Conrotatory ring

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. 

Figure 15.23 State correlation diagram for the dis- and conrotatory ring-closure of butadiene...

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... 

The two-step Staudinger reaction has been investigated in depth and 13-azadienes 56 have been isolated in some cases. The evidence suggests that the enol ether group is crucial in the stabilization of the diene and also plays an important role in promoting the conrotatory ring closure process . 

Tius and co-workers investigated a number of cationic cyclopentannelations of allenyl ethers [113] and found that 1-lithio-l-alkoxyallenes 180 react with a,/3-unsatu-rated carbonyl compounds 181 leading to highly functionalized cyclopentenones 182 (Scheme 8.44). The primary products are a-allenyl ketones 183, which form pentadienyl cations 184 by protonation. The latter undergo a thermally allowed 4jt-conrotatory ring closure to intermediates 185, which with elimination of R1 finally lead to the expected products 182 (Scheme 8.45). 

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 ... 

The HOMO of the pentadienyl cation is j/, which is antisymmetric, so a conrotatory ring closure occurs, consistent with the four electrons involved in this reaction. The HOMO of the pentadienyl anion is /2, which is symmetric, so a disrotatory ring closure occurs, consistent with the six electrons involved in this reaction. 

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.

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