Silver(I) and solvent effects on alkene photochemistry
Abstract
The direct irradiation of simple alkenes in solution with an excitation wavelength of 254 nm has been investigated. Addition of silver trifluromethanesulfonate (triflate) to acetonitrile solutions of disubstituted alkenes results in an increase in the rate of E/Z isomerization of Z-, but not E-2-heptene is increased in the presence of silver. The photostationary state obtained from direct irradiation in the presence or absence of AgOTf is different from that obtained from triplet sensitization. Silver nitrate supported on silica gel irradiated as a stirred suspension in a cyclohexane solution of an alkene is also effective as a promoter of E/Z isomerization. These results indicate that the enhancement of the isomerization reaction is primarily due to excitation of a preassociated alkene-silver complex to a Rydberg-type state, which dissociates in the excited state and relaxes back to ground state alkene. Irradiation of tetrasubstituted alkenes with 254 nm light results in solvent-dependent photochemistry. Irradiation of 2,3-demithyl-2-butene (TME) in acetonitrile solution with 254 nm light gives a different distribution of products than had been previously reported, with an apparent selectivity for one of two products which are reported to originate from the same carbene intermediate. Irradiation of TME in decane with 254 nm light results in a product distribution similar to that which had been reported. Addition of AgOTf to acetonitrile solutions of TME results in an increase in the amount of double-bond migration observed in the product mixture, in addition to an increase in the rate of loss of starting material. 2-Propenylidenecyclohexane (IPrCH) also exhibits solvent-dependent photochemistry upon irradiation at 254 nm. The product ratio increases in favor of double-bond migration products upon addition of silver to the solutions. The quantum efficiencies for IPrCH loss and for product formation exhibit a large solvent and silver(I) dependence. Deuterium-labelling studies indicate that the double-bond migration from IPrCH occurs via an intermolecular pathway in all cases except for irradiation of IPrCH-d$\sb{O}$ in CD$\sb3$CN with AgOTf. The change in mechanism has not been explained. The observed photochemistry of the tetrasubstituted alkenes is consistent with population of a Rydberg excited state, ionization of the excited state to form the alkene radicalcation and a solvated electron, followed by reactions of the radical cation which may include recombination with the solvated electron. Hydrogen-abstraction reactions from the solvent and from other alkene molecules are an integral part of the chemistry observed, which was examined through deuterium-labelling studies.
Degree
Ph.D.
Advisors
Morrison, Purdue University.
Subject Area
Organic chemistry
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