C-H activation and functionalization of unsaturated hydrocarbons by transition-metal boryl complexes

Transition-metal boryl complexes of the form Cp'Fe(CO)LBcat and (CO)(5)MBca t, where Cp' = C5H5, C5Me5, M = Mn, Re, L = CO, PMe3, and cat = 1,2-O2C6H4, were synthesized by reaction of ClBcat with [Cp'Fe(CO)L](-) or [M(CO)(5)]( -). X-ray crystal structures of CpFe(CO)(2)Bcat, Cp*Fe(CO)(2)Bcat, and (CO) (5)MnBcat were obtained. Upon irradiation, these metal boryl complexes reac ted with arenes and alkenes to form aryl- and vinylboronate ester products in moderate to high yields. Monosubstituted arenes with methyl, chloro, tri fluoromethyl, methoxy, and dimethylamino substituents were used as substrat es, and the resulting ratios of ortho- to meta- to para-substituted arene p roducts were measured. No significant electronic effects were observed, ind icating that the chemistry is not occurring through a typical electrophilic aromatic substitution pathway. Competition experiments between toluene and other substituted arenes were conducted. Reactivity differences were small , but anisole was found to have the fastest rate of reaction. Kinetic isoto pe effects were measured for the reaction of CpFe(CO)(2)Bcat, (CO)(5)MnBcat , or (CO)(5)ReBcat with benzene/benzene-d(6) mixtures and were found to be 3.3 +/- 0.4, 2.1 +/- 0.1, and 5.4 +/- 0.4, respectively. This difference in isotope effect along with differences in selectivities with substituted ar senic reagents rules out a mechanism by which a common free Beat radical at tacks free substrate. Several experiments were also conducted to probe for CO loss. A (CO)-C-13-labeling experiment, CO inhibition experiment, and PMe 3 trapping experiment indicate that the mechanism most likely proceeds thro ugh irreversible CO loss to form a. 16-electron intermediate. Functionaliza tion of alkenes to form vinylboronate esters was also observed, and mechani stic studies showed the absence of a measurable kinetic isotope effect for reaction of CpFe(CO)(2)Bcat or (CO)(5)ReBcat with ethylene/ethylene-d(4) mi xtures or for reaction with ethylene-d(2).