Monocyclic Enediynes:  Relationships between Ring Sizes, Alkyne Carbon Distances, Cyclization Barriers, and Hydrogen Abstraction Reactions. Singlet−Triplet Separations of Methyl-Substituted p-Benzynes

The Bergman-type cyclizations of parent, 2,3-dimethyl, and monocyclic (ring sizes = 7−12) enediynes were studied in detail at the Becke−Lee−Yang−Parr (BLYP) density functional (DFT) level with 6-31G* as well as 6-311+G** basis sets for geometry optimizations and relative energy evaluations, respectively. Pure DFT methods work reasonably well for these reactions; the errors are somewhat larger (ca. 3−7 kcal mol^-1) than for the much more time-consuming complete active space (CASPT2) and coupled-cluster [CCSD(T)] (both in error by ca. 2 kcal mol^-1) methods with high-quality basis sets. The hybrid method B3LYP is unsuitable for this type of chemistry (errors of 14−20 kcal mol^-1). The singlet−triplet energy separations (ΔE_ST) for p-benzynes are underestimated systematically by about 2 kcal mol^-1 at BLYP; the ΔE_ST of 2-methyl-p-benzyne (−3.1 kcal mol^-1) is close to that of p-benzyne (−3.8 kcal mol^-1, i.e., singlet ground state) but the 2,3-dimethyl-p-benzyne ΔE_ST is only −0.6 kcal mol^-1 due to singlet destabilization (methyl repulsion). 2,3-Dialkyl-p-benzynes thus have nearly degenerate singlet and triplet states. While we find that there is clearly no predictive relationship between the alkyne carbon distance (d) and the cyclization activation enthalpy (ΔH^⧧) for monocyclic enediynes, Nicolaou's empirically determined “critical range” of 3.31−3.2 Å, where spontaneous cyclization should occur at room temperature, should be extended to 3.4−2.9 Å. However, ring strain effects may become more important than distance arguments. Dimethyl substitution increases the endothermicity of the Bergman reaction (by about 12 kcal mol^-1). The cyclization of a nine-membered enediyne is only mildly endothermic; larger rings give larger endothermicities. The formation of final products via double hydrogen abstraction from 1,4-cyclohexadiene is highly exothermic. The exothermicity decreases with increasing ring size due to unfavorable H···H repulsion in the products.