Examples of Electrocyclic Reactions

The analysis in Section 22.3 indicates that the thermal interconversion of a diene with a cyclobutene should occur by conrotation. The reaction is allowed in both directions, as long as a conrotatory motion is followed. However, usually only the conversion of the cyclobutene to the diene is observed because the cyclobutene is destabilized by angle strain and is present only in trace amounts at equilibrium. An example of the opening of a cyclobutene to form a diene is provided by the following equation  

The reverse process, the conversion of a diene to a cyclobutene, can be accomplished photochemically. Although the cyclobutene is less stable, it is possible to selectively excite the diene because it absorbs longer-wavelength light (see Section 15.2). An example is shown in the following equation  

In this reaction, light of appropriate energy is used to selectively excite 1,3-cyclohepta-diene. The diene closes to a cyclobutene by a disrotatory motion. Although the product, because of its strained cyclobutene ring, is much less stable than the reactant, it is unable to revert back to the diene by an allowed pathway. It does not absorb the light used in the reaction, so the photochemically allowed disrotatory pathway is not available. A conrotatory opening is thermally allowed but results in a cycloheptadiene with a trans double bond. Such a compound is much too strained to form. Therefore, the product can 

Build a model of the following compound. Then replace the connector for the bond that is part of both rings with two separate connectors. Do a conrotation and a disrotation to see the strain that is incurred in each process. 

The thermally allowed cyclization of a triene to form a cyclohexadiene occurs by a disrotatory motion, as illustrated in the following equation. In this case the product is favored at equilibrium because it has one more sigma bond and one fewer pi bond than the reactant. (Sigma bonds are stronger than pi bonds.) 

Again we must point out that many of the examples we will cite have not been shown to be concerted but are consistent with the electrocyclic rules. It is possible that some of these involve intermediates however, in some cases the intermediates will be subject to the electrocyclic rules. 

hr Bicyclo[4.2.0]-oct-7-ene, mole % ciSttrans-X, -Cyclooctadiene, mole % cis,cis-, 4-Cyclooctadiene, mole % 

In many cases the transformations may be more complex than indicated by Eqs. (9.89)-(9,100). An example of this is the photochemistry of cis,cis-, 3-cyclooctadiene [Eq. (9.94)]. A close examination of this reaction indicates that bicyclo[4.2.0]oct-7-ene is formed but in low relative yields during the initial reaction (see Table 9.9). In addition, the m,rroiu l,3-cyclooctadiene is formed and then consumed as the reaction proceeds. Fonken showed that the bicyclooctene initially formed, however, was not from thermal isomerization of the m,rroRs-diene. Still a third reaction was the 1,3 sigmatropic hydrogen shift to form the c/s,cij-l,4-cyclooctadiene  

Vitamin D chemistry provided some of the first examples of both thermal and photo electrocyclic reactions 

It is interesting to note that while the electrocyclic reaction shown in Eq. (9.104) has been developed into a very useful synthetic reaction, not all stilbene-type systems cyclize. For the reaction to occur, the sum of the free valence indices (2 F ) for the first excited state at atoms between which the new bond is formed must be greater than unity -  

In addition to the 3,4-dimethylcyclobutene case discussed in Section 10.5.1, there are many other examples of electrocyclic ring opening of cyclobutanes, and cis- and fra 5-3,4-dichlorocyclobutene have been examined carefully. The products are those expected for conrotation. In the case of the franx-isomer, the product results from outward rotation of both chlorine atoms, in agreement with the calculated substituent effect. The c/x-isomer, in which one of the chlorines must rotate inward, has a substantially higher E.  

A particularly interesting case involves the bicyclo[2.2.0]hexa-2,5-diene system. This ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. Attempts prior to 1960 to prepare Dewar benzene derivatives failed, and the pessimistic opinion was that such efforts would be fruitless because Dewar benzene would be so unstable as to immediately revert to benzene. Then in 1962, van Tamelen and Pappas isolated a stable Dewar benzene derivative 9 by photolysis of l,2,4-tri-(/-butyl)benzene. The compound was reasonably stable, reverting to the aromatic starting material only on heating. Part of the stability of this particular derivative can be attributed to steric factors. The /-butyl groups are farther apart in the Dewar benzene structure than in the aromatic structure. 

van Tamelen. S. P. Pappas, and K. L. Kirk, J. Am. Chem. Soc., 93, 6092 (1971) this paper contains references to die initial work and describes subsequent studies. 

This compound is less stable than 9 and reverts to benzene with a half-life of about 2 days at 25°C, with A/f = 23 kcal/mol. Nevertheless, the relative kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. Furthermore, the central bond is not only strained but also /x-allylic. The kinetic stability of Dewar benzene is related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway would 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,E-cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. 

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The concepts of electron-transfer catalysis and so-called hole-catalysis [1] are closely related. It is now generally accepted that many organic reactions that are slow for the neutral reaction system proceed very much more easily in the radical cation. Although hole-catalysis is now well documented experimentally [2], there is surprisingly little mention of the corresponding reductive process, in which a reaction is accelerated by addition of an electron to the reacting system. Although the concept of electron-catalysis is not as well known as hole-catalysis, there are experimental examples of electrocyclic reactions that proceed rapidly in the radical anion, but slowly or not at all in the neutral system [3], For reasons that will be outlined below, we can expect that, in many cases, difficult or forbidden closed-shell reactions will be very much easier if an unpaired electron is introduced into the system by one-electron oxidation or reduction. Thus, if a neutral reaction A - B proceeds slowly or not at all, the radical cation (A" -> B" ) or radical anion (A" B" ) may be facile... 

A beautiful example of electrocyclic reactions at work is provided by the chemistry of the endiandric acids. This family of natural products, of which endiandric acid D is one of the simplest, is remarkable in being racemic—most chiral natural products are enantiomerically pure (or at least enantiomerically enriched) because they are made by enantiomerically pure enzymes (we discuss all this in Chapter 45). So it seemed that the endiandric acids were formed by non-enzymatic cyclization reactions, and in the early 1980s their Australian discoverer, Black, proposed that their biosynthesis might involve a series of electrocyclic reactions, starting from an acyclic polyene precursor. 

In contrast to cycloadditions, which almost invariably take place with a total of (An + 2) electrons, there are many examples of electrocyclic reactions taking place when the total number of electrons is a (An) number. However, those electrocyclic reactions with (An) electrons differ strikingly in their stereochemistry from those reactions mobilising (An + 2) electrons, as revealed when the parent systems are stereochemically labelled with substituents. The stereochemistry is not dependent upon the direction in which the reaction takes place, but it does depend upon whether there are (An) or (An + 2) electrons. 

Simple examples of electrocyclic reactions are the formation of cyclobutene from butadiene and cyclohexadiene from hexatriene ... 

A worked problem follows and several examples of electrocyclic reactions occur in the problems at the end of the chapter. The important principle is that for 4n electrons, conrotation will give the favoured Mobius transition state, whereas for An+ 2 electrons, disrotation will give the favoured Htickel transition state. 

Two examples of electrocyclic reactions for synthesis of carbocycles were reported. Diels-Alder reaction of D-glucose-derived diene 20 with quinone 21 gave 22, which epimerized on base catalysis to the trans-fused analogue. Levoglucosenone-derived enediene 23, was elaborated to triene 25 (via 24), thermal [3+3] rearrangement giving cyclohexadiene 26, then converted to 27, an... 

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. 

In electrocyclic reactions as already mentioned there is ring-opening or ring-closure of polyenes. In these reactions an open chain conjugated olefinic system with nn-electrons undergoes ring-closure to a system with (n -2) n-electrons + one o-bond and converse of this process is also feasible under changed reaction conditions. Some examples of electrocyclic reactions are cited below ... 

Although 2,8, etc., electron electrocyclic reactions are less common, they do exist, so it is useful to summarize the stereochemical outcomes quite generally (Table 18.1). You don t need to memorize the whole of this table—if you remember just one entry, and the fact that 4h and 4 + 2 are different, and thermal and photochemical are different, then you have it all. Some examples of electrocyclic reactions are given in Figure 18.34. 

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