Borane coupling reactions

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

Bischirogenic unsymmetric (S,S)-BisP 6b (cf. (S,S)-BisP -borane 81b) were synthesized from the coupling reaction between synthon 79 and the lithiated (S)-alkylmethylphosphine-boranes 87 in reasonable to quantitative yields with enantioselectivity over 97% (Scheme 15) [94]. These phosphines constitute the unsymmetric version of BisP in that they bear different groups on both phosphorus atoms, breaking the C2-symmetry character of BisP [32,94]. 

Shmaryahu Hoz of Bar-Ilan University reports (J. Org. Chem. 68 4388, 2003) that alkyl boranes couple with dinitro aromatic rings such as 1 to give the alkylated aromatic, with loss of one of the nitro groups. This reaction shows remarkable regioselectivity, as illustrated by the formation of 2. Much more complex alkyl boranes participate also, as illustrated by the coupling of the 9-BBN derivative 3. The reaction proceeded to give 4 as a single diastereomer. 

Polystyrene-bound trialkylboranes, which can be prepared by hydroboration of support-bound alkenes with 9-BBN, undergo palladium-mediated coupling with alkyl, vinyl, and aryl iodides (Suzuki coupling Entries 1 and 2, Table 5.3 for vinylations, see Section 5.2.4). Because boranes are compatible with many functional groups and do not react with water, these coupling reactions could become a powerful tool for solid-phase synthesis. To date, however, few examples have been reported. 

Early findings by Suzuki and co-workers [109] showed that the palladium-catalyzed iminocarbonylative cross-coupling reaction between 9-alkyl-9-BBN derivatives, t-butylisocyanide, and arylhalides gives access to alkyl aryl ketones 132 after hydrolysis of the corresponding ketimine intermediates 131. Presumably, the concentration of free isocyanide is kept to a minimum by its coordination with the borane. Formation of an iminoacylpalladium(II) halide 130 by insertion of isocyanide to the newly formed arylpalladium complex followed by a transmetallation step afford the ketimine intermediates 131 (Scheme 8.52). 

Boranes and, to a lesser extent, boronic acids can undergo slow hydrolysis (protode-boration) in the presence of protic solvents. This unwanted reaction can become predominant if a cross-coupling reaction only proceeds slowly (e.g. with electron-rich, sterically demanding, or unreactive halides Scheme 8.20 see also Scheme 8.14) or if the boron derivative is particularly sensitive, for example 2-formylphenylboronic acid. In such instances the reaction should be performed under anhydrous conditions in an aprotic solvent with a boronic acid ester [151] or a stannane. 

Miyaura, N. Yamada, K. Suginome, H. Suzuki, A. Novel and convenient method for the stereo- and regiospecific synthesis of conjugated alkadienes and alkenynes via the Pd-catalyzed cross-coupling reaction of 1-alkenyl-boranes with bromoalkenes and bromoalk-ynesj. Am. Chem. Soc. 1985, 107, 972-980. 

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