Fumio Kita

Localized triplet diradicals as a probe for electronic substituent effects in benzyl-type radicals: The Δ D scale

The D parameter, readily determined by EPR spectroscopy, is a sensitive function of the avera e distance r of separation between the unpaired electrons in the localized triplet 1,3-diradicals 2 (D cc llr ). For convenience, we have defined the difference between the D values of the benzylic substituent X and the parent system (X = H), i.e. AD = D, - D,, as a measure of spin delocalization by the aryl group at the radical site. The additivity of the AD values of the monosubstituted diradicals 1 versus the symmetrically disubstituted diradicals 2 demonstrates that such triplet diradicals can be described as a composite of two geometrically fixed cumyl radical fragments. The D parameter correlates well with the experimental hyperfine coupling constants (ap), with the calculated a spin densities (p,), and the calculated resonance stabilization energies (RSE) for substituted cumyl radicals. These results manifest that the novel AD scale constitutes a reliable spectral tool to determine electronic substituent effects in benzyl-type radicals and may serve as a probe to assess the importance of polar substituent effects in chemical (3,d scales. B

Two-photon chemistry in the laser jet: generation of radical intermidiates and their photochemical transformations

Abstract The novel laser jet technique provides sufficiently high photon densities to permit the observation of the photochemistry of photochemically generated radicals (two-photon chemistry) in the liquid phase. Four recent applications of this novel photochemically useful method are presented: these include the photochemistry of hydroxydiphenylmethyl, 9-hydroxyxanthenyl, diphenylmethyl, and benzoyl radicals under laser jet and normal photolysis conditions. The regioselectivity of cross-coupling reactions of hydroxydiphenylmethyl or 9-hydroxyxanthenyl radicals with solvent-derived radicals changes when these species are electronically excited, i.e. under the high intensity conditions of the laser jet, cross-coupling at the para position (head-to-tail combination) is significantly enhanced relative to the normal coupling mode at the hydroxy-bearing radical site (head-to-head combination). Semiempirical calculations of the spin density distributions for the ground and first excited states of the radicals confirm the change in spin density from the hydroxy-bearing carbon atom to the conjugating benzene rings in these radical species on photoexcitation. For the diphenylmethyl radical, two reaction pathways have been observed under the high photon densities of the laser jet: the electronically excited diphenylmethyl radical can either abstract a chlorine atom from carbon tetrachloride through an electron transfer process or can be photoionized on further photoexcitation (multiphoton chemistry). The resulting benzhydryl cation was trapped by methanol as the corresponding ether product, which unequivocally demonstrates that carbene formation by photoejection of a hydrogen atom does not take place under laser jet photolysis conditions. An advantage of the high photon densities produced in laser jet photolysis is the high steady state concentration of short-lived transients that are generated, which enable unprecedented intermolecular reactions to be observed. Thus, about a millimolar concentration of tert -butoxy radicals can be obtained in the laser jet photocleavage of tert -butyl peroxide. When the tert -butoxy radicals are produced in the presence of benzaldehyde, the main product is tert -butyl benzoate. If carbon tetrachloride is also present, chlorobenzene can be detected. This is rationalized as the product derived from chlorine abstraction by phenyl radicals, which are presumably produced by the photodecarbonylation of benzoyl radicals. An alternative method of obtaining benzoyl radicals is the two-photon cleavage of benzil. The laser jet photolysis of benzil in tert -butyl peroxide yields mainly tert -butyl benzoate, whereas in carbon tetrachloride, benzoyl chloride, chlorobenzene and α,α,α-trichloroacetophenone are observed. The first two products result from chlorine atom abstraction by the photochemically generated benzoyl and phenyl radicals, and the last product from in-cage cross-coupling between benzoyl and trichloromethyl radicals. Such product studies provide detailed mechanistic information on the photochemical behaviour of electronically excited, short-lived transients which complements nicely the kinetic and spectral data of time-resolved laser flash studies. Consequently, the laser jet technique constitutes a valuable tool for determining the mechanism of two- photon reactions.

Benzophenone-sensitized two-photon chemistry of azoalkanes in the “laser/liquid jet”: evidence for photoinduced hydrogen 1,2-shift in 1,3-cyclopentadiyl triplet diradicals.

Abstract While the triplet-sensitized photolysis of the bicyclicazoalkanes 1 led on denitrogenation to the corresponding bicycloalkanes 2, the laser/liquid jet photochemistry gave additionally also the cyclopentenes 3 by hydrogen 1,2-shift; the amount of the latter increased with increasing lifetime of the intermediary cyclopentadiyl triplet diradicals 4, which suggests that two-photon chemistry may operate.

The D parameter (zero-field splitting) as a direct measure of structural end electronic effects in localized triplet 1,3-diradicals

The zero-field splitting parameter D of the localized triplet diradicals 3–11, generated in a 2-methyltetrahydrofuran (MTHF) glass matrix at 77 K through photochemical deazetation of the corresponding azoalkanes, have been determined by EPR spectroscopy. It is demonstrated that the D parameter depends on structural (changes in the interspin distance d between the spin-bearing sites) effects, owing to differing ring size annelation, as well as electronic (changes in the spin density ρα at the radical sites) effects, i.e. D= f(d, ρα). The Dcalcd values calculated from the theoretical d and ρα values determined by semiempirical MO calculations (PM3) for the triplet diradicals 1–11, correlate well (r2= 0.961) with the Dexp values determined experimentally. Interestingly, heteroatom substitution at the 4,5-position in the cyclopentane-1,3-diyl moiety causes only a nominal spin delocalization onto the nitrogen atoms for the diradicals 11, which are also classified as localized triplet diradicals.