Abstract
The search for aliphatic nitrenium ions from solvolysis of N-carboxylates and N-sulfonates
Highlights
The aminoxyl radical 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO, TO) is widely used for the trapping of free radicals,[1,2] and has been extensively utilized in controlling living free radical polymerization.[3]
We have been carrying out studies of the reactions of ketenes with aminoxyl radicals,[4] as exemplified by the addition of TEMPO to phenylketene, which is found to proceed by radical attack at the carbonyl carbon forming α-acyl radical 1 which is trapped by a second TEMPO giving the 1,2-diaddition product 2 (Scheme 1)
A different interpretation of these results was proposed by Hoffman, et al, who studied the reactions of the N-substituted arenesulfonate esters 17 and found only products attributable to rearrangements following departure of sulfonate leaving groups (Scheme 6).8c-e They suggested rearrangement was concerted with leaving group departure so that discrete nitrenium ions were not formed
Summary
The aminoxyl radical 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO, TO) is widely used for the trapping of free radicals,[1,2] and has been extensively utilized in controlling living free radical polymerization.[3]. The ionic chemistry of N-substituted amines has long been studied, and is implicated in the Stieglitz, Curtius, Hoffman, Lossen, and Schmidt rearrangements.[5] These reactions have been considered to involve putative divalent positively charged nitrogen species, or nitrenium ions. Arylnitrenium ions[6,7] have been of major recent interest because they have been implicated as cancer causing agents, and their reactions have been well studied, including time resolved study by UV and by IR These species are readily prepared in ionizing solvents from a variety of precursors such as the N-pivalyl 4-. In the product 6 the positive charge is largely delocalized away from nitrogen into the aryl ring in these species as shown in 6b, as evidenced by the formation of products of nucleophilic attack on the aryl ring.[6,7,10] This structure is confirmed by the analysis of the time-resolved IR spectrum of Ph2N+ (7), shown to have a linear singlet structure (Scheme 3).7e. Methyl p-nitrobenzoate and 13 were proposed to result from transesterification of 8 by methanol (Scheme 4)
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