Catecholborane with chiral additives

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

In addition to BH3, alkyl- and alkoxy-substituted boranes can also undergo insertion into a B-H bond. The higher stability and commercial availability of these boranes (e.g., BBN, catecholborane) has made them attractive as hydroboration reagents. These substituents can also be utilized to effect stereocenters upon insertion. Since hydroboration proceeds with controlled cis stereochemistry, the use of chiral boron substituents... 

Addition to C=C. Chiral boranes obtained from Rh-complex-catalyzed hydroboration with catecholborane are transformed into amines with retention of configuration on sequential treatment with MeMgCl and H NOSOjH. 

The third approach to obtain diarylmethylpiperazine derivatives uses the highly stereospecific chiral oxazaborolidine-catalyzed reduction, using catecholborane as the reductant of the 4-bromobenzophenone chromium tricarbonyl complex, as described by Corey and Helal [59], followed by the stereospecific displacement of the hydroxyl benzyl group by the /V-substituted-piperazine [44]. As outlined in Scheme 2, Delorme et al. [44] used this approach for the enantioselective synthesis of compound 31, (+)-4-[ (aS)-a-(4-benzyl-l-piperazinyl)benzyl]-lV,lV-diethylben-zamide. Lithiation of the readily available benzene chromium tricarbonyl with n-BuLi in the presence of TMEDA in THF at —78 °C, followed by addition of... 

It was reported that chiral oxazaborolidine, prepared from the reaction of n-BuBCl2 with (S)-l,l-diphenylpyrrolidinemethanol, can be obtained in high purity (Scheme 23.38). A catalyst prepared in this way was used in the reduction of the Torgov diketone with catecholborane and yielded the hydroxy ketone in 87% yield and 92% ee. Based on this observation, the role of additive N,N-diethylaniline was clarified. ... 

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