Alkoxide homoleptic

Transfer of the hydride from the Cu to the electrophilic carbon and cleavage of the copper alkoxide by the silane regenerates 69. Recent reports point to the influence of the type of the counter ion X" of the homoleptic 66-67 on the activity, the BF being superior to the PF analogue this effect has been attributed to differences in the rate of active catalyst generation from the homoleptic [Cu(NHC)2] X and NaO Bu due to solubility differences of the inorganic salts formed during the displacement of the NHC by BuO" [54] (Scheme 2.10). 

The addition of a base, typically ammonia, to mixtures of transition metal halides and alcohols allows the synthesis of homoleptic alkoxides and phenoxides for a wide range of metals. Anhydrous ammonia was first used in the preparation of titanium alkoxides where the reaction is forced to completion by the precipitation of ammonium chloride.41 Although useful for the synthesis of simple alkoxides and phenoxides of Si, Ge, Ti, Zr, Hf, V, Nb, Ta and Fe, as well as a number of lanthanides,42-47 the method fails to produce pure /-butoxides of a number of metals.58 Presumably, secondary reactions between HC1 and Bu OH take place. However, mixing MC14(M = Ti, Zr) with the Bu OH in the presence of pyridine followed by addition of ammonia proves successful, giving excellent yields of the M(OBul)4 complexes.59... 

In order to highlight the major aspects of the physical and spectroscopic properties of metal alkoxides and phenoxides, representative examples of homoleptic compounds of stoichiometries M(OR)v or M(OAr)x will be considered. 

In the case of the alkoxides and phenoxides of molybdenum(III) the interaction between metal atoms becomes dominant, leading to diamagnetic compounds of stoichiometry Mo2(OR)6 or Mo4(OR )i2(R/ = the less bulky Me or Et).78 These compounds are readily sublimable, (70-90 °C/ 10-3 Torr for Mo OPr1 ) and retain either their dimeric or tetrameric nature both in solution and the gas phase, as determined by cryoscopic measurements and mass spectrometry, respectively.78 An analogous tungsten(III) complex W2(OBu )6 can be obtained, but for other alkoxides the simple homoleptic compounds cannot be isolated.79,168... 

The major types of the molecular structures for the heteroleptic derivatives appear to be the same or very close to those characteristic of homoleptic alkoxides. They can be classified as follows ... 

The derivatives of lower alcohols and also Al(OR )3, Al(OC5HnI,e°)3 are solids derivatives of other amyloxides and hexyloxides are liquids substituted phenoxides are crystalline matters. The oxocomplexes display a much clearer trend to crystallization than the homoleptic alkoxides. Nearly all the derivatives of aluminium can be sublimed or distilled without decomposition. 

In contrast to its analogs, thallium forms the homoleptic alkoxides only in a +1 oxidation state (the trivalent derivatives are known only as TlR OR Jj ). The TIOR homologous series is represented by a considerable number of derivatives that has been studied for 140 years. A number of them were first synthesized along with the other main derivatives of thallium in 1860s by Lamy [988], who discovered this element (simultaneously with Crookes). TIOEt was isolated as a viscous oil that separates from solution on dissolution of TlOH in hot EtOH. TIOMe crystallizes on action of MeOH on the ethoxide. The easily accessible ethoxide has become the most useful reactant in the synthesis of other inorganic and organic derivatives of T1(I). 

The homoleptic derivatives of Mo and W(VI) are rather scarcely studied. The only structurally characterized complex, W(OMe)5, possesses the molecular structure analogous to those of alkoxide halids, i.e. a dimer built up of two edge-sharing octahedra. The structure of monooxo homometallic derivatives is unknown and their individuality appears questionable. The only dioxocom-plex of molybdenum(V) isolated as pyridin solvate demonstrates the [Ti(OMe)w]-type structure (Table 12.19). 

Nearly all the methods described in Chapter 2 have been applied for the preparation of uranium derivatives. The reaction of metallic uranium with alcohols in the presence of halogenes (method 1) doesn t lead, as it turned out, to homoleptic alkoxides, but is a facile route to alkoxide halides [79, 1669] ... 

Some actinide alkoxide complexes can exist in equilibrium, for example, the homoleptic complex Th2(OCH-/-Pr2)8, obtained from a metallacyclic precursor after treatment with HOCH-/-Pr2 (5.50) ... 

The first reports of LA ROP using yttrium complexes focused on homoleptic alkoxide complexes, such as cluster complexes of the form Ln5(p-0)(0R)i3 [27]. A patent, and preprint, published by DuPont described the application of a homoleptic yttrium alkoxide, Y(OCH2CH2NMe2)3, formed in situ by reaction of yttrium fm-Ao-propoxide with ALV-dimethy I am i noethanol. The complex showed a very high rate (kob = 0.5 s 1, [Y]0 = 3 mM) and reasonable polymerization control [28]. 

Arnold and colleagues have reported a series of chiral homoleptic yttrium and lanthanide fra(alkoxide) complexes [49, 50], These initiators (including complex 1) show high degrees of iso-selectivity and rapid rates, even at low temperatures. Thus, using the racemic mixture of the lanthanide initiator, stereoblock PLA was produced with a P, of 0.81 so far, this is the only known type of yttrium initiator able to exert such stereocontrol and a very exciting finding. Analysis of the stereoerrors indicates that an enantiomorphic site control mechanism is responsible for the iso-selectivity. 

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