Alkoxide complexes

Double Alkoxides. Complex double alkoxides are formed when a solution of an alkaU or alkaline earth metal alkoxide is added to a solution of an alkoxide of aluminum, titanium, or tirconium and a series of such compounds have been prepared (44). 

The lithium aluminum hydride-aluminum chloride reduction of ketones is closely related mechanistically to the Meerwein-Ponndorf-Verley reduction in that the initially formed alkoxide complex is allowed to equilibrate between isomers in the... 

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, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. 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. 

Scheme 5-47 Asymmetric hydrophosphonylation of a cyclic imine catalyzed by heterobimetallic rare earth/alkali metal/BI-NOL complexes or by chiral titanium alkoxide complexes...

Similar studies were carried out with methoxycyclohexanones.138 3-Methoxy groups showed no evidence of chelation effects with these reagents and the 2-methoxy group showed an effect only with Zn(BH4)2. This supports the suggestion that the effect of the hydroxy groups operates through deprotonated alkoxide complexes. 

Cobalt(II) alkoxides are known and monomeric forms are part of a wider review.413 The interest in these compounds pertains to a potential role in catalysis. For example, a discrete cobalt(II) alkoxide is believed to form in situ from a chloro precursor during reaction and performs the catalytic role in the decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide.414 A number of mononuclear alkoxide complexes of cobalt(II) have been characterized by crystal structures, as exemplified by [CoCl(OC(t-Bu)3)2 Li(THF)].415 The Co ion in this structure and close relatives has a rare distorted trigonal-planar coordination geometry due to the extreme steric crowding around the metal. 

A series of zinc alkoxide complexes were characterized of the form [RZnOR ]ra where n 2 or 4 (Figure 4). Complexes of the form [Zn3 0(2,6-i-Pr2C6H3) 4R2] were produced by stoichiometry... 

Kimura and co-workers have synthesized a series of alkoxide complexes with the alcohol functionality as a pendent arm.447 674 737 A zinc complex of l-(4-bromophenacyl)-l, 4,7,10-tetraaza-cyclododecane was also synthesized by the same workers to mimic the active site of class II aldolases. The X-ray structure shows a six-coordinate zinc center with five donors from the ligand and a water molecule bound. The ketone is bound with a Zn—O distance of 2.159(3) A (Figure 12). Potentiometric titration indicated formation of a mixture of the hydroxide and the enolate. Enolate formation was also independently carried out by reaction with sodium methoxide, allowing full characterization.738... 

Under ambient pressures, the Alin-porphyrinato-alkoxide complex (250) traps C02971,972 and subsequent addition of EO affords the cyclic carbonate.973 However, at higher pressures (50 kg cm 2) (TPP)AIX complexes initiate the copolymerization of PO and C02, although activities are... 

This species adds a ketone yielding the alkoxide complex (84) which, after reductive elimination of the corresponding alcohol, generates the 16-electron species (85). This intermediate undergoes oxidative addition of 2-propanol (species (86)) and subsequent reductive elimination of acetone, regenerating the hydride complex (83). 

The synthesis of group 4 alkoxide complexes grafted on the surface of silica illustrates this approach.44-47 Two routes were considered, which are schematically represented in Equations(2) and (3) they differ in the nature of the precursor complex, which is either the tetra-alkoxide M(OR)4 (Equation(2)), or the tetra-alkyl complex MR4, M = Ti, Zr, Hf (Equation(3)) ... 

Mixed carbanion/alkoxide complexes were formed from combining n,sBu2Mg, TuOM (M = Na or K), and TMEDA to form the dimeric CIPs [(Bu)2(tBuO)MMg(TMEDA)]2 (M = Na and K) 437, 438.442 While the two structures are identical in their atom connectivity, they are not precisely isostructural in that K shows a bias toward C-coordination while Na is inclined toward N. The alkali metal cations are both formally five-coordinate bonding with two a-C(Bu) atoms, two N(TMEDA) atoms, and a single 0(cBu) atom. The dominant feature in both structures though is the [(Bu)2Mg(/x-tBuO)2MgBu2]2 dianion. 

R.R. Schrock, M.I.T. Have you or anyone else prepared platinum(IV) metallacyclobutane complexes with alkoxide ligands in place of chlorides One might expect the alkoxide complexes to behave considerably differently than the chloro complexes, perhaps like early transition metal complexes. 

Catalysts such as HRuX(PPh3)3, where X is an optically active car-boxylate (e.g., / -mandelate) gave only 0.4% ee using 2-ethylhex-l-ene as substrate (124). Soluble Ziegler-Natta catalysts comprised of triiso-butylaluminum with the optically active alkoxide complex, titanium tetra-(-)-menthoxide, hydrogenated racemic terminal olefins such as 3,4-di-methyl pent- 1-ene, but with zero ee (323). 

Chae, H. K. Payne, D. A. Xu, Z. Ma, L. 1994. Molecular structure of a new lead titanium bimetallic alkoxide complex [PbTi2( t4-0)(00CCH3)(0CH2CH3)7]2 Evolution of structure on heat treatment and the formation of thin-layer dielectrics. Chem. Mat. 6 1589-1592. 

Monomeric arrangements are rare for alkoxide complexes of bismuth and require excess anionic ligands or bulky substituents. Otherwise the alkoxide ligands typically impose dimerization or multinuclear clustering, which are expressed in a variety of currently unusual structural arrangements (Table I). 

An early example came from the report in 1985 by Darensbourg et al. on the reactions of [HCr(CO)5p and [HCr(CO)4P(OMe3)3] with aldehydes and ketones, in the presence and absence of acids [27]. Paraformaldehyde reacts readily with PPN+[HCr(CO)5] PPN+ = N(PPh3)J giving the alkoxide complex [(CO)5CrOCH3] through insertion of formaldehyde into the Cr-H bond (Eq. (18)). The addition of HOAc produced methanol (Eq. (19)). 

The cationic tantalum dihydride Cp2(CO)Ta(H)2]+ reacts at room temperature with acetone to generate the alcohol complex [Cp2(C0)Ta(H01Pr)]+, which was isolated and characterized [45]. The mechanism appears to involve protonation of the ketone by the dihydride, followed by hydride transfer from the neutral hydride. The OH of the coordinated alcohol in the cationic tantalum alcohol complex can be deprotonated to produce the tantalum alkoxide complex [Cp2(C0)Ta(01Pr)]. Attempts to make the reaction catalytic by carrying out the reaction under H2 at 60 °C were unsuccessful. The strong bond between oxygen and an early transition metal such as Ta appears to preclude catalytic reactivity in this example. 

The proposed mechanism for this H/D exchange is shown in Scheme 7.9. The formation of the alkoxide complex likely proceeds by displacement of the water ligand by the alcohol, forming an unobserved alcohol complex that transfers D+ to the OD ligand, producing an OD2 ligand. 

C). The exact role of the base is not clear, but it may accelerate the formation of the molybdenum alkoxide complex from a bound alcohol ligand. 

Path B in Scheme 17.5 involves hydrolysis or alcoholysis of the formate complex, yielding formic acid and a hydroxide or alkoxide complex, which then undergoes hydrogenolysis. This pathway would explain the observations that water... 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

Takaya and co-workers (256) disclosed that chiral copper alkoxide complexes catalyze the transesterification and kinetic resolution of chiral acetate esters. Selec-tivities are very poor (E values of 1.1-1.5) but it was noted that the Lewis acid BINAP CuOTf was not an effective catalyst. The observation thatp-chlorophcnyl-BINAP-CuOf-Bu complex gave faster rates than BINAP-CuOt-Bu suggests that both the Lewis acidic and Lewis basic properties of the copper alkoxide are required for optimal reactivity. 

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