Radical ring-opening illustration

The preparation of the radical ring-opening polymerization monomer, 5-methylene-2-phenyl-l,3-dioxolan-4-one, (I), is illustrated in Eq. (1). 

Figure 2. Schematic illustration of radical ring-opening copolymerization of cyclic monomers with vinyl monomers.

There are some reports in the literature of ring-opening polymerizations by free-radical mechanism. One is a polymerization of substituted vinyl cyclopropanes.The substituents are radical stabilizing structures that help free-radical ring-opening polymerizations of the cyclopropane rings. This can be illustrated as follows ... 

A novel synthesis of a C-3-branched glycal derivative, by application of a cyclopropylmethyl radical ring-opening reaction, is illustrated in Scheme 4. ... 

The formation of butanes by reduction of arylethenes may arise by radical-radical coupling of two radical-anions giving a dianion, which is then protonated. An alternative route is by nucleophilic addition onto one neutral molecule of the radical-anion, followed by further reduction and protonation. In support of this alternative, cyclobutanes have been isolated from electrochemical reduction of phenylvinylsulphones [21] and vinylpyridines [22], A mechanism for the latter process is illustrated for the case of 2-vinylpyridine 7. Nucleophilic attack of a radical-anion on the substrate gives an intermediate and this disproportionates to form the cyclobutane and a 1,4-diary Ibutane. Cyclobutanes are themselves reduced with ring opening to form the 1,4-diarylbulane. 

Cyclic siloxanes can undergo a ring-opening polymerization that is a chain-growth process. Free radicals are not useful as initiator species, because of the nature of the siloxane bond, but anionic and cationic initiators are very effective. The reaction is illustrated using the most common cyclic oligomers, the trimer hexamethylcyclotrisiloxane... 

Illustrations of the application of LSV for studies of reactions under purely kinetic conditions include the oxidation of 9-substituted fluorenide ions [99] and the reduction of 2,6-diphenylpyrylium ions [100], in both cases leading to the neutral radical, which dimerizes in an RR-type reaction, the oxidation of 1,4-dithiafulvenes into tetrathiafulva-lenes [101], the oxidative ring opening of arylcyclopropanes [102], the reduction of fluoro-alkoxyarenes in liquid ammonia [103], and the competition between protonation and dimerization during the reduction of cinnamic acid esters in MeOH [104]. 

An electron transfer strategy has been used in the irradiation of the cyclopropylamine derivatives (87) and other related molecules.The reactions are carried out in acetonitrile/water with DCB as the sensitiser and the radical cation of (87) formed ring opens to afford a radical cation of the type illustrated as (88). Within this intermediate only cyclization occurs and there is no evidence for a hydrogen transfer path. There are two modes of cyclization that occur and both are 5-cxo-radical in type giving the products shown in Scheme 3 in good yield. 

FIGURE 3.4 Illustration of a radical clock. The ratio of unarranged hydroxylation product and rearranged product reflects the ratios of rebound rate constant (kT) and ring opening of the radical intermediate (kc). In this case, kc — 3.4 x 1011 s 1. 

Silverman and Zieske have rationalized how a protein nucleophile other than flavin is involved in MAO inactivation reactions, and why different inactivator compounds specifically react with flavin, protein amino acids, or both (100). Hydrogen atom donation from a cysteine residue to the flavin semiquinone radical would produce a thiyl radical, which could then capture the primary or secondary alkyl radical generated on cyclopropyl ring opening from the amine radical cation of the inactivator. The hydrogen atom abstraction reaction between the flavin and active site amino acid may be an equilibrium process such that either species could be present at any turnover. Hence, a combination of steric constraints and proximity to either the flavin semiquinone radical or the thiol radical will determine the site of adduct formation for a particular inactivator structure. A two-dimensional representation is shown in Scheme 23 (compounds 40-42), which illustrates the proposed equilibrium between the flavin semiquinone radical and amino acid as well as the proposed intermediates for the inactivation of MAO by A-(l-methylcyclopropyl)benzylamine 40 (104), rrradical center relative to the particular protein radical is consistent with proposed site of attachment of inactivator to protein 40 is near the flavin radical, such that exclusive flavin attachment occurs, 41 is positioned closer to the amino... 

The nature of the reactions of hydroxyl radicals with polynuclear aromatic hydrocarbons is illustrated by studies of products observed with naphthalene (Fig. 6.25)." Addition of the hydroxyl radical to the aromatic ring produces ring opening with the... 

Fig. 2.14 Scheme illustrating the synthesis of ABC type tri-block polymer with hydrophilic PEO middle block using AROP. 1 Anionic polymerization followed by hydrogenation, 2 Anionic ring-opening polymerization, and 3 Chain end functionalization followed by atom transfer radical polymerization... 

Sugiyama and co-workers (69,70) report a very interesting template process by the ring-opening polymerization induced by radicals. The polyethylene template was connected with 2,2-diphenyl-4-methylene-l,3-dioxolane groups by the covalent bonds. The process can be illustrated by the following reaction ... 

The difference and the similarity between allotropy and polymorphism can be illustrated by considering sulfur. The rhombic and monoclinic crystalline forms both consist of puckered Ss rings, and these two modifications can interconvert by heating and cooling. It is tempting to call this relationship crystal allotropy, but the correct term is polymorphism because both structures involve the same compound (i.e., atomic connectivity). When heated to above 160 °C, the Ss rings open by means of a free radical reaction to form polymeric chains. In contrast to crystal allotropy, the relationship between the polymeric chains and the Sg rings can be termed chemical allotropy. However, since polymorphism is the preferred term for crystal allotropy, chemical allotropy can be shortened to allotropy. 

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