How the Conical Intersection Seam Controls Chemical Selectivity in the Photocycloaddition of Ethylene and Benzene

Abstract

The photocycloaddition reaction of benzene with ethylene has been studied at the CASSCF level, including the characterization of an extended conical intersection seam. We show that the regioselectivity is, in part, controlled by this extended conical intersection seam and that the shape of the conical intersection seam can be understood in terms of simple VB arguments. Further, the shape and energetics of the asynchronous segment of the conical intersection seam, suggests that 1,2 (ortho) and 1,3 (meta) will be the preferred regioselectivities with similar weight. The 1,4 (para) point on the conical intersection is higher in energy and corresponds to a local maximum on the seam. VB analysis shows that the pairs of VB structures along this asynchronous seam are the same and thus the shape will be determined mainly by steric effects. Synchronous structures on the seam are higher in energy and belong to a different branch of the seam separated by a saddle point on the seam. On S 1 we have documented three mechanistic pathways corresponding to transition states (with low barriers) between the reactants and the conical intersection seam: a mixed asynchronous/synchronous [1,2] ortho path, an asynchronous [1,3] meta path and a synchronous [1,3] meta path. 2 I IntroductionIn general, a photochemical reaction path has two branches: a branch on the excited state and a branch on the ground state. The two branches are connected at a point where the potential energy surfaces become degenerate known as a conical intersection (CI) [1][2][3][4][5][6][7] . However, a conical intersection is not an isolated point, but a collection of points, which we will refer to as a conical intersection "seam" 1,[8][9][10] . In this work we shall explore this feature in some depth for the case of the photochemical reactivity of ethylene and benzene. In the photocycloaddition of an arene and an alkene there are several possible regioselectivities (Scheme I). We will show that this regioselectivity is in part controlled by such an extended conical intersection seam.Experimental Background: Cycloadditions with alkenes are important and characteristic photochemical reactions of aromatic compounds. The prototype example of such a photocycloaddition is the reaction of benzene with ethylene [11][12][13][14][15][16][17][18][19][20][21][22] . Three cycloaddition modes (regioselectivity) can be distinguished (See Scheme 1): ortho-cycloaddition [1,2], meta-cycloaddition [1,3], and para-cycloaddition [1,4], and many applications of these reactions in organic synthesis have been described, as they afford the possibility to obtain polycyclic compounds in one step, which is important in the design of more complex molecular frameworks. Scheme 1 3The photocycloaddition reactions of arenes with alkenes have been extensively studied in order to rationalize the formation of the three possible cycloaddition products 11,16 . The meta-cycloaddition mode ([1,3] in our simplified notation) has been applied most extensively and is used as an important step in ...

In This Molecule There Must Be a Conical Intersection

“…32,88 At first sight this system would seem intractable for the VB approach. 32 For the addition of ethylene and benzene, there are three regioselectivities as shown in Figure 3.23. Accordingly, our strategy, similar to the 6 orbitals with 6 electrons case, is to analyze the computed conicalintersection structures obtained at the ab initio level using the MMVB method 2 and thus deduce the VB analysis.…”

“…19-21 Zimmerman, 22 Michl, 23 and Ruedenberg 24 were among the first to suggest that internal conversion occurring at a conical intersection was the key Figure 3.1 Schematic representation of photochemical reaction involving a conical intersection. Adapted from Serrano-Perez et al 32 In This Molecule There Must Be a Conical Intersection feature to enable understanding of certain photochemical mechanisms. The Y axis (X 1/2 ) is a compound coordinate, corresponding to a vector that lies in the plane spanned by X 1 X 2 .…”

(ππ*/πσ*) Conical Intersection Seam Experimentally Observed in the S–D Bond Dissociation Reaction of Thiophenol-d1

“…Theoretical descriptions of conical intersections have been very successful in explaining many important nonadiabatic chemical processes. These include photochemical organic reactions, 10 photoisomerization, 11,12 the radiation-protecting mechanism of DNA bases, 13,14 or photodissociation reactions. 15,16 Spectroscopic perturbations observed in photoabsorption or photoionization have given strong experimental evidence of the existence of conical intersections.…”

Effect of Initial Orientation on the Laser-Induced Cycloaddition Reaction of Benzene and Ethylene

“…Their energies (relative to benzene + ethylene in ground state) and some International Journal of Photoenergy significant geometrical parameters are presented in Table 1. The geometries of CIs, which are very similar to references [5,6], are displayed in Figure 10. Although Robb's group had calculated this system by using CASSCF, the overlaps of ethylene moiety and benzene ring in synchronous and asynchronous mode were neglected.…”

“…This low energy CI follows the formation of one C-C bond between benzene and ethylene. Subsequent studies [6] suggest that the chemical selectivity is, in part, controlled by this extended conical intersection seam and that the shape of the conical intersection seam can be understood in terms of simple VB arguments. Further, the shape and energetics of the asynchronous segment of the conical intersection seam suggest that the 1,2 (ortho) and 1,3 (meta) products are equally preferred.…”