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1 Wiadomości Chemiczne

1 2008

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Synteza koroli

Koszarna B., Gryko D.T.

Wiadomości Chemiczne

2008

[Z] 62, 3-4

165-185

Corroles, one carbon shorter analogues of porphyrins emerged a few years ago as an independent research area (Fig. 1) [1-3]. Their coordination chemistry, photophysics, synthesis, chemical transformations, electrochemistry and other properties have recently been studied in great detail [7-11]. Discovered in 1964 [15], for many years they have been mainly synthesized via a,c-biladienes cyclization [19-20]. Multistep a,c-biladienes preparation discouraged broader range of chemists from studying corroles (Scheme 2). Since initial reports by Gross [17, 18] and Paolesse [16] revealing one-pot syntheses of meso-substituted corroles from aldehydes and pyrrole, this reaction has been subjected to many refining studies (Scheme 4, 5; Table 2) [23-28]. As a result yields have been improved to ~ 15%. The one-pot synthesis of meso-substitued A3-corroles from aldehydes and pyrroles consists of two independent steps. The first step is an acid-mediated electrophilic substitution to yield a mixture of various aldehyde-pyrrole oligocondensates including bilane (tetrapyrrane) - the direct precursor of corrole (Scheme 6). The second step is the oxidative ring-closure. Maximizing bilane formation while minimizing the formation of dipyrromethanes, tripyrrane and higher oligocondensate is a difficult task due to the similar reactivity of all these compounds [30-32]. Recently new and efficient conditions for the synthesis of meso-substituted corroles have been developed [33]. The first step, namely the reaction of aldehydes with pyrrole, was carried out in a water-methanol mixture in the presence of HCl. A relatively narrow distribution of aldehyde-pyrrole oligocondensates was controlled by their solubility in the reaction medium. After thorough optimization of various reaction parameters high yields of bilanes were obtained. As a result, many A3-corroles were obtained in the highest yield (~25-30%) reported to date. Corroles bearing two various substituents at meso positions were synthesized for the first time from dipyrromethanes and aldehydes (Scheme 10) [35]. This method continues to prevail in the literature [36-38]. Performance of this reaction in H2O/MeOH/HCl mixtures allows to obtain trans-A2B-corroles in yields up to 56% (Scheme 11) [33]. Last developments in the chemistry of corroles make these compounds more accessible than respective porphyrins.