Date of Award
Doctor of Philosophy (PhD)
William J. Leigh
The photochemistry of a homologous series of aryldisilanes (9, 12, 34, and 35) has been studied in detail by steady state and nanosecond laser flash photolysis (NLFP) techniques. Direct irradiation in hydrocarbon or acetonitrile solution results in competitive formation of the corresponding transient simple silenes by concerted dehydrosilylation, 1,3,5-(1-sila)hexatrienes (silatrienes) by [1,3]-silyl migration, and silyl free radicals by Si-Si bond homolysis. Simple silenes and silatrienes are derived from rearrangements in the disilane lowest excited singlet state and are the major products in hydrocarbon solvents. Silyl free radicals are triplet-derived and are the major products in polar solvents such as acetonitrile. Steady-state photolysis in the presence of acetone affords two products consistent with addition of acetone to the corresponding silatriene: a silyl ether (56) from a formal ene reaction, and a 1,2-siloxetane (57) from formal [2+2]-cycloaddition. Product consistent with ene reaction of the corresponding simple silenes with acetone are also detected. Aryldisilane photolysis in the presence of 2,3-dimethyl-1,3-butadiene (DMB) affords both formal ene- and [2+2]- or [2+4]- addition products of the corresponding silatrienes, while irradiation of oxygen-saturated disilane solutions results in the formation of phenyltrimethylsilane and silanone oligomers. Steady-state photolysis in the presence of methanol affords complex mixtures of at least five primary products, four of which are isometric products consistent with addition of the trapping reagent to the corresponding silatriene, while the other is the simple-silene adduct. Analogous complex product mixtures are obtained upon photolysis in the presence of a series of other alcohols and methoxytrimethylsilane. Nanosecond laser flash photolysis of deoxygenated acetonitrile (ACN), isooctane (OCT), or tetrahydrofuran (THF) solutions of aryldisilanes affords readily detectable transient absorptions in the 300-340 and 400-540nm ranged which have been assigned to the corresponding silyl radicals (and simple silenes in some cases), and silatrienes, respectively. Silyl radical absorptions are significantly more intense in polar solvents such as ACN compared to hydrocarbons. Simple Silenes, silyl radicals, and silatrienes can be detected selectively by the addition of the appropriate combination of trapping reagents to the disilane solutions. Increasing phenyl substitution at the silenic silicon atom results in red-shifts in the silatriene absorption maxima consistent with increasing conjugation in the chromophore. The silatrienes form complexes with THF solvent molecules; the equilibrium constant for complex formation is dependent upon the degree of phenyl substitution in the chromophore. Absolute rate constants for reactions of several silatrienes with carbonyl compounds, alkenes, dienes, oxygen, alkyl halides, dimethyl sulfoxide, and alcohols have been determined by NLFP techniques. Silatriene reactions with acetone, DMB, and oxygen occur via stepwise mechanisms involving rate-determining formation of biradical intermediates which undergo subsequent decomposition to afford the detected products. Silatriene quenching by water, methanol, ethanol, tert-butanol, 1-pentanol, and ethylene glycol affords curved quenching plots, while the analogous plots for quenching by 2,2,2-trifluorethanol, acetic acid, and 1,3-propanediol are linear. The addition of water and methanol are subject to deuterium kinetic isotope effects (KIE's) of 1.7 and 1.9, respectively, while silatriene quenching by acetic acid-Od does not exhibit an observable KIE. These results are consistent with a mechanism which involves rapid, reversible formation of an alcohol/silatriene complex followed by competing, rate-determining, intra- and intermolecular proton transfer. Direct photolysis of 1,2-di-tert-butyl-1,1,2,2-tetraphenyldisilane (40) affords tert-butyldiphenylsilyl radicals (41) exclusively. Bimolecular rate constants for the reactions of tert-butyldiphenylsilyl radicals with a variety of alkyl halides, alkenes, dimethyl sulfoxide, and acetone have been determined.
Sluggett, Gregory Wilton, "The photochemistry of aryldisilanes and the reactivity of transient silenes" (1993). Open Access Dissertations and Theses. Paper 3887.