Pyrolysis esters

Ester Pyrolysis.If the structural components required for syn-elimination in the Hofmann and Cope elimination could be modified to use more readily available synthetic precursors, the reaction might have wider applicability. One possibility is to replace the amino group with an oxygen moiety so there is a 

Chapter 2. Acids, Bases, Functional Group Exchanges Me 

The importance of the syn relationship of hydrogen and the leaving group is seen in hydrindanes 227 and 228, which produced 229 upon pyrolysis. The cis-derivative (227) required temperatures at least 200°C higher than the trans-acetate (228).209b synthetic example that involves ester pyrolysis is taken from Fallis synthesis of (-t-)-longifolene (230), formed in 56% yield by pyrolysis of 231. 10 

Carboxylic esters 1, having an O-alkyl group with a /3-hydrogen, can be cleaved thermally into the corresponding carboxylic acid 2 and an alkene 3. This reaction often is carried out in the gas phase generally the use of a solvent is not necessary. 

The reaction proceeds by an Ei-mechanism. The /3-hydrogen and the carboxy-late are cleaved synchronously from the substrate molecule, while forming a new bond. This elimination reaction belongs to the class of -eliminations in the case of the ester pyrolysis, the substrate molecule passes through a six-membered cyclic transition state 4  

For the formation of the new double bond, the general rules for eliminations do apply. Following Bredt s rule, no double bond to a bridgehead carbon atom will be formed. If the elimination can lead to a conjugated system of unsaturated groups, this pathway will be favored. Otherwise the Hofmann rule will be followed, which favors an elimination towards the less substituted carbon center. 

With cyclic substrates, the formation of the new double bond depends on the availability of a c i -/3-hydrogen, which is required for the yw-elimination 

In the case of the cyclohexane derivative 7 however, that bears an equatorial acetate group, two axial cis-/3-hydrogens are available, and elimination in both directions is possible. The pyrolysis of 7 yields the two elimination products 8 and 6. Formation of product 8 is strongly favored, because the new double bond is in conjugation to the ester carbonyl group.  

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The thermolysis (pyrolysis) is generally carried out at temperatures ranging from 100-250 °C. Similar to the closely related ester pyrolysis the reaction mechanism is of the Ei-type, which involves a six-membered cyclic transition state 7 ... 

This mechanism of a -elimination reaction is supported by experimental findings with " S- and C-labeled starting materials." The Chugaev reaction is analogous to the ester pyrolysis, but allows for milder reaction conditions—i.e. it occurs at lower temperatures. It is less prone to side reactions, e.g. the formation of rearranged products, and is therefore the preferred method. 

Similar to the Ester pyrolysis the mechanism is formulated to proceed by a Ej-mechanism however in this case via a five-membered transition state 5 ... 

For the regioselectivity similar rules as for the ester pyrolysis do apply. With simple, alkylsubstituted amine oxides a statistical mixture of regioisomeric olefins is obtained. On the other hand with cycloalkyl amine oxides the regioselectivity is determined by the ability to pass through a planar, five-membered transition state. This has been demonstrated for the elimination reaction of menthyl dimethylamine oxide 10 and neomenthyl dimethylamine oxide 11 ... 

Rearrangements and other side-reactions are rare. The ester pyrolysis is therefore of some synthetic value, and is used instead of the dehydration of the corresponding alcohol. The experimental procedure is simple, and yields are generally high. Numerous alkenes have been prepared by this route for the first time. For the preparation of higher alkenes (> Cio), the pyrolysis of the corresponding alcohol in the presence of acetic anhydride may be the preferable method." The pyrolysis of lactones 9 leads to unsaturated carboxylic acids 10 ... 

Ester pyrolysis has been shown to be a syn elimination in the case of formation of stilbene by the use of deuterium labels.335... 

McFadyen-Stevens reaction, 13 571 McGaskell rotary pressure filter, 11 374 McLafferty ester pyrolysis reaction, 20 43 MCLs (maximum contaminant levels), 17 804... 

Buxbaum 61 ester pyrolysis no free radical reactions Pyrolysis, oxidation, hydrolysis, radiation No 280 ... 

The pyrone ring then acts as the dienophile in the intramolecular Diels-Alder cycloaddition, which was conducted in a microwave oven. The final step of this synthesis is a high-temperature ester pyrolysis to introduce the exocyclic double bond of longifolene. 

Ester Pyrolysis is a syn-elimination yielding an alkene, similar to the Cope Elimination, for which -hydrogens are needed. The carboxylic acid corresponding to the ester is a byproduct.The cyclic transition state can only be achieved if the steric environment is not too demanding. 

Ester Pyrolysis also obeys this preference, and the Hofmann Rule is generally followed whenever a reaction passes through a cyclic transition state. 

C is a primary alcohol and is quite prone to undergo Carbon skeleton rearrangement when treated with hoi acid. To obviate the carbon skeleton rearrangement and effect the dehydration so that B is the major product, it is suggested that the acetate ester of C be formed and pyrolyzed as shown below. The acetate ester pyrolysis is known to yield olefins in which there is no carbon skeleton rearrangement. 

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