Porphyrin blocking group

Figure 32. Porphyrin blocking groups offer the possibility of further supramolecular deri-vatization. Amide-linked rotaxane with porphyrin blocking groups 85, its Zn complex 86, and an unsymmetrically capped rotaxane 87.

The precursor C to the rotaxane E, called prerotaxane, is obtained in one step from a gold porphyrin macrocycle A and a difunctionalized thread B, thanks to the gathering properties of the transition metal (black dot). Construction of the porphyrin blocking groups is the kineticaUy templated key step leading to the metal-complexed rotaxane structure D. The desired rotaxane E is obtained after removd of the metal template from D. 

The second strategy was adopted by Dolphin and coworkers [59] and Baldwin et al. [56, 57]. Here the porphyrin cyclization was achieved during the final step. The synthesis of capped porphyrins (16) was an adaptation of the Rothemund synthesis. All four aldehyde groups destined to form the meso positions are provided from the pyromellitoyl derivative obtained in two steps from salicylaldehyde (Scheme 4). Dolphin adopted another strategy based on the cyclization of a bis(dipyrrylmethane) in which the dipyrrylmethane units are linked at the pyrrolic -positions by a pure hydrocarbon strap. In numerous examples of these compounds the straps contain a great variety of bulky blocking groups (Scheme 5). 

The impetus for this study was threefold. First, we sought to place the Mn capped porphyrin within the general framework of metalloporphyrin electrochemistry as recently reviewed by Kadish . Second, we hoped to discover and exploit the unique reactivity of the Mn capped porphyrin afforded by the presence of the nondisplaceable steric blocking group on one side of the macrocycle. Third, we wished to explore the chemistry of high valent Mn capped porphyrin. 

Functionalization of porphyrins via Wittig reactions using intermediates with CH2PPh3Cl groups as building blocks 98SL1297. 

As just mentioned, phosphorus porphyrins have unique photochemical properties. Their photophysics is also interesting. Emitter-quencher assemblies based on porphyrin building blocks have attracted attention due to their potential to serve as models in photosynthetic research (see [90] for an example) or for the development of photoswitches that could be used for the fabrication of molecular electronic/optical devices. In this context, Maiya and coworkers constructed a P(VI) porphyrin system 59b with two switchable azobenzene groups positioned in the apical positions of the pseudo-octahedral phosphorus atom [92]. Photoswitch ability (luminescence on/off) was demonstrated as... 

The meso-phenyl groups ofthe porphyrin building blocks have been omitted for clarity. 

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