Density Functional Study of Bergman Cyclization of Enediynes

Abstract

Bergman cyclization of the enediynes (Z)-3-hexene-1,5-diyne (1), (Z)-3-heptene-1,5-diyne (2), (Z)-4-octene-2,6-diyne (3), (Z)-1-cyclononene-3,8-diyne (4), (Z)-1-cyclodecene-3,9-diyne (5), and (Z)-1-cycloundecene-3,10-diyne (6) has been studied by density functional methods. The reaction of 1 was first studied using the BP86, BLYP, BPW91, and B3LYP functionals with the 6-311G** basis set and the large ANO basis set for the latter two functionals. The BPW91/6-311G** calculations yielded results comparing well with those of high-level ab initio computations. Thus, BPW91/6-311G** was employed to study the reactions of 2−6. Geometry optimizations and harmonic frequency calculations were applied for every reactant, transition structure, and product; frequency calculations were also carried out for other optimized stationary points. The optimized structure of the conformer of 6 with the lowest energy agrees excellently with the X-ray diffraction crystallographic structure. IRC (intrinsic reaction coordinate) calculations were carried out for the transition structures of 4, 5, and 6 to establish the reaction path. The zero-point energy corrected reaction barriers for 1−6 are 25.16, 27.93, 32.25, 12.09, 20.87, and 26.42 kcal/mol, respectively. Thermodynamic data, ΔH, ΔS, ΔG, ΔH a, ΔS a, and ΔG a, have been evaluated at several temperatures. The temperature effect on the free energy is insignificant. The critical distance, which is the distance between the two carbon atoms forming a new bond, in the transition states of all six reactions is approximately 2.0 Å. The IRC analysis shows that the reaction coordinate is close to the critical distance, and the reactant with a larger critical distance is relatively more stable and has a higher barrier. Therefore, a smaller ring, possessing a larger strain energy and a shorter critical distance, has a lower barrier.

Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals?

“…This has lead to the conclusion [17][18][19][20] that hybrid functionals are inappropriate for a description of 3 and related systems. Actually, if the basic error in the RDFT description is corrected by using BS-UDFT (Figure 1) it will turn out that hybrid functionals such as B3LYP combined with a sufficiently large basis set lead in general to a higher accuracy than GGA functionals such as BLYP [11].…”

“…Ref. [20]). In the case that the biradical character of a given molecule is not known one can avoid an erroneous use of RDFT by making an appropriate stability test [11,59] of the RDFT energy.…”

The Cyclization of Parent and Cyclic Hexa-1,3-dien-5-ynes—A Combined Theoretical and Experimental Study

“…The reason is the reduction in ring strain by forming the less-strained cyclization product. [18,21,22] For similar reasons, the barriers for small cyclic dienynes 11 should decrease and make the molecules cyclize at lower temperatures than parent dienyne 3.…”

Quantum Chemical Characterization of Singlet and Triplet Didehydroindenes

“…For the same (DFT) geometries, the DFT and CASPT2 energies relative to singlet (S-T splitting, kcal/mol) ) -1.99( 1 H hfs, G) -0.30 (6) (S-T splitting, kcal/mol) ) -1. 39( 1 H hfs, G) -9.48 (5) 3,4-DDI tend to be in rather good agreement. In the case of the triplet σ,σ-DDIs, which should have the lowest degree of multiconfigurational character, the average absolute difference in relative H 298 values is 0.5 kcal/mol (recall that all ZPVE and thermal contributions come from DFT, so the comparison is equivalent to electronic energies).…”

“…Focus has been almost exclusively on the benzynes, [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] including substituted benzynes, 27,37,[44][45][46][47][48] annelated benzynes, 8,36,[49][50][51][52] and benzynes incorporating heteroatoms in the aromatic ring. [53][54][55][56][57] Much less well studied have been aryl systems having rings of size other than six-membered.…”