Other 5-membered Heterocyclic

Other 5-membered Heterocyclic Derivatives. Several other classes of 5-membered heterocyclic derivatives exist Most are polymerized via a step-growth mechanism. A dibenzo fused derivative of polythiophene (figure 15), which is also partially formed upon doping of poly(phenylene sulfide), can also be synthesized by reaction of dibromo benzothiophene and sodium sulfide. Carbazoles have beoi polymerized by in situ coupling of their Grignards as well by as coupling of diiodocarbazole in molten iodine Thin films of carbazole polymers have been prepared by 

These examples involve pendant groups that interact with each other. There are, of course, several examples of polymers containing pendant chromophores, particularly as liquid crystal mesogens, that have been investigated as nonlinear optical matoials. In these materials, the electronic interaction between chromophores does not generally contribute to the material s nonlinear optical susceptibility.  

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This reasoning is supported by the observation that the reaction of 2-nitrofuran with trichloromethyl carbanion proceeds in both 3- and 5-positions, because in this case the base-induced p-elimination of HCl from the intermediate o adducts is a fast process [54]. Also the VNS reactions of nitro derivatives of other 5-membered heterocycles, imidazoles [20, 56] and thiazoles [34], with a variety of a-halogeno carbanions have been shown to proceed efficiently. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

The conversion of furans by oxidative acetylation or methoxylation to 2,5-diacetoxy- or 2,5-dimethoxy-2,5-dihydrofurans respectively, and their subsequent hydrogenation to the corresponding tetrahydrofurans, provides a useful source of protected 1,4-dicarbonyl compounds capable of conversion inter alia into the other five-membered heterocycles [Pg.142]

Furans, thiophenes and pyrroles have all been obtained by addition of alkynic dienophiles to a variety of other five-membered heterocycles, as illustrated in Scheme 104. As the alkynic moiety provides carbons 3 and 4 of the resulting heterocycle, this synthetic approach provides an attractive way of introducing carbonyl containing substituents at these positions, especially as many of the heterocyclic substrates are readily generated. Such reactions do... 

In Table 29 the u(C=0) and other characteristic bands are given for some saturated five-membered heterocycles, and compared with the corresponding absorption frequencies for cyclopentanone. Adjacent NH groups and sulfur atoms have the expected bathochromic effect on r (C=0), whereas an adjacent oxygen atom acts in the reverse direction. The CHa vibrations of cyclopentanone are repeated to a considerable extent in the heterocyclic analogs. 

A Hiickel model used for calculating aromaticity indicated that the isoxazole nucleus is considerably less aromatic than other five-membered heterocycles, including oxazole and furan. SCF calculations predicted that isoxazole is similar to oxazole. Experimental findings are somewhat difficult to correlate with calculations (79AHC(25)147). PRDDO calculations were compared with ab initio values and good agreement for the MO values was reported... 

Prototropic tautomerism of isoxazole derivatives has been well studied over a number of years and has recently been reviewed in context with similar behavior in other five-membered heterocycles (70C134, 76AHC(Sl)l, 79AHC(25)147, p. 202). Several generalizations are summarized below. 

SYNTHESIS FROM OTHER HETEROCYCLIC COMPOUNDS 4.17.10.1 From Six-membered Heterocycles... 

The proton affinities (gas phase) of thiirane and other three-membered heterocycles have been determined azirane (902.5), thiirane (819.2), phosphirane (815.0), oxirane (793.3 kJ moF ) (80JA5151). Increasing s character in the lone electron pairs decreases proton affinities. Data derived from NMR chemical shifts in chloroform indicate the order of decreasing basicity is azirane > oxirane > thiirane (73CR(B)(276)335). The base strengths of four-, five- and six-membered cyclic sulfides are greater than that of thiirane. 

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