Tracking “Invisible” Alkylchlorocarbenes by Their σ → p Absorptions:  Dynamics and Solvent Interactions

Abstract

Contrary to implications in the literature, the sigma --> p absorptions of alkylchlorocarbenes (RCCl) are readily acquired by laser flash photolysis with UV-vis detection in solution at ambient temperature. Examples include RCCl with R = methyl, benzyl, t-butyl, 1-adamantyl, and cyclopropyl. These absorptions permit direct monitoring of carbene reactions and the formation of carbene-solvent complexes. The kinetics of the reactions of "free" and complexed MeCCl and PhCH2CCl were directly followed with tetramethylethylene and 1-hexene. Particularly effective complexation was provided by anisole and 1,3-dimethoxybenzene, which modulated the rates of intermolecular carbene additions. Computational studies are presented, which aid in understanding the carbene absorption spectra and the nature of the solvent complexes.

A Half‐Century Odyssey in the Realm of Reactive Intermediates

“…Nevertheless, it proved possible to visualize these species by means of their weak s!p absorptions, [40] enabling studies of carbene solvation, reactivity modulation by p-complexation to aromatic molecules, and analysis of the equilibria between the carbenes and those complexes. [41] A burgeoning area in which we have not participated is the chemistry of stable carbenes.…”

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).…”