A laser flash photolysis and theoretical study of hydrogen abstraction from phenols by triplet alpha-naphthoflavone.

The hydrogen abstraction (HA) reaction by the triplet of alpha-naphthoflavone (1) has been investigated experimentally by the use of laser flash photolysis (LFP) and theoretically with density functional theory (DFT) and atoms in molecules (AIM). The triplet excited state of 1, in acetonitrile, has an absorption maximum at 430 nm and lifetime of 10 micros. The quenching rate constants for the triplet of 1 with 1,4-cyclohexadiene, substituted phenols and amines were determined. The low reactivity of this ketone with respect to HA from 1,4-cyclohexadiene is in accord with a pi,pi* excited state. HA from phenols in acetonitrile is proposed to occur in a diffusion controlled reaction from free phenol based upon the determination of the Abraham beta(H)(2) value for acetonitrile and correction of the quenching rate constants for hydrogen bonding of the phenols to acetonitrile. A molecular orbital analysis of the triplet (SOMO and SOMO-1) of 1 reveals contributions from the carbonyl oxygen atom, but principally from the alpha-carbon and the associated pi-bond network, consistent with a pi,pi* excited state. From a thermodynamic point of view, the triplet HA from phenol to oxygen of the carbonyl group is 17 kcal mol(-1) less demanding than the transfer to the alpha-carbon, consistent with the acidic nature of the phenolic hydrogen atom. DFT and AIM analysis of the hydrogen abstraction reaction reveals that the transition state (TS) is pseudo-symmetrically polarized and that HA in the hydrogen bonded exciplex occurs in a concerted manner but not necessarily by simultaneous electron and proton transfer.

Nucleophilicity toward ketenes: rate constants for addition of amines to aryl ketenes in acetonitrile solution.

Second-order rate constants (k(Nu)) have been measured for the addition of amines to ketenes 4-6 in acetonitrile solution by the laser flash photolysis technique. These ketenes are formed from a photochemical Wolff rearrangement of diazoketones 1-3, respectively. For all diazoketones studied, the presence of amines as nucleophiles in the reaction medium results in the formation of an intermediate that later converts to the amide. The rate of formation of these intermediates is linearly dependent on amine concentration. Various classes of amines, such as primary, secondary, and tertiary, aromatic, and aliphatic, have been used to investigate the ketene reactivity, and rate constants in the range 10(4)-10(9) M(-1) s(-1) have been measured. Reaction rates are dependent upon steric effects in both the ketene and the nucleophile, which is consistent with a reaction mechanism involving nucleophilic attack at Calpha in the molecular plane of the ketene. On the basis of these data, a set of N(+) parameters for the reaction of amines with ketenes was determined.