Steric Effects in Reactions of Decamethyltitanocene Hydride with Internal Alkynes, Conjugated Diynes, and Conjugated Dienes

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• 作者：Ji艡铆 Pinkas ; R贸bert Gyepes ; Ivana C铆sa艡ov谩 ; Ji艡铆 Kubi拧ta ; Michal Hor谩膷ek ; Karel Mach
• 刊名：Organometallics
• 出版年：2014
• 出版时间：July 14, 2014
• 年：2014
• 卷：33
• 期：13
• 页码：3399-3413
• 全文大小：635K
• ISSN：1520-6041

文摘

Titanocene hydride [Cp*₂TiH] (Cp* = 畏⁵-C₅Me₅) (1) readily inserts simple internal alkynes R¹C鈮R² into its Ti鈥揌 bond, yielding titanocene alkenyl Ti(III) compounds of two structural types. The less sterically congested products [Cp*₂Ti(R¹C鈺怌HR²)] (2a鈥?b>e) contain a 蟽¹-bonded alkenyl group, whereas the products bearing at least one trimethylsilyl substituent and other bulky substituents (R¹ = SiMe₃; R² = SiMe₃, 4a; CMe₃, 4b; and Ph, 4c) possess a remarkable Ti鈥揌 agostic bond of the 蟽¹-bonded alkenyl group. This feature is consistent with solution EPR spectra of 4a鈥?b>4c showing a doublet due to coupling of the hydrogen nucleus with the Ti(III) d¹ electron. Compound 1 reacts with one molar equivalent of conjugated buta-1,3-diynes (RC鈮)₂ to give 畏³-butenyne complexes (R = SiMe₃, 5a; CMe₃, 5b). The Ti(III) complexes 2a鈥?b>2e and 5a and 5b were oxidatively chlorinated with PbCl₂ to give Ti(IV) chloro-alkenyl complexes [Cp*₂TiCl(R¹C鈺怌HR²)] 3a鈥?b>3e and chloro-alkenynes 6a and 6b, respectively. ¹H and ¹³C NMR spectra of 3a鈥?b>3e and 6a and 6b revealed that these compounds form equilibria of two atropisomers differing by the anti- and syn-position of the chlorine and the alkenyl hydrogen atoms. Such atropisomers are denoted by appended (a) and (b), respectively. Compound 1 reacted with 1,3-butadiene to give a thermally stable 蟺-bonded 1-methylallyl complex (7) and with penta-1,3-diene to give a thermally labile 1,3-dimethylallyl complex (8). In toluene-d₈ solutions 7 dissociated at 80 掳C and 8 at room temperature to give [Cp*Ti(C₅Me₄CH₂)] and corresponding alkenes. Other methyl-substituted dienes, isoprene, 4-methylpenta-1,3-diene, and 2,3-dimethylbuta-1,3-diene, did not yield observable 蟺-bonded allyl products; the dienes were, however, hydrogenated to olefins with concomitant formation of [Cp*Ti(C₅Me₄CH₂)]. Compound 1 was shown to catalyze the hydrogenation of the alkynes and dienes to olefins and ultimately to alkanes under lower than atmospheric hydrogen pressure at room temperature. Single-crystal structures were determined for 3d(a), 3e(a), 4a鈥?b>4c, 5a, 6b, and 7.