| Research Abstract |
Recently, Noth and Manning reported that alkenes and alkynes can be hydroborated with catecholborane in the presence of Rh-catalyst under extremely mild conditions. Although catalytic hydrometalations such as hydrosilylation were widely examined, only a few reports were published on the catalytic hydroboration, because the reaction with most of hydroborating reagents proceeds readily without any catalysts. We applied the catalytic method for the following synthetic reactions.
The hydroboration of alkenes can tolerate various functionalities such as cyano and alkoxycarbonyl groups, however, more reactive keto group must be protected. On the other hand, the catalytic hydroboration of allyl acetone with catecholborane in the presence of 1 mole% of RhCl(PPh_3)_3 at 0 ^ﾟC proceeds chemoselectively at the carbon-carbon double bond but not at the keto group. The boronyl ketones thus obtained were subjected to coupling with aryl and 1-alkenyl halides to give corresponding ketones in 62-69% yiel
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ds. The coupling reaction was successfully induced by thallium (I) carbonate in the presence of palladium catalyst.
Although the hydroboration approach is simple and convenient, the method is not applicable to usual 1, 3-dienes because the hydroboration mainly occurs at the terminal double bond to give homoallylic boranes. We found that a catalytic 1, 4-hydroboration of 1, 3-dienes with catecholborane in the presence of Pd (PPh_3)_4 or Rh_4(CO)_<12> to afford (Z)-allylic boronates with high isomeric purity, over 99%.
Several rhodium (I) complexes containing chiral phosphine, (+) DIOP, (+) BINAP, (S, S) CHIRAPHOS, and (S) (R) BPPFA, have been found to be effective as catalyst for the asymmetric hydroboration of prochiral alkenes with catecholborane (1, 3, 2-benzodioxaborole) to give optically active 2-alkyl-1, 3,2-benzodioxaboroles. Among the ligands examined, DIOP has been recognized to be most effective to give high asymmetric inducation. We obtained a 74% ee for the hydroboration of indene. Less
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