Rotaxanes with porphyrin stoppers

The precursors used and the synthetic route followed to prepare a bis-copper(l)-complexed [3]-rotaxane fitted with porphyrin stoppers are shown in Figures 52 and... 

The synthesis of [3]- (figuratively shown as 7) and a [5]rotaxane (8) with one central and two terminal porphyrins in the open configuration has been reported <96AG(E)906> also a rotaxane with two Ru(terpy>2 stoppers has appeared <96CC1915>. A pseudorotaxane comprised of a macroring of 2,9-diphenyl-1,10-phenanthroline unit and a molecular string... 

Fig. 7 Principle oftransition metal-templated construction of a [2]-rotaxane with two different porphyrin stoppers. The white diamond is a zinc porphyrin, and the hatched diamond is a gold porphyrin. The black disk is Cu(i).

An important question in biological electron transfer is related to through-space and through-bond processes.74 Whereas through-bond processes were studied with [2]-rotaxanes 102 and 107, [2]-rotaxane 111 of Figure 2.38 was synthesized with the purpose of addressing the through-space question in a novel approach.75,76 In such a rotaxane the donor (Zn porphyrin stoppers) and the acceptor (Au porphyrin appended to the macrocycle) components are maintained in the same molecule by mechanical bonds only. It is therefore... 

Figure 2.38. Copper(I)-templated synthesis of a Zn(II)/Zn(II) bis-porphyrin-stoppered [2]-rotax-ane with Au(III) porphyrin-containing ring as its Cu(I), Ag(I), and Li(I) complexes (110,112, and 113, respectively), and as the free [2]-rotaxane 111.

Figure 43 Principle of transition metal-templated construction of a [2]-rotaxane with two different porphyrinic stoppers (same conventions as in Figure 42). (i) Macrocycle (A) is threaded onto chelate (B), end-blocked by a porphyrin at one extremity and fiaictionalized with a reactive group X, which is a precursor to the second porphyrin stopper, affording prerotaxane (C) (ii) construction of the second porphyrin, leading to metal-complexed [2]-rotaxane (D).

Removal of the Cu" template cation of compound (135) with KCN produced [2]-rotaxane (138) (Figure 45). As observed in some other cases, the demetallation reaction is accompanied with a pirouetting of the macrocycle placing its dpp chelate outside the cleft formed by the porphyrinic stoppers [126, 132]. 

Figure 51 Principle of transition metal-templated synthesis of a [3]-rotaxane, from two chelating macrocycles (B) and a bis-chelate-containing molecular thread (A) functionalized with reactive end groups X (same conventions as in Figure 43). (ii) Threading step, affording prerotaxane (C) construction of the porphyrin stoppers providing copper(I)-complexed [3]-rotaxane (D).

To test the influence of the metal cation incorporated in the porphyrin stoppers on the outcome of the demetalation reaction, (Cu2)-[3]rotaxane containing a Zn(II)- and a Au(III)-porphyrin as stoppers was reacted with a slight excess of KCN in a CH2CI2/H2O mixture. Unexpectedly, total removal of the Cu(l) templating ions took place, leaving the free [3]rotaxane 26" as product. It was isolated in 59% yield after chromatography. 

The systems studied by Schuster and Guldi are reported in Fig. 22 56 and 57" are analogous to rotaxanes 33 and 40 with two zinc(II) porphyrins as stoppers, a Cu(I)(dpp)2 as the spacer and a C6o as the electron acceptor are appended to the macrocycle or included in it. SS is a catenane derived from 56 upon axial binding by a bidentate ligand to the zinc ions of the porphyrin stoppers. Rotaxanes 59" and 60 differ from 56 and 57" in having two 50 electron acceptors as stoppers and a zinc porphyrin electron donor appended to the macrocycle and differ from each other in the distance between the electron donor and the copper complex, which has been increased by insertion of a phenylamido group. 

Figure 72. Sequence of reactions leading to [2]-rotaxane 154. Prerotaxane 129 is used as a dialdehyde in the condensation reaction with aldehyde 130 and dipyrrylmelhane 131, which affords Cu(l)-complexed [2]-rotaxane 132. After metallation of the porphyrin stoppers with Zn(ll), to afford 133, the template metal is removed with KCN, affording the free [2]-rotaxane 134.

Another recent example using PET reaction-based molecular shuttle has also been designed with a CBPQT-based rotaxane, with the thread composed of a porphyrin (Por) as the photoactive unit, Ceo as the electron acceptor, tetrathiafiilvalene (TTF) and dioxynaphthalene as the two tt-electron-donating stations, and substituted 2,6-diisopropylbenzene as the stopper (Figure 36). ° In... 

Molecular systems in which two porphyrins are held apart by a covalently linked spacer function give rise to bis-porphyrins with flexible or constrained geometries. Rotaxane architectures incorporating similar or dissimilar porphyrins as stoppers have been widely developed in our group, with the goal of building such systems capable of allowing modulation of electron transfer. 

The Cu(I)-complexed [2]-rotaxane 102 of Figure 2.36, containing Zn(II) and Au(III) porphyrins as stoppers, was designed and synthesized with the purpose of mimicking the ET processes occurring within the SP/BCh/BPh trichromophoric fragment described above. 

The precursors and the synthetic route leading to rotaxane 102 are represented in Figure 2.37.57b First macrocycle 58 was threaded onto the presynthesized Au(III) porphyrin-substituted phenanthroline 103, a semidumbbell molecule, in the presence of Cu(I), to afford prerotaxane 104 quantitatively. The second stopper (and functional end cap) was installed by the meso-porphyrin construction method. Thus reaction of 104 with 4,4/-dimethyl-3,3/-diethyl-2,2/-dipyrrylmethane 105 and 3,5-di-/m-bulyI benzaldehyde 84, followed by oxidation of the intermediate porphyrinogen with chloranil 85, gave the free-base Cu(I)-complexed [2]-rotaxane 106 in 25% yield. After metal-lation with Zn(0Ac)2 2H20 and exhaustive purification, Cu(I) complex 102 was obtained. It was subsequently demetallated with KCN, to afford the free [2]-rotaxane 107 in quantitative yield. 

The formation of [3]-rotaxane (136) does not involve any participation of the benzaldehyde derivative (132). As shown schematically in Figure 54, two molecules of prerotaxane (A) condense with two molecules of dipyrrylmethane in structure (C) the central stopper is a free-base porphyrin, which is shared by each threaded macrocycle, and the peripheral stoppers are gold(lll) porphyrins. The formation of [5]-rotaxane (149) is even more remarkable, because three porphyrins are formed simultaneously, involving the condensation in one molecule of 10 molecular precursors As before, the formation of the central porphyrin in (D) from two molecules of (B) does not involve the participation of 3,5-di-tert-butylbenzaldehyde... 

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