Disrotatory electrocyclic ring closure thermal

The first electrocyclic ring closure involves eight electrons, so it is conrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the tetraene, which are both in, become trans. The second electrocyclic ring closure involves six electrons, so it is disrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the triene, which are both out, become cis. This is the arrangement observed in the natural product. 

The initial photochemical reaction is intramolecular ortho cycloaddition leading to bicyclo[4.2.0]octa-2,4-diene derivatives. These normally are not detected because they undergo rapid thermal disrotatory ring opening to all-cis cyclooctatrienes which have large extinction coefficients and absorb a second photon to undergo electrocyclic ring closure to the photostable bicyclo[4.2.0] octa-2,7-dienes. 

Figure 1 The four stereochemically distinct modes of electrocyclic ring closure wherein the notations inward and outward refer to the axes of rotation as viewed with respect to ( ). The principle of the conservation of orbital symmetry predicts disrotatory (dis) or coniotatory (con) ring closure for the thermally or photochemically induced processes, reqwctively. The same modes (dis or con) are predicted for the reverse ring opening processes...

Which thermal electrocyclic ring closure mode, conrotatory or disrotatory, would the cation in problem 12.1 follow ... 

The thermal disrotatory [n6] electrocyclization of cis-1,3,5-hexatriene systems has been extensively employed for the synthesis of cyclic hydrocarbons. The average enthalpy of activation is in the range of about 120 kJ mol 1 [36]. The incorporation of two of the hexatriene double bonds in phenyl rings (stil-bene, 1) stabilizes the precursor significantly and necessitates temperatures of 1050°C to obtain a 30% yield of phenanthrene (2, see Scheme 2, [37]). An enthalpy of activation of (250 20) kJ mok was estimated for the conversion of 9,9 -bifluorenylidene (3) to benz[e]indeno[l,2,3-hi]acephenanthrylene (4), a reaction that is accompanied by the radical initiated isomerization of 3 to diben-zo[g,p]chrysene (5, Scheme 2, [38]). It is assumed that both reactions 1 —> 2 and 3 —> 4 are initiated by an electrocyclic ring closure forming a 4 a,4 fr-dihydro-phenanthrene (la) intermediate. 

In Summary Conjugated dienes and hexatrienes are capable of (reversible) electrocyclic ring closures to cyclobutenes and 1,3-cyclohexadienes, respectively. The diene-cyclobutene system prefers thermal conrotatory and photochemical disrotatory modes. The triene-cyclohexadiene system reacts in the opposite way, proceeding through thermal disrotatory and photochemical comotatory rearrangements. The stereochemistry of such electrocychc reactions is governed by the Woodward-Hoffmann rules. 

If the stereochemistry of the reaetion product 5 is exactly the opposite of what is expected by the mechanism in Scheme 28.8, it is evident that we should consider other alternatives for the eyclization step. An attractive option eould be to suppose that the eyclization of lidiiated compound 14 is a pericyclic reaction. In fact, if we discard the stepwise mechanism for die eyclization of 14 and consider a 6n disrotatory thermal electrocyclic ring closure in this intermediate (better indicated by the canonical form 18), the result would be c -tricyclic enolate 19. Quenching with an electrophile will give the reaction product 5 with total stereoselectivity (Scheme 28.9). 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

The thermal ring closure reaction of a 1,3,5-triene to a 1,3-cyclohexadiene occurs by a concerted disrotatory electrocyclic mechanism. An example of the latter is the oxepin-benzene oxide equilibrium (7) which favors the oxepin tautomer at higher temperatures (Section 5.17.1.2). Oxepin (7) was found to rearrange to phenol during attempted distillation at normal pressure (67AG(E)385>. This aromatization reaction may be considered as a spontaneous rearrangement of the oxirane ring to the dienone isomer followed by enolization (equation 7). 

Why is the thermal stability of diarylethene derivatives enhanced by replacing phenyl groups with furan or thiophene groups In molecular orbitals calculation, the photochromic reaction is treated as a typical electrocyclic reaction between hexatriene and cyclohexadiene. The thermal reaction proceeds disrotatorily and the photoreaction, conrotatorily. Disrotatory cyclization of A to B requires an increase in free energy larger than 138 kJ/mol, and hence no thermal ring-closure occurs in the case of either phenyl- or furan-substituted molecules (see... 

We have just seen why the configuration of the product formed under photochemical conditions is the opposite of the configuration of the product formed under thermal conditions The ground-state HOMO is symmetric—so disrotatory ring closure occurs, whereas the excited-state HOMO is asymmetric—so conrotatory ring closure occurs. Thus, the stereochemical outcome of an electrocyclic reaction depends on the symmetry of the HOMO of the compound undergoing ring closure. 

Sol 12. (b) The product, 7,8-dimethyl cycloocta-l,3,5-triene (1) with tmns stereochemistry is obtained upon disrotatory ring closure of octatetraene system, which occurs only under photochemical conditions. Similarly, a bicyclic product (11) with cis stereochemistry at fused carbons is obtained when a triene system undergoes disrotatory electrocyclization under thermal conditions. 

Let s first look at the electrocyclic reaction of (2 ,4 6 )-octatriene. The ground-state HOMO (i/ s) of a compound with three conjugated bonds is symmetric (Figure 28.3). This means that ring closure under thermal conditions is disrotatory. In disrotatory ring closure of (2 , 4. 6 )-octatriene, the methyl groups are both pushed up (or down), which results in formation of the cis product. 

The cis relationship between methyl and ring junction methine hydrogen atom in 7 indicates a disrotatory process. Spirotricyclic product 8 was also isolated in 17% yield from this reaction, presumably as a consequence of a cascade of two pericyclic reactions. A thermally allowed [1,7]-hydrogen shift leads from 6 to triene 9 that undergoes 6ti-disrotatory ring closure to 8. In both photochemical as well as thermal electrocyclizations, there may be multiple... 

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