Free radical coupling

When 3-methoxybenzyl acetate is photolyzed, however, the products expected from free radical coupling are decreased and a substantial amount of 3-methoxybenzylalcohol results. A carbonium ion intermediate has been proposed in the photolysis of this meta ester ... 

Didehydroferulic Acid 23 Formation. It should be noted that no direct evidence has ever been obtained in vivo to prove that didehydroferulic acid 23 is formed via free radical coupling reactions. This mechanism is assumed, since in vitro incubation of ferulic acid 1 and wheat pentosan, with... 

Attachment of Hydroxycinnamic Acids to Structural Cell Wall Polymers. Peroxidase mediation may also result in binding the hydroxycinnamic acids to the plant cell wall polymers (66,67). For example, it was reported that peroxidases isolated from the cell walls of Pinus elliottii catalyze the formation of alkali-stable linkages between [2-14C] ferulic acid 1 and pine cell walls (66). Presumably this is a consequence of free-radical coupling of the phenoxy radical species (from ferulic acid 1) with other free-radical moieties on the lignin polymer. There is some additional indirect support for this hypothesis, since we have established that E-ferulic acid 1 is a good substrate for horseradish peroxidase with an apparent Km (77 /tM), which is approximately one fifth of that for E-coniferyl alcohol (400 /iM) (unpublished data). 

Aromatic Ring Cleavage of Phenolic 0-0-4 Substructure Model Compounds by Laccase. When vanillyl alcohol was used as a substrate, only biphenyl formation (C5-C5 linked) occurred and no evidence for the formation of any ring-opened products was obtained (26). Hence, we also examined the effect of laccase on the sterically hindered 4,6-di-<-butylguaiacol substrate 50, as it would be unlikely to undergo such free-radical coupling reactions... 

The above reactions proceed via free radical coupling. An alternative system for photochemically driven hydrocarbon functionalization evidently proceeds via the carbanion, which is obtained from reduction of the initially formed free radical3. The carbanion reacts with acetonitrile to give, after in situ hydrolysis, the methyl ketone, e.g., formation of (tricyclo[3.3.1.13-7]dec-1-yl)ethanone6. 

Bipyridyl (20) (63AHC(2)179) is synthesized by oxidative dimerization of pyridine over hot Raney nickel, while 4,4 -bipyridyl (21) (B-79MI10705) is made by free radical coupling of the pyridine radical anion generated by sodium in liquid ammonia, followed by air oxidation (Scheme 6). Quaternization of (21) with methyl chloride gives paraquat while reaction of (20) with 1,2-dibromoethane gives diquat. 

Fraser-Reid s stereocontrolled synthesis of the Woodward reserpine precursor 195 relied upon a tandem 5-exol6-exo radical cyclization of pyranose-derived dienes [76-77]. As outlined in Scheme 36, a,P-unsaturated ester 188 was prepared by free radical coupling of iodide 187 with a tin acrylate. After hydrolysis of 188 (MeONa, MeOH, 100%) to give primary alcohol 189, the silicon tethered diene 190 was installed by silylation. Treatment of 190 with n-BujSnH led to the desired cage molecule 192 in high yield via a 5-exo-trig cyclization to intermediate 191 followed by a 6-exo cyclization. Tamao oxidation of tricycle 192 led to diol... 

The mechanism of polyamine hydrogenation (Fig. 6.8) is believed to involve successive electron transfer (from polyamine to fullerene) - proton transfer (from polyamine radical cation to fullerene radical anion) steps (Briggs et al. 2005 Kintigh et al. 2007). At or near room temperature, aliphatic amines and polyamines are known to hydroaminate [60]fullerene (Miller 2006), likely also involving preliminary electron transfer - proton transfer steps followed by free radical coupling of C and N based radicals (Fig. 6.8). At elevated temperatures in polyamine solution, however, this latter free radical coupling step becomes uncompetitive with... 

VII. The Free Radical-Coupled Copper Active Site. 36... 

Whittaker, J. W., 1994, The free radical-coupled copper active site in galactose oxidase. Metal Ions in Biology, Editors Sigel, H. and Sigel, A., Marcel Dekker publishers, 30 3159 360. 

The generation of a tertiary radical by the later route is the predominant reaction. The tertiary radicals preferentially undergo disproportionation, resulting in degradation rather than crosslinking. The number of crosslinks increased at room temperature after exposure of EPR to y-rays radiolysis [38] at —196 °C. It is because the life time of free radicals is less at room temperature than at lower temperature and the free radicals couple instantly at room temperature to cause crosslinking. 

The structure of III shows that a C-H bond, presumably that of the tertiary carbon of the isopentyl substituent, is somehow being replaced with a C-C linkage, a process customarily achieved by means of free radical couplings. However, such a radical, as well as any sort of carbanion, would be unlikely in aqueous phosphoric acid. There seem to be few alternatives left at this point. 

Biosynthetic pathways Assignments NH protons Paramagnetic probes Metal ions Free radicals Coupling constants... 

Barton and Cohen 10) and Erdtman and Wachtmeister 11) have related the concept of free radical coupling of phenols to the biogenesis of natural products, and suggested that bisbenzylisoquinoline alkaloids are formed by this reaction from benzylisoquinoline units. The mechanism of the reaction consists of generation of the resonance-stabilized phenoxy free radical by one-electron oxidation of the phenoxy anion, followed by coupling and tautomerization to form hydroxylated diphenyls or diphenyl ethers. Intermolecular coupling may involve any... 

The only bisbenzylisoquinoline alkaloids whose structures preclude the simple oxidative pairing mode of synthesis are those containing three diphenyl ether linkages (trilobine, isotrilobine, menisarine, normenis-arine, and micranthine). However, Barton and Cohen (10) have proposed a mechanism for the formation of the dibenzo-p-dioxin system of these alkaloids which comprises phenoxy free-radical coupling with a subsequent migration reaction. 

Thus, peroxidases may potentially effect direct free radical coupling reactions between aniline and humic substances or create additional substrate sites within the fiilvic or humic acid molecules for nucleophilic addition by aniline. A model for the latter pathway can be found in the work with guaiacol and 4-chloroaniline by Simmons et al. (19). Peroxidase catalyzed the coupling of guaiacol, itself not a substrate for nucleophilic addition, into the extended quinonoid dimer which subsequently underwent nucleophilic attack by the chloroaniline ... 

Both evidence of the browning caused by the high-intensity absorption of conjugated chromophoric groups and upward shifts in the ultraviolet-visible spectral baselines indicate that higher hydroquinone concentrations produce more polymers. Free radical coupling is probably involved in the polymerization since the existence of free radicals is correlated to hydroquinone concentration. The ultraviolet-visible absorption profiles of these polymers formed in the systems that are not sterilized (Kung and McBride, 1988) appear to be similar to those of phenol-derived polymers in the systems free... 

---