The synthetic potential, regio-and stereoselectivity of titaniom dialkylamide and dialkylthioamide enolates and "ate" complexes in reaction with some conjugate cat&my1 compounds are investigated Titanium enolatcs react preferentially in 1.2.position while "ate" complexes afford 1,4-regi~ntrol.
The stercochcmical behavioor of the latta follows in general the lithium and potassium precursors. Marked intluence of solvent and of electrophile geometry on 1,4-mltivity was found with the amides but not with the thioanalogs. The stereochemistry of the amide titanium "ate" complexes is wrre.latcd with a cyclic transition state. The solvent effect is explained by involvement into the transition structure. Some advantages of the titanium "ate" complexes are discussed. The wide variety of data concerning simple diastereoselectivity of the Michael reaction with alkali metal enolates allowed some use&l generalisations to be made, connecting the structural features of the reactants with the stereochemical outcome by means of a cyclic eight -membered transition state. 1 Our continued investigations on diastereoselective C-C bond formation in bitinctionalized 2,3-diphenyl-or 2-phenyl-3-methyl propanic model systems contribute to and are entirely consistent with the above generakations.2-8 Meanwhile, new types of reagents displaying some advantages over classical enolates appeared. Of special interest are organotitanium compounds. It had been discovered that titanation of the classical carbanions results in species with reduced basicity and reactivity, thus providing in general better chemo-, regio-and stereoselectivity. On the other hand the different number of ligands in enolates and "ate" complexes alters the steric environment of the titanium center and the formal charge in the titanium compounds, consequently their reactivity and stereochemical hehaviour.9 While titanium reagents are widely used in the aidol addition9, the examples of the vinylogous Michael reaction are scarce. lo-l6 In a recent study, the stereoselectivity of conjugate addition of ketone and ester enolate "ate" complexes was systematically investigated. l7 It was established that the stereochemical outcome parallels or opposes that of the parent lithium enolates depending on the donor and acceptor used. The results were not rationalised in terms of the transition state models assumed.
Having in hand our previous results with lithium enolates, it came natural to turn attention to the titanium reagents. Herein we report our investigations on the synthetic utility, regioselectivity and stereochemistry of the titanium mediated conjugate addition of phenylacetic acid N,N-dialkylamides and thioamides to cinnamic aldehyde, benzalacetone, chalcone and E-and Z-configurated methyl cinnamate ( Scheme 1 ). Both types of reagents -enolates la and "ate" complexes lb were used. An attempt has been made to connect the stereochemical data with probable transition state geometry.