Nickel complexes stereoselectivity

A related stereoselective route to glutamic acid derivatives consists of the addition of the nickel complex 5 to various activated olefins, i.e., 2-propenonitrile, 2-propenal and a,/ -unsaturated esters. 

Nickel complexes formed in situ by the reaction of NiCl.S-COD) with the iini-dazolium salts IMesHCl or IPrHCl in the presence KO Bu catalyse the hydrosilylation of internal or terminal alkynes with EtjSiH. Interestingly, Ni tri-butylphosphine complexes are inactive in this hydrosilylation reaction. The monosilylated addition products were obtained with slow addition rates of the alkyne in the reaction mixture and were formed with variable degree of stereoselectivity, depending on the type of the alkyne, the silane and the ligand on Ni [50],... 

Most studies on nickel-catalyzed domino reactions have been performed by Ikeda and colleagues [287], who observed that alkenyl nickel species, obtained from alkynes 6/4-41 and a (jr-allyl) nickel complex, can react with organometallics as 6/4-42. If this reaction is carried out in the presence of enones 6/4-43 and TM SCI, then coupling products such as 6/4-44 are obtained. After hydrolysis, substituted ketones 6/4-45 are obtained (Scheme 6/4.12). With cyclic and (5-substituted enones the use of pyridine is essential. Usually, the regioselectivity and stereoselectivity of the reactions is very high. On occasion, alkenes can be used instead of alkynes, though this is rather restricted as only norbornene gave reasonable results [288]. 

The catalytic cyclo-oligomerization of 1,3-butadiene mediated by transition-metal complexes is one of the key reactions in homogeneous catalysis.1 Several transition metal complexes and Ziegler-Natta catalyst systems have been established that actively catalyze the stereoselective cyclooligomerization of 1,3-dienes.2 Nickel complexes, in particular, have been demonstrated to be the most versatile catalysts.3... 

The cross-coupling route to allylsilanes is effective with either aromatic or aliphatic a-silylated Grignard reagents16, and palladium catalysts are more reactive and stereoselective than the corresponding nickel complexes. Unsubstituted or i+monosubstituted alkenyl bromides work well but the Z-substituted bromides give lower yields and an inferior enantiomeric excess. The enantiomeric excess increases quite markedly with decreasing temperature, and optimum results are obtained at 0 C or below. 

Amino-acid Complexes. A small, but reproducible, stereoselective effect has been observed in both the free energy and enthalpy changes associated with formation of [Ni(L,L-methioninate)2]. The meso-complex [Ni(o-Met)(L-Met)] is more stable in AH than the optically active NiL2 by 1.0 (0.1) kJ mol" L The stereoselectivity is attributed to terdentate co-ordination and so supports weak co-ordination of the thioether group. Formation constants of nickel(ii) and copper(ii) with N -benzyl-L-histidine and N N -dibenzyl-L-histidine and of the ternary complexes with d- and L-histidine have been measured. Stabilization of ternary complexes is small but significant stereoselectivity is found with ternary nickel complexes, when the meso configuration is preferred in each case with copper, stereoselectivity is small or absent. The i.r. spectra of tra s-[Ni(Gly)2(H20)2] and its 0-, N-, 1- C-, and 2- C-labelled... 

Transition-metal-catalyzed stereoselective reductions of vinylic sulfones with Grignard reagents are achieved with excess n-BuMgCl in tetrahydrofuran at room temperature.196,48 Better yields and selectivities are obtained with palladium catalysts [Pd(acac)2] than with nickel complexes such as [Ni(acac)2], especially if external ligands such as DABCO, triethylamine (TEA), or (n-Bu)2P are used.196,48 Nickel catalysts are used more often for reducing sulfonyl-1,3-dienes than palladium catalysts. When using this method, it is very important to remove the catalyst before isolation of the products in order to avoid isomerization of the... 

Nickel complexes also exhibit high catalytic activity toward the hydrosiiyiation of conjugated dienes under mild conditions. The reaction usually occurs by 1,4-addition, but the regio- and stereoselectivity is low compared with that achieved by palladium catalysts. 

Similar stereoselectivities arc observed with nonconjugaled dienes which give bicyclic cyclobu-tanes with trans fusion of the annulated ring. Thus, 1,7-octadiene, treated with the same photochemically activated nickel complexes, gives trans-bicyclo[4.2.0]octane (6) in 21 % yield, after the resulting metallacycle is exposed to air24. 

Platinum complexes show none of the catalytic activity found in palladium and nickel complexes. This chemical inertness makes platinum a useful model for the more active catalysts. There has been a suggestion that platinum complexes of chiral a-diimines might lead to stereoselective olefin polymerization. Chiral camphor-based ligands have been employed in palladium complexes for ethylene polymerization, but there was no mention of stereoselectivity in hexene polymerizations. 

Asymmetric carbon-carbon bond formation reaction under solvent-free conditions was carried out by Bolm et al. in ball mill [58]. Here, nickel(II) complex 206 was used as a chiral auxiliary in alkylation with various bromides (Schane 2.67). Optimized reaction conditions were set to increase the stereoselectivity however, the desired monoalkylated product 207 was often accompanied by small amount of doubly alkylated side product 208. Two different bases were used (NaOMe/MgS04 or CS2CO3) and grinding of nickel complex 206 with bromides for 30-75 min afforded alkylation products in moderate to high yields and with complete stereoselectivity (selected examples. Table 2.54). 

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