Stabilization of reactive species

In the various analyses given above, we considered reactive carbon species such as car-benes and allyl radicals coordinated to transition metals. This is a hallmark of organotransi-tion metal chemistry the stabilization of reactive species by coordination to a metal. Sometimes these ligands retain reactivity patterns analogous to their standard organic reactivity, but often completely new reaction pathways are induced due to the metal coordination. 

Steric protection using bulky substituents is one of the most effective methods for stabilization of reactive species such as dimerization and self-condensation that undergo facile bimolecular decomposition. Typical examples of highly reactive... 

The usefulness of the bridged calix[6]arene framework for the stabilization of reactive species was demonstrated by the synthesis of a stable sulfenic acid. ° Thermolysis of tert-butyl sulfoxide 12 in toluene produced the corresponding sulfenic acid 13 almost quantitatively the acid was isolated as stable crystals (Scheme 11.6). 

The development of molecular cavities based on a more inert framework that does not contain heteroatoms is desired for the kinetic stabilization of reactive species without perturbation of their intrinsic properties. For prevention of bimolecular decomposition of reactive species such as dimerization and self-condensation, it is considered essential to surround the functional group from all sides and from a distance in any conformation of the molecule. A simpler acyclic molecule with larger conformational flexibility could work as a molecular cavity for the stabilization of reactive species if it meets these criteria. 

The synergy of high reactivity and stability in the well-defined isolated reaction environments of nanoscale molecular systems is expected to open undiscovered applications of reactive species to various purposes. Although the covalent frameworks of the molecular cavities described here are very secure and chanically stable, they are static in nature. Interest in stabilization of reactive species by using mechanical bonding systems such as rotaxanes has grown recently. - ... 

Steric stabilization of reactive species is a standard strategy in organometallic chemistry. 

The results described in this review show that matrix stabilization of reactive organic intermediates at extremely low temperatures and their subsequent spectroscopic detection are convenient ways of structural investigation of these species. IR spectroscopy is the most useful technique for the identification of matrix-isolated molecules. Nevertheless, the complete study of the spectral properties and the structure of intermediates frozen in inert matrices is achieved when the IR spectroscopy is combined with UV and esr spectroscopic methods. At present theoretical calculations render considerable assistance for the explanation of the experimental spectra. Thus, along with the development of the experimental technique, matrix studies are becoming more and more complex. This fact allows one to expect further progress in the matrix spectroscopy of many more organic intermediates. 

Although there is no controversy about the basic definition of stability constants, physical chemists and biochemists handle the concepts involved and the resulting calculations differently. Physical chemists think in terms of reactive species and biochemists in terms of total concentrations of components, A further source of confusion is the differing definitions of apparent constant. To a physical chemist the stability constant for MgATP formation... 

Often the initial electrode product will undergo subsequent chemical or electron-transfer reactions, but the stability of the species depends greatly on the reactivity of the medium in that it is produced. 

Carbon-Halogen Vicinal-Dieations Trihalomethyl cations are shown to have enhanced reactivities in superacid solution, while poly-halomethanes in the presence of excess AlBr3 or AICI3 exhibit the properties of aprotic superacids.79 The trihalomethyl cations CX3+ (178, X=C1, Br, I) have been characterized by NMR and IR spectroscopy. The stability of these species is attributed to substantial resonance-stabilization by back-donation from the nonbonded electron pairs of the halogen atoms.22 Trihalomethyl cations are capable of hydride abstraction from alkanes and alkyl groups when the reactions are carried out in the presence of Bronsted or Lewis superacids (eq 46-48).80... 

All these features are also crucial in enzyme catalysis. The role of hydrogen bonding in stabilizing reactive intermediates has been recognized (196) and experimental studies on the stabilization of anionic species are numerous (197). 

Parylene polymerization proceeds with well-defined chemical species, whereas plasma polymerization proceeds via variety of not-well-defined chemical species, which are created in the luminous gas phase. The reactive species for parylene polymerization is para-xylylene, which has features of (1) difunctional (e.g., diradicals), (2) reactive but relatively stable, and (3) highly selective reactivity (see Fig. 2.1). The exact nature of reactive species involved in glow discharge polymerization is not well known however, (1) they are not exclusively bifunctional, (2) they are highly reactive, and (3) consequently they have very low selectivity. The difference in the stability or the selectivity of reactive species is reflected in the distinctively different characters of polymer depositions of these two processes. 

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