Mechanistic variations in the formation of benzocyclooctatetraene from benzobicyclo[4.2.0]octa-2,4,7-triene

Abstract

Benzocyclooctatetraene (i.e., COT, 2) is the major product from the thermolysis and direct photolysis of 2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (i.e., triene 1). It is also a minor product from the sensitized irradiation of 1. The mechanism of the reaction 1 → 2 was investigated using deuterium-labelled triene (i.e., 1a/b), which was prepared in a two-step procedure from a known triene mixture (1a) containing deuterium at both C-4 (43%) and C-5 (97%): the thermolysis of 1a led to labelled COT 2a, which upon direct irradiation (λ ≥ 310 nm) gave the deuterated triene mixture 1a/b in good chemical yield (61%) but in low quantum efficiency (Φ ≈ 0.0001). The COT produced from the direct irradiation (Corex filter) of 1a/b possessed different deuterium distributions than when generated thermally or from triplet sensitization (p-dimethylaminobenzophenone), as determined by 1H NMR integration. The observed deuterium-labelling patterns are in accord with a Zimmerman di-π-methane rearrangement being involved in formation of the COT from S1, of triene 1, while a mechanism proceeding by initial cleavage of the cyclobutene C1—C6 bond appears to operate during the thermal or sensitized (T1) generations of 2. Key words: mechanisms, rearrangements, photochemistry, di-π-methane.

An Alternative Platform for the Assessment of Hückel and Möbius Transition States in Concerted Reactions

“…Experimentally, [27] direct photolysis of the starting tricycle 24c in Scheme 9 did, indeed, give some benzocyclooctatetraene 22. It may have formed, in part, by disrotatory opening but labelling experiments implicated diradical intermediates, hence, a concerted hypothesis is not necessary to rationalize part of the observed results.…”

“…Following a Least Motion argument, [26] a suitable conformation for cyclooctatetraene 3 to reach the transition state for the formation of 4 is pictured in Figure 23. An interesting pair of thermolyses have been reported [27] (see Scheme 9). The stark difference in conditions for these apparently closely-related thermolyses (Scheme 9) presents a challenge to rationalize.…”

“…The stark difference in conditions for these apparently closely-related thermolyses (Scheme 9) presents a challenge to rationalize. Bender [27] offers the suggestion that the thermal conversion of 24c into 22 must proceed through a non-aromatic intermediate (shown in brackets in Scheme 9), thus making it more difficult. Nonetheless, reaction conditions are sufficiently different that he also proposed that the higher temperature pyrolysis may proceed in a stepwise fashion, presumably through a diradical, to give benzocyclooctatetraene 22.…”