Nickel complexes racemization

Nickel, tris(l, 10-phenanthroline) racemization, 1,24. 466 solid state, 1, 467 structure, 1,64 Nickel complexes, 5,1-300 acetylacetone alcoholysis, 2, 380 pyridine complexes, 2, 386 solvolysis, 2,379 structure, 2,388 amidines... 

Taylor Busch, 1967). In solution, these isomers are quite stable towards racemization - a property not normally characteristic of high-spin nickel complexes containing open-chain ligands. 

Early attempts at an asymmetric hydroalumination utilized a chiral -butylsalicylidenime complexed to a nickel(n) complex 117.128 When racemic 3,7-dimethyl-1-octene 116 was treated with 0.2mol% of the nickel complex 117 and 0.3 equiv. of TIBA at 0°C, followed by hydrolysis, the alkene 118 with 1.2% ee was obtained. The unreacted olefin 119 was recovered and found to have an ee of 1.8% (Scheme 14). 

Caeiro and colleagues coupled racemic benzylic bromides 1 with alkynylindium compounds 21 (M=In) catalyzed by a nickel complex generated from 10 mol% NiBivdiglyme and 13 mol% of (S, 5)-5b (entry 30) [74]. Reasonable to good ee values of 77-87% were observed for the benzyl alkyne products. The isolated yields were, however, a rather moderate 35-70%. 

The following subsections describe the asymmetric polymerization of isocyanides using four classifications based upon the mechanism of the asymmetric induction. The first two subsections deal with homo- and copolymerization of chiral, non-racemic isocyanides, and asymmetric polymerization of achiral isocyanides by chiral enantiopure nickel complexes are described in the final two subsections. 

We applied a similar approach under kinetic resolution conditions (Figure 5) (19). First a mixture of 2 and racemic 4-methylcaprolactone (4-MeCL) was reacted until all (S)-4-MeCL had been converted, i.e., to about 50 % total monomer conversion. Subsequently, oxygen was removed from the reaction medium by several consecutive freeze-pump-thaw cycles. The ATRP was initiated by adding MMA and Ni(PPh3)2Br2 and raising the reaction temperature to 80 C. In this case the nickel complex acts as both ATRP initiator and enzyme inhibitor thereby preventing any side reactions caused by the enzyme. Chiral block copolymers were obtained by this approach as evident from SEC analysis. 

Introduction of branched terminal chains, providing that they are long enough, resulted in the reduction, as expected, of the temperature of the transition crystal-to-mesophase, as well as the thermal stability. " The nickel complex with the racemic 2-methyloctyl chains showed two mesophases, Cr 98 SmC 103 N 118 I, whereas that with the chiral 5-5-methylheptyl chains presented an SmC between 119°C and 159°C. 

Fu et al. showed that a combination of NiCla and bis(oxazoline) 11 or 17 catalyses asymmetric Kumada coupling of racemic a-bromoketones with aryl Grignard reagents, giving a-arylketones in good yields and enantiomeric excess (Scheme 14.8). The reactions proceeded at low temperature, which enabled the asymmetric synthesis of racemisation-prone a-arylketones. The Pincer-nickel complex [(N2N)Ni-Cl] (18) catalyses Kumada reaction of primary allqrl bromides/iodides or secondary all l iodides with alkyl, (hetero)aryl or all nyl Grignard reagents (Scheme 14.9). The reactions... 

Intramolecular Rearrangement of Tris-chelate Complexes.— Rate data for the dissociation and racemization of [M(phen)3] + ions (M = Cr, Fe, Co, or Ni) are collected in Table 39. Only the nickel(ii) complex racemizes at a rate comparable to the ligand... 

Scheme 5.49 Palladium- and nickel-catalyzed enantioselective reaction of aryl triflates with enolates derived from racemic ketones 149, catalyzed by palladium or nickel complexes with ligand 150.

Among the J ,J -DBFOX/Ph-transition(II) metal complex catalysts examined in nitrone cydoadditions, the anhydrous J ,J -DBFOX/Ph complex catalyst prepared from Ni(C104)2 or Fe(C104)2 provided equally excellent results. For example, in the presence of 10 mol% of the anhydrous nickel(II) complex catalyst R,R-DBFOX/Ph-Ni(C104)2, which was prepared in-situ from J ,J -DBFOX/Ph ligand, NiBr2, and 2 equimolar amounts of AgC104 in dichloromethane, the reaction of 3-crotonoyl-2-oxazolidinone with N-benzylidenemethylamine N-oxide at room temperature produced the 3,4-trans-isoxazolidine (63% yield) in near perfect endo selectivity (endo/exo=99 l) and enantioselectivity in favor for the 3S,4J ,5S enantiomer (>99% ee for the endo isomer. Scheme 7.21). The copper(II) perchlorate complex showed no catalytic activity, however, whereas the ytterbium(III) triflate complex led to the formation of racemic cycloadducts. 

Enantioselectivities were found to change sharply depending upon the reaction conditions including catalyst structure, reaction temperature, solvent, and additives. Some representative examples of such selectivity dependence are listed in Scheme 7.42. The thiol adduct was formed with 79% ee (81% yield) when the reaction was catalyzed by the J ,J -DBFOX/Ph aqua nickel(II) complex at room temperature in dichloromethane. Reactions using either the anhydrous complex or the aqua complex with MS 4 A gave a racemic adduct, however, indicating that the aqua complex should be more favored than the anhydrous complex in thiol conjugate additions. Slow addition of thiophenol to the dichloromethane solution of 3-crotonoyl-2-oxazolidinone was ineffective for enantioselectivity. Enantioselectivity was dramatically lowered and reversed to -17% ee in the reaction at -78 °C. A similar tendency was observed in the reactions in diethyl ether and THF. For example, a satisfactory enantioselectivity (80% ee) was observed in the reaction in THF at room temperature, while the selectivity almost disappeared (7% ee) at 0°C. 

Figure 21. A1) True racemic composition for enantiomer separation196 of frons-2,3-dimethyloxirane by complexation gas chromatography on nickel(II) bis[3-heptafluorobutanoyl-(D )-camphorale] at 80CC. Integration with a Spectra-Physics SP4100 instrument (peak areas are equal).

C) Deviation from the expected 1 1 ratio (first peak diminished) upon enantiomer separation133 of racemic 2-melhyl-3-phenyloxiranc on nickel(II) bis[3-(heptafluorobutanovl)-(l fi)-catnphoralc] by complexation gas chromatography at 80 °C. 

Ni(04C2)(tet-a)-3H20 (tet = 5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradecane a = meso isomer b = racemic isomer) were prepared by the reaction of a concentrated solution of sodium oxalate with an aqueous solution of the appropriate nickel(II) amine complex.1780,1781 In the dinuclear complex [Ni2(04C2)(en)4](N03)2 (234)1740-1741 the bridging oxalato group is planar and symmetrically bonded to the two nickel atoms. The same structure occurs in the complex Ni2(04C2)(0N0)2(py)6 which was obtained as a by-product in a very low yield when a pyridine solution of methanenitrosolic acid and nickel(II) were allowed to stand for several months.1741... 

The coordination chemistry of macrocyclic ligands has been extensively studied and aspects of isomerism have been considered in numerous systems.241 Methods whereby two diastereomers of complexes of tetra- N-methylcyclam may be isolated have been discussed previously.184 This, however, is a relatively simple system and it is usually necessary to consider isomerism due to the presence of asymmetric atoms in the chelate arms, as well as that due to asymmetric donor atoms that may be rendered stable to inversion by coordination. An example of a system exhibiting this level of complexity is afforded by the nickel(II) complexes of the macrocyclic ligands generated by reduction of the readily prepared macrocycle (46). These ligands contain two asymmetric carbon atoms and four asymmetric nitrogen atoms but, because AT-inversion is rapid, it is conventional to consider that only three separable stereoisomers exist. There is an enantiomeric pair, (47a) and (47b), which constitutes the racemic isomer (R, R ), and an achiral (R, S ) diastereomer (47c), the meso isomer. 

The examples shown in Table 7.2 do not show a particularly high enantioselectiv-ity. However, with an enantiomeric excess of 50% (75/25), approximately optically pure material may be obtained in five cycles, and for chromatographic resolutions separation factors a > 1 are sufficient for efficient resolution processes132,331. Nickel(II) complexes of the type shown in Table 7.2 have been used to modify ion-exchange resins which were used for racemate separations1321, and derivatives of (S),(S)-ppm with functional groups that may be fixed to supports are readily available. ... 

If one enantiomer obtained by resolution is the desired one, the other recovered from the mother liquor of the resolution should be racemized to use for the next resolution. In general, whether the racemization method for unwanted enantiomer is known or not will be key to producing optically active compounds on an industrial scale. In most cases, optical resolution and racemization are non-divisable for the realization of a practical and economical process. For instance, nickel chloride complex of DL-a-amino-e-caprolactam has been successfully resolved. The complex, dissolved in ethanol, was heated and inoculated with enantiopure complex, then racemization and crystallization of the complex occurred at the same time. As a result,... 

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