Disrotatory electrocyclic ring opening

All three levels of theory predict the ring expansion of singlet phenylcarbene ( A -la) to cycloheptatetraene (3a) to occur in two steps, via bicy-clo[4.1.0]hepta-2,4,6-triene (2a) as an intermediate. The first step is addition of the carbene carbon to an adjacent 7t bond of the ring. The second step involves a six-electron, disrotatory, electrocyclic ring opening, which is allowed by orbital symmetry67 and thus proceeds by a highly delocalized transition state. Fig. 4... 

Cyclopropyl ions and radicals (23) can undergo conversion to allyl (24) by typical electrocyclic ring opening processes we have carried out calculations for ring opening by both conrotatory and disrotatory paths. Table 5 shows calculated activation energies for the various processes. 

In the first step, Ag+ promotes the departure of Cl- to give a cyclopropyl carbocation. This undergoes two-electron disrotatory electrocyclic ring opening to give the chloroallylic cation, in which the empty orbital is localized on Cl and C3. Then 09 can add to C3 desilylation then gives the product. 

The article is organized as follows. In the next Section we present a brief outline of the theoretical background for the present work. Section 3 contains summaries of the SC models for the electronic mechanisms of the gas-phase Diels-Alder reaction between butadiene andethene [11] and the 1,3-dipolar cycloaddition of fulminic acid to ethyne [12]. In Section 4 we provide, for the first time, a description of the SC model for the electronic mechanism of the gas-phase disrotatory electrocyclic ring-opening of cyclohexadiene. Conclusions and final comments are presented in Section 5. 

It would be difficult, at this stage, to present a properly argued preference between schemes A and B in the Introduction as the more faithful representation for the disrotatory electrocyclic ring-opening of cyclohexadiene. Chemical intuition (at least that of the authors) would suggest scheme B, but scheme A is on the textbook pages and, as has been shown by the SC description of the 1,3-dipolar addition of fulminic acid to ethyne, a heterolytic scheme remains a definite possibility. 

The SC descriptions of the electronic mechanisms of the three six-electron pericyclic gas-phase reactions discussed in this paper (namely, the Diels-Alder reaction between butadiene and ethene [11], the 1,3-dipolar cycloaddition offulminic acid to ethyne [12], and the disrotatory electrocyclic ring-opening of cyclohexadiene) take the theory much beyond the HMO and RHF levels employed in the formulation of the most popular MO-based treatments of pericyclic reactions, including the Woodward-Hoffmarm mles [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman model [4—6]. The SC wavefunction maintains near-CASSCF quality throughout the range of reaction coordinate studied for each reaction but, in contrast to its CASSCF counterpart, it is very much easier to interpret and to visualize directly. 

Consider the electrocyclic ring-opening reaction of cyclobutene. The molecule is formally divided into two fragments the double bond and the single 0 bond which is cleaved.9 The frontier orbital interactions (0,7t ) and (0, it) relevant to the conrota-tory and disrotatory reactions are given in diagrams 4, 5, 6 and 7, respectively. The net overlap is positive for 4 and 5, but zero for 6 and 7. The conrotatory process is therefore allowed, and the disrotatory process forbidden. 

Fig. 7. Disrotatory electrocyclic ring opening of 1,3-cyclohexadiene to 1,3,5-hexatriene. The excited state process, forbidden in the disrotatory mode, as indicated on the right, is allowed in the eonrotatory mode (not shown).

The majority of yc/n-dichIorocycIopropane substrates examined in this study provided the desired a-chlorocyclopentenones as a result of sequential electrocyclic ring opening and Nazarov cyclization. In general, those substrates lacking additional substitution on the cyclopropane moiety provided products 75 selectively as a result of regioselective elimination to deliver the more electron-rich olefin. The mechanism for this transformation is believed to involve disrotatory halocyclopropane ring opening... 

Figure 3. Symmetry-unique spin-coupled orbitals for the disrotatory electrocyclic ring-opening of cyclohexadiene.

We can now go back to the reaction that introduced this section—the photochemical electrocyclic ring opening of ergosterol to give provitamin D2. By looking at the starting material and product we can deduce whether the reaction is conrotatory or disrotatory. 

There are two things to note here—firstly, the geometry of the double bond is nothing to do with whether the reaction is conrotatory or disrotatory. As you know, this 4re electron electrocyclic ring opening must be conrotatory. but as there is no substituent on the other end of. the diene product we can t tell. Secondly, notice that, in this 12-membered ring, a trans double bond is not only possible, but probably preferred. We introduced irradiation as a means of interconverting double bond isomers in Chapter 31. 

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