Allylic and Propargylic Carbanions

Benzylic organometallic compounds may react with electrophiles at either the benzylic or ortho position. Benzylic Grignard or organozinc reagents react with elec- 

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Scheme5.11. y-Alkylations of allylic and propargylic carbanions [123, 125,126]. Ar=2,6-Ph2C6H3.

Allylic and propargylic heteroatom-substituted carbanions can yield rearranged or unrearranged products on treatment with an electrophile. The regio- and stereoselectivity of these reactions depends on the precise structure of the carbanion, on the metal and solvent chosen [199], and on the structure of the electrophile [150, 200-203], and can be difficult to predict. 

The addition of a-sulfinyl carbanions to carbonyl compounds followed by further functional group modification has proved to be an efficient and highly stereoselective approach to epoxides, a-aminoketones, a-aminoaldehydes, a-hydroxy esters, and allylic and propargylic alcohols [76-80]. 

Uses of Saturated Sulphides, Selenides, and Tellurldes in Synthesis.— These applications are mostly based on the C—C bond-forming reactions of sulphenyl carbanions the extensive uses of vinyl sulphides and their allyl and propargyl analogues are discussed in a later section. Some methods for C—S or C—Se bond cleavage, which are used to terminate a use in synthesis, have been discussed in an earlier section other methods, notably the conversion of the sulphide into a sulphoxide followed by pyrolytic elimination of the sulphenic acid, are covered in later sections. 

The sequential trans-addition of a carbon nucleophile and a carbon electrophile across an arene double bond in (arene)Cr(CO)3 was first reported in 1983 [35]. Since then this methodology has undergone extensive development, with recent efforts mainly directed towards enantioenriched products [36]. Anionic (cy-clohexadienyl)Cr(CO)3 complexes are very soft nucleophiles and this places restrictions on the electrophiles that can be used in this sequence. Specifically these reactions are successful when carbanion dissociation from the intermediate anionic cyclohexadienyl complex is slow compared to the reaction with the carbon electrophile. The sequential addition is usually carried out as a one-pot reaction and the proposed reaction sequence is that shown in Scheme 11. In contrast to the nucleophile addition/protonation sequence, products form with excellent 1,2-regioselectivity. It is likely that this is due to an irreversible transfer of the acyl, allyl, or propargyl group to one of the two termini of the cyclohexadienyl ligand. 

This chapter focuses the attention on the reactions of nonstabilized carbanionic compounds such as alkyl, vinyl, aryl, alkynyl metals, etc., and the chemistry of the stabilized system, i.e. allylic, propargylic or oxaallylic carbanions is presented in Volume 2 of this series. Electrophiles with C=X bonite which are discussed include aldehydes, ketones, epoxides, aziridines, acetals, orthoesters and imines, all of which turn into highly reactive electrophiles in the presence of Lewis acids. 

In an attempt to alleviate the problems associated with the instability of diethyl 1-lithio-l, 1-difluoromethylphosphonate, a study of the reactivity of the corresponding organocadmium compound was performed in 1981. It reacts with a variety of allyl, propargyl, and benzyl bromides or iodides in THF at room temperature to afford the alkylated products in 7-64% yields (Scheme 3.71, M = CdBr, X = Br, I), but these compounds are contaminated with variable quantities of protonated carbanion. ... 

Scheme 6.7. The effects of allylic strain on the stereoselectivity of alkene formation [32]. (a) AL2 strmn and the selective formation of 2-alkenes. (b) AL3 strain causes selective formation of -a kenes. (c) If one or both of the partners cf Figure 6.5a, R,. R2, or Z) is hydrogen, the selectivity is diminished, d) Al.3. t ain produces 100% selectivity when the carbanionic carbon is propargylic [48].

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