Cycloaddition ground-state forbidden

Cydization. 5rf Photocyciizalion Cycloaddition. 404-35. See also phoiocydoaddilion aromatic compounds, 416-23 carbonyl group. 424-33 crossed, 231.340. 342 enanlioseleclive. 471 formaldehyde 4 ethylene, 430 ground-state forbidden. 230. 34) mixed. 410. 419. 433 photosensitized. 470-71 rcgiochemisti y. 417-23 two ethylene molecules. 202-3. 237, 333-.36.. 3.39. 417... 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

In a photochemical cycloaddition, one component is electronically excited as a consequence of the promotion of one electron from the HOMO to the LUMO. The HOMO -LUMO of the component in the excited state interact with the HOMO-LUMO orbitals of the other component in the ground state. These interactions are bonding in [2+2] cycloadditions, giving an intermediate called exciplex, but are antibonding at one end in the [,i4j + 2j] Diels-Alder reaction (Scheme 1.17) therefore this type of cycloaddition cannot be concerted and any stereospecificity can be lost. According to the Woodward-Hoffmann rules [65], a concerted Diels-Alder reaction is thermally allowed but photochemically forbidden. 

In this way Hoffmann and Woodward have established the following simple rule a concerted m + n cycloaddition will be symmetry allowed in the ground state and symmetry forbidden in the excited state if m + n = 4q + 2 (q = 0, 1, 2...) if m + n = 4q the reaction will be symmetry allowed in the excited state and symmetry forbidden in the ground state. This rule applies to ms + ns cycloadditions and to ma + na processes. 

The forbidden retro-[ls -I- 2s]-cycloaddition can now be treated using a simple curve-crossing model analagous to the Marcus-Hush theory of electron-transfer [11]. The ground state at the quadricyclane-like geometry is the... 

The ortho cycloaddition is thermally forbidden in a suprafacial-suprafacial manner and the photochemical reaction is forbidden with S benzene and ground-state alkene. On the basis of these considerations, it could be understood that the ortho addition had only been observed with systems where the alkene is the lowest excited singlet species (as with maleimides [37,74,75] or where either the alkene or the arene has marked acceptor properties (the only examples known at that time were benzene-acrylonitrile [127] and benzonitrile + a mono-olefin [1,73], Benzene-acrylonitrile and benzonitrile-olefin systems do not display charge-transfer absorption, but charge transfer could well follow excitation. Bryce-Smith further stated that irradiation of benzene in the presence of simple mono-olefins normally provides B2u (Si) benzene as the lowest excited singlet species, which leads to meta rather than ortho addition, but the latter process might, in principle, be able to occur under conditions where a Biu (S2) state of benzene is populated. 

From the orbital correlation diagram derived by Bryce-Smith [38], it was deduced that the ortho cycloaddition is forbidden from the lowest excited singlet state of benzene and the ground state of ethene. Van der Hart et al. [189] have constructed molecular orbital and state correlation diagrams for the ortho photocycloaddition of benzene to ethene. The molecular orbital correlation diagram differs from that given by Bryce-Smith, because natural correlations have been used. From a topological point of view, it seems less desirable to correlate the tt... 

According to the Woodward-Hoffmann rules, the concerted [2S + 2J cycloaddition with two alkenes is photochemically symmetry-allowed, but is symmetry-forbidden at the ground state [9]. Photochemical [2 + 2] cycloaddition, in which one of two alkene partners is electronically excited, has been applied to the synthesis of cage hydrocarbons [10]. In such transformations, the intramolecular version of the reaction is particular efficient. The transformation of compound 1, in which two... 

The HOMO of the excited ethylene molecule has the same symmetry as the LUMO of a ground-state ethylene. An excited molecule can react with a ground-state molecule to give cyclobutane (Figure 15-22). The [2 + 2] cycloaddition is therefore photo chemically allowed but thermally forbidden. In most cases, photochemically allowed reactions are thermally forbidden, and thermally allowed reactions are photochemically forbidden. 

In contrast, the state correlation diagram for the forbidden cycloaddition (Fig. 6.20) is not so simple. The ground state of the starting materials on the left, %2%2, is overall symmetric, because both terms are squared. Following the... 

Whereas a [2 + 2] pericyclic reaction is essentially forbidden in the ground state, a [2+1] open-shell reaction is feasible. In this respect, the radical cations detected in this context represent distinct stages of pericyclic, radical-cation catalyzed cycloaddi-tions/cycloreversions. In Fig. 7.11, three distinct stages, a tight (cyclobutane-like), an extended (bis ethene), and a trapezoid, of a hole- (or radical-cation) catalyzed cycloaddition/cycloreversion are presented in a schematic way. °... 

The FMO argument for the [2+2]-cycloaddition in the ground state (i.e. under thermal conditions) is that the LUMO (rt ) of one ethene and the HOMO (tt) of another ethene are phase mismatched for the supra-supra [2s+2s]-cycloaddition (Fig. 8.26). Thus, it is symmetry forbidden under thermal conditions (two molecules in the ground state). Similarly, the antara-antara mode is symmetry forbidden. The anatra-supra or supra-anatara mode... 

Ground state [2s+2s]-cycloaddition reaction is symmetry forbidden... 

The cycloaddition is, therefore, forbidden in the ground state (no thermally activated reaction) and allowed via an excited state (photochemistry). 

A similar analysis of [ 2 -h,r 2]-cycloaddition shows the [s -f s] and [a + a] modes to be forbidden in the ground-state but allowed in the excited state, and vice versa for the [s + a] mode. The latter is illustrated in Fig. 1.7, in which the molecules come together in the off-orthogonal approach recommended by Woodward and Hoffmann [1, p. 69], and then turn and/or twist towards one another in order to increase the favorable HOMO-LUMO overlap. Then, regardless of how the orbital phases are chosen, the reaction is always allowed. 

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