Polyenes electrocyclic reactions

Conjugated polyene, electrocyclic reactions of, 1181-1186 molecular orbitals of, 1179-1180 Conjugated tricne, electrocyclic reactions of, 1182 Conjugation, 482... 

To summarize, for a linear polyene with 4, 8, 12,. ..Jt electrons involving in its thermal electrocyclic reaction, the reaction will follow a conrotatory pathway conversely, its photochemical electrocyclic reaction will adopt a disrotatory pathway. On the other hand, for a linear polyene electrocyclic reaction with 6, 10, 14,. .. r electrons involved, the pathways of its thermal and photochemical electrocyclic reactions will be disrotatory and conrotatory, respectively. 

When we recall the symmetry patterns for linear polyenes that were discussed in Chapter 1 (see p. 33), we can fiorther generalize the predictions based on the symmetry of the polyene HOMO. Systems with 4 n electrons will undergo electrocyclic reactions by conrotatoiy motion, whereas systems with 4 4- 2 n electrons will react by the disrotatoiy mode. 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

This type of reaction, whether it involves the cyclisation of a polyene, as here, or the ring-opening of a cyclic compound to form a polyene, is known as an electrocyclic reaction. 

Electrocyclic reactions were first described by Woodward and Hoffmann in their classic series of articles. One very interesting aspect of such reactions is, that for a given conjugated polyene photochemical transformation leads to the opposite stereochemical outcome than the thermal one314). 

This technique applies to many open-chain compounds, as discussed in later chapters. Pertinent here is the intramolecular cyclization of polyenes (an electrocyclic reaction). 

In electrocyclic reactions of conjugated polyenes, one double bond is lost and a single bond is formed between the terminal C s to give a ring. The reaction is reversible. 

With this model, we need only apply the method already used to derive the selection rules for electrocyclic reactions (p. 53). From the Coulson equations, we can deduce that in the in conrotatory cyclization of pentadiene, the MO generates a destabilizing C5-C4 secondary interaction, a stabilizing and Fg a destabilizing interaction. The absolute values of these contributions rise steadily because the terminal coefficients increase from Fg to Fg. Therefore, the sign of their sum is given by the HOMO contribution. If R is an attractor, the HOMO is Fg and rotation inwards is favored. If R is a donor, the HOMO is 4T and rotation inwards is disfavored. As the Coulson equations are valid only for polyenes, these conclusions are correct insofar as R can be modeled by a carbon 2p orbital. It follows that the Rondan-Houk theory works better for conjugative than for saturated substituents. 

An electrocyclic reaction is the formation of a new a bond across the ends of a conjugated polyene or.the reverse... 

This rotation is the reason why you must carefully distinguish electrocyclic reactions from all other pericyclic reactions. In cycloadditions and sigmatropic rearrangements there are small rotations as bond angles adjust from 109° to 120° and vice versa, but in electrocyclic reactions, rotations of nearly 90° are required as a planar polyene becomes a ring, or vice versa. These rules follow directly from application of the Woodward-Hoffmann rules—you can check this for yourself. 

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. 

Electrocyclic reactions of conjugated polyenes create chiral molecules through stereospecific conrotatory or disrotatory processes. In solution, the two enantiom-... 

The stereochemistry of an electrocyclic reaction is determined by the symmetry of the polyene HOMO. 

In general, polyenes with odd numbers of double bonds undergo disrotatory thermal electrocyclic reactions, and polyenes with even numbers of double bonds undergo conrotatory thermal electrocyclic reactions. 

It turns out that there is an alternating relationship between the number of electron pairs (double bonds) undei going bond leorganization and the stereochemistry of ring opening or closure. Polyenes with an even number of electron pairs undergo thermal electrocyclic reactions in a conrotatory sense, whereas polyenes with an odd number of electron pairs undergo the same reactions In a disrotatory sense. 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

To apply the selection rules for electrocyclic reactions, count the number of tt electrons in the open-chain polyene. 

The frontier orbital analysis of electrocyclic reactions focuses on the HOMO of the open-chain polyene. 

---