Sonication hydroboration

Another example of great synthetic interest, involves the hydroboration reaction of alkenes [62], In general, the addition of borane to alkenes proceeds stepwise, the final product being the trialkylborane. However, hindered alkenes react slowly, especially when the dialkylborane precipitates from the medium. It was found that trialkyl bor-anes could be obtained rapidly under sonication, even with highly hindered substrates (Eq. 3.5). Applications of this useful modification were published, among which were the reduction-hydroxylation of vinyl groups by 9-BBN [63,64]. 

Catalyzed hydroborationt. S prepared in the presence of 10 nnol Many catalysts have been discc Sonication with activated nickel ether) significantly increases the 1 fioselectivity for the hydroboraiioi reactions using bis(mesityl)niobiui cause the major hydroborating ages a-Halo boronic acids. The catecholborane without solvent. 

Brown et ah discovered a simple preparation of organoboranes inaccessible by hydroboration, e.g., aromatic, benzylic, allylic, by sonication of the halides, most of the time bromides, with magnesium and trifluoroborane etherate. The Grignard reagent formed in situ undergoes metal exchange in high yields (Eq. 35). 70... 

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