Laser flash photolysis of 1,2-diketopyracene and a theoretical study of the phenolic hydrogen abstraction by the triplet state of cyclic alpha-diketones.
Laser flash photolysis (LFP) studies, atoms in molecules (AIM) studies, and density functional theory (DFT) calculations have been performed in order to study the mechanism of the hydrogen abstraction by alpha-diketones in the presence of phenols. Laser irradiation of a degassed solution of 1,2-diketopyracene in acetonitrile resulted in the formation of a readily detectable transient with absorption at 610 nm, but with very low absorptivity. This transient decays with a lifetime of around 2 micros. The quenching rate constant for substituted phenols, kq, ranged from 1.10x10(8) L mol-1 s-1 (4-cyanophenol) to 3.87x10(9) L mol-1 s-1 (4-hydroxyphenol). The Hammett plot for the reaction of the triplet of 1,2-diketopyracene with phenols gave a reaction constant rho=-0.9. DFT calculations (UB3LYP/6-311++G**//UB3LYP/6-31G*) of the triplet complex ketone-phenol revealed that hydrogen transfer has predominantly occurred and that the reaction with alpha-diketones are generally 7 kcal/mol less endothermic than the respective reactions of the monoketones. These results together with the geometries obtained from the DFT calculations, natural bond order (NBO) analysis, and AIM results indicate that hydrogen abstraction for alpha-diketones is facilitated by the electrophilicity of the ketone, instead of neighboring group participation by the second carbonyl group.
de Lucas
Nanci C
NC
Departamento de Química Orgânica, Universidade Federal do Rio de Janeiro, Cidade Universitária, Rio de Janeiro, 21949-900, RJ, Brazil.
Correa
Rodrigo J
RJ
Albuquerque
Ana Carla C
AC
Firme
Caio L
CL
Garden
Simon J
SJ
Bertoti
Ada Ruth
AR
Netto-Ferreira
José C
JC
D007659:Ketones / Q000737:chemistry* / Q000528:radiation effects*; D007834:Lasers; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry* / Q000528:radiation effects*; D010782:Photolysis; D011083:Polycyclic Compounds / Q000737:chemistry* / Q000528:radiation effects*; D014466:Ultraviolet Rays
C520329:1,2-diketopyracene; D007659:Ketones; D010636:Phenols; D011083:Polycyclic Compounds