Mechanistic Studies of Organocopper Chemistry by NMR Spectroscopy: Conjugate Additions to Alkynes and Alkenes
Doctoral thesis, 1997
This thesis focuses on the mechanisms of organocopper reactions, in particular the reactions with activated alkynes (ynoates and ynones). These reactions are of interest in the formation of new highly functionalized double bonds with control of the stereochemistry. The solution structures of intermediate complexes and reaction products were determined with the aid of low temperature 13C NMR spectroscopy and the use of 13C-labelled substrates. It is shown that .pi.-complexes are formed reversibly between ynoates and t-BuCu(CN)Li before carbon-carbon bond formation. The subsequent reaction products have alkenylcopper or allenolate structures. The substrate, cuprate, solvent and additive all determine which one of these reaction products that predominates in solution. For example, ynoates preferably form alkenylcopper adducts in reactions with Me2CuLi·LiI, whereas ynones rather form products with an allenic structure under the same conditions. The adducts formed in reactions with ynoates and Me2CuLi·LiI (i.e. the lithium methyl (1-carboxyalkenyl)cuprates) are mixed cuprates and also give .pi.-complexes upon treatment of an excess ynoate. The equilibration between the E- and Z-alkenylcopper adducts is proposed to proceed via the lithium allenolate. This equilibration could not be detected by 13C NMR, but its presence is supported indirectly through the observed effect of solvent, LiI and TMSCl.
In order to gain further understanding of the mechanism and the dependence on the structure of the reagent, the cuprates used in this thesis were studied by NMR. Cyanocuprates, made from 15N-labelled copper cyanide in tetrahydrofuran or diethyl ether, were investigated by 15N NMR at low temperature. The lithium dimethylcuprate in diethyl ether or dichloromethane was studied for the first time using 6Li,1H-HOESY.
Product studies on the reaction between a chiral .gamma.-alkoxy enoate and various organocopper reagents gave additional information on the reactivity of the reagents, as this substrate is easily reduced and can be used as a sensitive mechanistic tool. The monoorganocopper reagent/TMSI combination provided the conjugate addition products smoothly, whereas the lithium dimethylcuprate only gave reduction.
13C and 15N NMR spectroscopy