Dialkyl zinc

Organometallics, such as dialkyl zinc or cadmium-alkyl or aryl lithium Alkali acetylides Diels-Alder Arndt-Eistert... 

Violent reactions have occurred between ozone and many chemicals, a small selection being acetylene, alkenes, dialkyl zincs, benzene/rubber solution, bromine, carbon monoxide and ethylene, diethyl ether, hydrogen bromide, and nitrogen oxide. 

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

Among recently described new Pd-catalysed enantioselective reactions, the ring opening of meso oxabicyclic alkenes with dialkyl zinc reagents in the presence of chiral P/P and P/N ligands reported by Tautens el al. constitutes a synthetically outstanding C-C bond-forming desymmetrization reaction. 

Scheme 1.22 Kitamura and Noyori s mechanism of the asymmetric addition of dialkyl zinc to aryl aldehydes.

Dialkyl zinc compounds form phosphine complexes of formula RZnP(SiMe3)2 on addition of one equivalent of bis(trimethylsilyl)phosphine. Solution and solid-state studies showed that the complexes are dimeric or trimeric in solution or the solid state. Bis(trimethylsilyl)phosphido-methylzinc crystallizes as a trimeric molecule with a Zn3P3 core in the twist-boat conformation. Bis(trimethylsilyl)phosphido- -butylzinc, shows a similar central Zn3P3 fragment. The sterically more demanding trimethylsilylmethyl substituent forms a dimeric species of bis(trimethylsilyl) phosphido-trimethylsilylmethylzinc. Solution studies of bis(trimethylsilyl)phosphido- .vo-propyl-zinc demonstrate a temperature-dependent equilibrium of the dimeric and trimeric species and the crystalline state contains a 1 1 mixture of these two oligomers. A monomeric bis(trimethyl-silyl)phosphido-tris(trimethylsilyl)methylzinc has also been synthesized.313... 

Feringa s group has demonstrated that cyclopentene-3,5-dione monoacetals as 2-47 can also be successfully applied as substrates in an asymmetric three-component domino Michael/aldol reaction with dialkyl zinc reagents 2-48 and aromatic aldehydes 2-49 [17]. In the presence of 2 mol% of the in-sitw-generated enantiomeri-cally pure catalyst Cu(OTf)2/phosphoramidite 2-54, the cyclopentanone derivatives 2-51 were formed nearly exclusively in good yields and with high ee-values (Scheme 2.11). 

The selectivity of the aldol addition can be rationalized in terms of a Zimmer -man-Traxler transition-state model with TS-2-50 having the lowest energy and leading to dr-values of >95 5 for 2-51 and 2-52 [18]. The chiral copper complex, responsible for the enantioselective 1,4-addition of the dialkyl zinc derivative in the first anionic transformation, seems to have no influence on the aldol addition. To facilitate the ee-determination of the domino Michael/aldol products and to show that 2-51 and 2-52 are l -epimers, the mixture of the two compounds was oxidized to the corresponding diketones 2-53. 

Addition of insoluble NaOMe and KOMe to benzene solutions of ZnR2, Scheme 57, gave soluble 1 2 zincates of the composition M[(R2Zn)2OMe] 73 (M = Na, K R = Et, Bu11, Bus, Me3SiCH2).128 By contrast, combining dialkyl-zincs with benzene-soluble KOBuc produces the dianionic zincate 74. 

Frankland s method orgchem Reaction of dialkyl zinc compounds with alkyl halides to form hydrocarbons may be used to form paraffins containing a quaternary carbon atom. frarik-lonz, meth-3d ... 

The Cu(I)-catalysed asymmetric conjugate addition of dialkyl zinc reagents to 3-nitrocoumarins 45 gives high yields of 3,4-dihydrocoumarins in a pH-dependent diastereoisomeric ratio. Subsequent decarboxylation gives optically active p-aryl nitroalkanes... 

Where possible, it may be most economical to effect a chiral transformation on a pre-formed, pro-chiral ring. Ben Feringa of the University of Groningen prepared (Chem Commun. 2005, 1711) the enone 2 from 4-methoxypyridine 1. Cu -catalyzed conjugate addition of dialkyl zincs to 2 proceeded in up 96% . Pd-mediated allylation of the intermediate zinc enolate led to 3, with the two alkyl subsituents exclusively trans to each other. 

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. 

In fact it can be assumed that, in the catalytic system TiCl4-bis[(S)-2-methyl-butyl]-zinc, dialkyl zinc alkylates the titanium atom (19) and that the titanium alkyl thus formed gives more stable complexes with the (S) olefin than with the (R) olefin, thus favouring the adsorption and polymerization of the (S) antipode (104). The influence exerted by the asymmetric groups bound to transition metals on the type of complexes formed by olefins with the same metal atom, has been recently investigated by Pajaro, Corradini, Palumbo and Panunzi (90). 

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). 

Cool to rt and dissolve in THF (45 mL). Confirm the formation of the dialkyl-zinc by performing an idolysis and a hydrolysis as described in Protocol 1. (This THF solution can be stored for several months at 0°C.)... 

As discussed in Sects. 2.1 and 2.2.8 control of molar mass is an important aspect in the large-scale polymerization of dienes. In Nd-catalyzed polymerizations the control of molar mass is unique amongst Ziegler/Natta catalyst systems as standard molar mass control agents such as hydrogen, 1,2-butadiene and cyclooctadiene which are well established for Ni- and Co-systems do not work with Nd catalysts [82,206,207]. The only known additives which allow for the regulation of molar mass without catalyst deactivation are aluminum alkyls, magnesium alkyls, and dialkyl zinc. 

However, an encouraging result was obtained very recently for the 1,4-conjugate addition of dialkyl zinc to a variety of Michael acceptors catalyzed by copper. Alexakis, Roland and coworkers have investigated the addition of diethyl zinc to cycloheptenone and observed an enantiomeric excess of 93% (95% yield) in the presence of Cu(OAc)2 and the silver carbene derivative of imidazolium 1 (Scheme 3) [10]. Silver carbene complexes are efficient transfer agents to copper(II) and therefore the potentially harmful use of a base to generate the catalytic species is avoided. 

Building on the success of Woodward s use of an achiral NHC to catalyze the conjugate addition of dialkyl zincs, Alexakis and Roland simultaneously reported the use of chiral NHCs to achieve the asymmetric addition into enones. In Alexakis s system, the catalyst was generated in situ by addition of BuLi to a suspension of imidazolium salt 19, Cu(OTf)2, and enone in toluene followed by addition of Et2Zn (Eq. 28) [63]. While conversions and yields were found to be nearly quantitative, ee values were moderate with 51% being the highest reported. 

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