Carbon-oxygen and related RX bond cleavages mediated by (silox)3Ti and other Group 4 derivatives (silox = tBu3SiO)

Halogen atom abstractions by (silox)3Ti (1) from CCl4, ClRh(PPh3)3, Br2 and I2 produced (silox)3TiCl (2), (silox)3TiBr (3) and (silox)3TiI (4), respectively. Treatment of 1 with MeI afforded a 1:1 mixture of 4 and (silox)3TiMe (5), regardless of [MeI], implicating a rough I abstraction rate constant of ka<2×105M−1s−1. Exposure of 2 to NaI (THF) or MeMgBr(Et2O) provided independent syntheses of 4 and 5, respectively. Br abstraction by 1 from the radical clock H2CCH(CH2)3CH2Br yielded 3 and (silox)3TiCH2(CH2)3CHCH2 (6), according to 1H NMR spectroscopy, and trapping of 1 by hexenyl radical is roughly kt>2×107 M−1s−1. A rationalization of the formation of (silox)3TiCH2CH2Ti(silox)3 (7) from 1 and C2H4 is presented. Na/Hg reduction of (silox)2TiCl2 (9) generated [(silox)2Ti]2(μ-Cl)2 (10) (μeff = 0.75 μB/Ti at 300.6 K). Quenching of 10 with CCl4 and C6h4O2 produced 9 and [(silox)2TiCl]2-(m̊:η1,η1-p-OC6H4O (11), respectively. Upon treatment of 10 with RCCR (REt, Ph) or C2H4, disproportionation to 9 and (silox)2TiCRCRCRCR (REt (12); Ph (13)), also prepared via Na/Hg reduction of 9 in the presence of alkyne, or (silox)2TiCR2(CH2)2CH2 (14) occurred. According to 1H NMR spectroscopy, exposure of 12 to C2H4 gave 14, and 10 catalytically hydrogenated Me2CCH2. Addition of THF to 1 yielded (silox)3TiOCH2(CH2)2CH2Ti(silox)3 (17) via metallaradical ring-opening, while inclusion of ≈ 10 equiv. of HSnPh3 provided a mixture of 17 and (silox)3TiOnBu (19). Addition of PhCH2MgCl to (silox)3MCl (MTi (2); Zr (22)) and (silox)2TiCl2 (9) produced (silox)3MCH2Ph (MTi (21); Zr (23)) and (silox)2Ti(CH2Ph)2 (24), respectively, but (silox)2Zr(CH2Ph)2 (26) was synthesized from addition of (silox)H to Zr(CH2Ph)4. While 21 and 23 were photolytically inactive, photolysis of 24 in THF produced dibenzyl and [(silox)2TiOCH2(CH2)2CH2]n (27, n = 2 (tentative)), while related photolysis of 26 afforded [(silox)2ZrOCH2(CH2)2CH2]2 (2821) and dibenzyl. Mass spectral analysis on dibenzyl derived from a 26:(silox)2Zr(CD2Ph)2 (26-d4) mixture showed that benzyl scrambling occurred. (Silox)2Zr(CH2-m-tolyl)2 (36) was prepared from Zr(CH2-m-tolyl)4 and H(silox). Crossover, i.e., detection of (silox)2Zr(CH2Ph)(CH2-m-tolyl) (38), occurred when a mixture of (silox)2Zr(CH2Ph)2 (26) and (silox)2Zr(CH2-m-tolyl)2 (36) was photolyzed, showing that benzyl scrambling, presumably via PhCH2, preceded THF scission. The mechanisms of THF ring-opening by 1 and, plausibly, (silox)2ZrCH2Ph (32), are discussed.