EPR studies of amine radical cations. Part 2. Thermal and photo-induced rearrangements of propargylamine and allylamine radical cations in low-temperature freon matrices

摘要

Matrix EPR studies and quantum chemical calculations have been used to characterize the consecutive H-atom shifts undergone by the nitrogen-centered parent radical cations of propargylamine (1b(center dot+)) and allylamine (5(center dot+)) on thermal or photoinduced activation. The radical cation rearrangements of these unsaturated parent amines occur initially by a 1,2 H-atom shift from C1 to C2 with pi-bond formation at the positively charged nitrogen; this is followed by a consecutive reaction involving a second H-atom shift from C2 to C3. Thus, exposure to red light (lambda > 650 nm) converts 1b(center dot+) to the vinyl-type distonic radical cation 2(center dot+) which in turn is transformed on further photolysis with blue-green light (lambda approximate to 400-600 nm) to the allene-type heteroallylic radical cation 3(center dot+). Calculations show that the energy ordering is 1b(center dot+) > 2(center dot+) > 3(center dot+), so that the consecutive H-atom shifts are driven by the formation of more stable isomers. Similarly, the parent radical cation of allylamine 5(center dot+) undergoes a spontaneous 1,2-hydrogen atom shift from C1 to C2 at 77 K with a t(1/2) of similar to 1 h to yield the distonic alkyl-type iminopropyl radical cation 6(center dot+); this thermal reaction is attributed largely to quantum tunneling, and the rate is enhanced on concomitant photobleaching with visible light. Subsequent exposure to UV light (lambda approximate to 350-400 nm) converts 6(center dot+) by a 2,3 H-shift to the 1-aminopropene radical cation 7(center dot+), which is confirmed to be the lowest-energy isomer derived from the ionization of either allylamine or cyclopropylamine. Although the parent radical cations of N, N-dimethylallylamine (9(center dot+)) and N-methylallylamine (11(center dot+)) are both stabilized by the electron-donating character of the methyl group(s), the photobleaching of 9(center dot+) leads to the remarkable formation of the cyclic 1-methylpyrrolidine radical cation 10(center dot+). The first step of this transformation now involves the migration of a hydrogen atom to C2 of the allyl group from one of the methyl groups (rather than from C1); the reaction is then completed by the cyclization of the generated MeN+(CH2) CH2CH2CH2 center dot distonic radical cation, possibly in a concerted overall process. In contrast to the ubiquitous H-atom transfer from carbon to nitrogen that occurs in the parent radical cations of saturated amines, the alternate rearrangements of either 1b(center dot+) or 5(center dot+) to an ammonium-type radical cation by a hypothetical H-atom shift from C1 to the ionized NH2 group are not observed. This is in line with calculations showing that the thermal barrier for this transformation is much higher (similar to 120 kJ mol(-1)) than those for the conversion of 1b(center dot+) -> 2(center dot+) and 5(center dot+)-> 6(center dot+) (similar to 40-60 kJ mol(-1)).