Regio- and stereochemistry of alkene expulsion from ionized sec-alkyl phenyl ethers

Photoionization mass spectrometry of isotopically substituted 3-phenoxyprop ane (iPrOPh), 2-phenoxybutane (sBuOPh), and 3-phenoxypentane (3AmOPh) permi ts the analysis of branching ratios for competing pathways by which the rad ical cations expel neutral alkene to yield ionized phenol. Ionization energ ies (IEs) of 2-phenoxyalkanes do not differ significantly between 7-phenoxy propane and 7-phenoxyoctane and are unaffected by deuterium substitution. I fs for 3-phenoxyalkanes are 0.04 eV lower than for the 2-phenoxyalkanes. Me asurements of PhOD.+:PhOH.+ ratios from deuterated analogues as a function of photon energy lead to a dissection of two mechanisms: direct syn elimina tion via four-member transition states (which differentiates between stereo chemically distinct positions on an adjacent methylene group) and formation of ion-neutral complexes (which affords hydrogen transfer from all positio ns of the side chain). Syn elimination from ionized sBuOPh partitions among trans-2-butene, cis-7-butene, and I-butene in a ratio of approximately 6:5 :4, exhibiting no systematic variation with internal energy. The proportion of ion- neutral complex formation for sBuOPh increases with energy, from v irtually nil at 9.6 eV to about 20% at 9.81 eV to slightly more than one-ha lf at 11.93 eV. Ion-neutral complexes from sBuOPh yield nearly equal propor tions of l-butene and 2-butenes, with little variation as a function of int ernal energy, while those from 3AmOPh yield about 80-90% 2-pentenes. DFT ca lculations confirm the preference for syn elimination from ionized iPrOPh a t low internal energies. The computed energy of that transition state agree s with published experimental determinations. Analysis of the electron dens ity using the atoms-in-molecules approach shows that the transition state d oes not possess cyclic topology, unlike vicinal eliminations from neutral m olecules (which pass through bona fide cyclic transition states). Cyclic to pology is seen for a structure that precedes the potential energy maximum, but that ring disappears at the top of the barrier. Both syn elimination an d ion-neutral complex formation from the radical cation proceed far along t he pathway for bond heterolysis before arriving at a point at which the two types of mechanism diverge from one another.