Dialkylzinc

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

Ligand exchange reactions can be used to prepare perfluoroalkylzinc compounds Solvated trifluoromethylzinc compounds can be synthesized via the reaction of dialkylzincs with bis(trifluoromethyl)mercury [36] (equation 27) A similar exchange process with bis(trif]uorometliyl)cadinium and diraethylzinc gives a mixture of tnfluoromethylcadmium and zinc compounds [77]... 

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). 

Asymmetric nucleophilic addition of dialkylzinc to 3,4-dihydroisoquinoline 1-oxides 98YGK11. 

CotLespondingly, tlie catalytic 1,4-addition of dialkylzinc reagents to 3-nitro-iimarin 89 iSdieme 7.24), witli a fixed irans orientation of tlie aiyl and nitro oiips, proceeds witli excellent yidds 190-99 96), bigli diastereoselectivity Id.r. up to ), and enantiosdectivities of up to 9296. Hydrolysis of tlie lactone moiety in 90 IS accompanied by decarboxylation, providing an asymmetric syntliesis of /J-aiyl-troalkane 91. 

Wlien 0.5 equivalents of dialkylzinc were used, ees of mote than 9096 were obtained, with reasonable isolated yields of up to 38 96 [52] of the Su2 -suhstituted... 

Pineschi and Feringa reported that chiral copper phosphoramidite catalysts mediate a regiodivergent kinetic resolution (RKR) of cyclic unsaturated epoxides with dialkylzinc reagents, in which epoxide enantiomers are selectively transformed into different regioisomers (allylic and homoallylic alcohols) [90]. The method was also applied to both s-cis and s-trans cyclic allylic epoxides (Schemes 7.45 and 7.46,... 

Cu(i(-catalyzed kinetic resolutions of racemic, cyclic 1,3-diene monoepoxides through the use of dialkylzinc [123] or trialkylaluminium reagents [124] have re-... 

Since the addition of dialkylzinc reagents to aldehydes can be performed enantioselectively in the presence of a chiral amino alcohol catalyst, such as (-)-(1S,2/ )-Ar,A -dibutylnorephedrine (see Section 1.3.1.7.1.), this reaction is suitable for the kinetic resolution of racemic aldehydes127 and/or the enantioselective synthesis of optically active alcohols with two stereogenic centers starting from racemic aldehydes128 129. Thus, addition of diethylzinc to racemic 2-phenylpropanal in the presence of (-)-(lS,2/ )-Ar,W-dibutylnorephedrine gave a 75 25 mixture of the diastereomeric alcohols syn-4 and anti-4 with 65% ee and 93% ee, respectively, and 60% total yield. In the case of the syn-diastereomer, the (2.S, 3S)-enantiomer predominated, whereas with the twtf-diastereomer, the (2f ,3S)-enantiomer was formed preferentially. 

Formation of C-C Bonds Using Dialkylzinc Reagents Under Chiral Catalysis... 

Table 26. Addition of Dialkylzinc Reagents to Aldehydes under Chiral Catalysis...

To obtain information about the structural requirements of a ligand capable of catalyzing the addition of dialkylzincs to aldehydes, various simple amines, alcohols and amino acid derived amino alcohols were tested as chiral catalysts (Table 27). 

Since amines, unlike alcohols, do not react with dialkylzincs but may coordinatively bind to the zinc atom, the dilithio salt of 2,5-diisopropylpiperazine is a more potent catalyst than the piperazine itself, presumably due to the greater nucleophilic power of the dianion10. Dimethylzinc reacts rather slowly with aryl aldehydes and although the product is obtained in high enantiomeric excess, the chemical yield is low10. 

The tridentate ligands C, L and M are effective catalysts for the enantioselective addition of dialkylzincs to aromatic aldehydes16,17. In particular, ligands L and M qualify as members of the chemical enzyme (chemzyme) class of synthetic reagents17, since they function in a predictable, clear-cut mechanistic way. As demonstrated by X-ray diffraction, the actual catalyst is a monomeric zinc chelate 2 formed in toluene at 50 C by reaction of L or M with one equivalent of diethylzinc. 

Polymer-supported amino alcohols and quaternary ammonium salts catalyze the enan-tioselective addition of dialkylzinc reagents to aldehydes (Table 31). When the quaternary ammonium salt F is used in hexane, it is in the solid state, and it catalyzes the alkylation of benzaldehyde with diethylzinc in good chemical yield and moderate enantioselectivity. On the other hand, when a mixture of dimethylformamide and hexane is used as solvent, the ammonium salt is soluble and no enantioselectivity is observed21. 

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. 

The polymer-bound catalysts A-C. (Table 31) are prepared by reaction of the corresponding amino alcohols with partially chloromethylated 1 -2% cross-linked polystyrene. In the case of A, the enantioselectivity of the addition of dialkylzincs to aldehydes is higher than with the corresponding monomeric ephedrine derivatives (vide supra). Interesting insights into the mechanism of the alkylation of aldehydes by dialkylzinc reagents can be obtained from the experi-... 

Optically active alkynyl alcohols can, however, be conveniently prepared by the addition of dialkylzinc reagents of alkynyl aldehydes catalyzed by the chiral ligand (S)-l-methyl-a,a-diphenyl-2-pyrrolidine methanol33-34. 

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