Aldol-Meerwein-Ponndorf-Verley reduction

We recently reported a modified Meerwein-Ponndorf-Verley reduction in which low-boiling alcohols such as EtOH and w-PrOH, but preferably i-PrOH, were used at temperatures near 225 °C in the absence of aluminum alkoxides [42]. The carbonyl moiety of an olefinic aldehyde such as cinnamaldehyde was reduced selectively to the alcohol without the carbon-carbon double bond being affected (Scheme 2.7). Since base was not present, aldol and Claisen-Schmidt condensations were avoided. 

Reactions of Aluminium Complexes with Carbonyls The Meerwein-Ponndorf-Verley Reduction/Oppenauer Oxidation and Murakaima Aldol Reactions... 

The synthetic method (a) is the regioselective reduction of an a,/ -unsaturated aldehyde or ketone (Section 5.18.2, p. 798), which is most conveniently effected by the Meerwein-Ponndorf-Verley procedure (Section 5.4.1, p. 520). The further disconnection shown of the a, -carbonyl compound is a retro-aldol condensation (Section 5.18.2, p. 799) however it should be emphasised that other routes to the unsaturated carbonyl compound, such as the Horner-Emmons reaction (Section 5.18.2, p. 799), may also be feasible. 

The use of Al(III) complexes as catalysts in Lewis acid mediated reactions has been known for years. However, recent years have witnessed interesting developments in this area with the use of ingeiuously designed neutral tri-coordinate Al(lll) chelates. Representative examples involving such chelates as catalysts include (1) asymmetric acyl halide-aldehyde cyclocondensations, " (2) asymmetric Meerwein-Schmidt-Ponndorf-Verley reduction of prochiral ketones, (3) aldol transfer reactions and (4) asymmetric rearrangement of a-amino aldehydes to access optically active a-hydroxy ketones. It is important to point out that, in most cases, the use of a chelating ligand appears critical for effective catalytic activity and enantioselectivity. 

In 2010, Crabtree and co-workers reported P-alkylation reaction of 1-phenylethanol (2 mmol) with benzyl alcohol was achieved by using simple alkali base KOH (2 mmol) under transition-metal free aerobic conditions, giving the corresponding ketones and alcohols in 78 and 21 %, respectively (Scheme 43) [236]. The reaction proceeded by an Oppenauer oxidation of these alcohols to give the ketone and aldehyde. Subsequently, base-assisted aldol reaction followed by Meerwein— Ponndorf—Verley (MPV) reduction and isomerization gave the p-alkylated products. 

---