α‐Phosphanyl Amino Acids: Synthesis, Structure and Reactivity of N‐Aryl‐α‐phosphanylglycines

Abstract

AbstractThe novel N‐aryl‐α‐phosphanylglycines 1a–d have beeneasily prepared by one‐pot three‐component reactions of diphenylphosphane, primary arylamines and glyoxylic acid hydrate in diethyl ether. The reactions proceed via intermediate arylammonium (diphenylphosphanyl)glycolates 2 and stop at this stage in the case of N‐secondary anilines, for example, the stable N‐methylanilinium phosphanylglycolate 3. The conversion of 2 into 1 is an equilibrium, as demonstrated by the reversible hydrolysis of 1b to 2b in [D8]THF containing varying amounts of water. The solid α‐phosphanyl amino acids are stable, but in THF or DMSO solution they suffer from slow decarboxylation, which becomes rapid on heating at about 80 °C, yielding N‐secondary phosphanylmethylanilines 4. Solutions of 1 in the polar protic solvent methanol are stable over long periods, but show rapid proton/deuterium exchange even of the α‐CH proton in deuteriomethanol and thus increased CH acidity compared with normal α‐amino acids. This and the sensitivity to hydrolysis hint at an acetal‐like character and partial protonation even at the phosphorus atom. Oxidation of the N‐aryl derivatives 1b,c by aqueous H2O2 proceeds more rapidly than hydrolysis and provides the first examples of phosphinoylglycines 5. Oxidation by sulfur is rapid even at room temperature in methanol, giving the thiophosphinoylglycines 6. The coordination properties of 1 at the phosphanyl group are characterized by the reaction of 1b with [MVI(CO)5(THF)] (MVI = Cr, Mo, W) and the coordination chemical shifts in the NMR spectra and νCO shifts in the IR spectra of the resulting [(1b)MVI(CO)5] complexes 7b–9b. The structures of the new compounds were elucidated by crystal structure analysis of 1b and 6b and the solution NMR spectroscopic data of all the compounds. Screening for the nickel‐catalysed oligomerization of ethylene showed the formation of active catalysts from 1 and Ni(cod)2.