Tetraphenylborate Anion as a Phenylating Agent: Chemical and Electrochemical Reactivity of BPh4--Rh Complexes toward Mono- and Dienes and Carbon Dioxide

A few aspects of the chemical and electrochemical reactivity of mono-, di-, and trinuclear η6-tertraphenylborate-Rh complexes [(C2H4)2Rh(η6-PhBPh3) (1), {[(C2H4)2Rh(η6-Ph)]2BPh2}O3SCF3 (2), and {[(C2H4)2Rh(η6-Ph)]3BPh}(O3SCF3)2 (3)] are described. The factors governing the phenyl transfer reaction from coordinated BPh4- anion to monoenes and dienes are highlighted. Phenyl transfer to ethylene affords, among other products, styrene and ethylbenzene; styrene is converted into trans-stilbene. Complexes 1 and 2 react with isoprene affording 1,1-dimethylindene and 1,2-dihydro-2-methyl- and 1,2-dihydro-3-methylnaphthalene, species formally involving phenyl transfer from B to coordinated isoprene and intramolecular cyclization of the resulting intermediate. The reactivity of 1-3 toward CO2 was studied and compared with that of (diphos)Rh(η6-PhBPh3). Using complexes 1-3, coordinated ethylene competes with CO2 toward coupling with phenyl. The electrochemical behavior of the dinuclear complex, 2, was studied. It undergoes two consecutive irreversible 1-electron reductions at -1.53 and -1.74 V vs. SCE, respectively Analogous experiments carried out in the presence of CO2 have clearly shown that the reduced species are both able to react catalytically with the heterocumulene under controlled-potential conditions (-1.5 and -1.8 V). CO2 was coupled with both ethylene and Ph affording propionic acid and HOBz. The former species was the only product under an ethylene/carbon dioxide atmosphere.