1,2-versus 1,4-addition to alpha-donor-cyclopenten-1-ones; a comparison ofcalculated and experimental data

A series of alpha-donor-cyclopenten-1-ones, 1 (alpha-donor; X = H, Cl, Br, OMe, pyrrolidin-1-yl, SPh and SePh) were treated with methyllithium, lithiu m dimethylcuprate and cyanomethyllithium in tetrahydrofuran at -78 degrees C. The reactions between 1 and methyllithium gave 1,2-adducts in 28-75% yie ld. No trace of the corresponding 1,4-adduct was observed. By treatment of 1 with lithium dimethylcuprate only 1,4-addition was obtained in 51-76% yie ld, except for X = OMe or pyrrolidin-1-yl, which gave no reaction at all. I n the reactions between 1 and cyanomethyllithium the 1,2-adducts were the m ain products (43-60%), but varying amounts of 1,4-adducts (8-30%) were also formed. The products formed have been fully characterized. Net atomic char ges obtained by density functional theory (DFT) calculations of the lithiat ed cations of 1 with X = H, Cl, Br or OMe showed very small differences bet ween the substituted and unsubstituted systems for all X, except for X = OM e. In this case the C-2 position was positive rather than negative. The C-3 position was found to be significantly more negative. The calculated relat ive energies of the methylated and cyanomethylated 1,2- and 1,4-adducts wit h X = H, Cl, Br and OMe indicated that the 1,4-adducts were thermodynamical ly favoured by 8 to 12 kcal mol(-1) for all but one case. In the cyanomethy lation of 1 with X = OMe the 1,2-adduct was predicted to be favoured by 12 kcal mol(-1). Thus, the predominance of 1,2-attack in the reactions involvi ng methyllithium and cyanomethyllithium cannot be explained by the relative thermodynamic stabilities of the products.