Sulfur-stabilized carbanions

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

Discuss the relevance of these observations to the structure of sulfur-stabilized carbanions and rationalize your conclusion about the structure of the carbanions in MO terms. 

Enolates of aldehydes, ketones, and esters and the carbanions of nitriles and nitro compounds, as well as phosphorus- and sulfur-stabilized carbanions and ylides, undergo the reaction. The synthetic applications of this group of reactions will be discussed in detail in Chapter 2 of Part B. In this section, we will discuss the fundamental mechanistic aspects of the reaction of ketone enolates with aldehydes md ketones. 

Sulfur stabilized carbanions Heterocycle stabilized carbanions... 

Sulfur-stabilized copper-zinc carbanions have been used [299] by Knochel et at A first type obtained from the reaction of the Simmons-Smith reagent with a copper(i) thiolate was treated with a... 

Most of the applications of sulfoxides in synthesis make use of the reactions of sulfur-stabilized carbanions with electrophiles [385, 386]. Thus the methylsulfinyl methylene carbanion, conveniently generated through the interaction of sodium hydride with DMSO [387], is a powerful nucleophile. 

From oxidative cleavage of 1,2-diols and 1,2-amino alcohols Dibutyltin oxide, 95 By reaction of alkyl halides with sulfur-stabilized carbanions Methylthiomethyl p-tolyl sulfone, 192 From reduction of carboxylic acids Vilsmeier reagent, 341 From terminal alkenes by addition reactions... 

The synthesis of 4-alkyl thioketones is possible by exploiting the stabilizing effect of a sulfur atom upon an adjacent carbanionic center. Ambident allylic anions react so that conjugate addition proceeds exclusively with the a-carbon of the nucleophile,129 243 244 as illustrated in equation (S3) 245 arylsulfinyl and arylsulfonyl groups normally246 behave similarly.247-249 Sulfur-stabilized vinylic carbanions can be prepared and function as Michael donors in difunctionalization sequences.250... 

Cyclization of 4-thiophenylbutan-2-ols 473, available by the reaction of sulfur-stabilized carbanions with epoxides, affords substituted thiochromans (Scheme 165) <2000JHC1527>. 

Lastly for sulfur-stabilized carbanions, the synthesis of carbaprostacyclin analogues has been performed using intramolecular cyclization of a dithiane anion and bromine as the key step in the process (equation 98)642. 

The closure of the macrocyclic ring by means of an intramolecular 8 2 reaction is a straightforward approach, and sulfur-stabilized carbanion alkylation has been successfully applied in the synthesis of 14-membered cembranoids. The synthesis of nephthenol (40) and cembrene A (59) is an example of this methodology. In Li and Yue s report, the total synthesis of ( )-sarcophytol M (17) was achieved from ger-aniol (137) through 12 steps and in 8.9% overall yield with an intramolecular nucleophilic addition of a sulfur-stabilized carbanion to a ketone as a key step. This example is the first of the closure of a macrocyclic ring with the intramolecular nucleophihc addition of a sulfur-stabilized carbanion to a ketone (Scheme 6-7). 

The closure of the macrocyclic ring by means of an intramolecular Sn2 reaction is a straightforward approach. For example, an intramolecular nucleophilic addition of sulfur-stabilized carbanion to epoxide was used in the synthesis of nephthenol (40) and cembrene A (59) from trani.tran -geranylhanlool (144) (Scheme 6-10). ... 

The first enantioselective total synthesis of l -(—)-cembrene A (59) and / -( )-nephthenol (40) were achieved by employing an intramolecular nucleophilic addition of sulfur-stabilized carbanion to asymmetric epoxide as the key step, starting from L-serine (Scheme 6-13). ... 

There has been much synthetic interest in sulfur-stabilized carbanions. These anions include sulfides, 1,3-dithianes, sulfoxides, sulfones and sulfoximides. Since the structural results in this area have been recently compiled and discussed in excellent detail by Boche, it will suffice to present only the list of compounds in Table 5 whose structures have been reported. Most of these anions have varied and unique... 

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