Diethylzinc determination

In a flame-dried Schlenk tube 0.37 g(1.88 mmol) of (-)-3-exo-(dimethylamino)isoborneol (C) and 200 mL of dry toluene are placed under an atmosphere of argon. 27 mL of 4.2 M diethylzinc (113 mmol) in toluene are added and the resulting solution is stirred at 15°C for 15 min. After cooling to — 78°C, lOg (94.2 mmol) of benzaldehyde are added and the mixture is wanned to O C. After stirring for 6 h, the reaction is quenched by the addition of sat. NH4C1 soln. Extractive workup is followed by distillation yield 12.4 g (97%) 98% ee [determined by HPLC analysis. Baseline separation of rac-1 -phenyl-1 -propanol was achieved on a Bakerbond dinitrobenzoyl phenylglycine column (eluent 2-propanol/hexanc 1 3 flow rate l.OmL/ min detection UV 254 nm)] [a] 0 —47 (c = 6.11, CHC13). 

Diethylzinc forms a 1 1 complex with DABCO, Et2Zn(DABCO) (94). An X-ray crystal structure determination revealed that Et2Zn(DABCO) exists in the solid state as a coordination polymer in which Et2Zn units are linked by the two coordinating nitrogen atoms... 

The next classes of reagents developed are those for the cyclopropanation of unfunctionalized alkenes. After early attempts at getting high enantioselectivities for the cyclopropanation of /3-methylstyrene using a chiral alcohol (21), bis(iodo)methylzinc, diethylzinc, dichloromethane and a Lewis acid (equation 90) , Shi and coworkers made a major breakthrough when they found that a simple dipeptide (22) derived from valine and proline could be used (equation 91) . However, in either case, the absolute stereochemistry of the cyclopropane has not been determined. 

In the (—)-DAIB-catalyzed reaction of diethylzinc and benzaldehyde, the rate is first-order in the amino alcohol. The initial alkylation rate is influenced by the concentration of diethylzinc and benzaldehyde but soon becomes unaffected by increased concentration. Thus, under the standard catalytic reaction conditions, the reaction shows saturation kinetics the rate is zeroth order with respect to both dialkylzinc reagent and aldehyde substrate. These data support the presence of the equilibrium of A-D, and alkyl transfer occurs intramolecularly from the dinuclear mixed-ligand complex D. This is the stereo-determining and also turnover-limiting step. 

The most fully understood system in this class of reactions, however, is the DAIB-catalyzed addition of diethylzinc to aldehydes, due to the very detailed mechanistic studies performed by Noyori et al.32-37 They were able to determine the structure of several intermediates involved in the reaction and established the kinetic law. Part of the catalytic cycle is depicted in Scheme 13. The origin of the asymmetric amplification lies in the formation of dimers of DAIB-zinc alkoxides. The heterochiral dimer is quite stable in the concentration range of the experiment (2 x 10 1 to 5 x 10 1 M in toluene for DAIB), whereas the homodimers are prone to dissociation and react further with diethylzinc to give a di-zinc complex that is the active species in the catalytic cycle. They react with benzaldehyde and give rise to the asymmetric transfer of the ethyl group. The final product, as a zinc alkoxide, does not interfere with the reaction (and hence there is no autoinduction), since it... 

Acetals of a,j8-unsaturated aldehydes with 3-0-alkylated derivatives of 1,2-0-isopropylidene- -D-fructopyranose and l,2-0-isopropylidene-/l-D-psicopyranose, which are readily available from D-fructose, were cyclopropanated with diethylzinc/diiodomethane with good stereoselectivity. The acetals were hydrolyzed and the aldehydes reduced to give cyclopropyl-substituted alcohols e.g. cyclopropanation of 103 to give 104 and hydrolysis and reduction to R,2R)-2-phenylcyclopropylmethanol (105). Extensive studies were carried out, with both exo- and endo-acQta structures, to determine the effects of the structure of the acetals on the enantioselec-tivity. Among various isomeric compounds, the asymmetric cyclopropanation reaction provided good enantioselectivity (consistent attack on the same face) with high chemical yields especially with en sfo-acetals of l,2-0-isopropylidene-3-0-(4-phenylbenzyl)-iS-D-fructopyranose. 

The atomic arrangement of the (2x4) reconstructed p-InP surface of a thin homoepitaxial layer has been determined from low-energy electron diffraction experiments and is shown in Figure 2.69. Samples with an In-rich surface have been prepared by MOVPE on a (100)-oriented InP wafer that was doped with Zn to 10 cm Thin films were also Zn-doped with a dopant concentration of 4 X 10 cm l As precursors in the deposition process, trimethylindium, tert-butylphosphine, and diethylzinc have been used for doping. The ratio of the... 

The procedure was applied to the determination of ethyl and butyl groups in a sample of distilled diethylzinc. This material contained some aluminium impurity originating in the triethylaluminium used in its preparation. The results obtained are presented in Table 6. 

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. ... 

We have polymerized propylene sulfide and propylene oxide of different enantiomeric composition with the same initiator system, diethylzinc-/ (-)-3,3-dimethyl-1,2-butanediol (1 1). Starting from a non racemic mixture we have to use Equation (1) for the determination of stereoelectivity ratio r. 

Table 11 Zn-C bond distances in dimethylzinc, diethylzinc, di-iso-propylzinc, and di-tert-butylzinc determined by gas electron diffraction and quantum chemical DFT calculations mean Zn-C bond dissociation energies derived from experimental thermochemical data and from calculated energies of isodesmic reactions combined with the experimental bond dissociation energy of Mc2Zn and net atomic charges of Zn and a-C atoms obtained by Mulliken population analysis...

Chiral alcohol (15,2/ )-52 (90-94% ee) was obtained as a major product by the chiral copper phosphoramidite-cata-lyzed alkylative ring opening of oxabenzonorbornadiene with diethylzinc. The esterification of alcohol (lS,2/ )-52 with (-)-CSDP acid yielded ester (15, 2/ )-(—)-53, which was recrystallized giving single crystals suitable for X-ray crystallography. The AC of (—)-53 was unambiguously determined based on the known AC of the CSDP acid moiety. 

---