Disrotatory, ring opening

Symmetry forbidden reaction (Section 10 14) Concerted re action in which the orbitals involved do not overlap in phase at all stages of the process The disrotatory ring opening of cyclobutene to 1 3 butadiene is a symmetry forbidden reaction... 

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

The molecular mechanisms for the ring openings of various cyclopropanone systems in the gas phase have been studied at the PM3 semiempirical level and shown to be disrotatory processes, while an experimental study of the stereomutation of 1,1-difluoro-2-ethyl-3-methylcyclopropane has confirmed the predicted preference for disrotatory ring opening and ring closure for this system. 

Figured. RHF, SC and CASSCF(6,6) energy profiles for the disrotatory ring-opening of cyclohexadiene alongthe CASSCF(6,6) IRC (all calculated with a 4-ilG basis).

Figure 5. Symmetry-unique SC orbitals for the disrotatory ring-opening of cyclohexadiene along the CASSCF(6,6) IRC at IRC = -1.2 amul 2 bohr (leftmost column), TS (IRC = 0) and IRC +1.2 bohr (rightmost column). The plot details are as for Fig. 1, except that the isovalue surfaces...

Figure 6. Composition of the active space spin-coupling pattern [ % in Eq. (2)]from the SC wavefunction for the disrotatory ring-opening of cyclohexadiene along the CASSCF(6,6) IRC, expressed in terms of Chirgwin-Coulson weights Pok [seeEq. (4)] in the Rumer basis [see Eq. (3)].

Unsaturation is also important in the metal-catalysed disrotatory ring opening of XXXII ( hexamethyl-Dewar-benzene , HMDB) to hexamethylbenzene. This formally forbidden process is catalysed by monomeric HMDBRhCl (the reaction being of order 1/2 in [HMDBRhCl]2 and order 1 in substrate) Closely related is the conversion of XXXIII(a) to XXXIV, presumably by way of an unsaturated intermediate XXXIII ( ), since free CO or added alkene ligands inhibit the process . ... 

On the other hand, it is conceivable that, if 7a undergoes disrotatory ring opening to 8a, energy remains in the disrotatory mode of methylene rotations for the short time that 8a takes to cross the TS for disrotatory closure to 7a. In this dynamical model, disrotatory ring opening to 8a, rather than resulting in preferential... 

Figure 22.9. Disrotatory ring opening of 1,1-difluorocyclopropane (9) to 2,2-difluorocyclo-propane-l,3-diyl (10). The in-phase comhination of 2p-n AOs in the highest occupied molecular orbital (HOMO) is stabilized by a bonding interaction with the 2p AO at C2 in the n combination of low-lying, C—F, antibonding orbitals.

Figure 22.11. 1,3 -Diradicals (15a and 15b) formed by conrotatory and disrotatory ring opening of cis- and franj-l,2-dimethylspiropentanes (14a and 14b).

Fig. 5a) does not correlate with the betaine HOMO (Fig. 5b), and it can be shown, therefore, that the thermal reactions are symmetry forbidden whereas photochemical reactions are symmetry allowed. Mechanistically, these reactions are analogous to the disrotatory ring opening of a-cyano-c/.s-stilbene oxide (450 - 451) which Huisgen and Markowski have demonstrated takes place thermally with a free energy of activation of 35.6 kcal mol at Clearly, although forbidden, these reactions are not... 

Figure 7.17 Orbital correlation diagram for conrotatory and disrotatory ring openings of cyclobutenes.

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