Transition metal-catalyzed hydroboration

Model studies on rhodium catalyzed hydroborations of sterically unbiased alkenes with catecholborane provided evidence for the role of d$\pi$-p$\pi$ bonding in these transformations. Catalyzed hydroborations of 5-substituted-2-methylene adamantanes occurred predominantly from the face opposite the most electron withdrawing group, while uncatalyzed hydroborations gave the reversed selectivities. Deuterium labeling studies on rhodium catalyzed hydroborations of allylic silyl ethers revealed some new mechanistic features. Vinylboronate esters were produced in these transformations in addition to the expected alkylboronate esters. The formation of such dehydrogenative borylation products can be explained by alkene insertion into the rhodium-boron bond of a metal complex produced from oxidative addition of catecholborane to the catalyst. Oxidation of the catalyst can have a pronounced effect on the product distributions. Catalyzed hydroboration of phenylethene produced 1-phenylethanol with freshly prepared Wilkinson's catalyst, but 2-phenylethanol was the predominant product if the catalyst had been in contact with oxygen. Enantioselective hydroborations of prochiral alkenes in the presence of chiral phosphine ligands produced optically active alcohol products. The best results were obtained with DIOP or BDPP ligands. Asymmetric amplification was not observed in these reactions. Bis(cyclopentadienyl) (tetrahydroborato(1-)) titanium promotes the addition of borohydride to alkenes. In the reaction of phenylethene the predominant products were tetraalkylborates. 1-Decene and $\beta$-pinene gave organoboron products that are involved in a dynamic equilibrium with borohydride. A mechanism is proposed which explains the observed experimental characteristics of this reaction. (o-Aminophenyl)diphenylphosphine was prepared from aniline in two steps. Optically pure (S) N-(tert-butoxycarbonyl)-2-amino-3-diphenylphosphinoboranepropyl (p-nitrophenyl)carbonate was prepared from scL-serine