Incipient double bond

Much evidence supports the conclusion that the elimination of the group HX from alkyl halides by bases is a trans elimination reaction. This means that the atoms H and X leave from the opposite site of the incipient double bond. It is mostly explained by assuming that the electrons which are left by the leaving proton and which will form the double bond prefer to attack the leaving group X from the rear (50). The transition state for the elimination, if it is concerted, is most stable if H, X, and the carbon atoms 1 and 2 lie on one plane, which in most molecules is best realized in the trans position (51). ... 

The E2 reaction occurs most easily if the molecule undergoing reaction can assume a conformation, 2, in which the leaving groups, H and X, are trans to each other and the atoms H—C3—Ca—X lie in one plane. Elimination then proceeds from opposite sides of the incipient double bond to give an alkene of structure 3. We shall call this mode of elimination antarafacial to distinguish... 

There was a tendency for the rate to be increased by substituents that can conjugate with the incipient double bond of the carbonyl group (Nos. 2, 5, 9, 11-15, 17, 19). 

The influence of an alpha methyl substituent is much less pronounced on the rate of gas-phase thermolysis of esters than of halides but it is still quite marked, suggesting a certain amount of C -0 bond heterolysis occurs in the transition state. On the other hand, beta methyl substitution has a slight retarding effect in the primary series but a slightly rate enhancing influence for secondary and tertiary acetates ". Possibly double-bond character is more developed in the transition states in the secondary and tertiary series and the mesomeric influence of beta methyl substituents stabilises the transition state. In the primary series, double-bond character is less developed and a balance between mesomeric stabilisation of this more incipient double bond and unfavourable deactivation of beta hydrogen acidity by the inductive effect of the alkyl substituent may explain its observed influence on the rate coefficient. 

An important group of alkene-forming reactions, some of which are useful in synthesis, are pyrolytic eliminations. Included in this group are the pyrolyses of carboxylic esters and xanthates, of amine oxides, sulfoxides and selenoxides. These reactions take place in a concerted manner, by way of a cyclic transition state and therefore proceed with syn stereochemistry, such that the hydrogen atom and the leaving group depart from the same side of the incipient double bond (in contrast to the eliminations discussed in Section 2.1) (2.14). 

In this section, only enediyne antibiotics bearing an epoxy moiety will be treated. The enediynes are a class of natural products that consist of a unique molecular architecture and exhibit phenomenal antitumor activity. They are stmcturally characterized by an enediyne core containing two acetylenic groups conjugated to a double bond or incipient double bond within a nine-membered ring, such as neocarzinos-tatin 301, or a 10-membered ring, such as dynemicin A 304a. The enediyne cores... 

Cram has studied the elimination from threo (equation 9) and 1,2-diphenyl-1-propyltrimethylammonium iodides (equation 10). Since the 2 mechamsm for elimination requires a trans-coplanar arrangement of the /3-proton, the carbon atoms of the incipient double bond and the ammonium ion in the transition state, these isomers should lead to different alkene isomers if the reaction follows the E2 path. With ethoxide ion as base in ethanol, trans elimination was observed for both isomers and a rate ratio (kthreol erythro) of 57 was calculated. This is in accord with the steric factors which hinder the... 

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