A Rational Approach to Crosslinking of Coordination Polymers Using the Photochemical [2+2] Cycloaddition Reaction

Summary: Polymerization as a result of photocrosslinking is one of the most versatile technologies for producing organic polymers. This article highlights the formation of crosslinking in metal coordination polymers in the solid‐state using a photochemical approach and its importance in material science. Past and current developments of solid‐state photochemical [2+2] cycloaddition in organic and inorganic systems relevant to the topic are reviewed. The challenges for the coordination chemist in utilising this rational approach to orient the coordination polymers and the scope of this work are delineated.

The CC‐containing molecules can be aligned by metal coordination bonds in such a way as to promote crosslinking by a photochemical [2+2] cycloaddition reaction.     

Metal–Organic Organopolymeric Hybrid Framework by Reversible [2+2] Cycloaddition Reaction

Organic polymers are usually amorphous or possess very low crystallinity. The metal complexes of organic polymeric ligands are also difficult to crystallize by traditional methods because of their poor solubilities and their 3D structures can not be determined by single‐crystal X‐ray crystallography owing to a lack of single crystals. Herein, we report the crystal structure of a 1D Zn^II coordination polymer fused with an organic polymer ligand made in situ by a [2+2] cycloaddition reaction of a six‐fold interpenetrated metal–organic framework. It is also shown that this organic polymer ligand can be depolymerized in a single‐crystal‐to‐single‐crystal (SCSC) fashion by heating. This strategy could potentially be extended to make a range of monocrystalline metal organopolymeric complexes and metal–organic organopolymeric hybrid materials. Such monocrystalline metal complexes of organic polymers have hitherto been inaccessible for materials researchers.     

Fluorine segregation in crystalline materials: structural control and solid-state [2+2] cycloaddition in CF(3)-substituted tetrathiafulvalene derivatives

The well-known influence of long perfluorinated chains on the structures and stability of amphiphilic molecules in liquid crystalline mesophases or mesoscopic micellar arrangements is evaluated here in the realm of crystalline materials based on rigid aromatic molecules bearing only a limited number of CF(3) moieties. Tetrathiafulvalene (TTF) derivatives bearing one or two CF(3) groups, that is, (Z)- and (E)-(CF(3))(2)TTF ((Z)-1, (E)-1), EDT-TTF-CF(3) (2), and EDT-TTF(CF(3))(2) (3) (EDT=ethylenedithio) are prepared from the 1,3-dipolar reaction of methyl 4,4,4-trifluorotetrolate with ethylenetrithiocarbonate. The structures of neutral (Z)-1, (E)-1, 2, and 3 as indicated by single-crystal X-ray diffraction measurements reveal the recurrent formation of layered structures with a strong segregation of the fluorinated moieties and formation of fluorous bilayers, attributed to the amphiphilic character of those TTF derivatives upon CF(3) functionalization, and without need for longer C(n)F(2n+1) (n>1) perfluorinated chains. The short intermolecular distance between outer C==C double bonds observed in the layered structure of (E)-1 allows a solid-state [2+2] photocyclization with formation of chiral dyads incorporating the characteristic cyclobutane ring. These dyads containing two dihydrotetrathiafulvalene moieties facing each other exhibit reversible oxidation to the mixed-valence radical cation state and organize in the solid-state into the same layered structures with fluorous bilayers.     

Gas-phase polar [4+ + 2] cycloaddition with ethyl vinyl ether: a structurally diagnostic ion-molecule reaction for 2-azabutadienyl cations

The intrinsic reactivity of eight gaseous, mass-selected 2-azabutadienyl cations toward polar [4(+) + 2] cycloaddition with ethyl vinyl ether has been investigated by pentaquadrupole mass spectrometric experiments. Cycloaddition occurs readily for all the ions and, with the only exception of those from the N-acyl 2-azabutadienyl cations (N-acyliminium ions), the cycloadducts are found to dissociate readily upon collision activation (CID) both by retro-Diels-Alder reaction and by a characteristic loss of an ethanol (46u) neutral molecule. Ethanol loss from the intact polar [4(+) + 2] cycloadduct functions therefore as a structurally diagnostic test: 72 u neutral gain followed by 46 u neutral loss, i.e., as a combined ion-molecule reaction plus CID 'signature' for N-H, N-alkyl and N-aryl 2-azabutadienyl cations. The two N-acyliminium ions tested are exceptional as they form intact cycloadducts with ethyl vinyl ether which dissociate exclusively by the retro-Diels-Alder pathway.     

Recent advances in "formal" ruthenium-catalyzed [2+2+2] cycloaddition reactions of diynes to alkenes

"Formal" and standard RuII-catalyzed [2+2+2] cycloaddition of 1,6-diynes to alkenes gave bicyclic 1,3-cyclohexadienes in relatively good yields. When terminal 1,6-diynes 1 were used, two isomeric bicyclic 1,3-cyclohexadienes 4 or 6 were obtained, depending on the acyclic or cyclic nature of the alkene partner. When unsymmetrical substituted 1,6-diynes 7 were used, the reaction with acyclic alkenes took place regio- and stereoselectively to afford bicyclic 1,3-cyclohexadienes 8. A cascade process that behaves as a "formal" RuII-catalyzed [2+2+2] cycloaddition explained these results. Initially, a Ru-catalyzed linear coupling of 1,6-diynes 1 and 7 with acyclic alkenes occurs to give open 1,3,5-trienes of type 3, which after a thermal disrotatory 6e(-) pi-electrocyclization led to the final 1,3-cyclohexadienes 4 and 8. When disubstituted 1,6-diyne 10 was used with electron-deficient alkenes, new exo-methylene cyclohexadienes 12 arose from a competitive reaction pathway.     

Bay-area selective thermal [4+2] and [4+4] cycloaddition reactions of triply linked ZnII diporphyrin with o-xylylene

A triply linked ZnII diporphyrin underwent site-selective cycloaddition reactions with thermally generated o-xylylene to provide a triply linked porphyrin-chlorin hybrid and a triply linked chlorin dimer in moderate yields. The former product is a symmetry-allowed [4+2] cycloadduct, while the latter is a symmetry-forbidden [4+4] cycloadduct. Oxidation of the latter product with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) provided a triply linked diporphyrin fused with a benzocyclooctatriene segment. This oxidized product and above [4+2] cycloadduct were structurally characterized by single crystal X-ray diffraction analysis. The observed site-selectivity is considered to arise from the large MO coefficients at the bay-area in the LUMO of the triply linked diporphyrin. The anomalous thermal [4+4] cycloaddition may be ascribed to the highly conjugated and quite perturbed electronic properties of triply linked ZnII diporphyrin.     

Unusual intramolecular [2 + 2] cycloaddition of allyl and vinylidene C=C bonds under mild conditions: a theoretical analysis

A theoretical analysis allows for the rationalization of the recently reported unusual formation under mild conditions of a cyclobutylidene ring from a diastereoselective [2 + 2] intramolecular cycloaddition of two C=C systems. The reaction takes place by heating in dichloromethane the vinylidene complexes [Ru((eta(5),eta(3)-C(9)H(7))[=C=C(R)H][kappa(1)-(P)-PPh(2)(C(3)H(5))](PPh(3))][BF(4)] (R = Ph, p-Me-C(6)H(4)) (1) yielding the bicyclic alkylidene complexes [Ru((eta(5),eta(3)-C(9)H(7))[kappa(2)-(P,C)-(=CC(R)HCH(2)CHCH(2)-PPh(2)](PPh(3))][BF(4)] (2). The proposed mechanism represents an alternative to the classical Woodward-Hoffmann's supra-antara approach.     

Formation of a cyclobutylidene ring: intramolecular [2 + 2] cycloaddition of allyl and vinylidene C=C bonds under mild conditions

Intramolecular [2 + 2] cycloaddition of two C=C bonds in vinylidene complexes [Ru(eta5-C9H7){=C=C(R)H}(PPh3){kappa1-(P)-PPh2(C3H5)][BF4] affords cyclobutylidene complexes [Ru(eta5-C9H7){kappa2-(P,C)-(=CC(R)HCH2CHCH2PPh2)}(PPh3)][BF4], which can be also obtained by reaction of terminal alkynes with [Ru(eta5-C9H7)(PPh3){kappa3-(P,C,C)-PPh2(C3H5)}][PF6]. The reaction proceeds under mild conditions via vinylidene complexes, and the activation parameters were determined by kinetic studies.     

Oligonucleotides containing arylacetylene residues: Synthesis and post-synthetic modification via [3+2] cycloaddition

The synthesis of a phosphoramidite reagent for 5′-modification of oligonucleotides by introducing an arylacetylene residue has been described. Using the reaction with 3-(perylen-3-yl)propyl azide as an example, it was shown that the acetylene derivatives of oligonucleotides synthesized using this reagent undergo Cu^I-catalyzed [3+2] dipolar cycloaddition. Fluorescent conjugates were obtained in high yields and characterized by mass spectra. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1220–1226, July, 2006.     

A Step‐by‐Step Assembly of a 3D Coordination Polymer in the Solid‐State by Desolvation and [2+2] Cycloaddition Reactions

Two solid‐state structural transformations that occur in a stepwise and a controlled manner are described. A combination of desolvation and cycloaddition reactions has been employed to synthesise a 3D coordination polymer (CP) from 1D CP [Cd(bdc)(4‐spy)₂(H₂O)]?2 H₂O?2 DMF (bdc=1,4‐benzenedicarboxylate, 4‐spy=4‐styrylpyridine) presumably via a 2D layered structure, [Cd₂(bdc)₂(4‐spy)₄]. In the absence of single crystals to follow the course of the photocycloaddition reaction, thermogravimetry, XAFS and NOESY NMR experiments were used to propose the formation of layered and pillared layered structures. Further, the present strategy enables us to synthesise new multidimensional architectures that are otherwise inaccessible by the self‐assembly process.