Boranyl radical

Alkyltriphenylborates were introduced as a new class of photooxidizable reagents and photoreduction of cyanoaromatics has been studied high yields of alkylated products were formed through a mechanism involving a boranyl radical in which a carbon-boron bond is homolytically cleaved [193,194,195], Electron transfer from tetraphenylborate to anthraquinone excited state has also been studied [196]. 

Alternatively, both [Mn(CO)5] or [Mn(CO)5(L)] radicals may couple with an alkyl or phenyl radical (R ) which are present due to the fragmentation of the unstable boranyl radical (Eqs 75-77). 

Cleavage rates for the boron-carbon bond in boranyl radicals have been estimated to exceed A = 10" S- [189, 190]. 

A detailed study was carried out on (benzophenonylmethyl)-tri- -butylammonium triphenylbutylborate (BTAB). Nano- and picosecond laser photolysis demonstrated electron transfer from the borate counteranion to the excited triplet state of the benzophenone moiety. This leads to formation of a benzophenone moiety and a boranyl radical that dissociates rapidly to form butyl radicals. In the nonpolar solvent benzene the short lifetime of the triplet state (300 ps) suggests an intra-ion-pair process. The addition of 1 % MeCN caused an increase in the triplet lifetime to 1.2 ns, suggesting formation of a solvent-separated ion pair. For a lO" m solution in neat MeCN triplet decay is a function of tetrabutylammonium triphenyl- -butylborate concentration. 

Rate coefficients for the alkoxyl radical attack on the phosphorus and boron centers are in Table 28. For phosphites some additional relaative rate coefficients are listed in Table 29. The decay of the i osphoranyl and boranyl radicals initially produced in the reaction have been measured by Griller et al. (1979), and their results are listed in Table 30. 

TABLE 30. Rate Constants for Decay of Phosphoranyl and Boranyl Radicals... 

Chatterjee, S., Davis, P.D., Gottschalk, R, Kurz, M.E., Sauerwein, B., Yang, X., and Schuster, G.B., Photochemistry of carbocyanine alkyltriphenylborate salts intra-ion-pair electron transfer and the chemsitry of boranyl radicals, /. Am. Chem. Soc., 112, 6329,1990. 

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