Zinc reagents Diethylzinc

Further optimization of this reaction was carried out with TFE as an achiral adduct, since reaction with TFE is much faster than that with neopentyl alcohol. We found that dimethyl- and diethylzinc were equally effective, and the chiral zinc reagent could be prepared by mixing the chiral modifier, the achiral alcohol and dialkylzinc reagent in any order without affecting the conversion and selectivity of the reaction. However, the ratio of chiral to achiral modifier does affect the efficiency of the reaction. Less than 1 equiv of the chiral modifier lowered the ee %. For example with 0.8 equiv of 46 the enantiomeric excess of 53 was only 58.8% but with 1 equiv of 46 it was increased to 95.6%. Reaction temperature has a little effect on the enantiomeric excess. Reactions with zinc alkoxide derived for 46 and TFE gave 53 with 99.2% ee at 0°C and 94.0% ee at 40°C. 

Ni(COD)2 alone catalyzes intramolecular alkylative cyclization of an alkynal with diethylzinc, while Ni(COD)2/PBu3 catalyzes reductive cyclization with the same zinc reagent (Scheme 87). 

Diethylzinc was obtained from Aldrich Chemical Company, Inc., and used without further purification. If the reaction flask has been carefully flushed with nitrogen, there sholild be very little smoke evident when the zinc reagent is added, Diethylzinc can be added with the syringe tip placed below the surface of the solvent rather than above it,... 

Representative results are shown in Fig. 2.1.2.1. Both reactions with mixtures of diphenylzinc and diethylzinc and those with pure diphenylzinc are catalyzed by ferrocene 9 as indicated by the comparisons between curves A and C as well as B and D, respectively. Apparently the reaction becomes slower when the mixture of the zinc reagents is applied (curve A versus curve B). We presume that this effect is - as hoped - due to a less pronounced background reaction as well as to a modification of the aryl source (potentially PhZnEt or complexes thereof). Consequently, a better control of the enantioselectivity at the expense of the reaction rate is observed. 

Duodomethane. 13,110-115 275-276 16,184-185 17,155 18, 139-140 19,128 Cyclopropanation. 1-Alkenylsilanols are transformed into cyclopropylsilanols by the Simmons-Smith reaction. A Grignard reagent may replace the zinc or diethylzinc... 

Figure 8.2) [4]. The initially obtained enantiomeric excess (ee) in this process was only 57%, and this was attributed to the competing background reaction. Later, Pu described protocols in which certain additives, such as methanol, were used to tame the reactivity of Ph2Zn [5]. The addition of diethylzinc to the reaction mixture led to the formation of a mixed zinc reagent, PhZnEt, which was found to be less reactive and thus more selective than Ph2Zn itself [6]. Additionally, Ph2Zn could be used in substoichiometric amounts (0.65equiv.), since both its phenyl groups were now available for the reaction.

Catalytic asymmetric propargylation has been reviewed (131 references). Aldehydes have been propargylated in up to 99% yield and 92% ee by allenyl-zinc reagents, generated in situ from the action of diethylzinc on iodoallene (or 3-iodopropyne), with a bulky diarylprolinol catalyst. The reaction is conveniently carried out in DCM at low temperature and avoids the use of tin reagents. ... 

Readily available chiral amines related to the Betti base [phenyl(2-hydroxy-l-naphthyl)methanamine] catalyse enantioselective addition of diethylzinc to aldehydes in moderate to excellent ee Observed enantioselectivities in addition of diethylzinc to aldehydes catalysed by a series of (5)-proline-derived pyrrolidines have been explained in terms of steric effects. New 2,5-diazabicyclo[2.2.1]heptanes have been applied to enantioselective addition of diethylzinc to benzaldehyde. (S)-2-(3-Methyl-2-pyridyl)-3,5-di-r-butylphenol (76) has been used as an enantioselective catalyst of diethylzinc addition to benzaldehydes. Reaction in toluene shows a significant variation in % ee with temperature, including observation of an inversion temperature with maximum ee. This value varies with the nature of the para-substituent in the aldehyde, and the overall behaviour may be due to a shift in the rate-determining step of the reaction. Other reports of zinc reagents include enantioselective addition of diethylzinc to aldehydes addition of diphenylzinc to aldehydes using a chiral ferrocene-based hydroxyoxazoline catalyst in up to 96% ee and 3-exo-morpholinoisoborneol has been proposed as a more convenient and efficient enantioselective catalyst of alkylzincs than Noyori s original 3-exo-dimethylamino catalyst. ... 

Oxygen accelerates the generation of the bromo-carbenoid reagent of zinc from diethylzinc and bromoform. 

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