Reactivity of a Titanocene Pendant Si–H Group toward Alcohols. Unexpected Formation of Siloxanes from the Reaction of Hydrosilanes and Ph₃COH Catalyzed by B(C₆F₅)₃

Abstract

The reaction of [Cp­(η5-C5H4CH2SiMe2H)­TiCl2] (1; Cp = η5-C5H5) and methanol in the presence of catalytic amounts of B­(C6F5)3 afforded a complex with a pendant silyl ether group, [Cp­(η5-C5H4CH2SiMe2OMe)­TiCl2] (2), in good yield. The analogous reaction of 1 and Ph3COH resulted in the unexpected formation of [CpTiCl2{μ-η5:η5-(C5H4)­CH2SiMe2OSiMe2CH2(C5H4)}­TiCl2Cp] (4). The formation of siloxanes from the reaction of 2 equiv of hydrosilane with Ph3COH mediated by B­(C6F5)3 has a general applicability and proceeds in two consecutive steps: (i) transfer of the hydroxyl group from the trityl moiety to the silicon atom and (ii) silylation of the silanol formed in situ with the second equivalent of hydrosilane. The different hydrosilane reactivity toward Ph3COH in comparison with other alcohols can be attributed to the easy generation of the borate salt [Ph3C]+[(C6F5)3B­(μ-OH)­B­(C6F5)3]− (5) under catalytic conditions. The intramolecular Si–H and Ti–Cl exchange in 1 is catalyzed by B­(C6F5)3 in the presence of no alcohol. This process affords presumably a transient titanocene hydrido chloride, which is either chlorinated to give [Cp­(η5-C5H4CH2SiMe2Cl)­TiCl2] (3) in CD2Cl2 or decomposes into several paramagnetic Ti­(III) species in toluene-d 8. Complex 3 was independently synthesized from 1 and Ph3CCl in a good yield.

B(C6F5)3 catalysis accelerates the hydrosilane chlorination by Ph3CCl

“…Based on the above results we propose, that the mechanism of hydrosilane chlorination (Scheme ) is very similar to the one published previously for hydrosilane hydroxylation . A reaction of B(C 6 F 5 ) 3 with Ph 3 CCl generates the key catalytic species [Ph 3 C] + [ClB(C 6 F 5 ) 3 ] − , which in turn reacts with the hydrosilane to form a corresponding silylium species [R 3 Si] + [ClB(C 6 F 5 ) 3 ] − as was previously demonstrated for an analogous reaction of hydrosilanes with [Ph 3 C] + [B(C 6 F 5 ) 4 ] − .…”

“…Similarly, the 11 B NMR spectrum showed a disappearance Based on the above results we propose, that the mechanism of hydrosilane chlorination (Scheme 3) is very similar to the one published previously for hydrosilane hydroxylation. [15] A reaction of B(C 6 F 5 ) 3 [20,21] However, the species [R 3 Si] + [ClB(C 6 F 5 ) 3 ] − is proposed to be unstable as it consists of a strongly electrophilic silylium cation in combination with nucleophilic chlorine atom. Therefore we propose its immediate decomposition into the corresponding chlorosilane and recuperation of B(C 6 F 5 ) 3 .…”

“…Recently, we published a convenient and fast hydroxyl group transfer from triphenyl methanol to hydrosilane under B(C 6 F 5 ) 3 catalysis . However, the subsequent reaction of the in situ formed silanol with hydrosilane resulted in ultimate formation of siloxanes.…”

“…[14] Recently, we published a convenient and fast hydroxyl group transfer from triphenyl methanol to hydrosilane under B(C 6 F 5 ) 3 catalysis. [15] However, the subsequent reaction of the in situ formed silanol with hydrosilane resulted in ultimate formation of siloxanes. The hydroxyborate [Ph 3 C] + [μ-OH{B(C 6 F 5 ) 3 } 2 ] − was proved as a key reaction intermediate for the hydroxyl functionality transfer.…”

A Highly Effective Route to Si−O−Si Moieties through O‐Silylation of Silanols and Polyhedral Oligomeric Silsesquioxane Silanols with Disilazanes

“…Classic methods are based on the condensation between various silicon compounds. [41][42][43][44][45][46][47][48][49][50] These protocols describe more-or less-sophisticateds olutions thatv ery often require expensive catalysts and harsh conditions. [39,40] Unfortunately, these reactions proceeda long with the evolution of various corrosiveb yproducts (e.g.,H Cl and HBr) or use toxic or sensitive materials.…”