Styrylpyridines

Pyridines can be functionalized by a range of metal complexes, notably ruthenium analogs. Ruthenium vinylidene complexes promote the reaction of pyridines with silylalkynes in both a regio- and stereoselective manner, affording 2-styrylpyridines (Equation (78)). 

During the course of this study, styrylpyridine based polyesters and polycarbonates as well as their related model compounds were synthesized and characterized by TGA, 01, and UV irradiation to determine the effect of the structure on thermal and UV stability. 

The preparation of hydroxy-terminated styrylpyridine has been described previously (18). The model esters and carbonate were prepared by an interfacial reaction. The scheme for the reaction was as follows ... 

Flammability Characterization of Styrylpyridine Polyesters and Polycarbonates and Their Related Model Compounds... 

The styrylpyridine based polycarbonates showed similar char yield and oxygen indices (Fig. 2 and Table II) to that of the polyesters. 

Photo-Fries Rearrangement of Styrylpyridine Based Ester and Carbonate-UV-Spectroscopic Studies... 

The ultraviolet (UV) rearrangement of polyarylesters and their related model compounds have been previously studied (20,21). The chemical changes which occur during the UV irradiation of styrylpyridine based ester and carbonate were investigated. The UV spectra of the p-VPPB and p,p -BVPDPC in 1,2-dichloroethane were monitored during the irradiation (Fig. 3 and 4). The maximum absorption for unirradiated p-VPPB was at 319 nm. After UV irradiation, the maximum peak shifted from 319 nm to 350 nm and the observed increased absorption in the... 

In the case of p,p -BVPDPC, there was no increased absorption at 270 nm probably due to steric effects associated with a large molecule (ponderal effect) and to increased aplanarity of the styrylpyridine ring to suppress the dimerization and isomerization of the double bond preventing cis-isomer formation. 

An increase of char yield is generally reflected as an improvement in oxygen index. In the styrylpyridine based polyesters and polycarbonate an intermolecular thermally induced Diels-Alder reaction has occurred through the double bond, this increased the char yield and decreased the flammability. The Fries rearrangement, as well as dimerization and isomerization, occurred simultaneously during the UV irradiation of p-VPPB, but no dimerization or isomerization occurred for p,p -BVPDPC, probably due to steric effects. 

Isomerization of unsaturated nitrogen-containing organic compounds, containing C=C double bonds, has been investigated less intensely than the isomerization of other related unsaturated compounds. Photoisomerization has been reported upon irradiation of a benzene solution of aqueous solution of its hydrochloride, and of the methiodide salt under nitrogen atmosphere. The corresponding cis-isomers are formed.308,309... 

Ultraviolet irradiation of the methiodide or hydrochloride of 2-styryl-pyridine (XCIII) in the solid state results in transformation of the trans-isomers to the corresponding dimers (XCVa, b) on the other hand irradiation in benzene solution gives both isomerization and dimerization.309 Dimer (XCVa) was produced in low yield on irradiation of XCIII in the powdered form in the presence of air.308 This is in contrast to the reported stability of XCIII toward photodimerization.109 Similar dimerizations have been reported in the case of 2,4-dichloro-3-cyano-6-styrylpyridine,164 2-styrylquinaldine (XCV) and frans-4,4 -diguanyl-stilbene bis(2-hydroxyethane sulfonate) (stilbamidine) (XCIV).82,83... 

The second stable adduct 60 of 2-styrylpyridine and dimethyl acetylenedicarboxylate was shown to be 2a,3,4,5-tetramethoxy-... 

C-Styrylpyridines undergo photocyclization to give azaphenanthrenes, and Y-styrylpyridinium cation forms an azoniaophenanthrene (523 — 524). 

Numerous condensation polymers such as polyamides (75MI11103) containing the pyridine nucleus in the backbone have been prepared from the corresponding pyridine diesters or diacid chlorides. The Knoevenagel condensation (Scheme 32) has provided another way of incorporating the pyridine nucleus into a condensation framework. Poly(styrylpyridines) (116) have been found to exhibit exceptional flame resistance and are useful in reinforced composites (79USP4163740). 

By analogy to the synthesis of poly(styrylpyridines) (see Section 1.11.2.3.1), poly(styryl-pyrimidines) (139) have been prepared (Scheme 40). The polymers were stated to have only trans double bonds and exhibited good thermal stabilities (71RRC1877). 

One or both of the carbocyclic aromatic rings in stilbenes or azobenzenes can be replaced by heterocycles. Examples include 2-styrylpyridine methiodide (138) and 4,4 -azopyridine (139) (53JCS1281). 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

Increasingly, this process has been adapted to the synthesis of heterocyclic systems. One or both of the phenyl substituents in stilbene can be replaced by a heteroaromatic system, and some of the more important of these cyclizations are recorded in Table I. The formation of benz[A]isoquinoline by photolysis of 4-styrylpyridine in cyclohexane is accompanied by the formation of 1- and 3-cyclo-hexylbenz[A]isoquinoline.346... 

The treatment of 3-cyano-4-styrylpyridine (109) with polyphos-phoric acid gave substance 110. This cyclization represents a rare example of an intramolecular Ritter reaction.128 This derivative was converted into compound 111 by the steps outlined. A similar condensation involving the treatment of compound 112 with polyphosphoric acid afforded l,8-dihydroxy-3,6-diphenyl-2,7-naph-thyridine (113) in 65% yield.129... 

A large number of t-1 derivatives are known to undergo photodimerization (56) however, quantitative details and cycloadduct stereochemistry are available in only a limited number of cases. Stilbenes substituted with electron donating ring substituents are reported to dimerize more efficiently than t-1 (57) styrylpyridines (58) and styrylthiophenes (59) less efficiently. 

The formation of styryl pyridines by condensation of anils with picolines was first reported by Crippa and Maffei.79 80 These workers found that 2-picoIine ethiodide (180) reacted with benzalaniline in methanol in the presence of piperidine to give 2-styrylpyridine ethiodide (181). However, it was also reported that 3-picoline ethiodide and 2-picoline itself failed to react under these conditions. 

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