Abstract
The reactions of 2,4-di-tert-butyl-6-(diphenylphosphino)phenol and various Michael acceptors (acrylonitrile, acrylamide, methyl vinyl ketone, several acrylates, methyl vinyl sulfone) yield the respective phosphonium phenolate zwitterions at room temperature. Nine different zwitterions were synthesized and fully characterized. Zwitterions with the poor Michael acceptors methyl methacrylate and methyl crotonate formed, but could not be isolated in pure form. The solid-state structures of two phosphonium phenolate molecules were determined by single-crystal X-ray crystallography. The bonding situation in the solid state together with NMR data suggests an important contribution of an ylidic resonance structure in these molecules. The phosphonium phenolates are characterized by UV-Vis absorptions peaking around 360 nm and exhibit a negative solvatochromism. An analysis of the kinetics of the zwitterion formation was performed for three Michael acceptors (acrylonitrile, methyl acrylate and acrylamide) in two different solvents (chloroform and methanol). Results revealed the proton transfer step necessary to stabilize the initially formed carbanion as the rate determining step. A preorganization of the carbonyl bearing Michael acceptors allowed for reasonable fast direct proton transfer from the phenol in aprotic solvents. In contrast, acrylonitrile not capable of forming a similar preorganization, is hardly reactive in chloroform solution, while in methanol the corresponding phosphonium phenolate is formed.
Supplementary materials
Title
Supporting data
Description
experimental procedures, plot of the solid-state structure of 2f, crystallographic data, NMR spectra, UV-Vis spectra and experimental and simulated time conversion plots for the zwitterion formation
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Title
structure data for 2a
Description
cif - file also available as CCDC 2287962
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Title
structure data for 2f
Description
cif - file also available as CCDC 2287963
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