Dialkylzinc complex

The investigation of the exchange of organyl gronps between varions dialkylzinc complexes, ZnR 2 and ZnR"2, has been performed by El MS. The observation of M of the mixed componnds ZnR R" confirmed that the alkyl exchange took place in the gas phase. ... 

The reaction of an aryl and an alkyl iodide in presence of complex (103) and the Zn-Cu couple leads to alkyl aryl ketones in good yields (equation 115).492 By-products such as ArR, ArH and Ar2 were found in some cases. Replacement of alkyl iodides by benzyl chlorides gave benzyl ketones, but the formation of by-products due to coupling reactions was significant. Dialkylzinc complexes were formed here and the proposed mechanism is given in Scheme 41. 

The reaction of iodopentafluorobenzene with dialkylzinc in the presence of Lewis bases quantitatively gives the pentafluorophenylzinc complexes [32], The pentafluorophenylzinc complexes can be prepared more easily via the direct reaction of bromopentafluorobenzene with zinc powder in DMF [/22] (equations 94 and 95). 

While the mechanism of the ammonium salt catalyzed alkylation is unclear, in polar solvents the enantioselectivity of the addition of dialkylzincs to aldehydes generally drops considerably, probably due to uncatalyzed product formation or complexation of the zinc reagent with the polar solvent rather than with the chiral auxiliary. 

Combination of nickel bromide (or nickel acetylacetonate) and A. A -dibutylnorephcdrinc catalyzed the enantioselective conjugate addition of dialkylzincs to a./Tunsaturated ketones to afford optically active //-substituted ketones in up to ca. 50% ee53. Use of the nickel(II) bipyridyl-chiral ligand complex in acetonitrile/toluenc as an in situ prepared catalyst system afforded the //-substituted ketones 2, from aryl-substituted enones 1, in up to 90% ee54. 

Kitamura and Noyori have reported mechanistic studies on the highly diastere-omeric dialkylzinc addition to aryl aldehydes in the presence of (-)-i-exo-(dimethylamino)isoborneol (DAIB) [33]. They stated that DAIB (a chiral (i-amino alcohol) formed a dimeric complex 57 with dialkylzinc. The dimeric complex is not reactive toward aldehydes but a monomeric complex 58, which exists through equilibrium with the dimer 57, reacts with aldehydes via bimetallic complex 59. The initially formed adduct 60 is transformed into tetramer 61 by reaction with either dialkylzinc or aldehydes and regenerates active intermediates. The high enantiomeric excess is attributed to the facial selectivity achieved by clear steric differentiation of complex 59, as shown in Scheme 1.22. 

Dialkylzinc compounds have a tendency to form colored, and sometimes paramagnetic, adducts with 7r-acceptor ligands. The potentially bridging nitrogen donor pyrazine (and 4,4 -bipyridine) formed dinuclear complexes, Scheme 31, while the chelating 2,2 - bipyridine formed mononuclear adducts with diisopropylzinc.78 In the bis(diisopropylzinc)-pyrazine adduct 33, the presence of an unpaired electron was detected, but the resolution of the ESR spectra was insufficient for an unambiguous structural assignment. 

These carbamate-rich species can be converted to conventional alkylzinc carbamates by treatment with excess dialkylzinc. Donor solvents force the dissociation of the alkylzinc carbamate aggregates to give complexes of lower nuclearity. For example, pyridine converts the tetrameric [MeZn(02CNPr1)2]4 to the dimeric [MeZn 02CN-(Pr1)2 (py)]2 142, shown in Figure 67.204... 

Hoveyda and co-workers also tested optically active A-heterocyclic carbenes and their silver complexes in copper-catalyzed reactions of allylic phosphates with dialkylzincs.402 The ratios of Sn2 SN2 products were higher than 98 2 and the ee varied from 34% to 98%. 

When Knochel and his co-workers attempted to use [PdC CF CN ] and related palladium(n) complexes as catalysts in the reactions of dialkylzincs with alkyl iodides, they observed the formation of the halogen-zinc exchange405 or cyclization406 products only. A recent paper of Zhou and Fu demonstrated that palladium complexes can also be used in the coupling reactions of alkylzinc bromides with alkyl iodides, bromides, chlorides, and... 

Since the discovery of amino alcohol induced dialkylzinc addition to aldehydes, many new ligands have been developed. It has recently been reported that chiral amino thiols and amino disulfides can form complexes or structurally strained derivatives with diethylzinc more favorably than chiral amino alcohols and thus enhance the asymmetric induction. Table 2 15 is a brief summary of such chiral catalysts. 

There is an equilibrium between the dimer and monomer, and molecular orbital study suggests that the heterochiral dimer is more stable than the homochiral isomer. The existence and behavior of the dimeric species were well confirmed by experiments such as cryoscopic molecular weight and NMR measurement. In the NMR study of a DAIB-catalyzed dialkylzinc addition reaction, noticeable changes were observed in the spectrum of the homochiral dimer on the addition of benzaldehyde, while the spectrum of the heterochiral complex remained the same. This may imply that the heterochiral complex is very stable and does not react, and the homochiral dimer leads to the reaction product. 

Enantioselective addition of R2Zn to aldehydes. Corey and Hannon2 have prepared the diamino benzylic alcohol 1 from (S)-proline and (lS,2R)-( + )-ephed-rine and report that the chelated lithium salt of 1 is an effective catalyst for enantioselective addition of diethylzinc to aromatic aldehydes. Thus benzaldehyde can be converted into (S)-( - )-3 with 95% ee, via an intermediate tridentate lithium complex such as 2 formed from 1. Similar reactions, but catalyzed by diastereomers of 1, show that the chirality of addition of dialkylzincs to aldehydes is controlled by the chirality of the benzylic alcohol center of 1. 

The catalysed reaction of a,p-unsaturated ketones with dialkylzincs and oxygen leads to the formation of chiral acyloxiranes. The initially formed intermediate complex between the chiral (3-hydroxyamine and the dialkylzine (cf. Scheme 12.9) is oxidized to the peroxyalkylzinc complex prior to the formation of the chiral oxirane (Scheme 12.13) [28]. 

Dialkylsilanols, sulfuric acid and, 1 410-411 Dialkyl strontium, 11 343-345 complexes of, 11 350 Dialkylzinc, 11 345-346 complexes of, 11 350-353 absorption bands of, 11 353... 

When we entered the field, Tomioka et al. had just introduced the dialkylzinc addition to N-sulfonylimines in the presence of chiral amidophosphine-copper(II) complexes, producing high levels of enantioselectivity (up to 94% ee) [27]. At the... 

The complex of Me2Zn with (5, 5 )-ebpe, 107, has been applied successfully as catalyst in the enantioselective reduction of ketones by polymethylhydrosiloxane and combines excellent product yields with high ee values . Its structure comprises the iV,iV-chelate coordination of the ebpe ligand to the MeiZn unit (Figure 51). It is remarkable that in this case the two secondary amine functionalities are coordinated to zinc and leave the Zn—C bonds unaffected. Indeed, usually secondary amines undergo a fast deprotonation reaction with dialkylzinc compounds. 

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