Electrocyclization mechanism

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). 

Deuterium labeling studies indicated that the electrocyclization mechanism necessitating the removal of a benzylic hydrogen, suggested earlier by Shudo and co-workers,183 is not operative here. Instead, experimental observations indicate the... 

Consequently, the main class of photochromic compounds that shows change in chirality by photochromic transformation is that based on electrocyclization. Fulgides [11-13], diarylethenes [14-16], and spiropyrans [17,18] are included in this class. It may be somewhat strange to put spiropyrans into this category because the colored merocyanine forms. ure known to take zwitterionic structures, which do not cyclize through the electrocyclization mechanism. However, in order to simplify the classification, we consider spiropyrans to cyclize from the neutral dienone structures as the merocyanine forms. 

All these photocyclizations are well explained in terms of an electrocyclic mechanism of nitrogen-containing, six 7r-electron conjugated system, according to the Woodward - Hoffmann rule, by postulating the intermediacy of a common trans cyclic structure from which respective types of products are formed depending on the reaction conditions either a nonoxidative, oxidative, or reductive condition. 

Photochemical conversion of stilbenes to phenanthrenes via a six 7t-elec-tron conrotatory cyclization according to an electrocyclic mechanism to the dihydrophenanthrenes and subsequent dehydrogenation is a very famous and useful synthetic reaction (103). 

Allyl phenyl ethers undergo an intramolecular [3,3]-sigmatropic rearrangement (the Claisen rearrangement) to form the C-alkyl derivative (Scheme 4.18). A consequence of the electrocyclic mechanisms is that the y-carbon atom of the allyl ether becomes attached to the aromatic ring. 

A particular case of addition to the double bond is the so-called citrylidene cyclization. When 5-hydroxychromenes derived from citral are heated in pyridine, they cyclize to citrylidene compounds [Eq. (19)]. The electrocyclic mechanism, recently proposed, accounts... 

Mechanism 2 (the semibenzilic acid mechanism) looks better, but labeling studies show that the two C atoms a to the ketone become equivalent in the course of the reaction, which is consistent only with mechanism 1 (the electrocyclic mechanism). The rearrangement of a-chloro-o-phenylacctonc to methyl hydrocinnamate is also consistent only with the electrocyclic mechanism if the semibenzilic mechanism were operative, then methyl 2-phenylpropionate would be the product. 

To summarize, when H atoms are present on the a-carbon opposite the leaving group, the electrocyclic mechanism usually operates when they are not, the semibenzilic mechanism operates. Why does the electrocyclic mechanism proceed more quickly than the more reasonable semibenzilic mechanism for eno-lizable a-haloketones Deprotonation and electrocyclic ring closing are both very rapid reactions—the latter even when a strained ring is formed—and they must simply be faster than HO- addition and migration, despite what our chemical intuition tells us. 

Scheme 2.113 One-pot synthesis of cesium pental

There are numerous examples of interconversion of 1,3,5-trienes and 1,3-cyclohexadiene systems by the electrocyclic mechanism. An especially interesting case of hexatriene-cyclohexadiene interconversion is the equilibrium between cyclo-heptatrienes and bicyclo[4.1.0]hepta-2,4-dienes. ... 

There are three double bonds each in a (3,y-position to the alcohol group. Two give identical products as shown in the top line of Figure S29 but the third gives a quite different pair of products from the same electrocyclic mechanism. Statistically, one might expect the yield of trimethylcyclohexene and hepta-l,6-diene-4-one to be 33%. In fact they only occur to the extent of 20%, indicating that there is some preference for the desired reaction pathway. The reasons for this probably lie in stereoelectronic factors. 

The Sj reaction has been featured in other indole syntheses, such as Molina and Fresneda s route to crypto-tackieine (Scheme 2, equation 1 [8]) and Orito s approach to indolo[2,l-fl]isoquinoline alkaloids (equations 2 and 3 [9]), even though neither substrate seems suitable for an Sj reaction. The successful indolization in equation 3 precludes a thermal electrocyclization mechanism because the gem-dimethyls block the required imine tautomerism. 

Insertion reactions for 1-methyl substituted allyl show that, with the exception of isonitriles, R NEC, insertion of substrate L takes place at the substituted end of the allyl ligand. An electrocyclic mechanism was proposed to explain the observed regioselectivity (Scheme 8). 

The rate law does, of course, not decide on the exact pathway by which the olefin dimer is formed from the excited 1 2 complex. One possibility is a radical-radical-dimerization with intermediate formation of a five membered metallacycle that could form the product by reductive elimination. Such a sequence is not subject to the restrictions of a concerted electrocyclic mechanism and the final stereochemistry of the cycloaddition product would be largely determined by the favored stereochemical arrangement in the 1 2 complex. An isolated and structurally characterized intermediate which is cited for support is the Ir complex 5, shown in Scheme 4, formed from [(COD)IrCl]2 and NBD followed by metathesis with 2,5-pentane-dionate [17]. 

As a team, consider the following historic preparation of a tris(l,l-dimethylethyl) derivative of Dewar benzene, B, by the photochemical isomerisation of l,2,4-tris(l,l-dimethylethyl) benzene by van Tamelen and Pappas (1962). B does not revert to A via either a thermal or a photochemical electrocyclic mechanism. Formulate a mechanism for the conversion of A to B and explain the kinetic robustness of B with respect to the regeneration of A. 

In Summary 2-Propenyloxybenzene rearranges to 2-(2-propenyl)phenol (o-allylphenol) by an electrocyclic mechanism that moves six electrons (Claisen rearrangement). Similar concerted reactions are undergone by aliphatic unsaturated ethers (aliphatic Claisen rearrangement) and by hydrocarbons containing 1,5-diene units (Cope rearrangement). 

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