Diradical coupling

R)-Reticuline, rewritten as in Figure 6.50, is the substrate for one-electron oxidations via the phenol group in each ring, giving the diradical. Coupling ortho to the phenol group in the tetrahydroisoquinoline, and para to the phenol in the benzyl substituent, then yields the dienone... 

Thus, the radical-monomer interaction involves a monoradical species which successively adds monomer units in an alternating copolymerization, while the monomer-monomer interaction involves the homopolymerization of a charge transfer complex through a diradical coupling... 

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. 

Mariano and coworkers have exploited this PET bond cleavage chemistry in intramolecular as well as intermolecular cyclization reactions [38]. The combined iminium-benzylsilane functionalities undergo intramolecular PET to provide an intramolecular amine radical/benzyl silane cation-radical pair by exciting either the iminium salt or the arene chromophore [38]. Cleavage of the benzyl C—Si bond presumably takes place with assistance of moderately weak n ldeophiles such as methanol. The diradical couples intramolecularly to provide an indolizidine derivative [38] in 90% yield. 

Another plausible mechanism fa- the Sml2-mediated Barbier reaction involves coupling of ketyl and alkyl radictds in a diradical coupling mechanism. Alternatively, addition of an alkyl radical to a Sm -activated ketone carbonyl may be invoked. ... 

These examples again have some mechanistic implications in that they appear to rule out cyclization via 5n2 displacement of the halide by a samarium ketyl. However, one cannot distinguish between a mechanism based on allylsamarium addition to the carbonyl versus an electron transfer mechanism as outlined for the alkyl hdide substrates above. Both mechanisms allow for isomerization of the double bond (via 1,3-allylic transposition in the case of an allylmetallic, or configurational instability in an allylic radical in a diradical coupling mechanism) and also provide reasonable routes for generation of butadiene. Further mechanistic work is clearly required in order to provide a more detailed understanding of all of these intramolecular Barbier-type reactions. 

Also present in these and other montane species are aryl—aryl dimmers, e.g. 34 from S. multicaulis (Ulubelen et al., 1997) and 35 from Decussocarpus rospigliossi, a large tropical conifer from the northern Andes (Amaro and Carroz, 1989) that call for diradical coupling as a likely key biosynthetic step. This suggests a role as a free radical trap to forestall molecular disruption by oxidative stress rather than a UV-B filter since there is no significant bathochromic shift in the dimmer relative to the aryl monomer because of the orthogonal conformation of vicinal aromatic rings. 

In a related but different vein, the cytochrome P450 enzyme StaP (CYP245A1) catalyzes a putative diradical coupling reaction involving one-electron oxidation of each of two indole rings to form the indolocarbazole alkaloid skeleton of staurosporine and rebeccamycin (Fig. 4.40) [230, 231]. The crystal stractuie of the protein-sub-... 

The propagation by diradical coupling could occur. However, it should be limited to low molecular moieties because we do not observe high molecular weight polymers (scheme 3, e,g, d2 + d3) ... 

Calculations have been carried out in order to assess the reactivity of cyclic and acyclic diaminocarbenes with CO2 in the presence of alcohols These calculations indicate that the formation of carbonate salts is favoured compared to the direct interaction with CO2. Because of their high values, these carbenes represent efficient systems for the capture and sequestration of carbon. Computations on the stereoselective transformation of 2-benzyloxyphenyl trimethylsilylketone to substituted benzofuran (Scheme 1) have been carried out. They revealed an exergonic 1,2-Brook rearrangement to an oxycar-bene, exhibiting two unpaired antiparallel electrons that feature in H migration followed by diradical coupling. 

Chen, Q.-Y. and Li, Z.-T., Pentafluorophenylation of aromatics with pentafluorophenyl perfluoro-and polyfluoroalkanesutfonates. A photoinduced electron-transfer cation diradical coupling process, /. Org. Chem., 58, 2599,1992. 

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