Over the past several years work utilising porphyrins has continued in the JKMS group. With a particular emphasis on the areas of Supramolecular chemistry and more recently Dynamic Combinatorial Chemistry (DCC) work involving many things from the development of catalyst to the formation of porphyrin capped -Rotaxanes has been the primary focus. This page wishes to spotlight only a few of the fascinating achievements over the last several years of research concerning porphyrins.
Metalloporphyrins as catalysts in Diels-Alder Reactions
In 1999 Moshe Nakash et. al. prepared new cyclic metalloporphyrin hosts, 6 and 7, obtained via Supramolecular Chemistry, which accelerated 65-fold and 840-fold respectively the reaction of diene 1 and dienophile 2 and also bind the hetero Diels-Adler product 3 very strongly (vide in figure 1). Small single crystals of solvated 6, 7 and the 6.3 complex were grown and their structures were determined. Comparison of these structures reveals that when the Diels-Alder product 3 is bound within the cavity, it induces significant structural changes in 6. This provides the first crystallographic structural evidence that accelerated product formation can be accompanied by substantial host distortion.
In the year 2000, Moshe Nakash et al. presented other interesting metalloporphyrin hosts, which accelerated 12-fold, 250-fold, 260-fold, and 1130-fold. It has been proposed that both host preorganisation as well as host flexibility were key features leading to high acceleration rates and product binding and that it was the delicate balance between the two structural features which leaded to maximum efficiency.
In the following year, Nakash et al. reported the results of the thermodynamic studies (binding constants) between the pyridine 1-3 and the metalloporphyrin hosts 4 and 6.
Several years ago Maxwell Gunter et al. showed that the self-assembly of a variety of porphyrin stoppered rotaxanes was possible by simply mixing of the constituent parts.
While mixing of all the three components of the rotaxanes at room temperature resulted in rapid rotaxane assembly irrespective of the order of addition (thermodynamic control), it was shown that at low temperatures it was possible to 'lock out' or 'lock on' the central thread unit under conditions of kinetic control. These concepts were further extended to the assembly of more complex multi-porphyrin arrays, where the central ring is a naphtocrown-strapped zinc porphyrin.
Bhaskar Maiy et al. in 2001 presented a new porphyrin trimer, which was self-assembled by employing the mutually non-interfering coordination properties of the ruthenium(II) and tin(IV) centers to form a multi-metal array. The photo-chemical metal array properties of the array were also reported.
Joane Hawley et al. described interactions of Sn(IV) porphyrins that bind oxygen-based ligandes and for which the Sn(IV)-O bond slowly exchanges on the NMR timescale.[i] A series of carboxylate complexes were employed to highlight the structural/geometrical features of porphyrin nonomers and cyclic oligomers.
Guido Kaiser et al. in the year 2000 using π-allyl transesterification as the reversible chemistry catalysed by palladium have reported the synthesis of three cyclic porphyrin dimers.[ii] The reaction was templated by a bidentate pyridyl ligand which improved the yield of one of the cyclic products 6-folds. And apparently it was the first time that reversible π-allyl palladium chemistry was applied in supramulecular chemistry.
In 2002, Eugen Stultz et al. reported that a cyclic porphyrin tetramer, consisting of two bis-phosphine substituted zinc(II) porphyrin units and two Rh(III)TPP units could be selected and amplified virtually quantitatively from a dynamic combinatorial library using 4,4'-bipy as a scaffold and using orthogonal binding modes (vide in figure 4).
Amy Kieran et al. reported that disulfide-linked cyclic porphyrin oligomers from dimer to trimer could be selected and amplified virtually quantitatively from a dynamic combinatorial library using bis-thiol substituted zinc(II) porphyrin units with appropriate amine donor templates (see section on disulfide exchange)
Eugen Stultz et al. reported that mixed metallo-porphyrin cages were selected and amplified from dynamic combinatorial libraries by using the appropriate templates. The cages were composed of two bisphosphate substituted zinc(II) and ruthenium(II) porphyrins as ligand acceptors, and were connected through metal-phosphorus coordination.
Interesting porphyrin chemistry
James Redman et al. in the year 2000, reported the synthesis of porphyrins with a disulfide-containing strap as an alternative to thiols for self-assemble of monolayer formation of gold surfaces. The strapped porphyrins add the advantage of greater air stability than their thiol analogues (Figure 5).
In 2001, James Redman et al. presented Rh(III) porphyrins complexes with bridging hydrazine, substituted hydrazine, disulfide and diselenide ligands, which were characterised in solution by proton NMR spectroscopy and solid state X-ray diffraction.
Dynamic chemistry can also be used to prepare thermodynamically-favoured products in the absence of templating:
A new, highly flexible porphyrin dimer was isolated in preparative scale from a dynamic disulfide library; this receptor adjusts to fit guests (e.g. C60, Figure 6) with a wide range of steric requirements.
A naphthyldiimide template amplifies the best fitting library members (with the most effective pi-pi host-guest stacking) from a disulfide library made of a porphyrin and a pi electron-rich aromatic building blocks. New macrocycles incorporating a porphyrin and a pi electron-rich aromatic were amplified, prepared and characterised (Figure 7).