Chiral Ni complex

This Mulheim chemistry has been highlighted by the discovery of the highly enantioselective hydrovinylation of styrene to produce chiral 2-phenyl-1-butene in 95.2% ee for a 10 kg-scale reaction (Scheme 60) (132). The Ni catalyst is very reactive and contains the unique chiral dimeric aminophosphine ligand derived from (R)-myrtenal and (S)-1-phenylethylamine. Computer simulations suggest that in this chiral Ni complex, the phenyl substituent of the chiral phenylethyl group acts as a windshield wiper across the catalytically active metal center. This... 

Interestingly, the specific environment of the ionic solvent system appears to activate the chiral Ni-catalyst beyond a simple anion-exchange reaction. This becomes obvious from the fact that even the addition of a 100-fold excess of Fi[(CF3S02)2N] or Na[BF4] in pure, compressed CO2 produced an at best moderate activation of Wilke s complex in comparison to the reaction in ionic liquids with the corresponding counter-ion (e.g., 24.4 % styrene conversion with 100-fold excess of Fi[(CF3S02)2N], in comparison to 69.9 % conversion in [EMIM][(CF3S02)2N] under otherwise identical conditions). 

Kumada and his co-workers (33) later showed that a chiral Ni(II) complex induced asymmetric hydrosilation of a-methylstyrene by meth-yldichlorosilane at 90°C for 60 hours. By use of the trans-(R) isomer of (PhCH2—PhMeP )2NiCl2, they isolated 8% PhMeC HCH2SiMeClH with [a]D + 6.43°and 31% PhMeC HCH2SiMeCl2 with [a] > + 6.50°. The latter compound was treated with methyllithium to prepare PhMeC HCH2SiMe3, [a], + 10°, which they estimated as 17.6% optically pure. 

Bis(oxazoline)-type complexes, which have been found useful for asymmetric aldol reactions, Diels-Alder, and hetero Diels-Alder reactions can also be used for inducing 1,3-dipolar reactions. Chiral nickel complex 180, which can be prepared by reacting equimolar amounts of Ni(C10)4 6H20 and the corresponding (J ,J )-4,6-dibenzofurandiyl-2,2 -bis(4-phenyloxazoline) (DBFOX/Ph) in dichloromethane, can be used for highly endo-selective and enantioselective asymmetric nitrone cycloaddition. The presence of 4 A molecular sieves is essential to attain high selectivities.88 In the absence of molecular sieves, both the diastereoselectivity and enantioselectivity will be lower. Representative results are shown in Scheme 5-55. 

The iodohydroxylation of 1,2-allenyl sulfoxide 171 with I2 in the presence of H20 exhibits E-selectivity leading to (E)-2-iodo-3-hydroxy-l-alkenyl sulfoxide [88]. By using Br2, CuBr2 or NBS, (E)-2-bromo-3-hydroxy-l-alkenyl sulfoxide is produced. For the chlorohydroxylation of a sulfoxide, CuCl2 and silica gel were used to mix with the sulfoxide to deliver the (E)-chlorohydroxylation products highly stereoselec-tively [88]. The chirality in the allene moiety can be efficiently transferred to the final allylic alcohols. Under the catalysis of a Pd or Ni complex, the C-X and C-S bonds can be coupled to introduce different substituent(s) into the different locations of the C=C bond. 

Beside [2+2+2] cycloaddition, [4+2] and [5+1] cycloadditions represent other approaches for the construction of six-membered ring systems. In parhcular, the intermolecular and intramolecular [4+2] cycloadditions of diene and alkyne have been extensively studied, and a variety of transition-metal complexes-including those of Fe, Ni and Rh-have been reported as efficient catalysts. The first enanh-oselective reaction was achieved with a chiral Rh complex, although the substrates were limited to dienynes with a substituent on the diene terminus [36]. Later, Shibata and coworkers developed an intramolecular and enanhoselective [4+2] cycloaddition using an Ir-BDPP (l,3-bis(diphenylphosphino)pentane) complex (Scheme 11.24) [37], where dienynes with an unsubstituted diene terminus were transformed into bicyclic cyclohexa-1,4-diene with up to 98% ee. 

Unlike CO, it is possible to polymerize isocyanides (R—N=C), isoelectronic analogs to CO. When R is a bulky group, such as tert-Bu, the polymer forms a stable helical structure. Asymmetric catalytic polymerization has been reported for t-Bu-NC using [Ni(T 3-allyl)(iV-trifluoroacetyl-proline)]2 providing (M)-helical polymer with 69% ee. The more stable helical polymer was prepared from 1,2-diisocya-nobenzene derivative initiated by a chiral Pd complex. (See Scheme 4.19.)... 

The asymmetric catalytic Pauson-Khand reaction met success in the late 1990s. Not only the conventional Co catalyst but also other metal complexes, such as Ti, Rh, and Ir, are applicable to the reaction. Asymmetric hydrocyanation of vinylar-enes is accomplished using Ni complex of chiral diphosphite. Further studies on the scope and limitation are expected. 

Bromostyrenes 162 react with 1-ethylphenylmagnesium chloride under Ni catalysis providing 1,3-diaryl-l-butenes 163. When chiral nickel complexes are used, the products... 

Attempts to aziridinate alkenes with iron catalysts in an asymmetric manner have met with only limited success to date [101], In an early report on the use of various chiral metal salen complexes, it was found that only the Mn complex catalyzed the reaction whereas all other metals investigated (Cr, Fe, Co, Ni etc.) gave only unwanted hydrolysis of the iminoiodinane to the corresponding sulfonamide and iodoben-zene [102], Later, Jacobsen and coworkers and Evans et al. achieved good results with chiral copper complexes [103]. 

Chiral BINOL-indium(ni) complexes have been employed in several enantioselective allylations (i) in the ionic liquid, hexylmethylimidazolium-PF6, for aldehydes,190 (ii) a moisture tolerant version, for a wide variety of aldehyde types,191 and (iii) a recyclable example, useful for aromatic, aliphatic, and a,/S-unsaturated ketones.192... 

Asymmetric Aziridination of Olefins with Chiral Ni domanganese Complexes... 

Addition of chiral LSR such as (+)tris[3-heptafluoropropyl hydroxymethylene]-D-[cam-phorato]Eu(III), Eu[(+)fpc]3 to a racemic mixture of diamagnetic Ni complex with propylenediamine backbone results in a composite of two overlapping spectra [92], The changes in the methyl doublet are shown in Fig. 10.27. Combined with spectral integration, this technique may be used for the analysis of mixtures of optically active complexes. 

Asymmetric Baeyer-Villiger oxidation reaction In 1994, Bolm et al used chiral Cu and Ni complexes (7.23) in catalytic amount with different oxidation systems. 

The chiral Ni(II) complex shown in Sch. 58 catalyzes the enantioselective hetero-Diels-Alder reactions of thiabutadienes with 3-(2-propenoyl)-2-oxazolidinone to afford optically active dihydrothiopyrans [204]. Similar results are obtained with analogous Cu(II) complexes where catalyst loading can be reduced when molecular sieves are added. 

Of the various possible asymmetric cross-coupling reactions, (1) asymmetric alkylation with secondary alkylmetals, (2) asymmetric biaryl synthesis, and (3) asymmetric allylation with allylic electrophiles have been most extensively studied with chiral Ni and Pd complexes [166]. The initial study in this area was reported as early as 1974 by Kumada and his co-workers, but only a meager range of 8-15% ee was reported [167]. By the end of the 1970s, however, the cross-coupling reaction had been sufficiently developed so that its application to the asymmetric synthesis was already practically attractive, as indicated by an asymmetric total sythesis of (R)-(—)-a-curcumene in five steps in 66% ee and 34% overall yield shown in Scheme 1-47 [168]. 

DFT calculations of polar charge distributions of the bitartrate-Ni complex [24] show the chiral transmission from the adsorbate to the surface is not restricted to a geometrical reorganization. In fact, the delocalised nature of the molecular... 

Asymmetric transfer hydrogenation with a chiral ruthenium complex is an alternative option for preparation of substituted phenethyl alcohols, which are important building blocks for the agricultural fungicide, (S)-MA20565 [47]. In the enantioselective synthesis of antidepressant sertraline (50), different chiral secondary alcohols have been proposed as pivotal intermediates (Scheme 14). Reduction of the keto ester 46 catdyzed by oxazaborolidine 45 provides chiral intermediate 47 in 90% ee [48]. Alternatively, reductive fragmentation of C2-symmetric oxa-tricyclic alkene 48 with DIBAL catalyzed by a BINAP-Ni complex generates a novel intermediate 49 in 88 % yield with 91% ee [49]. 

Diisocyanobenzene derivatives yield helical polymers via a cyclopolymerization mechanism by the polymerization with Pd and Ni complexes. Optically active polymers were initially obtained by the method illustrated in Figure 8.139 143 Monomer 66 was reacted with an optically active Pd complex to form diastereomeric pentamers 67, which were separated into (+)- and (—)-forms by HPLC. The polymerization of 68 using the separated 69 led to a one-handed helical polymer.139 The polymerization of 68 using the initiators having chiral binaphthyl groups, 69—71, also produced optically active polymers.142 The helix-sense selectivity in the polymerization using 69... 

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