Coordination of Lewis acid to η2-enonepalladium(0) leading to continuous structure variation from η2-olefin type to η3-allyl type

The reaction of alpha,beta -unsaturated carbonyl compounds, a palladium(0) complex, and Lewis acids led to the formation of a new class of complexes showing a wide variety of structures with eta (2)-type and eta (2)-type coordination of the carbonyl compounds. The reaction of Pd(PhCH=CHCOCH3)(PPh3)(2) with BF3. OEt2 or B(C6F5)(3) quantitatively gave palladium complexes 1a,b having BX3-coordinated eta (2)-enonepalladium structure, as revealed by X-ray structure analysis of the B(C6F5)(3) adduct 1b. On the other hand, the reaction of Pd-(PhCH=CHCHO)(PPh3)2 with BF3 . OEt2 or B(C6F5)(3) gave distorted zwitterionic eta (3)-allylpalladium complexes 3a,b, where the Pd-carbonyl carbon distance in 3a (2.413(4) Angstrom) is much shorter than that (2.96(1) Angstrom) in 1b. The values of the P-P coupling constant and C-13 chemical shift for carbonyl carbon are useful criteria for predicting how the eta (3)-coordination mode contributes to the structure of the enone-palladium-Lewis acid system. Molecular orbital calculations an the series of model complexes suggest that orbital overlap in the highest occupied molecular orbital between the palladium and carbonyl carbon is enlarged by coordination of the Lewis acid to the carbonyl group. Palladium-catalyzed conjugate addition of R-M (R-M = AlMe3, AlEt3, ZnEt2) and its plausible reaction path are also reported.