Activation of alkyl halides

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

With respect to the co-catalytic activity of alkyl halides, BF3 occupies a special position, since these (other than fluorides) cannot form complexes with BF3 for steric reasons. It has indeed been found [31a] that in MeCl solution the n-butenes are not polymerised by BF3. MeCl cannot act as co-catalyst in this system and some other (e.g., S02) was required. The mode of action of S02 is still obscure, but it is possible that H2S03 was the real co-catalyst. 

The cocatalytic activity of alkyl halides in the cationic polymerization of styrene in the presence of stannic chloride (17), in the polymerization of butadiene with Et2AlCl-cobalt compound (10) and R3Al-cobalt compound (23) catalyst systems and in the cationic polymerization of isobutylene (12) and styrene (13) in the presence of Et2AlCl is well documented. It is reasonable to propose that a reaction between Et2AlCl and a labile chlorine atom on PVC results in the generation of a carbonium ion on the polymer backbone. 

Garegg,95,96 and others97,98 involved the activation of alkyl halides over S -alkyl/aryl glycosides. Subsequently, other examples of suitable LGa for selective activations have become available trichloroacetimidates,99,100 phosphite,101 phosphate,102 thioi-midate,103-106 thiocyanate,107,108 hemiacetal,109,110 sulfoxide,111,112 selenoglyco-side,113-116 orthoester,76,117-119 O-hetaryl,120-122 and others.123-129 Many of these couplings were performed with the use of S -alkyl/aryl, fluoro, or O-al kenyl/hetary 1 moieties as LGb. 

A series of well-defined PMMA were successfully prepared by reduction of Cu(II) to Cu(I) upon UV irradiation and subsequent activation of alkyl halide. A linear relationship was observed between monomer consumption and polymerization time and evolution of molecular weight versus... 

The reactivities of alkyl halides and pseudohalides in the activation process depend upon the structure of both the alkyl group and the nature of the transferable (pseudo)halogen. An early study demonstrated that the activity of alkyl halides that mimic halogen-capped dormant polymer chains in reactions of activation mediated by the Cu complex of di(5-nonyl)-2,2 -bipyridne (dNbpy) in acetonitrile decreased in the order Et-BriB (kact = 0.60M s )>l-PhEtBr (kact = 8.5 x 10 M s )>Me-BrP... 

In systems where Cu and Cu can coexist due to disproportionation, activation of alkyl halides by copper in both oxidation states can take place. From a thermodynamic point of view, the relative activity of Cu and Cu as ATRP activators can be correlated to the ability of the Cu complex to disproportionate. The disproportionation of a Cu complex yields the corresponding Cu complex and Cu . Even if one assumes that the formed Cu is so finely... 

As shown in Table IV, the highest catalytic activity of metal halides used as Lewis acid for the alkylation reaction of ferrocene with 2 was observed in methylene chloride solvent. Among Lewis acids such as aluminum chloride, aluminum bromide, and Group 4 transition metal chlorides (TiCl4, ZrCU, HfCU), catalytic efficiency for the alkylation decrea.ses in the following order hafnium chloride > zirconium chloride > aluminum chloride > aluminum bromide. Titanium chloride... 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Activation rate constants (k) in ATRP/ATRA are typically determined from model studies in which copper complex is reacted with alkyl halide in the presence of radical trapping agents such as TEMPO [127,128,129], Rates are determined by monitoring the rate of disappearance of alkyl halide in the presence of large excess of the activator (Cu X/L) and TEMPO. Under such pseudo-first order conditions, the activation rate constant can be calculated ln([RX]0/[RX]() vs.t plots (slope =-k) Cu C/... 

Although the tin hydride reductions of alkyl halides seem simple, one must be careful because these reactions occur by a free radical mechanism. This is important, because the carbon radical produced in the reaction can isomerize68,78 and one often obtains two different stereoisomers from the synthesis. Another problem is that chiral centres can be lost in tin hydride reductions when an optically active halide is reduced. One example of this is the reduction of benzyl-6-isocyanopenicillanate with tributyltin deuteride78 (Scheme 14). The amount of isomerization depends on the temperature, the concentration of the tin hydride and the presence of and /-substituents78-82. However, some authors have reported tin hydride reductions where no racemization was observed78. 

The next question which presents itself is whether we can explain why in some systems solvent co-catalysis occurs, whereas in others, apparently similar, it does not. Let it be said first that in fact there is very little experimental evidence on this point. From the thermochemical point of view one can say that alkyl halide co-catalysis is the more probable, the lower the heterolytic bond dissociation energy of the alkyl halide, the more stable the cation derived from the monomer, and the smaller the anion derived from the metal halide. It must, however, be remembered that the non-occurrence of alkyl halide co-catalysis may be due to a kinetic prohibition, i.e., an excessively high activation energy for a reaction which is thermodynamically possible. 

Acrylonitrile, polymerization, 120 Activity of phase-transfer catalysts Sjj2 reactions, 170-175 weak-nucleophile Sj.Ar reactions, 175-182 Acyltetracarbonyl cobalt compound, cleavage in the carboxyalkylation of alkyl halides, 150 Addition reactions, Michael, catalytic asymmetric, 69,70f... 

Thus it has been shown that some metal-carbonyl compounds can be activated by electrochemical reduction generating reactive anionic species. Without going into details, it is worth pointing out that the synthesis of aldehydes can be obtained by electrolyzing a stoichiometric mixture of alkyl halides and ironpentacarbonyls (Eq. 17) [124, 125] ... 

Results of a chemical activation induced by ultrasound have been reported by Nakamura et al. in the initiation of radical chain reactions with tin radicals [59]. When an aerated solution of R3SnH and an olefin is sonicated at low temperatures (0 to 10 °C), hydroxystannation of the double bond occurs and not the conventional hydrostannation achieved under silent conditions (Scheme 3.10). This point evidences the differences between radical sonochemistry and the classical free radical chemistry. The result was interpreted on the basis of the generation of tin and peroxy radicals in the region of hot cavities, which then undergo synthetic reactions in the bulk liquid phase. These findings also enable the sonochemical synthesis of alkyl hydroperoxides by aerobic reductive oxygenation of alkyl halides [60], and the aerobic catalytic conversion of alkyl halides into alcohols by trialkyltin halides [61]. 

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