A Mechanistic Dichotomy in the Reactions of Cp2M(CH2CHMe) (M = Nb, Ta) with Catecholborane: Generation of Boryl Complexes by Propylene Hydroboration and Propylene Loss

A mixture of endo - and exo -Cp 2 TaH(CH 2 CHMe) ( 1a ) and catecholborane (HBCat, Cat = 1,2-O 2 C 6 H 4 ) reacted to give endo -Cp 2 TaH 2 (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 1 H 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 2 H NMR spectrum integrated in a 40:60 ratio. 1 H 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 -Cp 2 NbH(CH 2 CHMe) ( 1b ) reacts with HBCat to generate n- PrBCat, propane, propylene, Cp 2 NbH 2 BCat ( 2b ), and Cp 2 NbH(BCat) 2 ( 3 ). The Markovnikov hydroboration product, i -PrBCat was not detected. Cp 2 NbH(BCat) 2 was isolated as lemon-yellow crystals in 21% yield by fractional crystallization from toluene. 1 H NMR indicates inequivalent boryl environments in compound 3, and two distinct boron resonances at δ 65 (Δ ν 1/2 = 250 Hz) and δ 60 (Δ ν 1/2 = 210 Hz) were resolved in the 11 B NMR spectrum (C 6 D 6, 60 °C). 1 H{ 11 B} spectra and isotopic labeling experiments indicated coupling between the niobium hydride and the 11 B 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. 1 H NMR spectra indicated two new hydride resonances reaction δ −4.40 and δ −6.00, and 1 H{ 11 B} spectra demonstrated that the resonance at δ −6.00 is coupled to boron. A NOESY spectrum revealed a cross-peak between the two hydride positions of intermediate 4 . Generation of 4 - d 1 from DBCat and exo - and endo - 1b proved that the hydride resonance at δ −6.00 arises from the borane. The deuteride resonance in 4 - d is shifted to higher field by 210 ppb relative to the hydride shift in compound 4 . A modest temperature dependence for the hydride chemical shifts was observed between −50 and 50 °C (toluene- d 8 ). Intermediate 4 isomerizes to endo - 2b, and reacts with CO (100 psi at 25 °C) to give the carbonyl compound, Cp 2 NbH(CO), and HBCat. Small quantities of intermediate 4 could be generated by heating a solution of endo - 2b . An equilibrium constant could not be accurately determined. On the basis of spectroscopic data and chemical reactivity, the structure exo -Cp 2 NbH( η 2 -HBCat) was assigned to intermediate 4 .