Km. Waltz et al., C-H activation and functionalization of unsaturated hydrocarbons by transition-metal boryl complexes, ORGANOMETAL, 18(17), 1999, pp. 3383-3393
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).