Functional groups by hydrogen

The reduction of stereochemical complexity can frequently be effected by stereoselective transforms which are not disconnective of skeletal bonds. Whenever such, transforms also result in the replacement of functional groups by hydrogen they are even more simplifying. Transforms which remove FG s in the retrosynthetic direction without removal of stereocenters constitute another structurally simplifying group. Chart 3 presents a sampling of FG- and/or stereocenter-removing transforms most of which are not disconnective of skeleton. 

The requirements for replacing a functional group by hydrogen may be summarised, in the simplest terms, as two electrons and one proton, according to the general expression ... 

This section lists examples of the replacement of miscellaneous functional groups by hydrogen (RX — RH)... 

Until the second half of the twentieth century, the structure of a substance—a newly discovered natural product, for example—was determined using information obtained from chemical reactions. This information included the identification of functional groups by chemical tests, along with the results of experiments in which the substance was broken down into smaller, more readily identifiable fragments. Typical of this approach is the demonstration of the presence of a double bond in an alkene by catalytic hydrogenation and subsequent determination of its location by ozonolysis. After-considering all the available chemical evidence, the chemist proposed a candidate structure (or structures) consistent with the observations. Proof of structure was provided either by converting the substance to some already known compound or by an independent synthesis. 

It is interesting to compare the parameters exhibited by the cavitands with and without the functional group improving hydrogen bonding. The effect of the functionalization is expected to increase the parameter a this effectively happens for 1 and 2. The stronger effect of polarization is instead observed for the polarization term. This explains the... 

FIGURE 3.5 Deprotection of functional groups by acidolysis.5 Protonation followed by car-bocation formation during the removal of benzyl-based protectors by hydrogen bromide. Two mechanisms are involved in generating benzyl bromide from the protonated substrates. 

FIGURE 3.6 Deprotection of functional groups by acidolysis. Protonation followed by carbocation formation during the removal of ferf-butyl-based protectors by hydrogen chloride.8 One mechanism is involved in generating the ferf-butyl cation, which is the precursor of two other molecules. 

The use of free-radical reactions in organic synthesis started with the reduction of functional groups. The purpose of this chapter is to give an overview of the relevance of silanes as efficient and effective sources for facile hydrogen atom transfer by radical chain processes. A number of reviews [1-7] have described some specific areas in detail. Reaction (4.1) represents the reduction of a functional group by silicon hydride which, in order to be a radical chain process, has to be associated with initiation, propagation and termination steps of the radical species. Scheme 4.1 illustrates the insertion of Reaction (4.1) in a radical chain process. 

These difficulties in the synthesis of Si-functional carbosilanes were resolved by applying the Si-phenyl group as a protective group once the mechanism of the cleavage of the Si-phenyl group by hydrogen halides was known8. The reaction proceeds as follows ... 

Figures 4 and 5 present the difference between the DTG curves obtained for samples after adsorption of acetaldehyde at saturation conditions and the initial ones. Analysis of the shapes of the curves indicates that at least two types of interactions exist. They are represented by two peaks, which partially overlap. The first one, centered about 373 K likely represents weak interactions of acetaldehyde with functional groups via hydrogen bonding. The presence of mesopores on the surface results also in such a weak adsorption since these interactions are not energetically favorable. Moreover, the surface acidic sites present mainly on the unmodified and...

Crabtree s catalyst, [Ir(COD)(PCy3)(Py)]PF6, is quite inert to other functional groups under hydrogenation conditions, as illustrated by the reduction of the enone (46), in which the gem-dibromocyclopro-pane survives. ... 

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