1.6- Dialkoxide

Alkylberyllium alkoxides (RBeOR ) can be prepared from BeR2 by a variety of routes such as alcoholysis with R OH, addition to carbonyls, cleavage of peroxides R OOR or redistribution with the appropriate dialkoxide Be OR )i, e.g. ... 

Alkoxides of all 3 metals are well characterized but it is those of titanium which are of particular importance. The solvolysis of TiCU with an alcohol yields a dialkoxide ... 

A chemical cross-hnking of MEEP was obtained by Shriver [606] by using polyethylene glycol (PEG) dialkoxide, which also forms polymer salt complexes. The cross-linked polymers were prepared by substituting a part (1 and 10 mole%) of the methoxyethoxyethoxy ethanol by PEG in the synthesis of MEEP. Contrary to the MEEP, the amorphous polymers obtained do not flow and are stable even at 140 °C. The maximum ionic conductivity at 30 °C, obtained after complexation with liSOjCFj, are 4.1x10" S cm for MEEP/PEG 1% complexed with 6.4 wt% salt and 3x10" S cm for MEEP/PEG 10% com-plexed with 8.9 wt% salt. These values are comparable with those obtained with the parent hnear polyphosphazenes. 

The coordinated macrocycle readily reacts with alkoxide ions to yield products of type (71) (Taylor, Urbach Busch, 1969). In so doing additional flexibility is imparted to the ring which may reduce ring strain and, in part, provide a driving force for the reaction. Thus the coordinated imine carbons appear predisposed to attack by such nucleophiles. Based on this knowledge, elegant template syntheses of three-dimensional derivatives have been performed. The syntheses involved the reaction of [M(taab)]2+ (M = Ni, Cu) with the dialkoxide ions derived from bis(2-hydroxyethyl)sulphide or bis(2-hydroxyethyl)methylamine (Katovic, Taylor Busch, 1969). The products were demonstrated to be monomeric square-pyramidal complexes of type (72). The condensation... 

Anionic alkoxide complexes of the type M,Th2(OPri)9 (M1 = Li, Na) have been mentioned in a review66 and Li2U(OMe)6 precipitates when UCU is added to the stoichiometric quantity of LiOMe in MeOH. Compounds of the type U[Al(OPri)4 4 and U[Al(OPri)4]2Cl2 are also known. The dialkoxide dichlorides, U(OR)2Cl2 (R = Me, Bu ), are obtained by reaction of UCU with the stoichiometric quantity of the sodium alkoxide in ether (R = Me) or THF (R = Bu ). 

Reaction of [Mo(NO)(Me2pzb)I(OR)] with silver acetate in the presence of the alcohol ROH leads to the bis(alkoxides) [Mo(NO)(Me2pzb)(OR)2]. The mixed alkoxides were similarly prepared.126 The structures of several dialkoxides (Et, Pr Pr , Pr Et, Et) have been investigated in detail in view of the rather bulky character of the ligands at molybdenum. 

The diethylzinc-alcohol (1 2) system was also extensively studied by Tsuruta and his co workers (85,86). Amorphous zinc dialkoxide was concluded to be an active species, because crystalline zinc alkoxide prepared from zinc chloride and lithium alkoxide proved to have only a very small catalytic activity. Based on kinetic studies of the polymerization of propylene oxide with the ZnEt2-CH3OH (1 2) catalyst system, the catalytically active species was concluded to be the complex formed by coordination of one molecule of monomer to the catalyst. In the polymerization of propylene oxide with the catalyst system, it was concluded that the monomer was polymerized by ring opening brought about by cleaving the CH2-0 bond (87). 

In all of, these cases substitution of the second alkyl can then occur to yield the dialkoxide or diphenoxide. This allowed the isolation of the monomeric beryllium phenoxide Be(OAr )2 (OAr = 2,6-di-t-butylphenoxide).98 The alkyls of the Group IV metals, MR (M = Ti, Zr, Hf), undergo rapid reactions with common alcohols and phenols yielding eventually the corresponding tetra-alkoxides or -phenoxides and four equivalents of alkane.97,100 With very bulky substituted alcohols or phenols the reactivity can be very sluggish, in some cases leading to only partial substitution (equation 28). 66,100... 

From the o-dichlorobenzene complex, reaction with a dialkoxide can produce a cyclic bisether. This idea has been applied in the preparation of benzo-crown ethers, as outlined in Scheme 6.61 The first substitution product (15) does not undergo cyclization, but a second intermolecular substitution to give (16) sets up a final cyclization to the bischromium complex (17) the overall yield is 27%. A bisthia analog was also prepared.61... 

The elegant asymmetrization methodology of a meso compound, achieved in high enantioexcess under chiral environment, was the highlight of the total synthesis of (+)-pancratistatin (94) reported by Trost and Pulley (31]. The synthesis commenced with ( )-conduritol-A (130), obtained from p-benzoquinone, (Scheme 18) which was converted into the acetonide 131 and thence, via the dialkoxide to the cis-bis carbonate 132 (Scheme 19). The chiral n-ailyl palladium complex A formed on treatment erf 132 with the catalyst generated from chiral bis-amide 133 and n-allyl palladium chloride underwent azide substitution from the less hindered face of the molecule to provide the monocarbonate 134 in excellent yield and with high optical induction. 

If a cyclophosphazene precursor molecule has more than two reactive sites (e.g. chlorine atoms), linkage reactions with diamines of dialkoxides will generate a three-dimensional structure rather than a linear chain. Such materials are known as cyclomatrix polymers (3.18). These species form a half-way stage between linear polymers and ceramics, and occupy a critical position in materials science. The term ultrastructure covers a broad range of materials of this type. 

Replacement of the two oxygen atoms in these dialkoxides by sulphur, a poorer bridging atom, decreases the tendency to inter-molecular association and prevents the formation of dimers. (66) The high-frequency 1,9Sn chemical shifts found for R2Sn(SR )2 compounds reflect this behaviour ... 

The changes in chemical shift as a function of concentration and temperature for the dibutyltin dialkoxides (63) are shown in Fig. 6. 

Thermochemical data for the dissociation of dialkyltin dialkoxide dimers (neat liquids)... 

Structure [7b], which is very similar to that suggested for the acyclic dimeric dialkoxides in the pure liquids, [5a], is identified by a low-... 

An equilibrium involving substituent exchange between magnesium dialkyls and magnesium dialkoxides recently has been postulated in order to explain stereoselectivity of certain Grignard reduction reactions (64). 

Monotin alkoxides, tin dialkoxides and cyclic tin alkoxides have been utilized as initiators in the ROP of cyclic esters. The tin alkoxides are known to form cyclic species during synthesis and the dibutyltin alkoxides are known to exist as monomers and dimers [74]. The cyclic tin alkoxides were originally studied because of their resistance towards hydrolysis [75]. The tin alkoxides have been reported to be effective transesterification catalysts initiating polymerization at moderate temperatures [76]. 

The trialkylaluminum compounds leaving this transalkylation reactor now contain primarily C -C10 alkyl groups simply by the law of mass action. The low molecular weight olefins are separated from the trialkylaluminum compounds by distillation (19). This is a specially designed flash unit which contains provisions for scrubbing the overhead olefin vapors with partially (2/3) oxidized alkylaluminum dialkoxide to remove any trialkylaluminum carried overhead (20). 

Scheme 35. Mechanistic steps proposed for the formation of an unusual dialkoxide ligand...

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

Diorgano tellurium dihalides react with sodium alkoxides in alkanols8, sodium phenoxides9 l0, sodium enolates" in benzene, and with oximates in methanol12,13, to yield diorgano tellurium dialkoxides and diphenoxides. 

Dimethyl tellurium dialkoxides/phenoxides and several diaryl tellurium dialkoxides/ phenoxides have been prepared. These compounds are oils or colorless solids that are quite stable towards moisture. 

When diorgano tellurium dihalides are reacted with sodium alkoxides with the alcohols serving as solvent2, diorgano tellurium dialkoxides are formed in good yields as white solids or as slightly-viscous, clear liquids that are quite stable towards air and moisture. 

Dialkoxides and diphenoxides of 5,10-dihydro-5-oxa-10-telluraanthracene were similarly prepared from the 10,10-dichloride1. Dibenzotellurophene dichloride and sodium oxinate (prepared from 8-hydroxyquinoline) reacted in anhydrous methanol at 20° to give bright-yellow crystals of the dibenzotellurophene dioxinates (m.p. 95° from methanol)2. 

From Diorgano Tellurium Dialkoxides and Alcohols or Phenols... 

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