Synthesis and photochemistry of small and medium ring mono and disilacycloalkenes

Abstract

This study is focused on the solution phase photochemistry of silacycloalkenes. The first chapter describes the photochemical generation and characterization of the reactive intermediate, trans-1,1,4,4-tetramethyl-1,4-disilacyclohept-2-ene. Direct, p-xylene sensitized, and methyl benzoate sensitized photolyses of 1,1,4,4-tetramethyl-1,4-disilacyclohept-2-ene (1) in acidified methanol produced methanol addition products. Plots of reciprocal quantum yield of these products against reciprocal sulfuric acid ( (H$\sb2$SO$\sb4$) $\sp{-1}$) gave comparable intercept/slope ratios for direct and $p$-xylene triplet sensitized photolyses, suggestive of a common intermediate produced via the singlet and triplet states. A metastable intermediate was trapped in the dark with acidified methanol, and a kinetic study indicated its half-life (second order) of 90.8 min at 20$\sp\circ$C. This species decayed by strict second order kinetics, giving a mixture of dimers from self-dimerization. The intermediate was assigned as the trans isomer of reactant ${\bf 1}$ from the structure of its Diels-Alder cycloadduct, produced upon addition of cyclopentadiene to the photolysate at -75$\sp\circ$C in the dark. With 11,12-dimethylene-9,10-dihydro-9,10-ethanoanthracene as the 1,3-diene a crystalline cycloadduct of trans-1 was obtained and its structure was determined by X-ray crystallography. Product structure proofs in the above study, which required the synthesis of a series of disilacycloalkenes having exocyclic double bonds, employed chloroplatinic acid catalyzed hydrosilylation of acetylenes as a methodology. The intramolecular version of hydrosilylation with silanes bearing pendant acetylenic groups, i.e., HMe$\sb2$Si (CH$\sb2$) $\sb{\rm n}$-SiMe$\sb2$C=CR (R = H or Ph, n = 2 or 3), is described for the first time in Chapter 2. In the third chapter mechanistic studies of the photochemical ring opening and cleavage reactions of 1,1-Dimethyl-1-silacyclobut-2-ene are described. The intermediacy of the 1,1-dimethyl-1-sila-1,3-butadiene, and 1,1-dimethyl-1-silaethene was established using tert-butyl alcohol-O-$d\sb1$ and methoxytrimethylsilane as trapping agents in conjunction with deuterium labeling. The major photochemical process was thus found to be electrocyclic ring opening of the excited silacyclobutene, whereas excited state rearrangement to a silacyclopropylmethylene was found to be only a minor process. Quantum yields as a function of concentration of trapping agent (alcohol) revealed that equilibration of s-cis and s-trans rotamers of 1,3-silabutadiene was rapid relative to the rate of trapping, with the equilibrium favoring the more stable trans species. In addition the reaction of the s-trans conformer with alcohol appeared to proceed via a solvent-mediated long range proton transfer mechanism.

Recommended Citation

Udayakumar, Bettakeri Subraya, "Synthesis and photochemistry of small and medium ring mono and disilacycloalkenes" (1990). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. AAI9035637.
https://epublications.marquette.edu/dissertations/AAI9035637

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