Choice of reagents

Selection of the right reagent for a reaction is made on the basis of efficiency, availability and its effect on environment. The selection of a particular reagent versus another reagent for the same transformation can effect the nature of by-products, percentage yield etc. (see Chapter 6 for details). 

Steroids possessing an epoxide grouping in the side chain have likewise been converted to fluorohydrins. Thus, 20-cyano-17,20-epoxides of structure (19) furnish the 17a-fluoro-20-ketones (20) after treatment of the intermediate cyanohydrins with boiling collidine. The epimeric 5a,6a 20,21-oxides (21) afford the expected bis-fluorohydrins (22). The reaction of the allylic 

Reaction of Epoxides with Boron Trifluoride-Etherate 

The boron trifluoride-ether complex has been employed mainly in the opening of 5,6-epoxides. This reaction was first studied by Henbest and Wrigley and affords products depending on the nature and stereochemistry of the 

An alternative to fluorohydrin formation is observed with the 6/ -methyl-5a,6a-epoxide (30), which rearranges, possibly in a concerted reaction, to the A-homo-B-norsteroid (31) (cf, chapter 14, Vol. II). 

In the case of the 3a-acetoxy-5a,6a-epoxide (33), the electronic and conformational factors are opposed to each other. Thus, the energy of the system is lowered when the axial 3a-acetoxy group is permitted to become 

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Selective cleavage to give either the more or less substituted derivativi possible with the proper choice of reagents. 

Alkyl mercuric hydrides are generated in situ by reduction of an alkyl mercuric salt with sodium borohydridc (Scheme 3.91). Their use as radical traps was first reported by Hill and Whitesides491 and developed for the study of radical-olefin reactions by Giese,489490 Tirrell492 and coworkers. Careful choice of reagents and conditions provides excellent yields of adducts of nucleophilic radicals (e.g. -hexyl, cyclohexyl, /-butyl, alkoxyalkyl) to electron-deficient monomers (e.g. acrylics). 

Both trans-l amino-2,3-diphenylaziridine and l-amino-2-phenylazi-ridine give a,/S-epoxyhydrazones that fragment in the desired manner between 100° and 200°, the choice of reagent being dictated by the ease of separation of the alkynone from the by-products, (F/)-stilbene and styrene, respectively. The diphenylaziridine is especially useful when the alkynone is relatively volatile and easily separable by distillation from (E)-stilbene, as is the case in the present example. The phenylaziridine... 

Reduction of aromatic rings with lithium or calcium " in amines (instead of ammonia—called Benkeser reduction) proceeds further and cyclohexenes are obtained. It is thus possible to reduce a benzene ring, by proper choice of reagent, so that one, two, or all three double bonds are reduced. Lithium triethylborohy-dride (LiBEtsH) has also been used, to reduce pyridine derivatives to piperidine derivatives." ... 

Fig. 10. Comparison of a typical homogeneous immunoassay (left) with a heterogeneous format (right). Depending on choice of reagents, there are many possible variations on these basic schemes...

Derivatives with various substituted sulfonamides have been developed and used to form enolates from esters and thioesters.137 An additional feature of this chiral auxiliary is the ability to select for syn or anti products, depending upon choice of reagents and reaction conditions. The reactions proceed through an acyclic TS, and diastereoselectivity is determined by whether the E- or Z-enolate is formed.138 /-Butyl esters give A-enolates and anti adducts, whereas phenylthiol esters give syn adducts.136... 

Indicate one or more satisfactory oxidants for effecting the following transformations. Each molecule poses issues of selectivity or the need to preserve a sensitive functional group. Select oxidants that can avoid the installation of protecting groups. In most cases, a one-pot reaction is possible, and in no case is a sequence of more than three steps required. Explain the reason for your choice of reagent(s). 

Thus by suitable choice of reagent, the hydroxylation of alkenes can be made stereoselectively SYN or ANTI at will. 

The choice of reagent scheme depends largely on the type of natural ore to be treated. The three main groups of reagents used in beneficiation of oxide copper and copper cobalt ores include (a) sulphidizers, (b) collectors and (c) modifiers and depressants. 

Solid-solid (S-S) systems are now being investigated in view of eliminating solvents from chemical reactions. Here the paradigm is the best solvent is no solvent. Just mixing two solids can often lead efficiently and cleanly to a product however, there are limitations that are mainly due to the choice of reagents and to mass and heat transport. ... 

The choice of reagent depends very much on the nature of the feed, but the introduction of organophosphinic acids either alone or in combination with carboxylic acids in a synergistic mixture offer great potential for commercial development. 

Hexamethylbiguanide (CLXIX) reacts additively in acetonitrile with a,P Unsaturated compounds activated by an electron-withdrawing group (e.g. with vinyl cyanide) to yield saturated adducts (CLXX). By a suitable choice of reagents, adducts bearing substituents other than the nitrile group (e.g. carbamoyl, carbalkoxy) are accessible 213) ... 

The choice of reagent determines whether a nitrosamine undergoes conversion to a nitramine by either nitrolysis or oxidation. An example is given for the conversion of 1,3,5-trinitroso-1,3,5-triazacyclohexane (109) to l,3,5-trinitro-l,3,5-triazacyclohexane (3) (RDX) - the use of 30 % hydrogen peroxide in 99 % nitric acid at subambient temperature goes via oxidation of the nitrosamine functionality, whereas dinitrogen pentoxide in pure nitric acid makes use of a nitrolysis pathway via C-N bond cleavage. 

As hydrogen fluoride functions with equal ease in alkylation with olefins, alkyl halides, or alcohols, and in acylation with acids, acid anhydrides as well as acyl halides, a wide choice of reagents is possible and a separate operation of the reconversion of them is often saved. With aluminum chloride the alkyl halides and acyl halides are the preferred reagents and frequently must be made from more plentiful, cheaper, and readily available substances. 

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