Ethylzincation

Diazirines (3) smoothly add Grignard compounds to the N—N double bond, giving 1-alkyldiaziridines. Reported yields are between 60 and 95% without optimization (B-67MI50800). The reaction is easily carried out on a preparative scale without isolation of the hazardous diazirines and may serve as an easy access to alkylhydrazines. The reaction was also used routinely to detect diazirines in mixtures. The diaziridines formed are easily detected by their reaction with iodide. Phenyllithium or ethylzinc iodide also add to (3) with diaziridine formation. 

Eor ethylzinc chloride, CH3CH2ZnCl, and ethylzinc bromide, CH3CH2ZnBr, there is a linear relationship between the observed chemical shift and the ratio of ethylzinc halide to diethylzinc. Extrapolation of these lines to x=l (mol fraction of CH3CH2Z11X) gives predicted values for the average chemical shift that closely match those measured for these species. This indicates that for these two organozinc halides, the Schlenk equilibrium lies heavily on the side of the ethylzinc halide in toluene. However, in the case of ethylzinc iodide, CH3CH2ZnI, there is a... 

Likewise, triphenyltin hydride reacts with ethylzinc chloride, or triphenyltin chloride with metallic zinc, to give the compound PhaSnZnCl, which is stable in the presence of a strongly coordinating ligand, but, in its absence, apparently undergoes an intermetallic shift of the organic group, so that protic acids react to liberate benzene (272). 

The enantioselective 1,4-addition addition of organometaUic reagents to a,p-unsaturated carbonyl compounds, the so-called Michael reaction, provides a powerful method for the synthesis of optically active compounds by carbon-carbon bond formation [129]. Therefore, symmetrical and unsymmetrical MiniPHOS phosphines were used for in situ preparation of copper-catalysts, and employed in an optimization study on Cu(I)-catalyzed Michael reactions of di-ethylzinc to a, -unsaturated ketones (Scheme 31) [29,30]. In most cases, complete conversion and good enantioselectivity were obtained and no 1,2-addition product was detected, showing complete regioselectivity. Of interest, the enantioselectivity observed using Cu(I) directly in place of Cu(II) allowed enhanced enantioselectivity, implying that the chiral environment of the Cu(I) complex produced by in situ reduction of Cu(II) may be less selective than the one with preformed Cu(I). 

H NMR data has been reported for the ethylzinc complex, Zn(TPP—NMe)Et, formed from the reaction of free-base N-methyl porphyrin H(TPP—NMe) with ZnEti. The ethyl proton chemical shifts are observed upheld, evidence that the ethyl group is coordinated to zinc near the center of the porphyrin. The complex is stable under N2 in the dark, but decomposed by a radical mechanism in visible light.The complex reacted with hindered phenols (HOAr) when irradiated with visible light to give ethane and the aryloxo complexes Zn(TPP—NMe)OAr. The reaction of Zn(TPP—NMe)Et, a secondary amine (HNEt2) and CO2 gave zinc carbamate complexes, for example Zn(TPP—NMclOiCNEti."" ... 

The monocation tris(diethylether) ethylzinc is formed from diethyl zinc as a tetrakis(penta-fluorophenyl)borate salt.85 Longer-chain linear and cyclic ether complexes (2) of zinc alkyls have also been observed. The reaction between zinc dialkyls and primary amines gives a number of structurally diverse products dependent on the reaction conditions and the amine.86 The... 

The introduction of various metal-catalyzed reactions, however, remarkably expanded the scope of the epoxidation of Q,.3-unsaturatcd ketones. Enders et al. have reported that a combination of diethylzinc and A-methyl-pseudoephedrine epoxidizes various o,. j-unsaturatcd ketones, under an oxygen atmosphere, with good to high enantioselectivity (Scheme 23).126 In this reaction, diethylzinc first reacts with the chiral alcohol, and the resulting ethylzinc alkoxide is converted by oxygen to an ethylperoxo-zinc species that epoxidizes the a,/3-unsaturated ketones enantioselectively. Although a stoichiometric chiral auxiliary is needed for this reaction, it can be recovered in almost quantitative yield. 

Charette et al. converted both primary and secondary alkyl iodides to the corresponding alkylzinc iodides, using either ethylzinc iodide or isopropylzinc iodide.34 The reactions, for example, Scheme 30, which were performed in UV-irradiated (X > 280 nm) chloroform solutions, gave conversions as high as 88% in less than 4h. 

The complex has crystallographic ///-symmetry, the mirror plane bisecting the unique benzyl group, the nitrogen atom to which it is attached, and the ethylzinc moiety. The pseudo-tetrahedral zinc atom has a short (1.930(4) A) zinc-ethyl bond, but comparatively long (2.230(2) A) nitrogen-zinc donor bonds. 

These salts are soluble in diethyl ether and dichloromethane, but relatively insoluble in petroleum ether. The ethylzinc complex 55b was investigated by single crystal X-ray techniques, and the structure of the cation is shown in Figure 26. 

Depending on the reaction conditions, either one or both of the amino hydrogens can be removed in these reactions, and occasionally more unusual products, such as the tetranuclear ethylzinc/ethoxyzinc/2,6-diisopropylphe-nylamido complex 76, Figure 40, were isolated. 

Chisholm et al. synthesized organozinc compounds with bulky biphenolates as catalysts for the ring-opening polymerization of lactides.196 The protonolysis of diethyzinc by the biphenols, in the presence of diisopropylmethanol, afforded the polycyclic, trimetallic zinc-di(ethylzinc) pre-catalyst 135, which polymerizes /m -lactide to polylactide, enriched in isi- and. sir-tetrads (Scheme 85). 

The interactions of dimethyl- and diethylzinc with bulky tris(hydroxyphenyl)methanes, Scheme 86, yielded, depending on the reaction conditions, a variety of alkylzinc alkoxides, featuring two-, three-, and four-coordinate zinc centers. These polynuclear compounds (Figure 63 shows the trinuclear ethylzinc derivative 136) are relatively poor catalysts for the co-polymerization of cyclohexene oxide and carbon dioxide.197... 

The mono- and dinuclear ethylzinc complexes 137 and 138, respectively, were obtained when a 1,3-dimethylether /j-BT-calixMarene was treated with one and two equivalents of diethylzinc (Scheme 87).198 Excess [ZnEt2(tmeda)], in turn, reacted with this calixarene to furnish a pentanuclear dicalixarene. The syntheses and structures of related diorganozinc calixarenes, featuring both identical and non-identical organozinc moieties, were reported recently.199 The solid-state structure of the bis(ethylzinc)-l,3-dibenzylether-/)-But-calix[4]arene 139, which bears a close resemblance to 138, is shown in Figure 64. 

The ethylzinc compound 143, created from a bulky diaminophenol by treatment with ZnEt2, as shown in Scheme 89, was synthesized for the purpose of creating a well-defined zinc catalyst for the polymerization of lactides.206... 

A chiral ethylzinc aminoalkoxide 147, synthesized by the addition of ZnEt2 to (cyclohexene oxide with C02 in almost quantitative yield and with an ee of 49%. This value is somewhat lower than that obtained by the same authors from the in situ generated monomeric form of the catalyst, which furnished product with an ee of 70%.213... 

Enantiomerically pure and racemic / -carbonyl sulfoximes were treated with diethylzinc to afford the corresponding ethylzinc enolates 148a-c in both racemic and optically active forms (Scheme 94).214 Despite their rather similar solution structures, these complexes exhibited markedly different solid-state structures and reactivities with electrophiles. 

Another functionalized alcohol, namely 2-aziridineethanol (azol), reacted with diethylzinc to yield almost quantitatively the ethylzinc alkoxide EtZn(azol), which is trimeric in solution.216 The introduction of dry dioxygen into solutions of EtZn(azol) gave the peroxoethylzinc alkoxide EtOOZn(azol), which co-crystallized with EtZn(azol) in a 1 1 ratio as a face-sharing di- o-heterocube 150 (Figure 70). 

Trosch and Vahrenkamp reported a bis(2-picolyl)(2-hydroxy-3,5-di-/< //butylbenzyl)amine ethylzinc complex 151 (Figure 65) as a model compound for the active site of zinc enzymes.217... 

Chang et al. synthesized a 2-methyl-l-[methyl-(2-pyridin-2-yl-ethyl)amino]propane-2-thiol, which when treated with dimethylzinc gave the methylzinc complex 159 (Figure 73).224 A somewhat similar ethylzinc complex 160, supported by the boron-free ligand 2-mercaptobenzyl-bis-(2-pyridylmethyl)amine, proved chemically unstable.225... 

The ethylzinc ethylthiolate cluster consists of an ethyl-studded (ZnS)9 core, having a wurtzite structure, which is capped with an ethylthiolate group on one end and an ethylzinc moiety on the other. All zinc and sulfur atoms are four... 

Scheme 1.61. Ethylzincation of alkenes catalyzed by Cp2ZrCI2-2EtMgBr.

The mechanism of this one-pot reaction is proposed to be as follows (Figure 4.3) firstly, a chiral alkoxide ethylzinc is prepared from diethylzinc and the chiral alcohol with the evolution of a gas, which is probably ethane (I). The chiral ethylperoxyzinc alkoxide is formed by insertion of oxygen into the carbon zinc... 

Another approach was followed by Bolm et al. [24], who prepared dendron ligands consisting of a chiral pyridyl alcohol connected to the core of Frechet type dendrons [25]. The chiral dendron ligands were used for the in situ generation of ethylzinc dendron ligand complexes which catalyze the addition of diethylzinc to benzaldehyde (see e.g., 15, Scheme 16). The size of the dendron appeared to have practically no influence on the enantioselectivity of this reaction. 

Ethylzinc-enamine (95) undergoes a condensation reaction with aldehydes, affording ethylzinc-aldolates (equation 20). Such ethylzinc-aldolates are dimers, due to bridging of the aldolate-oxygen atoms between two zinc atoms. The structural features of this type of compounds will be discussed elsewhere in this chapter. 

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