New homogeneous rhodium catalysts for the regioselective hydroboration of alkenes

Article abstract of DOI:10.1021/ja00049a019

Multinuclear (¹H, ¹¹B, ¹³C, ¹⁹F, and ³¹P) NMR spectroscopy was used to monitor primary products from transition metal-catalyzed addition of catecholborane (HBcat) to a variety of alkenes. Hydroboration of vinylarenes (indene, vinylnaphthalene, and X-C₆H₄-CH=CH₂, where X = p-OMe, p-F, p-Cl, and m-F) with HBcat, employing [Rh(η³-2-Me-allyl){(Pr¹₂PCH ₂)₂}] (9) as catalyst precursor, proceeded with excellent activity and regioselectivity (>99%) in favor of the corresponding internal boronate ester. Analogous hydroboration reactions carried out in the presence of [Rh(COD){(Ph₂PCH₂CH₂)₂}]BF ₄ (11) (COD = 1,5-cyclooctadiene) or Wilkinson's catalyst, [RhCl(PPh₃)₃] (1), also gave internal boronate esters with regioselectivities comparable to those of 9. With 1, unlike 9 and 11, however, small amounts (ca. 5%) of hydrogenation products were observed consistently. With (E)-1-phenylpropene, 9 and 11 again gave internal boronate ester (>99%), whereas 1 gave a 70:30 mixture of internal:terminal boronate esters, with 5% n-propylbenzene as a side product. The effect of varying the chelating bis(phosphine) in [Rh(η³-2-Me-allyl)(P₂)] was examined for catalyzed hydroborations of 2-phenylpropene. Higher chemical yields and Markovnikov selectivities were observed for arylphosphines compared with corresponding bulky alkylphosphines. Unlike cationic [Rh(diene)(P₂)]⁺ catalyst precursors, hydroborations of aliphatic alkenes using 9 proceeded with complete regiocontrol to give terminal alkylboronate esters. Hydroboration of 2,3-dimethylbut-2-ene with HBcat was also catalyzed by 9 and 11; complex 1 failed to catalyze hydroboration of this sterically demanding substrate. Complex 9 reacts with excess HBcat to form zwitterionic [Rh(η⁶-catBcat){(Pr¹₂PCH₂) ₂}] (12), which is proposed to be the resting state of the active hydroboration catalyst. In situ monitoring of catalytic reactions using allylrhodium precursors by ³¹P{¹H} NMR spectroscopy showed that only the zwitterionic species were present in observable quantities at any time during or after completion of catalysis. Conversely, hydroborations using 11 gave several phosphinorhodium complexes, leading eventually to catalyst decomposition. The new zwitterionic catalysts can also be generated conveniently from [Rh(acac)(η-C₂H₄)₂] (acac = acetylacetonate), phosphine ligands, and HBcat.