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 CP 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.
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