Theoretical and experimental studies on stability of the C-ON bond in new ketone functionalized N-alkoxyamines

摘要

Three new ketone functionalized N-alkoxyamines derived from 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) were prepared: N-(1-phenylpropyloxy) -2,2,6,6-tetramethylpiperidin-4-one, 1-phenyl-1-(2,2,6,6- tetramethylpiperidinoxy)propanone, 1-phenyl-1-(4-oxo-2,2,6,6- tetramethylpiperidinoxy)propanone. The rate constants of C-ON bonds homolysis in the synthesized alkoxyamines were determined over a range of temperatures via nitroxide-exchange experiments using HPLC to monitor the concentration. The Arrhenius parameters of homolysis for the investigated alkoxyamines were determined (lnA, E_a). Homolytic bond dissociation energies (BDE) of the C-ON bond in the synthesized compounds were determined from quantum-mechanical calculations at the B3-LYP/6-31G(d) and BMK/6-311+G(3df,2p) levels. Ketone functionalization of the alkyl fragment of alkoxyamine in β position dramatically increases the rate constant of homolysis (by a factor of ca. 500 at the temperature of 363 K) suggesting that the new ketone functionalized N-alkoxyamines should be effective as C-radical precursor and unimolecular initiators in NMRP at lower temperatures than the alkoxyamines applied earlier. The analyses of natural bond, frontal orbitals and spin distribution indicated that the decrease in the strength of C-ON bonds in ketone fuctionalized alkoxyamines in the alkyl fragment predominantly originates from a substantially smaller HOMO-LUMO gap and more delocalized spin density in leaving alkyl radicals as compared with unfunctionalized alkoxyamines.