Sulfides carbanions from

The range of monomers that can be incorporated into block copolymers by the living anionic route includes not only the carbon-carbon double-bond monomers susceptible to anionic polymerization but also certain cyclic monomers, such as ethylene oxide, propylene sulfide, lactams, lactones, and cyclic siloxanes (Chap. 7). Thus one can synthesize block copolymers involving each of the two types of monomers. Some of these combinations require an appropriate adjustment of the propagating center prior to the addition of the cyclic monomer. For example, carbanions from monomers such as styrene or methyl methacrylate are not sufficiently nucleophilic to polymerize lactones. The block copolymer with a lactone can be synthesized if one adds a small amount of ethylene oxide to the living polystyryl system to convert propagating centers to alkoxide ions prior to adding the lactone monomer. 

This transannular addition of a-sulfinyl carbanions to nonactivated double bonds is utilized as the key step in a synthesis of trans-1-thiadecalin (70) in enantiomerically pure form. The required ( )-thiacy-clodec-4-ene 5-oxide (66a,b) was prepared via several steps frtm (/ ,/ )-l,6-dibromo-3,4-hexanediol. Upon treatment with butyllithium, a 4 1 mixture of isomeric sulfoxides (66a and 66b) undergoes smooth cyclizadon to give a mixture of isomeric bicyclic sulfoxides (67a and 67b) in the same 4 1 ratio as the starting material, suggesting that the cyclizadon is essentially stereospecific. The major isomer (67a) is reduced with PCb to a sulfide (68) from which the desired (70) is derived via a thiaoctaline (69 heme 16). 

A method similar to that used for phosphole carboxylate synthesis has been employed for the formation of ketones (279), (280), and (281) (Scheme 86) here the carbanion from a phosphole sulfide is reacted with ethyl acetate and the sulfur then removed with a phosphine <76T2395>. 

Freparatively useful induced diastereoselectivities have been reported mainly for 1,1-di-substituted allyllithium derivatives which bear carbanion-stabilizing substituents. l-[Methyl-thio-l-(trimethylsilyl)-2-propenyl]lithium106 and the appropriate 1-phenylthio107 derivative, generated from the allylic sulfide with sec-butyllithium, in the reaction with tetrahydropyranyl-protected pregnolone, furnish a single diastereomer. 

Since alkyllithium compounds and their carbanions have an isoelectronic structure with alkoxides, their reaction behavior with carbenes is expected to be similar to that of alkoxides, showing enhanced reactivity in both C-H insertion and hydride abstraction.35 In this reaction, the hydride abstraction cannot be followed by recombination and, therefore, can be differentiated from the insertion. Indeed, the reaction of alkyllithium compounds 70 or nitrile anions (see Section IV.B) with ethyl(phenylthio)carbenoid, which is generated by the reaction of 1-chloropropyl sulfide 69 with BuLi, takes place at the -position of 70 more or less in a similar manner giving both insertion product 71 and hydride abstraction products 72 and 73, respectively. This again supports a general rule C-H bonds at the vicinal position of a negatively charged atom are activated toward carbene insertion reactions (Scheme 22). 

An explanatory mechanism for the formation of vinyl sulfides is shown in Scheme 24. In route a, (phenylthio)carbene 77 generated from chlorosulfide 75 reacts with the nitrile anion to form (phenylthio)carbanion 79, which then undergoes elimination of cyanide ion to produce vinyl sulfide 76. In route b, 75 reacts first with the nitrile anion 74 to produce P-(phenylthio)nitrile 78 followed by base-catalyzed P-elimination. However, route b is unlikely because 79 cannot be generated from 68 due to a larger pKa value of its ot proton than that of the nitrile. In fact, the reaction of chlorosulfide 75a with lithionitrile 80 gave a different product 81 in 63% yield. 

STEVENS REARRANGEMENT. Migration of an alkyl group from a quaternary ammonium salt to an adjacent carbanionic center on treatment with strong base. The product is a rearranged tertiary amine, sulfonium, or sulfide. 

Dienes from ally lie 2-pyridyl sulfides. Treatment of the carbanion of the 2-pyridyl sulfide 2 with tri- -butyl(iodomethyl)tin (1) results in formation of 1-phenylbutadiene (3) directly. 

From this, it can be seen that the amount of KOH within the hydroxide mixture would probably be critical in removing organo-sulfur from coal. While the particular role of KOH has not been determined, evidence from the literature has shown that the size of the cation may be important in stabilizing intermediate carbanions. Wallace et al. (J ) conducted a series of base- catalyzed, beta-elimination reactions with isopropyl sulfide and measured the amount of olefin production. The proposed mechanism involved initial abstraction of a proton by the t-butoxide base, and formation of a carbanion, with subsequent elimination of the sulfur moiety (which can be considered a good leaving group) to form the olefin (Equation 5). 

In the case of thiophenoxide and dichlorodifluoromethane, such a reaction forms chlorodifluoromethyl carbanion, which dissociates to difluorocarbene and chloride anion. Difluorocarbene reacts with the thiophenoxide anion to form phenylthiodifluoromethyl anion. This unstable species reacts in two ways. (1) It abstracts chlorine from another molecule of dichlorodifluoromethane and forms chlorodifluoromethyl phenyl sulfide Y. (2) It reacts with hydrogen of water to form difluoromethyl phenyl sulfide Y. Product Y can also react with two molecules of thiophenoxide anion to give bis(thiophenyl)difluoromethane Y" [SO]. 

Dithioacetals of aldehydes are sources of carbanions and hence may be used to form new C-C bonds in reactions in which the formerly electron-deficient character of the aldehydic carbon has been reversed. The 1,3-dithianes derived from formaldehyde or a higher aldehyde may be metallated and then alkylated (Scheme 2.27). Hydrolysis of the dithioac-etal is usually carried out in the presence of a thiophilic (sulfur seeking) metal salt such as a mercury salt. The insoluble sulfides cause the equilibrium to move in favour of the parent carbonyl compound. 

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