alpha-Hydrogen spirocyclic alkoxyamines synthesis: ESR verified Nitroxides generation and their potential application in NMP polymerization

Herein, we report a synthetic methodology towards the construction of new cylic alkoxyamines without the prior obtention of their respective nitroxides. These systems are constituted by an indene core connected to a pyrrolidine nucleus through a spiro carbon atom, containing an adjacent a-hydrogen to the N atom, in order to exhibit the 1-azaspiro[4.4]nonane frame. Alkoxyamines were synthesised by a free radical-based spirocyclization, starting from functionalized oxime ethers, involving aryl and neutral alkoxyaminyl radicals in 16-40 % yields. Then, the novel spirocyclic aminoxyl radicals (nitroxides) were detected in ESR spectra, when alkoxyamines were subjected by the thermal homolytic cleavage of their -C-O-N- bond. In general, the temperatures found were in accordance with the increase in the stability of the carbon-centred radical for the same type of nitroxides. In addition, a spirocyclic alkoxyamine 8 b with a-hydrogen was employed as initiators/mediators in the styrene bulk homopolymerization, obtaining high conversions and relatively narrow dispersity (similar to 1.5). This suggests that the synthetic methodology used in this research work to obtain a-hydrogen spirocyclic alkoxyamines would have the potential to be applied in nitroxide-mediated polymerization (NMP).

Automated summary

In this article, a synthetic methodology for the construction of new cylic alkoxyamines is reported. These alkoxyamines are constituted by an indene core connected to a pyrrolidine nucleus through a spiro carbon atom, exhibiting the 1-azaspiro[4.4]nonane frame. The alkoxyamines were synthesised through a free radical-based spirocyclization, and the corresponding spirocyclic aminoxyl radicals (nitroxides) were detected in ESR spectra. Moreover, a spirocyclic alkoxyamine with an α-hydrogen was found to be effective as an initiator/mediator in the styrene bulk homopolymerization.