The transition metal catalyzed hydroboration of enamines

The addition of catecholborane (HBcat, cat = 1,2-O2C6H4) to 9-vinylcarbazole can give either the branched or linear hydroboration product depending upon the judicious choice of metal catalyst used in these reactions. Analogous reactions with pinacolborane (HBpin, pin = 1,2-O2C2Me4) and HBBzpin (Bzpin = 1,2-O2C2Ph4) using catalytic amounts (5 mol%) of either Rh(acac)(dppb) or [Cp∗IrCl2]2 gave the linear hydroboration product selectively. Hydroborations of 1-pyrrolidino-1-cyclopentene and 1-pyrrolidino-1-cyclohexene were complicated by a competing dehydrogenative borylation pathway. The branched isomer was not observed to any significant extent in these reactions, suggesting that the directing effect of the nitrogen atom is negligible. Although catalyzed additions of HBcat to 1-vinyl-2-pyrrolidinone gave complicated product distributions arising from competing reactions, addition of HBpin effectively generated the corresponding linear hydroboration product in good yields.

Graphical abstractThe addition of catecholborane (HBcat) to 9-vinylcarbazole can give either the branched or linear hydroboration product selectively depending upon the judicious choice of metal catalyst used in these reactions. Hydroborations with 1-pyrrolidino-1-cyclopentene, 1-pyrrolidino-1-cyclohexene and 1-vinyl-2-pyrrolidinone, however, all gave products arising from competing dehydrogenative borylation pathways.Figure optionsDownload full-size imageDownload as PowerPoint slide