Preparation and structure of a tubular addition polymer: a true synthetic nanotube.

The structure of a synthetic nanotube prepared by the solid-state polymerization of a stacked column of diacetylene-based macrocycles has been determined. A polyether macrocycle monomer with two parallel diacetylene functionalities was prepared. Its crystal structure revealed that the compound crystallizes with structural parameters suitable for topochemical polymerization. Slow annealing of a single crystal for 35 days brought about a single-crystal-to-single-crystal polymerization resulting in the first experimentally determined structure of a tubular addition polymer.

Weak interactions dominating the supramolecular self-assembly in a salt: a designed single-crystal-to-single-crystal topochemical polymerization of a terminal aryldiacetylene.

Single-crystal-to-single-crystal (SCSC) topochemical polymerizations of diacetylenes can yield nearly defect-free conjugated polymer crystals unattainable by other methods. Aryl-substituted diacetylenes with their potentially greater conjugation have been targeted for years, but until now no one has reported a SCSC polymerization of any aryl-substituted diacetylene. This is presumably due to the rigidity of such diaryl-substituted monomers as well as the lack of control over the supramolecular structure. To address this problem, the polymerization of a terminal phenyldiacetylene was targeted. It was assumed that a terminal diacetylene should demonstrate greater flexibility in the solid state. To establish the necessary (approximately 4.9 A) repeat distance, commensurate with the repeat distance in the polymer, a host-guest system was designed. The chosen diacetylene guest, the amine DABzNH(2), was to be crystallized with the oxalamide dicarboxylic acid host, H(2)og. The plan required a segregation of the hydrogen bonds, amide-amide hydrogen bonds to establish the 4.9 A spacing, and the carboxylate to ammonium ion hydrogen bonds to organize the guest. Prior to carrying out the diacetylene synthesis a series of model salts were studied. Consistent with the hydrophobic effect it was found that amines with large "greasy" substituents assembled according to the design. Once the model studies established that weak interactions could dominate the supramolecular structure of a salt, the actual design was put to the test. The targeted guest, DABzNH(2), was synthesized and crystals of the host-guest salt (DABzNH(3))(2)og were prepared. The resulting crystal structure was in complete accordance with the design. A SCSC polymerization was achieved by a slow annealing treatment lasting about three months. The crystal structure of the resulting polymer not only confirmed the first example of a poly(aryldiacetylene) single crystal, it also revealed an unexpected reaction pathway that shows a major movement involving the rigid aromatic substituent.

Single-crystal-to-single-crystal topochemical polymerizations by design.

The polymerization of simple conjugated dienes has long been of interest: polydienes occur throughout Nature, and polyisoprene and its analogues form the basis of entire industries. In contrast, the polymers of similar small conjugated compounds, diacetylenes, trienes, and triacetylenes, are either unknown or laboratory curiosities. For 40 years, the only viable synthetic method for the 1,4-polymerization of a diacetylene was a topochemical polymerization in a condensed phase. But such an approach is hit or miss: if the diacetylene monomers have a solid-state structure preorganized at distances matching the repeat distance in the final polymer, then thermal or photochemical energy can bring about the polymerization. However, most monomers lack the proper structural parameters and simply do not react. As discussed in this Account, we have developed a supramolecular host-guest strategy that imposes the necessary structural parameters upon a diacetylene monomer that otherwise does not polymerize. We apply this strategy in the synthesis of new types of conjugated polymers made from diacetylenes, triacetylenes, and trienes. To implement the host-guest strategy, we chose a host that would self-assemble into a supramolecular structure with the requisite intermolecular spacing. For diacetylenes, the ideal spacing is 4.9 A, and the oxalamides, which routinely crystallize with a spacing of 5 A, make ideal host molecules. We chose specific oxalamide host substituents that bind to the diacetylene guest molecule, typically through hydrogen bonding. We have focused upon the single-crystal-to-single-crystal polymerizations, allowing us to obtain and characterize the polymers in perfect crystalline form and to define and better understand the reaction trajectories. We have prepared several new classes of polydiacetylenes using this strategy, including the first terminal polydiacetylenes and an aryl-substituted diacetylene. Interestingly, to prepare poly(diiododiacetylene), we used halogen bonds to bind the host and guest. The simplest polydiacetylene known, poly(diiododiacetylene), lacks the side chains that complicate the structures of similar previous polymers. Future studies should provide insights into the role of such side chains in conjugated materials. We further demonstrated the strength of the host-guest strategy by moving from the polydiacetylenes to the polytriacetylenes. Although the structural requirements for a triacetylene polymerization had been stated decades ago, no one had ever found a triacetylene with the requisite spacing of 7.4 A. We designed a series of pyridine-substituted vinylogous amide hosts to achieve this spacing. Cocrystallization of these host molecules with a triacetylene dicarboxylic acid gave us the desired structure. Using thermal annealing, we completed the synthesis of the triacetylene polymer.

Pressure-induced polymerization of diiodobutadiyne in assembled cocrystals.

Diiodobutadiyne forms cocrystals with bis(pyridyl)oxalamides in which the diyne alignment is near the ideal parameters for topochemical polymerization to the ordered conjugated polymer, poly(diiododiacetylene) (PIDA). Nonetheless, previous efforts to induce polymerization in these samples via heat or irradiation were unsuccessful. We report here the successful ordered polymerization of diiodobutadiyne in these cocrystals, by subjecting them to high external pressure (0.3-10 GPa). At the lower end of the pressure range, the samples contain primarily monomer, as demonstrated by X-ray diffraction studies, but some polymerization does occur, leading to a pronounced color change from colorless to blue and to the development of intense Raman peaks at 962, 1394, and 2055 cm-1, corresponding to the poly(diacetylene). At higher pressures, the samples turn black and contain primarily polymer, as determined by solid-state NMR and Raman spectroscopy. Both density functional theory calculations (B3LYP/LanL2DZ) and comparisons to authentic samples of PIDA have confirmed the data analysis.

Designed cocrystals based on the pyridine-iodoalkyne halogen bond.

Diiodobutadiyne (1) and diiodohexatriyne (2) form cocrystals with bispyridyl oxalamides and ureas, based on the halogen bond between the alkynyl iodine and pyridine nitrogen. In each cocrystal, the oxalamide or urea host forms one-dimensional hydrogen-bonded networks, aligning the diiodopolyyne for potential topochemical polymerization with a repeat distance matching the host repeat.

Single-crystal-to-single-crystal topochemical polymerizations of a terminal diacetylene: two remarkable transformations give the same conjugated polymer.

Host-guest chemistry was used to prepare a cocrystal of a dipyridyl substituted oxalamide host and a resorcinol derivative of a terminal diacetylene. X-ray crystallography revealed that the molecules assemble into a triple-helix with the diacetylene functionalities aligned in the center of the helix. Upon heating, the diacetylenes polymerize to give the corresponding polymer. In a second experiment, the X-ray structure of the crystalline hydrate of the same diacetylene also showed a suitable alignment of the diacetylene functionalities. These crystals polymerize at only 50 degrees C with the C1 carbon end of the diacetylene undergoing an unprecedented 2.4 A inward swinging movement. This results in a remarkable 9% increase in crystal density. These are the first structurally characterized polymerizations of any terminal diacetylene. A detailed comparison of the two sets of structural changes offers an interesting insight into the precise trajectories of polymerization reactions.

The topochemical 1,6-polymerization of a triene.

The first topochemical 1,6-polymerization of a triene has been observed. The required supramolecular structure for this polymerization was achieved by the pi-pi stacking of the isonicotinate functionality. The crystal environment of this polymerization reaction controlled both the molecular and supramolecular structure of the polymer and allowed its structure to be determined by single-crystal X-ray diffraction.

Intramolecular Diels-Alder Reaction of 2-Cyano-1-aza-1,3-butadienes.

It has been demonstrated that a 2-cyano substituent is sufficient to activate 1-aza-1,3-butadienes with respect to the intramolecular Diels-Alder reaction. This reaction is successful for the preparation of the indolizidine and quinolizidine ring systems. The stereochemistry of the isomeric products was assigned by a careful analysis of their NMR spectral data.

Intramolecular Diels-Alder Reaction of N-Alkyl-2-cyano-1-azadienes: A Study of the Eschenmoser Cycloreversion of Dihydrooxazines as a Route to N-Alkyl-2-cyano-1-azadienes.

In connection with the development of the intramolecular Diels-Alder reaction (IMDA) of 1-azadienes, the 5,6-dihydro-4H-1,2-oxazine 12has been evaluated as a synthon equivalent of the 2-cyano-1-azadiene system. It was found that the dihydrooxazonium salt 27, generated in situ from the cyclic hydroxamic acid derivative 26, is converted directly to azadiene 4a via tautomerization to the corresponding enamine and a particularly facile Eschenmoser type cycloreversion process. Conditions were subsequently found for the preparation of synthon 12. N-Alkylation of this intermediate with alkyl bromides in the presence of Ag(+) ion also resulted in direct formation of the 2-cyano-1-azadiene products 38a-dand 4a. Microwave irradiation of a benzene solution of azadiene 4a proved to be a convenient means to effect its IMDA conversion to indolizidine 5a. To avoid decomposition of azadiene 38c, its intramolecular cycloaddition giving 40 (60%) was achieved by flash vacuum thermolysis.

Reactivity of N-Phenyl-1-Aza-2-Cyano-1,3-Butadienes in the Diels-Alder Reaction.

It is found that N-phenyl-2-cyano-1-azadiene 4, prepared via a two-step, one-pot, sequence from acrylanilide, undergoes efficient [4 + 2] cycloaddition with a complete range of electron rich, electron poor, and neutral dienophiles under remarkably mild thermal conditions (90-120 degrees C for 20-48 h). Regiospecific formation of the alpha-cycloadduct wherein the dienophile substituent is alpha to nitrogen is observed for vinyl ethers and styrene, whereas the Diels-Alder reactions with methyl acrylate and methyl vinyl ketone (MVK) produce alpha/beta mixtures in which the alpha-cycloadduct is the major regioisomer (approximately 4-5:1). An essentially identical reaction pattern was observed in the Diels-Alder reaction of N-(p-methoxyphenyl)-2-cyano-1-azadiene 18 and the 4-methyl-substituted azadiene 27. For compound 19 derived from cycloaddition of 18 with ethyl vinyl ether, facile conversion to the dihydropyridine 21 through loss of EtOH on brief acid treatment was also noted. The 2,4-cis-disubstitution pattern confirmed by X-ray diffraction for the major cycloadduct 29 isolated from the reaction of 27 with styrene provides evidence for the endo mode of cycloaddition in the Diels-Alder reaction of N-phenyl(aryl)-2-cyano-1-azadienes. Calculation of the frontier orbital energies and coefficients, as well as the transition state geometries for the [4 + 2] cycloaddition of N-phenyl-2-cyano-1-azadiene 4 with methyl vinyl ether, styrene, and MVK were carried out at the RHF AM1 level (MOPAC, Version 5.0). The FMO treatment indicates that the reaction of 4 with methyl vinyl ether occurs under LUMO(diene) control, whereas in contrast, the corresponding cycloaddition with MVK occurs preferably under HOMO(diene) control. A high degree of asynchronicity is observed in the calculated transition states for reaction of 4 with the three representative dienophiles. In all cases the transition states leading to the alpha-cycloadducts are lower in energy than those giving the beta-products. However, at the AM1 level the exo cycloaddition mode is found to be the preferred, this result contrasting with experimental results for azadiene 27.