Reactions with group-0 cations

In addition to its interesting structure, the triethylsilylium-aromatic complex has proved useful in preparing other cations. Reaction with 1,1-diphenylethylene, for example, provided the cation 95, the first example of a persistent p-silyl substituted carbocation (i.e., where decomposition by loss of the silyl group did not occur). 

Lehn synthesised guanidinium-based cationic steroids incorporating an acylhy-drazone linker using the approach shown in Fig. 9 [141]. The synthesis was developed from a polyamine scaffold by guanidination of the primary amino groups and alkylation of the secondary amine with methyl chloroacetate to introduce the ester moiety required to form a hydrazide group by reaction with hydrazine monohydrate. Cationic steroid hydrazones were then prepared via an acetic acid catalysed reaction with cholestanones, which demonstrated high transfection efficiency and low toxicity in a variety of cell lines [141]. 

Iron complexes can also catalyze allylic amination [31,32]. Enders et al. have demonstrated the nucleophilic addition of various acyclic and cyclic amines to the optically active l-methoxycarbonyl-3-methyl-(T)3-allyl)-tetracarbonyliron cation 49 formed in high yield from reaction of 48 with iron carbonyls. Oxidative removal of the tetracarbonyliron group by reaction with CAN gives 50 with high optical purity and retention of the stereochemistry (Eq. (12)) [31]. The reaction proceeds well for the different amines, and has been used for the synthesis of a compound showing cytotoxic activity against diverse cell lines [31b]. 

Another method consists of the double condensation of 3-chloro- or 3-methylthio-l,2-dithiolylium cations with a carbonyl compound. In the structure thus obtained, the carbonyl group is afterwards transformed into a thiocarbonyl group by reaction with phosphorus pentasulfide. 

One route to weak cation exchangers based on styrenic resins also starts from the chloromethylated resin. The chloro group is converted to a cyano group by reaction with KCN, and the cyano group is then hydrolyzed to a carboxylate. Exchangers based on this chemistry are rarely used in HPLC. A more convenient route to weak cation exchangers is through acrylic add derivatives. 

A cationic r -furfuryl complex of palladium (33) has been prepared from CMF (Scheme 12), which has the potential to activate the methyl group towards reactions with various nucleophiles [138]. 

A second common reaction theme of a carbonyl group is reaction with a proton or another Lewis acid to form a resonance-stabilized cation. Protonation increases the electron deficiency of the carbonyl carbon and makes it more reactive toward nucleophiles. The reaction is followed by removal of a proton to give a tetrahedral carbonyl addition compound. 

The electron-releasing R group helps stabilize this cation. As with anionic polymerization, the separation of the ions and hence the ease of monomer insertion depends on the reaction medium. The propagation reaction may be written as... 

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

A reactive dye for ceUulose contains a chemical group that reacts with ionized hydroxyl ions in the ceUulose to form a covalent bond. When alkaH is added to a dyebath containing ceUulose and a reactive dye, ionization of ceUulose and the reaction between dye and fiber is initiated. As this destroys the equihbrium more dye is then absorbed by the fiber in order to re-estabUsh the equUibrium between active dye in the dyebath and fiber phases. At the same time the addition of extra cations, eg, Na+ from using Na2C02 as alkaH, has the same effect as adding extra salt to a direct dye. Thus the addition of alkaH produces a secondary exhaustion. 

The major difference in reactivity between CF3OF and FCIO3 lies in the capacity of the former to react with olefins without the benefit of an electron releasing group and even with electron deficient olefins such as a,y5-un-saturated ketones. Reactions with nonactivated double bonds indicate the presence of an oc-fluoro cationic intermediate [e.g., (64)] as exemplified by the reaction with the -3-ketone (63), which yields the fluorophenol (65). 

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