Diels-Alder adducts of 4-chloro-1,6-dihydrophosphinine derivatives: A new precursor of 2-phosphapropene

The 4-chloro-1,6-dihydrophosphinine derivatives, prepared as a mixture of 3- and 5-methyl isomers from the thermolysis of dichlorocarbene adducts with 1-R-3-methylphospholene oxide (R  Me, MeO, EtO, n-PrO, i-PrO), participate in Diels-Alder reactions with dimethyl acetylenedicarboxylate and N-phenylmaleimide. The former reactant is of special value since the 2-phosphabicyclo[2.2.2]octa-5,7-diene framework is thermally labile and undergoes retrocycloaddition by a different path, eliminating the C P bridging unit as a low-coordinate species. Thus, in the case of R  Me, the initial phosphine oxides (a mixture of isomers) have been reduced to the phosphines and these undergo straightforward fragmentations on heating at 50°C to produce 2-phosphapropene. The reactions of this transient species with water and alcohols have been investigated. Unlike the case of stabilized phosphaalkenes, hydration occurs smoothly without catalysis to produce Me₂P(O)H; addition of alcohols gives phosphinites, Me₂POR. The structure of the Diels-Alder adducts was confirmed by ³¹P, ¹H, and ¹³C NMR spectroscopy. Each of the isomeric 1,6-dihydrophosphinine oxides gives two diastereomeric adducts; partial separation of some of the resulting four-component mixtures was achieved with silica gel chromatography.