The differing mechanisms of photo-formation of 7-cyanobenzocyclooctatetraene from 7- and 6-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene

Abstract

It is known that 7-cyanobenzocyclooctatetraene (COT 2) is a product of the thermolysis and direct photolysis of 7- and 6-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (1 and 5), though the mechanisms of these rearrangements have not been reported. In the present study experiments have been carried out using the deuterium-labelled trienes 1a (93%-d1 at C-6) and 5a (93%-d1 at C-8), which were formed from 2π + 2π photo-closure (direct irradiation) of COT 2a labelled at C-8. The results reveal that whereas the thermolysis of 1a and 5a and the direct irradiation of 5a reform COT 2a, probably via cleavage of the strained cyclobutene C-1—C-6 bond, the direct irradiation of triene 1a gives a differently labelled COT product, containing the deuterium exclusively at C-9. The mechanism proposed for the latter phototransformation is a modification of the Zimmerman di-π-methane rearrangement.Key words: mechanisms, rearrangements, photochemistry, di-π-methane rearrangement.

5.06 Di-π-methane, Oxa-di-π-methane, and Aza-di-π-methane Photoisomerization

“…The complex reaction mechanisms of the photochemical transformations of such cyano substituted tricyclic compounds have been systematically investigated. 67,68 Similar studies have also been performed with the unsubstituted core structure and a methyl derivative. 69 A photochemical rearrangement observed with C 60 fullerene derivatives has been explained by a di-p-methane photoisomerization.…”

Chemoselective Photorearrangements of Diazinobarrelenes. Deuterium Labeling Study

“…The chemical shifts at δ 3.28 (dt, J = 2.4, 6.4 Hz, 1H), δ 3.67 (q, J = 6.4 Hz, 1H), and δ 3.31 (t, J = 6.4 Hz, 1H) were assigned to the cyclopropyl protons. The observed multiplicity at δ 3.67 is characteristic of tricyclo[3.3.0.0 2,8 ]octene systems . In the case of quinoxalinosemibullvalene 8 , the proton spectral features at the fused cyclopentanoid moiety proved almost directly superimposable upon that of pyrazinosemibullvalene 7 .…”

A tetrabromo-1,4-ethanonaphthalene and related dibromo-1,4-ethenonaphthalene

“…We recently reported the crystal and molecular structure of the useful intermediate 3,4-dibromo-6,7-benzobicyclo[3.2.1]octa-2,6-diene, (III) (Johnson et al, 2011) (Scheme 1). We report here on two related structures which provide access to a variety of substituted benzobarrelenes (Ç akmak & Balcı, 1989;Bender et al, 2003). Compound (I) (Fig.…”

“…2). We have used (II), inter alia, in the preparation of specifically labelled deuterated species such as 2-bromo-3-deuterobenzobarrelene (Bender et al, 2003).…”