Hammick intermediate

An interesting class of elusive neutral species is heterocyclic ylids, as represented by the so-called Hammick intermediate that was postulated 65 years ago to explain the accelerated decarboxylation of 2-picolinic acid [146, 147]. An analogous dissociation takes place in ionized 2-picolinic acid in the gas-phase and was employed to generate the pyridine ion isomer 35+ (Scheme 13) [148]. Collisional neutralization of 35+ with A/,AT-dimethylaniline produced neutral ylid 35, which can also be represented as a singlet a-carbene (Scheme 13). Ylid 35 showed a survivor ion in the +NR+ mass spectrum, which was further char-... 

Lavorato D, Terlouw JK, Dargel TK, Koch W, McGibbon GA, Schwarz H. Observation of the Hammick intermediate reduction of the pyridine-2-yhd ion in the gas phase.JAm Chem Soc. 1996 118 11898-11904. 

The relatively easy decarboxylation of a- (682) and y-carboxylic acids is a result of inductive stabilization of intermediate ylides of type (683) (cf. Section 3.2.1.8.2). By carrying out the decarboxylation in the presence of aldehydes or ketones, products of type (684) are formed (Hammick Reaction). 

It seems reasonable to assume a dinitrosoethylene structure (74) (or dinitrosobenzene, for the benzofuroxans) as intermediate. This was originally suggested by Hammick et al.3A and supported by Mallory et al.31,99 from bond energy arguments, and also by consideration of the entropy of activation. This was found to be small, but positive in sign, perhaps indicating a... 

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