NBu 4BH 4 reduction of 2-cyclohexenones

Abstract

We noticed previously an inversion in the regioselectivity of 2-cyclohexenones reduction with LiBH 4 in THF by addition of cryptand [2.1.1]. Therefore, the regioselectivity of attack on the α-enone should be connected to its interaction with the Li + cation; that is the reason why we studied the NBu 4BH 4 reduction of 2-cyclohexenones (1 and 2), the interaction C C C O· N +Bu 4 being certainly very loose, inducing thus the 1–4 reduction. Some reactions in phase transfer catalysis conditions have also been investigated. ▪ The regioselectivity of NBu 4BH 4 reduction of 1 is quasi independent upon the aprotic solvent nature (1–2/1–4 is about 10 90 in 8 aprotic solvents: Et 20, THF, toluene, DME, HMPA, dioxane, CH 3CN and CH 3NO 2) the yields being nearly quantitative at room temperature. On the other hand, the reaction rates seem to be related to the solvent acceptor power (the rate increases when the solvent A. N. decreases). The orientation is affected by the intervention of hydrogen bonding with the α-enone carbonyl when the NBu 4BH 4 reduction is performed in CH 2Cl 2, CH 3OH or H 20 (1–2/1–4 ⋍ 40 60 ). NBu 4BH 4 reduction of 2 in THF or toluene at 60 °C leads to a saturated alcohol, resulting from 1–4 addition followed by reduction, with very good yield and selectivity (about 95%). On the other hand, in the liquid-liquid phase transfer catalysis conditions (toluene-water), a regioselectivity decrease is noticed which can be attributed to an interaction between H 20 and the isophorone carbonyl. Some experiments performed in solid-liquid phase transfer catalysis conditions give low yields, unless cryptand is used, the ion exchange being difficult.