Site selectivity and reversibility in the reactions of titanium hydrazides with Si–H, Si–X, C–X and H+ reagents: TiNα1,2-silane addition, Nβalkylation, Nα protonation and σ-bond metathesis

Abstract

We report a combined experimental and computational comparative study of the reactions of the homologous titanium dialkyl- and diphenylhydrazido and imido compounds Cp*Ti{MeC(NⁱPr)₂}(NNR₂) (R = Me (1) or Ph (2)) and Cp*Ti{MeC(NⁱPr)₂}(NTol) (3) with silanes, halosilanes, alkyl halides and [Et₃NH][BPh₄]. Compound 1 underwent reversible Si–H 1,2-addition to TiN_α with RSiH₃ (experimental ΔH ca. −17 kcal mol⁻¹), and irreversible addition with PhSiH₂X (X = Cl, Br). DFT found that the reaction products and certain intermediates were stabilised by β-NMe₂ coordination to titanium. The Ti–D bond in Cp*Ti{MeC(NⁱPr)₂}(D){N(NMe₂)SiD₂Ph} underwent σ-bond metathesis with BuSiH₃ and H₂. Compound 1 reacted with RR′SiCl₂ at N_α to transfer both Cl atoms to Ti; 2 underwent a similar reaction. Compound 3 did not react with RSiH₃ or alkyl halides but formed unstable TiN_α 1,2-addition or N_α protonation products with PhSiH₂X or [Et₃NH][BPh₄]. Compound 1 underwent exclusive alkylation at N_β with RCH₂X (R = H, Me or Ph; X = Br or I) whereas protonation using [Et₃NH][BPh₄] occurred at N_α. DFT studies found that in all cases electrophile addition to N_α (with or without NMe₂ chelation) was thermodynamically favoured compared to addition to N_β.