Transannular Ring Closure of 10-Membered Cyclic Diynes: Model Calculations.
The transannular ring closure reaction of the 10-membered cyclic diynes cyclodeca-1,6-diyne (9a), 1,6-diazacyclodeca-3,8-diyne (9b), and 1,6-dioxacyclodeca-3,8-diyne (9c) to the corresponding bicyclo[4.4.0]deca-1,6-diene-2,7-diyl systems (11a-c) has been investigated using quantum mech. methods. To optimize the geometries of the ground states, 9a-c, the transition states, and the intermediates, 11a-c, we employed the CASSCF(8,8)/6-31G* procedure. The inclusion of dynamic correlation (CASPT2 and CASPT2[g1]) approxns. was necessary to reproduce the thermodn. parameters obtained exptl. for 9a -> 11a. Our calcns. reveal that the activation energy for the ring closures 9 -> 11 depends on the distance between the triple bonds. The alternative of the ring closure of 9a to bicyclo[5.3.0]deca-1,6-diene-2,6-diyl is predicted to occur at a higher activation energy (ca. 7 kcal/mol) than that for ring closure to 11a. A comparison with the Bergman cyclization reveals late transition states for 9a-c -> 11a-c, in which the conjugation of the developing 4p system plays no role.
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