Zincates compounds

In contrast to Grignard reagents, dialkylzinc compounds are inert to carbon monoxide at atmospheric pressure either in the absence of solvents or in solution in THF, diglyme, ether, or benzene. Although Rathke and Yu reported in 1972 that the addition of potassium tert-butoxide in stoichiometric amounts promoted the absorption of carbon monoxide, it is now known that the actual reactive species was the corresponding zincate compound (R2ZnO-t-Bu) K+. The only zinc carbonyl species reported so far has been observed by NMR studies of CO on zeolite ZnY and poly crystalline ZnO, at low temperature. " Before this report, such species were only suggested from infrared and photoelectron studies of CO weakly bound to ZnO. 

The more difficult cross-coupling reaction between Csp -Csp can be accomplished by mixed zincate compounds, prepared by the reaction of diorganozinc derivatives with Me2CuMgCl-MgCl(CN) (equation 59). Substitution reaction at Csp centers are also efficiently catalyzed by Ni salts when the alkyl halide contains a double bond in remote position (Scheme 22). Changing the double bond for a triple bond leads to an intramolecular carbozincation reaction (equation 60). 

Formation of Zincate Complexes. Zincate compounds such as K(ZnCH2SiMe3)3 are formed in a transmetalation reaction of bis[(trimethylsilyl)methyl]zinc with elemental potassium, or by reaction with potassium salts. Lithium zincates such as Li[Zn(CH2SiMe3)2R] [R = Me, Ph, N(SiMe3)2] can be prepared by reacting the title compound with the corresponding organo-lithium compounds.Transmetalation with alkaline earth metals such as strontium, barium, and calcium affords zincates of the type illustrated in eq... 

The 1 1 zincate reagent is believed to be dimeric. At higher ratios of organolithium compounds, 2 1 and 3 1 species can be formed.174... 

Additional bonds are thus donor bonds, and to accept electron pairs from neutral and anionic ligands, zinc uses the two remaining 4p orbitals to form sp2 and sp3 hybrids. In the absence of steric effects, discrete, homoleptic, anionic tri- and tetraorganozinc compounds (zincates) have almost always ideal trigonal-planar and tetrahedral geometries, respectively. 

The first structurally characterized triarylzincate [Li(TMEDA)2][Zn 2,4,6-(Pr1)3G6H2 3] 59 was a byproduct of the lithiation of VC13(THF)3 with Li(2,4,6-(Pr1)3C6H2)3, in which the vanadium compound had been pre-reduced with metallic zinc.116 In the solid state, the compound consists of isolated, tetrahedrally coordinated lithium ions and trigonal-planar tris(tris(isopropyl)phenyl)zincate ions. The Zn-G bonds are equidistant (2.039(7) A), but the C-Zn-C bond angles are variable, ranging from 112.2(3)° to 125.4(3)°. 

Although the existence and structures of tetraorganozincates in the solid state are now well established, the reactivity patterns of these compounds in organic synthesis are still largely unknown. Homoleptic and heteroleptic tetrasubstituted dianionic zincates of the type [ZnR3X], X = Me, CN, SCN, were prepared as shown in Scheme 58... 

For purposes of completeness, palladium-catalyzed reactions for the synthesis of biaryls using zincates and arylhalides (Negishi Couplings) [254] or using sihcon compounds [255, 256] need to be mentioned. 

Organomagnesium compounds react with imines, prepared from 3-methoxy-2-naphth-aldehydes by a 1.4-addition mechanism. This reaction can be performed with high diastere-oselectivity. The method was applied for the synthesis of optically pure S-tetralones . Vinyhnagnesium bromide reacts as an acceptor with a ketone dimethyl hydrazone zincate 207, yielding a 1,1-bimetallic species, which can be reacted sequentially with two different electrophiles (equations 131 and 132) . The reaction proceeds via a metalla-aza-Claisen rearrangement, where the dimethylhydrazone anion behaves as an aza-allylic system . 

In an approach similar to the synthesis of 12, the neopentyl derivatives [(Me3CCH2)3Zn] K(C6H6) (17) and [(Me3CCH2)3Zn]Na (18) have been prepared from the reaction of (Me3CCH2)2Zn and potassium and sodium, respectively. Also, the structures of these compounds were crystallographically determined and are closely related to the structures of the (trimethylsilyl)methyl zincates described above . 

So far, only organozincates have been considered in which the three or four organic groups bound to zinc are identical (homoleptic zincates). It is well established that heteroleptic organozinc compounds undergo the so-called Schlenk equilibrium (equation 12). This is also true for heteroleptic zincates (equation 13). In this case, the equilibria are even more complicated, due to the possible formation of a variety of homo- and heteroleptic species. 

Such equilibria are driven by thermodynamics and therefore a selective synthetic route towards one of these species and isolation of such heteroleptic zincates in pure form is often very difficult or impossible. Only if one of the species has a sufficiently enhanced thermodynamic stability compared to the others in the equilibrium is its isolation as a pure compound possible. This is often the case when the various groups bound to zinc have a sufficiently different electronegativity, for example when one of the groups is bound to zinc via a heteroatom, or when the steric requirements of the groups bound to zinc are rather different. Sometimes it is possible to isolate one of the species present in the Schlenk equilibrium as a solid material, for example when one of the species preferentially crystallizes from solution. 

According to NMR data, reaction of lithium tetramethylpiperidine (TMP) and t-Bu2Zn, in a 1 1 molar ratio (Scheme 6) affords the heteroleptic zincate [f-Bu2Zn(TMP)]Li (36) . An interesting feature of this compound is that it is capable of metalate arenes that contain a directing metalating group (DMG) in ortho-posilion and thus allow further derivatization by reaction of the zincate intermediate with electrophiles (E+). 

The mutual diastereoselection between the alkenyllithium and the zincated allylic ether turned out to be the same as for a crotylzinc species, and could be rationalized by kinetic addition of a cisoid -alkoxyallylic organozinc compound. 

Nevertheless, chiral propargylic amines remain interesting substrates for achieving the diastereoselective addition of substituted allylic organozinc compounds to metallated alkynes. Besides crotylzincation, one example of diastereoselective addition of zincated allyl ethyl ether to 328 has also been reported183. 

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