoxazoline ligands, nonracemic hexahydro-2H-furo[3,2-b]pyrrol-2-ones could be obtained, as exemplified in Scheme 13 [42].
The gaseous CO-free conditions elaborated group of Gracza for the carbonylation of 4-ene-1,3-diols, involving the use of liquid [Fe(CO)5] as in situ CO source (Scheme 6), have also been successfully employed by the same research team in the Pd(II)-catalyzed carboylative double cyclization of N-protected 5-aminopent-1-en-3-ols, as exemplified in Scheme 14 [34].
A general approach leading to carbonylative double cyclization is the intramolecular Pauson-Khand reaction starting from suitable enyne substrates. Since several excellent reviews have been published on this reaction [43,44,45], even in the most recent literature [46,47,48], this process will not be treated here; however, a particularly striking example is shown in Scheme 15, to give the reader an idea of the powerfulness of this synthetic method for the construction of complex carbonylated polycyclic compounds [49].

3. Functionalized Acetylenic Substrates

Starting from suitably functionalized acetylenic substrates, bearing a nucleophilic group in appropriate position, Kato and coworkers have reported an interesting Pd(II)-catalyzed carbonylative double cyclization process without CO incorporation into the rings, which leads to di(hetero)cyclic ketones [50]. According to Scheme 16, the process, which has been called by the authors “cyclization-carbonylation-cyclization coupling reaction” or ”CCC reaction”, starts with the electrophilic activation of the triple bond by a Pd(II) species, followed by intramolecular nucleophilic attack to the activated triple bond, with formation of the first ring (only the endo cyclization mode is shown in Scheme 16 for simplicity). The ensuing cyclic vinylpalladium intermediate then undergoes carbon monoxide insertion followed by coordination of another molecule of the acetylenic substrate. This opens the way to a second cyclization, which is followed by reductive elimination to give the final product and Pd(0). The latter is reoxidized to catalytically active Pd(II) by the use of an external oxidant, usually benzoquinone. The system CuCl2/O2 has also been occasionally used [51], although it should be noted that in this case the CO−O2 mixture employed fell within the explosion limits for a CO−O2 mixture (the CO-O2 mixtures are potentially explosive over a large range of composition: the flammability range for CO in O2 is 16.7-93.5 % at room temperature and it becomes even larger at higher temperatures [52]).
This method has been successfully employed by the group of Kato for the synthesis of a variety of di(heterocyclic)ketones; representative examples are shown in Table 2, entries 1-8. Entry 9 of Table 2 shows a recent extention of the concept to allenic substrates (2-methyl-1-phenyl-2,3-dien-1-ones, in particular) for the synthesis of bis(3-furanyl)methanones.
The Pd(II)-catalyzed carbonylative cyclization of functionalized acetylenic derivatives with CO incorporation into the cycle was disclosed by our research group a few years ago [61,62,63,64] using the PdI2/KI catalytic system already successfully employed by us for promoting a plethora of carbonylation reactions [65,66,67,68,69]. Thus, starting from readily available 4-yne-1,3-diols under oxidative conditions (using oxygen from air as benign oxidative agent), novel dihydrofurofuranone derivatives with antitumor activity have been synthesized (Scheme 17). In particular, 5,5-dimethyl-6a-phenyl-3-(trimethylsilyl)-6,6a-dihydrofuro[3,2-b]furan-2(5H)-one showed a significant antiproliferative activity in vitro on human breast cancer cell lines, including the most aggressive triple negative breast cancer cells (MDA-MB-231 and MDAMB-468), while being practically non-toxic on normal cells (human mammary epithelia cells, MCF-10A, as well as murine fibroblasts 3T3-L1) [61,62,63].
Mechanistically, the process involves an initial 5-exo-dig heterocyclization [by intramolecular nucleophilic attack of the terminal hydroxyl group to the triple bond coordinated to Pd(II)], followed by carbon monoxide insertion. Intramolecular nucleophilic displacement then takes place (probably, through the formation of a palladacycle followed by reductive elimination), to give the product and Pd(0). The latter is then reoxidized to PdI2 according to the mechanism we demonstrated several years ago in the PdI2/KI-catalyzed oxidative dialkoxycarbonylation of alkynes [70], which involves oxidation of 2 mol of HI (formed during the process) by oxygen to give I2, followed by oxidative addition of I2 to Pd(0) (Scheme 18; anionic iodide ligands are omitted for clarity) [63].
The method has then been extended to the use of 2-(3-hydroxy-1-yn-1-yl)phenols as substrates to give furo[3,4-b]benzofuran-1(3H)ones in the ionic liquid BmimBF4 (1-butyl-3-methylimidazolium tetrafluoroborate) as unconventional solvent (Scheme 19) [71]. The catalyst-solvent system could be conveniently recycled several times without appreciable loss of activity. Interestingly, this process turned out to be unselective when carried out in a classical solvent (such as DME or MeCN), with formation of mixtures of the desired furobenzofuranone derivative and the simple benzofuran product deriving from non-carbonylative heterocyclization.
In a similar way, starting from 2-(hydroxyprop-1-ynyl)anilines as the substrates, 3,4-dihydrofuro[3,4-b]indol-1-ones were synthesized in one step with yields up to 98% from an initial 5-endo-dig N-heterocyclization followed by cyclocarbonylation (Scheme 20) [64].
Interestingly, the use of the analogues substrates bearing a secondary propargylaminic moiety rather than the propargylacoholic group [that are, 2-(3-(alkylamino)prop-1-yn-1-yl)anilines] led to a complex reaction mixture when allowed to react under conditions similar to those shown in Scheme 20. However, a selective and novel double cyclization process was observed with the N-acyl derivatives, i.e., in the case of N-(3-(2-aminophenyl)prop-2-yn-1-yl)acetamides, with formation of 4,6-dihydro-5H-[1,3]oxazino[5,6-c]quinolin-5-ones. (Scheme 21) [72]. In this case, the reaction begins with the intramolecular nucleophilic attack by the amide carbonyl oxygen to the coordinated triple bond, with 6-endo-dig ring closure and formation of a vinylpalladium intermediate stabilized by coordination of the aniline amino group. Carbon monoxide insertion followed by intramolecular nucleophilic displacement (possibly, through the formation of a palladacycle) then delivers the product (Scheme 21) [72].
A striking carbonylative tetracyclization process was observed in the case of 2-(3-amino-3-methylbut-1-yn-1-yl)anilines, bearing a primary propargylaminic moiety, which led to 7,7,16,16-tetramethyl-5H,14H-benzopyrido[3'',4'':5',6']pyrimido[2',1':2,3][1,3]oxazino[5,6-c]quinoline-6,15-diones in one step (Scheme 22) [72].
In fact, these substrates firstly underwent PdI2-catalyzed oxidative carbonylation of the primary amino group to give the corresponding urea [73,74], which then reacted through O-6-endo-dig cyclization from the ureidic carbonyl group followed by two cyclocarbonylations in sequence to yield the final product (Scheme 23) [72].
More recently, we have studied the reactivity of thiophenecarboxylic acids bearing an ω-hydroxyalkynyl substituent in vicinal position under PdI2/KI-catalyzed oxidative carbonylation consitions, and found that also these substrates are able to undergo carbonylative double cyclization to give previously unknown 1H-furo[3,4-b]thieno[3,2-d]pyran-1,5(3H)-dione, 4H-furo[3,4-b]thieno[2,3-d]pyran-4,8(6H)-dione, 3,4-dihydro-1H,6H-pyrano[4,3-b]thieno[3,2-d]pyran-1,6-dione, and 6,7-dihydro-4H,9H-pyrano[4,3-b]thieno[2,3-d]pyran-4,9-dione derivatives (Scheme 24) [75].
The process begins with 6-endo-dig cyclization from the carboxylic group followed by cyclocarbonylation, as exemplified in Scheme 25 for the synthesis of 1H-furo[3,4-b]thieno[3,2-d]pyran-1,5(3H)-diones from 3-(3-hydroxyprop-1-yn-1-yl)thiophene-2-carboxylic acids [75].
We have also found that even sulfurated acetylenic substrates, under appropriate conditions, can undergo PdI2/KI-catalyzed carbonylative double cyclization. However, considering the instability of the free thiol group under the oxidative conditions employed [76,77], it is necessary in this case to protect the sulfur atom with a methyl group, which can be easily removed, after cyclization, under the reaction conditions thanks to the presence of the excess of iodide anions. Accordingly, starting from 5-(methylthio)-1-yn-3-ols, we have been able to synthesize 6,6a-dihydrothieno[3,2-b]furan-2(5H)-ones as a new class of S,O-bicyclic heterocycles, as shown in Scheme 26 [78].
Mechanistically, the process begins with 5-exo-dig S-cyclization by intramolecular nucleophilic attack of the sulfur of the thiomethyl group to the triple bond coordinated to PdI2. This is followed by demethylation of the ensuing sulfonium cation by the iodide anion, with formation of the corresponding vinylpalladium intermediate and methyl iodide. The latter readily reacts with water [initially present as impurity and then also formed in the final Pd(0) reoxidation step] to give MeOH and one mol of HI. On the other hand, the vinylpalladium intermediates undergoes carbon monoxide insertion followed by nucleophilic displacement to give tha final product together with Pd(0) and a second mol of HI. Lastly, Pd(0) is, as usual, oxidized back to PdI2 by its reaction with 2 mol of HI and 0.5 mol of O2 (Scheme 27) [78].

4. Functionalized Halides

Under non-oxidative conditions, suitably functionalized halides may undergo interesting Pd(0)-catalyzed double cyclization processes leading to high value added polycyclic heterocyclic compounds. Thus, 1,2-dibromoarenes have been reported by Beller and Wu to undergo carbonylative double cyclization when allowed to react with 2-aminobenzyl amine using Pd(OAc)2 in the presence of BuPAd2 (Ad = 1-adamantly) as the catalyst precursor, in N,N-dimethylacetamide (DMA) as the solvent, with Et3N as base and under 10 atm of CO. In this manner, several isoindolo[1,2-b]quinazolin-12(10H)-one derivatives (analogues of the anticancer agent batracylin) have been prepared in 36-84% yield (Scheme 28) [79].
Mechanistically, the process starts with the oxidative addition of a C‒Br bond to Pd(0), followed by CO insertion. Then, nucleophilic displacement by the more nucleophilic benzylic amino group of the diamine takes place, followed by further oxidative addition and CO insertion from the second C‒Br bond, intramolecular nucleophilic displacement by the second amino group, and intramolecular condensation (Scheme 29) [79].
In a similar way, and under similar conditions, isoindolo[1,2-b]quinazolin-10(12H)-ones have been synthesized by the Wu’s group starting from 2-bromoanilines and 2-bromobenzyl amines, as shown in Scheme 30 [80].
In this case, it is the 2-bromoaniline derivative that undergoes initial oxidative addition to Pd(0), followed by CO insertion (Scheme 31). This is followed by nucleophilic displacement by the 2-bromobenzyl amine, oxidative addition to Pd(0) of the second C‒Br bond, CO, insertion, intramolecular nucleophilic displacement and intramolecular condensation (Scheme 31) [80].
The group of Beller and Wu also reported the Pd(0)-catalyzed reaction of 2-bromoanilines with 2-bromobenzaldehyde and CO, which resulted in a carbonylative double cyclization leading to 5H-benzo[4,5,1,3]oxazino[2,3-a]isoindole-5,11(6aH)-diones (Scheme 32) [81].
As shown in Scheme 33, the process begins with the oxidative addition of 2-bromobenzaldehyde to Pd(0), followed by CO insertion and nucleophilic displacement by the amino group of the 2-bromoaniline derivative. Then, intramolecular nucleophilic attack of the nitrogen of the newly formed amido group to the formyl group takes place, followed by a second oxidative addition of the second C‒Br group to Pd(0), CO insertion, and intramolecular nucleophilic displacement by the hydroxyl group. 2-Bromobenzoic acid could also be employed as substrates in this reaction in place of 2-bromobenzaldehyde [81].
2-Bromobenzonitriles also underwent carbonylative double cyclization when allowed to react with 2-bromoanilines, to afford isoindolo[1,2-b]quinazoline-10,12-diones (Scheme 34) [82].
The first steps of the mechanistic pathway are similar to those seen above for the reaction with 2-bromobenzaldehyde. Thus, oxidative addition of the 2-bromobenzonitrile derivative was followed by CO insertion, nucleophilic displacement by the 2-bromoaniline, and intramolecular nucleophilic attack of the nitrogen of the newly formed amido group to the cyano group, with formation of the corresponding 2-(2-bromophenyl)-3-iminoisoindolin-1-one derivative (Scheme 35). Then, an unexpected isomerization of this intermediate takes place, probably due to steric effects, to give a (Z)-3-((2-bromophenyl)imino)isoindolin-1-one intermediate. Oxidative addition of the C‒Br bond of the latter to Pd(0), followed by CO insertion and intramolecular nucleophilic displacement deliver the final product (Scheme 35) [82].
The carbonylative double cyclization of substrates bearing two aryl halide bonds and a suitable nucleophile (such as an enolate, formed in situ under basic conditions) in appropriate position has also been reported, as shown by the synthesis of 5H-isochromeno[3,4-b]quinoline-5,12(7H)-diones starting from N-(2-bromophenyl)-2-(2-iodophenyl)acetamides (Scheme 36) [83].
In this case, the more reactive C‒I bond gives the initial oxidative addition to Pd(0), followed by CO insertion (Scheme 37). Then, intramolecular nucleophilic displacement by the enolate oxygen takes place, which leads to the first cyclization. Oxidative addition of the C‒Br bond to Pd(0), followed by Csp2‒H activation, CO insertion, and reductive elimination eventually gives the final product [83].

4. Conclusions and Future Perspectives

Catalytic carbonylative double cyclization is a powerful methodology for the construction of two novel rings in sequential order with the simultaneous incorporation of carbon monoxide into the final product, which allows, starting from simple building blocks, the direct synthesis of high value added, complex molecular architectures.
So far, several important examples have been reported in the literature, starting, in particular, from suitably functionalized olefinic, acetylenic, or halide substrate and under the catalysis of either Pd(II) or Pd(0) species. These reactions have led to the formation of important polycyclic heterocyclic derivatives, which have shown important biological activities (including anticancer activity) or that have been used as precursors for the synthesis of bioactive natural products.
In the next future, progress in catalysis is expected to give a further impulse to this very attractive field of synthetic chemistry, with the discovery of novel and more efficient catalytic processes able to afford in one step polycyclic heterocycles with potential applications in many fields of science (including material science and pharmaceutical chemistry).

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Preprints 75152 sch001
Scheme 1. Synthesis of 3,3a,5,9b-tetrahydro-2H-furo[3,2-c]isochromen-2-ones from 1-(2-(hydroxymethyl)phenyl)prop-2-en-1-ols [7].
Scheme 1. Synthesis of 3,3a,5,9b-tetrahydro-2H-furo[3,2-c]isochromen-2-ones from 1-(2-(hydroxymethyl)phenyl)prop-2-en-1-ols [7].
Preprints 75152 sch001
Preprints 75152 sch002
Scheme 2. Synthesis of tetrahydrofuro[3,2-b]furan-2,5-diones from 3-hydroxy-4-pentenoic acids [8].
Scheme 2. Synthesis of tetrahydrofuro[3,2-b]furan-2,5-diones from 3-hydroxy-4-pentenoic acids [8].
Preprints 75152 sch002
Preprints 75152 sch003
Scheme 3. Synthesis of tetrahydrofuro[3,2-b]furan-2(3H)-ones from 4-ene-1,3-diols [9].
Scheme 3. Synthesis of tetrahydrofuro[3,2-b]furan-2(3H)-ones from 4-ene-1,3-diols [9].
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Preprints 75152 sch004
Scheme 4. Kinetic resolution of (±)-pent-4-ene-1,3-diol leading to enantioenriched (3aR,6aR)-tetrahydrofuro[3,2-b]furan-2(3H)-one [32].
Scheme 4. Kinetic resolution of (±)-pent-4-ene-1,3-diol leading to enantioenriched (3aR,6aR)-tetrahydrofuro[3,2-b]furan-2(3H)-one [32].
Preprints 75152 sch004
Preprints 75152 sch005
Scheme 5. Kinetic resolution of (±)-pent-4-ene-1,3-diol in [bmim][NTf2] leading to enantioenriched (3aS,6aS)-tetrahydrofuro[3,2-b]furan-2(3H)-one [33].
Scheme 5. Kinetic resolution of (±)-pent-4-ene-1,3-diol in [bmim][NTf2] leading to enantioenriched (3aS,6aS)-tetrahydrofuro[3,2-b]furan-2(3H)-one [33].
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Preprints 75152 sch006
Scheme 6. Carbonylative double cyclization of pent-4-ene-1,3-diol using [Fe(CO)5] as in situ CO source [34].
Scheme 6. Carbonylative double cyclization of pent-4-ene-1,3-diol using [Fe(CO)5] as in situ CO source [34].
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Preprints 75152 sch007
Scheme 7. Carbonylative double cyclization of 4-ene-1,3-diols using [Fe(CO)5] as in situ CO source under flow conditions [36].
Scheme 7. Carbonylative double cyclization of 4-ene-1,3-diols using [Fe(CO)5] as in situ CO source under flow conditions [36].
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Preprints 75152 sch008
Scheme 8. 5-exo-trig O-cyclization followed by cyclocarbonylation with 6-membered ring closure [37].
Scheme 8. 5-exo-trig O-cyclization followed by cyclocarbonylation with 6-membered ring closure [37].
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Preprints 75152 sch009
Scheme 9. Formation of 6-hydroxyhexahydro-2H-furo[3,2-b]pyrrol-2-one derivatives from N-protected 5-aminopent-1-en-3-ols [39].
Scheme 9. Formation of 6-hydroxyhexahydro-2H-furo[3,2-b]pyrrol-2-one derivatives from N-protected 5-aminopent-1-en-3-ols [39].
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Preprints 75152 sch010
Scheme 10. Synthesis of 2-oxo-N-phenylhexahydrofuro[3,2-b]pyridine-4(2H)-carboxamide from 1-(4-hydroxyhex-5-en-1-yl)-3-phenylurea [39].
Scheme 10. Synthesis of 2-oxo-N-phenylhexahydrofuro[3,2-b]pyridine-4(2H)-carboxamide from 1-(4-hydroxyhex-5-en-1-yl)-3-phenylurea [39].
Preprints 75152 sch010
Preprints 75152 sch011
Scheme 11. Synthesis of benzyl (3aR,6R,6aS)-6-hydroxy-2-oxohexahydro-4H-furo[3,2-b]pyrrole-4-carboxylate, a precursor for the formation of glycosidase inhibitor derivatives [40].
Scheme 11. Synthesis of benzyl (3aR,6R,6aS)-6-hydroxy-2-oxohexahydro-4H-furo[3,2-b]pyrrole-4-carboxylate, a precursor for the formation of glycosidase inhibitor derivatives [40].
Preprints 75152 sch011
Preprints 75152 sch012
Scheme 12. Synthesis of benzyl (3aR,6S,6aS)-6-hydroxy-2-oxohexahydro-4H-furo[3,2-b]pyrrole-4-carboxylate from ((2S,3S)-2,3-dihydroxypent-4-en-1-yl)carbamate [41].
Scheme 12. Synthesis of benzyl (3aR,6S,6aS)-6-hydroxy-2-oxohexahydro-4H-furo[3,2-b]pyrrole-4-carboxylate from ((2S,3S)-2,3-dihydroxypent-4-en-1-yl)carbamate [41].
Preprints 75152 sch012
Preprints 75152 sch013
Scheme 13. Kinetic resolution of (±)-N-(3-hydroxypent-4-en-1-yl)-4-methylbenzenesulfonamide leading to enantioenriched (3aR,6aR)-4-tosylhexahydro-2H-furo[3,2-b]pyrrol-2-one [42].
Scheme 13. Kinetic resolution of (±)-N-(3-hydroxypent-4-en-1-yl)-4-methylbenzenesulfonamide leading to enantioenriched (3aR,6aR)-4-tosylhexahydro-2H-furo[3,2-b]pyrrol-2-one [42].
Preprints 75152 sch013
Preprints 75152 sch014
Scheme 14. Carbonylative double cyclization of tert-butyl (3-hydroxypent-4-en-1-yl)carbamate using [Fe(CO)5] as in situ CO source [34].
Scheme 14. Carbonylative double cyclization of tert-butyl (3-hydroxypent-4-en-1-yl)carbamate using [Fe(CO)5] as in situ CO source [34].
Preprints 75152 sch014
Preprints 75152 sch015
Scheme 15. Synthesis of 2a-(ethoxycarbonyl)-8-oxododecahydropentaleno[1,6-cd]pentalene-1-carboxylic acid from ethyl 5-acetoxy-1-(but-3-en-1-yl)-2,3,4,5-tetrahydropentalene-3a(1H)-carboxylate by Pauson-Khand−type intramolecular reaction [49].
Scheme 15. Synthesis of 2a-(ethoxycarbonyl)-8-oxododecahydropentaleno[1,6-cd]pentalene-1-carboxylic acid from ethyl 5-acetoxy-1-(but-3-en-1-yl)-2,3,4,5-tetrahydropentalene-3a(1H)-carboxylate by Pauson-Khand−type intramolecular reaction [49].
Preprints 75152 sch015
Preprints 75152 sch016
Scheme 16. The “cyclization-carbonylation-cyclization coupling” concept leading to di(hetero)cyclic ketones [50].
Scheme 16. The “cyclization-carbonylation-cyclization coupling” concept leading to di(hetero)cyclic ketones [50].
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Preprints 75152 sch017
Scheme 17. Synthesis of 6,6a-dihydrofuro[3,2-b]furan-2(5H)ones by PdI2/KI-catalyzed carbonylative double cyclization of 4-yne-1,3-diols [61,62,63].
Scheme 17. Synthesis of 6,6a-dihydrofuro[3,2-b]furan-2(5H)ones by PdI2/KI-catalyzed carbonylative double cyclization of 4-yne-1,3-diols [61,62,63].
Preprints 75152 sch017
Preprints 75152 sch018
Scheme 18. Proposed mechanism for the by PdI2/KI-catalyzed carbonylative double cyclization of 4-yne-1,3-diols leading to 6,6a-dihydrofuro[3,2-b]furan-2(5H)ones [63].
Scheme 18. Proposed mechanism for the by PdI2/KI-catalyzed carbonylative double cyclization of 4-yne-1,3-diols leading to 6,6a-dihydrofuro[3,2-b]furan-2(5H)ones [63].
Preprints 75152 sch018
Preprints 75152 sch019
Scheme 19. Synthesis of furo[3,4-b]benzofuran-1(3H)ones by PdI2/KI-catalyzed carbonylative double cyclization of 2-(3-hydroxy-1-yn-1-yl)phenols in ionic liquid BmimBF4 [71].
Scheme 19. Synthesis of furo[3,4-b]benzofuran-1(3H)ones by PdI2/KI-catalyzed carbonylative double cyclization of 2-(3-hydroxy-1-yn-1-yl)phenols in ionic liquid BmimBF4 [71].
Preprints 75152 sch019
Preprints 75152 sch020
Scheme 20. Synthesis of 3,4-dihydrofuro[3,4-b]indol-1-ones by PdI2/KI-catalyzed carbonylative double cyclization of 2-(hydroxyprop-1-ynyl)anilines [64].
Scheme 20. Synthesis of 3,4-dihydrofuro[3,4-b]indol-1-ones by PdI2/KI-catalyzed carbonylative double cyclization of 2-(hydroxyprop-1-ynyl)anilines [64].
Preprints 75152 sch020
Preprints 75152 sch021
Scheme 21. Synthesis of 4,6-dihydro-5H-[1,3]oxazino[5,6-c]quinolin-5-ones by PdI2/KI-catalyzed carbonylative double cyclization of N-(3-(2-aminophenyl)prop-2-yn-1-yl)acetamides [72].
Scheme 21. Synthesis of 4,6-dihydro-5H-[1,3]oxazino[5,6-c]quinolin-5-ones by PdI2/KI-catalyzed carbonylative double cyclization of N-(3-(2-aminophenyl)prop-2-yn-1-yl)acetamides [72].
Preprints 75152 sch021
Preprints 75152 sch022
Scheme 22. Synthesis of 7,7,16,16-tetramethyl-5H,14H-benzopyrido[3'',4'':5',6']pyrimido[2',1':2,3][1,3]oxazino[5,6-c]quinoline-6,15-diones by PdI2/KI-catalyzed carbonylative tetracyclization of 2-(3-amino-3-methylbut-1-yn-1-yl)anilines [72].
Scheme 22. Synthesis of 7,7,16,16-tetramethyl-5H,14H-benzopyrido[3'',4'':5',6']pyrimido[2',1':2,3][1,3]oxazino[5,6-c]quinoline-6,15-diones by PdI2/KI-catalyzed carbonylative tetracyclization of 2-(3-amino-3-methylbut-1-yn-1-yl)anilines [72].
Preprints 75152 sch022
Preprints 75152 sch023
Scheme 23. Plausible mechanism for the PdI2/KI-catalyzed carbonylative tetracyclization of 2-(3-amino-3-methylbut-1-yn-1-yl)anilines to give 7,7,16,16-tetramethyl-5H,14H-benzopyrido[3'',4'':5',6']pyrimido[2',1':2,3][1,3]oxazino[5,6-c]quinoline-6,15-diones [72].
Scheme 23. Plausible mechanism for the PdI2/KI-catalyzed carbonylative tetracyclization of 2-(3-amino-3-methylbut-1-yn-1-yl)anilines to give 7,7,16,16-tetramethyl-5H,14H-benzopyrido[3'',4'':5',6']pyrimido[2',1':2,3][1,3]oxazino[5,6-c]quinoline-6,15-diones [72].
Preprints 75152 sch023
Preprints 75152 sch024
Scheme 24. Synthesis of 1H-furo[3,4-b]thieno[3,2-d]pyran-1,5(3H)-diones, 4H-furo[3,4-b]thieno[2,3-d]pyran-4,8(6H)-diones, 3,4-dihydro-1H,6H-pyrano[4,3-b]thieno[3,2-d]pyran-1,6-diones, and 6,7-dihydro-4H,9H-pyrano[4,3-b]thieno[2,3-d]pyran-4,9-diones by PdI2/KI-catalyzed carbonylative double cyclization of thiophenecarboxylic acids bearing an ω-hydroxyalkynyl substituent in vicinal position [75].
Scheme 24. Synthesis of 1H-furo[3,4-b]thieno[3,2-d]pyran-1,5(3H)-diones, 4H-furo[3,4-b]thieno[2,3-d]pyran-4,8(6H)-diones, 3,4-dihydro-1H,6H-pyrano[4,3-b]thieno[3,2-d]pyran-1,6-diones, and 6,7-dihydro-4H,9H-pyrano[4,3-b]thieno[2,3-d]pyran-4,9-diones by PdI2/KI-catalyzed carbonylative double cyclization of thiophenecarboxylic acids bearing an ω-hydroxyalkynyl substituent in vicinal position [75].
Preprints 75152 sch024
Preprints 75152 sch025
Scheme 25. Proposed mechanism for the PdI2/KI-catalyzed carbonylative double cyclization of 3-(3-hydroxyprop-1-yn-1-yl)thiophene-2-carboxylic acids to give 1H-furo[3,4-b]thieno[3,2-d]pyran-1,5(3H)-diones [75].
Scheme 25. Proposed mechanism for the PdI2/KI-catalyzed carbonylative double cyclization of 3-(3-hydroxyprop-1-yn-1-yl)thiophene-2-carboxylic acids to give 1H-furo[3,4-b]thieno[3,2-d]pyran-1,5(3H)-diones [75].
Preprints 75152 sch025
Preprints 75152 sch026
Scheme 26. Synthesis of 6,6a-dihydrothieno[3,2-b]furan-2(5H)-ones by PdI2/KI-catalyzed carbonylative double cyclization of 5-(methylthio)-1-yn-3-ols [78].
Scheme 26. Synthesis of 6,6a-dihydrothieno[3,2-b]furan-2(5H)-ones by PdI2/KI-catalyzed carbonylative double cyclization of 5-(methylthio)-1-yn-3-ols [78].
Preprints 75152 sch026
Preprints 75152 sch027
Scheme 27. Proposed mechanism for the PdI2/KI-catalyzed carbonylative double cyclization of 5-(methylthio)-1-yn-3-ols to give 6,6a-dihydrothieno[3,2-b]furan-2(5H)-ones [78].
Scheme 27. Proposed mechanism for the PdI2/KI-catalyzed carbonylative double cyclization of 5-(methylthio)-1-yn-3-ols to give 6,6a-dihydrothieno[3,2-b]furan-2(5H)-ones [78].
Preprints 75152 sch027
Preprints 75152 sch028
Scheme 28. Carbonylative double cyclization of 1,2-dibromoarenes with 2-aminobenzyl amine to yield isoindolo[1,2-b]quinazolin-12(10H)-ones [79].
Scheme 28. Carbonylative double cyclization of 1,2-dibromoarenes with 2-aminobenzyl amine to yield isoindolo[1,2-b]quinazolin-12(10H)-ones [79].
Preprints 75152 sch028
Preprints 75152 sch029
Scheme 29. Proposed mechanism for the carbonylative double cyclization of 1,2-dibromoarenes with 2-aminobenzyl amine leading to isoindolo[1,2-b]quinazolin-12(10H)-ones [79].
Scheme 29. Proposed mechanism for the carbonylative double cyclization of 1,2-dibromoarenes with 2-aminobenzyl amine leading to isoindolo[1,2-b]quinazolin-12(10H)-ones [79].
Preprints 75152 sch029
Preprints 75152 sch030
Scheme 30. Carbonylative double cyclization of 2-bromoanilines with 2-bromobenzyl amines to yield isoindolo[1,2-b]quinazolin-10(12H)-ones [80].
Scheme 30. Carbonylative double cyclization of 2-bromoanilines with 2-bromobenzyl amines to yield isoindolo[1,2-b]quinazolin-10(12H)-ones [80].
Preprints 75152 sch030
Preprints 75152 sch031
Scheme 31. Proposed mechanism for the carbonylative double cyclization of 2-bromoanilines with 2-bromobenzyl amines to yield isoindolo[1,2-b]quinazolin-10(12H)-ones [80].
Scheme 31. Proposed mechanism for the carbonylative double cyclization of 2-bromoanilines with 2-bromobenzyl amines to yield isoindolo[1,2-b]quinazolin-10(12H)-ones [80].
Preprints 75152 sch031
Preprints 75152 sch032
Scheme 32. Carbonylative double cyclization of 2-bromoanilines with 2-bromobenzaldehyde to give 5H-benzo[4,5,1,3]oxazino[2,3-a]isoindole-5,11(6aH)-diones [81].
Scheme 32. Carbonylative double cyclization of 2-bromoanilines with 2-bromobenzaldehyde to give 5H-benzo[4,5,1,3]oxazino[2,3-a]isoindole-5,11(6aH)-diones [81].
Preprints 75152 sch032
Preprints 75152 sch033
Scheme 33. Proposed mechanism for the carbonylative double cyclization of 2-bromoanilines with 2-bromobenzaldehyde to give 5H-benzo[4,5,1,3]oxazino[2,3-a]isoindole-5,11(6aH)-diones [81].
Scheme 33. Proposed mechanism for the carbonylative double cyclization of 2-bromoanilines with 2-bromobenzaldehyde to give 5H-benzo[4,5,1,3]oxazino[2,3-a]isoindole-5,11(6aH)-diones [81].
Preprints 75152 sch033
Preprints 75152 sch034
Scheme 34. Carbonylative double cyclization of 2-bromoanilines with 2-bromobenzonitriles to give isoindolo[1,2-b]quinazoline-10,12-diones [82].
Scheme 34. Carbonylative double cyclization of 2-bromoanilines with 2-bromobenzonitriles to give isoindolo[1,2-b]quinazoline-10,12-diones [82].
Preprints 75152 sch034
Preprints 75152 sch035
Scheme 35. Proposed mechanism for the carbonylative double cyclization of 2-bromoanilines with 2-bromobenzonitriles to give isoindolo[1,2-b]quinazoline-10,12-diones [82].
Scheme 35. Proposed mechanism for the carbonylative double cyclization of 2-bromoanilines with 2-bromobenzonitriles to give isoindolo[1,2-b]quinazoline-10,12-diones [82].
Preprints 75152 sch035
Preprints 75152 sch036
Scheme 36. Carbonylative double cyclization of N-(2-bromophenyl)-2-(2-iodophenyl)acetamides to give 5H-isochromeno[3,4-b]quinoline-5,12(7H)-diones [83].
Scheme 36. Carbonylative double cyclization of N-(2-bromophenyl)-2-(2-iodophenyl)acetamides to give 5H-isochromeno[3,4-b]quinoline-5,12(7H)-diones [83].
Preprints 75152 sch036
Preprints 75152 sch037
Scheme 37. Proposed mechanism for the carbonylative double cyclization of N-(2-bromophenyl)-2-(2-iodophenyl)acetamides to give 5H-isochromeno[3,4-b]quinoline-5,12(7H)-diones [83].
Scheme 37. Proposed mechanism for the carbonylative double cyclization of N-(2-bromophenyl)-2-(2-iodophenyl)acetamides to give 5H-isochromeno[3,4-b]quinoline-5,12(7H)-diones [83].
Preprints 75152 sch037
Table 1. Representative examples of the Pd(II)-promoted carbonylative double cyclization of enediol derivatives in the synthesis of natural and bioactive products.
Table 1. Representative examples of the Pd(II)-promoted carbonylative double cyclization of enediol derivatives in the synthesis of natural and bioactive products.
Entry Conditions Substrate Product Yield (%) Ref.
1 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 41 h Preprints 75152 i001 Preprints 75152 i002 63 [10]
2 PdCl2(MeCN)2, (10 mol%), CuCl2 (2.4 equiv), CO (1 atm), THF, 25 °C, 24 h Preprints 75152 i003 Preprints 75152 i004 65 [11]
3 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 8 h Preprints 75152 i005 Preprints 75152 i006 85 [12]
4 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (4 equiv), CO (1 atm), AcOH, 25 °C, 24 h Preprints 75152 i007 Preprints 75152 i008 93 [13]
5 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 33 h Preprints 75152 i009 Preprints 75152 i010 38 [14]
6 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 15 h Preprints 75152 i011 Preprints 75152 i012 >80 [15]
7 Pd(OAc)2 (1.5 equiv), CO (1.1 atm), THF, 23 °C, 4 h Preprints 75152 i013 Preprints 75152 i014 87 [16]
8 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 15 h Preprints 75152 i015 Preprints 75152 i016 81 [17]
9 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C Preprints 75152 i017 Preprints 75152 i018 63 [18]
10 PdCl2, CuCl, AcONa, CO, AcOH Preprints 75152 i019 Preprints 75152 i020 33 [19]
11 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 10 h Preprints 75152 i021 Preprints 75152 i022 85 [20]
12 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 23 °C, 24 h Preprints 75152 i023 Preprints 75152 i024 75 [21]
13 Pd(OAc)2 (1.5 equiv), N-methylmorpholine (3 equiv), CO, THF Preprints 75152 i025 Preprints 75152 i026 58 [22]
14 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 20 h Preprints 75152 i027 Preprints 75152 i028 65 [23]
15 Pd(OAc)2 (10 mol%), CuCl2 (3 equiv), AcONa (39 equiv), CO (1 atm), AcOH, 25 °C, 15 h Preprints 75152 i029 Preprints 75152 i030 63, 70 [24,25]
16 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 24 h Preprints 75152 i031 Preprints 75152 i032 33 [26]
17 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 24 h Preprints 75152 i033 Preprints 75152 i034 87 [27]
18 PdCl2 (10 mol%), CuCl2 (3 equiv), AcONa (3 equiv), CO (1 atm), AcOH, 25 °C, 12 h Preprints 75152 i035 Preprints 75152 i036 61 [28]
19 PdCl2(MeCN)2 (10 mol%), CuCl2 (5 equiv), AcOLi (5 equiv), [Fe(CO)5] (0.5 equiv), AcOH, 60 °C, 1 h Preprints 75152 i037 Preprints 75152 i038 47 [29]
20 PdCl2(MeCN)2 (10 mol%), Cu(OAc)2 (4 equiv), LiCl (4 equiv), [Fe(CO)5] (0.25 equiv), AcOH, 60 °C, 15 min Preprints 75152 i039 Preprints 75152 i040 67 [30]
21 PdCl2(MeCN)2 (10 mol%), CuCl2 (4 equiv), AcOLi (4 equiv), [Fe(CO)5] (0.3 equiv), AcOH, 60 °C, 30 min Preprints 75152 i041 Preprints 75152 i042 75 [31]
Table 2. Examples of the “cyclization-carbonylation-cyclization coupling” concept leading to di(hetero)cyclic ketones.
Table 2. Examples of the “cyclization-carbonylation-cyclization coupling” concept leading to di(hetero)cyclic ketones.
Entry Conditions Substrate Product Yields (%) Ref.
1 Pd(tfa)2 (5 mol%), Preprints 75152 i043 (10 mol%), p-benzoquinone (2 equiv), CO (1 atm), MeOH, 0 °C, 5-12 h Preprints 75152 i044 Preprints 75152 i045 90-92 [50]
2 Pd(L)(tfa)2 (5 mol%), L =Preprints 75152 i046 p-benzoquinone (1.5 equiv), CO (1 atm), MeOH, 7 °C to 25 °C, 18-48 h Preprints 75152 i047 Preprints 75152 i048 24-89 [53]
3 Pd(tfa)2 (5-10 mol%), Preprints 75152 i049 (7.5-12 mol%), p-benzoquinone (2 equiv), CO (1 atm), MeOH, ‒30 to 0 °C, 2-53 h Preprints 75152 i050 Preprints 75152 i051 71-99 [54]
4 Pd(tfa)2 (5 mol%), Preprints 75152 i052 (7.5 mol%), p-benzoquinone (2 equiv), CO (1 atm), MeOH, 25 °C, 1-63 h Preprints 75152 i053 Preprints 75152 i054 10-89 [55]
5 Pd(L)(tfa)2 (5 mol%), L =Preprints 75152 i055 p-benzoquinone (1.5 equiv), CO (1 atm), MeOH, ‒5 °C to 25 °C, 1-46 h Preprints 75152 i056 Preprints 75152 i057 70-94 [56]
6 Pd(tfa)2 (5 mol%), Preprints 75152 i058 (7.5 mol%), p-benzoquinone (1.5 equiv), CO (1 atm), MeOH, ‒30 °C to 25 °C, 24-144 h Preprints 75152 i059 Preprints 75152 i060 75-100 [51,57]
7 Pd(L)(tfa)2 (5 mol%), L =Preprints 75152 i061 p-benzoquinone (1.5 equiv), CO (1 atm), iPrOH, ‒5 °C to 15 °C, 47-72 h Preprints 75152 i062 Preprints 75152 i063 73-92 [58]
8 Pd(L)(tfa)2 (5 mol%), L =Preprints 75152 i064 p-benzoquinone (1.5 equiv), CO (1 atm), MeOH, ‒20 °C to 0 °C, 24-76 h Preprints 75152 i065 Preprints 75152 i066 70-94 [59]
9 Pd(L)(tfa)2 (5 mol%), L =Preprints 75152 i067 p-benzoquinone (1.5 equiv), CO (1 atm), MeOH, 0 °C to 25 °C, 24-55 h Preprints 75152 i068 Preprints 75152 i069 12-86 [60]
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