Site selectivity and reversibility in the reactions of titanium hydrazides with Si-H, Si-X, C-X and H+ reagents: Ti = N-alpha 1,2-silane addition, N-beta alkylation, N-alpha protonation and sigma-bond metathesis

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((NPr)-Pr-i)(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 Ti=N-alpha with RSiH3 (experimental Delta H 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 beta-NMe2 coordination to titanium. The Ti-D bond in Cp* Ti{MeC(NiPr)(2)}(D){N(NMe2)SiD2Ph} underwent sigma-bond metathesis with BuSiH3 and H-2. Compound 1 reacted with RR'SiCl2 at N-alpha 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 Ti N-alpha 1,2-addition or N-alpha protonation products with PhSiH2X or [Et3NH][BPh4]. Compound 1 underwent exclusive alkylation at N-beta with RCH2X (R = H, Me or Ph; X = Br or I) whereas protonation using [Et3NH][BPh4] occurred at N-alpha. DFT studies found that in all cases electrophile addition to Na (with or without NMe2 chelation) was thermodynamically favoured compared to addition to N-beta.