Tetrahydrofurans and tetrahydrothiophenes

In contrast to the 1,3-epoxides where a whole variety of substituted oxetanes have been polymerized, there is only one 1,4-epoxide of major importance. This monomer is commonly called tetrahydrofuran, but it has the systematic name 1,4-epoxybutane or tetramethylene oxide. The polymer derived from it is as often called polytetramethylene oxide as poly tetrahydrofuran. We have chosen to use the names tetrahydrofuran (THF) and polytetrahydrofuran (PTHF). An alternate abbreviation for THF that is found in some recent literature is H4furan . 

The polymerization of tetrahydrofuran has been reviewed in detail [5, 58]. There a thorough discussion of many aspects of the mechanism of polymerization and the very many initiators used was presented. Interested readers are referred to the earlier reviews for these aspects of the polymerization. Here attention is ain focused primarily on the kinetics of polymerization. 

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Miscellaneous. Lithium amides of chiral pyrrolidines, tetrahydrofurans, and tetrahydrothiophenes are evaluated as chiral ligands in the condensation of BuLi to o-tolualdehyde (eq 74). ... 

The conformational preferences of the 4-thiohexofuranoses with a- and /J-D-galacto- and -o-manno, and a-D-ta/o- configurations have been assessed from Jhh values on the basis of a Karplus equation, the parameters for which were obtained by molecular mechanics calculations for tetrahydrofuran and tetrahydrothiophene. ... 

Heteronuclear njnj. studies have been carried out on 5-amino-selenazofurin (3) and its parent compound (4) to determine their conformations in solution. The conformational preferences of 4-thiohexofuranose derivatives with a- and /9- D-galacto-, a-and fi-thinanno-, and a-D-m/o-conflgurations have been assigned from J values on the basis of a Karplus equation, the parameters for which were obtained 1 molecular mechanics calculations for tetrahydrofuran and tetrahydrothiophene. An analysis of their 620 MHz H-n.m.r. spectra indicated that, contrary to reports in the literature, the methyl 2-deoxy-D- //iro-pentofuranosides (5) are unsuitable as conformational models of the diphosphate (6) incorporated in abasic DNA. ... 

The electric dipole moments in units 1 X 10 18 e. s. u. of these molecules and their derivatives by hydrogenation measured19 in benzene solution are the following furan, 0.670 2,5-di-hydrofuran, 1.53 tetrahydrofuran, 1.68 pyrrole, 1.80 pyrroline, 1.42 pyrrolidine, 1.57 thiophene, 0.54 and tetrahydrothiophene, 1.87. We now give a very rough interpretation of these quantities based on the bond moments given... 

Strain energies of 23.5, 24.8 and 8.3 kJ mol 1 were estimated for tetrahydrofuran, pyrrolidine and tetrahydrothiophene, respectively (74Pmh(6)199). The larger sulfur covalent radius of 1.04 A lowers angular strain. 

These show marked similarities to their acyclic counterparts, e.g. tetrahydrofuran closely resembles diethyl ether. Minor differences arise due to the less sterically hindered nature of the heteroatoms in the cyclic compounds. The basicities are tetrahydropyrrole (pKa 10.4), tetrahydrofuran (— 2.1) and tetrahydrothiophene (thiolane) ( — 4.5). 

The structure of biotin consists of fused imidazole and tetrahydrothiophene rings and a carboxyl-containing side chain (Figure 38-21). In oxybiotin, which can substitute for biotin in most species, the sulfur of the tetrahydrothiophene ring is replaced by oxygen, making it a tetrahydrofuran ring. 

Significant changes in the ligand-to-metal charge-transfer transitions and the f-f transitions in the electronic spectra of (CjHjljYb are observed, when bases like pyrrolidine, triethylphosphine, tetrahydrofuran, or tetrahydrothiophene are added to the complex in benzene solution (Schlesener and Ellis, 1983). The photoluminescence of (C5H5)3Tb and (CH3CjH4)3Tb in tetrahydrofuran solutions have been studied at different temperatures (Brittain et al., 1983). 

The ring exchange of tetrahydrofuran to tetrahydrothiophene is also catalyzed by basic sites and showed a decrease in activity with addition of HCl [8, 235]. However, pyridine adsorption also decreased the catalytic activity of the reaction on alkali-exchanged zeolites. In this case, acid and basic sites seem to be necessary for catalysis. 

One approach (.40) has been to conduct the reaction in the presence of a more electropositive metal, often as an alloy. In the presence of magnesium, tin reacts with ethyl bromide to give tetraethyl tin, and various additives promote the reaction, the sequence of effectiveness being carbitols I > tetrahydrofuran, tetrahydrothiophene > ether triethylamine Br the ions ClOj, PFg, BFj, and BPhj are without effect. It is suggested that this reflects the coordination of the additive (L) to the Grignard reagent that is first formed, making it more reactive towards metallic tin. 

Dihydro- and tetrahydrofuroisoxazoline rings were constructed by intramolecular cycloaddition of nitrile oxides or nitrones, generated from oximes . Thus, oxime 30 and sodium hypochlorite afforded furoisoxazolines 31 (equation 14). Similarly, furanyl or thienyl oximes 32 in the presence of NaOCl afforded tricyclic products 33 in 35-90% yields (equation 15). Nitrostyrenes (ArCH=CHN02) and various nucleophiles (for example, allyl mercaptan) also generated hydroximoyl chlorides which underwent similar cycloaddition leading to bicycUc tetrahydrothiophene and tetrahydrofuran derivatives ... 

Pentafluorophenyl)(tetrahydrothiophene)gold(I) is a white crystalline solid, which is air and moisture stable at room temperature. It decomposes before melting (109 °C). It is very soluble in acetone, benzene, dichloromethane, diethyl ether, nitromethane, and tetrahydrofuran, soluble in ethanol and methanol, and insoluble in hexane. 

The fluorination of oxathiane is described in Section 25.1.1.3.84 1,4-Dithiane reacts quite differently107 from dioxane and oxathiane over potassium tetrafluorocobaltate(III). The major products (ca. 60 % of the product mixture, itself in ca. 50 % yield) are the result of rearrangement to the 2-methyl-l, 3-dithiolane skeleton, e.g. 1-3. Fluorinated 1,4-dithiancs comprised about 30% of the product the major product is 4. The extent of fluorination, that is the number of fluorine atoms introduced, is much greater than that of dioxane under comparable conditions (tetrahydrothiophene and tetrahydrofuran show a similar contrast). 

The ions (w) resulting from loss of cyclopropane from the molecular ions were only observed for the sulfur, selenium and tellurium analogues. The alternative mode of fragmentation in which the hydrocarbon fragment (y) carries the charge provides the base peak for the tetrahydrofuran spectrum, but is only a minor feature of the spectra of the selenium and tellurium analogues. The hydrocarbon ion C4H/ (z) is a minor feature of the tetrahydrothiophene spectrum but provides the base peak of the spectra of the selenium and tellurium analogues. 

The dipole moments of furan (0.72 D) and thiophene (0.53 D) are smaller than those of the corresponding saturated heterocycles (tetrahydrofuran, 1.68 D tetrahydrothiophene, 1.87 D.) Many authors35-38 seem to believe that, also in these cases, the direction of the dipole is from the heteroatom (positive pole) to the ring (negative pole) others, however, are of a different opinion.12, 32, 39, 40... 

Three dihydropyrrole 33-35 and one tetrahydropyrrole 36 ring systems can occur. Trivial names for the dihydropyrroles are A1-, A2-, and A3-pyrroline, where A indicates the position of the remaining double bond. Tetrahydropyrroles are known as pyrrolidines. Systematic naming of the reduced rings is illustrated by the following examples 2,3-dihydro-furan 37 (X = 0) and 2,5-dihydrothiophene 38 (X = S). 2,3,4,5-Tetrahydrofuran 39 is the well-known solvent THF. Tetrahydrothiophene 40 has an unpleasant smell and is sometimes used as an odorant in natural gas. 

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