N dimers

The pectin network revisited.--The importance of the interconnections of the pectic polysaccharides to the integrity of the pectin network has been highlighted by the recent discovery that RG-II is present in primary walls as a mixture of monomers and dimers [54]. The dimers are covalently cross-linked by borate diesters [55,56]. If single molecules of homogalacturonan are covalently attached to both RG-I and RG-II, the covalently cross-linked RG-n dimers would explain how the network of the three types of pectic polysaccharides is covalently connected and covalently cross-linked. 

Figure 22. Catalytic (C) and noncatalytic (N) dimer spheres and the collision events that occur on interaction of the A and B species with each sphere.

Very few examples of singlet n-n dimerizations are reported, the reactions of 2-butenes in Eqs. 36 and 37 being examples with observable stereochemistry. The reactions are stereospecific in agreement with theory. The unsensitized reactions of styrene 115> and some stilbene derivatives 116> may also proceed via excited states, Eq. 47 and... 

The most rapid reaction is N—N-dimerization (the rates of reactions A, B, C are related as 1 0.15 0.02 [94], Naphthylaminyl radicals recombine with the formation of N—C-dimers only [95], probably because voluminous naphthalene rings sterically hinder N—N-dimerization. A correlation between the rate constant of hyperfine splitting on the nitrogen atom of the aminyl radical and the rate constant of recombination of substituted ( (YC6H4)2N ) diphenyl-aminyl radicals was observed [95],... 

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. 

Phenylhydrazones were oxidized by NiOa at 0 °C via a radical mechanism to afford C-C and C-N dimers. ... 

It needs to be noted that Q + Q = Q + equilibrium is possible due to the rapid consumption of Q reducer. More often, the interaction of two identical ion-radicals can lead to their dimerization, but not disproportionation. For instance, the anion-radicals of tetracyanoethylene undergo dimerization (Lu et al. 2003). According to Miller and Novoa (2007), this dimerization proceeds as four-center carbon-carbon bonding. Rosokha and Kochi (2007) proposed the term n interaction for dimerization of the cation-radicals of tetrathiafulvalene. Because the n dimers are double-charged species, solvent polarity is essential for their formation. The n interaction overcomes the electrostatic repulsion, even in moderately polar enviromnents. 

For instance, inclusion of l,T-dimethyl-4,4 -dipyridyl cation-radical (the cation-radical of methyl viologen) into a cavity of sulfocalixarenes prevents the disadvantageous n dimerization. This is important for more effective use of viologens as components of electrochromic displays and electric condnctors (Gno et al. 2007, references therein). 

Radicals (5) and (6) were shown by GAUSSIAN-82 calculation to be stable. They should be easily (activation energy 1.3 2.0 kcal mol ) derived from the parent NH compounds by elimination of H atoms and ought to be highly reluctant (activation energy > 33 kcal mol ) to N—N dimerize <88CCC2128>. These expectations were largely confirmed for the radical (5) synthesized at almost the... 

Daphnodorin N Dimers (chalcone-flavan-3-ol) C30H22O10 Daphne acutiloba Thymelaeaceae 143... 

Two aminyl radicals combine to form the N—N-dimer with the rate constant proportional to the spin density on the nitrogen atom [96]. This recombination also depends on the steric factor. Thus, diethylaminyl radicals recombine with a rate constant of 109 L mol 1 s 1, while diisopropylaminyl radicals are only able to disproportionate with a rate constant of... 

The coordinative environment of the metal atom in the structures of Fe(OEC)Cl (Fig. 14b) and Fe(OEC)Ph (Fig. 14c) is similar to that found in the p-oxo complex but in both complexes the ligand assumes a more planar conformation. The a-bonded complex exists in the crystal as a n-n dimer, the intermolecular distance being 3.5—3.6 A. 

Complete electron transfer from arenes results in aromatic cation radicals, which differ dramatically in their donor/acceptor properties. As such, cation radicals can interact with the parent donor. The resulting (CT) interaction accounts for the self-assodation (n-dimerization) of aromatic donors with their corresponding cation radicals and results in the formation of charge-resonance (CR) complexes or 7r-mers [49]. 

The results seem to indicate that arylamines do not follow the same mechanism that presumably underlies the coupling of oxygen-substituted aryls. According to the authors, N-N dimerization (70 —> 126) is a competitive pathway that is favored with hindered substrates. This N-N coupling can eventually lead to both the C-C coupled product (path a) and the carbazole product (path b). The ratio depends upon the structure of the starting material (Scheme 31). 

Another example of covalent-ionic resonance is provided by VB studies of N-N dimers of HNO [61]. It has been calculated that the barrier to rotation around an N-N bond involves a substantial contribution from three-electron bond... 

Several theoretical studies of diazirinyl radicals have been undertaken. Multiconfiguration self-consistent field (MCSCF) calculations have been used to investigate the stability of, and isomerization between, the diazirinyl anion 6 and the open-chain biradical anion 7 (Scheme 1) <1995JPC6548>. Dimerization of aryldiazirinyl radicals has been explored by computational and experimental methods <2004PCCP756>. Both approaches support the predominance of the N-N dimer 9 over the isomeric C-N form 8. Furthermore, bimolecular dimer decomposition was shown to be considerably more favorable than the alternative unimolecular pathways. 

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