Transition state radical addition

However, the situation is not as clear-cut as it might at first seem since a variety of other factors may also contribute to the above-mentioned trend. Abuin et a/.141 pointed out that the transition state for addition is sterically more demanding than that for hydrogen-atom abstraction. Within a given series (alkyl or alkoxy), the more nucleophilic radicals are generally the more bulky (i.e. steric factors favor the same trends). It can also be seen from Tabic 1.6 that, for alkyl radicals, the values of D decrease in the series primary>secondary>tertiary (i.e. relative bond strengths favor the same trend). 

The configuration of a center in radical polymerization is established in the transition state for addition of the next monomer unit when it is converted to a tetrahedral sp1 center. If the stereochemistry of this center is established at random (Scheme 4.1 km = k,) then a pure atactic chain is formed and the probability of finding a meso dyad, P(m), is 0.5. 

The extent to which the radicals react according to Eqs. 6 or 7 depends on the nature of Ri, Ra, and R3. If Ri = Rj = H and R3 = H through NO2, the ratio (6) (7) > 20. The addition reactions observed with these systems are characterized by strongly negative activation entropies, which can be rationalized in terms of immobilization of water molecules by the positive charge at C in the transition state [15]. That the transition state for addition has pronounced electron-transfer character concluded from the fact [15] that the rate constants for addition depend on the reduction potential of the nitrobenzene in a way describable by the Marcus relation for outer-sphere electron transfer. 

Figure 1 Transition state for addition of radicals to alkenes...

Fig. 1. Typical polar transition state for addition of perfluoroalkyl radical to an electron-rich olefin...

An orbital symmetry analysis showed that a transition state for nucleophilic addition formed from a (4n + 1) cation-radical, e.g., 18, and a halide ion would involve orbital interactions energetically unfavorable in symmetry terms and thus prejudice such a transition state in favor of electron transfer to which similar restrictions do not apply. On the other hand, 19 is isoelec-tronic with the anthracene anion-radical and is thus a (4n + 3) species. The transition state for addition of halide becomes energetically favorable in symmetry terms in this instance. 

Factors that influence the ground state conformation of 1 are also important in the transition state for addition of a radical at the carbon bearing the carboxamide. For I, the proximal methyl of the pyrrolidine protects the back face of the alkene, and addition occurs predominantly from the front, or Si, face. Figure 2 shows the relative rates of addition of cyclohexyl radical to 1 and 2 at 22 C [9]. The proximal methyl group on 1 reduces the rate of cydohexyl radical addition to the Re face of the alkene by a factor of 20 at room temperature, relative to addition to the model alkene 2. Addition to the 57 face of 1 occurs some 30% faster than addition to the... 

These additions must be radical in nature in that an adduct is presumably formed first and then pairs with another cation radical (e.g., eq. 26). At the same time, however, the unsaturated compound is inactivated by electron withdrawing groups (e.g., as in propargyl chloride), so that the transition state to addition must also have much cationic character. 

Any discussion based on reactivity ratios is kinetic in origin and therefore reflects the mechanism or, more specifically, the transition state of a reaction The transition state for the addition of a vinyl monomer to a growing radical involves the formation of a partial bond between the two species, with a corre sponding reduction of the double-bond character of the vinyl group in the monomer ... 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mosdy owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (18). AppHcation of frontier molecular orbital theory may be more appropriate to explain certain reactions (19). 

The initial discussion in this chapter will focus on addition reactions. The discussion is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chi iter 1, Section 1.10). 

Thus one of the transferred hydrogens conies from the aluminum reagent, and the other one from the solvent. In addition to the mechanism via a six-membered cyclic transition state, a radical mechanism is discussed for certain substrates. ... 

No single mechanism accounts for all the reactions. One pathway involves a concerted one-step process involving a cyclic transition state. This of necessity affords a c -product. Another possibility, more favoured in polar solvents, involves a cationic 5-coordinate intermediate [IrX(A)(CO)L2]+, which undergoes subsequent nucleophilic attack by B-. Other possibilities include a SN2 route, where the metal polarizes AB before generating the nucleophile, and radical routes. Studies are complicated by the fact that the thermodynamically more stable isolated product may not be the same as the kinetic product formed by initial addition. 

Figure 1.1 Transition state for methyl radical addition to ethylene. Geometric parameters are from ab initio calculation with QCISD(T)/6-31GT(d) basis set.29...

Radical additions are typically highly exothermic and activation energies are small for carbon30-31 and oxygen centered32,33 radicals of the types most often encountered in radical polymerization, Thus, according to the Hammond postulate, these reactions are expected to have early reactant-like transition states in which there is little localization of the free spin on C(J. However, for steric factors to be important at all, there must be significant bond deformation and movement towards. sp hybridization at Cn. 

Various ab initio and scmi-cmpirical molecular orbital calculations have been carried out on the reaction of radicals with simple alkenes with the aim of defining the nature of the transition state (Section 1.2.7).2I>,j , 6 These calculations all predict an unsymmetrical transition state for radical addition (i.e. Figure 1.1) though they differ in other aspects. Most calculations also indicate a degree of charge development in the transition state. 

Stereoelectronie factors may also become important in polymerization when bulky substituents may hinder adoption of the required transition state. They may help explain why rate constants for addition of monomeric radicals may be very different from those for addition of dimeric or higher radicals.4... 

Moreover, the radical orbitals, p(D) and q(A) are in phase. The direct through-space interaction between the radical centers, i.e., the p...q interaction, thermodynamically stabilizes the singlet 1,3-diradicals in addition to the cyclic orbital interactions through the bonds. However, the through-space interaction can also stabilize the transition states of the bond formation between the radical centers and kinetically destabilize the diradicals (which will be discussed in Sect. 3.4.2). 

Mayo and Walling, who have given a penetrating critique of the Q,e scheme, point out that it represents in essence merely a transcription to equation form of the reactivity series of Table XX and the po-larity series of Table XXII. Regardless of the manner of interpretation adopted, it is apparent that monomer reactivity in copolymerization depends on two factors. One of these relates to the intrinsic characteristics of the monomer (and of the activated complex produced from it as well) as they tend to favor its addition to a radical. As we have seen, the capacity for resonance stabilization in the transition state is of foremost importance in determining the general level of monomer reactivity. The second factor has to do with the specificity... 

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