Substitution reaction summary

A large number of nucleophilic substitution reactions involving interconversions of pyridopyrimidines have been reported, the majority of which involve substituents in the pyrimidine ring. This subject has been reviewed previously in an earlier volume in this series which dealt with the theoretical aspects of nucleophilic re-activiti in azines, and so only a summary of the nucelophilic displacements of the substituent groups will be given here. In general, nucleophilic substitutions occur most readily at the 4-position of pyrido-... 

Substituent effect, additivity of, 570 electrophilic aromatic substitution and, 560-563 summary of. 569 Substitution reaction, 138 Substrate (enzyme), 1041 Succinic acid, structure of, 753 Sucralose, structure of. 1006 sweetness of, 1005 Sucrose, molecular model of. 999 specific rotation of, 296 structure of, 999 sweetness of, 1005 Sugar, complex, 974 d, 980 L, 980... 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

Summary Several lithium l,3-diphospha-2-sila-allyl complexes 3a-f and the diphosphino-dichlorosilane 2 have been prepared and characterized. The hydrolysis and substitution reactions of these compounds are described yielding a number of phosphino- and diphosphino-silaphosphenes 5a-d, 4a,b and 6. The compounds have been characterized by NMR and by X-ray analyses in the cases of 2, 3a-c and 4a. 

A general summary of the chemistry of H4Os4(CO)12 is given in Scheme 10 (159, 166). Substitution reactions with phosphine, phosphites, or pyridine have been observed. An addition reaction occurs with I2 to yield the anionic species [H3Os4(CO)12I] The X-ray structure of this anion has established that the iodide is bridging between two metal centers, to yield an Os4 "butterfly configuration in which one of the metal- metal bonds of the initial Os4 tetrahedron has been broken (Os-Os = 3.817 A) (Fig. 38). Interestingly, if the two Os-1... 

A summary of the in situ use of the azobenzene probases is given in Table 2. Apart from the generation of ylid, referred to above, the main applications have been for N- and C-alkylation of weak nitrogen and carbon acids, for the promotion of condensation and substitution reactions involving carbanions such as the cyano-methyl anion, for an interesting carboxylation reaction (entries 4 and 17), and for base-promoted cyclisations (entries 5 and 6). 

It is well known that not all attempts to explain the reactivity of individual positions in electrophilic substitution reactions have been successful. There are three main lines along which attempts have been made to remove discrepancies between theory and experiment (for a summary, see ref. 147) (1) introduction into the HMO treatment of additional empirical parameters (inductive effect), (2) invoking the addition-elimination mechanism, and (3) invoking different reactivity of the protonated and unprotonated forms. 

In the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

Summary of Electrophilic Substitution Reactions of A -iV-Dimethylaniline and Acetanilide t ... 

Table 5—Summary of Electrophilic Substitution Reactions of Phenol, Diphenyl Ether, and Anisole... 

In summary, the data for a wide range of activating and deactivating substituents in aromatic substitution reactions involving the replacement of hydrogen or other metalloid groups are correlated with truly remarkable accuracy by the o-+-parameters. 

The free radical substitution reactions, other than phenylation, of pyridine and its derivatives have received but scant attention. Alkylation of pyridine itself has been studied briefly, the alkyl radicals being generated either by the thermolysis of diacyl peroxides or of lead tetraacetate in acetic acid, or by electrolysis of the carboxylic acid precursor (for summary, see Norman and Radda369). Most of the available results are summarized in Table XIV. These figures on isomer ratios are not very reliable since the analyses were carried out by... 

Table 10.2 Summary of Substitution Reactions—cont d Reaction...

A reaction that has been discussed in terms of high acid concentrations is the nitration that takes place upon electrolysis of a suspended aromatic hydrocarbon in dilute (1-2 m) nitric acid (for summaries, see Allen, 1958 Fichter, 1942 Tomilov, 1961 Tomilov and Fiochin, 1963). The proposal has been that nitric acid would be formed in high concentration in the anode region, thus causing nitration in an ordinary electrophilic substitution reaction. This is an entirely reasonable idea, as it has since been shown (Cauquis and Serve, 1970) that nitrate ion is oxidized to dinitrogen pentoxide using nitromethane as a solvent (59). In an aqueous solution this... 

Although other types of reactions will be described, much of the remainder of this chapter is devoted to a description of substitution reactions from the standpoint of factors affecting their rates. However, before presenting a discussion of substitution reactions, a brief summary of several types of reactions will be given. 

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