Carbon-oxygen and related R-X bond cleavages mediated by (silox)(3)Ti and other Group 4 derivatives (silox=Bu-t(3);SiO)

Halogen atom abstractions by (silox)(3)Ti (1) from CCl4, ClRh(PPh3)(3), Br-2 and I-2 produced (silox)(3)TiCl (2), (silox)(3)TiBr (3) and (silox)(3)TiI (4), respectively. Treatment of 1 with MeI afforded a 1:1 mixture of 4 and (silox)(3)TiMe (5), regardless of [MeI], implicating a rough I abstraction rate constant of k(a) < 2 x 10(5) M-1 s(-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)(3)CH2Br yielded 3 and (silox)(3)TiCH2(CH2)(3)CH=CH2 (6), according to H-1 NMR spectroscopy, and trapping of 1 by hexenyl radical is roughly k(t) > 2 x 10(7) M-1 s(-1). A rationalization of the formation of (silox)(3)TiCH2CH2Ti(silox)(3) (7) from 1 and C2H4 is presented. Na/Hg reduction of (silox)(2)TiCl2 (9) generated[(silox)(2)Ti](2)(mu-Cl)(2) (10) (mu(eff)=0.75 mu(B)/Ti at 300.6 K). Quenching of 10 with CCl4 and C6H4O2 produced 9 and [(silox)(2)TiCl](2)-(mu:eta(1),eta(1)-p-OC6H4O) (11), respectively. Upon treatment of 10 with RC=CR (R=Et, Ph) or C2H4, disproportionation to 9 and (silox)(2)TiCR=CRCR=CR (R=Et (12); Ph (13)), also prepared via Na/Hg reduction of 9 in the presence of alkyne, or (silox)(2)TiCH2(CH2)CH2 (14) occurred. According to H-1 NMR spectroscopy, exposure of 12 to C2H4 gave 14, and 10 catalytically hydrogenated Me2C=CH2. Addition of THF to 1 yielded (silox)(3)TiOCH2(CH2)(2)CH2Ti(silox)(3) (17) via metallaradical ring-opening, while inclusion of similar to 10 equiv. of HSnPh3 provided a mixture of 17 and (silox)(3)Ti-(OBu)-Bu-II (19). Addition of PhCH2MgCl to (silox)(3)MCl (M=Ti (2); Zr (22)) and (silox)(2)TiCl2 (9) produced (silox)(3)MCH2Ph (M=Ti (21); Zr (23)) and (silox)(2)Ti(CH2Ph)(2) (24), respectively, but (silox)(2)Zr(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)(2)TiOCH2(CH2)(2)CH2](n) (27, n=2 (tentative)), while related photolysis of 26 afforded [(silox)(2)ZrOCH2(CH2)(2)CH2](2) (28(2)) and dibenzyl. Mass spectral analysis on dibenzyl derived from a 26:(silox)(2)Zr(CD2Ph)(2) (26-d(4)) mixture showed that benzyl scrambling occurred. (Silox)(2)Zr(CH2-m-tolyl)(2) (36) was prepared from Zr(CH2-m-tolyl)(4) and H(silox). Crossover, i.e., detection of (silox)(2)Zr(CH2Ph)(CH2-m-tolyl) (38), occurred when a mixture of (silox)(2)Zr(CH2Ph)(2) (26) and (silox)(2)Zr(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)(2)ZrCH2Ph (32), are discussed. (C) 1997 Elsevier Science S.A.