Theoretical studies of hetero-diels-alder reactions involving N-sulfinyl dienophiles
The gas-phase hetero-Diels-Alder reactions between butadiene and X-substituted sulfinyl dienophiles, O(-)-S(+)=N-X, are investigated theoretically at the B3LYP/6-31G level. The Z-forms of the dienophiles are found to be more stable (by 5-7 kcal mol(-)(1)) than the E-forms. Four modes of cycloadducts are considered: Z-endo; Z-exo; E(X)(-)(endo)(); E(X)(-)(exo)(). Five factors are responsible for the decreasing energetic preferences of the adducts in the order E(X)(-)(endo)() > E(X)(-)(exo)() > Z-endo > Z-exo: (i) The sigma-sigma proximate charge-transfer interactions in the TS; (ii) the relative sizes of the LUMO AO coefficients on S and N atoms; (iii) steric hindrance in the TS; (iv) the levels of the ground state and the LUMOs of the dienophile; (v) bond energies of the C-S and C-N bonds that are formed in the TS. All the reactions proceed concertedly, but the adduct formation is asynchronous. The endo-additions are favored over the exo-additions kinetically (lower DeltaG()) as well as thermodynamically (lower DeltaG degrees ). The major secondary orbital interaction determining the endo preference is that between the lone pair on N (n(N)) and the d(3) (C(3)-C(4)) sigma orbital (n(N)-sigma(d3)) interactions, whereas the larger AO lobe (LUMO) sizes on S favor a greater degree of d(5) (C-S) bond formation than d(6) (C-N) bond. The solvent, C(6)H(6), uniformly lowers the activation barriers so that the energetic preferences in the gas phase between various modes are maintained in solution.