Initiators diethylzinc

In this model, the intermediacy of a monomeric zinc species is postulated. To support this assumption, an examination of the effect of stoichiometry and solvent in cyclopropanation involving the 2,4-pentanediol auxiliary was preformed [59]. In the initial reaction protocol, a large excess of both diethylzinc and diiodo-methane is employed. Such excessive conditions are justified on account of the instability of the zinc carbenoid under the reaction conditions. To minimize the un-... 

The cyclopropanation of 1-trimethylsilyloxycyclohexene in the present procedure is accomplished by reaction with diiodomethane and diethylzinc in ethyl ether." This modification of the usual Simmons-Smith reaction in which diiodomethane and activated zinc are used has the advantage of being homogeneous and is often more effective for the cyclopropanation of olefins such as enol ethers which polymerize readily. However, in the case of trimethylsilyl enol ethers, the heterogeneous procedures with either zinc-copper couple or zinc-silver couple are also successful. Attempts by the checkers to carry out Part B in benzene or toluene at reflux instead of ethyl ether afforded the trimethylsilyl ether of 2-methylenecyclohexanol, evidently owing to zinc iodide-catalyzed isomerization of the initially formed cyclopropyl ether. The preparation of l-trimethylsilyloxybicyclo[4.1.0]heptane by cyclopropanation with diethylzinc and chloroiodomethane in the presence of oxygen has been reported. "... 

Pritchett et al.119 found that Ti(OPr )4 did not react with the bis(sulfon-amide) ligand itself, so they postulated that a chiral ligand initially reacted with the diethylzinc and was subsequently transferred to the titanium in the next step. Based on this assumption, they presented an improved procedure for the asymmetric alkylation of aldehyde to overcome the poor solubility of the li-... 

Stannyl radicals are usually generated by homolytic substitution at hydrogen in a tin hydride, or at tin in a distannane, or, conjugatively, at the y-carbon atom in an allylstannane.453 The initiator is commonly AIBN at ca. 80 °C. In the presence of a trace of air, organoboranes are oxidized by a radical chain mechanism, and triethylborane is now commonly used as an initiator at temperatures down to —78°C,519 and it can be used in aqueous solution.520 9-Borabicyclo[3.3.1]nonane (9-BBN) has similarly been used to initiate the reaction of tin hydrides at 0 and —78°C,521 and diethylzinc works in the same way.522... 

Radical cyclization of polyfunctional 5-hexenyl halides mediated by Et2Zn and catalyzed by nickel or palladium salts has been demonstrated to produce stereoselectively polyfunctional 5-membered carbo- and heterocycles [56, 57]. Based on this strategy a formal synthesis of methylenolactocin (11) was achieved (Scheme 20). The acetal 130, readily being built up by asymmetric alkylation of aldehyde 127 followed by reaction with butyl vinyl ether and NBS, served as the key intermediate for the construction of the lactone ring. Nickel(II)-catalyzed carbometallation was initiated with diethylzinc to yield exclusively the frans-disubstituted lactol 132, which could be oxidized directly by air to 134. Final oxidation under more forcing conditions then yielded the lactone (-)-75 as a known intermediate in the synthesis of (-)-methylenolactocin (11) [47aj. 

In early 1990 we began to work on catalyzed carboligations using organozinc reagents [1]. The reactions commonly involved diethylzinc 2, which was added enantioselectively to aldehydes 1 affording secondary alcohols 3. Initially, bipyridine 4 [2-6], pyridine 5 [3], and sulfoximines 6 [7] were applied as catalysts, which in some cases led to enantioselectivities greater than 95% ee [8, 9]. 

At low temperatures in inert solvents (such as methylene dichloride) a controlled polymerization can be effected using various acids and alkylating agents. These initiators include boron trifluoride etherate, triethylaluminum, trityl hexachloroantimonate, triethylam-monium hexachloroantimonate, diethyloxonium hexafluoroantimonate, p-toluenesulfonic acid and diethylzinc or cadmium-1,2-dioI complexes. Crystalline, high molecular weight... 

Diethylzinc, one of the first organometallic compounds to be isolated (Fran-kland, 1849), epitomizes these characteristics. The compound is easily prepared by heating powdered zinc or zinc-copper alloy with ethyl iodide under an atmosphere of dry nitrogen or CO2 (the initial product is actually C2H5ZnI, which disproportionates on distillation to zinc iodide and diethylzinc) ... 

In the (—)-DAIB-catalyzed reaction of diethylzinc and benzaldehyde, the rate is first-order in the amino alcohol. The initial alkylation rate is influenced by the concentration of diethylzinc and benzaldehyde but soon becomes unaffected by increased concentration. Thus, under the standard catalytic reaction conditions, the reaction shows saturation kinetics the rate is zeroth order with respect to both dialkylzinc reagent and aldehyde substrate. These data support the presence of the equilibrium of A-D, and alkyl transfer occurs intramolecularly from the dinuclear mixed-ligand complex D. This is the stereo-determining and also turnover-limiting step. 

Methylidenation of allylic thioethers. Methylidenation of an allylic phenyl-thioether with methylene iodide-diethylzinc is accompanied by a 2,3-sigmatropic rearrangement to a homologous allylic phenylthioether. The rearrangement is also initiated by ethylidene iodide. Cyclopropanation is not observed. The Simmons-Smith reaction with allylic sulfides results only in formation of an insoluble polymer. 

Studies of the polymerisation of alanine A-carboxylic acid anhydride with diethylzinc or triisobutylaluminium as the catalyst have revealed the relatively fast deprotonation of the monomer NH group by the metal alkyl as the first reaction step. This reaction (easily detectable by evolution of the ethane or isobutane) produces A-metallated initiating species (substituted metal carbamates) which, however, are not monomeric but undergo association via metal-heteroatom coordination bonds [75,175]. 

It may be mentioned that the use of ionic nucleophilic initiators, instead of zinc-based coordination catalysts, in order to promote propylene oxide/carbon dioxide copolymerisation, did not result in the formation of any copolymer but led to the cyclic carbonate, propylene carbonate [194,236,237]. Also, zinc-based coordination catalysts with non-condensed zinc atoms in their molecules (formed by the reaction of diethylzinc with a monoprotic compound such as... 

The course of the reactions was rationalized by initial reduction of the Pd(II) precatalyst to a Pd(0) species 190A by diethylzinc. This electron-rich complex reduces the alkyl iodide 187 to a radical 187 A, which undergoes cyclization to the alkene unit. The diastereoselectivity of the cyclizations can be explained by a Beckwith-Houk... 

In this work, the synthesis of high surface densities of chlororopropyl groups covalently grafted on mesoporous micelle templated aluminosilicates (Al-MTS) of various initial pore diameters is presented. The hybrid chiral materials resulting from halogen substitution are applied in the enantioselective addition of diethylzinc to benzaldehyde. 

This method was originally used by Frankland to prepare diethylzinc. It is described as an oxidative insertion see Insertion) of zinc metal inside the carbon-halogen bond of an organic halide. The initial product of the reaction is an alkylzinc halide derivative (RZnX), from which the diaUcylzinc compound (R2Zn) is formed as the Schlenk-type equilibrium establishes (equations 1 and 2). 

Polymerization. Complexes of BINOL-derived crown ethers with KO-t-Bu or BuLi have been used as initiators in the asymmetric polymerization of methacrylates. Thus optically active polymers are obtained with 80-90% isotacticity. Complexes of BINOL with Diethylzinc or CdMe2 also initiate the asymmetric... 

The complex of n-butyllithium with diethylzinc has also been studied as a polymerization initiator [180] for methylmethacrylate. In pure toluene the results obtained with this system were complex and not readily interpretable, but in presence of 1—10% tetrahydrofuran a simpler behaviour was reported. Monomer disappears by a first order process, and the observed first order coefficients can be expressed in terms of initiator, monomer and tetrahydrofuran concentrations in the following way... 

Chromium catalysts are notorious for the difficulty of initiating polymerization after a "turnaround." When process equipment is taken out of service for maintenance, the interior of reactors may be exposed to ambient air. This introduces oxygen and water, severe poisons for chromium catalysts. Even after inert gas (nitrogen) is re-introduced after maintenance activities are completed, trace amounts of poisons adhere to interior surfaces. Diethylzinc is aggressively reactive with water and oxygen and is used to scavenge these poisons from polymerization reactors. When reactors are started up again, polymerization initiates more readily. 

Using these ligands, the Davis group has reported the as)mimetric additions of diethylzinc to aromatic aldehydes as the initial exploration of the asymmetric induction realized by amino sugar-based ligands [180]. As shown inO Scheme 49, three aromatic aldehydes 198 were treated with diethylzinc in the presence of 194 or 195. The addition products 199 were obtained in... 

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