Solvation and complexation of carbenes – a perspective

Abstract

Complexes of several carbenes with aryl ethers are documented by laser flash photolysis with UV‐visible spectroscopic detection. The carbenes include methylchlorocarbene, benzylchlorocarbene, phenylchlorocarbene, p‐nitrophenylchlorocarbene, and dichlorocarbene, whereas the complexing ligands are anisole, various methylanisoles, dibenzo‐18‐crown‐6, dibenzofuran, 1,3‐dimethoxybenzene, and 1,3,5‐trimethoxybenzene. The carbene complexes are characterized both by spectroscopy and computation. Complexation retards the additions of the carbenes to olefins. The complexation of phenylchlorocarbene by 1,3,5‐trimethoxybenzene is shown to be an equilibrium process, and the pertinent thermodynamic parameters are obtained by experiment and computed from theory. Copyright © 2011 John Wiley & Sons, Ltd.

Unusual solvation through both p-orbital lobes of a carbene carbon

“…2, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Especially, relevant has been the UV-vis/IR-LFP detection of solvent-carbene metastable complexes, previous to the system evolution to typical carbene stable products such as ylides and cyclopropanes, depending of the case. [21][22][23][24][25][26][27][28] This occurs in solvents having nonbonding electrons such as ethers, haloalkanes, and halobenzenes, and solvents having π electrons such as benzene and other unsaturated species. [21][22][23][24][25][26][27][28] Those solvents generate specific solvation by electron density donation to the empty p carbene carbon orbital, temporarily stabilizing the system.…”

“…[21][22][23][24][25][26][27][28] This occurs in solvents having nonbonding electrons such as ethers, haloalkanes, and halobenzenes, and solvents having π electrons such as benzene and other unsaturated species. [21][22][23][24][25][26][27][28] Those solvents generate specific solvation by electron density donation to the empty p carbene carbon orbital, temporarily stabilizing the system. 11,12,[21][22][23] This complexation retards reactions such as additions to olefins to form cyclopropanes, 22,23 or the formation of ylides, by factors of up to 2 orders of magnitude in their rate constants, with respect to the case of absence of stabilizing solvents.…”

“…[21][22][23][24][25][26][27][28] Those solvents generate specific solvation by electron density donation to the empty p carbene carbon orbital, temporarily stabilizing the system. 11,12,[21][22][23] This complexation retards reactions such as additions to olefins to form cyclopropanes, 22,23 or the formation of ylides, by factors of up to 2 orders of magnitude in their rate constants, with respect to the case of absence of stabilizing solvents. 24 Parallel to the experimental findings, there have been theoretical efforts for describing this specific carbene solvation.…”

“…24 Parallel to the experimental findings, there have been theoretical efforts for describing this specific carbene solvation. 12,[21][22][23][24][25][26][27][28] In some of these studies, it has been proposed 1:1 complexes, with respect to the carbene:solvent molecular relationship. Candidate carbene-solvent structures are subjected to geometrical optimizations via traditional methods, usually within the framework of the density functional theory (DFT).…”

“…Candidate carbene-solvent structures are subjected to geometrical optimizations via traditional methods, usually within the framework of the density functional theory (DFT). By means of this procedure, Moss et al 12,[21][22][23][25][26][27] have postulated two classes of complexes: one of "π" type, where the carbene carbon interacts directly with the π system of the solvent aromatic ring. The other is the so-called "ylide complex," wherein a lone pair of a solvent molecule interacts with the carbene carbon.…”