I= CHIRAL RECOGNITION OF NITRILES, SELENIDES AND IODIDES BY 1H NMR SPECTROSCOPY IN PRESENCE OF A CHIRAL DIRHODIUM COMPLEX
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Title of Thesis
CHIRAL RECOGNITION OF NITRILES, SELENIDES AND IODIDES BY 1H NMR SPECTROSCOPY IN PRESENCE OF A CHIRAL DIRHODIUM COMPLEX

Author(s)
Shahid Hameed
Institute/University/Department Details
Department of Chemistry/ Quaid-i-Azam University of Islamabad
Session
1997
Subject
Organic Chemistry
Number of Pages
299
Keywords (Extracted from title, table of contents and abstract of thesis)
chiral recognition, nitriles, selenides, iodides, 1h nmr spectroscopy, chiral dirhodium complex

Abstract
The great surge of interest in the enantioselective syntheses since 1980€™s has led to a rapid progress in the last ten years in developing accurate, reliable and convenient methods of measuring enantiomeric purity. In the early 1960€™s chiroptical methods were used to assess the enantiomeric purity of a chiral molecule. This method has a number of disadvantages. Nowadays, the methods in use are the chromatographic methods and the nuclear magnetic resonance methods. Although the chromatographic methods, especially the gas chromatographic methods and the high performance liquid chromatographic methods are being used to an increasing extent, most of the practicing organic chemists use NMR methods for chiral recognition. The chiral auxiliaries used in NMR methods are the chiral derivatizing agents, chiral solvating agents and chiral lanthanide shift reagents. Though the chiral lanthanide shift reagents have been used most extensively for this purpose, still there are certain classes of compounds which fail to bind to these auxiliaries. Recently, dirhodium tetra-(R)-α-methoxy-α-(trifluoromethyl)phenylacetate has been proved suitable for NMR investigations of less polar compounds.

In the present work the potential of this complex at chiral recognition of some less polar classes of compounds like nitriles, selenides and iodides has been explored. A number of nitriles with different structural features were synthesized and their 1H NMR spectra recorded in CDCl3 in the absence and in the presence of the dirhodium complex. It was observed that the signals of almost all the protons were shifted to higher frequencies to different extent and some of them showed diastereotopic dispersions due to complexation. These diastereotopic dispersions allow the determination of enantiomeric ratio of the nitriles. Tentative statements concernmg the binding mode of the cyano group at the rhodium atom are presented.

Selenides is another important class of weak donors and chiral recognition of these compounds fail with the classical lanthanide shift reagents. The ability of the dirhodium complex at the chiral recognition of a representative class of selenides was studied. These studies were further extended to iodides and bromides. It was observed that the signal shifts and diastereotopic dispersions were less pronounced in selenides and iodides as compared to the nitriles where they were moderate to high but still they are large enough to allow the determination of enantiomeric purities. The bromides fail to bind to the dirhodium complex and do not show similar effects.

Download Full Thesis
4910.69 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
276.09 KB
2 1 Introduction 1
218.45 KB
3 2 Background 12
2100.63 KB
  2.1 Chiral Recognition 12
  2.2 General Methods For Synthesis 100
4 3 Plan Of Work 147
81.95 KB
  3.1 Synthesis 147
  3.2 NMR Shift Experiments 153
5 4 Results And Discussion 154
1486.75 KB
  4.1 NMR Results 154
  4.2 Synthetic Methods 196
  4.3 Spectroscopy Of The Compounds Synthesized 221
6 5 Experimental 237
661.06 KB
  5.1 Experimental Notes 237
  5.2 Synthesis Of Nitriles 238
  5.3 Synthesis Of Selenides 269
  5.4 Synthesis Of Dirhodium Tetra-(R)-‘- Methoxy - ‘-( Trifluoromethyl ) Phenylacetate (5) 274
7 6 References
448.29 KB