Zinc reagents catalytically active

This chemistry was extended to a catalytic enantioselective alkenylation and phenylation of aldehydes and a-ketoesters. Using CuF-DTBM-SEGPHOS complex, products were obtained with excellent enantioselectivity from a wide range of aldehydes including aromatic and aliphatic aldehdyes, [Eq. (13.26)]. Previously catalytic enantioselective vinylation and phenylation are restricted using the corresponding zinc reagents. The active nucleophile is proposed to be an alkenyl or phenyl copper, based on NMR studies. The chiral CuF catalyst can also be applied to a catalytic enantioselective aldol reaction to ketones... 

In Raney s method a catalytically active metal is alloyed with a catalytically inactive one and then treated with a reagent that dissolves out the inactive metal. The catalytically inactive component that is to be dissolved out may be aluminum, silicon, magnesium, or zinc. The catalytically active metal is usually nickel, cobalt, copper, or iron. Noble-metal catalysts can, however, also be produced by Raney s method if an aluminum-platinum alloy (40% of platinum) or a zinc-palladium alloy (40% of palladium) is decomposed by hydrochloric acid.153... 

Zinc chloride was used as a catalyst in the Friedel Crafts benzylation of benzenes in the presence of polar solvents, such as primary alcohols, ketones, and water.639 Friedel-Crafts catalysis has also been carried out using a supported zinc chloride reagent. Mesoporous silicas with zinc chloride incorporated have been synthesized with a high level of available catalyst. Variation in reaction conditions and relation of catalytic activity to pore size and volume were studied.640 Other supported catalytic systems include a zinc bromide catalyst that is fast, efficient, selective, and reusable in the /wa-bromination of aromatic substrates.641... 

Zinc was effectively activated from zinc chloride using lithium and a catalytic amount (10%) of naphthalene in order to prepare secondary or tertiary alkylzinc bromides 517 (starting from the corresponding aUcyl bromides 516). These reagents react with acyl chlorides or a,/3-unsaturated ketones to give the expected ketones 15 and 518 (Scheme 143). 

The enzyme contains six Zn2+ per molecule, two per R subunit. The zinc is not required for catalytic activity, but is essential for the maintenance of the quaternary structure. The structure has been determined to a resolution of 2.8 A in the presence and absence of CTP.528 The zinc-binding site is located in the C-terminal region of the R chains, and involves four cysteinyl residues, with tetrahedral geometry. The zinc domain represents the major site of interaction between the R and C chains, explaining the importance of zinc for the association of the subunits and the dissociative effect of mercurial reagents. When E. coli is grown in a zinc-deficient medium, some 70% of the enzyme is found to be dissociated into subunits.529... 

The reactions of a range of aryl, benzylic, and heterocyclic zinc reagents with iodo- and bromoarenes were reported at ambient temperature under biphasic conditions with [C4mmim][PF6] and toluene. The biaryl products were readily isolated by decanting the toluene phase, with yields of 70-92% achieved after several minutes. However, attempts to recycle the catalytic ionic liquid solution resulted in significantly decreased activities. 

Certain transition metal complexes exhibit activating properties and act with turnover on the metal center analogously to the catalytically active zinc ion in the active center of liver alcohol dehydrogenase. Various chiral europium shift reagents, for example Eu(hfc)3, induce reduction of (9b) by 1,4-dihydroni-cotinamides. Turnovers of about 100 are obtained on the metal complexes and methyl mandelate is formed with enantiomeric excesses of 25-44%. ... 

Oguni has reported asymmetric amplification [12] ((-i-)-NLE) in an asymmetric carbonyl addition reaction of dialkylzinc reagents catalyzed by chiral ami-noalcohols such as l-piperidino-3,3-dimethyl-2-butanol (PDB) (Eq. (7.1)) [13]. Noyori et al. have reported a highly efficient aminoalcohol catalyst, 2S)-3-exo-(dimethylamino)isobomeol (DAIB) [14] and a beautiful investigation of asymmetric amplification in view of the stability and lower catalytic activity of the het-ero-chiral dimer of the zinc aminoalcohol catalyst than the homo-chiral dimer (Fig. 7-5). We have reported a positive non-linear effect in a carbonyl-ene reaction [15] with glyoxylate catalyzed by binaphthol (binol)-derived chiral titanium complex (Eq. (7.2)) [10]. Bolm has also reported (-i-)-NLE in the 1,4-addition reaction of dialkylzinc by the catalysis of nickel complex with pyridyl alcohols [16]. 

Reduction of Cp2TiCl2 by zinc produces an active species for the radical cyclization reactions of substituted a-(prop-2-ynyloxy) epoxides.1162 Cp2TiCl2 is reduced with powdered zinc in THF to produce Cp2TiCl in situ, Nugent s reagent, an efficient catalytic agent for organic reactions.1163,1164... 

Ishizaki and Hoshino prepared optically active secondary alkynyl alcohols (up to 95% e.e.) by the catalytic asymmetric addition of alkyl zinc reagents to both aromatic and aliphatic aldehydes. The chiral ligands studied were based on the pyridine scaffold. Of the three aryl substitutions studied, the a-napthyl derivative was found to be superior (Scheme 21.10). Mechanistically, it was proposed that (S)-l would react with dialkynyl zinc alkoxide A and ethyl zinc alkoxide B. Coordination of additional di-alkynyl zinc and alkynylethyl zinc with these alkoxides (A, B) would give C and D, respectively (Scheme 21.11). More bulky alkoxide (C) would have severe steric interactions with the alkynyl group and pyridine moiety, which might cause undesired conformational changes of the l-zinc complexes. Consequently, the enatioselectivity would be decreased. 

It should be noted that addition of Lewis acidic salts, such as MgBt2, is critical in order to achieve an effective catalytic transformation when using arylzinc compounds. This observation indicates that the difficult step of the catalytic cycle is the transmetaUation of the aryl group from the zinc reagent to the catalytically active iron complex [42]. While the involvement of an intermediate radical species or a single electron-transfer process is suspected, mechanistic details of these iron-catalyzed cross-coupling reactions remain unclear. 

Sonochemical reduction of nickel salts such as chloride with zinc powder gives catalytically active nickel. Under these conditions the excess of metallic zinc also gets activated and reduces the water present in the medium producing hydrogen gas. In this way, not only the catalyst but also the reagent is produced in situ with maximum efficiency and safety. This process has been used for the reduction of carbon-carbon double bonds in a,P unsaturated carbonyl compounds the C-C double bond is reduced much faster than the carbonyl group. The variation in the conditions, especially pH permits the selective reduction of C = C in preference to C = O (Scheme 16). 

Using stoichiometric amounts of an alkylzinc reagent and catalytic amounts of Ni(0)/DBU, Mori was able to prepare a number of p-aUcylated or arylated a,-P-unsaturated carboxylic acids from alkynes and CO2 (Scheme 5.12) [56, 57]. Through a frans-metalation reaction with the oxanickelacyclopentenone intermediate, the alkyl zinc reagent effected the reductive elimination of the product from the nickel center with concomitant regeneration of the active zerovalent catalyst. 

Unfortunately, catalytic asymmetric additions of organic alanes to imines are almost unknown, and admittedly, zinc reagents are better applicable for this purpose. But the characteristics of aluminum organyls reveal their potential the high Lewis acidity should enable activation of the rather imreactive CNl-double bonds and the low Br0nsted basicity should preclude deprotonation and formatiOTi of the respective azaenolates. Because chiral a-tertiary and a-secondaiy amines are ubiquitous structural motifs in natural products and synthetic bioactive compounds, exploration of alane additions is highly relevant. 

Titanium catalysts have long been used in electron transfer reactions involving epoxides, mostly as stoichiometric reagents. Gansauer et al. have developed a catalytic version of these reactions using titanocenes along with zinc metal to generate the active catalyst (Scheme 60). In situ reduction of Ti(IV) with zinc metal provides Ti(III) species 231, which coordinates... 

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