The Photohydration ofN-Alkylpyridinium Salts: Theory and Experiment

Abstract

Bicyclic aziridines formed by the irradiation of pyridinium salts in basic solution have recently been recognized to have great synthetic potential. We have undertaken a joint computational and experimental investigation of the mechanism of this photoreaction. We have computationally determined the structures and relative energies of the relevant stationary points on the lowest potential energy surface (PES) of the pyridinium and methylpyridinium ions. Two important intermediates are shown to be bound minima on the ground-state PES: azoniabenzvalene and a 6-aza[3.1.0]bicyclic ion with an exo-oriented substituent (analogous to prefulvene). We advance a mechanism which involves initial formation of this exo-bicyclic ion, followed by nitrogen migration around the ring via the azoniabenzvalene intermediate. Thus, the barrier separating the two intermediates is the factor that determines the degree of scrambling observed in the photoproducts when the carbon atoms are labeled with deuterium or substituted with additional methyl groups. For N-methylpyridinium, the exo-methyl bicyclic ion was computed to be approximately 1 kcalmol(-1) lower in energy than N-methyl-azoniabenzvalene. The transition state was computed to lie several kcal mol(-1) above the exo-methyl bicyclic ion (+8.4kcalmol(-1), 6-31G* RHF; +3.7kcalmol(-1), 6-31G* B3LYP), but still well below the energy available from the 254 nm excitation of the N-methylpyridinium ion. The computed relative energies correspond splendidly with several experimental findings which include the preference for exo products, the results of deuterium labeling, and the impact of additional substituent methyl groups on the product distribution.

Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes

“… 78 We now confirm this feature for silabenzene at the CASPT2//CASSCF level ( Figure 5 ), and also for the pyridinium cation, which has a photochemistry which is of higher synthetic value than that of pyrazine. 79 − 84 Yet, at the lower CASSCF level, which contains only static electron correlation, there are still activation barriers, although they are smaller than for S 1 -state benzene. Thus, the activation barrier observed in the S 1 state of benzene is traceable to its high symmetry (for comparisons between the S 1 states of benzene, silabenzene, and pyridinium ion, see Table S19 ).…”

The synthetic potential of pyridinium salt photochemistry

“…In the original reports describing the photochemistry of pyrylium and pyridinium salts, the assumption was made that the heteroatom containing benzvalene cations 42 serve as initially formed intermediates and/or as intermediates in ring rearrangement pathways. The results of more recent theoretical studies 23 show that tricyclic cations (42, X = NR) derived from pyridinium salts have a higher energy content than the corresponding bicyclicaziridine cations (43, X = NR). Thus, it appears that neighboring group stabilization of the allylic cation in 43 (X = NR) by nitrogen is prevented by increased ring strain associated with formation of the tricyclic framework.…”

Photochemical transformations of pyridinium salts: mechanistic studies and applications in synthesis

“…In 2001, prompted by the renaissance of the synthetic photochemistry of pyridinium salts, Burger and L üthi et al reported their insights into the events originating from p → p* excitation. 18 Completely optimised geometries for the equilibrium, intermediate and transition state structures of species arising from pyridinium and methyl pyridinium ions were determined using a combination of restricted Hartree-Fock (RHF) and density functional theory DFT (B3LYP) calculations (Fig. 1).…”