Chiral dimeric

Table 4 Examples of the crystallization of 5 OEt with or without seeding and their photoreaction into the chiral dimer."...

In 2005, Moyano et al. [60] reported a new type of chiral dimeric ferrocene palladacycle 43 that lacked the element of planar chirality and involved three... 

J. P. I. Hearn, R. V. Cobley, and B. J. Howard, High resolution spectroscopy of induced chiral dimers A study of the dimers of ethanol by Fourier transform microwave spectroscopy. J. Chem. Phys. 123, 134324 (2005). 

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

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-... 

The ethylzinc alcoholate (149) prepared from racemic a-terr-butyl-6-phenyl-2-pyridyl-methanol and Et2Zn exists in the solid state as a hetero chiral dimer (Figure 78). Also in this case the alcoholate oxygen atoms bridge between two zinc atoms, affording a central Zn—O—Zn—O plane. The pyridyl nitrogen atoms are coordinated in such a way to the zinc atoms [Zn—N 2.185(2) A] that they are in awfi-position with respect to this plane. 

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... 

Wynberg studied stereochemistry of the McMurry reductive dimerization of camphor in detail (64). In Scheme 37, A and B are homochiral dimerization products derived by the low-valence Ti-promoted reduction, while C and D are achiral heterochiral dimers. The reaction of racemic camphor prefers homochiral dimerization (total 64.9%) over the diastereomeric heterochiral coupling (total 35.1 %). Similarly, as illustrated in Scheme 38, oxidative dimerization of the chiral phenol A can afford the chiral dimers B and C (and the enantiomers) or the meso dimer D. In fact, a significant difference is seen in diastereoselectivity between the enaritiomerically pure and racemic phenol as starting materials. The enantiomerically pure S substrate produces (S,S)-B exclusively, while the dimerization of the racemic substrate is not stereoselective. In the latter case, some indirect enantiomer effect assists the production of C, which is absent in the former reaction. Thus, it appears that, even though the reagents and reaction conditions are identical, the chirality of the substrate profoundly affects the stability of the transition state. 

Having optimized the catalytic enantioselective phase-transfer alkylation system, the group explored the scope and limitations. A variety of electrophiles were reacted with the benzophenone imine glycine tert-butyl ester 1 catalyzed by 5 mol% of the selected chiral dimeric PTCs, benzene-linked-l,3-dimeric PTC 37, 2 -F-benzene-linked-1,3-dimeric PTC 41, and naphthalene-linked-2,7-dimeric PTC 39, at reaction temperatures of 0°C or — 20 °C (Scheme 4.8). 

Hasegawa et al. reported another example of a [2 + 2] asymmetric transformation in a chiral crystal. [16] Ethyl 4-[2-(pyridyl)ethenyl] cinnamate 15 crystallizes in a chiral space group Phhh and upon irradiation yields a chiral dimer 16 with 92-95% ee. 

A. King, B. Howard, A microwave study of the hetero-chiral dimer of butan-2-ol. Chem. Phys. Lett. 348, 343-349 (2001)... 

Fig. 19 Achiral calix[4]arenes with different substituents on the urea residues lead to diastereoisomeric chiral dimeric capsules...

Kimura, Shirai and coworkers used two chiral dimeric porphyrins 95 and 96 to investigate their self-assembling behavior [162,163]. While incorporation into fibers made of the alkylamide derivatives of (fl,fl)-DACH, 95 formed stable well-resolved fibrous assemblies as visualized by transmission electron microscopy, the fluorescence of which was not quenched by external electron acceptors [162]. However, the induced CD was not detected indicating an inability of 95 to form chirally orientated aggregates under the applied conditions. In contrast, 96 was able to produce optically active inter molecular self-assemblies with an enhanced chiroptical response through the //-oxo bridging in an alkali solution, while intramolecular //-oxo dimer formation was excluded on the basis of steric reasons [163]. 

In 1978, Lahav et al. succeeded in performing an absolute asymmetric synthesis of chiral dimers and oligomers through crystallization of an achiral monomer in a chiral crystal, followed by the four-center type photopolymerization in the crystalline state9. ... 

The effects of the role of matrix birefringence, photoselection, and rotational diffusion on measuring the circular dichroism of chiral excited triplets embedded in a rigid matrix have been examined. The techniques described in this paper are applied to chiral dimer-like molecules in the binaphthyl and spirobifluorene series. Natural and magnetic circular dichroism spectra of selenofenchone provide evidence for a singlet-triplet component of the n - IT transition. ... 

For the description of the CD spectra of molecules with two (or more) identical or at least similar chromophores in a chiral arrangement the method of coupled oscillators (exciton chirality method) has proven particularly successful. Such systems can occur either as chiral dimers, or they can be obtained by introducing suitable chromophores into a chiral molecule. The best-known example is given by the dibenzoates derived from 1,2-cyclo-hexanediol of the general formula 4 ... 

In agreement with this suggestion, irradiation of crystals of racemic R,S) (2) yielded chiral dimers with the same stereochemistry as from (+)-(S) (2), but as a mixture of all possible diastereoisomers represented in Scheme 4. 

On the basis of these data, an asymmetric synthesis of chiral dimers and oligomers with quantitative enantiomeric yield is expected in the entire range of solid solubility... 

Scheme 7 displays a possibility of the synthesis of chiral 2-arylpropionic acids via the oxidative tranformation of (7 )-3-aryl-l-butenes. The requisite chiral olefins may be obtained by transition metal-catalyzed asymmetric coupling between a benzylic Grignard reagent and vinyl bromide (93 % optical yield) [28] or, more attractively, asymmetric hydrovinylation of an aromatic olefin with ethylene. The asymmetric combination of styrene and ethylene, giving the adduct 25 in 95 % ee, has been performed on a 10-kg scale with a dinuclear Ni catalyst formed from ( -allyl)NiCl2 and a unique chiral dimeric aminophosphine obtainable from (/ )-myrtenal and (5)-l-phenylethylamine [7a],... 

Oguni has reported asymmetric amplification [12] ((-i-)-NLE) in an asymmetric carbonyl addition reaction of dialkylzinc reagents catalyzed by chiral ami-noalcohols such as l-piperidino-3,3-dimethyl-2-butanol (PDB) (Eq. (7.1)) [13]. Noyori et al. have reported a highly efficient aminoalcohol catalyst, 2S)-3-exo-(dimethylamino)isobomeol (DAIB) [14] and a beautiful investigation of asymmetric amplification in view of the stability and lower catalytic activity of the het-ero-chiral dimer of the zinc aminoalcohol catalyst than the homo-chiral dimer (Fig. 7-5). We have reported a positive non-linear effect in a carbonyl-ene reaction [15] with glyoxylate catalyzed by binaphthol (binol)-derived chiral titanium complex (Eq. (7.2)) [10]. Bolm has also reported (-i-)-NLE in the 1,4-addition reaction of dialkylzinc by the catalysis of nickel complex with pyridyl alcohols [16]. 

An optical induction of up to 24% enantiomeric excess for the formation of the chiral dimer vch from butadiene can be obtained using optically active 1,3,2-dioxaphospholanes L in the catalyst system [Ni(cod)2]/L. 

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