Lewis acids diethylzinc additions

With regard to the preparation of the Zn/Cu couple, several methods have been developed Dibromomethane may be used instead of diiodomethane when a specifically prepared Zn/Cu couple is applied . The use of Zn/Ag couple often gives better results . Ultrasound irradiation of the reaction mixture has been shown to facilitate the reaction . The cyclopropanation of alkenes with diiodomethane and diethylzinc can be carried out in hydrocarbon solvents and is particularly suitable with easily polymerizable olefins such as vinyl ethers . It has been reported that molecular oxygen remarkably promotes the reaction of diiodomethane with diethylzinc and substantially increases the yield of the adducts . A convenient modification which avoids the handling of pyrophoric diethylzinc has been reported . In reaction of olefins which are sensitive to the unavoidably produced zinc iodide (the Lewis acid), the addition of one equivalent of dimethoxyethane (DME) to the solvent has been recommended . Zinc iodide is then precipitated as the 1 1 DME complex as it is formed. Zinc salts, which often complicate the workup of the reaction mixture, can also be removed as precipitates by the addition of pyridine prior to the workup . ... 

Lewis Acid. In addition to the numerous examples of the type throughout this article, several specific examples of TMSCl as a Lewis acid are given here. Addition of diethylzinc to various imines is promoted by different Lewis acids, such as TMSCl, BF3 Et20, and ZnCh, to offer a variety of secondary... 

DFT has been used to probe the mechanism of allylation of ketones by allylboronates in the presence of diethylzinc. Results favour a double y-addition stepwise route, rather than concerted Lewis acid. Diethylzinc is found to be weakly catalytic, with the addition of ethanol substantially accelerating the reaction via Zn(OEt)2 catalysis, with Zn(OH)2 and ZnF2 efficiencies also being calculated. 

Nitroolefins are good Michael acceptors and their reactivity toward diethylzinc is different depending on the presence or absence of Lewis acids. Since the nitro group can be further transformed to a variety of useful containing functionalities,76,7641 the asymmetric 1,4-additions of nitroolefins may provide an easily accessible pathway to highly versatile optically active synthons. 

DOLate was formed which was tested in the Lewis acid catalyzed enantioselective addition of diethylzinc to benzaldehyde. The authors note that the catalyst can be recovered by simple filtration and was active for at least more than twenty runs (refer also to Section 4.2.3) [102]. 

Dialkylzincs are much less reactive than phenyl or alkynylzincs. In 2002, Kozlowski et al. developed a chiral salen-based catalyst 62 that can promote the diethylzinc addition to a-ketoesters in high yield, [Eq. (13.38)]. In their catalysis, titanium acts as a Lewis acid, and amine nitrogen acts as a Lewis base (63). The enantioselectivity was up to 78% ee ... 

Quite recently, there has been significant expansion and development in the alkylation of imines with organozinc reagents using chiral Lewis acid catalysts. In 2000, Tomioka and co-workers reported a copper(II)-chiral amidophos-phine 4-catalyzed asymmetric process for the addition of diethylzinc to N-sul-fonylimines (Scheme 2) [8]. Excellent enantioselectivities (90-94%) and yields (83-99%) were obtained in reactions of N-sulfonylimines derived from arylal-dehydes. 

Their 3,3 -substituents are utilized not only for their steric bulk, but also for the coordination to metals. Yamamoto and coworkers employed a boron complex of 3,3 -bis(2-hydroxyphenyl) BINOL in the asymmetric Diels-Alder reaction of cyclopentadiene and acrylaldehyde (equation 70) . The ligand possesses two additional hydroxy groups and forms a helical structure on coordination. The catalyst is considered to function as a chiral Brpnsted acid and a Lewis acid. The complex was also used in the Diels-Alder reactions and aldol reactions of imines. Although addition of diethylzinc to aldehydes gives low ee using BINOL itself or its 3,3 -diphenyl derivative, the selectivity can be increased when coordinating groups are introduced at the 3,3 -positions. Katsuki and... 

Triethylborane or diethylzinc has been proposed to have multiple roles in radical additions to C=N bonds, including both initiation and chain propagation, offering the potential for a radical chain process without tin hydride [6, 7]. Accordingly, we attempted triethylborane mediated tin free additions of various halides, simply deleting BujSnH from the conditions outlined in Table 2.3 and using InCb as the Lewis acid (Table 2.5). As in the case of tin mediated additions, the secondary iodides worked quite well in additions to the propionaldehyde hydrazone (entries 2 4). Although other primary radicals were ineffective, chloroiodomethane did lead to... 

SCHEME SJ. Addition of diethylzinc to hcnzaldc-hyde, catalyzed by chiral Lewis acids [7],... 

The enantioselective addition of functionalized dialkylzinc reagents to aldehydes is possible using a chiral Ugand and a Lewis-acid catalyst. The dialkylzinc reagents can be prepared by transmetallation (from the corresponding iodides or boranes) with diethylzinc. Selectivities are often best with aromatic aldehydes or... 

The addition of nucleophiles to carbonyl groups is a fundamental process in organic synthesis. The addition of diethylzinc to aldehydes occurs with high ee in the presence of a wide range of aminoalcohol ligands and also titanium-based Lewis acids. This methodology has recently been extended to the enantioselective addition of alkenyl, alkynyl and arylzincs and also to the more challenging addition to ketones. 

The catalytic enantioselective addition of diethylzinc to a-ketoesters was developed by Kozlowski [56]. Modular bifunctional Ti-salen [N,N -bis(salicylidene) ethylene-diamine] type catalysts that contain Lewis acid and Lewis base activating groups accelerate the carbonyl addition to a much greater extent than the competing carbonyl reduction. a-Hydroxyesters containing new quaternary stere-ogenic centers are obtained in high yields and moderate enantiomeric excesses (Scheme 14.10). 

Use as a Dual Catalyst in Addition of Alkynylzinc Species to Aldehydes. A dual Lewis acid/base system was reported involving NMI, Ti(OiPr)4, and (/f)-BINOL in the enantioselective alkynylation of aldehydes with diethylzinc at room temperature. Other methods to access these products have been hindered by the need for higher temperatures to generate the alkynylzinc species. A variety of propargy lie alcohols could be prepared from aromatic aldehydes in high yields and with excellent enantioselectivities (eq42). ... 

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