Atom-or group-transfer reactions

1 ATOM- OR GROUP-TRANSFER REACTIONS The general formulation of a group transfer reaction is A+B—C - A—B + C 

In this section, group transfer reactions in which the product molecule A-B is vibrationally excited but still in its ground electronic state are considered. (Transfer reactions that produce electronic excitation are discussed in Section 3.4.) The available experimental evidence is tabulated. Only typical examples are described. The principal points discussed are the limitations that experimental technique has imposed on observation and interpretation of this type of chemi-excitation and the extraction of generalizations concerning this class of reaction. 

Although the novelty of observing chemically produced vibrational excitation provided an initial impetus, the main purpose of the studies to date has been to determine in detail the relative proportions of excited molecules in the various energy states, the fraction of the reaction energy that goes into internal excitation, which products are excited, and the fate of the excited molecules. Such data are used as aids in the construction of potential energy surfaces to be used, in turn, to describe the dynamics of the reactions. In short, the studies have been in the hands of kineticists. As interest in the subject has spread, more attention has been paid to applications laser action and the reactions of the excited molecules. 

Several generalizations may be made about observed excitation in this class of reactions. They are presented here as a guide to the discussion not all are apparent from the data in Table 1. 

The primary location of vibrational excitation is in the product molecule in which a new bond is formed, i.e., A-B. Excitation of the other product, C, occurs but to a lesser extent. 

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Research Focus Air-stable and high-activity ruthenium-based catalysts containing Schiff base ligands useful in olefin metathesis agents or in atom or group transfer reactions. 

Propagation steps are the heart of any chain and generally fall into two classes atom or group transfer reactions and addition reactions to tr-bonds (or the reverse elimination). The rate of the chain transfer step is especially important in synthetic planning because, by fixing the maximum lifetime that radicals can exist, it determines what reactions will (or will not) be permitted. Termination steps are generally undesirable but are naturally minimized during chain reactions because initiation events are relatively uncommon. 

Almost all of the reactions of radicals can be grouped into three classes redox reactions, atom (or group) transfer reactions and addition reactions. A detailed discussion of these reactions is beyond the scope of this chapter, but a summary of some important features (with references to more in-depth discussions) is essential. Although addition reactions will receive the most attention, redox and atom transfer reactions are important because nearly all radicals formed by addition reactions will be removed from the radical pool to give nonradical products by one of these methods. 

Possible dimensions now being explored for proton transfers involving carbon (where the approximation that this can be treated as a simple dimension breaks down) are bond formation to a proton and bond breaking to a proton (a similar pair of dimensions is being examined for hydrogen atom transfer). This is the test case for the more general problem of atom or group transfer reactions. 

To achieve low radical concentrations, most radical reactions are traditionally performed as chain reactions. Atom or group transfer reactions are one of the two basic chain modes. In this process the atom or group X is the chain carrier. A metal complex can promote such chain reactions in two ways. On one hand, the catalyst acts only to initiate the chain process by generating the initial radical 29A from substrate 29 (Fig. 10). This intermediate undergoes the typical radical reactions, such as additions or cyclizations leading to radical 29B, which stabilizes to product 30 by abstracting the group X from 29. A typical example is the use of catalytic amounts of cobalt(II) salts in oxidative radical reactions catalyzed by /V-hydroxyphthalimide (NHPI), which is the chain carrier [102]. 

Fig. 10 Transition metal catalysts as initiators in atom or group transfer reactions.

Choose the initiator which is appropriate to your system. AIBN, although capable of doing many things, is often incapable of triggering atom or group transfer reactions (try lauroyl peroxide instead). 

There are several fundamental types of radical reactions. Radicals can abstract hydrogen or other atoms from many types of solvents and reagents. This is a particularly important example of an atom or group transfer reaction. 

In these rhenium nitrosyl complexes the nitrosyl ligand takes different functions (1) as an ancillary NO ligand mimicking isoelectronic metal carbonyls (2) as a frans-influence ligand to activate donor ligands for atom or group transfer reaction. 

Although this model was initially developed for reactions occurring in an adiabatic potential energy surface, it can be extended to weak interaction systems under the assumptions mentioned above, because the reaction path is obtained in terms of independent stretches of the two reactants. This contrasts with the Agmon-Levine approach, which is restricted to atom or group transfer reactions, where reactant and product are connected via a family of parallel curves each defined by the same positive bond order, maintained constant through the compensation between the decrease in the reactant bond order and the increase in the product one. One caution must be observed in our analysis the extensions x are not directly related to the intersection of the reactants curves, although it is convenient to represent them that way. Therefore, we can use the transition state expression to estimate the rates of ET reactions... 

II. Intra-Pair Reactions Back electron transfer Proton, atom, or group transfer Coupling... 

Radical ion pairs also react by proton, atom, or group transfer. We illustrate proton transfer in reactions of aromatic hydrocarbons with tertiary amines. These reactions cause reduction or reductive coupling. In the reduction of naphthalene, the initial ET is followed by H" transfer from cation to anion, forming 67 paired with an aminoalkyl radical the pair combines to generate... 

Cyclizations of aromatic diazonium salts138 (intramolecular Meerwein arylations) are pieparatively related to atom transfer reactions because a radical cyclization is terminated by the transfer of an atom or group other than hydrogen. However, the two methods are not mechanistically related. In the atom transfer method, the atom that is transferred to the cyclic product always derives from the radical precursor, but in the cyclizations of aryldiazonium salts, die atom or group transferred derives from an added reagent. This means that many different products can be prepared from a single diazonium precursor, but it... 

For the latter reason atom or group transfer may sometimes also take place in outer-sphere processes, and it has even been suggested that atom transfer can be part of an outer-sphere mechanism, if only for the case of hydrogen atom transfer. Such a case is the Fe(II)—Fe(III) self-exchange reaction in water where hydrogen bonding between two ligands in the transition state [2] would... 

Some of the new theoretical relations, the cross-relation between the rates of a cross-reaction of two difierent redox species with those of the two relevant selfexchange reactions, were later adapted to non-electron transfer reactions involving simultaneous bond rupture and formation of a new bond (atom, ion, or group transfer reactions). The theory had to be modified, but relations such as the crossrelation or the effect of driving force (—AG°) on the reaction rate constant were again obtained in the theory, in a somewhat modified form. For example, apart from some proton or hydride transfers under special circumstances, there is no predicted inverted effect. Experimental confirmation of the cross-relation followed, and an inverted effect has only been reported for an H+ transfer in some nonpolar solvents. The various results provide an interesting example of how ideas obtained for a simple, but analyzable, process can prompt related, yet different, ideas for a formalism for more complicated processes. 

In homogeneous chemistry, pure electron transfer reactions are seldom encountered. Indeed, with the exception of a few examples, electron transfers are often associated with atom or group transfers. This usually results in a confused notion of the nature of oxidation-reduction reactions. For example, the reaction of a ketone with sodium in alcohol to afford the corresponding alcohol, via the sequence in Eq. (1) [2],... 

If the transient radicals R transform rapidly into other transient radicals R , for example by fragmentation, rearrangement, addition to an unsaturated molecule, atom or group transfer, or by any other reaction, then the cross-reaction products between R - and Y become dominant. The transformation of Y to another persistent Y leads to the selective formation of R—Y. ... 

The reactions of organic synthesis are related to many steps (bond breaking, bond formation, atom or group transfer) frequently triggered, or anyway associated, with electron transfer processes [11], In principle, every step may be able to affect the... 

Roberts et al. (1982) concluded that the multistep mechanism involving an electron transfer process can be excluded, considering of the a-value of a Br nsted plot (a=0.5) observed over a wide range of cationic substrates, which agrees with the Marcus theory for atom transfer. The Br nsted a for an atom or group transfer depends on the position of a substituent and the tightness of the transition state (t) as well as on the resemblence of the transition state to reactants and/or products. The Marcus theory predicts that t can be related to the rates of symmetrical reactions. Rates and equilibrium constants were measured for the reactions of 10-methylacridane with a series of 1-benzyl-3-cyanopyridinium ions substituted in the... 

Fig. 3. Schematic representations of the energy profile along the reaction coordinate for (a), atom or group transfer and (b) addition-fragmentation transfer.

With the exception of propadiene, the addition of sulfonyl halides and seleno-sulfonates to allenes can be totally regioselective (equation (57)) [116], The attack of sulfony] radical on the central carbon atom, which leads to a stabilized ally] radical, is probably less reversible, if at all, than the addition to the terminal carbon. The ensuing atom or group transfer occurs at the less substituted end of the allyl radical. Therefore, the reaction results in 1,2-addition to the less substituted double bond. Subsequent oxidation of the adducts when X = SePh gives rise to allylic alcohols since the [2, 3]-sigmatropic rearrangement of selenoxide is much faster than the elimination of PhSeOH. 

Repetition of atom or group transfer addition reactions to alkenes leads to the formation of living polymers possessing heteroatom functionality X at the co-polymer end (Scheme 2(a)). The effect of heteroatom compounds... 

Scheme 7 A general scheme for radical addition reaction to an alkene followed by atom or group transfer.

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