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

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; λ max ∼265 nm). The second species exhibits λ max = 310 nm and much lower reactivity toward alcohols and is consistent with any of E -1,2-dimethyl-1,2-bis(trimethylsilyl)silene ( E - 9 ), the corresponding Z -isomer ( Z - 9 ), or 1,2,2-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 3 and 9 ( 8g ) with methanol, methanol- Od, 2-propanol, cyclohexanol, tert -butyl alcohol, tert -butylamine, and acetic acid have been determined in hexane solution at 24 °C. In all cases but one, the ratios of the rate constants (relative to tert -butyl alcohol) for reaction of silene 3 compare favorably with the relative rate data reported previously for this silene in diethyl ether solution at 100 °C. The rate constant for addition of methanol to 3 in hexane at 25 °C 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 0−60 °C range and a maximum at 24 °C, consistent with a stepwise mechanism for reaction with methanol. The transient UV spectra and reactivity observed in THF solution at 24 °C indicate that 3 complexes strongly with the ether solvent, resulting in 10 3 - to 10 4 -fold reductions in its reactivity toward aliphatic alcohols compared to hexane solution. Smaller (ca. 20-fold) reductions in reactivity are observed for the minor silene product(s) under the same conditions.

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