Radical zinc chloride

Alkyl Isoquinolines. Coal tar contains small amounts of l-methylisoquinoline [1721-93-3] 3-methylisoquinoline [1125-80-0] and 1,3-dimetliylisoquinoline [1721-94-4J. The 1- and 3-methyl groups are more reactive than others in the isoquinoline nucleus and readily oxidize with selenium dioxide to form the corresponding isoquinoline aldehydes (174). These compounds can also be obtained by the hydrolysis of the dihalomethyl group. The 1- and 3-methyhsoquinolines condense with benzaldehyde in the presence of zinc chloride or acetic anhydride to produce 1- and 3-styryhsoquinolines. Radicals formed by decarboxylation of carboxyUc acids react to produce 1-aIkyhsoquinolines. 

This study suggests a radically new explanation for the nature of Lewis acid activation in the Simmons-Smith cyclopropanation. The five-centered migration of the halide ion from the chloromethylzinc group to zinc chloride as shown in TS2 and TS4 has never been considered in the discussion of a mechanism for this reaction. It remains to be seen if some experimental support can be found for this unconventional hypothesis. The small energy differences between all these competing transition states demand caution in declaring any concrete conclusions. 

In the presence of zinc chloride, stereoselective aldol reactions can be carried out. The aldol reaction with the lithium enolate of /-butyl malonate and various a-alkoxy aldehydes gave anti-l,2-diols in high yields, and 2-trityloxypropanal yielded the syn-l,2-diol under the same conditions.633 Stoichiometric amounts of zinc chloride contribute to the formation of aminoni-tropyridines by direct amination of nitropyridines with methoxyamine under basic conditions.634 Zinc chloride can also be used as a radical initiator.635... 

Lewis acid catalyst is normally required when ammonium polyhalides are used, although recourse does not have to be made to strong acids, such as aluminium trichloride. Bromination and iodination reactions are normally conducted in acetic acid in the presence of zinc chloride [32], but chlorination using the ammonium tetrachloroiodate in acetic acid does not require the additional presence of a Lewis acid [33]. Radical chlorination of toluenes by benzyltrimethylammonium tetrachloroiodate in the presence of AIBN gives mixtures of the mono-and dichloromethylbenzenes [34], Photo-catalysed side-chain chlorination is less successful [35], Radical bromination using the tribromide with AIBN or benzoyl peroxide has also been reported [36, 37],... 

If cation-radicals are in fact formed, there should be the same number of anion-radicals as counterparts. This stoichiometry is not established. Neither anion-radical nor anionic propagation has been detected. Since anionic propagation would form homopoly AN, the total polymer was extracted with saturated zinc chloride solution. There was, however, no extractable fraction. 

Russian workers have recently claimed that polymerization of methyl methacrylate in the presence of zinc chloride yields isotactic polymer. Polymer 6, made according to their procedures, does not appear to be significantly different from what would be normally expected for free radical polymerization at or near room temperature (28). 

F. W. Billmeyer. The free-radical polymerization of methyl methacrylate, acrylonitrile, and other polymer monomers can be accelerated by adding Lewis acids, like zinc chloride or alkylaluminum chloride. The polar monomer forms a complex with the Lewis acid and becomes more electron accepting. In the presence of a nonpolar olefin or conjugated diene, the complexed polar monomer transfers its charge and copolymerizes readily, as described by N. G. Gaylord and A. Takahashi. 

T he free radical initiated polymerization of polar monomers containing pendant nitrile and carbonyl groups—e.g., acrylonitrile and methyl methacrylate—in the presence of metal halides such as zinc chloride and aluminum chloride, is characterized by increased rates of polymerization (2, 3, 4, 5,10, 30, 31, 32, 33, 34, 53, 55, 65, 66, 75, 76, 77, 87). Imoto and Otsu (30, 33, 34) have attributed this effect to the formation of a complex between the polar group and the metal halide. The enhanced reactivity of the complexed monomer extends to copolymerization with uncomplexed monomers, such as vinylidene chloride, which are readily responsive to... 

The free radical initiated homopolymerization of acrylonitrile and methyl methacrylate in the presence of zinc chloride was characterized by an increase in the reaction rate and the molecular weight with increas-... 

The increased reactivity of propagating radicals complexed with the metal salt towards uncomplexed monomer has been postulated (87) to account for the increased susceptibility of methyl methacrylate to ultraviolet light or free radicals in the presence of aluminum chloride or aluminum bromide. A similar explanation has been considered to account for the enhanced reactivity in the methyl methacrylate-zinc chloride and acrylonitrile-zinc chloride polymerization systems. However, the complex shown in Reaction 20 must equally be considered. 

Russian workers have proposed that the increased activity of allyl acetate and allyl alcohol in free radical or gamma ray initiated polymerization in the presence of zinc chloride may be connected with the decreased degradative chain transfer with complexed monomer or the activation of the stabilized allyl radical in the complexed monomer—i.e., the conversion of degradative chain transfer to effective transfer (55, 87). However, these explanations have been partially rejected as inadequate. 

The most significant observation in the radical copolymerization of methyl methacrylate with vinylidene chloride in the presence of zinc chloride is the increase in the Q and e values of methyl methacrylate, the increase in the rx value of methyl methacrylate, and the decrease in the r2 value of vinylidene chloride (30). Although it has been proposed that these results arise from the increased reactivity of the complexed methyl methacrylate monomer, a more likely explanation is the homopolymerization of a methyl methacrylate-complexed methyl methacrylate complex accompanied by the copolymerization of methyl methacrylate with vinylidene chloride. 

The free radical copolymerization of methyl methacrylate or acrylonitrile in the presence of zinc chloride with allylic compounds such as allyl alcohol, allyl acetate, and allyl chloride or butene isomers such as isobutylene, 1-butene, and 2-butene is characterized by the incorporation of greater amounts of comonomer than is noted in the absence of zinc chloride (35). Analogous to the radical homopolymerization of allylic monomers in the presence of zince chloride, the increase in the electron-accepting capability of the methyl methacrylate or acrylonitrile as a result of complexation results in the formation of a charge transfer complex which undergoes homopolymerization and/or copolymerization with a polar monomer-complexed polar monomer complex. 

In view of the failure to produce any polymer in the presence of peroxide and the absence of zinc chloride, probably caused by the slow rate of radical formation at 60°C., it is also unlikely that the excess polar monomer enters by the coupling of a growing chain with a radical from the diradical complex. It is significant, however, that a very small concentration of radicals is sufficient to open the complex. 

Owing to the insolubility of the polar monomer-zinc chloride complex, handling of the reaction mixture is difficult. However, a second patent (73) describes an improved process wherein the polar monomer is utilized in considerable excess with no effect on the polar monomer content of the resulting copolymer, in contrast to the results from a conventional free radical polymerization. This is consistent with the mechanism shown in Reaction 23 and essentially eliminates the participation of a polar monomer-complexed polar monomer complex. 

The process improvement patent (73) also describes the preparation of an acrylonitrile-isoprene copolymer using azobisisobutyronitrile as free radical initiator and zinc chloride as complexing agent. The reaction is carried out using 1-50 moles of acrylonitrile per mole of isoprene and a 10 1 molar ratio of acrylonitrile to zinc chloride. The copolymer, obtained in 16.8% yield (calculated as a 1 1 copolymer), contains 48.2 weight % of acrylonitrile, corresponding to a 1.2 1 acrylonitrile-isoprene molar ratio, and is soluble in acetone and chloroform, in marked contrast to the solubility of the corresponding copolymer prepared with free radical initiators. 

Further evidence for the hypothesis was found in the patent describing the isoprene—acrylonitrile—zinc chloride system (23). On adding a four-fold excess of isoprene to an equimolar mixture of acrylonitrile and zinc chloride, in the absence of a free radical catalyst, an exothermic reaction occurs after approximately 30 minutes. The recovered polymer is insoluble in hydrocarbons, chloroform, and acetone. This eliminates polyisoprene and the alternating copolymer. The yield of product is 12%, calculated a polyacrylonitrile, compared with the 16.8% yield of copolymer obtained when excess acrylonitrile and a free radical catalyst are used. 

As shown in Table II, in the absence of a free radical catalyst, the reaction of isoprene with acrylonitrile in the presence of zinc chloride results in a less than 10% yield of product containing 66-74 mole % acrylonitrile. In the presence of both free radical catalyst and zinc chloride the yield of product is greatly increased. The product contains more than 85 mole % acrylonitrile when the isoprene and catalyst are added to precomplexed acrylonitrile. 

In the absence of zinc chloride, the free radical catalyzed polymerization of isoprene and acrylonitrile gave a 19% yield of copolymer containing more than 40% acrylonitrile. 

The effects of an equimolar amount of zinc chloride and the presence of a free radical catalyst in the copolymerization of equimolar amounts of acrylonitrile and isoprene at 60°-70°C. are summarized in Table III. 

In the absence of a free radical catalyst, Reaction 25 is dominant, particularly when precomplexed acrylonitrile is present, accompanied by Reaction 27. In the presence of both a free radical catalyst and zinc chloride, Reactions 25, 26, and 27 occur simultaneously. In addition, Reactions 26 and 27 are initiated by free radicals. 

The microstructure of the copolymer produced in the absence of zinc chloride is reasonably consistent with the analysis of a commercial free radical 1 1 copolymer—i.e., 20.6% cis-1,4 and 73.8% trans-1,4 structure. The increased 1,4 content in the polymer produced in the presence of zinc chloride is consistent with the postulated charge transfer complex mechanism. 

Hirooka has proposed that the products are alternating copolymers produced through complex copolymerization and that the latter process differs from that of Imoto and Otsu (30, 33, 34) in which a free radical initiator is necessary for the random copolymerization of olefins with acrylonitrile or methyl methacrylate in the presence of zinc chloride. 

As previously discussed, the copolymers produced in the zinc chloride-free radical system are not necessarily random copolymers but are probably the result of the copolymerization of the acrylonitrile-complexed acrylonitrile complex with the olefin-complexed acrylonitrile complex. Further, the olefin-alkylaluminum halide complexed acrylonitrile complex only differs from the olefin—zinc chloride complexed acrylonitrile complex in degree rather than in kind—i.e., the former is an unstable charge transfer complex capable of spontaneous uncoupling of the diradical system followed by intermolecular diradical coupling, while the latter is a stable charge transfer complex requiring radical attack to uncouple the diradical system. 

Two older direct methods are of limited scope. Tetrafluorohydrazine reacts thermally with alkenes at SO C, 6 h to give a,P-bis(difluoroamino) compounds in good yields.Tetramethyltetrazene-zinc chloride complex, a source of dimethylamino radicals, adds to a-methylstyrene in 30-40% yield.Bar-... 

Enholm [26] has reported the first examples of asymmetric radical cy-clizations on soluble polymer supports. The stereocontrol element employed consists of a (+)-isosorbide group attached by a 4-carbon chain to each subunit of a soluble succinimide-derived ROMP backbone. Treatment of the radical cychzation substrate 162 with tributyltin hydride in the presence of zinc chloride followed by hydrolysis of the resulting polymer-supported ester 163 gave the desired product 164 in 80% yield and > 90% ee (Scheme 38). The use of alternative Lewis acids, such as magnesium bromide etherate and ytterbiiun (III) triflate, resulted in lower enantioselectivities, 84% and 72% respectively. No such decrease in selectivity was observed in analogous reactions carried out off-support [27], suggesting that the polymer backbone is somehow responsible for this phenomenon. 

The reaction reaches equilibrium rapidly if the hydroxyl group is attached to a reactive radical, as in tertiary alcohols. In such cases, excess of the halogen acid is shaken with the alcohol, and the mono-halogen compound is separated. With primary and secondary alcohols, equilibrium is reached more slowly, and a catalyst (such as zinc chloride in the case of hydrochloric acid, and sulfuric acid in the case of hydrobromic acid) is used. On account of its cost, hy-driodic acid is not used, but iodine and phosphorus, which react as follows ... 

The comonomer complexes may be anchored on the cellulose, analogous to structures I and II, or through the interaction of zinc chloride and the cellulosic hydroxyl groups aqueous solutions of the metal halide are known to break the hydrogen bonds in cellulose and reduce crystallinity. Although radicals generated on the cellulose as the result of reaction with the catalyst may initiate polymerization of the comonomer... 

Catalyzed cyclization reactions for the synthesis of isoquinolines were the focus of various reports. 4-Aryl-l,2,3,4-tetrahydroquionlines were synthesized in good yield via a quinone methide mediated cyclization in the presence of zinc chloride <04TL7487>. The intramolecular radical cyclization of a, 3-unsaturated amides 85 was reported for the synthesis of isoquinoline analogs 86. In this study a-unsubstiuted acrylamides afforded 6-exo products exclusively. On the other hand, substrates bearing an a-chlorine (X = Cl) substituent provided the 1-endo benzazepine derivatives <04TL2335>. 

Inorganic compounds can also act as radical initiators. Zinc chloride (ZnCl2) has been used to initiate radical reactions at —78 °C. In following case (Scheme 19), zinc chloride acts as a radical initiator as well as a chelating agent. 

---