Chiral dimeric complexes

One of the most famous chiral titanium complexes is the Sharpless catalyst (16), based on a diisopropyl tartarate complex. Nmr studies suggest that the complex is dimeric ia nature (146). An excellent summary of chiral titanium complexes is available (147). 

The catalytic asymmetric cyclopropanation of an alkene, a reaction which was studied as early as 1966 by Nozaki and Noyori,63 is used in a commercial synthesis of ethyl (+)-(lS)-2,2-dimethylcyclo-propanecarboxylate (18) by the Sumitomo Chemical Company (see Scheme 5).64 In Aratani s Sumitomo Process, ethyl diazoacetate is decomposed in the presence of isobutene (16) and a catalytic amount of the dimeric chiral copper complex 17. Compound 18, produced in 92 % ee, is a key intermediate in Merck s commercial synthesis of cilastatin (19). The latter compound is a reversible... 

Fig. 1. UV-Vis absorption spectra of the precatalyst Scheme 2. Possible working model for the HKR chiral Co(salen) and monomer and dimer complex. of terminal epoxides catalyzed by C0-AIX3...

It would be ideal if the asymmetric addition could be done without a protecting group for ketone 36 and if the required amount of acetylene 37 would be closer to 1 equiv. Uthium acetylide is too basic for using the non-protected ketone 36, we need to reduce the nucleophile s basicity to accommodate the acidity of aniline protons in 36. At the same time, we started to understand the mechanism of lithium acetylide addition. As we will discuss in detail later, formation of the cubic dimer of the 1 1 complex of lithium cyclopropylacetylide and lithium alkoxide of the chiral modifier3 was the reason for the high enantiomeric excess. However, due to the nature of the stable and rigid dimeric complex, 2 equiv of lithium acetylide and 2 equiv of the lithium salt of chiral modifier were required for the high enantiomeric excess. Therefore, our requirements for a suitable metal were to provide (i) suitable nucleophilicity (ii) weaker basicity, which would be... 

Kitamura and Noyori have reported mechanistic studies on the highly diastere-omeric dialkylzinc addition to aryl aldehydes in the presence of (-)-i-exo-(dimethylamino)isoborneol (DAIB) [33]. They stated that DAIB (a chiral (i-amino alcohol) formed a dimeric complex 57 with dialkylzinc. The dimeric complex is not reactive toward aldehydes but a monomeric complex 58, which exists through equilibrium with the dimer 57, reacts with aldehydes via bimetallic complex 59. The initially formed adduct 60 is transformed into tetramer 61 by reaction with either dialkylzinc or aldehydes and regenerates active intermediates. The high enantiomeric excess is attributed to the facial selectivity achieved by clear steric differentiation of complex 59, as shown in Scheme 1.22. 

Che has reported that both achiral and chiral rhodium catalysts function competently for intramolecular aziridination reactions of alkyl- and arylsulfonamides (Scheme 17.29) [59, 97]. Cyclized products 87 are isolated in 90% yield using 2 mol% catalyst, PhI(OAc)2, and AI2O3. Notably, reactions of this type can be performed with catalyst loadings as low as 0.02 mol% and display turnover numbers in excess of 1300. In addition, a number of chiral dimeric rhodium systems have been examined for this process, with some encouraging results. To date, the best data are obtained using Doyle s Rh2(MEOX)4 complex. At 10 mol% catalyst and with a slight excess of Phl=0, the iso-... 

Asymmetric C-H insertion using chiral rhodium catalysts has proven rather elusive (Scheme 17.30). Dimeric complexes derived from functionalized amino acids 90 and 91 efficiently promote oxidative cychzation of suifamate 88, but the resulting asymmetric induction is modest at best ( 50% ee with 90). Reactions conducted using Doyle s asymmetric carboxamide systems 92 and 93 give disappointing product yields ( 5-10%) and negligible enantiomeric excesses. In general, the electron-rich carboxamide rhodium dimers are poor catalysts for C-H amination. Low turnover numbers with these systems are ascribed to catalyst oxidation under the reaction conditions. 

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... 

The initial study in this area employed the meso-1,3-dimethylallyl ligand.432 The chloride dimer was generated from 2-pentene, and a variety of optically active phosphine ligands were added to form the chiral bisphosphine complexes. Reaction of these allyl complexes with sodiodiethyl malonate resulted in optical yields in the range of 2-29% (equation 351). 

The activation of a racemic catalyst by a chiral additive was achieved by Mikami in a chiral titanium complex-catalyzed asymmetric carbonyl-ene reaction (Scheme 9.21) [39], The racemic catalyst ( )-BINOL-Ti-(0-i-Pr)2 37 (10 mol %) is activated by adding (R)-BINOL (5 mol %), and the ene product 38 with 90% ee is obtained. (R)-BINOL is selectively associated with (/f)-BIN0L-Ti-(0-i-Pr)2 to give a dimeric catalyst whose activity is kinetically calculated to be 25.6 times greater than that of the remaining (S)-BIN0L-Ti-(0-i-Pr)2. 

Asymmetric amplification, i.e. ee of the product is higher than that of the chiral catalyst, is observed with some chiral (3-aminoalcohols 73g and 3.3° Formation of the less reactive dimeric complex from (+) and (-) catalysts increases the ee of the higher reactive monomeric catalyst. The remaining monomeric chiral catalyst with higher ee than that of the total chiral catalyst affords sec-alcohols with higher ee.3°... 

Scheme 1 a The [2 + 2] cycloaddition product of prochiral trans 2-butene with Si dimers of the Si(100) surface leads to chiral adsorbate complexes, b Hydrogenation of prochiral a-keto esters over platinum is a heterogeneously catalyzed reaction leading to chiral alcohols. Using cinchonidin as chiral modifier makes this surface reaction enantioselective. In a similar fashion, TA-modified nickel is a highly enantioselective catalyst for /3-keto ester hydrogenation... 

Further results on asymmetric hydrogenations of activated carbonyl compounds catalyzed by bis(dimethylglyoximato) cobalt (Il)-chiral amine complexes have been reported (55,56). Some chiral reductive dimerizations were observed (55). 

When a chiral scandium complex of 2,6-bis-(oxazolinyl)pyridine, [Sc(/f)-py-box](OTf)3, 0Tf=0S02CE3, is employed instead of Sc(OTf)3, a 2 2 chiral Jt-dimer complex of 1,4-napthosemquinone radical anion (NQ ) with Sc (R)-... 

NQ )2—(Sc " (/ )-pybox)2] by 1 equiv ofDDQ results in reproducing NQ and Sc (7 )-pybox (110). Such formation and dispersion cycles of the chiral 7t-dimer complex in response to ET reduction and oxidation are highly reversible and can be repeated many times as shown in Fig. 40 (110). Thus, the effective redox control on building affinity of the chiral supramolecules affords reversible formation and dispersion of chiral assemblies in response to a simple external signal, for example, an electron, giving achiral-chiral switchability (111, 112). 

The hfac ligand forms very stable and volatile complexes, therefore there is an opportunity for their use in separation and metal vapor deposition reactions. An analogous reaction with PbO will lead to the formation of dimer complexes, which exist in the solid state as a chiral enantiomer connected with the third diglyme oxygen of one unit oriented toward the Pb center of the other unit. Despite the dimer strucmre, this complex is... 

In 1999 Trost and Schroder reported on the first asymmetric allylic alkylation of nonstabilized ketone enolates of 2-substituted cyclohexanone derivatives, e.g. 2-methyl-1-tetralone (45), by using a catalytic amount of a chiral palladium complex formed from TT-allylpaUadium chloride dimer and the chiral cyclohexyldiamine derivative 47 (equation 14). The addition of tin chloride helped to soften the lithium enolate by transmetala-tion and a slight increase in enantioselectivity and yield for the alkylated product 46 was observed. Besides allyl acetate also linearly substituted or 1,3-dialkyl substituted allylic carbonates functioned well as electrophiles. A variety of cyclohexanones or cyclopen-tanones could be employed as nucleophiles with comparable results . Hon, Dai and coworkers reported comparable results for 45, using ferrocene-modified chiral ligands similar to 47. Their results were comparable to those obtained by Trost. 

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