Laser Flash Photolysis Studies of the Reaction of Arylhalocarbenes with Tetramethylethylene as a Function of Solvent

Abstract

Absolute rate constants of reaction of chlorophenylcarbene, bromophenylcarbene, and chloro-p-nitrophenylcarbene with tetramethylethylene (TME; k TME ) are similar in alkane solvent, Freon-113, benzene, tetrahydrofuran, and acetonitrile. Solvation of the carbenes has little influence on k TME . Strong specific solvation which impedes cycloaddition reactions of the arylhalocarbenes with TME is not found with these carbenes, but the existence of weakly bound complexes with spectra and kinetics similar to those of free carbenes cannot be ruled out.

Matrix and Time-Resolved Infrared Spectroscopy of Chloro-p-nitrophenylcarbene and Related Species

“…This is in accord with our kinetic studies of this system. 13 LFP of 2 in either Freon-113 (CF 2 ClCFCl 2 ) or carbon tetrachloride ( Figure 2) leads to the presence of a new transient species absorbing at 1316 cm -1 . This species grows and decays with comparable time constants.…”

“…This spectrum is attributed to ylide 4, previously observed by UV-vis spectroscopy. 4,11,13 The ylide is formed with observed rate constant 5.8 × 10 6 s -1 and has a lifetime of several microseconds at ambient temperature.…”

Solvation and complexation of carbenes – a perspective

“…This is ascribed to specific solvation of the carbene rather than to ylide formation. Of course, Platz notes that ‘the simplest and most direct evidence of specific solvation would be the discovery of a unique spectroscopic signature of a specifically solvated carbene distinct from that of a free carbene and an ylide.’ Such direct evidence was not obtained in the study of the amidochlorocarbene (or its fluorocarbene analog) with either dioxane or THF, nor was it found in earlier studies of phenylchlorocarbene (PhCCl) or phenylfluorocarbene (PhCF) with benzene or THF, or of PhCCl, phenylbromocarbene (PhCBr), or p ‐nitrophenylchlorocarbene with benzene, THF, or acetonitrile …”

Carbene Complexes of Nonmetals

“…[50] Ethers were found to affect the CϪX vibrations and the reaction rates with TME only slightly. [50,51] Arylhalocarbenes have singlet ground states, due to electron-donation from the halogen atoms to the divalent carbon. Therefore, complexation with external donors does not contribute strongly to the stability of these species.…”

“…In fact, the reaction of arylhalocarbenes 18 with TME proceeded more slowly in benzene than in aliphatic hydrocarbons, by factors of 2Ϫ3. [51] The CϪX vibrational fre-Scheme 17. Complexation of halocarbenes with benzene quencies of arylhalocarbenes, on the other hand, are not significantly affected by the presence of benzene.…”