Steric and kinetic effects of changing solvent and reactant in grignard reactions: Conformationally mobile versus rigid δ-keto esters |
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Authors: | A. Colantoni G. Di Maio E. Vecchi E. Zeuli |
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Affiliation: | Istituto di Chimica Organica dell''Università, Rome, Italy |
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Abstract: | trans-7α-carbomethoxy-decal-1-one (2) yields a mixture of the two oxy-esters 6 and 7 on reacting with MeMgX. Ratios were measured for reactions performed in benzene (with X = I) and in THF (with X = Cl). The small variations of the ratios as compared to those obtained in analogous experiments performed with methyl (2-oxo-cyclohexyl)-propionate 4 and methyl 4-methyl-5-oxo-hexanoate 5 suggest that conformational mobility plays a fundamental role in determining the variations of stereospecificity with varying the reaction conditions. Competitive Grignard reactions among 2,4 and 5 show that their reactivities are in the order 4>2>5 ( = 1.7; =0.8) when reactions are performed in benzene with X I and 2>4>5 (= 0.56; =0.25) when reactions are performed in THF with X C1. The experimental data are interpreted in terms of anchimeric assistance given by the ester group to the reactions of the keto group in conformationally mobile δ-keto esters. The occurrence of this effect depends on the reaction conditions which can favour, or not, folded transition states. |
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