Synthesis and Characterization of β‐Phosphaenones An Investigation on the Conjugative Properties of the PC Bond

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Abstract
Thermally and air‐stable β‐phosphaenones were synthesized by functionalization of Mes*PCCl2 (1; Mes* = supermesityl = 2,4,6‐tri‐tert‐butylphenyl). At low temperature, 1 was lithiated by halogen‐metal exchange with n‐butyllithium to give the phosphanylidene carbenoid (Z)‐Mes*PC(Cl)Li [(Z)‐2] which reacted with acid chlorides to furnish the C‐carbonyl‐substituted phosphaalkenes (Z)‐Mes*PC(Cl)R (3: P = COtBu; 4: R = COPh; 5: R = COOEt). The reaction of (Z)‐2 with carbon dioxide furnished the carboxylate 6, which was converted by treatment with pivaloyl chloride or trimethylsilyl chloride into the phosphaalkenes 7 and 8 functionalized at the carbon atom by an anhydride or a trimethylsilyl ester function, respectively. Acidification of 6 or hydrolysis of 8 with water in chloroform solution afforded the novel carboxylic acid (Z)‐Mes*PC(Cl)COOH (9). Spectroscopic investigations (NMR, UV, IR) of 3–9 and the X‐ray structures of 3 and 4 are presented. Based on these properties and on theoretical calculations, the occurrence of conjugation in the β‐phosphaenone system is discussed and compared with the well‐known conjugation in normal enones.

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