Dialkylzinc, reaction with

For a stereoselective dialkylzinc reaction with a phosphinoylimine, see Addition to Organometallics below a resolution via a Schiff base is described under Enolates. 

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). 

Kitamura and Noyori have reported mechanistic studies on the highly diastere-omeric dialkylzinc addition to aryl aldehydes in the presence of (-)-i-exo-(dimethylamino)isoborneol (DAIB) [33]. They stated that DAIB (a chiral (i-amino alcohol) formed a dimeric complex 57 with dialkylzinc. The dimeric complex is not reactive toward aldehydes but a monomeric complex 58, which exists through equilibrium with the dimer 57, reacts with aldehydes via bimetallic complex 59. The initially formed adduct 60 is transformed into tetramer 61 by reaction with either dialkylzinc or aldehydes and regenerates active intermediates. The high enantiomeric excess is attributed to the facial selectivity achieved by clear steric differentiation of complex 59, as shown in Scheme 1.22. 

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. 

Primary and secondary alkylzinc iodides and benzylic zinc halides also undergo Ni-catalyzed reactions with various primary alkyl iodides and bromides.407-409 According to the procedure by Knochel and co-workers, the transformations with alkylzinc iodides, which are less reactive than the corresponding dialkylzincs, require the presence of two additives Bu4NI and 4-fluorostyrene (Scheme 155).407,408... 

Terminal propargylic mesylates are converted to alkylallenylzinc compounds by reaction with lithiotrialkylzincate reagents (Scheme 9.32) [117]. The latter are formed in situ from dialkylzinc and alkyllithium species. Deuterolysis of the allenylzinc intermediates gave rise to deuterated allenes (Eq. 9.138). 

Chiral aminoalcohols both catalyze reactions of simple dialkylzinc reagents with aldehydes and also induce a high degree of enantioselectivity, even when used in only catalytic amounts. Two examples are given below. Indicate how the aminoalcohols can have a catalytic effect. Suggest transition states for the examples show which would be in accord with the observed enantioselectivity. 

The complex of Me2Zn with (5, 5 )-ebpe, 107, has been applied successfully as catalyst in the enantioselective reduction of ketones by polymethylhydrosiloxane and combines excellent product yields with high ee values . Its structure comprises the iV,iV-chelate coordination of the ebpe ligand to the MeiZn unit (Figure 51). It is remarkable that in this case the two secondary amine functionalities are coordinated to zinc and leave the Zn—C bonds unaffected. Indeed, usually secondary amines undergo a fast deprotonation reaction with dialkylzinc compounds. 

A novel approach to allenylzinc reagents, reported by Oku and coworkers, employs propargylic mesylates as starting materials12. Treatment with triorganozincates, prepared in situ by the reaction of dialkylzinc compounds with organolithium reagents, affords transient allenyl dialkylzinc intermediates. These intermediates were found to react with various... 

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. 

Conjugate addition to enals. In the presence of Ni(acac), diarylzinc reagents undergo 1,4-addition to a, 3-enals in satisfactory yield however, the same reaction with dialkylzinc reagents gives low yields (15%, one example). 

In the course of the continuing study [9a,b] on the enantioselective addition of dialkylzincs to aldehydes by using chiral amino alcohols such as diphenyl(l-methyl-2-pyrrolidinyl)methanol (45) (DPMPM) [48] A. A -dibutylnorephedrine 46 (DBNE) [49], and 2-pyrrolidinyl-l-phenyl-1-propanol (47) [50] as chiral catalysts, Soai et al. reacted pyridine-3-carbaldehyde (48) with dialkylzincs using (lS,2/ )-DBNE 46, which gave the corresponding chiral pyridyl alkanols 49 with 74-86% ee (Scheme 9.24) [51]. The reaction with aldehyde 48 proceeded more rapidly (1 h) than that with benzaldehyde (16 h), which indicates that the product (zinc alkoxide of pyridyl alkanol) also catalyzes the reaction to produce itself. This observation led them to search for an asymmetric autocatalysis by using chiral pyridyl alkanol. 

The dialkylzincs up to the dibutyl derivatives readily ignite and bum in air. The higher alkyls fume but do not always ignite [1]. During preparation of dialkylzincs, reaction of the copper—zinc alloy with mixed alkyl bromides and iodides must begin (exotherm, often after a long induction period), before too much halide mixture is added, or violent explosions may occur [2]. Reactions with water may be explosive [3]. 

In another study Feringa et al. [20] reported a catalytic enantioselective three-component tandem conjugate addition-aldol reaction of dialkyl zincs. Here, zinc enolates were generated in situ via catalytic enantioselective Michael addition of dialkylzinc compounds to cydohexenone in the presence of a chiral Cu catalyst. Their diastereoselective reaction with an aldehyde then gave trans-2,3-disubstituted cyclohexanones in up to 92% yields and up to >99% ees (Scheme 9.11). 

Finally, when aziridines bear a 2-alkenyl substituent, they can engage in an SN2 reaction with dialkylzinc reagents using copper catalysts. Enantioselectivity can also be induced by including a chiral ligand, such as the binaphthyl phosphoramidite 174. For example, the aziridinyl cyclohexane 175 provided exclusively the trans-, 4 adduct with 83% ee upon treatment with dimethylzinc in the presence of copper(II) triflate and ligand 174 <03TL8559>. 

---