Influence of tetrahydrofuran on reactivity, aggregation, and aggregate structure of dimethylcuprates in diethyl ether

dThe 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 center dot Lil (1 center dot Lil) and Me2CuLi center dot LiCN (1 center dot 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 center dot Lil a pronounced acceleration effect upon addition of distinct amounts of THF, whereas the reactivity of 1 center dot LiCN continuously decreases with the addition of THE Series of NMR diffusion measurements as well as H-1-Li-7 heteronuclear Overhauser effect (HOE), and H-1-H-1 nuclear Overhauser effect (NOE) spectra show different structural. influences of THF on 1 center dot Lil and 1 center dot LiCN. For 1 center dot Lil, small salt units are separated from the cuprate aggregate by THF. In contrast to this, THF disaggregates the oligomeric structures of 1 center dot LiCN, while the core structures remain intact with salt attached. Thus, the reactivity of 1 center dot Lil seems to be fine-tuned through distinct amounts of salt or THF, whereas the decreasing reactivity of 1 center dot 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 H-1-C-13 HOE- and H-1-H-1 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.