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

Attempted synthesis of the ylide dianion [2,4,6‐Me 3 C 6 H 2 P(CHR) 3 ] 2– (2,4,6‐Me 3 C 6 H 2 = mesityl, R = H or Me) by the reaction of mesitylphosphonium iodides [2,4,6‐Me 3 C 6 H 2 PR 3 ] + I – (R = Me, 1 ; R = Et, 2 ) with t BuLi 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‐Me 2 C 6 H 3 )CH 2 PR 3 ] + I – (R = Me, 6 ; R = Et, 7 ), in which the phosphonium centre, the R 3 P group, is transferred to an ortho ‐CH 3 group. In situ 31 P NMR spectroscopic studies show that the reaction is base‐activated and stoichiometric with respect to t BuLi. 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.