Penta-alkoxides

A new series of niobium derivatives (62) of the IV-phenylurethanes has been obtained586 from the insertion reaction between niobium penta-alkoxides and phenyl isocyanate. Most of the products are yellow liquids, one is white and the rest are brown or black solids or semi-solids. They exhibit v(C=0) in the range 1690— 1730 cm-1. Insertion of up to three molecules of phenyl isocyanate occurs readily at... 

Thomas prepared tantalum and niobium penta-alkoxides by the method of Eq. (2.101). For example, the alcoholysis of tris-(dialkylamido) monoalkylimidotan-talum yielded pentaalkoxides very conveniently ... 

The fragmentation pattern for tantalum penta-alkoxides Ta(OR)5 (R = Me, Ft, Pr, CH2CH20Me), in their mass spectra is similar to that for alkoxides of zirconium and hafnium, niobium, molybdenum, and tungsten. For example, at first the loss of OR groups occurs, followed by elimination of the molecules of unsaturated hydrocarbons (usually, with the same number of carbon atoms as R). However, the decomposition of Ta(OMe)s differs from that of other homologues by the loss of HCHO molecules and H atoms. Elimination of ethers, R2O, which is accompanied by the formation of the metal oxoalkoxide ions of the above mentioned alkoxides is typical. In the absence of X-ray crystallographic data, considerable effort had been directed earlier to throw light on the molecular complexities of aluminium trialkoxides in different (solid, liquid, or vapour) states. 

With niobinm and tantalum " penta-alkoxides, the degree of insertion of phenyl isocyanate may be controlled, resulting in the formation of mono-, di-, tri-, tetra-, and... 

Bradley D.C., Holloway R.C. Nuclear magnetic resonance on niobium and tantalum penta-alkoxides. J. Chem. Soc. 1968 A 219-223... 

Whether the formation of alkene 3 proceeds directly from alkoxide 4 or via a penta-coordinated silicon-species 6, is not rigorously known. In certain cases—e.g. for /3-hydroxydisilanes (R = SiMes) that were investigated by Hrudlik et al —the experimental findings suggest that formation of the carbon-carbon bond is synchronous to formation of the silicon-oxygen bond ... 

The presence of alkoxide ion would enhance the rate of ammonolysis, and the formation of bis(amido) derivatives by an ortho-ester mechanism (see Section VI, p. 110) would be partially suppressed in the competitive set of reactions. Thus, ammonolysis of penta-O-benzoyl-D-glucose in the presence of 5 mmolar proportions of sodium meth-oxide showed a decrease of 11% in the yield of the bis(benzamido)-glucitol derivative as compared with the same reaction conducted without added methoxide ion.47... 

There have been few investigations of the reactions of polyhydroxy compounds with alkali metal alkoxides in nonaqueous media at room temperature.. Percival" reported the preparation of D-glucose-NaOEt and D-glucose NaOMe by treating d-glucose with sodium ethoxide, and penta-O-acetyl-D-glucopyranose with sodium methoxide, respectively. The adducts were dried under vacuum at 60° for 24 hours, a treatment which would make the presence of any significant proportion of alcohol of solvation unlikely. Any traces of moisture in the preparative medium preclude the formation of an aJkoxide adduct. 

Diorganotin alkoxides in the solid show a strong tendency to undergo dimer associations by intermolecular O —> Sn coordination forming a planar four-membered ring with penta-and even hexacoordinate tin atoms. 

The chemistry involved in nucleophilic aromatic substitution is well reflected in the reactions of a variety of nucleophiles with methyl penta-fluorophenyl ether (Ingemann et al 1982a). For most of the nucleophiles such as alkoxide, thiolate, enolate and (un)substituted allyl anions, the dominant reaction channel is the attack upon the fluoro-substituted carbon atoms, as is the case for OH-. The latter ion reacts approximately 75% by attack upon the fluoro-substituted carbon atoms and the remaining 25% by Sn2 (20%) and ipso (5%) substitution as summarized in (41). In the attack upon the fluorinated carbon atoms, the interesting observation is made that a F- ion is displaced via an anionic o-complex to form a F- ion/molecule complex, which is not observed to dissociate into F- as a free ionic product. 

Organosilicon spirocyclic compounds consisting of two five-membered rings linked through the silicon atom, readily form rather stable complexes with bases the central silicon atom in such complexes is penta-coordinate. Thus, bis(ethylene-l,2-dihydroxy )silane forms adducts with alkali metal alkoxides 232.233) which event is not... 

Alkoxide and aryloxide ligands are excellent ligands for the actinides. As a result, these ligands have been studied extensively in the coordination chemistry and reactivity of tri-, tetra-, penta-, and hexavalent actinides. The alkoxides and aryloxides can be synthesized by a variety of routes the two most popular routes include direct reaction of actinide halides with alkali metal salts of the alkoxide or aryloxide of interest and protonolysis of actinide amides by alcohols. 

The structurally characterized penta- and hexanuclear aggregates are all oxo or hydroxo clusters. A/4(M4-0)06 cores (Fig. IF) having an adamantanelike structure are found for lead and barium alkoxides. The Af5(p5-0)0y (Fig. IG) core seems to display a special stability for large di- or trivalent metals. Hexanuclear aggregates derive from the pentanuclear by the addition of a sixth... 

A., A penta-indium oxo alkoxide cluster with a central 5-co-ordinate oxygen. Preparation and X-ray crystal structure of (InO Pr)5(p2-0 Pr)4(p3-OPr)4( i-50), J. Chem. Soc. Chem. Commun. 1258 (1988). 

Aluminium alkoxides have also been utilized as efficient catalysts for the lactide polymerization. The most popular one is aluminium isopropoxide (Al(0 Pr)3). Duda verified that Al(0 Pr)3 exists in two types of aggregates a trimer (A3) and a tetramer (A4) as shown in Scheme 1.9. The A3, consisting of penta-coordinate aluminium ion, can initiate the lactide polymerization whilst A4, having hexa-coordinate aluminium ion, is ineffective for the ROP. Since A4 is equilibrated with A3, the ROP of lactide is slowly initiated in the presence of A4. All of the alkojq l ligands in the aluminium alkoxides are involved in the initiation because interligand exchange of the propagating metal alkoxides is fast. 

By contrast, the alkali alkoxide route appears to be inapplicable for the synthesis of zirconium tetra-alkoxides or niobium (tantalum) penta-aUcoxides, as these tend to form heterobimetallic alkoxides with alkali metal alkoxides (Chapter 3, Section 3.2.1.1), which volatilize out during final purification, whereas alkali titanium alkoxides, even if formed, dissociate readily to give volatile titanium alkoxides. 

An early example was the failure to obtain penta-tertiary alkoxides of niobium owing to the preferential formation of the oxo compounds Nb20(0R)g and NbO(OR)3 in contrast to tantalum which formed thermally stable Ta(OBu )5. This behaviour was explained in terms of the greater tendency of niobium to form the metal-oxo double bond. In the reactions of W(NMe2)e with ROH the hexa-alkoxides W(OR>6 (R = Me, Et, Pr , allyl) were formed but not the tertiary butoxide which gave WO(OBu )4 instead. " The tendency of MoO(OR)4 compounds to form Mo02(OR>2 by ether elimination has also been noted (Eq. 5.4). ... 

The formation of multiple metal-oxo bonds by penta or hexa valent transition metals (d°) is not surprising but the more recent discovery that scandium, yttrium, and the tervalent lanthanides form oxo-isopropoxides rather than triisopropoxides was unexpected. Considerable efforts have been devoted to ensuring that the formation of these pentanuclear oxo-alkoxides [M5(/r5-0)(/r,3-0Pr )4(/r-0Pr )4(0Pr )5] was not due to hydrolysis, and there is evidence that elimination of ether is involved. An alternative mode of decomposition would be alkene elimination with concomitant formation of alcohol (Eq. 5.5). 

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