Thiocarbonyl insertions

A rather complex mixture of products is obtained from the reaction of benzyl-idenetriphenylphosphorane and CS2.48 The major product from the reaction of diphenyl disulphide with methylenetriphenylphosphorane is tris(phenylthio)-methane (44) and only a trace of the insertion product bis(phenylthio)methane is isolated.49 Presumably the salt (45) is deprotonated before it can react with the phenylthioate anion (Scheme 12). a-Thiocarbonyl-stabilized ylides (46) are obtained from the reaction of ylides with S-alkyl thiolcarboxylates.50... 

Insertion of imsymmetrical heterocumulenes such as 0=C=X (X = S, NR), into an N-Si bond can produce two different constitutional isomers [8]. In the case of compound 1 and X = NR, R = alkyl or aryl, there is rapid insertion into both N-Si functions and the resulting situation is extremely complex with respect to the configurational (0,N-silylation) and conformational isomerism [5, 8, 10]. In contrast, 0=C=S inserts rapidly into only one N-Si bond of 1, there is no evidence for any second insertion even after several days. The formation of small amounts of pyrazine as oxidation product even under strictly anaerobic conditions suggests some electron transfer reactivity. Spectroscopic [10] and especially the structural evidence given below show that of the two conceivable constitutional isomers [11] of the insertion reaction only the 0-Si bonded species 2 with intact thiocarbonyl function is formed. 

In summary, the insertion of 0=C=S into the Si-N bond of reactive 1 proceeds to give only one apparently rather stable primary product which owes its stability to the matching of thiocarbonyl n-acceptor and dihydropyrazine 7t-donor capacities. 

Interception of reactive intermediates provides access to an even broader range of products. For example, insertion of CO leads to conjugated lactones and thioe-sters (Scheme 4). A wide variety of sulfur derivatives can be used in these processes producing an array of interesting synthetic intermediates. These organosulfur compounds can be free thiol [164, 167-170, 175, 190, 193, 197, 202, 204], disulfide [160, 165, 183, 207], thiocyanate [179, 182, 186], thioborate [161], thiostannane [196], sulfenamide [180], and thioester [187, 189, 191, 194, 195]. Palladium catalyzed thiocarbonylation, and the development of multicomponent reactions centering on hydrothiolation with free thiols and disulfides has been reviewed extensively [19, 20, 23-26, 167, 227-231]. 

A probable mechanism for the thiocarbonylation of allylic alcohols is outlined in Scheme 3. It is well-known that Pd(OAc>2 is easily reduced to Pd(0) in situ in the presence of phosphine ligands and carbon monoxide. Oxidative addition of protonated allylic alcohol to Pd(0) gives the tr-allylpalladium complex A, which may undergo substitution of H2O by SPh to form the tr-allylpalladium sulfide complex B. Insertion of CO into B affords the acylpalla-dium complex C. Reductive elimination of Pd(0) would form the /3,-y-unsaturated thioesters. 

The latter carbonylation involves the formation of PtH(SR)(PPh3)2 by the oxidative addition of RSH to the zero-valent platinum complex. A possible pathway may include the CO insertion into the S-Pt bond of PtH(SR)(PPh3)2. Then, acylplatination of alkynes generates p-thiocarbonyl-substituted vinylplatinum intermediate, which undergoes reductive elimination to give the a,p-unsaturated thioesters with regeneration of the catalyst. 

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