Oxazaborolidine borane reduction acetophenone

After the identification of aprepitant as a clinical candidate, Merck invested considerable process research toward an improved synthesis of aprepitant, which culminated in the elegant manufacturing process shown in Scheme 6.21,22 The key step relies on displacement of a trifluoroacetate from intermediate 48 by the optically active alcohol intermediate 49. The synthesis of 49 was accomplished via an oxazaborolidine-catalyzed borane reduction of the corresponding acetophenone. Although the displacement resulted in an almost equal mixture of the two diastereomers 50 and 51, the desired diastereomer 50 could be recovered in high yield by base-catalyzed equilibration of the mixture and crystallization. Addition of p-fluorophenyl magnesium bromide followed by hydrogenolysis afforded the key intermediate 40, which can be readily converted to 1 as detailed in the previous synthesis. 

Reaction of ATBH with trimethyl borate in THF presumably affords the B-methoxy oxazaborolidine, which effectively catalyzes asymmetric borane reduction of prochi-ral ketones. Thus the borane reduction of acetophenone with the reagent prepared in situ from 0.1 equiv of ATBH and 0.12 equiv of trimethyl borate provides... 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

Kragl and Wandrey made a comparison for the asymmetric reduction of acetophenone between oxazaborolidine and alcohol dehydrogenase.[59] The oxazaborolidine catalyst was bound to a soluble polystyrene [58] and used borane as the hydrogen donor. The carbonyl reductase was combined with formate dehydrogenase to recycle the cofactor NADH which acts as the hydrogen donor. Both systems were run for a number of residence times in a continuously operated membrane reactor and were directly comparable. With the chemical system, a space-time yield of 1400 g L"1 d"1 and an ee of 94% were reached whereas for the enzymatic system the space-time yield was 88 g L 1 d"1 with an ee of >99%. The catalyst half-life times were... 

Boranes have opened the door to asymmetric reduction of carbonyl compounds. The first attempt at modifying borane with a chiral ligand was reported by Fiaud and Kagan,75 who used amphetamine borane and desoxyephedrine borane to reduce acetophenone. The ee of the 1-phenyl ethanol obtained was quite low (<5%). A more successful borane-derived reagent, oxazaborolidine, was introduced by Hirao et al.76 in 1981 and was further improved by Itsuno and Corey.77 Today, this system can provide high stereoselectivity in the asymmetric reduction of carbonyl compounds, including alkyl ketones. 

The reduction of dialkylketones and alkylaryl ketones is also conveniently accomplished using chiral oxazaborolidines, a methodology which emerged from relative obscurity in the late 1980s. The type of borane complex (based on (,V)-diphenyl prolinol)[39] responsible for the reductions is depicted below (10). Reduction of acetophenone with this complex gives (/ )-1 -phenylethanol in 90-95% yield (95-99% ee) [40]. Whilst previously used modified hydrides such as BiNAL-H (11), which were used in stoichiometric quantities, are generally unsatisfactory for the reduction of dialkylketones, oxazaborolidines... 

The proposed catalytic cycle for reduction of acetophenone is illustrated in Figure 1.28. The (5)-oxazaborolidine catalyst (5)-28A has both Lewis acidic and basic sites, and its borane adduct 28B acts as a chiral Lewis acid. The B center in the borolidine ring selectively interacts with a stericaUy more accessible electron... 

A modified oxazaborolidine 2 catalyzing the enantioselective reduction of acetophenone or tetralone with borane proved to give ttn values in the same order of magnitude [10, 11]. Using a special hydroxyproUne-based polymer-enlarged oxazaborolidine 3, a ttn of 1400 for the reduction of tetralone was achieved (Fig. 3.1.3, 3) [5, 12]. 

In 1987, Corey and co-workers proved that highly enantioselective reduction of ketones could be achieved by using stoichiometric borane in the presence of catalytic amounts of the oxazaborolidine 28a11 (Scheme 4.3j). Compound 28a, synthesized by heating (S)-(-)-2-(diphenylhydroxymethyl)pyrrolidine at reflux in THF with 3 equivalents of BH3 THF, shows excellent catalytic activity for the asymmetric reduction of acetophenone and other ketones. The B -methylated analog 28b was later synthesized to improve the air and moisture sensitivity associated with 28a. The third analog, 28c, with a 2-naphthyl substituent on the oxazaborolidine ring, has proven to be the best to afford the alcohol 29 with superb levels of enantioselectivity. 

The use of membrane reactors is favorable not only with respect to an increase in the total turnover number. In certain cases the selectivity can also be increased by applying high concentrations of the soluble catalyst together with making use of the behavior of a continuously operated stirred-tank reactor. Basically, this is also possible with a catalyst coupled to an insoluble support, but here the maximum volumetric activity is limited by the number of active sites per mass unit of the catalyst. This has been shown for the enantioselective reduction of ketones (eq. (2)) such as acetophenone 5 with borane 6 in the presence of polymer-enlarged oxazaborolidines 8 and 9 [65-67]. 

Halo-substituted acetophenones such as m-bromo- [74] or p-chloroacetophe-none [46] were reduced with borane in high enantioselectivity in the presence of oxazaborolidines 4b and 47, respectively. Other important halogen-containing ketones are chloromethyl or bromomethyl ketones. Oxazaborolidine reduction of co-chloro- or co-bromoacetophenone gives enantio-enriched halohydrins that can be converted into chiral oxiranes [20]. Martens found that the sulfur-containing oxazaborolidine catalysts 60 show high enantioselectivity in this kind of reduction [44, 86, 87]. Enantiopure halohydrins were obtained as shown in Scheme 8. 

Diazaborolidines that are structurally analogous to oxazaborolidine were prepared from a chiral diamine and borane [109]. Their catalytic activity is similar to that of oxazaborolidine. iV-Sulfonyldiazaborolidine derived from 2-amino-methylpiperidine was used in the reduction of acetophenone (>95% 72% ee)... 

Since the first asymmetric reduction of ketones with chiral borohydrides by Itsuno et al. [ 1 ], a number of studies on the asymmetric reduction of ketones with chiral borane reagents have been demonstrated [2]. Corey s oxazaborolidines are some of the most successful reagents [3 ]. The effect of fluorine substituents was examined in the asymmetric reduction of acetophenone with LiBH4 by the use of chiral boronates (73) obtained from substituted phenyl boronic acid and tartaric acid [4]. Likewise, 3-nitro, fluorine, and trifluoromethyl groups on the 3- or 4-position provided enhanced stereoselection (Scheme 5.20). 

The first attempt to use a chiral ligand to modify borane was Kagan s attempt at enantioselective reduction of acetophenone using amphetamine-borane and desoxy-ephedrine-borane in 1969 [18]. However, both reagents afforded 1-phenyl ethanol in <5% ee. The most successful borane-derived reagents are oxazaborolidines, introduced by Hirao in 1981, developed by Itsuno, and further developed by Corey several years later (reviews [19,20]). Figure 7.2 illustrates several of the Hirao-Itsuno and Corey oxazaborolidines that have been evaluated to date. All of these examples are derived from amino acids by reduction or Grignard addition. Hirao... 

Enantiomerically pure boranes have a long history in the reduction of prochiral ketones/ Amongst the early results using stoichiometric oxazaborolidines, the work of Itsuno is of particular interest. For example, acetophenone (3.32) could be reduced with the oxazaborohdines (3.113) or (3.114) where the ratio of amino alcohol to borane was 1 2, implying that one equivalent of oxazaborolidine and one equivalent of borane were present in the transition state. Itsuno also reported that the oxazaborohdine reagent (3.114) could be used catalytically in the reduction of prochiral ketoxime ethers. ... 

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