TETRAPHENYLBORATE ANION AS A PHENYLATING AGENT - CHEMICAL AND ELECTROCHEMICAL REACTIVITY OF BPH(4)(-)-RH COMPLEXES TOWARD MONOENE AND DIENE AND CARBON-DIOXIDE

A few aspects of the chemical and electrochemical reactivity of mono-, di-, and trinuclear eta(6)-tetraphenylborate-Rh complexes [(C2H4)(2)Rh(eta(6)-PhBPh(3)) (1), {[(C2H4)(2)Rh(eta(6)-Ph)](2)BPh(2)}O-3-SCF3 (2), and {[(C2H4)(2)Rh(eta(6)-Ph)](3)BPh}(O3SCF3)(2) (3)] are described. The factors governing the phenyl transfer reaction from coordinated BPh(4)(-) 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-methyl-naphthalene, species formally involving phenyl transfer from boron to coordinated isoprene and intramolecular cyclization of the resulting intermediate. The reactivity of 1-3 toward carbon dioxide has been investigated and compared with that of (diphos)Rh(eta(6)-PhBPh(3)). Using complexes 1-3, coordinated ethylene competes with CO2 toward coupling with phenyl. The electrochemical behavior of the dinuclear complex, 2, has been investigated. It undergoes two consecutive irreversible one-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 phenyl affording propionic and benzoic acid. The former species was the only product under an ethylene/carbon dioxide atmosphere.