Conrotatory process, electrocyclic

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 ... 

Considerable effort has been expended on elucidating the mechanism of the photocyclizations. Di-p-tolylamine can be cyclized to 3,6-dimethylcar-bazole either photolytically in petrol or thermally at 880°C. Each process was viewed as electrocyclic, proceeding via cis and trans versions of 312 (R = Me, = H) produced by dis- or conrotatory processes, respectively. ... 

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. 

This reactivity proved to be a general process, providing the unique products in moderate yields following cyclopropanation and immediate treatment with silver tetrafluoroborate. These structures revealed that a cascade sequence was proceeding stereoselectively in every case to furnish a single product as the result of conrotatory 4jt electrocyclization, electrophilic aromatic substitution at the least hindered position on the arene moiety (para to the MeO) in favor of six-membered ring formation, and desilylation with protonation from the exo face of the bicyclic product. Dehydrochlorination to form a second cationic intermediate did not occur in this case, due to structural restrictions imposed by the bridged architecture of 81. 

The electrocyclic reactions of phenyl fulgides and l,8a-DHNs can be photo-induced by a conrotatory process. The electrocyclic ring-opening reaction of cyclohexadiene systems (l,8a-DHNs) can also be induced thermally via a disrota-tory process. 

There are abundant examples in the literature which reveal that the thermal electrocyclization of 8e systems to form eight-membered rings proceeds with lower activation energies than for the lower vinyl-og, (Z)-hexatrienes. The helical geometry of the transition state leads to less steric congestion about Ae reacting termini of the octatetraene and this likely accounts for the facility of the thermal 8e conrotatory process. Minor structural differences can induce cycloreversion of the process. ... 

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 photochemical transformation is a 6tt electron (4n + 2) electrocyclic ring opening. The selection rules predict a conrotatory process as illustrated ... 

The second reaction involves a reverse Diels-Alder reaction and an electrocyclic opening of the dobutene product. This is a four-electron conrotatory process. The two substituents may both. otate out to give the , -diene or both in to give the 2,Z-diene. 

The Nazarov reaction, in which the key electrocyclic step is the conrotatory process 6.505, has one more atom in the ring but the same number of electrons. The question with respect to torquoselectivity now, since this reaction is taking place in the opposite direction, namely ring-closing, is which reacts faster, a dienone... 

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... 

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). 

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

However, there are other possibilities. For example, a chemical reaction involving the molecule in its excited singlet state can occur. In the case of electrocyclic reactions, such photochemical processes occur with a symmetry opposite to those of the thermal reactions, that is, a (thermally) conrotatory process will occur in a (photochemically) disrotatory way and vice versa since antibonding orbital(s) are now occupied. The dimerization of ethene (ethylene, H2C=CH2), a thermally disallowed 2, -r 2, reaction, is one example that will be encountered later in in this chapter. Other photochemical processes will be examined when carbonyl compounds (Chapter 9) are considered. 

There are also examples of electrocyclic reactions that follow the stereochemical outcomes (conrotatory vs. disrotatory) expected for reactions under orbital symmetry control. For example, the photochemical ring opening of Eq. 16.24 should be a six-electron, conrotatory process, and indeed the product has the predicted trans double bond. An important biological example of such a process is the photochemical conversion of ergosterol to pre-vitamin D (Eq. 16,25), a key event in the synthesis of vitamin D. 

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

Other examples of sequences where electrocyclization is the final step include the atom-economical syntheses of [4.6.4.6.] fenestradienes. A tandem hydrogenation/8 t-> 6 t electrocyclization cascade from en-yn-enes 110 linked to a preexistent cyclobutane-containing tricycle was developed (Scheme 5.40) [118, 119]. Overall yields for the sequence range from 63-93%, leading to these fenestradienes 112 with full diastereoselectivities. These results may be explained by the high torquoselectivities observed during the 8jt-conrotatory process that forms the cyclooctatrienering intermediate 111 [120]. 

Vitamin D2 is produced by two pericyclic reactions. One of them is photochemicaUy initiated the second thermally initiated. The first step is a photochemical electrocyclic reaction in which a cyclohexadiene of the B ring is isomerized to a triene. The reaction involves six k electrons and is the reverse of the photochemical cyclization reaction discussed in Section 28.4. Thus, by the principle of microscopic reversibility, this photochemicaUy allowed ring opening involving a 4 +2 71 system must occur by a conrotatory process. 

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. 

---