A Convenient Synthesis of New Isolable Phosphaalkenes Using the Base-Induced Rearrangement of Secondary Vinylphosphines

The secondary vinylphosphines ArFP(H)C(R)CH2 [2a, ArF = 2,6-(CF3)2C6H3, R = CH3; 2b, ArF = 2,6-(CF3)2C6H3, R = C6H5; 2c, ArF = 2,4,6-(CF3)3C6H2, R = CH3] were prepared by treating the corresponding dichlorophosphine ArFPCl2 (1) with H2CC(R)MgBr. In the presence of catalytic base (DBU or DABCO) the vinylphosphines (2a − c) undergo quantitative 1,3-hydrogen migration over 3 d to give stable and isolable phosphaalkenes ArFP=C(R)CH3 (3a, ArF = 2,6-(CF3)2C6H3, R = CH3; 3b, ArF = 2,6-(CF3)2C6H3, R = C6H5; 3c, ArF = 2,4,6-(CF3)3C6H2, R = CH3). Under analogous conditions, only 90% conversion is observed in the base-catalyzed rearrangement of MesP(H)C(CH3)CH2 to MesPC(CH3)2. Presumably, the increase in acidity of the P−H group when electron-withdrawing groups are employed (i.e. 2a − c) favors quantitative rearrangement to the phosphaalkene tautomer (3a − c). Thus, the double-bond migration reaction is a convenient and practical method of preparing new phosphaalkenes with C-methyl substituents.