Furans hydrogenation

Reduction of furan and its homologs is readily achieved by catalytic hydrogenation using palladium [419, nickel [420] or Raney nickel [421 as catalysts. Too energetic conditions are to be avoided since hydrogenated furan rings are easily hydrogenolyzed [422,423. ... 

Hydrogenated furans can be prepared from the reaction of 7-olefins with formaldehyde in trifluoroacetic acid kinetics suggest that die reactions proceed via equilibrium addition of the protonated aldehyde.184... 

Kinetics data suggest that the mechanism for the formation of hydrogenated furan from a-alkenes and formaldehyde involves the reversible addition of protonated formaldehyde to the alkene 45... 

An analogous reaction of 2-acetoxy-1,4-naphthoquinone with 1-morpholinopropene gives the dihydronaphthofuran 27 (equation 18)37 similarly, enamines add to styrene oxide and 2-chlorotropone to yield the hydrogenated furans 2838 and 2939, respectively (equation 19). 

C6H3CI9O3, a-[2,4,6-Tris(trichloromethyl)-1,3,5-trioxane], 46B, 303 C6H3CI9O3, p-[2,4,6-Tris(trichloromethyl)-1,3,5-trioxane], 46B, 303 C6H3KO5, Potassium hydrogen furan-3,4-dicarboxylate, 44B, 278 CeHijClflOft, 2,2,4a,5a-Tetrachloro-la, 3a-dihydroxycyclopentane-l,4-carbolactone, 43B, 384... 

QUANTUM-CHEMISTRY STUDY OF THE FORMATION MECHANISM OF HYDROGENATED FURANS IN TRIFLUOROACETIC ACID BY PRINS REACTION... 

The value of Gibbs free energy of the nonsubstituted dioxolenium ion 24 cyclization equals -11.1 kJ/mol and shows that this reaction is a spontaneous process (Fig. 8.2). The substituent at the 1st carbon atom in the hydroxyethyl radical on the dioxolenium ion raises the thermodynamic advantage of the hydrogenated furans formation in the case of the methyl substituent the AG° is -15.3kJ/mol, the ethyl group (26) reduces the value of the Gibbs free energy to -21.4 kJ/mol. 

It is theoretically proved that the mechanism of formation of 3-al-kylsubstituted hydrogenated furans in trifluoroacetic acid by interaction between terminal alkenes and formaldehyde medium which includes recyclization of 3-(2-hydroxyethyl)-l-trifluoromethyl-2,5-dioxolenium ions is preferable. 

The Pd—C cr-bond can be prepared from simple, unoxidized alkenes and aromatic compounds by the reaction of Pd(II) compounds. The following are typical examples. The first step of the reaction of a simple alkene with Pd(ll) and a nucleophile X or Y to form 19 is called palladation. Depending on the nucleophile, it is called oxypalladation, aminopalladation, carbopalladation, etc. The subsequent elimination of b-hydrogen produces the nucleophilic substitution product 20. The displacement of Pd with another nucleophile (X) affords the nucleophilic addition product 21 (see Chapter 3, Section 2). As an example, the oxypalladation of 4-pentenol with PdXi to afford furan 22 or 23 is shown. 

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

Uses. Furfural is primarily a chemical feedstock for a number of monomeric compounds and resins. One route produces furan by decarbonylation. Tetrahydrofuran is derived from furan by hydrogenation. Polytetramethylene ether glycol [25190-06-1] is manufactured from tetrahydrofuran by a ring opening polymeri2ation reaction. Another route (hydrogenation) produces furfuryl alcohol, tetrahydrofurfuryl alcohol, 2-methylfuran, and 2-methyltetrahydrofuran. A variety of proprietary synthetic resins are manufactured from furfural and/or furfuryl alcohol. Other... 

As can be seen, most of the furfural produced in this country is consumed as an intermediate for other chemicals. Hydrogenation to furfuryl alcohol is the largest use. Some of the furfuryl alcohol is further hydrogenated to produce tetrahydrofurfuryl alcohol. The next major product is furan, produced by decarbonylation. Furan is a chemical intermediate, most of it is hydrogenated to tetrahydrofuran, which in turn is polymerized to produce polytetramethylene ether glycol (PTMEG). 

Catalytic hydrogenation of furan to tetrahydrofuran is accompHshed in either Hquid or vapor phase. Hydrogenation of the double bonds is essentially quantitative over nickel catalysts but is generally accompanied by hydrogenolysis over the noble metals. 

The oxidative reaction of furan with bromine in methanol solution or an electrochemical process using sodium bromide produces 2,5-dimethoxy-2,5-dihydrofuran (19), which is a cycHc acetal of maleic dialdehyde. The double bond in (19) can be easily hydrogenated to produce the corresponding succindialdehyde derivative. Both products find appHcation in photography and as embalming materials, as well as other uses. 

In the now-obsolete furfural process, furfural was decarboxylated to furan which was then hydrogenated to tetrahydrofuran (THF). Reaction of THF with hydrogen chloride produced dichlorobutene. Adiponitrile was produced by the reaction of sodium cyanide with the dichlorobutene. The overall yield from furfural to adiponitrile was around 75%. 

Many of the physical characteristics of pyrrole indicate at least partial association. In particular, the boiling point is 98°C higher than that of furan. It has been postulated that various associated dimeric and higher stmctures occur because of hydrogen bonding (9,10). 

Manufacture of thiophene on the commercial scale involves reactions of the two component method type wherein a 4-carbon chain molecule reacts with a source of sulfur over a catalyst which also effects cyclization and aromatization. A range of suitable feedstocks has included butane, / -butanol, -butyraldehyde, crotonaldehyde, and furan the source of sulfur has included sulfur itself, hydrogen sulfide, and carbon disulfide (29—32). 

The oxidation reaction between butadiene and oxygen and water in the presence of CO2 or SO2 produces 1,4-butenediol. The catalysts consist of iron acetylacetonate and LiOH (99). The same reaction was also observed at 90°C with Group (VIII) transition metals such as Pd in the presence of I2 or iodides (100). The butenediol can then be hydrogenated to butanediol [110-63-4]. In the presence of copper compounds and at pH 2, hydrogenation leads to furan (101). 

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

As might be anticipated from the behaviour of the parent heterocycles, C-2 of indole, benzo[i]furan and benzo[i]thiophene (Table 13) is shifted to lower field than C-3. However, the shifts for C-2 (O, 144.8 Se, 128.8 S, 126.1 NH, 124.7 Te, 120.8) and C-7a (O, 155.0 Se, 141.3 S, 139.6 NH, 135.7 Te, 133.0) in the benzo[i] heterocycles vary irregularly (80OMR(l3)3l9), and the sequence is different to that observed for C-2 in the parent heterocycles, namely 0>Se>Te>S>NH. Also noteworthy is the upheld position of C-7, especially in indole and benzofuran, relative to the other benzenoid carbons at positions 4, 5 and 6. A similar situation pertains in the dibenzo heterocycles (Table 14), where not only are C-1 and C-8 shifted upheld in carbazole and dibenzofuran relative to the corresponding carbons in dibenzothiophene and fluorene, but similar, though smaller, shifts can be discerned for C-3 and C-6 in the former compounds. These carbon atoms are of course ortho and para to the heteroatom and the shifts reflect its mesomeric properties. Little variation in the carbon-hydrogen coupling constants is observed for these dibenzo compounds with V(qh) = 158-165 and V(c,h) = 6-8 Hz. 

The high reactivity of pyrroles to electrophiles is similar to that of arylamines and is a reflection of the mesomeric release of electrons from nitrogen to ring carbons. Reactions with electrophilic reagents may result in addition rather than substitution. Thus furan reacts with acetyl nitrate to give a 2,5-adduct (33) and in a similar fashion an adduct (34) is obtained from the reaction of ethyl vinyl ether with hydrogen bromide. 

The chemical consequences of /3-protonation are illustrated further by the ring-opening reactions of furans with methanolic hydrogen chloride and of (V-substituted pyrroles with hydroxylamine hydrochloride (Scheme 11) (82CC800). 

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