Oxygen-centred

When rationalizing the significant difference of the hydrocarbon- and ether-bridged radical anions, the main aspect will certainly be the conformation of the oxyethylene chain, which brings the electrophores into closer contact. An additional aspect follows from the ability of the oxygen centres along the chain to chelate the counterion and thus to fix the cation between the electrophores. It is not possible from the available experimental evidence to discriminate between the two effects. The role of ion pairing and the relative position of the counterion and carbanion will be dealt with below. 

Smith and Hanson, 1971). Upon addition of NaBPh4, the free anion was converted into the ion pair, and upon addition of dibenzo-18-crown-6 [11] the ion pair was converted into the free anion. The results (Table 26) show the free anion to be much more reactive than the ion pair towards both methyl iodide and methyl tosylate. The increase in rate is again mainly due to the enhanced reactivity of the hard oxygen centre. The ratio of the rates of alkylation at the two sites of the ambident anion (kN/k0) decreased from about 1.2 for the ion pair to 0.55 and almost zero for the free anion. Increased alkylation of the hard... 

The ammonium catalyst can also influence the reaction path and higher yields of the desired product may result, as the side reactions are eliminated. In some cases, the structure of the quaternary ammonium cation may control the product ratio with potentially tautomeric systems as, for example, with the alkylation of 2-naph-thol under basic conditions. The use of tetramethylammonium bromide leads to predominant C-alkylation at the 1-position, as a result of the strong ion-pair binding of the hard quaternary ammonium cation with the hard oxy anion, whereas with the more bulky tetra-n-butylammonium bromide O-alkylation occurs, as the binding between the cation and the oxygen centre is weaker [11], Similar effects have been observed in the alkylation of methylene ketones [e.g. 12, 13]. The stereochemistry of the Darzen s reaction and of the base-initiated formation of cyclopropanes under two-phase conditions is influenced by the presence or absence of quaternary ammonium salts [e.g. 14], whereas chiral quaternary ammonium salts are capable of influencing the enantioselectivity of several nucleophilic reactions (Chapter 12). 

Transformations of the radical cations of 2,3- and 2,5-dihydrofuran (DHF), radi-olytically generated in Freon matrices, were investigated by low-temperature EPR. The 2,3-DHF+ radical cation is stable at 77 K but at higher temperatures is transformed into dihydrofuryl radical, DHF. The oxygen-centred radical cation 2,5-DHF+ is unstable at 77 K and transforms via an intramolecular H-shift into a transient distonic radical cation 2,4-DHF+ which at higher temperatures yields the DHF radical. 

Addition of chromium(ll ) [114] or tin(li) [115] ions to the electrolyte strongly favours the formation of head-to-head pinacols from the reduction of enones. It is believed that these ions direct tlie dimerization step by coordination through the oxygen centre... 

Radical-anions of carbonyl compounds are basic and undergo protonation at the oxygen centre generating highly reactive radical species. 

A single example has been reported of a bis(aluminium) oxide-alkoxide based on an Al(/x-0)2Al motif where one oxygen centre represents the oxide while the other one is part of the organooxide moiety [47]. [(i-Pr)OH]3 ClAl(/x-0) [fi-0 (f-Pr)]AlCl2 reveals a 4-membered metallocycle with the sterically-available bridging oxide centre also coordinating to a molecule of AICI3. 

Several other classes of polydentate amide ligands in which the ligating atoms are various types of nitrogen (and occasionally oxygen) centres have been investigated. Prominent examples of these are the podand type of ligand as shown in G-J, introduced by Verkade, Schrock, Gade and coworkers. 

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