2-Styrylpyridine

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... 

Such reactions are, however, not unexpected, as the reactivity of the methyl groups in 2- and 4-picolines is well documented, as exemplified by their condensation with aromatic aldehydes to give trans 2- and 4-styrylpyridines.82... 

Both 2- and 4-(26) yielded the saturated hydrodimers using Baizer s procedure.54 Utley and Caddy confirmed the macroscale synthesis of the hydrodimer of 4-(26).55 4-Styrylpyridine gave the isomeric meso and dl hydrodimers (28) as did the p-tolyl and p-anisyl analogs of styrylpyridine.56... 

A case in question is the ac electrolysis of the complex Re(trans-SP)2(C0)3C1 (SP = 4-styrylpyridine) (59). It was shown before that the coordinated ligand SP undergoes a photochemical trans/cis isomerization (60). The reactive excited state is the lowest mr intraligand (IL) state, which is not luminescent. The ac electrolysis leads also to the trans/cis isomerization of the coordinated ligand (59). Hence it is a reasonable assumption that the electrolysis proceeds via the generation of the ttu IL state ... 

The /nms- as-4-styrylpyridine isomerization is up-hill energetically by 2 kcal/mole,123 and, therefore, such an isomerization represents movement away from the thermodynamic ratio of these two isomers. Photolysis at 436 nm of ClRe(CO)3(fnms-4-styryl-pyridine)2 was found to ultimately yield completely (sequentially ) ClRe(CO)3 (c -4-styrylpyridine)2. Further, the coordinated cfs-4-styrylpyridine could be exchanged thermally with excess tazns-4-styrylpyridine in the medium, and the sequence of events in reactions (36)—(39) was proposed123 to account for the photoinduced iso-... 

The fact that the intraligand transition is lowest in energy in ClRe(CO)3-(fnms-4-styrylpyridine)2 also allows a direct assessment of the effect of the coordination on the decay properties of the intraligand excited state. Data for both the photoisomerization by direct irradiation, Table 19, and the triplet sensitized isomerization, Table 20, of ClRe(CO)3(fra s-4-styrylpyridine)2 and frans-styrylpyridine... 

Styrylpyridine photochemistry has been important in one other system. The photoprocesses in the complexes W(CO)sX (X = pyridine, 2-styrylpyridine, and 4-styrylpyridine) have been investigated.129 Unlike the XRe(CO)3(taz s-styryl-pyridine)2 and Ru(II)-styrylpyridine complexes having lowest IL and CT states, respectively, the W(CO)5X complexes have lowest LF excited states with only a small contribution from W - pyridyl CT. The one-electron diagram for low-spin d6, Cnv complexes shown in Scheme 7 is appropriate here. Both photosubstitution and photoisomerization reactions are found for the W(CO)5X complexes and some quantum yield data are found in Table 23.129 The data show that substitution efficiencies for... 

Styrylpyridine-3-carboxamide (17) underwent cyclization to 3-phenyl-3,4-dihydro-2,7-naphthyridin-1 (2//)-one (18) (P2O5/H3PO4, heat 90%), easily dehydrogenated to 3-phenyl-2,7-naphthyridin-1 (2//)-one (Pd/C, p-cymene, reflux 91%).188,cf"9... 

Mitsui et al. studied the selectivity of transition metal catalysts in the hydrogenation of various pyridine V-oxides in ethanol at room temperature and atmospheric pressure with Raney Ni, Pd-C, Pt-C, Rh-C, and Ru-C, and at elevated temperatures and pressures with Raney Co, Raney Cu, and copper-chromium oxide.229 The IV-oxide groups of 4-benzyloxy- and 4-styrylpyridines were hydrogenated in preference to the other functional groups over Raney Cu, copper-chromium oxide, Raney Co, and Ru-C, similarly as over Raney Ni. Pt-C and Rh-C also showed similar selectivity in... 

Shi F, Zhang G, Zhou D et al (2009) An unexpected green and facile synthesis of 2, 6-diaryl-4-styrylpyridines via multi-component reactions in microwave-assisted solvent-free conditions. Chin J Chem 27 1569-1574... 

In a particularly interesting development, Kunkely et al. have described the first examples of the generation of electronic excited-state species by a.c. electrolysis, using Re[(trans-SP)2(CO)3Cl] in MeCN-Bu4NC104, where SP=4-styrylpyridine. Substitution of CO in Cr(CO)6 by MeCN was achieved in like manner. We can expect to hear more applications of this new approach. 

On the whole, fra/w-cto-isomerizations store less energy than valence isomerizations, but may be easier to sensitize to visible light 436 nm light, for example, absorbed by XRe(CO)8(/raw,s-3- or 4-styrylpyridine)2, where X = Cl, Br, or I, photoassists the frans-cfa-isomerization of excess styrylpyridine, which does not itself absorb at that wavelength. The mechanism is as shown in Scheme 1,... 

Methyl-4-styrylpyridine (S -Methyl-y-stilbazole, -va.-tolyl-2-y-pyridyle(hylene)... 

Methyl 4-styrylpyridine (i Methyl HbcaoU, l- 4alyl-2-y-pyridykthykne). 

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