EPR Studies of Amine Radical Cations. Part 2. Thermal and Photo-Induced Rearrangements of Propargylamine and Allylamine Radical Cations in Low-Temperature Freon Matrices

Abstract

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*+) and allylamine (5*+) 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*+ to the vinyl-type distonic radical cation 2*+ which in turn is transformed on further photolysis with blue-green light (lambda approximately 400-600 nm) to the allene-type heteroallylic radical cation 3*+. Calculations show that the energy ordering is 1b*+ > 2*+ > 3*+, so that the consecutive H-atom shifts are driven by the formation of more stable isomers. Similarly, the parent radical cation of allylamine 5*+ undergoes a spontaneous 1,2-hydrogen atom shift from C1 to C2 at 77 K with a t1/2 of approximately 1 h to yield the distonic alkyl-type iminopropyl radical cation 6*+; 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 approximately 350-400 nm) converts 6*+ by a 2,3 H-shift to the 1-aminopropene radical cation 7*+, 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*+) and N-methylallylamine (11*+) are both stabilized by the electron-donating character of the methyl group(s), the photobleaching of 9*+ leads to the remarkable formation of the cyclic 1-methylpyrrolidine radical cation 10*+. 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* 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*+ or 5*+ 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 (approximately 120 kJ mol-1) than those for the conversion of 1b*+ --> 2*+ and 5*+--> 6*+ (approximately 40-60 kJ mol-1).

Reaction of benzophenone triplet with aliphatic amines. What a potent neurotoxin can tell us about the reaction mechanism

“…These results confirm that there is no barrier associated with the ring-opening of 11 Å+ . Because these results correspond with previously published studies, [21][22][23][24] it can be said with confidence that these computational methods can also be utilized to examine 12 Å+ ?15 Å+ .…”

“…As noted, calculations pertaining to the ring opening chemistry of 11 Å+ have been previously reported, [21][22][23][24] thereby allowing us to validate the computational methods before examining the chemistry of compounds 12 Å+ ?15 Å+ . The following are the specific issues to be addressed: will a tertiary aminyl radical cation such as 12 Å+ be more stable than primary radical 11 Å+ and thus have a barrier to ring opening?…”

“…19 EPR experiments in a solid matrix have provided evidence for the distonic radical cation (ring-opened) structure based upon hyperfine coupling constants and have shown that the aminyl radical cation ring-opens to the distonic radical cation rapidly upon electron removal. 21,22 Subsequent molecular orbital calculations at the MP2 and other levels of theory have suggested that the removal of an electron from N-cyclopropylamine occurs simultaneously with ring opening; the only stable structure is the distonic radical ion, which collapses to a more stable aminopropenyl ion. 23,24 Studies on MPTP and the cyclopropyl analogs 7 and 8 by online coupling of electrochemistry and mass spectrometry (EC-ESI-MS) have been reported.…”

Enhancement of Two-Photon Initiating Efficiency of a 4,4′-Diaminostyryl-2,2′-bipyridine Derivative Promoted by Complexation with Silver Ions

“…MDEA is usually used as a coinitiator which undergoes a hydrogen abstraction from excited triplet species or radical cations. 33,47,48 This reaction then produces α-aminoalkyl radicals (MDEA • ) which can both initiate free radical photopolymerization 49 and reduce silver cations. 32,33,50−52 Therefore, the sequential steps for the dye regeneration should be described as follows: The first reaction step corresponds to the hydrogen transfer reaction which also produces the acid form of DES (eq 5).…”

Unusual Structures of Radical Ions in Carbon Skeletons: Nonstandard Chemical Bonding by Restricting Geometries

“…In cyclophanes, the geometric restrictions force the aromatic constituents to interact through space. [11][12][13][14][15] Much less persistent radical cations with remarkable electronic structures can be produced by g-irradiation of small hydrocarbons [16][17][18] in freon matrices at low temperatures. Analogously, such short-lived species can also be detected by CIDNP spectroscopy.…”