STRUCTURALLY WELL CHARACTERIZED BINAPHTHOLATE TITANIUM CHLORIDE LEWIS-ACIDS - EVIDENCE FOR ACTIVE DINUCLEAR CATALYSTS IN A DIELS-ALDER PROCESS

Well characterize Ti2(diolate)2Cl4 (diolate=3,3'-dimethy-1,1'-bi-2-nap htholate (Me2BINO), 10,10'-bi-9-phenanthrolate (BIPHENO), or 5,5',6,6' ,7,7',8,8'-octahydrobinaphtholate (H8BINO)) complexes have been isolat ed as red-orange crystalline solids from diethyl ether, toluene, or me thylene chloride, respectively. An X-ray crystal structure was perform ed on the Ti2(Me2BINO)2Cl4 complex, which was found to be a dimer supp orted by unidentate bridging naphtholate units. Solution molecular wei ght studies confirmed that the complexes remain dimeric in solution. H omochiral (R,R*)-Ti2(Me2BINO)2Cl4 and (R*,R*)-Ti2(H8BINO)2Cl4 are the only spectroscopically observable products from syntheses beginning w ith 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 monodi olate complexes is less thermodynamically favorable than equilibria le ading to related Ti2(diolate)(0-i-Pr)6 species. Ti2(Me2BINO)2Cl4 and T i2(H8BINO)2Cl4 catalyze a Diels-Alder reaction between cyclopentadiene and methyl acrylate. The reactions are rigorously first order in tita nium, diene, and dienophile, indicating that the rate determining step for the reaction is the cycloaddition process (Ti2-(Me2BINO)2Cl4: DEL TAH(double dagger) = 7.1 +/- 0.5 kcal/mol, DELTAS(double dagger) = -58 +/- 2 eu). Spectroscopic studies show that the binding constant for t he dienophile to the titanium catalysts is very small. The catalyst sh ows no element of non first order rate dependence, demonstrating that the titanium dimer does not homolytically cleave into mononuclear tita nium active sites. The percent enantiomeric excess (ee) of the product prepared with (RR)-Ti2(Me2BINO)2Cl4 is dependent on the initial dieno phile to titanium ratio, suggesting that the catalyst has two similar spacially proximal active sites. This observation favors a dinuclear c omplex as the catalytically active species. Structural data for Ti2(Me 2BINO)2Cl4 at 294 K: a = 15.307(3) angstrom, b = 12.007(1) angstrom, c = 21.948(2) angstrom, beta = 99.13(1)-degrees, V = 3983(1) angstrom3, Z = 4, D(calc) = 1.438 g cm-3, space group P2(1)/n (No. 14), R(F) = 0 .036, R(w)(F) = 0.049 for 4159 unique intensities (I > 3.00sigma(I)).