Zinc methoxide

A related N-methylated zinc methoxide (323) also polymerizes cis- and trans- BO with inversion of stereochemistry. In this case, coordination of the monomer trans to the axial alkoxide is hindered by the N-Me group, so a modified mechanism has been proposed based on cis-coordination of the monomer, but which still involves the simultaneous participation of two metal centers. 

The major application of Zn(OR)2 is connected with their catalytical activity in the polymerization of olefine oxides. It is rather interesting that the activity of file freshly prepared amorphous samples of the zinc methoxide, ethoxide, or Zn(OMe) (OEt)2.n as heterogeneous catalysts in these reactions is noticeably higher than for soluble R ZnOR >ZnR2 as homogeneous catalysts [808, 1604],... 

Catalysts for epoxide polymerisation of quite different characteristics comprise metalloporphyrins of aluminium and zinc, such as (5,10,15,20-tetraphe-nylporphinato)aluminium chloride [(tpp)AlCl], methoxide [(Mtpp)AlOMe] or 1-propanethiolate [(tpp)AlSPr] and (5,10,15,20-tetraphenyl-21-methylporphi-nato)zinc methoxide [(Mtpp)ZnOMe] [32 35] ... 

It was most desirable for us to elucidate the stereocontrol mechanism in terms of molecular level considerations rather than a phenomenological approach. No information, however, was available concerning the chiral structure of d - and l -catalyst sites, because none of the active catalysts possesses a well-defined structure. The active zinc methoxide, for instance, was a disordered powdery substance and its catalyst efficiency was extremely low. 

Reaction of alkylmagnesium chlorides with zinc methoxide in ether provides a highly valuable method to prepare salt-free dialkylzinc reagents, such as 34 or 35 (in ether solution [58] or solvent free [59]), that are otherwise difficult to obtain (Scheme 4.8). The approach takes advantage of the low solubility of magnesium methoxide in ether, which allows for its removal by filtration. 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

Many methods for the conversion of acid copolymers to ionomers have been described by Du Pont (27,28). The chemistry involved is simple when cations such as sodium or potassium are involved, but conditions must be controlled to obtain uniform products. Solutions of sodium hydroxide or methoxide can be fed to the acid copolymer melt, using a high shear device such as a two-roU mill to achieve uniformity. AH volatile by-products are easily removed during the conversion, which is mn at about 150°C. A continuous process has been described, using two extmders, the first designed to plasticate the feed polymer and mix it rapidly with the metal compound, eg, zinc oxide, at 160°C (28). Acetic acid is pumped into the melt to function as an activator. Volatiles are removed in an extraction-extmder which follows the reactor-extmder, and the anhydrous melt emerges through a die-plate as strands which are cut into pellets. 

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... 

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... 

The cyclizations to obtain cyclic thioureas have been performed using thiocarbonyldiimidazole.232 Reaction of methyl acetoacetate, thiourea and an aliphatic aldehyde in the presence of the zinc iodide (Znl2) was studied. Under the normal pressure, reaction has not been occurred whereas at high pressure (300 MPa) conditions 3,4-dihydropyrimidine-2-thione was obtained only in 10% yield.233 The same one-pot three-component cyclocondensation reaction in the presence of iodide (I2) provides a variety of 3,4-dihydropyrimi-dine-2-thione in high yields.234 Condensation reaction of thioureas with a,p-unsaturated ketones in the presence of the sodium methoxide in methanol affords 3,4-dihydropyrimidine-2-thione derivatives.235,236 Acylation of N,N -disubstituted thioureas with methyl malonyl chloride followed by base-catalysed cyclization leads in the formation of l,3-disubstituted-2-thiobarbituric acids (Scheme 78).237... 

Tetrakis(ethylthio)uranium, 3096 Titanium butoxide, 3730 Titanium(III) methoxide, 1317 Titanium tetraisopropoxide, 3579 Tungsten hexamethoxide, 2604 Zinc ethoxide, 1704... 

As seen in many of the above examples, acetic anhydride and zinc chloride each make effective condensation catalysts for the free bases, presumably by efficient coordination with the ring nitrogen. The quaternary salts condense readily in the presence of piperidine. Potassium hydroxide, methoxide or piperidinium acetate are suitable for condensations with 2- and 4-methylpyridine AAoxides. For example, the Claisen condensation is effective with these Af-oxides using ethoxide catalyst (Scheme 45) but the reaction fails with the parent picolines unless activated by nitro substitution (69JHC775). 

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