Reagent classes, properties

Throughout this book, and in association with the various reactions presented, various reagents were presented that, due to their specific properties, react in very specific ways. These reagents differ in their basicity, nucleophilicity, and preferred sites of reaction. Table 8.1 summarizes the various properties of the reagent classes presented. 

Reagent Class (Class Name) Examples Properties Uses... 

As noted earlier, the chloride ligand in Cp3 And (An = Th, U, Np) is labile and this feature has facilitated the synthesis of a great many tris(cyclopentadienyl)actinide(IV) alkyl derivatives by metathesis with alkyllithium or Grignard reagents. The properties and reactivity of these complexes have been extensively studied and reviewed. They exhibit remarkable thermal stability. As members of the general class of CpsAnX complexes, they adopt the ubiquitous pseudotetrahedral coordination geometry that was illustrated earlier in (2). 

If an unknown compound gives a positive test with the 2 4-dinitrophenylhydrazine reagent, it then becomes necessary to decide whether it is an aldehyde or a ketone. Although the dimedone reagent (Section 111,70,2) reacts only with aldehydes, it is hardly satisfactory for routine use in class reactions. It is much simpler to make use of three other reagents given below, the preparation and properties of which have already been described (Section 111,70). 

Separations based upon differences in the chemical properties of the components. Thus a mixture of toluene and anihne may be separated by extraction with dilute hydrochloric acid the aniline passes into the aqueous layer in the form of the salt, anihne hydrochloride, and may be recovered by neutralisation. Similarly, a mixture of phenol and toluene may be separated by treatment with dilute sodium hydroxide. The above examples are, of comse, simple apphcations of the fact that the various components fah into different solubihty groups (compare Section XI,5). Another example is the separation of a mixture of di-n-butyl ether and chlorobenzene concentrated sulphuric acid dissolves only the w-butyl other and it may be recovered from solution by dilution with water. With some classes of compounds, e.g., unsaturated compounds, concentrated sulphuric acid leads to polymerisation, sulphona-tion, etc., so that the original component cannot be recovered unchanged this solvent, therefore, possesses hmited apphcation. Phenols may be separated from acids (for example, o-cresol from benzoic acid) by a dilute solution of sodium bicarbonate the weakly acidic phenols (and also enols) are not converted into salts by this reagent and may be removed by ether extraction or by other means the acids pass into solution as the sodium salts and may be recovered after acidification. Aldehydes, e.g., benzaldehyde, may be separated from liquid hydrocarbons and other neutral, water-insoluble hquid compounds by shaking with a solution of sodium bisulphite the aldehyde forms a sohd bisulphite compound, which may be filtered off and decomposed with dilute acid or with sodium bicarbonate solution in order to recover the aldehyde. 

These 108 reagents are used in the Reactivity Charts that have been prepared for each class of protective groups. The reagents and some of their properties are described on the following pages. 

Quinones are an interesting and valuable class of compounds because of their oxidation-reduction, or redox, properties. They can be easily reduced to hydroquinones (g-dihydroxybenzenes) by reagents such as NaBH4 and SnCl2/ and hydroquinones can be easily reoxidized back to quinones by Fremy s salt. 

Another class of silicon-containing polymers that have great potential to be extremely useful precursor materials are poly(chlorocarbosilanes).14f 46 Poly (chlorocarbosilanes) are not useful without modification because of the rapid hydrolysis of Si—Cl bonds, forming HC1 and an insoluble crosslinked polymer network. However, nucleophilic substitution of these Si—Cl bonds with various reagents produces materials widi a broad range of properties that are determined by the nature of the nucleophile used.47 Poly(chlorocarbosilanes) can be easily synthesized by ADMET (Fig. 8.18) without any detrimental side reactions, since the Si—Cl bond is inert to both catalysts 12 and 14. Early studies produced a polymer with Mn = 3000.14f... 

Salts with di(alkynyl)aurate(i) anions are very common reagents for the preparation of other gold(i) complexes. A large number of examples with R taken from all sorts of organic functionalities have been prepared and characterized, mainly in a search for new classes with potentially useful photophysical properties (below). 

The simultaneous ionic and covalent character of quaternary ammonium compounds (197a) is central to most of their applications. TV-Quatemized heteroaromatic compounds possess many of the properties of 197a, and will be mentioned occasionaly in this chapter. In Table 4 are listed some quaternary ammonium compounds that have found industrial application. Many analytical methods make use of this class of compounds both as essential reagents or as accessories however, in the present chapter quaternary ammonium compounds will appear only as analytes. 

Enantioselective conjugate addition [40] has become truly useful with the aid of dialkylzinc, cationic copper catalyst, and a chiral ligand (Eq. 1, see also Chapt. 7) [41]. Magnesium-based reagents have found use in quantitative fivefold arylation of Cgo (Eq. 10.2) [42] and threefold arylation of C70 [43], paving ways to new classes of cyclopentadienyl and indenyl ligands with unusual chemical properties. 

The largest subfamily by far is class A. It comprises almost 90% of all GPCRs (1). Various family members have been subjected to detailed study at both the molecular and the structural levels (1). The definition of the properties of these receptors has resulted in the isolation of putative receptors (4-10) that have become reagents for drug discovery (10). 

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