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Base-Induced 1,3-Sigmatropic Rearrangement of Mesitylphosphonium Salts



Attempted synthesis of the ylide dianion [2,4,6-Me3C6H2P(CHR)3]2– (2,4,6-Me3C6H2 = mesityl, R = H or Me) by the reaction of mesitylphosphonium iodides [2,4,6-Me3C6H2PR3]+I (R = Me, 1; R = Et, 2) with tBuLi at reflux does not result in the anticipated deprotonation of the phosphorus-bonded R groups. Instead, quantitative 1,3-sigmatropic rearrangement occurs to give new benzylic phosphonium salts [(3,5-Me2C6H3)CH2PR3]+I (R = Me, 6; R = Et, 7), in which the phosphonium centre, the R3P group, is transferred to an ortho-CH3 group. In situ 31P NMR spectroscopic studies show that the reaction is base-activated and stoichiometric with respect to tBuLi. DFT calculations support the conclusion that the rearrangement is thermodynamically favourable in the gas phase and in THF and show that the rearrangement is enthalpically driven.