Elimination—addition pyrolytic

A recent pnblication demonstrated the bioconversion of ricinoleate to undecylenic add and to sebacic acid, eliminating the pyrolytic step or the alkali fusion, respectively, as synthetic steps to produce these useM monomers (Song et al., 2014). Indnstrial implementation of this approach conld resnlt in lower energy input and additional environmental benefit from prodnction and utiUza-tion of these castor oil-based polymers. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

The possibility that reacting species prefer to react along those paths in which they undergo the least modification has always been intuitively attractive. At one time or another, so-called principles of minimum structural change or deformation, configurational change, and minimum atomic and electronic motion have been invoked (Wheland, 1960 Hine, 1966). To account for Michael s rule of favored anti 1,2-addition, Pfeiffer formulated acetylenes as tram-heat structures in 1904 Frankland (1912) suggested that anti elimination is favored by an inherent tendency to centric symmetry. The more conscious applications of PLM by Muller after 1886, are probably misapplications of the principle, since they were usually concerned with complex pyrolytic reactions above 1000° (Muller and Peytral, 1924). 

The slopes of the lines obtained from the Taft correlations of aliphatic primary, secondary and tertiary chlorides obtained at different temperatures by the extrapolation p%2lP%i T IT2 indicate that the positive nature at the carbon reaction center of the C—Cl bond in the transition state increases from a primary to a tertiary carbon atom (Table 8)70. An additional fact is that for each type of alkyl halide, the degree of positive charge at the carbon reaction center tends to decrease as the temperature increases. This means that the pyrolytic eliminations tend to be more concerted and less polar at very high temperatures. These data support Maccoll s theory on the heterolytic character of the alkyl halides pyrol-yses in the gas phase1. 

Pyrolytic reactions can appear to be much more complicated compared to other reactions. However, this is mainly due to subsequent reactions that occur after the initial elimination step. A common cause of this problem is related to the fact that the reactions do not actually take place in ideal gas phase. Some pyrolytic processes may take place in true condensed phase. Multiple reaction paths and the interaction of the resulting molecules are, therefore, inevitable. Also, additional issues may affect the practical results of a pyrolysis. Some are related to the fact that the true pyrolysis can be associated with reactions caused by the presence (intentional or not) of non-inert gases such as oxygen or hydrogen that may be present during the heating. Also, the pyrolyzed materials may be in contact with non-inert surfaces that can have catalytic effects. In order to diminish these effects in the pyrolysis done for analytical purposes, an inert gas frequently is present during the pyrol ic reaction. 

Pyrolytic reactions often occur in the gas phase, without the addition of another reagent. For example, heating carboxylate esters readily produces an alkene and a carboxylic acid. There is a requirement for a c/s-(3-hydrogen in order for this reaction pathway to proceed, because the reaction proceeds via a six-membered cyclic transition state. This cyclic pyrolytic elimination is labelled the Ei mechanism, which stands for intramolecular, or internal, elimination. In those versions of the Ei mechanism that involve a four- or five-membered cyclic transition state, there is a requirement that all the atoms are co-planar or virtually so. 

In an alternative approach to (83) Danishefsky and his co-workers have achieved what is effectively the same transformation by reaction of the diene (81) directly with dienophiles of the general type (87) (R = H or alkyl, R = alkyl or O-alkyl). Pyrolytic elimination of the phenylsulphinyl group occurs during the addition, and subsequent hydrolysis then furnishes thb dienone (83). 

In principle (and in some cases, in practice), gas-phase and low-temperature (ca. 5K), elimination reactions can be photochemically induced. Such processes are of restricted utility and limited application. Some gas-phase reactions can also be induced thermally. While pyrolytic decomposition of halides has been investigated on some small molecules in detail, most elimination studies have been carried out in solution where the loss of HX or X2 (X = F, Cl, Br, I) with the introduction of unsaturation is generally effected, for the former, by the addition of base and, for the latter, by using suitably reactive metals. 

Without additional reagents Coupled pyrolytic elimination of carboxylic acid esters... 

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