Enantioselectivity Meerwein-Ponndorf-Verley reduction

Evans reported an enantioselective Meerwein-Ponndorf-Verley reduction using a catalytic amount of chiral samarium complex 26 prepared from samarium (III) iodide and a chiral amino diol (Scheme 9.16) [34], Even when a partially resolved ligand (80% ee) was used, the enantiopurity of the resulting alcohol 27 reached 95% ee, which is the same value as that obtained when the enantiopure amino diol was used. 

It is important to realise that chirality is a symmetry property of a whole molecule and cannot be localised in a particular centre or group (although it may be associated with the presence of a particular stereogenic unit). For this reason the intramolecular transfer of chirality is an impossibility. Transfer of chirality can only occur between two molecules and is quite uncommon. [An example is the enantioselective Meerwein-Ponndorf-Verley reduction of a ketone with a chiral alcohol.] What some authors refer to incorrectly as transfer of chirality is, in fact, retention of chirality in the course of modification of the stereogenic units present. 

As shown in Figure 1.26, a chiral Sm(III) complex catalyzes asymmetric reduction of aromatic ketones in 2-propanol with high enantioselectivity. Unlike other late-transition-metal catalysis, the hydrogen at C2 of 2-propanol directly migrates onto the carbonyl carbon of substrate via a six-membered transition state 26A, as seen in the Meerwein-Ponndorf-Verley reduction. ... 

Yu and coworkers reported that use of PrSBEt2 as an additive accelerates the chiral Zr-catalyzed asymmetric allylation reaction and suppresses the concomitant Meerwein-Ponndorf-Verley reduction. The presence of the additive is thought to dissociate the product from the reaction complex and to regenerate the chiral catalyst [28]. This method was further extended to asymmetric propargylation with allenyltributylstannane by the same group [19]. In contrast, Taghavini and Umani-Ronchi and their group have shown that an enantioselective allylation of... 

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. 

Although the Meerwein-Ponndorf-Verley (MPV) reduction was discovered over 80 years ago, it has not been until relatively recently that catalytic variants utilising chiral aluminium have been realised. " In 2002, the Nguyen group reported a practical, enantioselective catalytic MPV reduction. Isopropanol was used as the hydride source, and trimethylaluminium with (R)-BINOL as the catalyst (Scheme 19.54). ... 

A highly enantioselective Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction of N-phosphinoyl ketimines by (BINOL)Al(iii)/2-propanol has been reported. High yields and enantiomeric excesses were observed for a wide range of structurally diverse ketimines. A [2.0.4] bicyclic chelation model was proposed to account for the... 

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