Alkoxide-exchange

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

Comparison of zinc alkoxide and zinc hydroxide bond energies has been made. The relative heterolytic bond energies for hydroxide, methoxide, ethoxide, and tert-butoxide were determined from studies of a series of alkoxide exchange equilibria using a four-coordinate monomeric zinc tris(pyrazolyl)borate compound.335... 

Metal alkoxides undergo alkoxide exchange with alcoholic compounds such as alcohols, hydro-xamic acids, and alkyl hydroperoxides. Alkyl hydroperoxides themselves do not epoxidize olefins. However, hydroperoxides coordinated to a metal ion are activated by coordination of the distal oxygen (O2) and undergo epoxidation (Scheme 1). When the olefin is an allylic alcohol, both hydroperoxide and olefin are coordinated to the metal ion and the epoxidation occurs swiftly in an intramolecular manner.22 Thus, the epoxidation of an allylic alcohol proceeds selectively in the presence of an isolated olefin.23,24 In this metal-mediated epoxidation of allylic alcohols, some alkoxide(s) (—OR) do not participate in the epoxidation. Therefore, if such bystander alkoxide(s) are replaced with optically active ones, the epoxidation is expected to be enantioselective. Indeed, Yamada et al.25 and Sharp less et al.26 independently reported the epoxidation of allylic alcohols using Mo02(acac)2 modified with V-methyl-ephedrine and VO (acac)2 modified with an optically active hydroxamic acid as the catalyst, respectively, albeit with modest enantioselectivity. 

Comparison of Exchange Barriers. Among the most interesting results we have obtained are the intrinsic barriers for halide and alkoxide exchanges, shown in Table II. Note that... 

In a very similar way, hydroxy functionalized ATRP initiators such as 2,2,2-tribromoethanol can be used for the simultaneous polymerization of eCL and MMA (Scheme 25) [83]. Purposely, the ROP of eCL is promoted by Al(OfPr)3 added in catalytic amount so that the rapid alcohol-alkoxide exchange reaction (see Sect. 2.4) activates all the hydroxyl functions. In order to avoid initiation by the isopropoxy groups of Al(0/Pr)3. The in-situ formed zPrOH is removed by distillation of the zPrOH/toluene azeotrope. On the other hand, the ATRP of MMA is catalyzed by NiBr2(PPh3)3. The two aforementioned one-step methods provide block copolymers with controlled composition and molecular weights, but with a slightly broad MWD (PDI=1.5-2). 

The carbonyl intermediate then reacts readily with a primary amine to afford an imine and water. A subsequent addition of the iridium hydride to the C=N double bond of the imine, followed by amide-alkoxide exchange, would then occur to release the product. 

Alkoxy- or aryloxypyridazines and the corresponding diethers are usually made from a halo- or dihalopyridazine and an equivalent amount of sodium alkoxide or phenoxide. As by-products 6-alkoxy-3(2 )-pyridazinones may be formed (particularly if aqueous bases are used) or other products may result from alkoxide exchange. A detailed examination of the reaction between 3,6-dichloropyridazine and various alkoxides revealed that the crude products, i.e., 3-alkoxy-6-chloropyridazines, are always contaminated with the starting material and the 3,6-bisalkoxy derivative. Lower temperatures and prolonged heating favor the preparation of 3-alkoxy-6-chloro-pyridazines and similar optimum reaction conditions for the synthesis of 3,6-dialkoxy- and phenoxypyridazines are reported. 

The exchange between different Ge tetraalkoxides can be easily obtained through alkoxide exchange. AlCls-catalyzed reactions and cleavage of cyclic ethers can also produce... 

Variable temperature NMR studies of the parent titanium-tartrate system (3,4, 5) revealed that isopropoxide exchange, fluxional interconversion (in which the coordinated ester and uncoordinated ester carbonyls, as well as tartrate alkoxides, exchange with respect to the titanium) and epoxidation rate are closely coupled. Thus, the faster the isopropoxide exchange, the more a vacant site becomes available for coordination of allyl alkoxide and the faster the latter anion associates with the titanium. Likewise, faster fluxional interconversion indicates more rapid dissociation of the ester carbonyls in 3 and thence more rapid coordination of the alkyl hydroperoxide to give 4. 

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.

The perovskite PbCr03 has been obtained from PbO and Cr02 at high pressure, and the compound shown to be a semiconductor with antiferromagnetic behaviour below a transition temperature of ca. 160 Tetrakis-(3,3-dimethyl-2-butoxy)chromium(iv) has been prepared by alkoxide exchange from tetra-t-butoxychromium(iv) with an excess of pure pinacolyl alcohol sealed in an ampoule at 70°C for 36 h. This new compound is very sensitive to both O2 and HjO but is otherwise remarkably stable. The electronic spectral and magnetic properties have been determined and are very similar to those of other chromium(iv) alkoxides. ... 

Most metal chlorides undergo only partial metathetical halide/alkoxide exchange upon reaction with alcohols or no reaction at all even at elevated temperatures. The metal alkoxide chlorides thus obtained, MClx(OR), have not been used in sol-gel processing (see, however. Section 7.10.3.3.2). In order to achieve the preparation of homoleptic metal alkoxides from metal chlorides basic conditions are essential in order to trap the liberated HCl. This can be achieved by reaction of metal chlorides with alcohols in the presence of a base such as ammonia or, less often, trialkylamines or pyridine (Equation (11a)). The base also increases the equilibrium concentration of alkoxide ions, which are a more powerful nucleophile for reaction with the metal chloride than the parent alcohol. For this reason the use of alkali alkoxides (M OR), mostly lithium, sodium, or potassium alkoxides, proves to be more successful (Equation (11b)). The use of LiOR has advantages for the preparation of insoluble metal methoxides because LiCl is soluble in methanol and is thus easily separated from insoluble metal alkoxides. 

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. 

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