Diastereoisomeric complexes

In a diastereoisomeric complex the two types of chirality inducers might have opposite rather than additional effects, a feature so far impossible to predict... 

Miscellaneous Reactions of Phosphines.- The role of chiral phosphines as ligands in the catalysis of reactions leading to the formation of chiral products has been reviewed.1111 A procedure for the determination of the enantiomeric excess in chiral phosphines has been developed, based on 13C n.m.r. studies of the diastereoisomeric complexes formed by phosphines with the chiral pinenyl nickel bromide complex. 111 Studies of the sulphonation of triphenylphosphine and of chiral arylphosphines have been reported in attempts to prepare water soluble ligands which aid... 

M. Mons, E Piuzzi, I. Dimicoli, A. Zehnacker, and F. Lahmani, Binding energy of hydrogen bonded complexes of the chiral molecule 1 phenylethanol, as studied by 2C R2PI Comparison between diastereoisomeric complexes with butan 2 ol and the singly hydrated complex. Phys. Chem. Chem. Phys. 2, 5065 5070 (2000). 

Ha is the more and HB is the less complexed host enantiomer in the aqueous phase. In the second type of experiment excess of racemic valine and optically pure (S)-[269] were distributed between two immiscible phases. In this experiment a 1 1 complex is formed in the non-aqueous phase in which L-valine dominates by an amount of 12.5% (CRFchc1j 1.28 and EDC 1.50). In terms of differences in free energy between the two diastereoisomeric complexes this means a difference in A(AG°) of 0.23 kcal mol-1 in favour of the (l)-S-[269] complex. Similar experiments have been carried out with crown ethers [270]—[280]. 

A new route to optically active amino-acids has been achieved by treatment of halogen compounds with Fe° or Ni° diastereoisomeric complexes of a-methylbenzyliminoglyoxylic acid ethyl ester. ... 

Pirkle type CSPs contain a chiral moiety having phenyl ring and, therefore, the formation of a it-it charge-transfer diastereoisomeric complex of the enantiomers (with the phenyl group) with CSP is supposed to be essential. In view of this, the re-acidic CSPs are suitable for the chiral resolution of re-donor solutes and vice versa. However, the newly developed CSPs containing both re-acidic and re-basic groups are suitable for the chiral resolution of both types of solute (i.e., re-donor and re-acceptor analytes). 

FIGURE 6 Graphical representation of the guest-host diastereoisomeric complex formation (a) in the presence of acid in the mobile phase and (b) in the absence of acid in the mobile phase, (c) Three-dimensional structures of the guest-host complexes formed between R- and 5-enantiomers of a-phenylethylamine and chiral 18-crown-6 ether CSP. 

Toda, F., Mori, K., Stein, Z., and Goldberg, I. (1989) Optical Resolution of Amine N-Oxides by Diastereoisomeric Complex Formation with Optically Active Host Compound, Tetrahedron Lett., 30, 1841-1844. 

Key words optical resolution, diastereoisomeric complexes, 0,0 -dibenzoyltartaric acid, hydroxycarboxylic acid esters, hydroxycarboxylic acids, chiral phosphine oxides, racemic alcohols. 

Biased classification of DBTA into the group of agents for diastereoisomeric salt formation was breaken through when Hatano and his co-workers recognized that trans-bicyclo[2.2. l]heptane-2,3-diamine (1, Scheme 2) form diastereoisomeric complexes with DBTA instead of regular salts [15]. Our laboratory has also reported such complex formations of eight N-alkylpipecolic acid-anilide derivatives (2, Scheme 2) with DBTA. [22]... 

In 1986, Minami and his coworkers described the optical resolution of the oxide of tram-his-1,2-(diphcnylphosphino)cyclobutanc (20, Scheme 10.) with stoichiometric amount of DBTA in methanol solution. [30] In the same year, Noyori and his coworkers published the synthesis of optically active BINAP via resolution of BINAP oxide (21) with DBTA. [31] In this process, hot chloroformic solution of racemic 21 was mixed with a molar equivalent amount of DBTA previously solved in ethyl acetate. Optically pure -21 was isolated in 79 % yield from the recrystallized diastereoisomeric complex by... 

Good crystallization ability of the (S)-25-DBTA complex gave a possibility for preparation (S)-25 in a through process . In that case, diastereoisomeric complex formation was accomplished starting from the crude reaction mixture of ( )-25 and the residue of the monomeric phosphine oxide was recycled into the previous oxidative coupling reaction from the filtrate of the crystalline diastereoisomeric complex. [34]... 

After filtration, the alcohol enriched in one enantiomer was obtained from the filtrate, the other one was isolated from the solid complex. Thermal methods such as distillation or sublimation were used for liberation of the optically active alcohols from the complexes. [38] Time-scaled experiments showed continous increasing of the enantiomeric excess (ee) in the crystalline diastereoisomeric complexes of 26 and 8 for 14 days, then these ee values practically did not change within the next 16 days (Figure 1). 

Practical advantage of this resolution method is that most of the investigated racemates could be solved in hexane but DBTA and the complex was insoluble in it. Consequently, complete separation of the solid diastereoisomeric complex and the remained free enantiomeric mixture of the alcohol is simple and chemists can avoid the exhausting work of solvent selection and concentration optimization. 

TABLE -5. Resolution of alcohols via diastereoisomeric complex formation with DBTA ... 

Rate of complex formation between chiral alcohols and DBTA monohydrate in hexane suspension is quite slow (see Figure 1) and numerous separation steps are necessarry for isolation of the alcohol isomers (filtration of the diastereoisomeric complex then concentration of the solution, decomposition of the complex, separation of the resolving agent and the enantiomer, distillation of the product). To avoid these problems, alternative methods have been developed for complex forming resolution of secondary alcohols. In a very first example of solid phase one pot resolution [40] the number of separation steps was decreased radically. Another novel method [41] let us to increase the rate of complex forming reaction in melt. Finally, first examples of the application of supercritical fluids for enantiomer separation from a mixture of diastereoisomeric complexes and free enantiomers [42, 43] are discussed in this subchapter. 

The melting point of racemic 37 is 43 °C, therfore this racemate and DBTA monohydrate was mixed in 2 1 molar ratio as two powders at ambident temperature, then the mixture was subjected to fractionated sublimation in vacuum (Scheme 14). At 50 °C, the unreacted alkohol sublimated and it contained (1R,2R)-37 in excess. At higher temperature (up to 100 °C) the diastereoisomeric complex decomposed slowly and the second sublimated fraction was enriched in (1S,2S)-37 isomer. Results of one, two and three weeks reactions are summarised in Table 7. 

Supercritical carbon dioxide is an apolar solvent, thus it is able to replace hexane during separation of the unreacted enantiomer from the diastereoisomeric complex containing reaction mixture. This idea was successfully applied in the complex forming resolution of tram-2-halogenocyclohexanols (35, 36, 37) and menthol (28). [42, 43] Diastereo-isomeric complex formation reaction was carried out in the mixture of the hexane solution of the racemic ligand and less then an equivalent amount of pulverised DBTA monohydrate. 

Kozma, D., Bocskei, Zs., Kassai, Cs., Simon, K., and Fogassy, E. Optical resolution of racemic alcohols by diastereoisomeric complex formation with 0,0 -dibenzoyl-(2R,3R)-tartaric acid, the crystal structure of the (-)-lR,2 S, 5R-menthol.O,0,-dibenzoyl-(2R,3R)-tartaric acid complex. J. Chem. Soc. Chem. Commun. 1996, 753-754. 

Fig. 5 Tetrameric diastereoisomeric complexes based on the assembling of achiral tris-dimelamine irnit with chiral isocyanuric acid...

Equimolar amounts of achiral urea 1 and the optically active calix[4]urea 5 give rise to the formation of a heterodimeric capsule with only one cyclic arrangement of the urea hydrogen bonds (Fig. 11), and in the presence of a chiral racemic substrate like norcamphor two diastereoisomeric complexes were present in a 1.3 to 1 ratio. 

Differently from the previous chiral softball, in this case the dissymmetric inner surface of the cavity easily sensed chiral guests, providing different diastereoisomeric complexes in different amounts. Each diastereoisomer is characterized by four different NH resonances because of the complete loss of symmetry of the complex. The system described was extensively investigated pursuing the synthesis of a total of six different glycoluril monomers (21-26) with various spacers in terms of size and electronic properties on the two sides of the bicyclic six-membered unit, and the relative homodimeric capsules were studied with a total of 20 chiral enantiopure guests (Fig. 23 and Table 2) [53]. 

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