Core modified oxybenziporphyrins: New aromatic ligands for metal-carbon bond activation

Article abstract of DOI:10.1039/b111155h

Successful syntheses of two new aromatic core modified oxybenziporphyrins by a simple '3 + 1' methodology and the first aromatic core modified oxybenziporphyrin palladium complex are reported.

Full text of DOI:10.1039/b111155h

Core modified oxybenziporphyrins: new aromatic ligands for
metal–carbon bond activation†
Sundararaman Venkatraman, Venkataramanarao G. Anand, Simi K. Pushpan, Jeyaraman Sankar
and Tavarekere K. Chandrashekar*
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India. E-mail: tkc@iitk.ac.in
Received (in Cambridge, UK) 6th December 2001, Accepted 17th January 2002
First published as an Advance Article on the web 5th February 2002
Successful syntheses of two new aromatic core modified
oxybenziporphyrins by a simple ‘3 + 1’ methodology and the
first aromatic core modified oxybenziporphyrin palladium
complex are reported.
atmosphere, condensation of reagents was found to be complete
and was monitored by TLC. The reaction was neutralised with
triethylamine and oxidised by DDQ. Purification was done by
column (alumina basic; grade III) chromatography using
dichloromethane as eluent, which yielded 6 in 28% yield and 7
in 15% yield. Reaction of 6 with PdCl2 in benzonitrile under
reflux followed by chromatography (alumina neutral) afforded
8 in 77% yield (Scheme 1).
The structures of new macrocycles 6, 7 and 8 were confirmed
by a variety of spectroscopic techniques.‡ The FAB mass
spectra confirmed the composition, and infrared spectra showed
a strong peak in the region of 1600 cm21 due to the presence of
a keto group. The detailed 1H and 2D NMR spectral
analysis† confirmed the proposed structure. Briefly, the inner
CH proton resonates as a sharp singlet at 23.5 ppm for 6, 24.32
ppm for 7, while the NH pyrrole proton resonates as a broad
singlet at 24.7 ppm for 6 and 22.09 ppm for 7 suggesting the
effect of diatropic ring current on these protons. The meso
hydrogens appear in the deshielding region and the estimated
Dd (difference in chemical shift of most shielded and the most
deshielded proton) is 15.2 ppm for 6 and 15.12 ppm for 7,
clearly confirming the aromatic nature of these macrocycles. As
expected hydrogens a and b to the keto group appear as a
doublet at ≈ 7.3 ppm and at ≈ 8.7 ppm respectively and their
chemical shifts are in accord with an a,b unsaturated keto
subunit.5b Additional evidence for the aromatic nature of 6 and
7 comes from UV-Visible spectral data, where strong Soret type
Syntheses of new aromatic porphyrin derivatives continue to
attract the attention of the chemists due to their diverse
applications in biology, medicine, catalysis and material
science.1 This led to the discovery of many new porphyrin
analogues such as carbaporphyrin2 1, N-confused porphyrin3 2,
and benziporphyrin4 3 etc. Even though macrocycles 1, 2 and 3
contain a CNNN core, 1 and 2 were found to be aromatic while
3 was non-aromatic. The non-aromatic nature of 3 was
attributed to strong 6p arene aromaticity of the benzene ring
relative to the porphyrinoid aromaticity. Addition of a suitably
placed hydroxy group on the benzene ring in 3 led to the
retention of aromaticity due to keto–enol tautomerism.2
A
recent report4c shows that this ligand is capable of forming a
Pd–C bond, which was not known previously. In this commu-
nication we wish to report the first examples of aromatic core
modified oxybenziporphyrins containing OCNN and SCNN
cores. Such core modifications not only alter the electronic
structure of the ring but also provides variable cavity size for
metal coordination, where M–C, M–N and M–X (X = O or S)
bonds can be created inside a single porphyrin unit.
A ‘3 + 1’ acid catalysed MacDonald condensation has been
successfully utilised by various groups5 to synthesize new
porphyrin analogues. In the present synthesis, a similar
methodology was followed by the reaction of modified
tripyrrane 4 or 5 with 5-formylsalicyaldehyde in the presence of
10 equivalents of trifluoroacetic acid in dichloromethane at
room temperature. After two hours of stirring under nitrogen
† Electronic supplementary information (ESI) available: See FAB mass,
UV-Vis, 1H, 2D NMR spectra. http://www.rsc.org/suppdata/cc/b1/
b111155h/
Scheme 1
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CHEM. COMMUN., 2002, 462–463
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(split) bands appear in the region 430–480 nm (log e ≈ 5) and
multiple Q-bands in the region 500–750 nm confirm the
porphyrinoid nature of macrocycles. These absorption bands are
found to be approximately 30 to 35 nm red shifted relative to the
all aza analogue, due to the substitution of a heteroatom in the
porphyrin core.6 The metallation led to the expected changes in
the absorption spectra (Fig. 1) where a lesser number of Q-
bands were observed, typical of metalloporphyrins.
The monocations 6a and 7a were generated by careful
titration with TFA (15 µL of 10% v/v in CDCl3). The inner CH
proton resonates as a sharp singlet at 25.03 ppm for 6a and at
24.85 ppm for 7a suggesting the retention of aromatic character
upon mono-protonation. However further addition of TFA
resulted in deshielding of this proton signal relative to 6a and 7a
and the magnitude of deshielding is proportional to the
concentration of TFA added. For example, the further addition
of 35 µL, 65 µL, 125 µL of 10% TFA–CDCl3 solution(v/v) to
6a resulted in the observation of CH proton signal at 24.62,
24.09 and 23.03 ppm while for 7a the signals were at 24.65,
24.38 and 23.78 ppm respectively. Furthermore the NH
protons of 6a were seen at 3.26 ppm and at 3.3 ppm at 273 K
while for 7a only a broad signal at 3.5 ppm was observed at 233
K. Excess addition of TFA can result in the formation of
dicationic species either as a non-aromatic form represented by
the diprotonated hydroxybenziporphyrin5b structures 6b and 7b
or as an aromatic protonated carbonyl moiety as in 6c and 7c.
The fact that the inner CH proton still experiences considerable
diatropic ring current even in excess TFA indicates that the
dicationic species can be better represented as 6c and 7c rather
than 6b and 7b. This is in contrast to the observation made for
the all aza analogue.5b The appearance of pyrrole NH signals at
the deshielded region is attributed to the structural change
experienced by the macrocycle upon diprotonation, typical of
meso aryl porphyrins.7 UV-Visible titration data with TFA also
support such a conclusion.†
The reaction of PdCl2 in benzonitrile with 6 results in the
formation of metal complex 8. The absence of inner CH and NH
signals, in the NMR spectrum, imply the coordination of
palladium with inner nitrogens, oxygen and carbon. The meso
protons are slightly deshielded corresponding to free base 6, an
observation typical of metalloporphyrins. The shift of the CNO
peak in the complex (gCNO = 1588 cm21) relative to free base
(gCNO = 1630 cm21) further supports the palladium coordina-
tion. UV-Visible spectral data for the palladium complex
indicate the retention of aromatic character. It should be pointed
out here that very recently Latos-Grazynski et al4c reported the
anionic palladium derivative of oxybenziporphyrin, which on
alkylation forms both O-substituted and C-substituted metal
complexes.§
In summary, we have described the syntheses of two new
heteroatom substituted aromatic oxybenziporphyrins and the
first metallated derivative of an oxa substituted oxybenzipor-
phyrin, where palladium is coordinated to pyrrole nitrogens,
furan oxygen and benzene carbon inside a single porphyrin unit.
Further studies on the use of 8 as a catalyst for organic
conversions and C–H bond activation are under way.
We thank Department of Science and Technology, New
Delhi, India for the research grant provided.
Notes and references
‡
Characterization data for 6 UV/Vis (CH2Cl2): lmax nm (log e) = 437
(4.92), 467 (4.66), 515 (3.79), 549 (4.44), 593 (3.86), 616 (3.79), 723 (3.56);
1H NMR (400 MHz, CDCl3): d = 24.70 (br s, NH), 23.50 (s, H), 1.82 (s,
6H), 1.83 (s, 6H), 2.57 (s, 6H), 7.24 (s, 4H), 7.33 (d, H), 8.26 (t, 2H), 8.67
(m, 3H), 8.90 (d, H), 9.04 (d, H), 9.53 (s, H), 10.50 (s, H); FAB mass m/z
(%): 575 [M + 1]; IR(KBr) gCNO: 1630 cm21
.
7 UV/Vis (CH2Cl2): lmax nm (log e) = 442 (5.09), 472 (4.89), 558
(4.00), 600 (4.18), 727 (3.68); 1H NMR (400 MHz, CDCl3): d = 24.32 (s,
H), 22.09 (br s, NH), 1.84 (s, 6H), 1.86 (s, 6H), 2.58 (s, 6H), 7.26 (s, 4H),
7.46 (d, H), 8.36 (d, 2H), 8.83 (d, H), 8.91 (d, H), 9.07 (m, 3H), 9.66 (s, H),
10.80(s, H); FAB mass m/z (%): 591 [M + 1]; IR(KBr) gCNO: 1628 cm21
.
8 UV/Vis (CH2Cl2): lmax nm (log e) = 404 (4.26), 468 (4.56), 489 (4.41),
582 (3.81), 672 (3.80); 1H NMR (400 MHz, CDCl3): d = 1.74 (s, 12H), 2.52
(s, 6H), 7.15 (s, 4H), 7.19 (s, H), 8.26 (dd, 2H), 8.55 (d, H), 8.67 (d, 2H),
8.78 (d, H), 8.90 (d, H), 9.38 (s, H), 10.63 (s, H); FAB mass m/z (%): 678
Fig. 1 Electronic absorption spectra of 6 (—) (1.91 3 1025 M) and 8 (---)
(1.23 310 21 M) in dichloromethane.
[M+]; IR(KBr) gCNO: 1588 cm21
.
§ After the submission of this manuscript a paper 4c appeared in the
literature and we thank one of the referees who brought it to our notice.
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therein.
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