Ozonation of 1,1,2,2-Tetraphenylethene Revisited: Evidence for Electron-Transfer Oxygenations

Abstract

Ozonolyses of 1,1,2,2-tetraphenylethene (TPE, 1) have been described many times in the literature, but the reports are contradictory. This reaction is particularly important for understanding the mechanism of alkene ozonolysis, in view of possible stabilization of reactive intermediates by aryl groups. Thus, systematic investigations of ozonolysis in both aprotic solvents and in protic solvents are reported here. Attention is directed to the following details that have been underestimated in the past: i) the actual electronic structure of ground-state ozone (O3), ii) differentiation between strained and unstrained alkenes, iii) the significance of both the O3 concentration and the TPE concentration, iv) the influence of various solvents, including pyridine, v) the influence of the reaction temperature, vi) the role of electron-transfer catalysis (ETC) and, vii) the effect of structural modifications. Our results suggest that ozonolysis of TPE (1) does not include a 1,3-dipolar reaction step, but represents a particularly interesting example of electron-donor (TPE)/electron-acceptor (O3) redox chemistry. The present investigations include several crucial results. First, pure 3,3,6,6-tetraphenyltetroxane (3, m.p. 221° (dec.)) and pure tetraphenylethylene ozonide (4, m.p. 153° (dec.)) are prepared for the first time, although 3 and 4 have long been known. Second, the singlet diradical character of O3, lessened by means of hypervalent-electron interaction and predicted by different calculations, is evidenced via reaction with the spin-trap galvinoxyl (2,6-bis(1,1-dimethylethyl)-4-{[3,5-bis(1,1-dimethylethyl)-4-oxocyclohexa-2,5-dien-1-ylidene]methyl}phenoxy; 8), and the zwitterionic reaction behavior of ground-state O3 is ruled out. Third, the electron-acceptor ability of O3 is evidenced by reactions with suitable tetraaryl ethylenes: it is enhanced by addition of catalytic amounts of protons or Lewis acids. Fourth, the observed distribution of the O3 O-atoms to the two different olefinic C-atoms of the unsymmetric alkene 27b is in full agreement with an initial single-electron transfer (SET) step, followed by a radical mono-oxygenation to cause the crucial C,C cleavage. Final dioxygenation should lead to the generally known products (ozonides, tetroxanes, hydroperoxides). The regioselectivity is found to be inconsistent with the expected decay of an intermediate primary ozonide. Finally, the treatment of 1,2-bis(4-methoxyphenyl)acenaphthylene (36) with O3 (simultaneous transfer of three O-atoms) leads to the same experimental result as a stepwise transfer of one O-atom followed by a transfer of two O-atoms.