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Anomalous NMR behavior of meso compounds with remote stereogenic centers on addition of chiral shift reagent or chiral solvating agent

Anomalous NMR behavior of meso compounds with remote stereogenic centers on addition of chiral shift reagent or chiral solvating agent
Anomalous NMR behavior of meso compounds with remote stereogenic centers on addition of chiral shift reagent or chiral solvating agent
A problem has arisen in using chiral shift reagents (CSR) and chiral solvating agents (CSA) to determine meso and racemic forms of diastereoisomers in which the stereogenic centers of the molecules are separated by achiral spacers. It is found that NMR signals of both meso and racemic forms of diastereoisomers may exhibit doubling on addition of CSR/CSA, which means that unequivocal assignments cannot be made without characterizing the effects for separate meso and racemic forms; this is particularly important for additions of CSR/CSA at relatively low concentrations, which always result in the splitting of some NMR signals of diastereoisomers. The phenomenon is demonstrated in the 31P NMR spectra of meso and racemic forms of three spermine-bridged gem-disubstituted cyclotriphosphazatrienes, 1a-c, and compared with analogous achiral molecules, the per-substituted spermine-bridged cyclotriphosphazatrienes 2a-d. As expected, only one set of 31P NMR signals was observed for the achiral compounds 2a-d, even on addition of CSA. Two sets of 31P NMR ABX multiplets corresponding to meso and racemic diastereoisomers were observed for compounds 1a-c; on addition of CSA, the signals of at least one of the multiplets for each compound separated into more than the expected groups of three lines with an intensity distribution of 2:1:1. To understand this phenomenon, the meso and racemic forms of 1a and 1b and the meso form of 1c have been separated and characterized by X-ray crystallography. On addition of CSA to the racemic forms of 1a and 1b, the 31P NMR spectrum shows the expected doubling of signals, but, unexpectedly, the same is observed for each of the meso forms of 1a-c. Analogous results using both CSA and CSR have been obtained for the meso and racemic forms of the diastereoisomeric piperazine-bridged macrocyclic-phosphazene compound, 3, whereas no effect was observed for the two meso forms of the doubly bridged macrocyclic-phosphazene compound 4. The phenomenon of doubling of the 31P NMR signals of the meso form of singly bridged cyclotriphosphazatrienes, 1a-c and 3, is explained by consideration of the equilibrium in solution of independent complexation of a chiral ligand with molecules that have two chiral cyclophosphazene moieties separated by an achiral spacer group. The results show that the stereogenicity of such diastereoisomeric molecules in solution cannot be characterized unequivocally by NMR measurements on addition of either CSR or CSA.
phosphorus-nitrogen compounds, cyclophosphazenes, spermidine
0002-7863
4943-4950
Beşli, Serap
63ec1cf4-d0d6-4982-9e0a-c632f1f98d08
Coles, Simon J.
3116f58b-c30c-48cf-bdd5-397d1c1fecf8
Davies, David B.
675b90e6-0c3c-468b-9996-326220517cc6
Eaton, Robert J.
e274676b-0499-47e1-8dbb-8d1ed908cdea
Hursthouse, Michael B.
57a2ddf9-b1b3-4f38-bfe9-ef2f526388da
Kiliç, Adem
53892b58-c3f9-4894-bdf9-6be70ab7611e
Shaw, Robert A.
a9ecde60-6fe9-4b5d-86e3-77cd5dcfaab8
Çiftçi, Gönül Yenilmez
ec263230-06cb-46be-b546-f366b4a38438
Yeşilot, Serkan
6d82d5b6-173c-4ca4-bf2e-02676c18c8d0
Beşli, Serap
63ec1cf4-d0d6-4982-9e0a-c632f1f98d08
Coles, Simon J.
3116f58b-c30c-48cf-bdd5-397d1c1fecf8
Davies, David B.
675b90e6-0c3c-468b-9996-326220517cc6
Eaton, Robert J.
e274676b-0499-47e1-8dbb-8d1ed908cdea
Hursthouse, Michael B.
57a2ddf9-b1b3-4f38-bfe9-ef2f526388da
Kiliç, Adem
53892b58-c3f9-4894-bdf9-6be70ab7611e
Shaw, Robert A.
a9ecde60-6fe9-4b5d-86e3-77cd5dcfaab8
Çiftçi, Gönül Yenilmez
ec263230-06cb-46be-b546-f366b4a38438
Yeşilot, Serkan
6d82d5b6-173c-4ca4-bf2e-02676c18c8d0

Beşli, Serap, Coles, Simon J., Davies, David B., Eaton, Robert J., Hursthouse, Michael B., Kiliç, Adem, Shaw, Robert A., Çiftçi, Gönül Yenilmez and Yeşilot, Serkan (2003) Anomalous NMR behavior of meso compounds with remote stereogenic centers on addition of chiral shift reagent or chiral solvating agent. Journal of the American Chemical Society, 125 (16), 4943-4950. (doi:10.1021/ja028871r).

Record type: Article

Abstract

A problem has arisen in using chiral shift reagents (CSR) and chiral solvating agents (CSA) to determine meso and racemic forms of diastereoisomers in which the stereogenic centers of the molecules are separated by achiral spacers. It is found that NMR signals of both meso and racemic forms of diastereoisomers may exhibit doubling on addition of CSR/CSA, which means that unequivocal assignments cannot be made without characterizing the effects for separate meso and racemic forms; this is particularly important for additions of CSR/CSA at relatively low concentrations, which always result in the splitting of some NMR signals of diastereoisomers. The phenomenon is demonstrated in the 31P NMR spectra of meso and racemic forms of three spermine-bridged gem-disubstituted cyclotriphosphazatrienes, 1a-c, and compared with analogous achiral molecules, the per-substituted spermine-bridged cyclotriphosphazatrienes 2a-d. As expected, only one set of 31P NMR signals was observed for the achiral compounds 2a-d, even on addition of CSA. Two sets of 31P NMR ABX multiplets corresponding to meso and racemic diastereoisomers were observed for compounds 1a-c; on addition of CSA, the signals of at least one of the multiplets for each compound separated into more than the expected groups of three lines with an intensity distribution of 2:1:1. To understand this phenomenon, the meso and racemic forms of 1a and 1b and the meso form of 1c have been separated and characterized by X-ray crystallography. On addition of CSA to the racemic forms of 1a and 1b, the 31P NMR spectrum shows the expected doubling of signals, but, unexpectedly, the same is observed for each of the meso forms of 1a-c. Analogous results using both CSA and CSR have been obtained for the meso and racemic forms of the diastereoisomeric piperazine-bridged macrocyclic-phosphazene compound, 3, whereas no effect was observed for the two meso forms of the doubly bridged macrocyclic-phosphazene compound 4. The phenomenon of doubling of the 31P NMR signals of the meso form of singly bridged cyclotriphosphazatrienes, 1a-c and 3, is explained by consideration of the equilibrium in solution of independent complexation of a chiral ligand with molecules that have two chiral cyclophosphazene moieties separated by an achiral spacer group. The results show that the stereogenicity of such diastereoisomeric molecules in solution cannot be characterized unequivocally by NMR measurements on addition of either CSR or CSA.

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More information

Published date: 23 April 2003
Keywords: phosphorus-nitrogen compounds, cyclophosphazenes, spermidine

Identifiers

Local EPrints ID: 19907
URI: http://eprints.soton.ac.uk/id/eprint/19907
ISSN: 0002-7863
PURE UUID: 29f1699a-9f53-48f5-b73d-0be1087b54d8
ORCID for Simon J. Coles: ORCID iD orcid.org/0000-0001-8414-9272

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Date deposited: 23 Feb 2006
Last modified: 09 Jan 2022 02:59

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Contributors

Author: Serap Beşli
Author: Simon J. Coles ORCID iD
Author: David B. Davies
Author: Robert J. Eaton
Author: Adem Kiliç
Author: Robert A. Shaw
Author: Gönül Yenilmez Çiftçi
Author: Serkan Yeşilot

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