Vinyllithiums configurational stability

Vinyllithium is tetrameric in THF solution. Alkenyllithium compounds containing a-hydrogen or-aUcyl substituents do not isomerize readily however, the presence of donor solvents promotes (Z)-to-( ) isomerization. Those compounds containing a-aryl substituents isomerize readily. The configurational stabilities of HDC=C(Li)X, where X is CN or C02Me, and RHC=C(Li)Y, where Y is OR or MesSi, have been investigated. 

Trigonally hybridized vinyllithium derivatives are characterized by higher configurational stability [34,35]. Panek [36] demonstrated that (E) and (Z)-l-lithium-l-phenyl-l-butene could be prepared stereospecifically, with retention of configuration, by a lithium-halogen exchange reaction between n-butyllithium and (Z) and ( )-l-bromo-l-phenyl-1-butene, respectively (Scheme 7). Both lithium derivatives are configurationally stable at... 

In a more recent study Knorr and von Roman 114) investigated the configurational stability of the synthetically interesting 115) a-trimethylsilyl- and a-alkoxy-vinyllithium species 167 and 168. 

Configurational stabilities of the products of a number of such M-X reactions, as well as from the direct reaction, RX-I- Li, have been explored. Alkyl-substituted vinyllithium reagents are configurationally stable (20) under severe conditions (refluxing ether, several hours), but a,/9-aryI-substituted reagents are quite labile, possibly suggesting a transition state or intermediate of the following nature ... 

Knorr R, von Roman TF (1984) Configurational stability of vinyllithium derivatives with 1-trimethylsilyl and 1-alkoxy substituents. Angew Chem Int Ed 23 366-368... 

A similar result was observed with a completely aliphatic substituted cyclopropyl derivative (Scheme 10). When optically pure (S)-(+ )-l-bromo-l-methylspiro[2,5]octane 19 was treated with t-butyllithium in ether solution at -78 C for 30 min, the acid 22 obtained on carbonation was isolated, with complete retention of optical activity and configuration [22]. This indicates that 1-methylcyclopropyllithium derivatives are configurationally more stable than alkyllithium (sp ) reagents and those vinyllithium reagents that contain a phenyl group in the a-position. Their stability is comparable with that of alkyl-substituted vinyllithium reagents. 

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