Chiral NMR spectroscopy

Wenzel,T.).,Wilcox, J. D. Chiral reagents for the determination of enantiomeric excess and absolute configuration using NMR spectroscopy. Chirality, 2003,15, 256-270. 

Chiral gas chromatography NMR spectroscopy Chiral resolving agent Diastereomeric methyl groups... 

Wenzel TJ, Chisholm CD. Assignment of absolute configuration using chiral reagents and NMR Spectroscopy. Chirality 2011 23 190-214. 

Duddeck H, Diaz Gomez E. Chiral recognition of ethers by NMR spectroscopy. Chirality 2009 21(l) 51-68. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

Unfortunately, in most of the previous examples, the extent of the asymmetry-induction was determined by chiroptical measurements (ORD, CD) that gave qualitative and not quantitative information. The NMR chiral shift efficiency of TRISPHAT 8 and other hexacoordinated phosphate anions was therefore considered as an excellent analytical tool to provide accurate measurement of the induced selectivity by NMR spectroscopy. 

In order to achieve a true comparison between both catalytic systems, colloidal and molecular, which display very different reaction rates, a series of experiments were carried out with the homogeneous molecular system, decreasing the catalyst concentration in the studied allylic alkylation reaction. The reaction evolution is monitored taking samples at different reaction times and analysing each of them by NMR spectroscopy (to determine the conversion) and HPLC chromatography with chiral column (to determine the enantioselectivity of I and II). For molecular catalyst systems, the Pd/substrate ratio was varied between 1/100 and 1/10,000. For the latter ratio, the initial reaction rate was found comparable to that of the colloidal system (Figure 2a), but interestingly the conversion of the substrate is quasi complete after ca. 100 h in... 

Table II presents H-NMR data of the hasubanan alkaloids obtained since 1976. During the period 1976-1986, the application of H-NMR spectroscopy was accelerated together with the improvement of measuring instruments. NOE, INDOR, and two-dimensional NMR experiments (7) have been undertaken to resolve the question of stereochemistry at the chiral centers.

Other chiral diphosphine ligands (374), (375), (376), and (377) bind to iridium with little diastereoselectivity.603 The complexes are characterized by 1H and 31P H NMR spectroscopy. 

Another possibility to find out more about the structure of these dendrimers was chosen by incorporating fluorine atoms. The use of 19F-NMR spectroscopy offered an additional tool to study the conformation of the dendrimer, especially with the fluorines attached close to the stereogenic centers [91 ]. Following our previously developed methods [92], fluorine-containing 1st- and 2nd-genera-tion chiral dendrimers such as 76 were synthesized (Fig. 24). 

Yeste et al.7 have developed chiral derivatization protocols for determination of enantiopurity of chiral diols by the H and 19F NMR spectroscopy. Three-component method has required the treatment of diol with 2-formylphenylboronic acid and a-methyl-4-fluorobenzylamine [4],... 

Gawronski et al.21 have used NMR spectroscopy, especially NOE experiments in order to assign conformation of the chiral calixsalen-type macrocycles, products from the [3 + 3] cyclocondensation of the trans-1,2-diaminocyclohexane with hydroxydialdehydes. It was shown that the macrocycles had C3-symmetrical structure and s-syn conformation of imine C—H/cyclohexane axial C—H bond systems and s-trans conformation of the bis-imine unit. 

Structure of the chiral poly-Schiff base [14] has been studied by NMR spectroscopy.29 It was shown that poly-Schiff bases have an interesting feature their chiral forms can be self-assembled into nanometer-size fibrous architecture, while their racemic analogues did not show any self-assembling properties. 

The interactions between the chiral Schiff bases being derivatives of 2-aminoalcohols and substituted salicylic aldehydes and titanium (IV) isopropoxide [33] have been monitored by aH NMR spectroscopy.81... 

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