Alkoxide donors

Figure 52 shows the structure of [Mn4" (HL33)(L33)(NCS)4], which can also be viewed as either a dimer-of-dimers or a severely distorted butterfly. The links between the two macrocycles are two triply bridging alkoxide donors, one from each macrocycle. The wings of the butterfly have closed to bring the two outer manganese ions quite close to one another an extension of this movement would result in formation of a cubane. 

One subset of Schiff base complexes are the hydrazone complexes which have a R—C=N—N functional group instead of the R C=N—C functionality. These compounds have been prepared as models for bromoperoxidase389 and other biological systems.390 In the solid state, the hydrazones have coordination patterns similar to those of the Schiff bases and the majority of the complexes are oxovanadium(V) hexadentate or pentacoordinate complexes with tridentate 02N donor sets (see Table 5). While most structures contain alkoxide donors,390-403 complexes have been reported with diols and catechols,129,404,405 hydroxamic acids,288,391 hydroxy quinolinate,406, 7 and benzoylhydrazine.408 In addition, dioxo,375,409-412 oxo-bridged... 

A competing reaction in any Birch reduction is reaction of the alkali metal with the proton donor. The more acidic the proton donor, the more rapid IS the rate of this side reaction. Alcohols possess the optimum degree of acidity (pKa ca. 16-19) for use in Birch reductions and react sufficiently slowly with alkali metals in ammonia so that efficient reductions are possible with them. Eastham has studied the kinetics of reaction of ethanol with lithium and sodium in ammonia and found that the reaction is initially rapid, but it slows up markedly as the concentration of alkoxide ion in the mixture... 

Many aromatic steroids submitted to the Birch reduction contain hydroxyl groups which are deprotonated to the corresponding alkoxides during the reduction, particularly if a tertiary alcohol is used as the proton donoi. The steroidal alkoxides and the one derived from the proton donor often precipitate and cause foaming of the reaction mixture, as was noted by Wilds and Nelson. These alkoxides can be kept in solution by adding an excess of the proton donor alcohol to the mixture the alcohol also assists in dissolving the starting hydroxylic steroid. A particularly useful reaction medium for hydroxylic steroids contains ammonia, tetrahydrofuran and -butyl alcohol in the volume ratio of 2 1 (Procedure 2, section V). This mixture... 

Aldol reactions, Like all carbonyl condensations, occur by nucleophilic addition of the enolate ion of the donor molecule to the carbonyl group of the acceptor molecule. The resultant tetrahedral intermediate is then protonated to give an alcohol product (Figure 23.2). The reverse process occurs in exactty the opposite manner base abstracts the -OH hydrogen from the aldol to yield a /3-keto alkoxide ion, which cleaves to give one molecule of enolate ion and one molecule of neutral carbonyl compound. 

Usually metal-free phthalocyanine (PcH2) can be prepared from phthalonitrile with or without a solvent. Hydrogen-donor solvents such as pentan-l-ol and 2-(dimethylamino)ethanol are most often used for the preparation.113,127 128 To increase the yield of the product, some basic catalyst can be added (e.g., DBU, anhyd NH3). When lithium or sodium alkoxides are used as a base the reaction leads to the respective alkali-metal phthalocyanine, which can easily be converted into the free base by treatment with acid and water.129 The solvent-free preparation is carried out in a melt of the phthalonitrile and the reductive agent hydroquinone at ca. 200 C.130 Besides these and various other conventional chemical synthetic methods, PcH2 can also be prepared electrochemically.79... 

Figure 2.22 Stable gold(lll) alkoxides stabilized by C-donor ligands.

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

Zinc hydroxide and alkoxide species are particularly relevant to catalytic processes, often forming the active species. The cooperative effects of more than one zinc ion and bridged hydroxides are exploited in some enzymatic systems. Zinc alkyl phosphate and carboxylate materials have been important in the formation of framework compounds, often containing large amounts of free space for the inclusion of guest molecules. Aldehyde and ketone compounds are of low stability due to the poor donor capabilities of the ligands however, a number of examples have recently been characterized. 

Kimura and co-workers have synthesized a series of alkoxide complexes with the alcohol functionality as a pendent arm.447 674 737 A zinc complex of l-(4-bromophenacyl)-l, 4,7,10-tetraaza-cyclododecane was also synthesized by the same workers to mimic the active site of class II aldolases. The X-ray structure shows a six-coordinate zinc center with five donors from the ligand and a water molecule bound. The ketone is bound with a Zn—O distance of 2.159(3) A (Figure 12). Potentiometric titration indicated formation of a mixture of the hydroxide and the enolate. Enolate formation was also independently carried out by reaction with sodium methoxide, allowing full characterization.738... 

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