Organocuprate Conjugate Addition: The Structural Features of Diastereomeric and Supramolecular p-Intermediates

In the reaction pathway of conjugate additions with organocuprate reagents, Cu(I) π-complexes and Cu(III) σ-complexes have been identified as central, NMR-detectable intermediate species. However, no experimental evidence for the structures of π-intermediates with extensive chiral enones or the principal aggregation level and aggregate structure of π-complexes in diethyl ether has been available so far. Furthermore, the structural characteristics of π-complexes which are essential for their high reactivities and diastereoselectivities have not yet been rationalized experimentally. Therefore, the π-intermediates of 4,4a,5,6,7,8-hexahydro-4a-methyl-naphthalen-2(3H)-one and Me2CuLi or Me2CuLi·LiX (X = I, CN) in diethyl ether are investigated in detail. For the first time, the formation of two intermediate cuprate enone π-complexes on both sides of the double bond is observed. In addition, the conformation of the enone adopted in the major β-face π-complex rationalizes the exclusive syn addition observed in the synthetic product. For the investigation of the aggregation level and structure, a NMR screening of π-complexes with Me2CuLi·LiX (X = I, CN) and three achiral enones is performed, which simplifies the spectra by the generation of enantiotopic π-complexes. Thus, NMR diffusion experiments on cuprate intermediates and the detection of scalar couplings across copper without isotope labeling are possible for the first time. Extensive NMR studies, including those of cyclohexanone complexes, show that, in principle, salt-free dimethylcuprate is able to complex the carbonyl group. However, in the presence of salt, the carbonyl-complexing aggregates are composed of salt and cuprate moieties. These mixed aggregates cause the formation of large supramolecular π-intermediate structures which control their reactivity. The π-complexing cuprate units show a bent geometry as a general structural feature that is unaffected by the presence or kind of salt and the type of enone. Thus, the high diastereoselectivity and the reactivity of organocuprate 1,4-addition reactions are for the first time rationalized on the basis of structural characteristics of selected π-intermediates