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.
Park
YS
Department of Chemical Education, Chonnam National University, Kwangju, 500-757 Korea, and Department of Chemistry, Inha University, Inchon, 402-751 Korea.
Kim
WK
Kim
YB
Lee
I