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Going round the outside: The palladium-catalyzed coupling of meso-bromoporphyrins with a variety of internal alkynes efficiently provides peripherally cyclopentadiene-fused porphyrins. This modification has a significant impact on the electronic system of the porphyrin rings (see scheme), which results in non-porphyrin-like absorption spectra and narrower HOMO–LUMO gaps. Oxidation of the outer CC bond is facilitated by the induced strain.
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Sahoo et al. (2006) studied this question.
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