Electron transfer followed by double fragmentation reactions: Mechanism of photogeneration of tertiary amines and radicals from tetraorganyl borates

The double fragmentation reactions of N,N,N-tributyl-N-acetobenzo[b]thiophene ammonium borates have been investigated. The primary step in the cleavage reactions is electron transfer from the borate anion to the excited acceptor. This generates radical pairs that decompose by rapid carbon-nitrogen and carbon-boron bond fragmentation. Transient spectra and photoproducts are consistent with this bond cleavage. Tertiary amines are formed as a result of reductive carbon-nitrogen bond scission. The boranyl radicals formed undergo oxidative carbon-boron bond cleavage to generate alkyl or phenyl radicals, depending on the structure of the borate. Both cleavage processes are irreversible, and their rates easily exceed the rate of the back electron transfer reaction. The net quantum yields for photodecomposition are also high. Detailed mechanistic studies were carried out.