Planar substitution reactions

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

Stereochemical analysis can add detail to the mechanistic picture of the Sj l substitution reaction. The ionization mechanism results in foimation of a caibocation intermediate which is planar because of its hybridization. If the caibocation is sufficiently long-lived under the reaction conditions to diffirse away from the leaving group, it becomes symmetrically solvated and gives racemic product. If this condition is not met, the solvation is dissymmetric, and product with net retention or inversion of configuration may be obtained, even though an achiral caibocation is formed. The extent of inversion or retention depends upon the details of the system. Examples of this effect will be discussed in later sections of the chapter. 

Aromaticity (Chapter 15 introduction) The special characteristics of cyclic conjugated molecules. These characteristics include unusual stability, the presence of a ring current in the 1H NMR spectrum, and a tendency to undergo substitution reactions rather than addition reactions on treatment with electrophiles. Aromatic molecules are planar, cyclic, conjugated species that have An + 2 7T electrons. 

Square planar complexes of palladium(II) and platinum(II) readily undergo ligand substitution reactions. Those of palladium have been studied less but appear to behave similarly to platinum complexes, though around five orders of magnitude faster (ascribable to the relative weakness of the bonds to palladium). 

Solvent paths and dissociate intermediates in substitution reactions of square planar complexes. R. J. Mureinik, Coord. Chem. Rev., 1978, 25,1-30 (133). 

Application of the principle of microscopic reversibility can be used to eliminate a mechanism suggested at one time for the nucleophilic substitution reactions of square-planar platinum(II) complexes. For the sake of specificity, we take PtCl - as a typical... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Rate constants for the substitution reactions of square-planar dithio-phosphates and dithiocarbonate complexes of Ni(II), Pd(II), and Pt(II), with ethylenediamine and cyanide ion as nucleophiles, have been measured in methanol. The results were compared with those obtained in previous investigations, and interpreted in terms of the stabilities of 5-coordinate species that are formed prior to substitution (377). 

Aromatic substitution, a quantitative treatment of directive effects in, 1, 35 Aromatic substitution reactions, hydrogen isotope effects in, 2, 163 Aromatic systems, planar and non-planar, 1, 203 Aryl halides and related compounds, photochemistry of, 20, 191 Arynes, mechanisms of formation and reactions at high temperatures, 6, I A-Se2 reactions, developments in the study of, 6,63... 

So the tertiary halide reacts by a different mechanism, which we call SnI- It s still a nucleophilic substitution reaction (hence the S and the N ) but this time it is a unimolecular reaction, hence the 1 . The rate-determining step during reaction is the slow unimolecular dissociation of the alkyl halide to form a bromide ion and a carbocation that is planar around the reacting carbon. 

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

Steric hindrance is well known to slow down the rates of ligand substitution reactions in square-planar metal complexes. An example for which steric hindrance controls the aquation rate is complex 9. The effect of 2-picoline on the rate of hydrolysis of CP trans to NH3 (cis to 2-picoline) is dramatic, being about 5 times as slow as the analogous CP ligand in the nonsterically hindered 3-picoline complex (Table I) (44). 

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