The reaction mechanism of Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions proceeds
in most cases via a cyclic six-membered transition state in which both reductant
and oxidant are coordinated to the metal centre of a metal alkoxide catalyst. Thermodynamics
gives insight into the strength of reductants and oxidants and yields quantitative
information about equilibrium conversions that may be reached in a specific MPVO
reaction. In aluminium-catalysed MPVO reactions, ligand exchange dominates the rate
of reaction. In lanthanide-catalysed MPVO reactions, ligand exchange is much faster
and reaction rates, therefore, are determined by either ligand exchange, hydride
transfer, or both. Traditional aluminium catalysts usually show too slow ligand exchange
to enable the use of catalytic amounts. Alkali and alkaline-earth metals have very
high ligand exchange rates, but low charge density (Na, K) or low coordination number
(Li) mostly prevents catalytic use in this case, too. Promising results are reported
for lanthanide catalysts and zirconium complexes and their hafnium analogues, in
which good Lewis acidic character is combined with excellent ligand exchangeabilities.
Metal alkoxide catalysts can be prepared by several methods. Direct synthesis proceeds
via metals or anhydrous metal halides. Exchange of the alkoxide group can be achieved
by alcoholysis or transesterification. Several heterogeneous systems have been developed,
with advantages like easy catalyst handling, workup, and recycling. Here again, lanthanide
and zirconium catalysts seem very promising. Various examples of selective reactions
are given. Chemo- and regioselectivity were found almost immediately after discovery
of the MPVO reactions, especially in the synthesis of steroids and other bio-related
compounds. Elegant modern examples show regio- and stereoselectivity in various forms.
Recently, highly enantioselective MPV reduction has been reported, in which the catalyst
was a chiral lanthanide(III) alkoxide. 1. Introduction 2. Mechanism and Related
Reactions 2.1. Reaction Mechanism 2.2. Side Reactions 3. Thermodynamics, Selection
of Oxidants and Reductants 4. Metal Alkoxide Catalysts 4.1. Choice of the Metal
Ion, the Ligands, and the Solvent 4.2. Preparation and Characterisation 5. Heterogeneous
Catalysts 6. Selectivity in MPVO Reactions 6.1. Chemo- and Regioselective MPVO Reactions
6.2. Stereoselective MPVO Reactions 6.3. Enantioselective MPVO Reactions 7. Conclusions
