Helix persistence and breakdown in oligoureas of metaphenylenediamine: Apparent diastereotopicity as a spectroscopic marker of helix length in solution

Jonathan Clayden*, Loïc Lemiègre, Gareth A. Morris, Mark Pickworth, Timothy J. Snape, Lyn H. Jones

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

67 Citations (Scopus)

Abstract

Oligomeric ureas derived from m-phenylenediamine with chain lengths of up to seven urea linkages were made by iterative synthetic pathways. Three families were synthesized: 4 and 20, bearing a terminal chiral sulfinyl group; 24, bearing a terminal rotationally restricted amide group, and 30 bearing a terminal achiral bromophenyl group. The distal end of the oligomers was capped with an N-benzyl group to act as a diastereotopic probe. With a terminal sulfinyl group, the 1H NMR signals arising from the CH2 group of the diastereotopic probe remained anisochronous even when separated from the stereogenic center by up to 24 bonds (in 20c). With a rotationally restricted amide, anisochronicity was no longer apparent beyond 17 bond lengths (in 24c). No anisochronicity was observable with a terminal bromophenyl group. We interpret these results as indicating that the oligoureas of short lengths adopt a defined helical secondary structure in solution, but that in longer oligomers the helicity breaks down and transmission of chirality in these systems is limited to about 24 bond lengths. We propose that "apparent diastereotopicity" (anisochronicity) provides a general empirical method for identifying secondary structure in solution.

Original languageEnglish
Pages (from-to)15193-15202
Number of pages10
JournalJournal of the American Chemical Society
Volume130
Issue number45
Early online date10 Oct 2008
DOIs
Publication statusPublished - 12 Nov 2008

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