Electron reactivity

Austin R, Buhks E, Chance B, DeVault D, Dutton PL, Frauenfelder H, Goldanskii VI (eds) (1987) Protein structure molecular and electronic reactivity. Springer, Berlin Heidelberg New York... 

In this cluster system the enclosure of sulfite anions with a correct orientation transforms the anions from innocent structural templates to electronically reactive, functional units. These can now release electrons to the cluster shell upon activation by heat-the sulfite groups in type 2 clusters are activated whereas those in type 1 are not. 

In the preceding explanation, we have ignored bonding factors. In the norbornanes (106), at least, frontier electron reactivity indices (Fukui and Fujimoto, 1965) actually appear to favor syn-exo elimination. Normally, syw-elimination is in violation of the orbital symmetry rule, and we are inclined to accept anti elimination as the standard. Even so, we believe that powerful steric factors may overcome apparently unfavorable bonding factors to produce novel 55, as in 92-95. 

The most important conclusion to be made from these studies is the great importance of hydrophobicity in the modulation of the potential for mutagenicity and carcinogenicity. Hansch and coworkers have showed that compounds that require S9 activation to become mutagenic in bacteria all have log Kow terms with coefficients near 1.0 (Debnath et al., 1994). Other QSARs show that where a direct chemical reaction with DNA appears to occur, without metabolic activation, no hydrophobic term enters into the equation (Hansch et al., 2001). In these cases, usually only the electronic (reactivity) properties are important. Notable examples of QSARs based on electronic terms and without a hydrophobic term relate to the mutagenicity to Salmonella of aniloacridines, di-platinum analogs, lactones, and epoxides. All of these examples are for chemicals that do not require activation (Hansch et al., 2001). 

Nickel catalysis is a very active field in organometallic and organic chemistry (selected reviews [3-7]). Complexes of all oxidation states are active in two-electron transfer processes, such as oxidative addition or reductive elimination as well as in single electron transfer initiating radical reactions. Through these processes, oxidation states from Ni(0) to Ni(III) can be easily accessed under mild conditions. Occasionally, Ni(IV) intermediates were also proposed. Apart from the vast number of Ni(II) complexes, a number of organonickel(I) complexes were characterized by X-ray crystallography and their potency as active species in catalytic cycles tested [8-10]. Either radical or two-electron reactivity was observed. Recently, the structure of some alkylnickel(III) complexes was also structurally elucidated [11]. 

It was pointed out in [2,3] that nuclear-configuration changes define chemical reactions so that nuclear reactivities should be defined and set on equal footing with the corresponding electronic reactivities. Thus nuclear Fukui functions , Eq. (59), and nuclear softnesses a Eq. (60), were defined in [2], and explicit Kohn-Sham expressions were found for them, Eqs. (61H64), as reviewed in Sect. 5. These are electron-transfer reactivities and are valid only for extended systems, leaving open the question of nuclear electron-transfer reactivities for localized systems and nuclear isoelectronic reactivities for all systems. 

Wolynes, P. (1987) In Protein Structure Molecular and Electronic Reactivity ... 

Raman transitions are of considerable interest as well. Furthermore, in the Appendix we consider briefly the literature concerning collision-induced electronic, reactive, and nonlinear light scattering processes. 

The rates of many electron transfer reactions have now been measured and, as new coordination complexes are prepared, characterized, and their solution properties studied, our understanding of fundamental structure (both geometric and electronic)-reactivity relationships continues to grow. Both inner- and outer-sphere reactions will be explored in Chapter 5. 

Figure 6-4. Molecular charge (electron or hole) transfer motion along a slow diffusion-like solvent or conformational nuclear mode with electronic reactive spill-over along a fast, friction-less local nuclear mode. Analogy to the diffusive electron or hole transfer shown in Fig. 6-3.

A delocalized 0-stannyl radical anion can also be generated from the reaction of an a,/ -unsaturated ketone or aldehyde with tributyltin hydride and radical initiator AIBN [3, 4, 5a, 5b]. Thus, a,/ -unsaturated carbonyl compound 4 (R or R = H or alkyl), can be reacted with wBu SnH under standard free-radical conditions to give allylic O-stannyl ketyl species (5 6), shown in Scheme 2. After hydrogen atom transfer to the -position of 6, a synthetically useful tin(IV) enolate is produced [5b, 5d, 5g. Allylic 0-stannyl ketyls have both one- (radical) and two-electron (anionic) sites for reactivity. These reactions can proceed in a sequential manner - a rapidly-evolving methodology in organic synthesis [2, 5, 8j. If the one-electron reactivity in 6 is used with a radicophile, then the tin enolate or two-electron reactivity can be used in reactions with suitable electrophiles (E ). Note that the carbonyl species. 

Since both alcoholic oxidation and O2 reduction are two-electron processes, the catalytic reaction is conceptually equivalent to a transfer of the elements of dihydrogen between the two substrates. Biological hydrogen transfer generally involves specialized organic redox factors (e.g., flavins, nicotinamide, quinones), with well-characterized reaction mechanisms. Galactose oxidase does not contain any of these conventional redox factors and instead utilizes a very different type of active site, a free radical-coupled copper complex, to perform this chemistry. The new type of active site structure implies that the reaction follows a novel biochemical redox mechanisms based on free radicals and the two-electron reactivity of the metalloradical complex. 

Studies of the kinetics and thermodynamics of proton transfer and hydride transfer reactions have led to a better fundamental understanding of the range of reactivity available, and how it is influenced by different metals and ligands. This information is also central to the rational development of molecular catalysts for oxidation of H2 and production of H2 described in Chapter 7, and in the broad context of other reactions pertinent to energy production and energy utilization that require control of multi-proton and multi-electron reactivity. 

Miller, J.R. Proceedings in Life Sciences Protein Structure Molecular and Electronic Reactivity, Austin, R., Buhks, E., Chance, B., de Vault, D.,... 

Based on the above experimental observation combined with the assumption of 16-electron reactive intermediates, Heck and Breslow proposed a mechanism for the cobalt-catalyzed olefin hydroformylation, which gives a qualitative explanation of the catalytic aldehyde formation by a sequence of hydrido-, alkyl-, and acylcobalt complexes according to Scheme 1 (99). 

The next critical step was to measure the alkene binding step. To investigate this, the partitioning of the 14-electron reactive intermediate between productive alkene metathesis (Step 2) reversion back to precatalyst (A .i) was probed. Unfortunately, the rate of alkene binding could not be directly determined instead, the metathesis efficiency ratio k /t2 was kinetically determined from a plot of 1//Toi,s versus [CygP]/[alkene]. 

H2lMes ligand. In a bis(NHC) complex, one NHC must be rendered as a better leaving group to allow dissociation to form a 14-electron, reactive intermediate. 

The microscopic models developed in this chapter will be directly applicable in explaining why elements form the compounds that they do (3.8, 3.9). Reactive atoms, such as chlorine, are reactive because they have seven valence electrons when eight are required for stability (3.7). Consequently, chlorine reacts with other elements in an attempt to gain an additional electron. Reactive atoms like chlorine can become environmental problems, especially when human activity moves them to places where they are usually not found. For example, chlorine atoms are transported into the upper atmosphere by synthetic compounds called chlorofluorocarbons. Once there, they react with the ozone layer and destroy it (3.10). 

Wada, T., Shinsaka, K., Namba, H., and Hatano, Y, Electron reactivity in liquid hydrocarbon mixtures. Can. J. Chem., 55, 2144,1977. 

G. Feher, M.Y. Okamura, and D. Kleinfeld, Electron transfer reactions in bacterial photosynthesis charge recombination kinetics as a structure probe, in "Protein Structure Molecular and Electronic Reactivity," R. Austin, E. Buhks, B. Chance, D. Devault, P.L. Dutton, H. Frauenfelder, and V.I. Goldanskii, eds.. Springer Ver-lag. New York (1987). 

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