Ab Initio Study of the Pathways and Barriers of Tricyclo[4.1.0.0 2, 7 ]heptene Isomerization Zhendong Zhao, Steven R. Davis,* and Walter E. Cleland Department of Chemistry and Biochemistry, UniVersity of Mississippi, UniVersity, Mississippi 38677, United States ReceiVed: June 25, 2010; ReVised Manuscript ReceiVed: August 23, 2010 The thermal isomerization of tricyclo[4.1.0.0 2, 7 ]heptene has been studied using computational chemistry with structures determined at the MCSCF level and energies at the MRMP2 level. Both the allowed conrotatory and forbidden disrotatory pathways have been elucidated resulting in cycloheptatriene isomers. Four reaction channels are available for the conrotatory pathway depending on which bond breaks first in the bicyclobutane moiety leading to enantiomeric pairs of (E,Z,Z)-1,3,5-cycloheptatriene and (Z,E,Z)-1,3,5-cycloheptatriene intermediates. The activation barrier is calculated to be 31.3 kcal · mol -1 for two channels and 37.5 kcal · mol -1 for the other two. The lower activation barrier leading to the (E,Z,Z)-1,3,5-cycloheptatriene enantiomeric pair is proposed to be due to resonance within the transition state. The same behavior was observed for the disrotatory pathway with activation barriers of 42.0 kcal · mol -1 and 55.1 kcal · mol -1 for the two channels, again with one transition state resonance stabilized. The barriers for trans double bond rotation of the intermediate cycloheptatrienes are determined to be 17.1 and 17.4 kcal · mol -1 , about 5 kcal · mol -1 more than that for the seven carbon diene (E,Z)-1,3-cycloheptadiene. The electrocyclic ring closure of the trans cycloheptatrienes have been modeled and barriers determined to be 11.1 and 11.9 kcal · mol -1 for the formation of bicyclo[3.2.0]hepta-2,6-diene. This structure was previously reported as the end product for thermolysis of the parent tricyclo[4.1.0.0 2, 7 ]heptene. The thermodynamically more stable cycloheptatriene can be formed from bicyclo[3.2.0]hepta-2,6-diene through a two step process with a calculated pseudo first-order barrier of 36.4 kcal · mol -1 . The trans-cycloheptatrienes reported herein are the first characterization of a small seven- membered ring triene with a trans double bond. Introduction The bicyclobutane system has been of interest because of its highly strained nature and its various isomerization pathways. 1-4 When the two methylene carbons in the bicyclobutane moiety are joined by a carbon chain, a tricyclo structure is formed (see Figure 1).The resulting six (TC6) and seven carbon (TC7) tricyclic compounds have received attention because their thermal isomerization pathways have been postulated to proceed through an (E,Z)-1,3-cyclodiene intermediate containing a trans double bond. 5-11 It is this trans double bond in a small six- or seven-membered ring that adds a significant amount of strain to the cyclic diene structure but has not been detected experimentally. 12-14 Once the ring size enlarges to eight or more carbons, the presence of a trans double bond in the 1,3- cyclodiene does not destabilize the ring as much and (E,Z)- 1,3-cyclooctadiene can be isolated at room temperature. 15,16 Christl and Bru ¨ntrup 6 first reported the synthesis of tricyclo[3.1.0.0 2, 6 ]hexane (TC6) and found the thermal isomer- ization produced only 1,3-cyclohexadiene as the product. The thermolysis pathway was conjectured to proceed through the (E,Z)-1,3-cyclohexadiene intermediate in a Woodward-Hoffmann allowed process. 6,7,17 The activation barrier of the thermolysis reaction was determined to be 43 kcal · mol -1 in solution. We reported 10 a calculated barrier of 42 kcal · mol -1 at the MRMP2 level; the opening of the bicyclobutane moiety was modeled after that reported by Gordon, et al. for the four-carbon bicyclobutane. 4 One of the main differences between the six and seven carbon analogues, TC6 and TC7, was that there were two thermolysis products in the seven carbon structure: bicyclo[3.2.0]heptane and 1,3-cycloheptadiene. The synthesis of tricyclo[4.1.0.0 2, 7 ]- heptene (TCE7) was also first reported by Christl 18 who also studied its thermal decomposition. A summary of the pathways that were postulated is shown in Figure 2. In C 2 Cl 4 solution, the thermolysis products were found to be bicyclo[3.2.0]hepta- 2,6-diene (BCD7) and cycloheptatriene (ZZZCT7) in a 90 to 10% ratio, respectively. In tetramethylethylenediamine, the sole product was BCD7, however. The reaction was assumed to proceed through an asynchronous pathway forming the proposed intermediates (E,Z,Z)-1,3,5-cycloheptatriene (EZZCT7) or (Z,E,Z)- 1,3,5-cycloheptatriene (ZEZCT7) that could then rearrange to form the products. At a temperature of 135 °C, the half-life of the TCE7 reactant was one hour. EZZCT7 was proposed to be the most likely intermediate based on deuterium labels as the positions of the D atoms in the final product depend on the pathway. The activation barrier for thermolysis of TCE7 was measured to be 32.4 kcal · mol -1 in solution, somewhat lower than the TC7 analog with a barrier of 37.4 kcal · mol -1 . The difference between TCE7 and TC7 is the presence of a double bond in the three carbon connecting chain. We were interested in how this double bond might effect the reaction pathway since the intermediates would require a conjugated triene with a trans double bond in the seven-membered ring. For TC6 thermal isomerization, the (E,Z)-1,3-cyclohexadiene * To whom correspondence should be addressed: E-mail: davis@ chemistry.olemiss.edu. Figure 1. Structure of TC6 and TC7. J. Phys. Chem. A 2010, 114, 11798–11806 11798 10.1021/jp105886c  2010 American Chemical Society Published on Web 09/30/2010