Issue 22, 2003

The charge delocalised β,β-carotene dication—preparation, structure elucidation by NMR and reactions with nucleophiles

Abstract

The reaction between β,β-carotene and BF3-etherates has been investigated, leading to structural elucidation of the blue product, formed in appropriate organic solvents, as a symmetrical charge delocalised dication (λmax 985 nm at room temperature in CHCl3) with considerable stability. The reaction, monitored by EPR studies at −25 °C, occurred via free radical intermediates. A C40H56BF3 intermediate was captured by EIMS. The detailed structure of the dication was established by COSY, HSQC, HMBC and 1D and 2D ROESY NMR techniques (600 MHz, CDCl3, −20 °C) leading to complete assignments of 1H and 13C chemical shifts and 3JH,H coupling constants. The effects of the two delocalised charges on chemical shift (charge distribution) and bond distance (3JH,H) were considered. The results are consistent with charge delocalisation mainly in the C-5–C-9 and C-5′–C-9′ regions and with bond inversion to retro shifted double bonds in the central C-13–C-13′ region. A convention for denoting the charge delocalisation and bond types is presented. The experimental results are discussed relative to previous theoretical calculations of the β,β-carotene dication structure. (All-E) and (15-Z)-β,β-carotene provided the same dication. The NIR spectra and stability of dications prepared in the same manner from the related carotenes 20,20′-dinor-β,β-carotene, heptapreno-β,β-carotene and nonapreno-β,β-carotene were examined for comparison. Reactions of the β,β-carotene dication with selected nucleophiles provided products including isocryptoxanthin, isocarotene and mutatochrome with H2O as nucleophile, and isocryptoxanthin methyl ether, 8-methoxy-7,8-dihydro-β,β-carotene and isocarotene with CH3ONa as nucleophile. The formation of these products is rationalised from the structure assigned to the dication.

Graphical abstract: The charge delocalised β,β-carotene dication—preparation, structure elucidation by NMR and reactions with nucleophiles

Supplementary files

Article information

Article type
Paper
Submitted
03 Jul 2003
Accepted
10 Sep 2003
First published
09 Oct 2003

Org. Biomol. Chem., 2003,1, 4064-4072

The charge delocalised β,β-carotene dication—preparation, structure elucidation by NMR and reactions with nucleophiles

B. F. Lutnaes, L. Bruås, G. Kildahl-Andersen, J. Krane and S. Liaaen-Jensen, Org. Biomol. Chem., 2003, 1, 4064 DOI: 10.1039/B307531A

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements