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

Abstract

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-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, Cp2NbH(CO), and HBCat. Small quantities of intermediate...

Reactivity of a Terminal Chromium Borylene Complex towards Olefins: Insertion of a Borylene into a CH Bond

“…Furthermore, a broad 1 H NMR singlet resonance (peak width at half height: 87.5 Hz) at d = À10.62 ppm suggested the presence of a hydride-like proton, and the 1 H{ 11 B} NMR spectra showed a sharper peak (width at half height: 17.5 Hz) for the same resonance, which is consistent with a Cr-H-B bridge existing in solution. [9] A COSY correlation between this signal and an olefinic proton served to reinforce the proposed connectivity of compound 3.…”

“…As a result, the C=C bond is in position to share its electron density with the metal, and at the same time the boron-bound hydrogen H1 is aligned trans to the C1-O1 carbonyl group. The C5 À C6 bond length (1.3723 (17) + , which has a CÀC distance of 1.393 (9) . [11] The C5ÀB1 distance is in the expected range of CÀB single bonds (1.5482(18) ) but is surprisingly unperturbed by the close Cr-H-B association, the distance agreeing well with the vinylborane complex [(OC) 4 Fe{H 2 C = CHBCH 3 N(CH 3 ) 2 }] (4) of Schmid et al, in which the boron atom is distant from the metal center.…”

Stretched σ-borane complexes of rhodium: A theoretical study

“…Garcia and coworker have recently reported the r-borane complexes of nickel(0) [10]. Synthesis and bonding of the transition metal complexes containing residual BÁ Á ÁH interaction have been reported [11][12][13][14][15][16][17][18]. Mechanistic studies have revealed that r-borane complexes can be intermediates in catalytic hydroboration and borylation reactions [13,[19][20][21].…”

3 Boron

“…153 This stepwise hydroboration^homologation approach has been shown to afford high enantioselectivity in the resulting products. 154 The synthesis, structures and reactivity of osmium boryl complexes of the type [Os(PPh 3 ) 2 (CO) n (C 8 H 4 Me-2)(Bcat)] (n 1,2) have been described and are claimed to be the ¢rst stable coordinatively unsaturated and saturated examples respectively of complexes containing simple boryl and s-bound ligands. Such complexes are interesting as they represent isolated intermediates in hydroboration reactions.…”