Organometallics, 20, 5707-5716 (2001).

2001 American Chemical Society

Direct Detection of Wiberg’s Silene (1,1-Dimethyl-2,2-bis(trimethylsilyl)silene) and Absolute Rate Constants for its Reactions in Solution

T.L. Morkin, W. J. Leigh,* T.T. Tidwell and A.D. Allen

Contribution from the Departments of Chemistry, McMaster University, Hamilton, ON  Canada  L8S 4M1 and The University of Toronto, Toronto, ON Canada M5S 3H6

Abstract: Photolysis of (pentamethyldisilanyl)(trimethylsilyl)diazomethane (6a) and the analogous ketene derivative (6b) in hydrocarbon solution in the presence of aliphatic alcohols affords alkoxysilanes from trapping of four isomeric silene reactive intermediates, in yields of ca. 78%, 6%, 9%, and 8%, respectively. Nanosecond laser flash photolysis (193 or 248 nm) of 6a,b allows detection of two transient species, the most prominent of which has been assigned as 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (3; lmax ~265 nm). The UV absorption spectrum (lmax = 310 nm) and low reactivity toward alcohols of the minor species are consistent with any of E-1,2-dimethyl-1,2-bis(trimethylsilyl)silene (E-9), the corresponding Z-isomer (Z-9), or 1,2,3-trimethyl-2-pentamethyldisilanylsilene (8g). The four silenes are the products expected from the various possible 1,2-migrations in the carbene intermediate formed by photoextrusion of nitrogen or carbon monoxide from the precursors. Absolute rate constants for reaction of the two silenes with methanol, methanol-Od, isopropanol, cyclohexanol, tert-butanol, tert-butylamine and acetic acid have been determined in hexane solution at 24 oC. In all cases but one, the ratios of the rate constants (relative to tert-butanol) for reaction of silene 3 compare favorably with the relative rate data reported previously for this silene in diethyl ether solution at 100 oC. The rate constant for addition of methanol to 3 in hexane at 25 oC correlates with those of a series of C-substituted 1,1-dimethylsilenes, and indicates this silene to be the most highly electrophilic derivative known. The rates exhibit a bell-shaped dependence on temperature over the 0-60C range and a maximum at 24 oC, consistent with a stepwise mechanism for reaction with methanol. The transient UV spectra and reactivity observed in THF solution at 24 oC indicate that 3 complexes strongly with the ether solvent, resulting in 103 to 104-fold reductions in its reactivity toward aliphatic alcohols compared to hexane solution. Smaller (ca. twenty-fold) reductions in reactivity are observed for the minor silene product(s) under the same conditions.


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