Reductive coupling of dimethylfulvene with alkaline earth metals yields inseparable mixtures of the tetramethylethylene-bridged complex Me 4 C 2 (C 5 H 4 ) 2 M and the unbridged metallocene (i-PrCp) 2 M (M ) Mg, Ca, Sr, Ba). In contrast, the reductive coupling of phenylfulvene with activated calcium in THF gives nearly quantitative yields of a mixture of cis-and trans-diphenylethanediyl-bridged ansa-calcocenes. The more soluble cis isomer can be removed by recrystallization, yielding the C 2 -symmetric trans-Ph 2 C 2 H 2 (η 5 -C 5 H 4 ) 2 Ca-(THF) 2 (trans-2). The molecular structure of this ansa-calcocene shows the cyclopentadienyl rings to be bound in an η 5 fashion to the calcium center. This versatile reagent gives high yields of C 2 -symmetric transition metal ansa-metallocene complexes via the facile reaction of trans-2 with metal chlorides (e.g., FeCl 2 and ZrCl 4 ). The molecular structure of the corresponding ansa-zirconocene dichloride complex is described.

The reaction of 1,3-diphenyl-2-propenyl acetate (9) with dimethylmalonate to give the substitution product 10 is effectively catalyzed by Pd complexes containing chiral imine-sulfide chelate ligands derived from amino acids. The ligand of choice, (S)-N-2'-chlorobenzylidene-2-amino-3-methyl-1-thiophenylbutane (6e), prepared in only two steps from (S)-valinol, gave an ee of 94%. Because the explanation of selectivity with the majority of other nitrogen-sulfur chelate ligands in this reaction assumes the selectivity to be controlled by an electronic bias, which contradicts our results, we characterized the Pd-allyl intermediate 14 by X-ray diffraction and solution NMR. The possible mechanism of chirality transfer is discussed. The site of nucleophilic attack on the allyl ligand is not trans to the perceived better pi-acceptor ligand (sulfur), which would be analogous to chiral nitrogen-phosphorus systems. This reaction occurs trans to the imine donor, and the enantioselectivity is ultimately controlled by the subtle steric environment of the chiral imine-sulfur chelate ligand, which predisposes the allyl unit of the reaction intermediate to a preferred reaction trajectory. In light of results that emphasize the power of electronic desymmetrization for chiral recognition, these results suggest that electronically dissimilar ligands may not give rise to chiral recognition through electronic dissimilarity.

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