Thiol esters cross-coupling

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

Savarin, C. Srogl, J. Liebeskind, L. S. Thiol ester-boronic acid cross-coupling. Catalysis using alkylative activation of the Pd thiolate intermediate. Org. Lett. 2000, 2, 3229-3231. 

Falck and co-workers report cross-coupling of (a-(acyloxy)alkyl)tributylstan-nanes with thiol chloroformate using a catalytic amount of copper salts. For example, (a-(acetyloxy)benzyl)tributylstannane 13 was treated with thiol chloroformate 14 in the presence of 8 mol% of CuCN in toluene at 75 °C for 21 h to give the thiol ester 15 with 98% yield (Eq. 14) [21]. 

In the presence of 5-10% palladium catalyst 34,20-45% Nal, and 4-6 equiv K2CO3, various boronic acids underwent cross-coupling with thio esters 32 in dimethylacetamide at 90-95 °C for 12-24 h to give the desired ketones in moderate to high yields. Tetrahydrothiophene, detected by GC/MS, was formed by intramolecular nucleophilic attack at the terminal carbon by a sulfur atom. Interestingly, in the absence of Nal, the bromobutyl thiol ester 32a gave only a trace amount of benzophenone when treated with phenylboronic acid in the presence of 34 in DMA at 95 °C for 22 h. In the presence of Nal, the bromide atom at the terminal carbon is replaced by an iodide atom, leading to sulfonium formation via intramolecular S-alkylative activation. 

Liebeskind and Srogl developed Pd-catalyzed cross-coupling of simple thiol esters with boronic acids to give ketones in the presence of Cu(I) thiophene-2-carboxylate (CuTC) under nonbasic conditions (Eq. 25) [50]. 

In 2002, Venkataraman published the synthesis of bisaryl- and arylalkyl-thioethers catalyzed by the Cul/neocuproine system [74]. At the same time, using excess ethylene glycol as a ligand, Cul-catalyzed coupUng of aryl iodides with thiols was found to take place at 80 °C [75]. Soon after, N,N-dimethylglycine (L2) (Table 9.15, entry 1) [76] and L-proline (LI) (entry 2) [77] were revealed to be effective for the same transformation. Recently, CuBr/ 3-keto ester (L19) was reported to be able to promote this cross-coupling reaction to afford the corresponding diaryl thioethers at 60-75 °C (entry 3) [27]. 

A common feature of the above described processes is that only aryl substituents are tolerated on the side of the acid chloride 4 and aryl iodide 6 due to ketene formation and jd-H elimination from aliphatic derivatives. However, primary alkyl-substituted alkynones 1 can be obtained by a palladium-catalyzed cross-coupling of corresponding thiol-esters 9 with terminal alkynes 5 in the presence of overstoichiometric amounts of copper (2003SL1512, 2010SL1239) (Scheme 4). 

In combination with catalytic amounts of Pd, CuTC has given rise to novel cross-coupling reactions under mild and neutral con-ditions that have furnished a number of very useful organic building blocks. Thus, an unprecedented cross-coupling of thiol esters with aryl and alkenylboronic acids in the presence of a catalytic amount of Pd(0) and 1.6 equiv of CuTC furnished a wide variety of aryl ketones under baseless conditions (eq 7). ... 

Arylthiocyclobutenediones may be considered as vinylogous thiol esters. As such, they were expected to participate in the Liebeskind-Srogl cross-coupling. Thus, (2) was exposed to a variety of horonic acids under the Lieheskind conditions to give the corresponding symmetrical cyclohutenediones (eq 16). ... 

Functionalized alkenyl diamino- and dialkoxyboranes have been produced regio-and stereoselectively through addition of carbon- or heteroatomgenerated from bromotrichloromethane, thiols, phosphines and tributyltin hydride) to ethynylbis(diisopropylamino)boranes. The synthetic utility of these reactions was illustrated by the preparation of stereodefmed (Z)- or ( )-alkenylboronic esters via palladium-catalyzed crossStille reaction and a Suzuki coupling under basic conditions can be further conducted (Scheme 9.14) [33]. 

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