Disrotatory process, electrocyclic

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

The closed and open forms, 4 and 5, respectively, represent the formal starting and end points of an electrocyclic reaction. In terms of this pericyclic reaction, the transition state 6 can be analysed with respect to its configurational and electronic properties as either a stabilized or destabilized Huckel or Mobius transition state. Where 4 and 5 are linked by a thermally allowed disrotatory process, then 6 will have a Hiickel-type configuration. Where the process involves (4q + 2) electrons, the electrocyclic reaction is thermally allowed and 6 can be considered to be homoaromatic. In those instances where the 4/5 interconversion is a 4q process, then 6 is formally an homoantiaromatic molecule or ion. 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

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. 

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

It should be pointed out that our description of electrocyclic reactions thus far has been qualitative. Woodward and Hoffmann (1965a) do refer to unpublished HMO calculations which back up the almost intuitive symmetry arguments. Nevertheless, Fukui (1965,1966) and Zimmerman (1966) outlined HMO treatments in which they obtained changes in energy for conrotatory and disrotatory processes. On the basis that paths involving minimum energy between reactants and transition states were favored, their predictions were in essential agreement with those of Woodward and Hoffmann. 

Electrocyclic reactions of conjugated polyenes create chiral molecules through stereospecific conrotatory or disrotatory processes. In solution, the two enantiom-... 

Two electrocyclic reactions, involving three electron pairs each, occur in this isomerization. The thermal reaction is a disrotatory process that yields two cis-fused six-membered rings. The photochemical reaction yields the rrans-fused isomer. The two pairs of n electrons in the eight-membered ring do not take part in the electrocyclic reaction. 

The selection rules can be applied to charged species as well as to neutral molecules. The only requirement is that the reaction be a concerted process involving electrons in overlapping p orbitals. For example, the conversion of a cyclopropyl cation to the allyl cation can be considered as a tt -electrocyclic process. For this process, the selection rules predict a disrotatory process. 

Now the disrotatory process is allowed and the conrotatory one forbidden - exactly the reverse of the thermal process. This result is quite general photochemical electrocyclic reactions always occur in the opposite sense to their thermal equivalents. 

In an electrocyclic reaction, the conrotatory process is the one in which the two end groups that rotate to form the new bond do so in the same direction. For the disrotatory process, they rotate in opposite directions. 

In this process, the Pd-catalyzed Stille cross-coupling reaction of enantiopure iodide 259 and stannane 258 afforded polyene 262, which underwent a conrotatory 87t-electrocyclization resulting in the cyclooctatrienes 263 and 264. These compounds reacted further in a disrotatory 67t-electrocyclization to a 5 1 mixture of the diastereomers shimalactone A (260) and B (261) in 55% and 11% yield, respectively (Scheme 14.40). 

Woodward—Hoffmann orbital symmetry rules can be applied to the charged systems as well. The conversion of a cyclopropyl cation to an allylic cation is the simplest one, which involves only 27r-electrons (Figure 2.13). This is an electrocyclic reaction of (4n + 2) type (n = 0) and should, therefore, be a disrotatory process. 

The next four items (examples 2-5, inclusive) in Table 6.6 are all electrocyclic reactions, clearly related to the cycloadditions and others already discussed earlier in this chapter and the symmetry controlled processes of Chapter 4. Example 2, a conrotatory four-electron 2% + 27t = 27t + 2d) process relating trans or ( )-3,4-dimethylcyclobutene to trans, trans or (2 ,4 )-hexadiene conserves C2 symmetry as shown in Figure 4.41 and again here in Equation 6.59. Examples 3,4, and 5 are six-electron disrotatory processes. 

A7.65 The thermal and photochemical electrocyclic reactions consist of two successive disrotatory processes and two successive conrotatory processes, respectively... 

Thermal 671-electrocyclizations are disrotatory processes. For example, heating triene 6 to 180°C in decalin leads to tricyclic product 7 in 57% yield (Scheme 19.3). ... 

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... 

The photochemical Nazarov electrocyclization of 4-pyrones and of 4,4-disubstituted 2,5-cyclohexadienones takes place by means of a disrotatory process as predicted... 

The ruthenium-catalyzed cycloaddition of 1,6-diynes to 1,3-dienes forming 1,3,5-cyclooctatrienes and vinylcyclohexadienes can formally be assigned to [4+2+2] cycloaddition. This reaction is actually a tandem process catalyzed by Ru(II) resulted in the formation of Z-tetraenes or vinyl-Z-trienes, followed by the ring closure in pure thermal conrotatory Sir- or disrotatory 6ir-electrocyclization processes, respectively (Scheme 2.88 and Table 2.13) [141]. 

If we now consider the HOMO of hexatriene (Figure 18.30), where the closure reaction is a six-electron process, to bring the orbitals together to make the new o-bond, the two orbitals must now rotate in opposite directions, one clockwise and one counterclockwise. This is a disrotatory process, and this rotation applies to all 4 + 2 electrocyclic reactions. As always, it is easier to see what is happening, when there are substituents (Figure 18.31). 

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry mles were formulated. We will discuss a few representative examples in the following paragraphs. 

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. 

Sulfur extrusion is the main decomposition pathway, with 1-benzothiepins being more stable than 3-benzothiepins. The required 6it disrotatory electrocyclic process leads to a loss of aromaticity for the [6]annulated benzene ring, whereas the [r/]benzene derivative 4 leads to a thianorcaradiene derivative with less disturbed aromaticity of the benzene ring. 

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