Structurally Well Characterized Binaphtholate Titanium Chloride Lewis Acids: Evidence for Active Dinuclear Catalysts in a Diels‒Alder Process

Well characterized Ti₂(diolate)₂Cl₄ (diolate = 3,3′-dimethyl-1,1′-bi-2-naphtholate (Me₂BINO),10,10′-bi-9-phenanthrolate (BIPHENO), or 5,5′,6,6′,7,7′8,8′-octahydrobinaphtholate (H₈BINO)) complexes have been isolated as red-orange crystalline solids from diethyl ether, toluene, or methylene chloride, respectively. An X-ray crystal structure was performed on the Ti₂(Me₂BINO)₂Cl₄ complex, which was found to be a dimer supported by unidentate bridging naphtholate units. Solution molecular weight studies confirmed that the complexes remain dimeric in solution. Homochiral (R*,R*)-Ti₂(Me₂BINO)₂Cl₄ and (R*,R*)-Ti₂(H₈BINO)₂Cl₄ are the only spectroscopically observable products from syntheses beginning with either optically pure or racemic ligands. The Me₂BINO and BIPHENO Ti₂(diolate)₂Cl₄ complexes establish equilibria with Ti₂(diolate)Cl₆ in the presence of added TiCl₄, although the formation of these monodiolate complexes is less thermodynamically favorable than equilibria leading to related Ti₂(diolate)(O-i-Pr)₆ species. Ti₂(Me₂BINO)₂Cl₄ and Ti₂(H₈BINO)₂Cl₄ catalyze a Diels‒Alder reaction between cyclopentadiene and methyl acrylate. The reactions are rigorously first order in titanium, diene, and dienophile, indicating that the rate determining step for the reaction is the cycloaddition process (Ti₂(Me₂BINO)₂Cl₄: ΔH^‡ = 7.1 ± 0.5 kcal/mol, ΔS^‡= −58 ± 2 eu). Spectroscopic studies show that the binding constant for the dienophile to the titanium catalysts is very small. The catalyst shows no element of non first order rate dependence, demonstrating that the titanium dimer does not homolytically cleave into mononuclear titanium active sites. The percent enantiomeric excess (ee) of the product prepared with (R,R)-Ti₂(Me₂BINO)₂Cl₄ is dependent on the initial dienophile to titanium ratio, suggesting that the catalyst has two similar specially proximal active sites. This observation favors a dinuclear complex as the catalytically active species. Structural data for Ti₂(Me₂BINO)₂Cl₄ at 294 K: a = 15.307(3) Å, b = 12.007(1) Å, c = 21.948(2) Å, β = 99.13(1)°, V = 3983(1) ų, Z = 4, D_calc = 1.438 g cm⁻³, space group P2₁/n (No. 14), R(F) = 0.036, R_w(F) = 0.049 for 4159 unique intensities (I > 3.00σ(I)).