Alkenes isohypsic transformations

As we have already shown, the creation of novel carbon-carbon bonds usually results in the formation of functionalized derivatives (the Wurtz coupling and Friedel-Crafts alkylation are probably the only exceptions). That is why a set of special reductive methods was devised to remove residual functionality that is unwanted in the final structure. The well-known hydrogenation of alkenes and alkynes belongs to this group of non-isohypsic transformations. Several other pathways available for the reductive removal of various functions will briefly be considered below. 

As we have seen above, alcohols and alkenes are produced routinely in the numerous reactions utilized in the formation of carbon-carbon bonds. These two functionalities are extremely useful for both the same-level oxidation interconversions and reduction-oxidation transitions (as stairways connecting different floors ). Therefore, it is not surprising to see an enormous number of methods developed to capitalize on the diverse options of alkene and alcohol transformations. These methods occupy a key place among the reactions known for isohypsic conversions of level 1 functionalities. 

As was amply demonstrated in the preceding sections of this chapter, numerous C-C bond-forming reactions are applicable for the preparation of products with a terminal double bond. Thus the sequence (i) introduction of the terminal alkene moiety, (ii) isohypsic double bond transformation leading to the derivatives like 127 or 128, and (iii) the C-C bond-forming step, may be considered as a reliable operation for carbon chain elongation. 

The basic route to the synthesis of epoxides and, in general, vicinal bifunctional derivatives is a non-isohypsic (oxidative) transformation of alkenes (to be discussed below), Epoxides can also be formed directly as a result of certain C-C bond-forming reactions such as the Darzens reaction (modification of a classical aldol-like reaction with a-chloro esters 130 as a methylene component)... 

A thoughtful reader would have noticed that, while plenty of methods are available for the reductive transformation of functionalized moieties into the parent saturated fragments, we have not referred to the reverse synthetic transformations, namely oxidative transformations of the C-H bond in hydrocarbons. This is not a fortuitous omission. The point is that the introduction of functional substituents in an alkane fragment (in a real sequence, not in the course of retrosynthetic analysis) is a problem of formidable complexity. The nature of the difficulty is not the lack of appropriate reactions - they do exist, like the classical homolytic processes, chlorination, nitration, or oxidation. However, as is typical for organic molecules, there are many C-H bonds capable of participating in these reactions in an indiscriminate fashion and the result is a problem of selective functionalization at a chosen site of the saturated hydrocarbon. At the same time, it is comparatively easy to introduce, selectively, an additional functionality at the saturated center, provided some function is already present in the molecule. Examples of this type of non-isohypsic (oxidative) transformation are given by the allylic oxidation of alkenes by Se02 into respective a,/3-unsaturated aldehydes, or a-bromination of ketones or carboxylic acids, as well as allylic bromination of alkenes with NBS (Scheme 2.64). 

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