Daisy de Brito Rezende

Laser-flash photolysis of α-brominated o-xylenes

Abstract The flash photolysis (λ=266 nm ) of four α-brominated o-xylenes in apolar solvents gives two transients characterized, depending on parent compound substitution, as either monoradicals or carbenes, and quinodimethanes. α-Bromomethylbenzyl radical is characterized and the novel species α,α′-dibromo-o-xylylene and o-(α,α-dibromomethyl)benzyl carbene are described for the first time.

Olefin photoaddition to 4-oxoisophorone

The intramolecular photocyclization of the enedione moiety to the olefinic moiety in 1, yielding the cage-compound 3, was first described in 1964 by Cookson et al.3 It has been extensively investigated since then, and is part of a proposed 4 solarenergy storage system. This reaction is not a concerted [2+2] cycloaddition and its mechanism ought to be formulated as in Scheme 1, in view of the cumulative results (see ref. 5a and references therein cited) published since 1964.

H-1 and C-13 NMR of some alpha-halo derivatives of o-xylene

Abstract 13C and 1H NMR data for a series of α-halo derivatives of o-xylene are presented. A dynamic 1H NMR investigation of α,α,α′,α′-tetrabromo-o-xylene (5) was performed and the spectrum of the most stable conformer at 213 K is presented. The free energy of activation for the rotation of the CHBr2 groups in 5 are determined for the first time.

Photolysis of a series of α-brominated ortho-xylenes in apolar solvents

The α-brominated ortho-xylenes have been subjected to 254 nm irradiation in deaerated benzene, isooctane and benzene–cyclohexene. The product analysis revealed that homolysis of the C–Br bond is followed by a series of hydrogen abstraction and radical recombination reactions resulting in xylenes more and less brominated than the starting compound. The less brominated products are formed with higher quantum yield when cyclohexene is present, due to hydrogen abstraction by the o-benzyl radical formed initially, together with cyclohexene dimers. Additionally, the formation of 2-bromo-2,4,4-trimethylpentane is observed when isooctane is the solvent. The quantum yields observed for the photolysis of 1 and 2 are higher in benzene than in isooctane, suggesting sensitization by benzene. A biradical intermediate of the type o-quinodimethane was expected in the case of (a) photolysis of the o-benzyl radical formed (biphotonic process) or (b) intramolecular hydrogen abstraction. However, the addition of cyclohexene failed to produce the expected Diels–Alder adduct. The synthesis of the novel α,α-dibromo-o-xylene 3 is reported.

The γ-cis effect in the mercury-199 NMR spectroscopy of substituted vinylmercury halides

The γ-cis effect is identified for the first time in the 199Hg NMR data presented here, being proposed as a new criterion for the distinction between E and Z isomers of substituted vinylmercury compounds.

Novel photorearrangement of a 4-chloromethylazulene

Abstract 1-Chloro-4,6,8-trimethylazulene is formed almost exclusively upon UV irradiation of a deaereted benzene solution of 4-chloromethyl-6,8-dimethylazulene. This photoproduct was identified by 1 H and 13 C NMR spectroscopy of both the isolated photoproduct and the one prepared by a thermal route. The mechanism proposed involves the recombination of the intimate radical pair initially formed by the photohomolysis of the CCl bond, followed by a sigmatropic [1,9] hydrogen shift.

An empirical method for the evaluation of electron affinities

An empirical correlation involving electron affinity (Eea), first ionization potential (Ei) and the lowest singlet excited-state energy (ΔE0–01) has been obtained through a multiple regression analysis performed on the literature data of model monocyclic substituted aromatic compounds. The Eea of a series of aryl alkyl sulphides has been evaluated using this correlation. The resulting data are within the expected range, when compared with those of the model compounds.