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A stable, chiral diaquo titanocene complex is an excellent catalyst for certain Diels-Alder reactions giving good enantioselectivity which indicates that catalysis occurs b y metal rather than by proton activation of the dienophile.The development of structurally defined, air stable and water tolerant transition metal based Lewis acid catalysts is likely to provide a convenient and rational approach to understanding stereochemical transformations promoted by these systems. 1 Such species obviate many of the inherent inconveniences and structural complexities of classical Lewis acids which include water sensitivity and a propensity to oligomerization and to ligand redistribution. We recently reported that the complexes, [R~(salen)(NO)(H~O)lSbF6~ and [ T ~( T ~-C ~M ~~) ~-(H20)2] (OTOz3 (salen = N,N'-bis(salicy1idene)ethylenediamine, OTf = CF3S03-), were excellent catalysts for certain Diels-Alder reactions. These structurally defined Lewis acids operate at low catalyst loadings (1-2 mol%) and are stable to and function in the presence of water. It was demonstrated for the ruthenium complex that the aquo ligand is readily displaced by the dienophile and similar metal coordination was assumed for the titanium complex. This metal binding is believed to serve in activating the dienophile to reaction. Although argum e n t ~* , ~ were presented which supported the exclusion of proton catalysis, it seems necessary to establish the existence of the metal-based path particularly since these titanium aquo complexes are known to be a ~i d i c . ~ Perhaps the most unambiguous means of resolving this issue is to demonstrate that aquo complexes of this type can produce high enantioselectivity for the Diels-Alder reaction. Our recent synthesis of the complex, [Ti ( (S)-biphenacene} (C1),I5 1 provides such a possibility.Reaction of 1 in dichloromethane solution with 2 equiv. of AgOTf leads to the formation of the air-stable, green diaquo complex, [Ti{ (S)-biphenacene} (H20)2](OTf)2 which was characterized by 1H and 19F NMR spectroscopy in dichloromethane solution and by elemental analysis. In dichloromethane solutions the NMR spectrum is complex and varies with temperature. It indicates the existence of rapid equilibration between the diaquo, aquo-triflato and bis-triflato complexes with the first of these species predominating at -78 "C. Thus at 20 "C a broad 19F NMR signal of OTf ion is observed which at temperatures of and below -10 "C becomes sharp (6 -78.6). The chemical shift of this 19F NMR resonance is the same as that found for Bun4NOTf in dichloromethane.When dissolved in dichloromethane the [Ti { (S)-biphenacene } (H20)2](OTf)2 complex catalyses Diels-Alder reactions between @unsaturated aldehydes and ketones with a variety of cyclic and acyclic dienes. These reactions occur rapidly and cleanly at 25 "C with 1 mol% of catalyst. The rate of catalysis is retarded when steric hindrance is present in the diene and dienophile and no catalysis occurs with dienophile esters even with intrinsically reactive dienes.The enantios...