The Anionic Mechanism

As above stated, the reaction steps in the anionic mechanism take place in reverse order than in the cationic mechanism (Fig. 5.8). Thus, in the anionic mechanism the deprotonation of the alkyne by the external base in complex 2 occurs first, followed by the iodide-for-phosphine substitution. The Gibbs energy profile obtained for the copper-free Sonogashira reaction with phenylacetylene (R = H) through the anionic mechanism is shown in Fig. 5.10. 

The deprotonation of the alkyne by the external base in complex 2 occurs through the transition state DA-TSl (23.0 kcal moP ) and leads to the formation of the ion-pair (DA-1) formed between the anionic Pd complex and the protonated base. 

Subsequently, this species evolves to the next intermediate, RE-1, by a iodide-by-phosphine substitution via the transition state DA-TS2 with a relative energy barrier of 13.1 kcal mol . Finally, RE-1 undergoes reductive eUmination giving rise to the coupled product and regenerating the catalytic species. The optimized structures for the transition states involved in this mechanism are shown in Fig. 5.11. 

Overall, the substitution of the iodide by the phosphine ligand in DA-1 through the transition state DA-TS2 has a global energy barrier 2.3 kcal mol higher than 

With the anionic reaction pathway established for phenylacetylene (R = H), we next examined the effect of the alkyne R substituent on the overall anionic mechanism by computing the corresponding Gibbs energy profile for the other 4-substituted phenylacetylenes (R = CF3, OMe, NMc2). The relative Gibbs energies obtained with these 4-substituted phenylacetylenes are collected in Table 5.3. 

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The anionic mechanism is similar to that postulated to explain the thermal polymerisation. The apparent general similarity of the polymers produced by the two methods of initiation justifies this. Cross-linking involves additions to the free CN groups, and regular networks with two or more interconnected polymer chains are possible. Thus the structure of the azulmine is highly complex. Termination must occur by reactions of the growing anions with H2CN+ ions formed in the reaction... 

The latter three are obtained by solution polymerization technique with alkyllithium initiator through the anionic mechanism. For these materials, the analysis of block sequences is also an interesting subject in the area of TLC application. However, because a somewhat different principle has to be applied to achieve separation by the difference in block sequences, this subject will be discussed in a subsequent section (cf. Section IV.2.). 

Thus, in the presence of alcohols or other proton donors the polymerization of epoxy compounds under the action of TA proceeds according to the anionic mechanism to give quaternary ammonium alcoholate as the active propagating site [Scheme (33)]. 

Radiation-induced polymerization of nitroethylene in the liquid state was studied by Yamaoka and one of the present authors (K. H.). They verified that nitroethylene polymerizes in the anionic mechanism by radiation too, from the following three observations (7). Tetrahydro-furan is the most favorable solvent for the polymerization, among nitro-ethane, ethylether and tetrahydrofuran. The addition of hydrogen chloride remarkably retards the polymerization. Finally, nitroethylene copolymerizes with acrylonitrile. 

The pH dependence of the action of fumarate hydratase indicates participation of both an acidic and a basic group with pfCa values of 5.8 and 7.1.56 See Chapter 9 for additional information. However, either anion or carbocation mechanisms might be possible. That the cleavage of the C-H bond is not rate limiting is suggested by the observation that malate containing 2H in the pro-R position is dehydrated at the same rate as ordinary malate. If the anion mechanism (Eq. 13-13) is correct, the 2H from the pro-R position of specifically labeled malate might be removed rapidly, while the loss of OH could be slower. If so, the 2H would be "washed out" of L-malate faster than could happen by conversion to fumarate followed by rehydration to malate. In fact, the opposite was observed. 

The beauty of the Szwarc procedure is that the chains can be terminated by hydrolysis, oxidation, carboxylation with COz, and so on, to give polymer with the same kind of groups on each end of the chain. Also, it is possible to form chains in which different monomers are present in blocks. The only requirements are that the different monomers polymerize well by the anion mechanism and contain no groups or impurities that will destroy the active ends. Thus one can start with ethenylbenzene (S), and when the reaction is complete, add methyl 2-methylpropenoate (M) to obtain a block copolymer of the type... 

T7177>. Ring-opening polymerization can proceed by two mechanisms depending on the initiator used the coordination-insertion mechanism or the anionic mechanism. While most studies focused on the synthesis of poly(/3-hydroxy butyrate), alternative PHAs were synthesized from a-methyl-/3-pentyl-/3-propiolactone... 

Super glue is a polymer of methyl cyanoacrylate. Because both the cyano and carbonyl groups of the monomer help stabilize carbanions, this compound is sensitive to polymerization by the anionic mechanism. The tube of glue contains very pure monomer, which does not polymerize until it contacts an initiator. However, contact with any nucleophile causes rapid polymerization. Therefore, when the tube is opened, polymerization is initiated by water in the air, by SiOH groups on a glass surface, by FeOH groups on an iron surface, or by various nucleophiles that are part of the proteins in skin. The adhesion between the polymer and the surface to which it is applied is very strong because the polymer chains are covalently linked to the nucleophiles that are part of the surface ... 

Since the epoxy resin cures primarily by a ring-opening mechanism, it exhibits a smaller degree of cure shrinkage than other thermosetting resins. In these reaction processes, the epoxy group may react in one of two different ways anionically and cationically. Both are of importance in epoxy resin chemistry. In the anionic mechanism, the epoxy group may be opened in various fashions to produce an anion, as shown in Fig. 2.10. 

Natta et al. (167,188,287,298,312) have built a strong case in favor of a coordinated anionic mechanism in which an electropositive metal complexes and polarizes the monomer and a polymer anion adds to the positively polarized carbon of the monomer. One of the points which was used to support the anionic mechanism was that the order of reactivity for ethylene, propylene and butene is opposite to that of cationic catalysts. The lower reactivity of propylene and butene versus ethylene was attributed to the electron releasing alkyl groups (287), but steric hindrance is believed to be a better explanation. Support for the steric effect is indicated by the influence of bulk placed at some distance from the double bond (116). For example, reactivity decreases sharply in the order pentene-1 > 4-methylpentene-l > 4,4-dimethylpentene-l, although basicity of the double bonds must change only very slightly. 

Scheme 5.5 Outline of the anionic mechanism for the Heck reaction...

In a number of papers Yamanka and co-workers 9-14) examined polymerization. Their main conclusion was that radiation induced polymerization througlt the anionic mechanism involving free ions and induced post-polymerization at icmpcratures below —ISO C. 

Hence, although the presence of radicals has been demonstrated, it has yet to be shown that the SET pathway is more than a minor competitive reaction, and for the remainder of this review the anionic mechanism set out in Scheme 1 will be assumed to apply. 

Special Features of the Anionic Mechanism Absence of Termination Processes Effect of Counterion (Initiator)... 

Because the monomer was not a vinyl compound and the active chain end was an alkoxide, this reaction was not considered an important case of anionic polymerization. Ironically, this reaction actually is a very good example of the anionic mechanism and can be satisfactorily studied because it is a homogeneous reaction. In fact, it was Flory (6) who first pointed out the unique consequences that arise from such a polymerization in which presumably the alkoxide chain end does not undergo any "side reactions," that is, termination. Flory remarked that in such a situation in which all the growing chains have equal access to the monomer, the chains will tend to reach similar lengths, that is, the molecular weight of the polymer will have the very narrow Poisson distribution ... 

The special features of the anionic mechanism that distinguish it from the other mechanisms can be classified as follows ... 

Equation 17 can then be plotted as a linear function of rate versus initiator concentration to yield k and k Kg. Furthermore, if Kg is determined, for example, from conducxivity measurements, then the absolute value of k is also available. A number of such measurements have been taken and yield rate constant values for various monomers (mainly styrenes and dienes) and various counterions and solvents (3 ). In general these data indicate that, although the free anions are only present in very small proportion (Kg lO ), they are responsible for most of the chain propagation because their rate constants (kp 10 -10 M sec ) are several orders greater than those of the ion pairs (k 10 M sec ). Hence, Reaction 14 seems to represent an adequate picture of the anionic mechanism in these systems. 

These monomers cannot be polymerized by the anionic mechanism, because of the susceptibility of the Si—H bond to cleavage in the basic medium. This is an important group of reactive polymers because the Si—H bond can be used for further reactions, e.g., with olefins and perfluoroolefins. 

The carbene mechanism is to be highly preferred over the anionic mechanism proposed earlier for the formation of the dihydrobenzofuran derivative. Further study revealed that the preferred mode of cyclization in most of the cases is the one which leads to five rather than six-membered rings. Thus o-propylbenzoyl- and o-phenoxybenzoyl phospho-nates give the five-membered ring products 13 and 14 In the latter case, the reaction involves expansion of the aromatic ring. 

According to Menger a simple nucleophilic catalysis is considered to occur in methanolysis of tetrachlorophthalic anhydride ia the presence of pyridine, and charge-transfer complex formation has been confirmed neither by kinetic studies nor by spectrometry. Also, the conductivity of a binary solution anhydride-tertiary amine is much lower than that of the ternary system containing an epoxide and does not change with time. Antoon and Koenig also reject the formation of zwitterions. Hence, the first modification of Fischer s mechanism performed by Tanaka and Kakiuchi is not appropriate, which was later admitted by the authors The formation of ionic species probably proceeds by the anionic mechanism... 

Polymerization initiated by electron transfer from a metal, or by an aromatic radical anion, represents only one of the anionic mechanisms. It is, of course. 

There is no general agreement about the mechanism of these polymerizations. Both anionic and free-radical mechanisms were proposed as the most probable reaction paths. The role of sodium chloride is not clear in this mixture, though it was shown that it is essential. It may act, perhaps, as a support for the catalyst and may be a part of some sort of lattice involving both sodium alkoxide and allylsodium. The anionic mechanism is pictured as follows ... 

Polymerization initiated by electron transfer from a metal, or by an aromatic radical anion, represents only one of the anionic mechanisms. It is, of course, possible to consider separately those polymerizations initiated directly by organometallic compounds. Of the latter, the organohthium compounds are probably the best examples, since they are soluble in a wider variety of solvents and are relatively stable. Furthermore, it is these organometallic compounds which are used commercially for the preparation of synthetic elastomers [161,162]. The mechanism of these polymerizations is somewhat simpler than in the case of sodium napththalene, since there is no electron transfer step thus... 

It is, of course, the anionic mechanism which is most suitable for the synthesis of block copolymers, since many of these systems are of the living polymer type, as described previously [144,150,165,194,262,263]. Thus it is possible to use organoalkah initiators to prepare block copolymers in homogeneous solution by sequential addition of monomers, where each block has a prescribed molecular weight, based on monomer-initiator stoichiometry, as well as a very narrow molecular weight distribution (Poisson) [185]. As would be expected, such block copolymers are very pure, due to the absence of any side reactions during the polymerization (e.g., termination, monomer transfer, branching). 

Much of the recent hteiature on the mechanism of the enantioselective intramolecular Mizoroki-Heck reaction has focused on the anionic mechanism, o-iodoanilide substrates, pathways involving neutral pentacoordinate palladium intermediates and the influence of additives. The new examples and mechanistic findings indicate that the potential may exist to control the stereoselectivity of the intramolecular Mizoroki-Heck reaction through pathways other than the cationic mechanism. However, further research is needed to obtain the level of effectiveness of the traditional cationic pathway. 

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