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Only trace amounts of the bis- and tris-adducts were detected
by MALDI-TOF MS analysis.
Attempts to react compound 1 with an excess of N-methyl-
glycine to obtain multiply cyclized adducts were not successful,
with prolonged reaction times and large excesses of N-methyl-
glycine affording complex mixtures of products. Subsequent
analysis of these compounds by proton NMR spectroscopy in
CDCl3 revealed that the N–H proton in 2 appeared as a sharp
singlet at 11.67 ppm, while the same proton appeared at 11.97 ppm
in the oxidized compound 3.12 This shift in the resonance signal to
a lower field indicated the existence of an intramolecular hydrogen
bonding interaction in both cases.
Fig. 1 1H NMR (400 MHz) spectral changes of (a) free base 1, (b) Pd(II)-
complex 4, and (c) compound 4 following the addition of TFA (CDCl3).
Since the double 1,3-dipolar cycloaddition of 1 with the
azomethine ylide in a single step was unsuccessful, the metal-
complex 4 was prepared separately before being subjected to
the 1,3-dipolar cycloaddition reaction. The reaction of 1 with Upon treatment with excess trifluoroacetic acid, all three
PdCl2 proceeded smoothly to give the corresponding Pd(II)- phenyl-Hs were clearly visible again. It was envisaged that the
complex 4 in high yield, which was reacted with the azomethine addition of excess acid to the Pd(II)-complex 4 would induce
ylide (generated in situ from the reaction of N-methylglycine demetallation. However, in practice, this treatment resulted in
with paraformaldehyde) to afford a diastereomeric mixture of the cleavage of the Pd(II)–C(Ph) bond only, and afforded com-
the bis-adduct 7 in 39% yield (Scheme 2).
pound 8 in quantitative yield.
Subsequent attempts to oxidize 7 with various oxidants,
The 1H NMR spectrum of compound 8 in CDCl3 revealed
however, were unsuccessful, and resulted in extensive decom- that there has been a significant down field shift of all the
position. The facile decomposition of 7 was attributed to the aryl protons (i.e., 2, 3 and 4) while maintaining the normal
severe steric hindrance between the diethylmalonyl group and coupling. This observation strongly suggested the existence of a
the annulated N-methylpyrrole ring when fully oxidized. Pleasingly, d-p back bonding interaction between the phenyl group and
however, the reaction of compound 4 with the azomethine ylide to Pd(II). It has been reported that the protonation can trigger the
afford adduct 5 also afforded Pd(II)-complex 6 quantitatively by rehybridization of C(22) from trigonal to tetrahedral whereas
subsequent oxidation with DDQ.
the positive charge can readily disperse at the six-membered
Different products were formed depending on the reaction ring preserving still the Pd(II)–C(22) coordination.13 But this is not
temperature, and it was therefore critically important to exercise the case with this heavily sterically hindered, conformationally
strict control over the reaction temperature to obtain the desired locked porphyrinoid.
product. As shown in Scheme 2, the mono-cyclized adduct 5 was
We proceeded to investigate the 1,3-dipolar cycloaddition
formed as the major product when the reaction was conducted reaction with the pentaphyrin analogue 9. As shown in Scheme 3,
at 80 1C, whereas bis-adduct 7 was formed as the major product the meso-diethylmalonylidene-(m-benzi)pentaphyrin 9 was reacted
at elevated temperatures (490 1C).
with the in situ generated azomethine ylide under similar condi-
1
As shown in Fig. 1, the H NMR spectrum of the free base tions to those used for the cycloaddition of compound 4 to give
porphyrin 1 displayed a broad singlet at 7.58 ppm, which was the mono-cyclized adduct 10 in 49% yield. Subsequent oxidation
attributed to H(22). Interestingly, this resonance signal was with DDQ afforded the fully oxidized compound 11 in quantitative
not present in the 1H NMR spectrum of the corresponding yield. Compound 11 was found to be unstable in solution and
Pd(II)-complex 4, indicating the Pd(II)–C(22) bond formation. underwent a slow reduction process to give compound 12 during
its purification by column chromatography over silica gel. The
resulting mixture of 11 and 12 was treated with NaBH4 to afford
pure compound 12. Given that our attempted one-pot double
cycloaddition reaction for the conversion of 9 to 14 failed in the
presence of an excess of the azomethine ylide, the cyclization was
repeated following the isolation of compound 12. In this parti-
cular case, the reaction proceeded smoothly to afford compound
13 in high yield, which was readily converted to compound 14 in
quantitative yield by DDQ oxidation. All of the products and
intermediates were characterized by 1H NMR, UV-vis and high
resolution mass spectrometry (ESI†).
The UV-vis absorption spectra of one pyrrole-extended com-
pound 3 showed clear hypsochromic shifts. Notably, a red shift
was observed in the Soret-like band (B30 nm) of the pyrrole-fused
Scheme 2 Attempted cycloaddition reaction of 4.
9278 | Chem. Commun., 2014, 50, 9277--9280
porphyrin (Fig. 2a). Fig. 2(b) shows the absorption spectra of the
This journal is ©The Royal Society of Chemistry 2014
Park, Dowoo
