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Pericyclicreactions are reactions in which a cyclic, conjugated system of electrons iscreated in the transition state. The Diels-Alder reaction is a common example.Electrocyclic reactions are a sub-type of pericyclic reaction which isunimolecular and in which the termini of a conjugated system become sigmabonded to each other to form a shortened pi system. The reverse of this, thecleavage of a sigma bond to generate a longer conjugated system, is sometimescalled a retroelectrocyclic reaction. As an example of the latter, cyclobuteneis cleaved thermally to yield 1,3-butadiene, relieving the extensive strain inthe cyclobutene system and gaining the resonance stabilization of theconjugated diene system (Scheme 1). The alkene pi bond of cyclobutene isextended to give the conjugated system of butadiene. The especially interestingthing about this reaction is its stereochemistry, as revealed, for example, inthe cleavage of cis- and trans-3,4-dimethylcyclobutadiene. The reaction is highlystereospecific, with the transisomer of the reactant yielding the E,E isomer of the diene and the cis isomerof the reactant giving the Z,E isomer of the diene. Note that in the IUPACnomenclature, E (entgegen) means trans and Z (zusammen) means cis. The observation of stereospecificity meansthat in both transition states (for the two isomers) a single rotatory pathwayis followed, yielding different isomers of the product. In particular thegroups attached to C3 and C4 must rotate in the same (both clockwise or bothcounterclockwise) direction. The term which is used in this connection is“conrotation”.
Scheme 1. Theconrotatory electrocyclic reactions of cyclobutenes.
Incis-3,4-dimethylcyclobutene, e.g., thetwo methyl groups both are shown as rotating clockwise, as shown in Scheme 1and also in Scheme 2, below. This yields the cis,trans (E,Z) isomer of the diene. On the other hand, in thetrans isomer of3,4-dimethylcyclobutene, if clockwise rotation is followed, the trans,trans (E,E)isomer of the diene is generated.
Scheme 2. Conrotationof C3 and C4 results in the formation of the E,E diene from the transcyclobutene and the E,Z diene from the ciscyclobutene.
Notewhat is not observed, viz., disrotation,in which the groups attached to C3 and C4 rotate in opposite directions (oneclockwise and one counterclockwise; shown in Scheme 3). In such a case, the cis cyclobutene isomer would give rise toE,E-2,4-hexadiene and the trans cyclobuteneisomer would yield E,Z-2,4-hexadiene, i.e., the opposite results from what areactually observed. So the reactions, as experimentally observed, are notdisrotatory, nor are they a mixture of disrotatory and conrotatory paths, butthey occur exclusively by a stereospecific conrotatory path. It will be ofinterest to examine the reasons for this preference. Incidentally,electrocyclic reactions in larger (or smaller) rings always have a strongrotatory preference, but not always for conrotation. That is, some strongly preferdisrotation, as we shall see.
Scheme 3. Thedisrotatory cleavage of cyclobutenes which is not observed.
Whatis the basis for the strong preference for conrotation over disrotation? To seethis, we can again use the theory of transition state aromaticity/antiaromaticityas applied to both of the competing transition states (Scheme 4).
Scheme 4. Thetopologies associated with the conrrotatory and disrotatory reaction modes.
Asyou will recall, the normal (Huckel) cyclobutadiene MO display is predicted bythe circle mnemonic in which a polygon of appropriate size is inscribed, withone apex down, inside a circle of radius 2b.In the Mobius topology, in which every MO has at least 1 nodal plane, theappropriate circle mnemonic is that in which the polygon is inscribed with oneside down, as illustrated below (Scheme 5). Therefore, Mobius cyclobutadienehas two bonding MO’s and can support four electrons.
Scheme 5. The circlemnemonic for Mobius cyclobutadiene
Inthe case of the electrocyclic reaction of 1,3-cyclohexadiene to1,3,5-hextratriene the converse result is observed, viz., disrotation (Scheme6). This is as expected because the disrotatory mode maintains a normalHuckel-type topology, and with six electrons, this is aromatic. It is alsopertinent to note that the actual reaction occurs in the forward electrocyclicsense, from hexatriene to cyclohexadiene, since the latter has little strain,and the conversion of a pi bond to a sigma bond is favorable in terms of bondstrength.
Scheme 6. Thedisrotatory electrocyclic reaction of hexatrienes to give 1,3-hexadienes.
The stereoelectronic relationships can perhaps be best seenby starting with the cyclic form (Scheme 7):
Scheme 7. Illustratinghow the disrotatory reaction mode converts E,E-2,4,6-hexatriene to cis-5,6-dimethyl-1,3-cyclohexadiene and the E,Z triene to trans-5,6-dimethyl-1,3-cyclohexadiene(looking at the reaction in the reverse sense)
Retroelectrocyclic Reaction of the CyclopropylCarbocation. An excellent example of aretroelectrocyclic reaction which occurs via a stereospecificdisrotatory mode is the cleavage of the cyclopropyl cation to the allyl cation(Scheme 8). Again, much strain is relieved and the large resonancestabilization of the allyl cation is gained. The stereochemistry is disrotatorybecause the cyclic, conjugated system present in the TS has two electrons andis aromatic in the Huckel topology.