Classes of directing group

In his review of 1990,104 Snieckus divided the typical ortholithiation-directing groups into classes, according to the ease with which they may be lithiated and the practicalities of their application in synthesis. It is clear that in the meantime certain metallation-directing groups have remained little more than curiosities, while others have become widely used. In this section, we shall survey metallation-directing groups in broad assemblies which share certain features. 

The most powerful classes, for several reasons, comprise carboxylic acid and carbonate-derived functions containing both nitrogen and oxygen secondary and tertiary amides,104 oxazolines,140102 and carbamates.104 We have grouped these functional groups into the N+O class . Their importance stems firstly from their amphoteric nature they are all functional 

Sulfones and sulfonamides - the S+O class are similarly powerful directors,141 and do not suffer electrophilic attack at sulfur (though occasionally suffer nucleophilic attack on the aromatic ring)134143 but are less useful because of their more limited synthetic applications.144 143 Aryl J-butylsulfoxides are also powerful ortho-directors,145 but less hindered diaryl sulfoxides are susceptible to attack by organolithiums at sulfur146 (see section 3.3.3 for discussion of this sulfur-lithium exchange ). 

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Nitro dyes are the most important class of direct hair dyes they are substituted derivatives of nitrobenzene or nitrodiphenylamine [3, Nitro and Nitroso Dyes]. By proper selection of donor groups and substitution site on the benzene ring, a spectrum of dyes from yellow to blue violet can be prepared [9, pp. 247-250], [40] (Scheme 3). 

Of late a new class of direct-dyeing cotton-colours has been introduced. These contain one or more free amido-groups capable of diazotisation, and are therefore suitable for direct production of azo-dyes on the fibre. Amongst these dyestuffs are Diamine Blue-Black E, Diamine Black B and R (see Table). These dyestuffs are combinations of diazotised bases of the Benzidine series with one or two molecules of an amidonaphtholsulphonic acid, and these amido-groups, in the naphthol compound remaining intact, may subsequently be diazotised. 

Direct heterolytic fission of the substrate-photoprotecting group bond is the required course for photorelease of most biologically important substrates. This process avoids the generation of destructive radicals that could result in reactions such as decarboxylation, radical dimerization, or redox processes. Thus, the effect of m-substitution on the photochemistry of benzyl, naphthyl, and other aromatic chromophores has become the object of many studies in search of alternatives to the o-nitrobenzyl class of protecting group. 

Most methods for their preparation convert one class of carboxylic acid derivative to another and the order of carbonyl group stabilization given m Figure 20 1 bears directly on the means by which these transformations may be achieved A reaction that converts one carboxylic acid derivative to another that lies below it m the figure is pracfical a reacfion fhaf converts if fo one fhaf lies above if is nol This is anofher way of saying fhaf one carboxylic acid derivative can be converted to another if the reaction leads to a more stabilized carbonyl group Numerous examples of reacfions of fhis fype will be pre senfed m fhe secfions fhaf follow... 

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