Zinc alkyls reactions with

Transition State Models. The stoichiometry of aldehyde, dialkylzinc, and the DAIB auxiliary strongly affects reactivity (Scheme 9) (3). Ethylation of benzaldehyde does not occur in toluene at 0°C without added amino alcohol however, addition of 100 mol % of DAIB to diethylzinc does not cause the reaction either. Only the presence of a small amount (a few percent) of the amino alcohol accelerates the organometallic reaction efficiently to give the alkylation product in high yield. Dialkyl-zincs, upon reaction with DAIB, eliminate alkanes to generate alkylzinc alkoxides, which are unable to alkylate aldehydes. Instead, the alkylzinc alkoxides act as excellent catalysts or, more correctly, catalyst dimers (as shown below) for reaction between dialkylzincs and aldehydes. The unique dependence of the reactivity on the stoichiometry indicates that two zinc atoms per aldehyde are responsible for the alkyl transfer reaction. 

Reaction XV. Action of Zinc Alkyl on Aldehydes, on certain Ketones, and on Acyl Chlorides. (A., 223, 162.)—As with other zinc alkyl reactions the corresponding Grignard reaction described in Reaction XIV. has replaced it almost completely, so that the following is only of more or less historical and theoretical interest ... 

Reaction XLIII. (c) Condensation of a-Halogen Fatty Acid Esters with Aldehydes and Ketones by means of Zinc or Magnesium (Reformatsky-Grignard). (C., (1901), I., 1196 II., 30 (1902), I., 856.)—This is an extension of the Grignard and zinc alkyl reactions which enables a-halogen esters to be condensed with carbonyl compounds as if they were simple alkyl halogen compounds. The zinc or magnesium alkyl derivative is neither prepared beforehand nor isolated in the reaction, but there is little doubt that some such compound is transitorily formed. Zinc is the metal... 

All lation. Several alkylation reactions are known either the olefin or chloro- group may be involved. The reactions of aEyl chloride with benzene are typical of reactions involving the double bond. In the presence of ferric or zinc chloride, the products are 2-chloropropylbenzene [10304-81-1] and 1,2-diphenylpropane [5814-85-7]. ... 

A carboxylic acid (not the salt) can be the nucleophile if F is present. Mesylates are readily displaced, for example, by benzoic acid/CsF. Dihalides have been converted to diesters by this method. A COOH group can be conveniently protected by reaction of its ion with a phenacyl bromide (ArCOCH2Br). The resulting ester is easily cleaved when desired with zinc and acetic acid. Dialkyl carbonates can be prepared without phosgene (see 10-21) by phase-transfer catalyzed treatment of primary alkyl halides with dry KHCO3 and K2C03- ... 

Halide exchange, sometimes call the Finkelstein reaction, is an equilibrium process, but it is often possible to shift the equilibrium." The reaction is most often applied to the preparation of iodides and fluorides. Iodides can be prepared from chlorides or bromides by taking advantage of the fact that sodium iodide, but not the bromide or chloride, is soluble in acetone. When an alkyl chloride or bromide is treated with a solution of sodium iodide in acetone, the equilibrium is shifted by the precipitation of sodium chloride or bromide. Since the mechanism is Sn2, the reaction is much more successful for primary halides than for secondary or tertiary halides sodium iodide in acetone can be used as a test for primary bromides or chlorides. Tertiary chlorides can be converted to iodides by treatment with excess Nal in CS2, with ZnCl2 as catalyst. " Vinylic bromides give vinylic iodides with retention of configuration when treated with KI and a nickel bromide-zinc catalyst," or with KI and Cul in hot HMPA." ... 

The analogous reactions of Zn and Cd derivatives arc less well studied. Zinc alkyls ZnRi (R = Et, n-Bu) react with transition-metal hydride complexes, H2M(Cp-(M = Mo, W) ... 

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

Notes on the preparation of secondary alkylarylamines. The preparation of -propyl-, ijopropyl- and -butyl-anilines can be conveniently carried out by heating the alkyl bromide with an excess (2-5-4mols) of aniline for 6-12 hours. The tendency for the alkyl halide to yield the corresponding tertiary amine is thus repressed and the product consists almost entirely of the secondary amine and the excess of primary amine combined with the hydrogen bromide liberated in the reaction. The separation of the primary and secondary amines is easily accomplished by the addition of an excess of per cent, zinc chloride solution aniline and its homologues form sparingly soluble additive compounds of the type B ZnCl whereas the alkylanilines do not react with sine chloride in the presence of water. The excess of primary amine can be readily recovered by decomposing the zincichloride with sodium hydroxide solution followed by steam distillation or solvent extraction. The yield of secondary amine is about 70 per cent, of the theoretical. 

Organozinc reagents have been used in conjunction with a-bromovinylboranes in a tandem route to Z-trisubstituted allylic alcohols. After preparation of the vinylborane, reaction with diethylzinc effects migration of a boron substituent with inversion of configuration and exchange of zinc for boron.176 Addition of an aldehyde then gives the allylic alcohol. The reaction is applicable to formaldehyde alkyl and aryl aldehydes and to methyl, primary, and secondary boranes. 

The reactivity of the zinc alkyl derivatives [TpBut]ZnR is typically lower than that of the corresponding magnesium derivatives (Section III,A,2,c.). For example, whereas [TpBut]MgMe undergoes insertion of C02 into the Mg-C bond at room temperature, no reaction is observed between [TpBut]ZnMe and C02 at 140°C. Similarly, whereas the magnesium alkyl derivatives [TpBut]MgR (R = Me, Et, Pr1, Bul) react immediately with 02 at room temperature to give alkylperoxo derivatives [TpBut]MgOOR, solutions of the zinc derivative [TpBut]ZnEt are stable in the presence of 02 at 100°C. 

The diastereoselective ultrasonically induced zinc-copper 1,4-addition of alkyl iodides to chiral a, (i-unsaturaied systems in aqueous media was studied by Suares and co-workers the Z-isomer gives good diastereos-electivities while reactions with the E-isomer are nonstereoselective.63 The 1,4-addition to chiral y, a-dioxolanyl-a, (i-unsaturated esters also proceeds with good yields (51-99%) (Eq.10.29).64... 

Clerici and Porta reported that phenyl, acetyl and methyl radicals add to the Ca atom of the iminium ion, PhN+Me=CHMe, formed in situ by the titanium-catalyzed condensation of /V-methylanilinc with acetaldehyde to give PhNMeCHMePh, PhNMeCHMeAc, and PhNMeCHMe2 in 80% overall yield.83 Recently, Miyabe and co-workers studied the addition of various alkyl radicals to imine derivatives. Alkyl radicals generated from alkyl iodide and triethylborane were added to imine derivatives such as oxime ethers, hydrazones, and nitrones in an aqueous medium.84 The reaction also proceeds on solid support.85 A-sulfonylimines are also effective under such reaction conditions.86 Indium is also effective as the mediator (Eq. 11.49).87 A tandem radical addition-cyclization reaction of oxime ether and hydrazone was also developed (Eq. 11.50).88 Li and co-workers reported the synthesis of a-amino acid derivatives and amines via the addition of simple alkyl halides to imines and enamides mediated by zinc in water (Eq. 11.51).89 The zinc-mediated radical reaction of the hydrazone bearing a chiral camphorsultam provided the corresponding alkylated products with good diastereoselectivities that can be converted into enantiomerically pure a-amino acids (Eq. 11.52).90... 

Zinc carbamate complexes are well known, and the structural types and stabilities can be compared with thiocarbamates and dithiocarbamates which are discussed in Sections 6.8.11.1.3 and 6.8.7.1.4482 Carbamates of zinc can be formed from the reaction of carbon dioxide with alkylzinc alkyl amides and further reaction with alkylzinc can give a distorted cubane structure.483 The tetrameric diethylcarbamate species initially formed can also be used to produce monomeric or dimeric carbamate structures in reaction with amines tetramethylethylenediamine forms a monomer [(Me2NCH2)2Zn(02CN(C2H5)2)2] with an octahedral zinc center and pyridine forms a dimer[CsH5NZn2Me(02CN(C2H5)2)3] with tetrahedral zinc centers.484... 

---