A Mechanistic Dichotomy in the Reactions of Cp2M(CH2CHMe) (M = Nb, Ta) with Catecholborane:  Generation of Boryl Complexes by Propylene Hydroboration and Propylene Loss

A mixture of endo- and exo-Cp2TaH(CH2CHMe) (1a) and catecholborane (HBCat, Cat = 1,2-O2C6H4) reacted to give endo-Cp2TaH2(BCat) (2a) and n-PrBCat as the major products. Small quantities of exo-2a are also formed during the reaction. When the reaction was monitored by 1H NMR, the resonances for endo-1a were diminished relative to exo-1a, and eventually all of the olefin complex was consumed. The analogous reaction employing DBCat led to deuterium incorporation at the α-methylene position of n-PrBCat and the deuteride positions of 2a. The alkylborane and deuteride resonances in the 2H NMR spectrum integrated in a 40:60 ratio. 1H NMR spectra indicate the α-methylene integration in n-PrBCat-d 0 - 1 is depleted by 50% of its normal value. A mechanism involving borane attack on a propylidene hydride intermediate is invoked to account for the labeling results. A mixture of endo- and exo-Cp2NbH(CH2CHMe) (1b) reacts with HBCat to generate n-PrBCat, propane, propylene, Cp2NbH2BCat (2b), and Cp2NbH(BCat)2 (3). The Markovnikov hydroboration product, i-PrBCat was not detected. Cp2NbH(BCat)2 was isolated as lemon-yellow crystals in 21% yield by fractional crystallization from toluene. 1H NMR indicates inequivalent boryl environments in compound 3, and two distinct boron resonances at δ 65 (Δν 1/2 = 250 Hz) and δ 60 (Δν 1/2 = 210 Hz) were resolved in the 11B NMR spectrum (C6D6, 60 °C). 1H{11B} spectra and isotopic labeling experiments indicated coupling between the niobium hydride and the 11B resonance at δ 60. Reaction of 1b with DBCat gave 2b-d 2, 3-d, propane-d 0 - 2 and n-PrBCat-d 0 - 2. The deuteride resonance in 3-d is shifted to higher field by 180 ppb relative to the hydride shift in compound 3. The chemical shift of the hydride resonance in compound 3 was temperature independent between −80 and 25 °C (THF-d 8).Compound 3 was crystallized as a yellow acetone solvate, and its molecular structure was determined. The Nb center lies on a C 2 axis, and the chemically inequivalent boryl groups are symmetry related. An Nb−B distance of 2.29(1) Å was found for compound 3, and the hydride position could not be reliably located. At low temperature the reaction between exo- and endo-1b with HBCat generates a persistent intermediate, 4, as the major Cp-containing component. 1H NMR spectra indicated two new hydride resonances reaction δ −4.40 and δ −6.00, and 1H{11B} spectra demonstrated that the resonance at δ −6.00 is coupled to boron. A NOESY spectrum revealed a cross-pe