The Equilibrium Between 2‐Lithium‐Oxazole(‐Thiazole, ‐Imidazole) Derivatives and Their Acyclic Isomers – A Structural Investigation
1997; Wiley; Volume: 130; Issue: 9 Linguagem: Inglês
10.1002/cber.19971300908
ISSN0009-2940
AutoresChristoff Hilf, Ferdinand Bosold, Klaus Harms, Michael Marsch, Gernot Boche,
Tópico(s)Catalytic Cross-Coupling Reactions
ResumoAbstract 2‐Metallated oxazoles, thiazoles and imidazoles are of interest for the preparation of derivatives of the respective heterocycles, and – especially in the case of metallated thiazoles – as acyllithium equivalents in organic syntheses. Metallation at C2 of these compounds, however, often leads (more so with Li‐oxazoles than with Li‐thiazoles and Li‐imidazoles) to products derived from the acyclic isomers of the metallated heterocycles. In order to obtain more information on the positions of the relevant equilibria, we have carried out a 13 C‐NMR study of substituted li‐ and ZnCl‐oxazoles, Li‐thiazoles and Li‐imidazoles. In the case of Li‐oxazoles, the equilibrium is completely on the side of the acyclic isomers. Addition of ZnCl 2 , however, leads to ring‐closure, and, even in the case of 2‐ZnCl‐substituted benzoxazole, only the ring‐closed isomer is observed. This demonstrates a dramatic gegenion effect on the equilibrium between two isomeric anions. In the case of Li‐thiazoles, depending on the substitution pattern, either solely the ring‐closed or an equilibrium with the ringopened isomer is observed. In the series of Li‐imidazoles, only the 2‐lithiated benzimidazole is in equilibrium with its ring‐opened isomer. The tendency of these compounds to undergo ring‐opening parallels the leaving group properties of the various subunits [OLi > SLi > R 2 NLi, and phenyl‐O(S, NR)Li > vinyl‐O(S, NR)Li]. The difference between the effects of Li + vs. ZnCl + in the (benz)oxazole series is in agreement with results of solid‐state structure investigations of 2‐lithiated and a 2‐zincated thiazole: in the Li compound the C–S bond is 2.9 pm longer than in the ZnCl species, indicating a more facile C–S bond cleavage with Li + as the gegenion.
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