Ketones, aryl hydride transfer

This observation has led to the preparation of more effective bicyclic oxaza-borolidines such as 1, prepared from (S)-(-)-2-(diphenylhydroxymethyl)pyrrolidine and BH3 (la) or methylboronic acid (lb). Both reagents catalyze borane reduction of alkyl aryl ketones to furnish (R)-alcohols in > 95% ee, by face-selective hydride transfer within a complex such as B. Catalyst lb is somewhat more effective than... 

Promise is held in MPV reactions carried out under catalytic conditions. Instead of, for example, stoichiometric amounts of aluminum as the metal ion activator, catalytic quantities of complexes of rhodium and iridium can sometimes be used to bring about the same reactions. Although the catalytic mechanisms have not been established, postulation of the usual six-membered transition state in the critical step of hydride transfer appears reasonable. The strongly basic conditions of the MPV reaction are avoided. Reductions of aryl ketones (69 equation 30) using (excess) isopropyl alcohol as hydrogen donor and at partial conversions have led to the formation of alcohol (70) in modest enantiomeric excesses with various chiral ligands. " ... 

The rates of hydride transfer with (123) are only moderate with aldehydes in THF solution. However if no solvent is used and reaction times of several days are tolerated both aliphatic and aryl ketones can be reduced to the corresponding secondary alcohols (equation 53).Although reactions are sluggish,... 

Recent refinements in hydride transfer reductions have enhanced the utility of oxazaborolidine- and BINAP-Ru (II) complex-catalyzed reductions. A review by Wills describes the development of catalysts for the syntheses of chiral nonracemic secondary alcohols from aryl ketones [5]. Among the more interesting catalysts discussed were f/ -arene ruthenium complexes, which utilize diamine and monotosylated diamine ligands. 

A proposed mechanism, shown in Scheme 1, involves a six-member cyclic transition state between the aryl ketone and the active form of the catalyst, 2 [6]. The stable catalyst precursor 1 is transformed to the active catalyst, 2, through the loss of HCl. Treatment with 2-propanol forms ruthenium hydride 3 as a single diastereomer. Complexation of an aryl ketone precedes the hydride transfer step, which results in the reduced product. The mild reaction conditions make this catalyst an excellent candidate for incorporation in an imprinted network. The reported enantiometric excesses (ee s, +90%) serve as a useful benchmark to evaluate the influence of the imprinted polymer on the reduction. To the extent that the ruthenium center is situated in an imprinted cavity, the MIP can influence the approach of the ketone to the metal ion or better accommodate a specific reduction product. 

Some time ago, McLoughlin8 and later Mitchell9 observed the reductions of various aryl ketones and aldehydes displaying an ortho-O-ncyl residue (Fig. 4.9). The transformation appeared to involve the transfer of two hydrides and the researchers speculated the formation of an o-QM intermediate that subsequently underwent 1,4-reduction. 

Reductions can also be performed in water. Systems for reduction of ketones in water can be water-compatible sodium and lithium borohydrides, amino acid-based cationic surfactants to reduce aryl ketones [19], iridium hydrides used in transfer hydrogenations, such as [Cp Irm(bpy)H]+ (Cp — q5-C5Mes, bpy = 2,2 - bipyridine) [20], and IrHCI2(cod) 2 with a chiral diaminodiphosphine ligand to form secondary alcohols in high enantioselectivity and almost quantitative yield (Equation 4.12) [21]. 

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