Acyloxyboranes, borane catalysts

Reetz and coworkers introduced the cyclic chlorodialkylboron Lewis acid (75) (Equation 48) [46], and Kiyooka and coworkers made use of acyloxyborane (76) (Equation 49) [47] in enantioselective Mukaiyama-aldol reactions that employ stoichiometric amounts of the respective boron Lewis acids. Both species give high enantioselectivity in the formation of the desired aldol adducts. After Kiyooka s report of (76), various boron catalysts derived from chiral amino acids appeared in the literature. As such, Masamune and coworkers introduced (77) and (78) [48], Kiyooka and co workers introduced (79) [49], and Corey and co workers introduced (80) [50] as chiral acyloxy borane catalysts for enantioselective aldol reactions (Figure 5.7). 

Acyloxyboranes. Yamamoto et a/.1 have used the known reactivity of borane with carboxylic acids to activate acrylic acids for Diels-Alder reactions. Thus addition of BH3-THF to acrylic acids at 0° furnishes an acyloxyborane formulated as 1, which undergoes cycloaddition (equations I and II). The reaction proceeds satisfactorily even when borane is used in catalytic amounts. A chiral acyloxyborane, BL, prepared from a tartaric acid derivative, can serve as a catalyst for an asymmetric Diels-Alder reaction (equation III). 

Asymmetric Diels-Alder Reaction of Unsaturated Carboxylic Acids. A chiral acyloxyborane (CAB) complex (1) prepared from mono(2,6-dimethoxybenzoyl)tartaric acid and 1 equiv of borane is an excellent catalyst for the Diels-Alder reaction of a,p-unsaturated carboxylic acids and dienes. In the CAB-catalyzed Diels-Alder reaction, adducts are formed in a highly diastereo- and enantioselective manner under mild reaction conditions (eq 2). The reaction is catalytic 10 mol % of catalyst is sufficient for efficient conversion, and the chiral auxiliary can be recovered and reused. 

Two related chiral Lewis acids are the acyloxyborane 68 [46] and the oxazabo-rolidine 69 [47] which are derived from (/ ,/ )-tartrate and L-valine, respectively. Each uses borane to generate the active catalyst, and they promote a variety of... 

With these experimental results at hand, it became of interest to evaluate the inducing ability of appropriate chiral auxiliary by introducing them into the acyloxyborane intermediate. Reaction of the monacylated tartaric acid with one equiv of borane should result in the formation of chiral acyloxyborane intermediate. Acrylic acid was then added to this catalyst and the mixture was treated with cyclopentadiene at -78 C After being stirred for 24 h, the product was isolated in high yield. The product showed a specific rotation of -86 which corresponded to 78% optical yield. 

Since our group (22) and Hehnchen s (23) independently announced a new class of chiral acyloxyboranes derive from iV-sulfonylamino acids and borane THF, chiral 1,3 -oxazaborolidines, their utility as chiral Lewis acid catalysts in enantioselective synthesis has been convincingly demonstrated (2(5). In particular, Corey s tryptophan-derived chiral oxazaborolidines 10a and 10b are highly effective for not only Mukaiyama aldol reactions (24) but also Diels-Alder reactions (25). More than 20 mol% of 10b is required for the former reaction, however. Actually, the reaction of the trimethylsilyl enol ether derived from cyclopentanone with benzaldehyde afforded the aldoI products in only 71% yield even in the presence of 40 mol%of 10b (24). We recently succeed in renewing 10b as a new and extremely active catalyst lOd using arylboron dichlorides as Lewis acid components (2(5). 

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