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

Timothy J. Boyle, Nancy W. Eilerts, Joseph A. Heppert, Fusao Takusagawa

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Abstract

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) Å3, Z = 4, Dcalc = 1.438 g cm−3, space group P21/n (No. 14), R(F) = 0.036, Rw(F) = 0.049 for 4159 unique intensities (I > 3.00σ(I)).

Original languageEnglish
Pages (from-to)2218-2229
Number of pages12
JournalOrganometallics
Volume13
Issue number6
DOIs
StatePublished - Jun 1 1994

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