Dipyrromethene

Naturally occurring porphyrins are usually symmetrically substituted about the 15-methine bridge. These porphyrins can be synthesized by the condensation of two dipyrroiic intermediates. Typical dipyrroiic intermediates in current use arc the dipyrromethanes and the dipyrromethenes. Both methods will shortly be described. This again is a highly specialized... 

A mild procedure which does not involve strong adds, has to be used in the synthesis of pure isomers of unsymmetrically substituted porphyrins from dipyrromethanes. The best procedure having been applied, e.g. in unequivocal syntheses of uroporphyrins II, III, and IV (see p. 251f.), is the condensation of 5,5 -diformyldipyrromethanes with 5,5 -unsubstituted dipyrromethanes in a very dilute solution of hydriodic add in acetic acid (A.H. Jackson, 1973). The electron-withdrawing formyl groups disfavor protonation of the pyrrole and therefore isomerization. The porphodimethene that is formed during short reaction times isomerizes only very slowly, since the pyrrole units are part of a dipyrromethene chromophore (see below). Furthermore, it can be oxidized immediately after its synthesis to give stable porphyrins. 

Conjugated dipyrrolic pigments, the dipyrromethenes, are synthesized by add-catalyzed condensation of an a-formyl pyrrole and an a-unsubstituted pyrrole. They are readily protonated and deprotonated and are difficult to purify by chromatography. 

The pyridine-like nitrogen of the 2H-pyrrol-2-yiidene unit tends to withdraw electrons from the conjugated system and deactivates it in reactions with electrophiles. The add-catalyzed condensations described above for pyrroles and dipyrromethanes therefore do not occur with dipyrromethenes. Vilsmeier formylation, for example, is only successful with pyrroles and dipyrromethanes but not with dipyrromethenes. 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

With the catalysis of strong Lewis acids, such as tin(IV) chloride, dipyrromethenes may aiso be alkylated. A very successful porphyrin synthesis involves 5-bromo-S -bromomethyl and 5 -unsubstituted 5-methyl-dipyrromethenes. In the first alkylation step a tetrapyrrolic intermediate is formed which cyclizes to produce the porphyrin in DMSO in the presence of pyridine. This reaction sequence is useful for the synthesis of completely unsymmetrical porphyrins (K.M. Smith, 1975). 

Dipyrromethenes rapidly exchange the hydrogen atom attached to nitrogen, and the two isomers of unsyramctrical compounds (e.g, 19 and 20) cannot be separately isolated/ Dipyrromethenes form meso-... 

Kollmannsberger M, Rurack K, Resch-Genger U, Daub J (1998) Ultrafast charge transfer in amino-substituted boron dipyrromethene dyes and its inhibition by cation com-plexation a new design concept for highly sensitive fluorescent probes. J Phys Chem A 102 10211-10220... 

To quantitate and analyze conjugated bile pigments, azo derivative formation is applied most frequently (Fig. 1). Such terms as azobilirubin, or azobilirubin conjugate, erroneously suggest tetrapyrrolic structures. The following trivial names appear to be more appropriate dipyrromethene azo derivative, azodipyrrole, dipyrrolic azo pigment, and the like. The term azodipyrrole will be employed in the present review. 

Aromatic species include the neutral molecules pyrrole, furan and thiophene (1 Z = NH, O, S) and the pyrrole anion (2). The radicals derived from these rings are named pyrryl, furyl and thienyl. The 2-furylmethyl radical is called furfuryl. Compounds in which two pyrrole nuclei are joined by a CH2 group are called dipyrromethanes when the linkage is by a CH group, they are named dipyrromethenes . The 2- (3) and 3-pyrrolenines (4) are isomeric with the pyrroles, but are nonaromatic as the ring conjugation is broken by an. s/r -hybridized carbon atom. 

Unfortunately it was discovered that after a few days in solution, a red discolouration was observed (particularly with compound 19). Mass spectrometry revealed the loss of two mass units, consistent with oxidation of the product to the dipyrromethene. 

Zhang X, Xiao Y, Qian X. Highly efficient energy transfer in the light harvesting system composed of three kinds of boron-dipyrromethene derivatives. Org Letters 2008 10 29-32. 

Dipyrromethenes (dipyrrins) can be deprotonated to form 7t-conjugated biden-tate monoanionic ligands (dipyrrinates) that readily form bis- and tris(dipyrrinato) metal complexes with a variety of transition metal ions. An excellent recent review of these ligands was recently published (79). The overall symmetry of meso-substituted dipyrrins is quite similar to that of the previously discussed acac ligands that were used to create MOFs. Indeed, dipyrrin ligands with donor atoms in the meso position were used in the construction of 1,2, and 3D strucmres. 

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