Fast Decarboxylation of Aliphatic Amino Acids Induced by 4-Carboxybenzophenone Triplets in Aqueous Solutions. A Nanosecond Laser Flash Photolysis Study

Quenching of the 4-carboxybenzophenone triplet ( 3 CB*) by H 2 N−CH 2 −CO 2 −Et and by amino acid anions of the general formula NR 2 −CR 2 −CO 2 -, where R is H or Me, has been investigated in basic aqueous solution. Spectral analysis of the primary products on the microsecond time scale showed that the major quenching process was electron transfer to 3 CB*, producing the CB •- radical anion and the R 2 N •+ −CR 2 −CO 2 - zwitterion aminium radical. However, on a nanosecond time scale, a small amount of (CBH) • was also formed, and this was attributed to a rapid proton transfer from about 10% of the aminium radicals to the CB •- anion radicals within the primary solvent cage. The values of the overall primary quenching rate constants were (8.5 ± 0.9) × 10 8 M -1 s -1 for N, N -dimethylglycine, (1.3 ± 0.1) × 10 8 M -1 s -1 for glycine, (1.5 ± 0.2) × 10 8 M -1 s -1 for alanine, (1.3 ± 0.1) × 10 8 M -1 s -1 for glycine ethyl ester, and (0.3 ± 0.03) × 10 8 M -1 s -1 for α-methylalanine. The introduction of methyl groups into the glycine structure resulted in a pattern of reactivity similar to that observed for amines. Except in the case of glycine ethyl ester, there were strong secondary growths of CB •- . This was attributed to the reduction of CB by the R 2 N− • CR 2 species produced from the decarboxylation of the R 2 N •+ −CR 2 −CO 2 - aminium species. The second-order rate constants for CB reduction by the aminoalkyl radicals are (3.2 ± 0.4) × 10 8 M -1 s -1 for H 2 N− • CH 2, (1.7 ± 0.3) × 10 9 M -1 s -1 for H 2 N− • C(Me)H, and (1.8 ± 0.3) × 10 9 M -1 s -1 for H 2 N− • CMe 2 . The transfer of protons from aminium radicals within the solvent cage gives rise to • NR−CR 2 −CO 2 - aminyl radicals, and these are known to undergo β-elimination of CO 2 •- . There was evidence for the presence of aminyl radicals in the case of α-methylalanine, where a small tertiary growth of CB •-, due to the reduction of CB by CO 2 •-, was observed. The magnitude of this growth matched the yield of (CBH) • from the spectral analysis of the primary products. In further experiments, the R 2 N •+ −CR 2 −CO 2 - zwitterion aminium radicals were deprotonated by bulk OH - when NaOH was added at concentrations in the range of 1−4 M. As expected, this produced a lowering of the CB •- yield from the R 2 N− • CR 2 radicals and an increase from the CO 2 •- species. An analysis of this effect was made assuming a diffusion-controlled rate of 1 × 10 10 M -1 s -1 for the transfer of the proton from R 2 N •+ −CR 2 −CO 2 - to OH - . It indicated that the rate constant for transfer of the proton from R 2 N •+ −CR 2 −CO 2 - to CB •- within the solvent cage was (6.9 ± 1.5) × 10 9 s -1 . The rate constant for the decarboxylation of the aminium species was estimated to be (8.7 ± 0.5) × 10 10 s -1 . The latter rate is at least 1 order of magnitude above those observed for decarboxylations of aliphatic acyloxyl radicals in aqueous media.