Furan reaction with atomic carbon

To conclude this section two compound specific reactions are worthy of mention. Firstly, the interaction of atomic carbon with benzene has been found to provide 1 albeit in 11% yield and as one of eleven hydrocarbon products46. Secondly, 4,5,7-tri-t-butylisobenzofuran has been reported to undergo furan-cyclopropenylaldehyde photorearrangement rather than An electrocyclization to the Dewar furan47. Steric factors undoubtedly dominate in yielding 2,3,5-tri-/-butylcyclopropabenzene-l-carbaldehyde the product was obtained in solution and characterized by H NMR only. 

For the thiophen mentioned above, all the carbon atoms stemmed from cysteine, carbohydrate apparently not having a role. On the other hand, theoretically, thiophens can be derived from furans simply by reaction with H2S. Belitz and Grosch251 postulate derivation from 2-mercaptoethanal (Strecker aldehyde from cysteine) and acrolein or butenal (aldol condensation product from acetaldehyde). Vemin and Parkanyi216... 

Halofuran and halothiophene derivatives undergo photochemical reactions with arylalkenes and arylalkynes and with benzo[6]furan513,514. With the arylalkenes and aryl-alkynes, heteroarylation takes place at the terminal alkene or alkyne carbon atom, while benzo[6]furan is substituted at position 2. The experimental results are interpreted in terms of solvent-separated or contact radical ion pairs. Iodothiophene and iodofuran derivatives can also be used to synthesize derivatives of benzimidazole by means of photochemical coupling515. The reaction of 5-iodothiophene-2-carboxaldehyde (157) with benzimidazole (158) giving the coupling product 159 is illustrated in equation 131. 

A related reaction occurs on irradiation of a hypochlorite (Scheme 2). The alkoxyl radical can again abstract a hydrogen atom in a l,S-shift, and the final chloro alcohol can be cyclized to form a tetrahydro-furan. Some processes lead directly to the cyclic product For instance, an alcohol with an accessible 8-hydrogen can be directly converted to a tetrahydrofuran on refluxing with lead tetraacetate. In a related reaction, treatment of an alcohol with silver carbonate and bromine can lead to the cyclic ether by initial formation of a hypohalite. The cyclization occurs when the rearranged radical is converted to a cation, either by oxidation with Pb(OAc)4 or by silver-assisted loss of halide ion. Hypoiodites are also fiequent-ly used, generated in situ. ... 

A heterocycle is a compound with a ring that has more than one kind of atom. Nitrogen, oxygen, and sulfur are often found along with carbon in heterocyclic rings. Saturated heterocyclic amines, ethers, and sulfides usually have the same chemistry as their open-chain analogs, but unsaturated heterocycies such as pyrrole, furan, and thiophene are aromatic. All three are unusually stable, and all three undergo aromatic substitution on reaction with electrophiles. 

A useful reaction for the synthesis of unsaturated seven-membered heterocycles is the (2 + 2)-cycloaddition of heteroaromatic compounds, e.g., 1 //-pyrrole, furan, or thiophene derivatives, with acetylenes. In combination with a subsequent intramolecular (2 + 2)-cycloreversion (Section IV,B,2) of the annulated cyclobutene moiety, ring enlargement with two carbon atoms can be achieved. 1-Heterocycloheptatrienes, such as benzol6]azepines,26,27,65,66 benzo[fc]oxepins,67,68 benzo[6]-thiepins,69,70 and thiepins,18,71 have been successfully prepared in this way other routes are either nonexistent or laborious.72 In these compounds the reacting carbon-carbon double bond constitutes part of a (4n + 2)7r-electron system and in the (2 + 2)-cycloaddition the resonance energy of the aromatic nucleus is lost. Just like the nonaromatic heterocycles, heteroaromatic compounds have been reported to undergo (2 + 2)-cycloaddition reactions both with electron-deficient and with electron-rich acetylenes. 

Different heteroaromatic rings (e.g., pyridine, furan and thiophene) and carbon-carbon double bonds are well tolerated under the reaction conditions. Moreover, reagent 130 can be easily recycled from the reaction mixture and reused. The mechanism of this reaction includes initial homolytic cleavage of the I—Cl bond providing a chlorine atom and the iodanyl radical 132, which is in equilibrium with the benzoyloxy radical 133. TEMPO (134) is oxidized by the chlorine atom to oxoammonium salt 135, which then oxidizes the alcohol 129 to the corresponding carbonyl compound 131 and is itself reduced to hydroxylamine 136. The benzoyloxy radical 133 accomplishes the regeneration of TEMPO 134 from hydroxylamine 136, giving rise to 2-iodobenzoic acid and the catalytic cycle is complete (Scheme 3.55) [158]. 

The third pattern of reaction of sulphonium ylides with alkynes results in the formation of furans. For example, the reaction which afforded (47) and (48) in benzene gave the furan (49) in high yield when DMSO was used as solvent. Furthermore, warming (47) in ethanol or DMSO effects conversion into the furan (49), perhaps indicating the actual mechanism of furan formation (i.e. enolate anion displacement of the sulphonium group). In this type of reaction, the acetylenic carbon atoms become C-3 and C-4 of... 

Bis(trifluoromethyl)furan 87a did not react with bromine, however lithiation allowed one to insert Br-atom regioselectively at the C2 position [143], Many other electrophilic reagents can be involved into the reaction with lithiated furan derivatives, for example, carbon dioxide, aldehydes, silyl chlorides, various Michael acceptors, etc. [74, 84, 144], Treatment of vinylation products 226, 227 with alkyl-lithium afforded the corresponding alkynes 228, 229 [64, 143],... 

Fluorine atom activates furan ring to oxidative Heck reaction with various alkenes [152] and to Pd-catalyzed arylation of the neighboring carbon atom [153]. 

In a similar reaction, but with reversed polarities in the starting materials 3-nitrobenzofuran adds to l-phenyl-2-pyrrolidinoacetylene to afford a mixture of three components, one being 5-nitro-3-phenyl-2-pyrrolidino-l-benzoxepin (3, 27 %).183 In the first step of this reaction, a bond between C2 of the furan and the carbon atom in the a-position to the phenyl group is formed to produce a dipolar intermediate that can react in different directions. 

Phosphorus pentasulfide is used to replace oxygen atoms with sulfur atoms the reaction is commonly carried out in a solvent heated under reflux. Solvents employed include carbon disulfide, aromatic hydrocarbons, and pyridine. If an oxygen atom is part of a heterocycle, then the reagent may replace it with sulfur, as in the formation of 2,1-benzisothiazoles from 2,1-benzisoxazoles.119 Such replacements are, however, not general some prior ring opening appears to be necessary before the reagent can act. For example, under normal conditions furan is not attacked. 

If the data obtained on the dehydration of the 2-(aWo-polyhydroxy-alkyl)benzimidazoles31 is considered, it is to be expected that no inversion occurs in the configuration of the carbon atom next to the furan ring. If this is so, the anhydride should have formula XXXIV. With the object of deciding this question, researches are being carried out32 on the reaction of ethyl acetoacetate with 3,6-anhydro-n-glucose and with D-altrose. 

Both uncatalyzed and catalyzed [4+2]-cycloaddition reactions of furans with the allenic esters have been reported (Table 12.6) [93]. The allene adds from the less hindered C1-C2 Jt-face. The unfavorable steric interaction between the a-hydrogen atom of the furan and the methyl group at C4 of the allene is responsible for this selectivity. The more reactive 2-methylfuran adds to the allenic ester also in a regio-selective manner. The C2 carbon atom of 2-methylfuran was exclusively attached to the Cl carbon atom of the allenic ester, providing a mixture of endo- and exoadducts. 

ETEROAROMATics FURAN AND THIOPHENE. The chemical transformation of thiophene at high pressure has not been studied in detail. However, an infrared [441,445] study has placed the onset of the reaction at 16 GPa when the sample becomes yellow-orange and the C—H stretching modes involving sp carbon atoms are observed. This reaction threshold is lower than in benzene, as expected for the lower stability of thiophene. The infrared spectrum of the recovered sample differs from that of polythiophene, and the spectral characteristics indicate that it is probably amorphous. Also, the thiophene reaction is extremely sensitive to photochemical effects as reported by Shimizu and Matsunami [446]. Thiophene was observed to transform into a dark red material above 8 GPa when irradiated with 50 mW of the 514.5-nm Ar+ laser line. The reaction was not observed without irradiation. This material was hypothesized to be polythiophene because the same coloration is reported for polymeric films prepared by electrochemical methods, but no further characterization was carried out. 

Reaction of Carbon Atoms with Furan. Attempts to trap oxacyclo-hexatriene in the reaction of arc generated C atoms with fiiran (87) in analogy with the reaction of carbon with 76 led to polymerization of 87, precluding identification of other products. However, reaction of chemically generated C... 

Many common reactions of aliphatic amines, ethers and sulfides (1) involve initial attack by an electrophilic reagent at a lone pair of electrons on the heteroatom salts, quaternary salts, coordination compounds, amine oxides, sulfoxides and sulfones are formed in this way. Corresponding reactions are very rare (c/. Section 3.3.1.3) with pyrroles, furans and thiophenes. These heterocycles react with electrophilic reagents at the carbon atoms (2-3) rather than at the heteroatom. Vinyl ethers and amines (4) show intermediate behavior reacting frequently at the (3-carbon but sometimes at the heteroatom. 

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