Time-resolved IR Studies on the Mechanism for the Functionalization of Primary C−H Bonds by Photoactivated Cp*W(CO)3(Bpin) (original) (raw)

2010, Journal of the American Chemical Society

Recently, transition-metal-boryl compounds have been reported that selectively functionalize primary C-H bonds in alkanes in high yield. We have investigated this process with one of the welldefined systems that reacts under photochemical conditions using both density functional theory calculations and pico-through microsecond time-resolved IR spectroscopy. UV irradiation of Cp*W(CO) 3 (Bpin) (Cp*) C 5 (CH 3) 5 ; pin) 1,2-O 2 C 2-(CH 3) 4) in neat pentane solution primarily results in dissociation of a single CO ligand and solvation of the metal by a pentane molecule from the bath within 2 ps. The spectroscopic data imply that the resulting complex, cis-Cp*W(CO) 2 (Bpin)(pentane), undergoes C-H bond activation by a σ-bond metathesis mechanismsin 16 µs, a terminal hydrogen on pentane appears to migrate to the Bpin ligand to form a σ-borane complex, Cp*W(CO) 2 (H-Bpin)(C 5 H 11). Our data imply that the borane ligand rotates until the boron is directly adjacent to the C 5 H 11 ligand. In this configuration, the B-H σ-bond is broken in favor of a B-C σ-bond, forming Cp*W(CO) 2 (H)(C 5 H 11-Bpin), a tungsten-hydride complex containing a weakly bound alkylboronate ester. The ester is then eliminated to form Cp*W(CO) 2 (H) in approximately 170 µs. We also identify two side reactions that limit the total yield of bond activation products and explain the 72% yield previously reported for this complex. alize C-H bonds of alkane solvents at room temperature, but the reactions are generally not selective for a specific type of C-H bond. 7 Recently, however, one of the authors' laboratories discovered transition-metal-boryl complexes that selectively 2,6,8-10 functionalize primary C-H bonds by both thermal 2,8 and photochemical 6,9,10 reactions with yields as high as 92%. The mechanism of C-H bond cleavage and functionalization by these compounds has been investigated experimentally 2,6,8-10 and theoretically, 8,11,12 but complexes proposed to be intermediates have not been observed previously by spectroscopic