Leaving group, in elimination reactions

Effect on Reactivity. The leaving groups in elimination reactions are similar to those in nucleophilic substitution. The E2 eliminations have been performed with the following groups NRj, PR, SRj, OHR", SO2R, OSO2R,... 

The role of the leaving group in elimination reactions can, in general, be looked at in a very similar way as in SN reactions. As illustrated by the relative amounts of elimination products formed by the base-catalyzed reactions of the pesticide 1,2-dibro-mo-3-chloropropane (DBCP, Eq. 13-10), bromide is a better leaving group than chloride (Burlinson et al., 1982) ... 

Ejfect on Reactivity. The leaving groups in elimination reactions are similar to... 

When ArNH2 is o-phenylenediamine (80), the reaction is poorly catalysed by the second amino group, but it is mainly catalysed by an external molecule of amine. As a consequence, internal catalysis by an intramolecular complex such as 81 is unlikely. In competition with the substitution (Scheme 34), when the nucleophile (or a base) attacks a hydrogen atom in a fi position with respect to the leaving group, an elimination reaction takes place. 

The number of reactions in this chapter may seem overwhelming at first. The key to success is to remember that nucleophiles react with electrophiles. If you can identify the nucleophile or base and the electrophilic carbon (the one bonded to the leaving group) in each reaction and recall the factors that affect the competition between the two substitution mechanisms and the two elimination mechanisms, the material you have to learn will be much more manageable. 

The leaving groups commonly employed in E2 reactions are listed in Table 2.1. As you can see, they are essentially the same as those displaced in nucleophilic substitution reactions (see Part 2), with two exceptions. First, protonated alcohols are not listed as substrates RX in Table 2.1, because they usually react by the El mechanism (as we shall see later) rather than the E2 mechanism. Secondly, the trimethylammonium and dimethylsulfonium groups have limited importance as leaving groups in substitution reactions, although they are particularly important in elimination reactions. In fact, the reaction involving trimethylammonium is known, after its discoverer, as the Hofmann elimination... 

When complex organic systems have evolved in nature, they are frequently made by the combination of carboxy carbonyl groups and the heteroatoms of alcohols, amines, or thiols. Why The addition-elimination processes introduced in Chapter 19 provide mechanistic pathways of relatively low activation energy for the interconversion of variously substituted carboxylic acid derivatives, many of which play central roles in biology (Chapter 26). Chemists find these compounds similarly useful, as we shall see in this chapter, which deals with the chemistry of four major carboxylic acid derivatives halides, anhydrides, esters, and amides. Each has a substituent, L, that can function as a leaving group in substitution reactions. We already know, for instance, that displace-O... 

The NR3 group in a quaternary amine, R —NRj, is a good leaving group in E2 reactions this enables the Hofmann elimination to take place. 

The Williamson ether synthesis (Sec tion 16 6) An alkoxide ion displaces a halide or similar leaving group in an Sn2 reaction The alkyl halide cannot be one that is prone to elimination and so this reaction is limited to methyl and primary alkyl halides There is no limitation on the alkoxide ion that can be used... 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

Fluoride-induced /S-elimination reactions of silanes having leaving groups in the position are important processes in synthetic chemistry, as, for, example in the removal of / -trimethylsilylethoxy groups. 

It s this ability to drive otherwise unfavorable phosphorylation reactions that makes ATP so useful. The resultant phosphates are much more reactive as leaving groups in nucleophilic substitutions and eliminations than the corresponding alcohols they re derived from and are therefore more likely to be chemically useful. 

ElcB reaction (Section 11.10) A unimolecular elimination reaction in which a proton is first removed to give a car-banion intermediate, which then expels the leaving group in a separate step. 

---