The enantiodifferentiating photosensitized cyclodimerization of cyclohexa-1,3-diene 1 was performed over a range of temperatures in the presence of chiral arene(poly)carboxylates, giving endo-and exo-[4 ϩ 2] cyclodimers (2a, 2b) and anti-and syn-[2 ϩ 2] cyclodimers (3a, 3b). Among the three chiral cyclodimers (2a, 2b, 3a), only 2b was obtained as an optically active species with an enantiomeric excess (ee) of up to 8.2%. The detailed reaction mechanism and the origin of the enantiodifferentiation have been elucidated, and the crucial role played by the 'microenvironmental polarity' around the chromophore in determining the photoreactivity and the ee of the product is also discussed.

Recently, much attention has been paid to transition metal complexes of polyhapto allenyl/propargyl ligands because of their unique bonding mode and reactivity. Typical examples include mononuclear 7/3-allenyl/propargyl complexes1 [type A {rj3, 4e)2] and dinuclear //-allenyl/propargyl complexes [type B {rj2, 4e)3 or type C {rj3, 6e)4 ] (Chart 1). Geometrically, the inherently linear unsaturated hydrocarbon framework might match a linear dinuclear moiety better than a mononuclear moiety, but only one such example has been reported in Ru2 and Os2 systems (B').5 On the other hand, there has been no example of a type D complex having such a more delocalized structure through resonance of j/3-allenyl and 7/3-propargyl forms, as is found in type A complex. We report here the synthesis, structure, and reactivity of type D complexes of Pd.

The mu-eta(3)-allenyl/propargyldipalladium complexes were synthesized by the reaction of the corresponding eta(1)-allenyl- or eta(1)-propargylpalladium complexes with Pd(2)(dba)(3). The X-ray diffraction analysis indicates that the dinuclear complex has a unique structure, in which two palladium, three carbon, two phosphorus, and one halogen atoms are in the same plane. These dinuclear complexes react with electrophiles, such as HCl or AcCl, at the central carbon of the mu-eta(3)-allenyl/propargyl ligand to give the mu-eta(3)-vinylcarbenedipalladium complexes. Intramolecular reaction proceeded smoothly to give cyclization products quantitatively. Addition of a catalytic amount of a palladium(0) complex dramatically accelerated the carbon-carbon bond formation. The MO calculations on the mu-eta(3)-allenyl/propargyl complexes indicated that the reaction proceeds via orbital control.