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Theoretical and experimental studies on stability of the C‐ON bond in new ketone functionalized N ‐alkoxyamines
Author(s) -
Megiel Elżbieta,
Kaim Andrzej,
Cyrański Michał Ksawery
Publication year - 2010
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1685
Subject(s) - homolysis , chemistry , ketone , nitroxide mediated radical polymerization , alkyl , radical , bond dissociation energy , bond cleavage , photochemistry , computational chemistry , medicinal chemistry , polymer chemistry , organic chemistry , dissociation (chemistry) , polymer , radical polymerization , polymerization , catalysis
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 (ln A , 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. Copyright © 2010 John Wiley & Sons, Ltd.