Diels-Alder reactions of nickel(II) N-confused porphyrins as dienophiles

Article abstract of DOI:10.1039/b203813g

Diels-Alder reactions of nickel(II) N-confused tetraarylporphyrins as dienophiles with o-benzoquinodimethane yield nickel(II) N-confused isoquinoporphyrins.

Full text of DOI:10.1039/b203813g

Diels–Alder reactions of nickel(II) N-confused porphyrins as dienophiles†
Ziwei Xiao, Brian O. Patrick and David Dolphin*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1,
Canada. E-mail: ddolphin@qltinc.com; Fax: 604-822-9678; Tel: 604-822-4571
Received (in Corvallis, OR, USA) 16th April 2002, Accepted 18th June 2002
First published as an Advance Article on the web 19th July 2002
Diels–Alder reactions of nickel(II) N-confused tetraar-
ylporphyrins as dienophiles with o-benzoquinodimethane
yield nickel(II) N-confused isoquinoporphyrins.
Based on correlations shown in a COSY experiment, the Ha/
Hf protons appeared as singlets at d 6.91 and 8.57 (see Scheme
1 for proton assignments) and the Hb–He protons appeared at d
7.11, 7.27 (2H) and 7.46. The cross peak of 1H d 6.91 with 15
N
An N-confused porphyrin is a porphyrin isomer with an
inverted pyrrolic ring. Since the independent report of its
preparation by Latos-Grazynski et al.1 and Furuta et al.2 in
1994, N-confused porphyrins have been studied for their ability
to form complexes containing metal–carbon bonds as well as
properties associated with the outward pointing nitrogen
atom.
It is known that the peripheral double bonds of porphyrins are
cross-conjugated and may react as alkenes,3 and vinyl substi-
tuted porphyrins may react as dienes in Diels–Alder reactions.4
Diels–Alder reactions of porphyrins as dienophiles have been
reported by Cavaleiro et al.5 Here we report on the reactions of
the nickel(II) N-confused porphyrins (1) with o-benzoquinodi-
methane 3 to yield the nickel(II) N-confused isoquinoporphyrins
5 (Scheme 1).
d 17 in the 1H/15N HSQC experiment showed that Ha is either
two or three bonds away from a nitrogen atom, which indicates
that cycloaddition occurred on the peripheral carbon–nitrogen
bond rather than carbon–carbon bonds. The structure of
compound 5b is analogous to that of 5a, as determined by NMR.
Compounds 5a and 5b were presumably formed by oxidation of
the chlorins 4a and 4b.
The high selectivity of the peripheral carbon–nitrogen bond
over the carbon–carbon bonds can be rationalized by the
resonance contributions to the overall structure of 1.8 Fig. 1
shows two such resonance structures. In canonical form II the
CNN is both ‘cross conjugated’ and in an iminium form which
is known to be electron-deficient and an active dienophile.9 The
original X-ray structure of 1b was disordered1 and the N(2) and
C(21) atoms could not be distinguished. We have obtained the
X-ray crystal structure of the N-iodooctyl derivative (6) and
bond lengths (Fig. 2) clearly demonstrate the uniqueness of the
CNN+R bond and confirm that it should be an effective
dienophile.9
Direct evidence for the structural assignment of compound 5a
came from X-ray diffraction analysis (Fig. 3).¶ The isoquino
group is in the same plane with the linked pyrrole subunit.∑
Unlike planar compound 1b, the porphyrin skeleton of
compound 5a is ruffled. The dihedral angles between the
pyrrole planes and the plane defined by N(22)N(23)N(24) are as
follows: C(21) 19.88, N(22) 16.85, N(23) 215.98, N(24)
213.87°. The Ni–N(C) bond distances (Å) are: C(21) 1.904(2),
N(22) 1.9239(18), N(23) 1.949(2), and N(24) 1.9276(18),
which are shorter than that of compound 1b (1.955 or 1.963
N-Confused porphyrins were prepared using the procedure
described by Lindsey et al.6 and nickel(II) N-confused porphyr-
ins were synthesized using the method of Latos-Grazynski et
al.1 A solution of nickel(II) N-confused tetraphenylporphyrin
(1a, 100 mg) and sultine 27 (1.50 g) was refluxed in benzene (50
mL) for 2 days. The solvent was removed by vacuum and the
residue was chromatographed using silica gel (14 g, 1+1
hexanes/CH2Cl2). The product, which has a higher Rf value than
that of 1a,‡ was obtained in 23% yield and identified as the N-
confused isoquinoporphyrin 5a on the basis of its mass and
1
NMR (1H, 13C, selective NOE, COSY and H/15N HSQC)
spectra.§
Å**), and this is a typical change from planar nickel(II
)
porphyrins to ruffled nickel(II) porphyrins.10 The bond distance
between N(2) and C(3) is 1.389(3) Å, much longer than that of
Fig. 1 Two canonical forms for a nickel(II) N-confused porphyrin.
Scheme 1
† Electronic supplementary information (ESI) available: further character-
ization data. See http://www.rsc.org/suppdata/cc/b2/b203813g/
Fig. 2 Peripheral bond lengths (Å) for compound 6.
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CHEM. COMMUN., 2002, 1816–1817
This journal is © The Royal Society of Chemistry 2002

benzoquinodimethane. However, the amount was too small to be charac-
terized by NMR spectroscopy.
§ Rf (silica–CH2Cl2/hexanes 2+1) 0.77; 1H-NMR (400 MHz, CD2Cl2) d =
6.91 (s, 1H), 7.11 (d, J = 8.6 Hz, 1H), 7.27 (m, 2H), 7.46 (m, 1H), 7.55–8.05
(m, 20H), 8.22 (m, 2H), 8.29 (m, 3H), 8.36 (d, J = 4.9 Hz, 1H), 8.57 (s, 1H);
UV-vis (CH2Cl2) lmax/nm (log e) 438 (5.07), 628 (4.62); MS (LSIMS) 770
(M, 100%); HRMS (LSIMS) m/z Calc. for C52H32N4Ni: 770.19799. Found
770.19797 (M); Anal. Calc. for C52H32N4Ni·0.5H2O: C, 80.02; H, 4.26; N,
7.18. Found: C, 79.88; H, 4.11; N, 6.95.
¶ Crystal data:
C52H32N4Ni (5a), M = 771.53, monoclinic, a =
13.2484(19), b = 15.1197(17), c = 19.255(3) Å, b = 110.821(3)°, V =
3605.1(8) Å3, T = 173(2) K, space group P21/c (No. 14), Z = 4, m(Mo-Ka)
= 5.84 cm21, 33795 reflections measured, 8021 unique (Rint = 0.085)
which were used in all calculations. The final wR(F2) was 0.0873 (all data).
There is a 50/50 disorder between N(2) and C(3) for complex 5a since N(2)
and C(3) are occupied half the time by nitrogen and half the time by carbon.
CCDC 169757. See http://www.rsc.org/suppdata/cc/b2/b203813g/ for crys-
tallographic data in CIF or other electronic format.
∑ Deviation from plane for the isoquinopyrrole unit is 0.066 Å.
** The C(21) and N(2) atoms in complex 1b are disordered and not
distinguishable in the X-ray structure.1
Fig. 3 An ORTEP drawing of compound 5a showing atomic labeling and
thermal ellipsoids at the 50% probability level.
1 P. J. Chmielewski, L. Latos-Grazynski, K. Rachlewicz and T. Glowiak,
Angew. Chem., Int. Ed. Engl., 1994, 33, 779.
2 H. Furuta, T. Asano and T. Ogawa, J. Am. Chem. Soc., 1994, 116,
767.
3 L. Jaquinod, in The Porphyrin Handbook, ed. K. M. Kadish, K. M.
Smith and R. Guilard, Academic Press, San Diego, 2000, vol. 1, ch.
5.
4 R. K. DiNello and D. Dolphin, J. Org. Chem., 1980, 45, 5196.
5 A. C. Tomé, P. S. S. Lacerda, M. G. P. M. S. Neves and J. A. S.
Cavaleiro, Chem. Commun., 1997, 1199.
6 G. R. Geier III, D. M. Haynes and J. S. Lindsey, Org. Lett., 1999, 1,
1455.
7 M. D. Hoey and D. C. Dittmer, J. Org. Chem., 1991, 56, 1947.
8 T. D. Lash, D. T. Richter and C. M. Shiner, J. Org. Chem., 1999, 64,
7973.
compound 1b (1.351 or 1.324 Å**), as N(2)–C(3) is in the
isoquino group and has only about 1/3 double bond character.
In conclusion, the peripheral carbon–nitrogen double bonds
of nickel(II) N-confused porphyrins are partially isolated from
the 18p conjugated aromatic system and react as dienophiles in
Diels–Alder reactions with o-benzoquinodimethane yielding
novel nickel(II) N-confused isoquinoporphyrins. To the best of
our knowledge, this is the first report on addition reactions of N-
confused porphyrins.
We thank NSERC of Canada for funding, Dr Elizabeth Cheu
for advice, and the departmental MS, NMR and Microanalysis
for services.
9 D. L. Boger and S. N. Weinreb, in Organic Chemistry, ed. H. H.
Wasserman, Academic Press, San Diego, 1987, vol. 47.
10 W. R. Scheidt, in The Porphyrins, ed. D. Dolphin, Academic Press, New
York, 1978, vol. 3, ch. 10.
Notes and references
‡ There is a compound with a higher Rf value than compound 5a. The parent
ion is observed at m/z 879 (LSIMS) suggesting the addition of two o-
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