Equilibrium Structure of cis-Hex-3-ene-1,5-diyne and Relevance to the Bergman Cyclization

Abstract

An accurate gas-phase structure determination of cis-hex-3-ene-1,5-diyne (1), the parent molecule for the Bergman cyclization reaction, has been achieved through a combination of Fourier transform microwave spectroscopy and computational quantum chemistry. The microwave spectra of seven isotopes of enediyne 1 have been observed using a pulsed-nozzle Fourier transform spectrometer. The ground-state rotational constants were corrected for vibrational effects and the equilibrium structure (R e ) was deduced. The equilibrium structure displays excellent agreement with that obtained from high-level ab initio calculations (CCSD(T)/cc-pVTZ). Both approaches confirm that the molecule has C 2V symmetry with slight deviations of the alkyne units (C-CtC-H) from linearity. These groups distort away from each other in contra-distinction to their distortion during the Bergman cyclization reaction. The C1-C6 distance (4.32 Å), which has been viewed as a critical parameter governing reactivity in the Bergman cyclization, is notably longer than an earlier, widely publicized value (4.12 Å). An important role of the macrocyclic ring in enediyne antibiotics is to overcome an intrinsic distortion of the enediyne moiety in the direction opposite that of the reaction coordinate for Bergman cyclization.

The bergman cyclizations of the enediyne and its N‐substituted analogs using multiconfigurational second‐order perturbation theory

“…1), identical to the previously predicted geometries. [7,13,18,19,[21][22][23][24][25][26][27] The CASPT2 optimized geometry of 1R s is in agreement with experiment [21] and the discrepancies for the bond lengths and angles are within 0.009 Å and 0.3 , respectively (see Fig. 1).…”

“…As shown in Table 1, calculations using the different methods predicted very different geometric parameters for 1R. For example, the C 1 AC 6 distance, a matter of significant interest in theoretical studies in the past, [21,25,27,31,32] ranges from 4.365 to 4.548 Å , all much larger than the experimental value of 4.321 Å . The DH = (DH) values of reaction 1, calculated using the different methods, were also very different, ranging from 22.0 (À0.7) to 41.9 (25.9) kcal/mol.…”

“…The activation enthalpy (DH = ) and reaction enthalpy (DH) of reaction 1 were experimentally determined to be 28.2 and 8.5 kcal/mol, 28 and 8 kcal/mol, and 32 and 14 kcal/mol by Roth et al, [29] Wenthold and Squires, [30] and Bergman and Jones, [11] respectively. The thermal Bergman cyclization of 1R was also investigated using various theoretical methods such as CCSD(T), MBPT, CASSCF, B3LYP, BLYP, and BPW91 by several groups, [7,13,18,19,[21][22][23][24][25][26][27] and the results are summarized in Table 1. As shown in Table 1, calculations using the different methods predicted very different geometric parameters for 1R.…”

Energetic and spectroscopic properties of the low-lying C7H2 isomers: a high-level ab initio perspective

“…On the contrary, the a-type transitions appear to be less sensitive to the angle of the chains with respect to the ring and therefore they can be predicted with greater accuracy than the b-type transitions. 35 Moreover, we also speculate that 3 can undergo Bergman cyclization 106 and would become 4.…”

Photochemistry of Diethynyl Sulfides: A Cycloaromatization for the Formation of Five-Membered Rings

“…7 In the simplest enediyne system, hex-3-ene-1,5-diyne (1), the a-b distance is 2.93 Å, while the c-d distance is 4.32 Å as determined by microwave spectroscopy. 8 The first microwave and crystal structures of free diethynyl sulfides were recently obtained, revealing the carbon-carbon distances critical for bond formation to obtain the 2,5-didehydrothiophene intermediate (Figure 1). 9 The a-b distances in diethynyl sulfides are shorter than those in enediynes, and the c-d distances are slightly longer.…”