229024-50-4

Upstream product

• 229024-39-9 (C₅H₅)2Ti(HBO₂C₆H₃C(CH₃)3)(C₆H₅CCC₆H₅) 
• 201230-82-2 carbon monoxide 
• 52705-45-0 carbonylbis(η-cyclopentadienyl)(η-diphenylacetylene)titanium 
• 188707-78-0 5-tert-butyl-1,3,2-benzodioxaborolane 
• 501-65-5 diphenyl acetylene 

Downstream Products

• 256234-72-7 4-MeOC₆H₄CH₂CH₂BO₂C₆H₃-4-t-Bu 

Relevant academic research and scientific papers

Titanocene borane σ-complexes

The chemistry of titanocene bisborane complexes Cp2Ti(HBcat')2 (1a-g) (HBcat' = catecholborane or a substitued catecholborane) and monoborane complexes Cp2Ti(HBcat')(L) (2-4) (L = PMe3, PhSiH3, or PhCCPh) is reported. These complexes are unusual σ-complexes. The B-H bond in the catecholborane of 1 acts as a two-electron-donor ligand. The 4-tert-butyl version 1a was studied in depth and underwent ligand substitution reactions with PMe3, CO, PhSiH3, and PhCCPh. The products of the reaction of 1a with PMe3 and PhSiH3 are the novel monoborane σ-complexes Cp2Ti(HBcat')(PMe3) (2a; HBcat' = HBO2C6H3-4-t-Bu) and Cp2Ti(HBcat')(PhSiH3) (3; HBcat' = HBO2C6H3-4-t-Bu), in which the catecholborane remains a two-electron- donating ligand. Reaction with CO formed Cp2Ti(CO)2. Reaction with PhCCPh formed Cp2Ti(HBcat')-(PhCCPh) (4; HBcat' = HBO2C6H3-4-t-Bu), which was observed in solution and reductively eliminated the vinyl boronate ester (Ph)(Bcat')C=C(Ph)(H). The rates for the reactions of 1a with these substrates showed a first-order dependence on the concentration of 1a and a zero-order dependence on the concentrations of both the departing HBcat' and the incoming ligand. The substitution reaction proceeded at the same rate ((3.8 ± 0.3) x 10-4) regardless of the identity of the incoming ligand. The entropy of activation was +30 ± 5 eu. These data are consistent with a dissociative substitution mechanism for the reaction of 1a with these substrates. The ΔH(+) value of 25 ± 3 kcal mol-1 for these reactions provides an upper limit for the strength of the borane-metal interaction. Electronic effects on the reaction rate support a bonding model involving back-donation from titanium to the borane, and the unusual steric effects allow a proposal for the geometric changes that occur upon formation of the transition state.

Mechanistic Studies of Titanocene-Catalyzed Alkene and Alkyne Hydroboration: Borane Complexes as Catalytic Intermediates

The bis(borane) complex Cp2Ti(HBcat′)2 is a highly active catalyst for the hydroboration of vinylarenes. We provide a detailed mechanistic analysis of this hydroboration process and of the hydroboration of alkynes catalyzed by titanocene dicarbonyl. The hydroboration of alkynes showed a reaction rate that was first order in the concentration of alkyne, inverse second order in the concentration of carbon monoxide, and first order in the concentration of borane. These data are consistent with a mechanism in which two equilibria involving Cp2Ti(CO)(PhCCPh) generate the alkyne borane complex Cp2Ti(HBcat′)(PhCCPh), which forms the hydroboration product. The hydroboration of alkenes catalyzed by the bis(borane) complex Cp2Ti(HBcat′)2 involves the similar intermediate Cp2Ti(CO)(RCH=CCH2), containing a coordinated alkene, rather than alkyne. The catalytic process for alkene hydroboration is inverse first order in borane and first order in alkene, indicating that the reaction occurs by reversible dissociation of borane and coordination of alkene to form an alkene borane complex that undergoes elimination of alkylboronate ester. Reactions of this complex that compete with the production of alkylboronate ester include the formation of vinylboronate esters, presumably by β-hydrogen elimination of the intermediate complex formed by addition of alkene to coordinated borane. This β-hydrogen elimination pathway was suppressed by using excess catecholborane.