MECHANISMS FOR INTERCONVERSIONS AMONG HYDROCARBONS: DEUTERIUM ISOTOPE EFFECTS AND INDEPENDENT GENERATION OF VINYLMETHYLENE INTERMEDIATES IN PHOTOISOMERIZATIONS OF ALLENES AND CYCLOPROPENES
Direct photolysis of 4,4-dimethyl-1,1-diphenyl-1,2-pentadiene (1) in tert-butyl alcohol yields 1-tert-butyl-3,3-diphenylcyclopropene (2), 4,4-dimethyl-methyl-1,1-diphenyl-2-pentyne (3), and 1-tert-butyl-3-phenylindene (4). Involvement of vinylmethylene as potential intermediates in 1 photoisomerization was assessed by independent generation with cyclopropene 2, the tosylhydrazone 11 of 2,2-dimethyl-5,5-diphenyl-4-pentene-3-one, and 3-tert-butyl-5,5-diphenyl-3H-pyrazole (13). Quantum yields for direct photolysis of 2 were determined for allene 1, indene 4 and indene 5. Allene 1, cyclopropene 2 and indene 4 were products of nitrogen photoxtrusion from 13, and similiar results were obtained from the sodium salt 12 of tosylhydrazone 11. Flash vacuum pyrolysis (FVP) of 2 at 400 C produced indene 4 and 3-tert-butyl-1-phenylindene (15). FVP of indene 4 also gave 15 and recovered 4. Allene 1 was recovered in 98% yield after FVP (400 C). Deuterium labelling of photoproducts was determined for 4,4-dimethyl-1,1-diphenyl-1,2-pentadiene-3-d1 (1-d1); a (PHI)(,H)/(PHI)(,D) = 3.07 suggest that the second 1,2-H shift is rate determining as opposed to the small (PHI)(,H)/(PHI)(,D) of 1.36 for total products, 2 + 3 + 4, implicating only a low excited-state barrier in the first step. KIEs are derived. Funneling from the singlet excited state to the ground state is suggested formation of 1 in photolyses of 2. Discrete ground-state vinylmethylenes adequately account for thermolysis products. Alternative mechanisms are discussed. The direct photolysis at 185 nm (far-UV) of 1,1,3,3-tetramethyl-1,3-disilacyclopentene (1) yielded a ring contraction product 1,1,3,3-tetramethyl-2-methylene-1,3-disilacyclobutane (2) and a cleavage product, 2,4,4-trimethyl-2,4-disilahex-5-yne (3). Photolysis at 214 nm gave yields of products which accounted for a higher total, of the primary photochemistry of 1. Sensitized photolysis at 254 nm using toluene as the triplet sensitizer gave no reaction. The photolysis of disilacyclopentene 1 at 214 nm in methanol and methanol-0-d1 gave products 2 and 3 in addition to an alcohol adduct. The photoreaction was elucidated as methanol addition and cleavage to form silyether 4a. Photolysis in methanol-0-d1 led to >99% incorporation of one deuterium, exclusively at the vinyl carbon (beta) to silicon of silylether 4a, with a high degree of stereospecificity favoring H(,B). Concentration vs. time profiles are constructed and quantum yields are determined using cis-cyclooctene actinometry. Direct photolysis of vinylidenecyclobutane (1) at 185 nm produces spiro 2.3 hexene-1-ene (2), ethynylcyclobutane (3), and 3-methylenecyclobutane (4). Highly volatile products were identified as butatriene, ethylene, 1,3-butatriene (7) and vinylacetylene (8). Plots of product concentration vs time plots clearly show that 2-4, butatriene, and ethylene are primary products whereas the other volatiles, 1,3-butadiene and vinylacetylene, are secondary photoproducts, most likely of butatriene. Butatriene is rapidly converted to 1,3-butadiene and vinylacetylene upon direct irradiation (185 nm = 254 nm) in heptane. However, an upward curvature of a concentration vs time plot for the photolysis of butatriene 5 in pentane appears indicative of a free radical process. Quantum yields were determined from initial slope of concentration vs time plots using as a actinometer the cis to trans isomerization of cyclooctene.
RICHARD THOMAS MAYES,
"MECHANISMS FOR INTERCONVERSIONS AMONG HYDROCARBONS: DEUTERIUM ISOTOPE EFFECTS AND INDEPENDENT GENERATION OF VINYLMETHYLENE INTERMEDIATES IN PHOTOISOMERIZATIONS OF ALLENES AND CYCLOPROPENES"
(January 1, 1985).
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