Stereochemistry conrotatory process

An example of preferred conrotatory cyclization of four-7c-electron pentadienyl cation systems can be found in the acid-catalyzed cyclization of the dienone 12, which proceeds through the 3-hydroxypentadienyl cation 13. The stereochemistry is that expected for a conrotatory process. 

The stereochemistry of the cyclobutene isomerizations and the reverse processes of this type, involving the formation of a bond between the ends of a linear system containing a number of 7i--electrons, has been discussed by Woodward and Hoffmann (1965). They term such processes electrocyclic and consider that their steric course is determined by the symmetry of the highest occupied molecular orbital of the open-chain isomer. In an open-chain system containing 4 7T-electrons (such as butadiene), the symmetry of the highest occupied ground-state orbital is such that bonding interaction between the ends of the chain must involve overlap between orbital envelopes on opposite faces of the system, and this can only occur in a conrotatory process ... 

The Nazarov2 is probably the most important of reactions like 3. The cation 6 is formed from a dienone 5 by protonation and cyclises to the allylic cation 7. Though this is presumably a conrotatory process, the stereochemistry is usually lost in the formation of the cyclopentenone 9. 

These selection rules can also be obtained from an analysis of polyene cyclizations. If the interaction between the terminal atoms Ca and Cn is bonding (antibonding), it will favor (disfavor) the cyclization. Figure 4.5 shows how the contribution of any given MO changes as a function of the reaction stereochemistry. When p is odd (even) the conrotatory process is disfavored (favored) and the disrotatory process is favored (disfavored). Obviously, the preferred pathway can be deduced by summing the contributions of all of the occupied MOs, up to and including the HOMO ... 

For cyclobutenes, there is another interesting aspect to the stereochemistry of the electrocyclic reactions. There are two stereochemically distinct possibilities for the conrotatory process. A substituent group at C(3) might move away from or toward the breaking bond. 

A common type of electrocyclic reaction is the ring-opening of a cyclobutene to a butadiene. The stereochemistry of the new alkene(s) in the diene can be interpreted on the basis of the Woodward-Hoffmann rules. For a four electron component, thermal ring-opening occurs by a conrotatory process (both terminal p-orbitals moving clockwise or anticlockwise), whereas the photochemical reaction... 

The Nazarov cyclization is a four-electron cyclization and occurs thermally by a conrotatory process. The stereochemical outcome across the new carbon-carbon bond is often obscured by the loss of a proton at one of these centres during the cyclopentenone formation. If, however, the proton loss occurs exo to the five-membered ring or if the aUyl cation is quenched by a nucleophile, then the stereochemistry can be observed. For example, trapping the allyl cation by reduction with... 

Another example of the preference for controtatory cyclization can be seen in the acid-catalyzed cyclization of the divinyl ketone 17. This occurs via a 3-hydroxypentadienyl cation and the stereochemistry of the product is as expected for a conrotatory process ... 

The photocyclization of N-aryl enamines derived from cyclic or acyclic ketones proceeds under mild conditions to produce 2,3-dihydroindole derivatives (178b). The stereochemistry of the products is predominantly trans, which follows from a photochemical electrocyclic process which should take place in a conrotatory manner (178c,I78d). However, the presence of some cis products is not as easily explained. 

Figure 4.41 (a) Orbital overlaps in conrotatory and disrotatory o bond formation in a ring-closure reaction, (b) Stereochemistry of preferred products in photochemical and thermal (dark) processes... 

Although the cyclopropyl anion itself does not appear to have been investigated, the isoelectronic aziridines are known to open in the predicted conrotatory sense. Thus heating the cis and trans isomers 39 and 40 in the presence of di-methylacetylenedicarboxylate leads with high stereospecificity, by way of a 1,3-dipolar addition, to the products indicated in Scheme 4. The stereochemistry is reversed, as predicted, for the excited-state process.87... 

Fig, 4,18 The stereochemistry of many reactions is easily predicted from the symmetry of molecular orbitals, usually the highest occupied n MO (n HOMO). In the ring closure of 1,3-butadiene to cyclobutene the phase (+ or —) of the HOMO (i//2) at the end carbons (the atoms that bond) is such that closure must occur in a conrotatory sense, giving a definite stereochemical outcome. In the example above there is only one product. The reverse process is actually thermodynamically favored, and the cis dimethyl cyclobutene opens to the cis, trans diene. No attempt is made here to show quantitatively the positions of the energy levels or to size the AOs according to their contributions to the MOs... 

Further studies on the stereochemistry of these processes indicate an allowed conrotatory ring opening for the benzocyclobutene radical ration, similar to the neutral process (there is no evidence for 38 as intermediate either uncharged or as radical cation). However, diffusional quenching by means of chloranil (9) as acceptor leads to SSIP s (Eqa. (22—25)). 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

The stereochemistry of a thermally induced 10e electrocyclization (predicted to be disrotatory) has not been firmly established and the main synthetic application is found in the formation of azulenes and ring-fused azulenes as in the transformation (452) to (453). Thermolysis of (454) with spontaneous elimination of dimethylamine from intermediate (455) afforded the fused azulene structure (456). The chemistry of even higher order (12e to 20e") pericyclic processes has been recently reviewed. An example of an unusual sequence of pericyclic processes is the transformation of heptahendecafulvadiene (457) to the pentacyclic hydrocarbons (462) and (463) in a 2 1 ratio. The pathway for this transformation can be viewed as an initial conrotatory 20e electrocyclization followed by a cascade of 10e and 6e pericyclic processes. ... 

The stereochemistry of such a process is termed conrotatory or antarafacial if the substituents at the interacting termini of the conjugated system both rotate in the same sense, for example,... 

Therefore, some conclusions have been generally accepted and have been summarized as follows the cycloaddition reaction is a stepwise reaction rather than a concerted one the reaction is initiated by nucleophihc attack of an imine to a ketene, giving rise to a zwitterionic intermediate a conrotatory eleclrocyclic ring-closure of the zwitterionic intermediate produces the final 2-azetidone product [85], As the stereochemistry of the structure of the P-lactams strongly affects their biological activity, the stereoselectivity of the process must be carefully considered. Uncatalysed as well as catalysed processes have been reported organometallic and organic catalysts have been utilized in procedures oriented to the syntheses of enantiopure P-lactams [90-92],... 

The cyclobutene ring first opens in an electrocyclic reaction 152. This must be conrotatory as it is a four electron process but there is no stereochemistry at this stage. Then an intramolecular Diels-Alder cycloaddition 153 closes the new six-membered ring. This is a particularly favourable reaction as the formation of the alkene completes a benzene ring. It would not be possible to prepare such an unstable diene so a tandem process is necessary. 

We have considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbital, orbital correlation diagrams, and transition state aromaticity. All arrive at the same conclusions about the stereochemistry of electrocyclic reactions. Reactions involving 4n + 2 electrons are disrotatory and involve a HUckel-type transition structure, whereas those involving 4n electrons are conrotatory and the orbital array are of the Mobius type. These general principles serve to explain and correlate many specific experimental observations. The chart that follows summarizes the relationship between transition stmcture topology, the number of electrons, and the feasibility of the reaction. 

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