Stereochemistry of substitution

Although most substitutions in octahedral complexes involve D or pathways, we consider the stereochemical implications only of the D mechanism since this involves a 

5-coordinate species which we can readily visualize (equation 26.39). 

A-configuration. Explain why this observation indicates that a trigonal bipyramidal-square-based pyramidal rearrangement is not competitive in terms of rate with the aquation reaction. 

4) can be envisaged. The argument certainly rationalizes the observations, but one has to accept an assumption that no rearrangement occurs during the lifetime of the 5-coordinate species. (We have previously discussed rearrangements in 5-coordinate complexes see for example the end of Section 2.11.) 

We have already seen in Section 4.8 that there is little energy difference between trigonal bipyramidal and square-based pyramidal structures, and that 5-coordinate complexes therefore tend to undergo ligand rearrange- 

The aquation reaction of A-cis-[Co(en)2Cl2] leads to cis-and tra 5-[Co(OH2)(en)2Cl], the cA-isomer retaining its A-configruration. Explain why this observation indicates that a trigonal bipyramidal-square-based pyramidal rearrangement is not competitive in terms of rate with the aquation reaction. 

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While it may be convenient to use optically active reactants to probe the stereochemistry of substitution reactions, it should be emphasized that the stereochemistry of a reaction is a feature of the mechanism, not the means of determining it. Thus, it is proper to speak of a substitution process such as the hydrolysis of methyl iodide as proceeding... 

Nucleophilic substitution in cyclohexyl systems is quite slow and is often accompanied by extensive elimination. The stereochemistry of substitution has been determined with the use of a deuterium-labeled substrate (entry 6). In the example shown, the substitution process occurs with complete inversion of configuration. By NMR amdysis, it can be determined that there is about 15% of rearrangement by hydride shift accon any-ing solvolysis in acetic acid. This increases to 35% in formic acid and 75% in trifiuoroacetic acid. The extent of rearrangement increases with decreasing solvent... 

Figure 15.6 FMO rationalizes the stereochemistry of substituted Diels-Alder reactions...

Peddle and Redl,0) were still rather pessimistic in 1970 Thus while it should be possible to resolve an optically active organotin compound with four carbon-tin bonds, it seems unlikely that such a compound would be very useful in investigating the stereochemistry of substitution at the tin atom 10). 

TABLE 9. Stereochemistry of substitution reactions on methylcyclohexyl bromides by VLnSiiLi and MesGeLi... 

The specific feature of the bonds also affects its chemical behaviour and the stereochemistry of substitution reactions. For example in the conversion of (-) trans -2, 3 diphenyl cyclopropane carboxylic acid into (+) 1, 3 diphenylallene the optical activity is retained. 

The stereochemistry of substitutions at sulfinyl sulfur that proceed with inversion has usually been explained in terms of the reaction proceeding through a trigonal bipyramidal intermediate (or transition state) [46] in which... 

The foregoing considerations show that, in order to reproduce the stereochemical trends, it is not necessary to introduce either d orbitals for the silicon atom or pseudo-rotations for the transition state. Conversely, the stereochemistry of substitution reactions on silicon compounds cannot be taken as a proof of d orbitals intervention in silicon chemistry. 

Grosu and co-workers also studied the complex configurational and conformational aspects of the unique stereochemistry of substituted spiro-l,3-oxathiane derivatives 100-102 by NMR spectroscopy <2001T8751>. 

Vicinal H, H coupling constants do not satisfactorily indicate the stereochemistry of substituted thiolanes. However, it has been found during an investigation of a number of 3,4-disubsti-tuted and 2,3,4-trisubstituted thiolanes, e.g., 3 and 4, that the temperature dependence of the trans coupling is much stronger than that of the cis coupling, which is virtually invariant505. 

The factors controlling the stereochemistry of substitution, i.e., the nature of the leaving group and the nucleophile. 

TABLE 2. Stereochemistry of Substituted Stilbene Photodimeri zati on ... 

This electrophilic assistance could account for the observed solvent and counterion effects. The stereochemistry of substitution of alkoxy, mercapto and fluorosilanes with alkoxide ion depends dramatically on the composition of the solvent4. As the percentage... 

A conformational study of /3-arteether 11, by H and 13C NMR, showed that the solution of /3-arteether 11 is similar to that of its X-ray structure <1999J(P2)2089>. The interaction of artemisinin 1 with manganese tetraphenylpor-phorin, in the presence of a reducing agent, was studied by UV-visible (UV-Vis) and H NMR <1997CR59>. A series of rules using 1-D and 2-D NMR spectroscopy for identification of relative stereochemistry of substituted antimalarials, related to artemisinin 1, were reported <1997SPL241>. 

The possibility of electrophilic substitution at saturated carbon as an independent mechanism was considered by Hughes and Ingold2 in 1935, but this mechanism was not kinetically demonstrated with metal alkyls as substrates until 1955, when Winstein and Traylor3 published their results on the acetolysis of dialkylmercurys. At about the same time, stereochemical studies on electrophilic substitutions at saturated carbon were commenced by Winstein and by Reutov, again using alkylmercury compounds as substrates. Notable studies on the kinetics and stereochemistry of substitution at saturated carbon have been carried out by Ingold and his co-workers and by Reutov and his co-workers. Ingold4... 

Thus, with compounds of type (a), the stereochemistry of substitution, insertion, and cleavage reactions can be studied (8, II), whereas with compounds of type (b), the mechanism of racemization and epimerization at the chiral metal atom can be investigated (II, 92). In the following sections, representative examples of both types of study will be given, from which it will become evident how optically active labels at metal centers in organo-transition-metal compounds contribute to the elucidation of the stereochemical course of reactions. 

Chapter 8 discussed the stereochemistry of substitution reactions—that is, what happened to the stereochemistry when the reaction occurred at a carbon chirality center. This section discusses the regiochemistry of the elimination reaction—that is, what happens when a reaction can produce two or more structural isomers. The structural isomers that can often be produced in elimination reactions have the double bond in different positions. As shown in Figure 9.5, elimination of hydrogen chloride from neomenthyl chloride produces two structural isomers but in unequal amounts. 

Another important argument is provided by the stereochemistry of substitution reactions at silicon. In the optically active complex 6 nucleophilic substitution by H, OH-, or OMe- occurs with inversion of configuration at silicon, while retention was observed with the related... 

An important aspect of hot-atom chemistry concerns the stereochemistry of substitution reactions, particularly if the reactions proceed via retention or (Walden) inversion. Cross sections for the reac-... 

Stereoselectivity will be dictated in most cases by the binding of the substrate to the hydroxylating enzyme. One exception to this occurs in the 6p-hydroxylation of 3-keto A -steroids. In this case the stereochemistry of substitution at C-6 of the product is determined largely by conventional stereoelec-tronic processes, as the mechanism is believed to involve axial addition of oxidant to a conjugate of the substrate and the hydroxylating enzyme (equation 24). ... 

These products have a varied and useful chemistry, for example, the stereochemistry of substitution with azide can be manipulated by optional formation of an intermediate aziridine (Scheme 90). ... 

Stereochemical Considerations. Much attention has been directed toward the stereochemistry of substitution reactions of alkyl complexes. Substitution of MeMn(CO)s leads to cw-MeC(0)Mn(C0)4L initially, although the cis complex isomerizes to the trans eventually ... 

Reference has already been made (p. 47) to the unusual retention of configuration in nucleophilic substitutions of 3j0-substituted-A -steroids (i). The stereochemistry of substitution is controlled by participation of the sr-electrons of the C(5) C(6) bond in a unimolecnlar solvolysis at C(S), to give, as the primary product, a mesomeric carbonium ion commonly represented by the non-classical structure (2) [19,20], although this is probably an over-simplification (see below). Carbonium ions related to the structure (2) are well-known outside the... 

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