Hydrogen bonds cation-radicals

Returning to the exchangeable proton, we assume that this is the hydroxyl proton. It is obvious that one can discriminate between the cation or the neutral hydrogen-bonded species by means of the hf parameters of this proton. Comparing the hf parameters of this proton with model systems for cations of the type (CHjOH) and for neutral hydrogen-bonded nitroxide radicals, it is derived in [11] that for the cation one would expect = 2.5-3.0 MHz and for the neutral hydrogen-bonded radicals ai = 3.Q-4.6 MHz and = 8-9 MHz neglecting of the nqi. Our data (ai , = 4 and =... 

Fig. 10 Side view of the cation radical stacks in the [Au(ppy)(C8H4S8)]2[PF6] (left) and [Au(ppy) (C8H4S602)]2[BF4] (right) salts. The packing motif is influenced by S—S contacts in the [Au(ppy) (C8H4S8)]2[PF6] salt and O-HC hydrogen bonds in [Au(ppy)(C8H4S602)]2[BF4]...

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

Product 34 predominates in the polar aprotic solvent (acetonitrile), while in the polar protic solvent (methanol) products 35 are formed preferentially. The different products are caused by the relative rate of deprotonation against desilylation of the aminium radical, that is in turn governed by the action of enone anion radical in acetonitrile as opposed to that of nucleophilic attack by methanol. In an aprotic, less silophilic solvent (acetonitrile), where the enone anion radical should be a strong base, the proton transfer is favoured and leads to the formation of product 34. In aprotic solvents or when a lithium cation is present, the enone anion radical basicity is reduced by hydrogen bonding or coordination by lithium cation, and the major product is the desilylated 35 (Scheme 4). 

Hydrogen bonding between an ion-radical and solvent may also enhance the ion-radical reactivity. Thns, the formation of hydrogen bond between methanol and the p-diketone cation-radical accelerates its deprotonation according to Scheme 5.14 (Jiao et al. 2007). 

Hydrogen bonding can facilitate the transition of conventional ion-radicals into their distonic forms. The distonic forms have been discussed in Section 3.2.4. It is sufficient to mark here the corresponding transformation of the pyridine cation-radical in the presence of water (Ibrahim et al. 2007). 

The formation of intramolecular hydrogen bonds can also anchor specific conformations of the amino acid cation-radicals. Using a combination of resonant two-photon laser ionization and... 

Simple alkenes such as 1-octene are completely resistant to this cation-radical hydrogenation. This makes it possible to reduce a more ionizable double bond selectively in the presence of a simple alkene moiety as illustrated for l,l-bis(anisyl)hexa-l,5-diene in Scheme hexa-1,5,7.5 (Mirafzal et al. 1993). 

Among the electron-rich alkenes, vinylsulfides are especially amenable to cation-radical reduction an important feature is the absence of hydrogenolysis of carbon-sulfur bonds. The reduction of [(phenylthio)methylene]cyclohexane is efficient (88%), and the retention of the phenylthio group clearly contrasts with catalytic hydrogenation (Mirafzal et al. 1993). This provides versatile functionality for further synthetic operations. 

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