“…Experimental analyses suggest that the Schmittel products 10 also are σ,π-biradicals just as the Myers−Saito intermediates. The DNA-cleaving ability of 10 strongly support their biradical nature. , …”
The Myers−Saito and the C2−C6 cyclization (“Schmittel” cyclization) of the parent enyne−allene
(Z)-1,2,4-heptatriene-6-yne were investigated with pure density functional theory (DFT) methods and were
compared to coupled cluster [CCSD(T)] and Brueckner doubles [BCCD(T)] high-level calculations. Both the
Becke−Lee−Yang−Parr (BLYP) and the Becke−Perdew−Wang (BPW91) DFT levels with the 6-31G(d)
basis sets are quite suitable to describe the cyclization barriers and are nearly as accurate as much more time-consuming high-level methods. As noted before for the Bergman-cyclization, the hybrid functional B3LYP
yields good geometries but is less suitable for energies due to Hartree−Fock mixing. Single-point energy
evaluations with the much larger cc-pVTZ basis set do not necessarily improve the results; some even become
worse. The computed enthalpy of formation (ΔH
f°) of the Myers−Saito product (107 ± 4 kcal mol-1) compares
relatively well to the experimental value (103 ± 3 kcal mol-1) where an upward correction within the error
bars seems indicated. Using isodesmic equations, the ΔH
f° of the Schmittel product is predicted to be 129 ±
4 kcal mol-1. Since BLYP describes the barriers of the parent enyne−allene system quite well, it was utilized
to compute the cyclizations of monocyclic enyne−allenes (ring sizes = 7−10 carbons). The Myers−Saito
cyclization of the nine-membered ring is associated with the smallest reaction barrier and the highest
exothermicity. While ring strain effects are not able to favor the Schmittel products much over the Myers−Saito products, the eight-membered ring closures should give rise to a mixture of both products. The singlet−triplet separations of all cyclization products are very small (1 kcal mol-1); the hydrogen-abstracting ability of
such cyclic systems should therefore be rather high.
“…Experimental analyses suggest that the Schmittel products 10 also are σ,π-biradicals just as the Myers−Saito intermediates. The DNA-cleaving ability of 10 strongly support their biradical nature. , …”
The Myers−Saito and the C2−C6 cyclization (“Schmittel” cyclization) of the parent enyne−allene
(Z)-1,2,4-heptatriene-6-yne were investigated with pure density functional theory (DFT) methods and were
compared to coupled cluster [CCSD(T)] and Brueckner doubles [BCCD(T)] high-level calculations. Both the
Becke−Lee−Yang−Parr (BLYP) and the Becke−Perdew−Wang (BPW91) DFT levels with the 6-31G(d)
basis sets are quite suitable to describe the cyclization barriers and are nearly as accurate as much more time-consuming high-level methods. As noted before for the Bergman-cyclization, the hybrid functional B3LYP
yields good geometries but is less suitable for energies due to Hartree−Fock mixing. Single-point energy
evaluations with the much larger cc-pVTZ basis set do not necessarily improve the results; some even become
worse. The computed enthalpy of formation (ΔH
f°) of the Myers−Saito product (107 ± 4 kcal mol-1) compares
relatively well to the experimental value (103 ± 3 kcal mol-1) where an upward correction within the error
bars seems indicated. Using isodesmic equations, the ΔH
f° of the Schmittel product is predicted to be 129 ±
4 kcal mol-1. Since BLYP describes the barriers of the parent enyne−allene system quite well, it was utilized
to compute the cyclizations of monocyclic enyne−allenes (ring sizes = 7−10 carbons). The Myers−Saito
cyclization of the nine-membered ring is associated with the smallest reaction barrier and the highest
exothermicity. While ring strain effects are not able to favor the Schmittel products much over the Myers−Saito products, the eight-membered ring closures should give rise to a mixture of both products. The singlet−triplet separations of all cyclization products are very small (1 kcal mol-1); the hydrogen-abstracting ability of
such cyclic systems should therefore be rather high.
“…In retrospect, such finding indicated that ring strain effects may also influence the regioselectivity of enyne–allene cyclizations (C 2 –C 7 versus C 2 –C 6 ). In spite of aryl or bulky substituents at the alkyne and allene termini, cyclopent‐enyne–allene 62 ( n = 1; R 1 = P(O)Ph 2 , R 2 = n Bu, Ph) underwent exclusively Myers–Saito cyclization (Scheme ) . The selectivity was rationalized by the notable ring strain differences for both regioselective variants.…”
Section: Factors Influencing the Schmittel (C2–c6) Cyclizationmentioning
The thermal C 2 -C 6 /ene cyclization of enyne-allenes has become an exciting venue for both mechanistic and synthetic studies. While at first most efforts were aimed at establishing the diradical nature of the thermal process and to derive therefrom efficient protocols for synthetic schemes, recent evidence has disclosed the C 2 -C 6 cyclization as a unique reaction heavily influenced by nonstatistical dynamic effects. Depending on the substituents at the enyne-allene, one can readily identify transitions between classical and nonstatistical behaviors on the basis of experimental data. We are grateful to him for his friendship and many scientific enlightenments.a M. Schmittel, C. Vavilala, M. E. Cinar FB 8 (
“…Up to now the influence of ring size effects on the regioselectivity has not been investigated systematically, although a few isolated examples 14 may indicate some effect. Herein, we would like to disclose 15 that the switch between the C 2 -C 7 and the C 2 -C 6 cyclisation can be initiated simply by ring size effects.…”
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