Cationic alkoxide

For the preparation of the sol, different categories of reagents were nominated all alkoxides (as a source of cations), alkoxides and salts, all salts, all salts with suitable polymers or other oiganics for gel formulation, and all partial colloidal sols. The use of crystalline seed for gel crystallization is also mentioned [12],... 

The macrocychc hexaimine stmcture of Figure 19a forms a homodinuclear cryptate with Cu(I) (122), whereas crown ether boron receptors (Fig. 19b) have been appHed for the simultaneous and selective recognition of complementary cation—anion species such as potassium and fluoride (123) or ammonium and alkoxide ions (124) to yield a heterodinuclear complex (120). 

Hydrolysis of aluminum alkoxides is also used commercially to produce precursor gels. This approach avoids the introduction of undesirable anions or cations so that the need for extensive washing is reduced. Although gels having surface area over 800 m /g can be produced by this approach, the commercial products are mosdy pseudoboehmite powders in the 200 —300 m /g range (28). The forming processes already described are used to convert these powders into activated alumina shapes. 

Dialkyl and diarylthaHium(III) derivatives are stable, crystalline soHds that melt at 180—300°C. The dimethylthaHium derivatives of CN , CIO, BF, and NO 2 contain linear (CH2)2T1 cations and the free anions (19). In aqueous solutions, they ionize to the (CH2)2T1(H20) ions, except those derivatives containing alkoxide, mercaptide, or amide anions, which yield dimeric stmctures (20,21). 

Salt effects on the reaction of 2,4-dinitrochlorobenzene with amines or alkoxides have been investigated.Reinheimer et al. have studied decelerative ion pairing of alkali metal methoxides in reaction with this substrate cations and anions in added salts have specific effects on ion pairing. 

Alcohols can be selectively bound to the same host type if they are combined with an amine and vice versa, considering that a cation and an anion will be formed through a proton transfer. The so-formed alkoxide anion will bind to the boron atom, while the ammonium ion will be complexed by the crown ether (147, Fig. 39). Competition experiments involving benzyl-amine have shown enhanced selectivity for the complexation of alcohols with... 

Ligand A anionic (e.g., halides, pseudohalides, alkyl, aryl, thiolate, alkoxide) ligand N neutral (e.g., amines, imines, phosphines, carbenes, nitriles, isonitriles, NO, CO) ligand C cationic (e.g., NO+)... 

An important improvement in the oxy-Cope reaction was made when it was found that the reaction is strongly catalyzed by base.212 When the C(3) hydroxy group is converted to its alkoxide, the reaction is accelerated by a factor of 1010-1017. These base-catalyzed reactions are called anionic oxy-Cope rearrangements, and their rates depend on the degree of cation coordination at the oxy anion. The reactivity trend is K+ > Na+ > Li+. Catalytic amounts of tetra-rc-butylammonium salts lead to accelerated rates in some cases. This presumably results from the dissociation of less reactive ion pair species promoted by the tetra-rc-butylammonium ion.213... 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

The Michael addition of alkoxides to nitroalkenes gives generally a complex mixture of products due to the polymerization of nitroalkenes.16 The effect of cations of alkoxides has been examined carefully, and potassium- or sodium-alkoxides give pure p-nitro-ethers in 78-100% isolated yield (Eqs. 4.12 and 4.13).17 When lithium-alkoxides are employed, the yields are decreased to 20-40%. 

Most of these reactions are promoted with an inorganic base such as KOH, NaOH, or K2C03 as an essential co-catalyst. For reaction without alkaline bases see Mizushima et al.137 Many of these complexes contain a chloride ligand, which is easily displaced by an alkoxide displacement/ /3-hydride elimination sequence in the presence of a base to remove HC1 formed (Scheme 25). In contrast, cationic LnM+ systems add the alcohol by formation of M—O bond, with the base... 

A particularly interesting aspect of the reaction is its apparent counter-cation dependence. Our initial screen of bases included inorganic bases with a variety of counterions, including Li+, Na+, K+, and MgX+, but only potassium bases were effective. The precise role of the counterion is not completely understood, but it is critical for success. To date, the use of KOf-Bu or KH have consistently provided the best results, with KHMDS used as a lower-yielding alternative if absolutely necessary (in cases where the substrate was unstable to alkoxide bases). 

Addition of alkyllithium to cyclobutanones and transmetallation with VO(OEt)Cl2 is considered to give a similar alkoxide intermediates, which are converted to either the y-chloroketones 239 or the olefinic ketone 240 depending on the substituent of cyclobutanones. Deprotonation of the cationic species, formed by further oxidation of the radical intermediate, leads to 240. The oxovanadium compound also induces tandem nucleophilic addition of silyl enol ethers and oxidative ring-opening transformation to produce 6-chloro-l,3-diketones and 2-tetrahydrofurylidene ketones. (Scheme 95)... 

Mixed carbanion/alkoxide complexes were formed from combining n,sBu2Mg, TuOM (M = Na or K), and TMEDA to form the dimeric CIPs [(Bu)2(tBuO)MMg(TMEDA)]2 (M = Na and K) 437, 438.442 While the two structures are identical in their atom connectivity, they are not precisely isostructural in that K shows a bias toward C-coordination while Na is inclined toward N. The alkali metal cations are both formally five-coordinate bonding with two a-C(Bu) atoms, two N(TMEDA) atoms, and a single 0(cBu) atom. The dominant feature in both structures though is the [(Bu)2Mg(/x-tBuO)2MgBu2]2 dianion. 

These observations are consistent with the reactive species being constituted from tight ion pairs between cations and the alkoxide anions resulting from abstraction of hydrogen atoms in A, B and C (Scheme 3.11). 

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