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

Abstract

Halogen atom abstractions by (silox) 3Ti ( 1) from CCl 4, ClRh(PPh 3) 3, Br 2 and I 2 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 k a<2×10 5M −1s −1. Exposure of 2 to NaI (THF) or MeMgBr(Et 2O) provided independent syntheses of 4 and 5, respectively. Br abstraction by 1 from the radical clock H 2CCH(CH 2) 3CH 2Br yielded 3 and (silox) 3TiCH 2(CH 2) 3CHCH 2 ( 6), according to 1H NMR spectroscopy, and trapping of 1 by hexenyl radical is roughly k t>2×10 7 M −1s −1. A rationalization of the formation of (silox) 3TiCH 2CH 2Ti(silox) 3 ( 7) from 1 and C 2H 4 is presented. Na/Hg reduction of (silox) 2TiCl 2 ( 9) generated [(silox) 2Ti] 2(μ-Cl) 2 ( 10) ( μ eff = 0.75 μ B/Ti at 300.6 K). Quenching of 10 with CCl 4 and C 6h 4O 2 produced 9 and [( silox) 2 TiCl] 2-( m ̊ :η 1,η 1-p- OC 6 H 4 O ( 11), respectively. Upon treatment of 10 with RCCR (REt, Ph) or C 2H 4, 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 TiCR 2(CH 2) 2CH 2 ( 14) occurred. According to 1H NMR spectroscopy, exposure of 12 to C 2H 4 gave 14, and 10 catalytically hydrogenated Me 2CCH 2. Addition of THF to 1 yielded (silox) 3TiOCH 2(CH 2) 2CH 2Ti(silox) 3 ( 17) via metallaradical ring-opening, while inclusion of ≈ 10 equiv. of HSnPh 3 provided a mixture of 17 and (silox) 3TiO nBu ( 19). Addition of PhCH 2MgCl to (silox) 3MCl (MTi ( 2); Zr ( 22)) and (silox) 2TiCl 2 ( 9) produced (silox) 3MCH 2Ph (MTi ( 21); Zr ( 23)) and (silox) 2Ti(CH 2Ph) 2 ( 24), respectively, but (silox) 2Zr(CH 2Ph) 2 ( 26) was synthesized from addition of (silox)H to Zr(CH 2Ph) 4. While 21 and 23 were photolytically inactive, photolysis of 24 in THF produced dibenzyl and [(silox) 2 TiOCH 2(CH 2) 2CH 2 ] n ( 27, n = 2 (tentative)), while related photolysis of 26 afforded [(silox) 2 ZrOCH 2(CH 2) 2C H 2] 2 ( 28 21) and dibenzyl. Mass spectral analysis on dibenzyl derived from a 26:(silox) 2Zr(CD 2Ph) 2 ( 26-d 4) mixture showed that benzyl scrambling occurred. (Silox) 2Zr(CH 2- m-tolyl) 2 ( 36) was prepared from Zr(CH 2- m-tolyl) 4 and H(silox). Crossover, i.e., detection of (silox) 2Zr(CH 2Ph)(CH 2- m-tolyl) ( 38), occurred when a mixture of (silox) 2Zr(CH 2Ph) 2 ( 26) and (silox) 2Zr(CH 2- m-tolyl) 2 ( 36) was photolyzed, showing that benzyl scrambling, presumably via PhCH 2, preceded THF scission. The mechanisms of THF ring-opening by 1 and, plausibly, (silox) 2ZrCH 2Ph ( 32), are discussed.