Metal alkoxides exchange reactions

Out of a number of preparative routes described in Section II for preparing metal alkoxides, those involving (a) the metal-carbon bond cleavage reaction (Section II.F), (b) the chloride-alkoxide exchange reaction (Section II.C. 1), (c) the amido-alkoxo exchange reaction (Section II.E), and (d) the alcoholysis reaction (Section II.D) appear to be more convenient and versatile. Schemes 1-6 summarize some of the typical reactions studied for the preparation of metal complexes of alcohol ligands with large steric requirements. 

Scheme 2. Preparation of some di-rerr-buty Imethoxide derivatives of metals via chloride-alkoxide exchange reactions.

In connection with the mechanistic interpretations of stereocontrol, either (LXII) or (LXIII) is consistent with the effect of a bulky alkoxide group. This is because alkyl-alkoxide exchange reactions could place an alkoxide group on aluminum where growth occurs. Structure (LXIII) seems reasonable in view of the known tendency of aluminum alkoxides to dimerize (McElvain and Davie, 1951 Hoffmann, 1960) however, a monomeric structure (LXII) cannot presently be excluded. The metal alkoxides, like alkyllithiums, probably involve equilibria among monomeric and associated species. Therefore, one or more active species may be involved. Whatever the species, it is not too difficult to imagine a complex composed of a tetravalent aluminum, in which monomer-chain or monomer-chain-alkoxide interactions favor one path for monomer addition. 

The amide-alkoxide exchange reactions has also been successfully applied for the synthesis of many interesting divalent group 12, group 14, and 3d transition metal alkoxides as depicted in Scheme 2.3. 

The process of inversion (racemization) without exchange is called isoinversion. This involves an ion-pair mechanism. Indeed, in the presence of a crown ether, to separate the ions, the percentage of racemate increases (307). In fact, the stereochemical course of many metal-alkoxide catalyzed reactions in nonpolar solvents can be drastically modified by addition of catalytic amounts of crown ethers to the medium. For this reason, ion pairs, in low dielectric media, play a remarkable role as intermediates in reactions in which the negative ion is a carbanion. For example. Cram studied the rate of exchange (ke) and racemization kj as a function of the substituents present with deuterium-labeled 9-methylfluorene (309). The most interesting case was kjk < 0.5, racemization without exchange. This happens for X = di-... 

Catalytic amounts of mercuric chloride are usually employed in this preparation. Aluminum isopropoxide is a useful Meerwein-Potmdorf-Verley reducing agent in certain ester-exchange reactions and is a precursor for aluminum glycinate, a buffering agent (see Alkoxides, metal). 

A similar steric effect was observed in the reaction of benzyl carboxylate (44). When 44a-d were treated with Bu OK under solvent-free conditions at around 100 °C for 30 min, the corresponding condensation products 45a (75%), 45b (66%), 45c (64%), and 45d (84%) were obtained in the yields indicated [9] (Scheme 6). When the same reactions of 44a-d and Bu OH were carried out in toluene under reflux for 16 h, no condensation product was obtained and 44a-d were recovered unchanged. In solution reactions, exchange of the alkoxy group occurs among the substrate, reagent, and solvent. Therefore, the alkoxy groups of the ester, metal alkoxide, and alcohol used as a solvent should be identical. 

In transfer hydrogenation with 2-propanol, the chloride ion in a Wilkinson-type catalyst (18) is rapidly replaced by an alkoxide (Scheme 20.9). / -Elimination then yields the reactive 16-electron metal monohydride species (20). The ketone substrate (10) substitutes one of the ligands and coordinates to the catalytic center to give complex 21 upon which an insertion into the metal hydride bond takes place. The formed metal alkoxide (22) can undergo a ligand exchange with the hydride donor present in the reaction mixture, liberating the product (15). 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

Epoxide polymerizations taking place in the presence of protonic substances such as water or alcohol involve the presence of exchange reactions. Examples of such polymerizations are those initiated by metal alkoxides and hydroxides that require the presence of water or alcohol to produce a homogeneous system by solubilizing the initiator. Such substances increase the polymerization rate not only by solubilizing the initiator but probably also by increasing the concentration of free ions and loose ion pairs. In the presence of alcohol the exchange... 

U(OEt)5 is easily prepared by oxidation of U(OEt)4 by bromine in ethanol, followed by addition of the calculated quantity of sodium ethoxide, a reaction which yields181 only Np(OEt)4Br in the case of Np(OEt)4. U(OEt)s is commonly used as the starting material for the preparation of other uranium(V) alkoxides by alcohol exchange reactions. A comprehensive review of metal alkoxides includes a useful discussion of the uranium(V) compounds.182... 

From Exchange Reactions Involving Metal Alkoxides 339... 

From Exchange Reactions Involving Metal Alkoxides 15.3.2.5.1 By alcohol interchange... 

An in-depth study of the industrially important hydrolysis of titanium alkoxides has been carried out by Bradley.234,235 A number of intermediate complexes were isolated and characterized. The alcohol exchange reaction has been discussed previously. The addition of hydrohalous acids to alkoxides is clearly related to the reverse reaction, the addition of alcohols to metal halides. In general, the products of these two reactions will be the same (equation 59). Hence, complete substitution will occur to give metal halides that are known to form only alcoholates with alcohols (equations 60 and 61),31,236... 

Application of halogenides under the same conditions results inMHal/OR), which does not decompose in excess of M OR or under refluxing. The patent [833] describes the general method of preparation of the stable metal alkoxide solutions (which are used in technology) it comprises a heterogeneous reaction of Zn, Cd, Y, Ln, In, Pb, Sn, Zr, Sb, Bi and Mn carboxylates in alcohol solutions with NH3 or amines with subsequent purification by ionic exchange. 

In [1677] complex alkoxides and alkoxide-carboxylates were compared as precursors for preparation ofBST films. In contrast to the introduction of alkaline earth carboxylates in the form of preliminary isolated salts, in this work metal alkoxide solution in methoxyethanol containing titanium and alkaline-earth metal was modified by addition of 2-ethylhexanoic acid with subsequent slow distilling off the solvent and repeated dilutions with fresh portions of methoxyethanol. During the distillation process, part of the alkoxide groups are substituted by the 2-ethylhexanoate ligands. The exchange reaction of Ti(OPr )4 with acid was studied in different solvents, and it was demonstrated that in the course of distillation the titanium oxoisopropoxy-2-ethylhexanoate is formed with elimination of ester ... 

---