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Carbanion rearrangements. Collison induced dissociations of deprotonated alkyl acetoacetates

1987; Wiley; Volume: 22; Issue: 2 Linguagem: Inglês

10.1002/oms.1210220210

2376-3884

P. Eichinger, John H. Bowie,

Asymmetric Synthesis and Catalysis

Abstract Deprotonation of methyl acetoacetate yields two enolate ions MeCOC̄HCO 2 Me ( a ) and C̄H 2 COCH 2 CO 2 Me ( b ). On collisional activation, ions a and b fragment differently. The major fragmentation of a is specific loss of MeOH through a four‐centred transition state to form − O(Me)CCCO. In contrast, ion b eliminates CH 2 CO to give − CH 2 CO 2 Me. Some rearrangement of b to a is also noted. Rearrangement of a to b is very minor under single collision conditions but at high collision gas pressure rearrangement of a to b is strongly promoted. Similar effects are observed in the collisional activation spectra of MeCOC̄(Me)CO 2 Me ( c ) and − CH 2 COC(Me)CO 2 Me ( d ). The loss of MeOH from ( c ) proceeds via a six membered transition state to − CH 2 COC(Me)CO; this is a stepwise process in which the deprotonation (step two) is not rate determining. A number of other decompositions occur, these have also been studied by deuterium labelling.