Homo- and Cross-[2+2]-Cycloaddition of 1,1-Diphenylsilene and 1,1-Diphenylgermene. Absolute Rate Constants for Dimerization and the Molecular Structures and Photochemistry of the Resulting 1,3-Dimetallacyclobutanes

Abstract

Absolute rate constants for the head-to-tail [2+2]-dimerization of 1,1-diphenylsilene and 1,1-diphenylgermene have been determined in hexane and isooctane solution at 23 °C by laser flash photolysis, using the corresponding 1,1-diphenylmetallacyclobutanes as precursors. This requires knowledge of the molar extinction coefficients at the UV absorption maxima of the two transient species, which have been determined by a transient actinometric procedure. The rate constants for dimerization of the two compounds are similar and within a factor of about 2 of the diffusional rate constant in both cases. The 1,3-dimetallacyclobutane dimerization products have been prepared by direct photolysis of the corresponding 1,1-diphenylmetallacyclobutanes in dry isooctane. In addition, 1,1,3,3-tetraphenyl-1-germa-3-silacyclobutane has been prepared by photolysis of a 1:1 mixture of the two metallacyclobutanes. The solid-state structures of the three 1,3-dimetallacyclobutanes have been determined by single-crystal X-ray crystallography, and their photochemistry has been studied. Laser flash and/or steady-state photolysis experiments indicate that all three compounds cyclorevert to the corresponding 1,1-diphenylmetallaenes upon photolysis in hydrocarbon solvents, most likely via the same (•M−C−M‘−C•) 1,4-biradical intermediates which link the metallaenes with their corresponding dimers via stepwise dimerization mechanism. The quantum yield for photocycloreversion of the digermacyclobutane is roughly one-fourth that of 1,1-diphenylgermacyclobutane. However, it is about 3 times higher than that for the silagermacyclobutane and roughly 15 times higher than that for the disilacyclobutane. Singlet lifetimes have been determined for the metallacyclobutanes and the disila- and digermacyclobutanes using single photon counting techniques. The implications of these results for the mechanisms of the head-to-tail [2+2]-dimerization of silenes and germenes are discussed.

The photochemistry of a potential diphenylsilylene precursor. Time-resolved spectroscopic studies of the reactivity of a transient vinylsilirane

“…20% conversion of 2 by 600 MHz NMR spectroscopy. The quantum yield ( −2 = 0.31 ± 0.05) was calculated from the relative slopes of concentration vs. time plots for 2 and 1,1-diphenylsilacyclobutane, and the reported quantum yield for the latter reaction (φ = 0.21 ± 0.02 [49]). …”

DFT Calculations of the Elimination Kinetics of Silacyclobutanes and its Methyl Derivatives in the Gas-Phase

“…9 Proceeding the reverse dimerization reaction of diphenylsilene nonconcertedly, it must involve the intermediacy of the corresponding 1,3-disila-1,4-butanediyl biradical. 10 In terms of the "principle of microscopic reversibility," the reverse reaction, a head-to-tail [2+2]cycloreversion of silenes, should also be a diradical process, [11][12][13] but not a concerted one. 14 The kinetics of the gas-phase decomposition of silacyclobutane 1 and its methyl derivatives 2 and 3 have been experimentally studied.…”

Synthesis and Structure of a 2,4‐Unsubstituted cis/trans‐1,3‐Disilacyclobutane by Dehydrofluorination of a Highly Hindered Fluorosilane

“…[14][15][16][17] As a consequence of the steric repulsion of the substituents at the silicon atoms, the Si-C distances within the ring [1.901(2)-1.909(2) Å] are the longest observed so far for 1,3-disilacyclobutanes without substituents at the ring carbon atoms. According to previous studies long Si-C bond lengths in the ring correspond to reduced puckering angles.…”