Alkoxides halide metathesis

The alkoxides of both oxidation states were obtained by metathesis of germanium halides (chlorides and iodides) with alkali alkoxides [1488, 1142, 857, 1535]. The yields can be increased by application of GeCl4 solvates with Py or NH3 or amines [3, 222] (method 5) and also by alcohol interchange of ethoxides (method 6) or alcoholysis of Ge[Si(NR3)2]2 [568, 1543] (method 4). The application of alkali alkoxides in the preparation of Ge(OR)4is possible in contrast to that of analogous derivatives of Sn(IV) and Zr due presumably to the much lower stability of corresponding alkoxogermanates the intermediate products of the corresponding reactions because of stability — of the tetrahedral coordination for Ge. The direct electrochemical preparation of Ge(OEt)4... 

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, halide, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal halide. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

In general, compounds of the type TiX4 (X = halide or alkoxide) are inexpensive and are accessible in quantity from commercial sources. Even such titanium compounds that are not readily available are basically easy to prepare by the metathesis reaction starting from simpler compounds according to the equations ... 

Five-coordinate W alkylidene complexes, where L can be alkoxide or halide or SO3CF3, are highly effective olefine metathesis catalysts in the presence of Lewis acids. The vast number of compounds synthesized can be understood by the variation and combination of different ligands. 

The complex Re(NAr)2NpCl2 (Ar = DIPP) undergoes dehydrohalogenation with the strong base DBU, and the resulting halide can be further transformed into the alkoxide derivatives (75) with LiOR, for example, (R = CH(Cp3)2, CMe(CF3)2) (Scheme 19). Contrary to earlier expectations, neither (75) nor its analogs, Re(NAr)2(CH-t-Bu)X (X = Cl, OR) and (58), are active for alkene metathesis or even polymerization of the very reactive norbornene. 

It is clear that electron-withdrawing alkoxides in M(NAr)(CHR)(OR )2 complexes dramatically increase the rate of reaction of an olefin with the M=CHR bond. In fact, no other X ligands in M(NAr)(CHR)X2 complexes are as successful as alkoxides for sustained metathesis activity, [66] either because they are not bulky enough to stabilize an electron deficient metal center and prevent bimolecular decomposition (e.g., halides) or because they donate too much electron density to the metal in a a and/or Ji fashion (e.g., amides or thiolates). A recent example is Mo(NAr)(CHR)(diamide) where the diamide is a N,N -disubstituted-2,2 -bisamido-l,r-binaphthyl ligand no ready reaction was observed between this complex and ethylene or even benzaldehyde. [108] Alkylidene complexes are now known for all metals in groups 4, 5, and 6, plus rhenium. For example, it was shown that Cp2Ti(CH2) could be trapped... 

Several common synthetic strategies for the formation of transition metal-heteroatom bonds can be used for the preparation of early transition metal alkoxides, as shown in Equation 4.57. These methods include (1) metathesis between metal halides and alkali metal salts of the corresponding alcohols (2) direct reaction of the alcohols with metal halides in the presence of a base and (3) protonolysis of metal alkyls, amides, or alkoxides with more acidic alcohols. 

SCHEME 20.2 Transition metal carbenes and metallacyclobutanes are key intermediates in the catalytic cycle of the olefin metathesis reaction (M = transition metal L = ligand, e.g., halide, phosphine, alkoxide, arylamido, cyclopentadienyl R, = alkyl group or, with some late transition metals, functional group). 

This procedure, sometimes referred to as transmetallation or (metathesis or salt-elimination) reaction, is by far the most versatile synthetic method for a wide range of d-and p-block metal alkoxide complexes. The alkali metal (usually sodium or potassium) alkoxide is treated in the presence of excess alcohol with the corresponding metal(loid) halide either in a hydrocarbon (generally benzene) or an ether solvent (Eq. 2.41) ... 

Metathesis of a Metal Halide with a Bimetallic Alkoxide of another Metal and an Alkali Metal (Method 2.2). This approach has been proposed for the case, where the alkoxide of one of the metals is not easily accessible or is an insoluble and inert solid as, for example, the alkoxides oflate transition and some main group metals (Mn(II), Fe(II), Fe(III), Co, Ni, Cu, Zn, Cd, Sn(II), Pb(II)). It has been applied also for the preparation of heterometallic derivatives of rare earth elements (see Bradley, 1978,2001 Turova, 2002). It is necessary to take into accoimt that the reaction ... 

Copper and silver fluoroalkoxides have been synthesized by primarily two methods. One method, ionic metathesis in ether solvents between al or alkaline earth salts and a copper or silver halide, usually results in a double metal alkoxide product. A second method, alcoholysis of organometallics or unfluorinated alkoxides avoids the presence of other metals but can result in mixed... 

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