Photochemistry of 5-Phenyl-1-pentene in the Gas Phase

Abstract

The photochemistry of the title compound has been studied in the gas phase using 254-nm irradiation. In addition to meta cycloadducts analogous to those observed in solution, population of S1 vib in the gas phase gives several products, the relative amounts of which depend on quencher gas pressure but not on excitation wavelength. For example, in the absence of butane, the major photoproduct is compound 5. This product is formed by a [1,5] hydrogen shift in the primary photoproduct, compound 4. Compound 4 is an intramolecular meta cycloadduct that is generated in the gas phase with sufficient excess vibrational energy to undergo rearrangement unless quencher gas is present. Likewise, there is evidence that two other meta cycloadducts (2 and 3) are also formed with appreciable vibrational energy in the absence of a quencher gas. A unique intramolecular ortho cycloadduct is also formed from 1 but only within a narrow range of quencher gas pressures. This is a two-photon product, with the initial cycloadduct (11) ring opening to a cyclooctatriene (12) that photochemically closes to 6. The pressure dependence of this ortho cycloaddition may be due to a requirement for vibrational deactivation of 11 (Scheme ) or a precursor species (Scheme ). The overall chemistry is outlined in Scheme .

Chromatography with silver nitrate: part 2

“…Ho and Morrison 79 undertook an extensive investigation encompassing the photochemistry of 5-phenyl-1-pentene (152) at 254nm. Such meta cycloadducts have been of considerable interest to both photo and synthetic chemists alike and revealed some intriguing aspects, for example, the relative amounts of products (153-156), purified using SNIS, depend on quencher gas pressure and not excitation wavelength (Scheme 27).…”

Conformation-Specific Spectroscopy and Excited State Photophysics of 5-Phenyl-1-pentene

“…We have pinned down the threshold for this lifetime reduction to an energy region of about 1000−1200 cm −1 , consistent with the observation that interconversion to the exciplex is quite efficient at room temperature. At the same time, in solution, formation of meta cycloaddition photochemical products is strongly preferred over other possible cycloaddition products, occurring with a quantum yield of about 0.1 …”

“…An initial sample (∼500 mg) of 5PPene (with a purity of ∼99%) was obtained from the Morrison group at Purdue for preliminary studies. When this sample was exhausted, a synthesis was performed by following the procedure outlined by Ho et al involving a Grignard reaction with 1-bromo-2-phenylethane and allylmagnesium bromide as reactants. 5PPene was produced with about 60% yield.…”

“…Possible exciplex formation of 5PPene was indirectly implicated in gas phase and solution photochemistry experiments at room temperature. − Morrison and co-workers surmised that a double bond, three carbons removed from a benzene ring, is efficient in trapping the S 1 state of the phenyl ring. − By forming an exciplex between the phenyl ring and the terminal vinylic double bond, the S 1 excited state of the phenyl ring is stabilized in configurations that bring the double bond over the phenyl ring. Exciplex formation in solution showed itself as a decrease in lifetime of the excited state from 35 ns in toluene (where exciplex formation is not possible) to 2.5 ns in 5PPene. , Exciplex formation was also consistent with the observed photocycloaddition products on both solution and gas phase . Because the primary products involve meta cycloadducts of interest in many natural products, this photochemistry has been studied in some detail over the intervening years …”

“…Possible exciplex formation of 5PPene was indirectly implicated in gas phase and solution photochemistry experiments at room temperature. [3][4][5] Morrison and co-workers surmised that a double bond, three carbons removed from a benzene ring, is efficient in trapping the S 1 state of the phenyl ring. [3][4][5] By forming an exciplex between the phenyl ring and the terminal vinylic double bond, the S 1 excited state of the phenyl ring is stabilized in configurations that bring the double bond over the phenyl ring.…”