J. Am. Chem. Soc., submitted.
© American Chemical Society, 2005
Rosa Becerra, Peter P. Gaspar,* Cameron R. Harrington, William J. Leigh,* Ignacio Vargas-Baca, Robin Walsh,* and Dong Zhou
Contribution from the Instituto de Quimica-Fisica Rocasolano, C.S.I.C., C/Serrano 119, 28006 Madrid, Spain; the Department of Chemistry, Washington University, St. Louis, MO 63130, U.S.A.; the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4M1 Canada; and the School of Chemistry, University of Reading, Whiteknights, Reading, RG6 6AD, UK.
The photochemistry of 1,1-dimethyl- and 1,1,3,4-tetramethylstannacyclopent-3-ene (4a and 4b, respectively) has been studied in the gas phase and in hexane solution by steady state and 193 nm laser flash photolysis methods. Steady state (lamp) or semi-preparative laser photolysis of the two compounds results in the formation of 1,3-butadiene (from 4a) and 2,3-dimethyl-1,3-butadiene (from 4b) as the major products, suggesting that cycloreversion to yield dimethylstannylene (SnMe2) is the main photodecomposition pathway of these molecules. Indeed, the stannylene has been trapped as the Sn-H insertion product upon photolysis of 4a in hexane solution containing trimethylstannane. Flash photolysis of 4a in the gas phase affords a transient absorbing in the 450-520 nm range that is assigned to SnMe2 on the basis of comparisons of its spectrum and reactivity to those previously reported from other precursors. Flash photolysis of 4b in hexane solution affords results consistent with the initial formation of SnMe2 (lmax ~ 500 nm), which decays on the microsecond timescale to form tetramethyldistannene (5b; lmax ~ 470 nm). The latter then decays over the next ca 50 microseconds to form at least two other longer-lived species that are assigned to higher SnMe2 oligomers. Time-dependent DFT calculations in support of the spectral assignments for SnMe2 and Sn2Me4 are reported, as well as calculations examining the variation in the bond dissociation energies with substituent (H, Me, and Ph) of disilenes, digermenes and distannenes, in order to address the possibility that dimerization of SnMe2 proceeds reversibly. Addition of millimolar amounts of methanol leads to reversible reaction with SnMe2 to form a new transient absorbing at lmax ~ 360 nm, which is assigned to the Lewis acid-base complex between the stannylene and the alcohol. These studies represent the first direct observations of both SnMe2 and Me2Sn=SnMe2 in solution.
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