New Porphyrin 4π-Cycloaddition Reagents and Their Use in the Preparation of Porphyrin-(Rigid Spacer)- 1,10-Phenanthrolines in Which Geometric “Tuning” of the Chromophores Is a Feature

Establishing a library of porphyrin building blocks for superstructured assemblies: porphyrin dienes and dienophiles for cycloaddition reactions

The synthesis and utility of a series of porphyrins with (masked) diene and dienophile functionality are described. The key porphyrin diene is synthesised from a sulfolenopyrrole by a 3+1 strategy. A range of Diels-Alder cycloadducts is readily accessed from the diene by mild thermal extrusion of sulfur dioxide from the sulfolenoporphyrin, which produces the reactive porphodimethylidene. Each of these cycloadducts is fused to the porphyrin nucleus through a cyclohexene ring thus retaining some conformational flexibility in the resultant structures. The structures can be rigidified by mild oxidation to the corresponding benzo-derivatives. Diels-Alder reaction of the porphyrin 1,3-diene resulting from the sulfolenoporphyrin with norbornadiene produces the norbornene derivative, which can serve as a dienophile or dipolarophile in subsequent cycloaddition reactions. Nevertheless, a preferred route to this structure is through a corresponding 1+3 route, where the norbornene component is part of the tripyrrane. Extension of the synthetic protocols allows ready access to a “mixed function” porphyrin, containing both diene and dienophile components. Likewise, the synthesis of a bis-norbornene porphyrin is described. A collection of each of these reactive components is the basis for a library of building blocks which allows easy and simple entry to a wide variety of complex porphyrin-containing superstructures.

Use of a 9,10-Dihydrofulvalene Pincer Cycloadduct as a Cornerstone for Molecular Architecture

Dimethyl-3,3a,3b,4,6a,7a-hexahydro-3,4,7-metheno-7H-cyclopenta[a]pentalene-7,8-dicarboxylate (3, a 9,10-dihydrofulvalene pincer cycloadduct) was found to be a versatile building block for the synthesis of polynorbornyl scaffolds containing various end functionalities such as porphyrins, pyrimidine, and the phenanthroline ligand in the free and complexed forms. The block is stable under various cycloaddition reaction conditions and has high π-bond reactivity. Molecular modelling using a semi-empirical based method (AM1) shows that incorporation of this diene block into polynorbornyl systems has an important influence on their overall architecture. In particular, incorporation of the block results in a straightening of the curvature normally observed in polynorbornyl molecules to form almost ideally linear polycyclic rigid spacers.

Probing the dimensions of semi-rigid inner functionalised U-shaped bis-porphyrin cavities

The synthesis of a series of open U-shaped bis-porphyrin cavity molecules is described, with bridged bicyclic backbones to confer rigidity, and a pendant substituted aromatic probe unit suspended on the inside of the cavity. The dimensions and flexibility of the bis-zincporphyrin cavity were probed using several different techniques. Initially the molecular ruler concept was employed, using flexible bidentate ligands as guests with a range of possible linear dimensions. Secondly, NMR methods were employed with rigid bidentate ligand guests of fixed lengths, and thirdly diffusion based NMR methods were utilised. The range of inter-porphyrin distances estimated by these methods suggests that these types of open-cavity systems have a surprising degree of flexibility.

Cross [2 + 2] cycloaddition of bicyclic alkenes with alkynes mediated by cobalt complexes: a facile synthesis of cyclobutene derivatives

Bicyclic alkenes 1a-e and 5 undergo [2 + 2] cycloaddition with a variety of alkynes PhC(triple bond)CPh, (TMS)C(triple bond)CH, HC(triple bond)C(CH(3))(2)OH, (TMS)C(triple bond)CCO(2)Et, PhC(triple bond)CCH(3), C(2)H(5)C(triple bond)CC(2)H(5), CH(3)C(triple bond)CC(3)H(7), and CH(3)C(triple bond)CC(2)H(5) in the presence of Co(PPh(3))(2)I(2), PPh(3), and Zn powder in toluene to afford the corresponding exo-cyclobutene derivatives 3a-m, 6, and 8a-g in fair to excellent yields. The yield of this cycloaddition is highly sensitive to the cobalt catalyst, solvent, ligand, and temperature used. A mechanism involving a metallacyclopentene intermediate is proposed to account for this cobalt-catalyzed cyclization.

Incorporation of a molecular hinge into molecular tweezers by using tandem cycloadditions onto 5,6-dimethylenenorbornene

Site-selective 1,3-dipolar coupling at the norbornene pi-bond of 5,6-dimethylenenorbornene 1 yields cycloadducts with an end-fused 1,3-diene system which have been reacted with N=N (or C=C) dienophiles to produce ribbon molecules, in which the internal diazacyclohexene (or cyclohexene) subunits are capable of acting as conformational hinges. Direct coupling of 5,6-dimethylenenorbornene with 1,3,4-oxadiazoles or dual coupling with bis(cyclobutene epoxides) afforded bis(1,3-dienes) that diastereoselectively react with dienophiles to produce new, conformationally mobile, molecular tweezers.

Bis-Porphyrin Racks with Space-Separated Co-Planar Porphyrin Rings

A porphyrin appended norbornenyl building block 8 has been isolated and coupled, using a 1,3-dipolar ACE reaction, to yield bis-porphyrin compounds in which the porphyrin moietes are angled upward relative to the norbornane backbone.

Photochemical cycloaddition reagents for rigidly attaching the 1, 4-dimethoxynaphthalene chromophore to scaffold alkenes

The norbornanecyclobutene epoxides 1a-1c containing a fused 1, 4-dimethoxynaphthalene chromophore have been reacted with cyclobutenes, cyclohexenes, norbornenes, 7-isopropylidenenorbornenes, 7-azanorbornenes, and other cyclic or electron-deficient alkenes at room temperature to form 1:1 adducts in stereoselective 1,3-dipolar cycloaddition reactions; alkynes can also participate in this reaction. The ability to form 2:1 adducts has also been demonstrated, thereby opening up opportunities for preparing functionalized products with large chromophore separations.