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 r eaction. When the reaction was monitored by H-1 MMR, the resonances for end o-1a were diminished relative to exo-1a, and eventually all of the olefin c omplex was consumed. The analogous reaction employing DBCat led to deuteriu m incorporation at the alpha-methylene position of n-PrBCat and the deuteri de positions of 2a. The alkylborane and deuteride resonances in the H-2 NMR spectrum integrated in a 40:60 ratio. H-1 NMR spectra indicate the alpha-m ethylene integration in n-PrBCat-d(0-1) is depleted by 50% of its normal va lue. A mechanism involving borane attack on a propylidene hydride intermedi ate 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, Cp(2)NbH(2)BCat (2b), and Cp2NbH(BCat)(2) (3). The Markovnikov h ydroboration product, i-PrBCat was not detected. Cp2NbH(BCat)(2) was isolat ed as lemon-yellow crystals in 21% yield by fractional crystallization from toluene. H-1 NMR indicates inequivalent boryl environments in compound 3, and two distinct boron resonances at delta 65 (Delta nu(1/2) = 250 Hz) and delta 60 (Delta nu(1/2) = 210 Hz) were resolved in the B-11 NMR spectrum (C 6D6, 60 degrees C). H-1{B-11} spectra and isotopic labeling experiments ind icated coupling between the niobium hydride and the B-11 resonance at delta 60. Reaction of 1b with DBCat gave 2b-d(2), 3-d, propane-d(0-2) and n-PrBC at-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 an d 25 degrees 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 relat ed. An Nb-B distance of 2.29(1) Angstrom was found for compound 3, and the hydride position could not be reliably located. At low temperature the reac tion between exo- and endo-1b with HBCat generates a persistent intermediat e, 4, as the major Cp-containing component. H-1 NMR spectra indicated two n ew hydride resonances reaction delta -4.40 and delta -6.00, and H-1{B-11} s pectra demonstrated that the resonance at delta -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 prove d that the hydride resonance at delta -6.00 arises from the borane. The deu teride resonance in 4-d is shifted to higher field by 210 ppb relative to t he hydride shift in compound 4. A modest temperature dependence for the hyd ride chemical shifts was observed between -50 and 50 degrees C (toluene-d(8 )). Intermediate 4 isomerizes to endo-2b, and reacts with CO (100 psi at 25 degrees C) to give the carbonyl compound, Cp2NbH(CO), and HBCat. Small qua ntities 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 e xo-Cp2NbH(eta(2)-HBCat) was assigned to intermediate 4.