Catalysis of organic reactions by molecular sieve systems: 1. Meerwein-Ponndorf-Verley reductions

Abstract

n-C 5 – C 12 alkanals underwent highly selective (9̆9% at 100 °C) Meerwein-Ponndorf-Verley (MPV) reduction in a continuous flow reactor, by applying as reducing agent a two-component hydrogen-transfer system consisting of NaX-type zeolite and isopropanol. Citronellal (1), which contains a Δ 6,7 double bond, underwent with MX-isopropanol systems (where M = alkali or alkaline-earth cation) two competing reactions, i.e., cyclization to isopulegol (2) or normal MPV reduction to citronellol (3). The direction of the reaction can be fully controlled by the type of exchangeable metallic ion in the X-type zeolite, CsX, yielding the MPV reduction product 3 with a selectivity of 92.3% at 150 °C, while LiX, NaX and CaX preferentially produce the cyclic compound 2. Non-substituted and alkyl-substituted cycloalkanones, having 4- to 15-membered rings were also investigated. The relative MPV conversions to the corresponding cycloalkanols at 100 °C and 180 °C were found to depend on the size of the ring system and, for substituted cycloalkanones, on the position and size of the alkyl substituent. Steric effects due to the geometry of the zeolite intracrystalline channel were also indicated. A limit in the applicability of the MPV reaction was reached with cyclopentadecanone, which was resistant to reduction. A mechanistic scheme for the zeolite-catalyzed MPV reductions is proposed.