Preparation and structure elucidation by NMR of the charge delocalised β,β-carotene dication

Article abstract of DOI:10.1016/S0040-4039(02)00978-4

The β,β-carotene dication (λ_max 985 nm at room temperature) prepared by treatment of β,β-carotene with BF₃-etherate had considerable stability. The reaction, monitored by EPR studies at -25°C, occurred by free radical intermediates. EIMS further revealed a C₄₀H₅₆BF₃ intermediate. The detailed structure of the charge delocalised β,β-carotene dication was established by COSY, HSQC, HMBC and 1D and 2D ROESY NMR techniques (600 MHz, CDCl₃, -25°C) leading to complete assignments of ¹H and ¹³C chemical shifts and ³J_H,H coupling constants. Considerable downfield shifts were observed relative to β,β-carotene. The results are consistent with charge delocalisation mainly in the C-5-C-9 and C-5′-C-9′ regions and with retro shifted double bonds in the central C-13-C-13′ region. The effect of the two delocalised charges on chemical shift (charge distribution) and bond distance (³J_H,H) is discussed.

Full text of DOI:10.1016/S0040-4039(02)00978-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5149–5152
Preparation and structure elucidation by NMR of the charge
delocalised b,b-carotene dication
Bjart Frode Lutnaes, Liv Brua˚s, Jostein Krane and Synnøve Liaaen-Jensen*
Department of Chemistry, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway
Received 19 April 2002; accepted 17 May 2002
Abstract—The b,b-carotene dication (umax 985 nm at room temperature) prepared by treatment of b,b-carotene with BF3–etherate
had considerable stability. The reaction, monitored by EPR studies at −25°C, occurred by free radical intermediates. EIMS further
revealed a C40H56BF3 intermediate. The detailed structure of the charge delocalised b,b-carotene dication was established by
COSY, HSQC, HMBC and 1D and 2D ROESY NMR techniques (600 MHz, CDCl3, −25°C) leading to complete assignments
1
3
of H and 13C chemical shifts and JH,H coupling constants. Considerable downfield shifts were observed relative to b,b-carotene.
The results are consistent with charge delocalisation mainly in the C-5–C-9 and C-5%–C-9% regions and with retro shifted double
bonds in the central C-13–C-13% region. The effect of the two delocalised charges on chemical shift (charge distribution) and bond
distance (3JH,H) is discussed. © 2002 Elsevier Science Ltd. All rights reserved.
Carotenoid cations are likely intermediates in E1-type
eliminations of allylic hydroxy, alkoxy and glycosyloxy
groups1 and have been hypothetic intermediates in reac-
tions of BF3 with carotenoids.2
The reaction between 1 and BF3–etherate provided a
complex with umax (CHCl3) 985 nm at rt and umax 920
nm at −25°C. The stability was monitored by UV–vis–
NIR studies. The absorption of the dication at −25°C
showed a decrease in intensity of less than 20% during
2 h. This is remarkably stable for a cationic species,
albeit consistent with the strong stabilisation from
extensive delocalisation.
Carotenoid cation radicals have been demonstrated in
photosystem II, which is a membrane bound pigment–
protein complex responsible for catalysing the oxida-
tion of H2O to O2 in oxygenic photosynthesis.3–6
Moreover, the presence of carotenoid dications have
been suggested in photosynthetic reaction centres.7
The reactive charged carotenoid species relevant for
biological systems have been studied by methods
including NIR,3,7–11 EPR,4,12 ENDOR5 and resonance
Raman spectroscopy,3 cyclic voltammetry,7–9 and AM1
calculations.13–16 The experimental data have not
resulted in definitive structural assignments of such
charged carotenoid species. No NMR data have been
published.
The reaction between b,b-carotene 1 and BF3–etherate,
studied by Zechmeister’s school in the 1950s,17,18 has
been reinvestigated by modern methods. Our studies
have resulted in the isolation and structural elucidation
of the dication of b,b-carotene 1 as a symmetrical
charge delocalised dication.
The reaction, also monitored by EPR studies at −25°C,
occurred via free radical intermediates. The EPR spec-
trum of the reaction mixture showed a weak signal with
a linewidth of 14 G, which is in agreement with
observed values for the b,b-carotene (1) cation radi-
Keywords: carotenoid cation; b,b-carotene; dication; preparation;
NMR; charge delocalisation; structure.
* Corresponding author. Tel.: +47 73 59 40 99; fax: +47 73 59 42 56;
e-mail: slje@chembio.ntnu.no
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00978-4

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B. F. Lutnaes et al. / Tetrahedron Letters 43 (2002) 5149–5152
cal.19 The presence of a radical species is also supported
by the observation that in freshly prepared samples the
NMR resonances are broadened at the beginning of the
recordings. Moreover, EIMS revealed an intermediate
with molecular ion m/z 604 compatible with C40H56BF3
and with fragmentations involving two consecutive
losses of C2H2.
1H NMR studies of the dication 3 were conducted in
CDCl3 at −25°C as an optimum temperature, and with
suppression of the methyl/ethyl signals of the etherate
reagent. However, no 1H–13C connectivities could be
observed due to dominant signals from the etherate
reagent. The dication was subsequently prepared using
BF3 complexed with (CD3)2O as Lewis acid in order to
minimise unwanted signals.
Figure 1. Through-space interactions in b,b-carotene dication
3 established by 1D and 2D ROESY.
calisation of the positive charges, evidenced by the large
downfield 13C shifts for C-5(5%). The effect of rotating
the b-ring on spin density and bond lengths in the
polyene chain has been studied in detail by the density
functional method B3LYP22 in the b,b-carotene cation
radical.
The 1H NMR spectrum demonstrated a symmetrical
1
dication. H and 13C chemical shifts were established
NMR shifts of the symmetrical structure 3 relative to
the b,b-carotene 1 (Table 1) served to identify the
charge distribution of the delocalised cation. The total
13C chemical shift change of 3 relative to 1 was 504
ppm, consistent with the formation of a dication.23 Also
the total shift change for 1H is consistent with the
expected value per positive charge24 for formation of a
polyene dication. The downfield shift is most pro-
nounced for the carbon atoms that can carry the posi-
tive charge by resonance (odd-numbered carbons)
(Table 1).
using homonuclear 1H–1H COSY and heteronuclear
1H–13C HSQC and HMBC NMR techniques (Table 1).
Initially the (15-Z)-configuration (2) was deduced20 on
the basis of a relatively small coupling constant for
21
J15,15%
.
However, detailed 1D and 2D ROESY experi-
ments led to a revised structure for the b,b-carotene
dication 3.
Whereas the assigned chemical shifts (1H and 13C) and
3JH,H coupling constants remained unchanged, the
interactions in space shown in Fig. 1 clearly differenti-
ated between structures 2 and 3 for the b,b-carotene
dication. The reorientation of the six-membered rings
relative to the polyene chain allows an increased delo-
The downfield shift increases towards the end of the
polyene chain, and in an almost linear fashion for the
odd numbered carbons (Fig. 2). Deviations for C-7 and
C-9 are probably due to C-7 being a secondary carbon,
whereas C-9 is a tertiary carbon, stabilising the positive
charge better. The effect for the even-numbered carbon
atoms not carrying the charge does not give a clear
picture.
1
Table 1. 13C and H NMR chemical shifts (ppm) relative
to TMS for b,b-carotene 1 and b,b-carotene dication 3 in
CDCl3, and chemical shift differences for 3 relative to 1
13C
1H
1
In the H NMR, the size of the downfield shift in 3
1
3
D
1
3
D
versus 1 is largest at H-7(7%), even when the increased
steric hindrance between 18(18%)-CH3 and H-7(7%) from
the rotation of the end groups is taken into consider-
ation. A similar effect is observed for C-7(7%) in the 13C
NMR, which are also the positions with the largest
change in chemical shift of all proton carrying carbons.
1/1%
2/2%
3/3%
4/4%
5/5%
6/6%
7/7%
8/8%
34.3
39.8
19.4
33.2
35.1
41.5
22.7
39.5
0.8
1.7
3.3
6.3
51.9
8.3
33.0
−0.9
45.8
11.2
28.5
2.9
27.0
13.3
12.9
−0.2
−0.2
2.9
1.47
1.62
2.03
1.63
1.72
2.68
0.16
0.10
0.65
129.3
138.0
126.7
137.8
136.0
130.8
125.1
137.3
136.4
132.4
130.0
29.0
181.2
146.3
159.7
136.9
181.8
142.0
153.6
140.2
163.4
145.7
142.9
28.8
6.16
6.15
8.56
7.39
2.40
1.24
9/9%
10/10%
11/11%
12/12%
13/13%
14/14%
15/15%
16/16%
17/17%
18/18%
19/19%
20/20%
6.15
6.65
6.37
7.27
8.40
7.03
1.12
1.75
0.66
6.27
6.63
1.03
1.03
1.72
1.97
1.97
7.10
7.40
1.35
1.35
2.39
2.64
2.43
0.83
0.77
0.32
0.32
0.67
0.67
0.46
29.0
21.7
12.8
12.8
28.8
24.6
15.2
14.2
2.4
1.4
Figure 2. 13C NMR downfield chemical shift differences
between b,b-carotene 1 and b,b-carotene dication 3.
252.3
12.12

B. F. Lutnaes et al. / Tetrahedron Letters 43 (2002) 5149–5152
5151
Steric effects also explain the small upfield shift of
C-8(8%), which will experience less steric hindrance in 3
versus 1. Thus, the chemical shift data support a sym-
metrical delocalised dication with the charge preferen-
tially in the C-5–C-9 and the C-5%–C-9% regions.
The b,b-carotene dication 3, therefore, emerges as an
example in which the effect of the cations on bond
lengths and charge distribution is differently experi-
enced in various regions of the polyene chain. The
effect of the positive charge (as deduced from the
chemical shifts) is largest towards the ends of the chain
(C-5–C-9 and C-5%–C-9%), whereas the effect on the
The size of the coupling constants in the polyene chain
of non-charged carotenoids has been generalised as
13.5–16.8 Hz for trans double bonds with a tendency to
decrease towards the central part of the conjugated
chain. The coupling constant across single bonds (s-
trans) are normally 10.5–12.0 Hz with the larger values
near the central part of the molecule.21 These generali-
sations were not made for retro-carotenoids25 in which
the positions of double bonds are shifted. For cis
double bonds, the range 11.5–12.8 Hz has been given.21
3
bond lengths (as deduced from the JH,H coupling con-
stants) is largest in the centre of the molecule (C-13–C-
13%) leading to a reversal of double and single bonds in
the centre. The regions C-9–C-13 and C-9%–C-13% are
characterised by delocalised bonds.
In relation to the previously proposed structure for the
b,b-carotene dication with a central cis-bond 2, it was
considered of interest to examine the dication obtained
from (15-Z)-b,b-carotene (15Z-1). The (15-Z)-isomer,
which was completely stable in CDCl3 in darkness at
As depicted in Fig. 3, the coupling constants J14,15
=
1
room temperature for 20 h, provided, according to H
14.6 Hz and J15,15%=12.0 Hz are of the same size as the
coupling constants in 1 in this region (J14,15=14.4 Hz
and J15,15%=11.9 Hz),26 and are compatible with
localised retro double/single bonds in the C-13,13%
region. The C-10,11 (J10,11=13.7 Hz) and C-11,12
(J11,12=12.4 Hz) bonds have similar bond character
compatible with bond reversals in this region.
NMR data, the same dication 3 with BF3 complexed
with (CD3)2O after approximately 1 h at −25°C. It is
concluded that the (15-Z)-bond isomerised as a conse-
quence of the formation of the preferred delocalised
dication structure.
The formation of the dication 3 and the C40H56BF3
intermediate via a radical type process is rationalised in
Scheme 1. The structure of the negative counter ion of
the dication 3 is tentative, formulated by analogy,27 and
not established. Reaction of the dication 3 with a
nucleophile (H2O in acetone) provided strongly E/Z
isomerised isocryptoxanthin 4 as the major product,
(E/Z)-retro-dehydro-b,b-carotene 5 and mutatochrome
6 according to HPLC, UV, MS and 1H NMR data. The
resulting E/Z isomerisation is expected via cationic
intermediates. The formation of the products 4–6 from
the dication 3 may be rationalised on a mechanistic
basis. Details will be presented elsewhere.
Structure 3 is also consistent with the value found for
the coupling constant J7,8 (15.1 Hz), which, though
being somewhat smaller than in 1 (16.1 Hz), indicates a
trans 7,8-double bond. Also, the C-5,6 bond is therefore
expected to have a high degree of double bond charac-
ter, forcing the C-4–C-5–C-6–C-1 structural element of
the end group in a planar arrangement, consistent with
the coupling constants in the 1H NMR spectra. A
planar polyene system is also a prerequisite for maxi-
mum electron delocalisation.
The total NMR evidence is best accommodated with
structure 3, with charge delocalisation in 3 primarily in
the C-5–C-9 and C-5%–C-9% regions. This structure is
compatible with previously published semi-empirical
Hartree-Fock AM1 calculations of the structure of the
b,b-carotene dication, suggesting that the b,b-carotene
dication would exist as a pair of charged solitons with
bond reversals.13,14
Other AM1 calculations of bond length modifications
in the b,b-carotene dication versus 1, showing the
largest changes in the central region compatible with
bond reversals, and almost no change towards the end
groups, do not contradict structure 3.15,16 These studies,
however, suffer from the fact that they do not take into
consideration the rotation of the end groups, thereby
predicting a too small charge delocalisation here.22
Acknowledgements
Dr. A. Kamlowski, Bruker, Karlsruhe is thanked for
the EPR spectra at −25°C. The gift of (15-Z)-b,b-
carotene from Dr. A. Giger, Hoffmann–La Roche,
Figure 3. Coupling constants in Hz recorded for b,b-carotene
dication 3.

5152
B. F. Lutnaes et al. / Tetrahedron Letters 43 (2002) 5149–5152
Scheme 1. Proposed formation of b,b-carotene dication 3 and its counterion from b,b-carotene 1 and BF3–etherate.
Basel, and a research grant from the latter company to
S.L.-J. is gratefully acknowledged.
Cornil, J.; Bre´das, J. L. Synth. Met. 1993, 55–57, 4707–
4713.
14. Ehrenfreund, E.; Moses, D.; Heeger, A. J.; Cornil, J.;
Bre´das, J. L. Chem. Phys. Lett. 1992, 196, 84–90.
15. Kispert, L. D.; Gao, G.; Deng, Y.; Konovalov, V.;
Jeevarajan, A. S.; Jeevarajan, J. A.; Hand, E. Acta
Chem. Scand. 1997, 51, 572–578.
16. Broszeit, G.; Diepenbock, F.; Gra¨f, O.; Hecht, D.;
Heinze, J.; Martin, H.-D.; Mayer, B.; Schaper, K.;
Smie, A.; Strehblow, H.-H. Liebings Ann./Recl. 1997,
2205–2213.
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