Coordination of Lewis acid to eta(2)-enonepalladium(0) leading to continuous structure variation from eta(2)-olefin type to eta(3)-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 s howing a wide variety of structures with eta (2)-type and eta (2)-type coor dination 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 dist orted zwitterionic eta (3)-allylpalladium complexes 3a,b, where the Pd-carb onyl 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 ch emical shift for carbonyl carbon are useful criteria for predicting how the eta (3)-coordination mode contributes to the structure of the enone-pallad ium-Lewis acid system. Molecular orbital calculations an the series of mode l complexes suggest that orbital overlap in the highest occupied molecular orbital between the palladium and carbonyl carbon is enlarged by coordinati on of the Lewis acid to the carbonyl group. Palladium-catalyzed conjugate a ddition of R-M (R-M = AlMe3, AlEt3, ZnEt2) and its plausible reaction path are also reported.