Rhodium(I) catalyzed hydroboration

Abstract

The stereochemical outcomes and trends of catalysis of hydroboration of alkenes by catecholborane using rhodium(I) catalysts is examined. For 1,2-chirality transfer in simple allylic alcohol derivatives with aliphatic substituents on the chiral center the syn diastereoisomer is formed preferentially, this result is stereocomplementary to the stereoselectivities observed in reaction with hindered alkyl borane reagents. Syn diastereoselectivity, in the catalyzed reaction, is found to increase with increasing electron withdrawing power of the substituent on oxygen and with its steric bulk. The observed stereochemical outcome of the catalyzed reactions is postulated to be the result of binding of the substrate to the low valent metal catalyst in such a way that it behaves as the best ligand for a transition metal, i.e. such that the $\pi$* of the alkene lowered by perturbation with the lowest energy $\sigma$* of a bond on the chiral center (electronic effect). This occurs when bond to the oxygen substituent is aligned with the lobes of the $\pi$* orbital. Steric interactions dictate that the metal bonds opposite the alcohol substituent, and the larger this group is, the larger its preference to occupy this position (steric effect). N-(benzyltosyl) allyl amines were subjected to catalyzed and uncatalyzed hydroboration conditions. The catalyzed reactions give syn selectivity as anticipated. However 9-BBN also gives syn products preferentially which was not expected from analogy of the behavior of this reagent with allylic alcohol derivatives. Borane-THF complex gives consistantly high anti selectivity with these substrates. Transfer of chirality from a monochiral rhodium catalyst to several simple prochiral alkenes is described. The efficiency of this enantioselective hydroboration process is strongly dependant on the type monochiral bisphosphine used to form the catalyst. A new class of monochiral bisphosphine which contains stereogenic phosphorus centers, and a chiral backbone linking them, were synthesized by alkylation of unsymmetrically substituted phosphide ions with 1,4-Ditosyl-2,3-O-isopropylidene-L-threitol. Separation of the resulting epimeric (at phosphorus) compounds was achieved by flash chromatography of molybdenum carbonyl complexes. Decomplexation to give pure diastereoisomers is carried by treatment with sodium naphthenalide.