Conrotatory, ring opening

Symmetry allowed reaction (Section 10 14) Concerted reac tion in which the orbitals involved overlap in phase at all stages of the process The conrotatory ring opening of cy clobutene to 1 3 butadiene is a symmetry allowed reaction... 

For Woodward-Hoffman allowed thermal reactions (such as the conrotatory ring opening of cyclobutane), orbital symmetry is conserved and there is no change in orbital occupancy. Even though bonds are made and broken, you can use the RHF wave function. 

As in the photolysis of protonated eucarvone, an acyclic intermediate is proposed in the mechanistic pathway. The protonated dienones 73 and 74 should be thermally stable, since a symmetry-allowed ring closure in the conrotatory mode is precluded in the cyclic system (Woodward and Hoffmann, 1970). Upon irradiation it can undergo a conrotatory ring opening however, to produce the acyclic cations 79 and 80 which in... 

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

When substituents are present at C5 and/or C6 of the cyclohexadienyl system, two possible stereoisomers can be formed via conrotatory ring opening. In general, however, the... 

A variation of this approach, based on interception of o-quinodimethanes provides access to l/f-cyclopropa[f>]n hthalenes. Halton exploited the conrotatory ring-opening of trans-1,2-diphenylbenzocyclobutene (31) to the o-quinodimethane... 

Figure 22.8. Conrotatory ring opening of cyclopropane (7) to what Hoffmann called the (0,0) conformation of propane-1,3-diyl (8). The in-phase combination of Ip-n AOs in the LUMO is destabilized by an antibonding interaction with the re combination of C—H bonding orbitals at C2. A lower energy MO, which is not shown, is stabihzed by h3fperconjugative electron donation that is, the bonding version of this interaction. The out-of-phase combination of 2p-n AOs in the HOMO has a node at C2 hence, it does not mix with the C—H orbitals at this carbon.

Figure 22.10. Rearrangement products (13), predicted to be formed from 1,1-disilyl-cyclopropanes (11) by conrotatory ring opening, followed by a 1,2-sbift of a trimethylsilyl group in the 1,3-diradicals (12) generated. Pyrolysis of 11b should lead to a mixture of E/2) isomers (13b), but pyrolysis of 11c is predicted to form only the E stereoisomer (13c). ...

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure 14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a a2s + 05 pericyclic reaction. 

Figure 7.17 shows that in conrotatory ring opening bonding orbitals cor-... 

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