Ozonation of 1,1,2,2-tetraphenylethene revisited: Evidence for electron-transfer oxygenations

Ozonolyses of 1,1,2,2-tetraphenplethene (TPE,1) have been described many ti mes 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 group s. Thus, systematic investigations of ozonolysis in both aprotic solvents a nd in protic solvents are reported here. Attention is directed to the follo wing details that have been underestimated in the past: i) the actual elect ronic structure of ground-state ozone (O-3), ii) differentiation between st rained and unstrained alkenes, iii) the significance of both the O-3 concen tration and the TPE concentration, iv) the influence of various solvents, i ncluding pyridine, v) the influence of the reaction temperature, oil the ro le of electron-transfer catalysis (ETC) and, vii) the effect of structural modifications. Our results suggest that ozonolysis of TPE (1) does not incl ude a 1,3-dipolar reaction step, but represents a particularly interesting example of electron-donor (TPE)/electron-acceptor (O-3) redox chemistry. Th e present investigations include several crucial results. First, pure 3,3,6 ,6-tetraphenyltetroxane (3, m.p. 221 degrees (dec.)) and pure tetraphenylet hylene ozonide (4, m.p. 153 degrees (dec.)) are prepared for the first time , although 3 and 4 have long been known. Second, the singlet diradical char acter of O-3, lessened by means of hypervalent-electron interaction and pre dicted by different calculations, is evidenced via reaction with the spin-t rap 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 r eaction behavior of ground-state O-3 is ruled out. Third, the electron-acce ptor ability of O-3 is evidenced by reactions with suitable tetraaryl ethyl enes: it is enhanced by addition of catalytic amounts of protons or Lewis a cids. Fourth, the observed distribution of the O-3 O-atoms to the two diffe rent olefinic C-atoms of the unsymmetric alkene 27b is in full agreement wi th an initial single-electron transfer (SET) step, followed by a radical mo no-oxygenation to cause the crucial C,C cleavage. Final dioxygenation shoul d lead to the generally known products (ozonides, tetroxanes, hydroperoxide s). The regioselectivity is found to he inconsistent with the expected deca y of an intermediate primary ozonide. Finally, the treatment of 1,2-bis(4-m ethoxyphenyl)acenaphthylene (36) with O-3 (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.