Substrate-centered reactivity

Since dioxiranes are electrophilic oxidants, heteroatom functionalities with lone pair electrons are among the most reactive substrates towards oxidation. Among such nucleophilic heteroatom-type substrates, those that contain a nitrogen, sulfur or phosphorus atom, or a C=X functionality (where X is N or S), have been most extensively employed, mainly in view of the usefulness of the resulting oxidation products. Some less studied heteroatoms include oxygen, selenium, halogen and the metal centers in organometallic compounds. These transformations are summarized in Scheme 10. We shall present the substrate classes separately, since the heteroatom oxidation is quite substrate-dependent. 

Since reactive metal centers were secured, attempts were initiated to achieve substrate selectivity in amide hydrolysis by the metal complexes [50, 51], The active site of... 

The presence of different reactive centers in the substrate, steric constraints, etc., may originate anomalous or unexpected results in Mannich synthesis. Thus, multifunctional substrates, or substrates containing reactive prochiral centers, give rise to the possibility of chemo-, regio-, and stereoselectivity (Secs. C.1-3), cyclizations and/or polymerizations (Sec. C.4 and Chap. III). 

The prominent goal of these studies was the detection of interactions between substrate and silyl esters with different organic groups R. The model substrates used include various kinds of silica, alumina, titania in addition to typical sandstones and silicon wafers. With respect to a contribution in the first conference report [3] one example may suffice here to demonstrate the procedure and the importance of the result obtained. The example concerns the treatment of a silicon wafer surface with 3-aminopropyltriethoxysilane (APTES) aimed at the detection of covalent bonding and possible interaction of the functional aminopropyl group with reactive surface centers. The method of choice is a... 

The metallation of acidic hydrocarbons (pK 15-30), such as ketones and esters, and their nitrogen-containing analogues are complicated because the substrates contain a reactive electrophilic center. Sterically hindered RjNM, (CgH5)3CM or nonnucleophilic... 

They are highly strained and have a singlet ground state.73,74 Because such intermediates have two reactive carbon centers they are attractive substrates for cycloaddition reactions. However, little consideration will be given to this important use of arynes and hetarynes because it is outside the scope of this article. Information dealing with the generation of new intermediates is included, although attempts have not yet been made to exploit them in syntheses. 

Strategy IV. Cooperative substrate-centered radical-type reactivity based on redox non-innocent substrates... 

They are defined as enzymes that catalyze the isomerization of a substrate through stereochemical reverse rearrangement of a substituent bound to a chiral center (usually a chiral carbon) in the substrate molecule. Racemases act on molecules containing only the asymmetric center concerned in the reaction. Epimerases act on substrates containing one or more asymmetric centers in addition to the reactive chiral center. 

In addition to the cationic, radical, and non-synchronous concerted mechanisms outlined above, many other proposals have been offered [9, 56], In a recent provocative paper, an organometallic mechanism was postulated for the activation of alkanes by a ferryl porphyrin model species [79]. Less reactive substrates such as H2, D2 and CH4 were observed to inhibit the reaction between the synthetic catalyst and cyclohexane. In the proposed mechanism, a 2 -t- 2 C-H addition across the Fe-O bond is preceded by coordination of the alkane to the metal center to form an intermolecular <7-adduct. Inhibition arises from preferential binding of the smaller substrates to the congested metal site. Attempts to identify a similar effect with sMMO have been unsuccessful the presence of H2 had no effect on the rate of reaction between methane and Q (A. M. Valentine, S. S. Stahl, S. J. Lippard, unpublished results). 

Many stereoselective substitution reactions (Sj 2) were first studied and optimized with steroidal substrates, because this class of compounds provides a large variety of pure enantiomers with just one reactive chiral center. Furthermore, the... 

If a radical is generated in a substrate that also has a it bond in the molecule, intramolecular cyclization can occur (as in 64 65). The highly reactive radical center is positioned to react with the it bond of the alkene, as shown, and the product (65) can be trapped to give the final product. Radical cyclization reactions will be discussed in Section 13.7. A synthetic example is taken from Rigby s synthesis of (+)-pancratistatin, in which photolysis of 66 generated an aryl radical that added to the alkene unit intramolecularly to give a 50% yield of 67. For a brief discussion of this and other pancratistatin syntheses, see Section 10.7. 

Figure 33D illustrates the fit of the model expressed by Eq. 8 to the enhanced tunneling current vs. substrate potential characteristics (z enh vs. Es). Jenh was obtained from the experimental data plotted in Fig. 33C after base line correction. The particular experiment led to reorganization energies A = 0.42 eV of the radical cation. The values of and y 1 indicate a complete potential drop at the reactive viologen center with the variation of r] and Ebias" For further details we refer to [269]. 

N-Alkylations (arylations) of tetrazoles are continuously reported, however regioselectivity is problematic with substrates containing other reactive azole centers. The combination of alcohols or epoxides, activated with either zinc triflate or dibenzyl N,N-diethylphosphoramidate, in acetonitrile, nitromethane or dichloromethane alkylates... 

Stimulated by recent success in the catalytic activation of other small molecules, an intensive effort has been made to activate molecular oxygen by coordination to a metal center with the ultimate goal being catalytic transfer of oxygen to a reactive substrate in a selective manner. It is with this approach to the chemistry of metal... 

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