Atom -transfer reaction

A synchronous suprafacial-supra-facial H2 transfer reaction. 

Equations 11.36 and 11.37 show two model reactions, the transfer of a pair of hydrogen ( H) atoms from ethane to perdeuterioethene and the transfer of a pair of hydrogen atoms from ethane to l,l,4,4-tetradeuterio-l,3-butadiene.  

For leading references and a theoretical study, see Frontera, A. Suher, G. A. Deya, P. M. /. Org. Chem. 1992, 57, 6731. The analogous intramolecular rearrangement is known as a dyotropic reaction. 

Orbital correlation diagram for ethane + ethene atom transfer reaction. (Adapted from reference 14.) 

Examples of this type of reaction, where A, B, and C are atoms, are hard to find. Clear, well understood examples are particularly rare, and one must look instead in the uncertain field of elementary steps postulated as parts of complex mechanisms. A necessary condition for the reaction to occur is for the AB bond to be much stronger than the BC bond. The chances for success are presumably increased if AB has a low lying electronically excited state. They are further increased if formation of AB in the electronic ground state is forbidden by spin conservation. Since there is little detailed knowledge of even the few processes of the above type which have been proposed, we can give only a cursory discussion. 

In diffusion flames of alkali metals with organic halides, emission from C2 (Swan bands) has been explained238 by 

Reactions of this type are nearly as scarce as those in which the diatomic product is electronically excited, and presumably for the same reasons. Again they tend to be proposed as parts of complex mechanisms. A relatively simple possibility is 

A number of reactions of type (3.4.2) have been discussed in connection with the excitation of sodium in the upper atmosphere244, viz. 

The last two are known to be exothermic. Tanaka and Ogawa245 have suggested the importance of mechanisms involving Na2 in producing Na(2P). 

In the above cases hydrophobic binding involves reactants that add to each other, and both appear in the product. It seemed to us likely that we would also see such effects in water with reactions such as reductions or oxidations in which one of the reagents simply transfers an atom, rather than itself adding to a substrate. This turned out to be true. 

Oiu first studies involved hydride reductions of carbonyl groups. We compared the 

We examined the epoxidation of carbon-carbon double bonds by oxygen atom transfer reactants. We used the competition between cinnamic acid derivatives and crotonic acid in water (D2O) and water with added 2-propanol. There was no selectivity when the oxidant 

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N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

The rates and chemiluminescent intensities of atom-transfer reactions are proportional to the concentrations of the reactants, but the intensity is inversely proportional to the concentration of inert gas present. The latter quenches the excited state through coUision with an efficiency dependent on the stmcture of the inert gas. Chemiluminescence Qc increases with temperature, indicating that excitation has a higher activation energy than the ground state... 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

Vitamin E actually consists of a family of compounds, the most active of which is a-tocopherol. The mechanism of the vitamin s action is not completely certain, but it seems likely that it might undergo hydrogen atom transfer reactions with free radicals to give a stable radical (see also Chapter 17, Problem 7). 

It is expected from simple Ihermochemical considerations that adjacent n-, 0-or lone pair orbitals should have a significant influence over the facility of atom transfer reactions. Thus, the finding that /-buloxy radicals show a marked preference for abstracting hydrogens a to ether oxygens (Figure 1.9) is not... 

The primary alkyl radical, H, is anticipated to be more reactive and may show different specificity to the secondary or tertiary radical, Tv In VAc and VC polymerizations the radical H appears more prone to undertake intermolecular (Sections 4.3.1.1 and 4.3.1.2) or intramolecular (4.4.3.2) atom transfer reactions. 

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

Since it thus appears that reactions other than the atom transfer process are occurring, one must consider the possibility that the low k (thermal)/ (10.5 volts/cm.) ratios may result from a variation of the relative importance of the atom transfer reaction channel with ion energy. Similarly, in some of the cases where (thermal) = (10.5 volts/cm.) the relative importance of the atom transfer process may also change with ion energy. Thus the value of k(thermal)// (10.5 volts/cm.) does not necessarily provide conclusive evidence for the interaction potential between the ion and the neutral molecules. 

Table III shows that in the gas phase at a pressure of 40 torr the relative rates of the H2 transfer reactions from the cyclopentane ion to the various additives differ drastically from those derived from liquid phase radiolysis experiments. This indicates that the changes in density may profoundly affect the relative rates of the two competitive reactions, Reactions 22 and 28. Experimental results, which will be described in a later publication, indicate that in the liquid phase an increased importance of the H2 transfer reaction to some of the additives occurs at the expense of the H atom transfer reaction, Reaction 23.

Electron-, Energy-, and Atom-Transfer Reactions between Metal Complexes and DNA H. Holden Thorp... 

The molybdenum and tunsten diphenylacetylene compounds have been chemically useful primarily as precursors to the quadruple metal-metal bonded dimers [M(Por)]2, formed by solid-state vacuum pyrolysis reactions. However. Mo(TTP)()/"-PhC CPh) is also a useful substrate in atom-transfer reactions, reacting with Sx or Cp2TiS i to form Mo(TTP)=S. The reaction can be reversed by treatment of Mo(TTP)=S with PPh (which removes sulfur as PhxP=S) and PhC CPh. The order of preference for ligand binding to molybdenum 11) has been established to be PPh < PhC CPh < 4-picoline. ... 

Our group has also reported that the alkylation products of 4-cyano-l,3-diox-anes can serve as substrates for radical atom transfer reactions [41]. One such example is shown below (Eq. 17). Slow addition of tributyltin hydride/AIBN to a refluxing solution of cyanohydrin 115 generated the radical nitrile transfer product 116. This method, though somewhat limited in scope, can provide access to syn-l,3-diols which maybe unstable to the vigorous Li/NHg reduction conditions. 

Quantification of antioxidant action usually relies on the reducing ability of antioxidants, measured either by electron transfer, reaction [16.15], or by hydrogen atom transfer reactions, reaction [16.16] ... 

M. S. Platz, Atom-transfer reactions of aromatic carbenes, Acc. Chem. Res. 1988, 21, 236. 

Radical reactions used in synthesis include additions to double bonds, ring closure, and atom transfer reactions. Several sequences of tandem reactions have been developed that can close a series of rings, followed by introduction of a substituent. Allylic stannanes are prominent in reactions of this type. 

Lewis, E. S. Isotope Effects in Hydrogen Atom Transfer Reactions. 74, 31-44 (1978). 

When alkyl radicals take part in atom transfer reactions as acceptors... 

An investigation of the competing halogen transfer from BrCCl3 and CCl45, 79 has shown that steric effects are also of importance in atom transfer reactions to alkyl and aryl radicals. Giese80 investigated very carefully the temperature depen-... 

The Hj ion, recently detected in the interstellar medium via infrared transitions,25 can subsequently react with a variety of neutral atoms present in the gas. The reaction with oxygen leads to a chain of reactions that rapidly produce the hy-dronium ion H30+ via well-studied H atom-transfer reactions ... 

In Sections III(l) and 111(2) the lability principle has been illustrated for processes involving the transfer of weakly bound electrons, including the reactions of solvated and trapped electrons and F-centers and processes of electrochemical generation of solvated electrons. In Sections IV and V, it will be illustrated also by atom transfer reactions and, in particular, by reactions involving adsorbed atoms. 

Figure 9. Linear reaction complex for the atom transfer reaction.

Metal Complexes as Catalysts for Oxygen, Nitrogen, and Carbon-atom Transfer Reactions... 

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