Formation of two deprotonated forms of benzyl acetate and higher homologues: some unusual fragmentations

Abstract

Deprotonation of RCO2CH2Ph (R = Me, Et, iso Pr) with HO- in the gas phase yields the two (M-H)- ions (R-H)--CO2CH2Ph and RCO-2-CHPh which do not interconvert under conditions of collisional activation. When R = tert Bu, deprotonation forms the benzylic anion exclusively. Both anions undergo fragmentation following extensive rearrangement. The enolate anions (R-H)--CO2CH2Ph lose CO2 following a Claisen type rearrangement. The benzylic anion RCO-2-CHPh undergoes two complex rearrangements, viz (i) a 1,2 Wittig type rearrangement yielding (Ph)(R)CHO- and CO, and (ii) loss of the elements of R. The latter reaction is not a simple cleavage of R·: deuterium labelling supports stepwise loss of [H· + (R-H)] and product ion studies suggest the reaction proceeds through the complex rearrangement pathway RCO2--CHPh → RCOCH(Ph)O- → RCH(O-)COPh → [PhCOCHO]-• + H· + (R-H). The last step may proceed via a cyclic radical anion/radical intermediate. © 1998 Elsevier Science B.V.