Dioxygen Activation by Group 4 tritox Alkyls (tritox = t-Bu3CO-): Insrtion and Oxygen Atom Transfer

Article abstract of DOI:10.1021/ja00236a021

Dioxygen treatment of (tritox)2MMe2 (M = Ti (1a), Zr (1b), Hf (1c)) and (tritox)TiMe3 (3) afforded (tritox)2M(OMe)2 (2a-c) and (tritox)TiMe_3-n(OMe)_n (n = 1 (4), 2, (5), 3 (6), respectively, depending on the stoichiometry.Solid 4 is dimeric, consisting of two symmetry-related square pyramidal units linked by basal μ-OMe bridges (C_2h); the Ti atoms of <(tritox)TiMe2>2(μ-OMe)2 (4₂) lie 0.71 Angstroem above the basal planes, and short Ti-O(C-t-Bu3) distance (1.752 (5) Angstroem) concomitant with the 174.4 (6)deg Ti-O-C(t-Bu3) angle indicates strong O->Ti ?-bonding.Crystal data: monoclinic, C2/m, a = 9.191 (3) Angstroem, b = 13.810 (5) Angstroem, c = 14.840 (4) Angstroem, β = 76.105 (21)deg, Z = 2, T = 25 deg C, and R = 0.073 (926 reflections where /F₀/ >/= 3?(/F₀/).Ligand exchange reactions were prevalent: 3 and 5 conproportionated to give 4; 4 and 6 yielded 5; 1b-d₆ and 1c swapped Me groups; 2b-d₆ and 2c exchanged methoxides; and 1b and 2c equilibrated (K ca. 1) to give 1c and 2b.The O2 reactivity implicated η²-OOMe intermediates.Upon exposure to dioxygen, (tritox)2MMe(O-E-CH2CR=CHR') complexes degraded, perhaps due to fast Lewis acidic opening of formed epoxides.From treatment of (tritox)2MMe(OCR2CH=CH2) (R = Me, M = Zr (16b), Hf (16c); R2 = -(CH2)4-, M = Zr (17) with O2, the thermally sensitive epoxy alkoxides, (R = Me, M = Zr (18b), Hf (18c); R2 = -(CH2)-, M = Zr (19), were obtained, apparently via O2 insertion followed by O-atom transfer, paralleling known TBHP epoxidations.The sporadic oxygenation rates of 1a, 16b,c, and 17, combined with inhibition and initiation studies, suggest that autoxidation involving propagation by MeO2 radical is the mechanism of O2 insertion.The observation that 16b-d₃ and 17 (ca. 1:1) epoxidize to 18b, 18b-d₃, 19, and 19-d₃ (ca. 1:1:1:1) supports this contention.Cp2ZrMe2 (23) reacted with O2 to give Cp2Zr(OMe)2 (24), but Cp2ZrMe(O-E-CRR'CH=CHR'') (R = R' = Me, R'' = H (25); RR' = -(CH2)-, R'' = H (26); R = H, R' = R'' = Me (27)) complexes exhibited increased decomposition rates relative to an inert atmosphere.These results suggest that tritox engenders a more electrophilic metal center than Cp, a feature crucial to the ligand exchanges and to S_H2 substitution processes occuring during autoxidation.The similarities of these O2 activations to main group autoxidation reactions are also discussed.