The photochemistry of 6- and 7-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene

Abstract

Chem. 64, 237 (1986).The direct irradiation of 7-cyanobenzocyclooctatetraene (5) led to 6-and 7-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (7 and 8; Q7 = Qg < 0.0001). On direct irradiation, 7 gave 1-cyanobenzosemibullvalene (6; Q, = 0.19), 5 (Q, = 0.022), and 2-cyanonaphthalene (Q, = 0.04), whereas 8 gave 7-cyanobenzosemibullvalene (16; Q

The photochemistry of 1-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene

“…Turning now to the direct irradiation of triene 10, the product types formed are the same as those found in the case of triene 1 (2), though the SB formation from 10 is much less predominant than from 1. Furthermore, the results from the deuterium runs otherwise, e= H indicate that the limited SB that is formed from 10 is furnished by two routes.…”

“…The formation of COT 9 from T I of 10 is a striking result, since all previous studies with benzobicyclo[4.2.0]octatrienes have found SBs and (or) naphthalenes to be the triplet products (1)(2)(3)(4)(5). The SB 4 formed in the case of 1 was rationalized by the Zimmerman DPM rearrangement depicted in Scheme 1 (2), and the high efficiency of the process Can : cleave '.,cleave : b and c '\, a andd = 0.68) was attributed to the presence of the radical delocalizing cyano group on the methane carbon (C-6) of the vinyl-vinyl DPM s y~t e m .~ For triene 10, the deuterium labelling experiments clearly rule out the involvement of a vinyl-vinyl DPM rearrangement in the formation of COT 9, since no deuterium was found at C-6 or at C-7 of the product (i.e., route ii, Scheme 4, does not operate). Also, the possible operation of a benzo-vinyl DPM rearrangement, which apriori might be promoted by the cyano group on the methane carbon (C-1), seems unlikely since this would give the octavalene 20 and subsequently5 the COTS 21 and 22 (see Scheme 6), neither of which was observed in photolysates of 10; both 21 and 22 are known compounds (10) and were used for gc comparisons.…”

“…The most dramatic is that found for the triplet reactivities: 1 (2) gives a semibullvalene (4) as the major product (90%), whereas 10 gives exclusively a COT (9) in 92% yield. The formation of COT 9 from T I of 10 is a striking result, since all previous studies with benzobicyclo[4.2.0]octatrienes have found SBs and (or) naphthalenes to be the triplet products (1)(2)(3)(4)(5).…”

“…The most dramatic is that found for the triplet reactivities: 1 (2) gives a semibullvalene (4) as the major product (90%), whereas 10 gives exclusively a COT (9) in 92% yield. The formation of COT 9 from T I of 10 is a striking result, since all previous studies with benzobicyclo[4.2.0]octatrienes have found SBs and (or) naphthalenes to be the triplet products (1)(2)(3)(4)(5). The SB 4 formed in the case of 1 was rationalized by the Zimmerman DPM rearrangement depicted in Scheme 1 (2), and the high efficiency of the process Can : cleave '.,cleave : b and c '\, a andd = 0.68) was attributed to the presence of the radical delocalizing cyano group on the methane carbon (C-6) of the vinyl-vinyl DPM s y~t e m .~ For triene 10, the deuterium labelling experiments clearly rule out the involvement of a vinyl-vinyl DPM rearrangement in the formation of COT 9, since no deuterium was found at C-6 or at C-7 of the product (i.e., route ii, Scheme 4, does not operate).…”

“…It has been shown recently that semibullvalenes (i.e., SBs) can be produced from the light-induced isomerization of benzobicyclo[4.2.0]octa-2,4,7-trienes (1)(2)(3)(4)(5). Mostly their formation can be rationalized by a Zimmerman di-71-methane rearrangement (6) involving initial bridging between C-5 and C-7 of the two vinyl portions of the trienes (e.g., 1 4 4; Scheme…”