Influence of Tetrahydrofuran on Reactivity, Aggregation, and Aggregate Structure of Dimethylcuprates in Diethyl Ether

Abstract

The comprehension of factors influencing the reactivity of organocuprates is still far from enabling a rational control of their reactions. Especially the degree of aggregation and structures of organocuprates are the focus of discussion about the factors affecting their reactivity. Therefore, this study combines kinetic measurements and NMR investigations to elucidate the influence of disaggregation via addition of tetrahydrofuran (THF) on the reactivity and aggregate structure of Gilman cuprates. As model systems, Me2CuLi·LiI (1·LiI) and Me2CuLi·LiCN (1·LiCN) in diethyl ether (DEE) were chosen; as model reaction, the 1,4-addition to 4,4-dimethylcyclohex-2-enone. The kinetic data show for 1·LiI a pronounced acceleration effect upon addition of distinct amounts of THF, whereas the reactivity of 1·LiCN continuously decreases with the addition of THF. Series of NMR diffusion measurements as well as 1H−7Li heteronuclear Overhauser effect (HOE), and 1H−1H nuclear Overhauser effect (NOE) spectra show different structural influences of THF on 1·LiI and 1·LiCN. For 1·LiI, small salt units are separated from the cuprate aggregate by THF. In contrast to this, THF disaggregates the oligomeric structures of 1·LiCN, while the core structures remain intact with salt attached. Thus, the reactivity of 1·LiI seems to be fine-tuned through distinct amounts of salt or THF, whereas the decreasing reactivity of 1·LiCN correlates with the disaggregation of oligomers via THF. Thus, for synthetic chemists with reactivity problems in specific reactions of iododialkylcuprates, the addition of small amounts of THF might be useful to enhance the reactivity. In addition to these structure−reactivity studies, the CN- group is shown to be directly attached to the cuprate moiety via a combination of 1H−13C HOE- and 1H−1H NOEs. This represents the first direct experimental evidence in solution for the position of the CN- group relative to the cuprate moiety in cyano-Gilman cuprates.