Tetrahydrofuran basicity

These show marked similarities to their acyclic counterparts, e.g. tetrahydrofuran closely resembles diethyl ether. The minor differences which arise between these two types of compounds are due to the less sterically hindered nature of the heteroatoms in the cyclic compounds. The basicities of tetrahydropyrrole (pHTa 10.4), tetrahydrofuran (-2.1) and... 

Note 9) in 500 ml. of dry tetrahydrofuran i.q added to the gtirred basic mixture heated to 65° over a period of approximately 8 hours a light nitrogen stream is used to carry the methylenecyclopropane into the cold trap. After the addition is complete, the reaction mixture is stirred and heated to 65° for 3 more hours (Note 10). The trap flask contains 58 g. (43%) of methylenecyclopropane (Note 11). 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

Stork and his colleagues, working with nonsteroidal compounds, have shown that only lithium enolates may be alkylated successfully in ammonia or tetrahydrofuran. The more basic sodium and potassium enolates undergo... 

The reactivities of compounds of type 6 with aniline in acetone correlate quite well with substituent effects, and autocatalysis is unimportant here. In the less polar tetrahydrofuran, where the hydrochloride is only partly soluble, the reaction shows autocatalysis when aniline and -chloro aniline are reactants but not when the more basic -toluidine is involved. In these cases the solubility of the acidic product may also influence the differential behavior observed. 

Finally, with compounds of type 7, which have one chlorine atom and two ZR substituents, the reactions are, as expected, more frequently acid catalyzed than with compounds of type 6 e.g., the reaction with aniline in acetone is distinctly acid catalyzed. Again, reactions stiU occur, e.g., with benzylamine in tetrahydrofuran, in which autocatalysis is absent, possibly because of a combination of the marked basicity of the reagent and the low solubility of the acidic product. 

The simple hexaalkylditins, RsSnSnRs, do not disproportionate on heating, but, in oxolane (tetrahydrofuran) or acetonitrile in the presence of a base such as a Grignard reagent, or in the more strongly basic solvent hexamethylphosphoric triamide (HMPT), disproportionation readily occurs at room temperature, and, in HMPT, addition occurs to such alkynes as phenylacetylene and diphenylbutadiyne. The disproportionation is considered to proceed by nucleophilic attack upon tin (259, 260), e.g.,... 

Reaction of 3,4-bis(phenylsulfonyl)-l,2,5-oxadiazole oxide isomers with ethanol and ethanethiol in basic medium gave the expected alkoxy- and alkylthio-substituted (benzenesulfonyl)furoxans, respectively <1996JHC327, 1997FES405>. Nucleophilic substitution of the sulfonyl group of 3,4-bis-(benzenesulfonyl)furoxan 222 in the presence of aqueous NaOH in tetrahydrofuran (THF) furnished the corresponding 3 -0-(3-benzenesulfonylfur-oxan-4-yl) derivative 223 in 79-92% yield (Equation 44) <2004JME1840>. 

The Williamson ether synthesis remains the most practical method for the preparation of tetrahydrofurans, as can be exemplified by the two examples shown in the following schemes. A simple synthesis of 2-substituted tetrahydrofuran-3-carbonitriles 84 is achieved by generating the alkoxide under a phase transfer condition via reaction between 4-chlorobutyronitrile and non-enolizable aldehydes <00SL1773>. A synthesis of 2-alkylidene-tetrahydrofuran 85 was recorded, in which a dianion can be generated through treatment of the amide shown below with an excess of LDA, and is followed by addition of l-bromo-2-chloroethane. In this way, the more basic y-carbon is alkylated and leads eventually to the nucleophilic cyclization <00SL743>. 

The initiation reaction in the polymerization of vinyl ethers by BF3R20 (R20 = various dialkyl ethers and tetrahydrofuran) was shown by Eley to involve an alkyl ion from the dialkyl ether, which therefore acts as a (necessary) co-catalyst [35, 67]. This initiation by an alkyl ion from a BF3-ether complex means that the alkyl vinyl ethers are so much more basic than the mono-olefins, that they can abstract alkylium ions from the boron fluoride etherate. This difference in basicity is also illustrated by the observations that triethoxonium fluoroborate, Et30+BF4", will not polymerise isobutene [68] but polymerises w-butyl vinyl ether instantaneously [69]. It was also shown [67] that in an extremely dry system boron fluoride will not catalyse the polymerization of alkyl vinyl ethers in hydrocarbons thus, an earlier suggestion that an alkyl vinyl ether might act as its own co-catalyst [30] was shown to be invalid, at least under these conditions. 

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

In order to demonstrate the use of laser flash photolysis in elucidation of the MDI based polyurethane photolysis mechanism, three polyurethanes, two aryl biscarbamate models, an aryl monocarbamate model, and an aromatic amine were selected. Two of the polyurethanes are based on MDI while the third is based on TDI (mixture of 2,4 and 2,6 isomers in 80/20 ratio). The MDI based polyurethanes all have the same basic carbamate repeat unit. The MDI elastomer (MDI-PUE) is soluble in tetrahydrofuran (THF). The simple polyurethane (MDI-PU) based on MDI and 1,4-butanediol is used in the tert-butoxy abstraction reactions since it does not contain a polyether backbone. (See page 47 for structures of polymers and models.)... 

Miscibility is an important consideration when selecting solvents for use in biphasic systems. Table 4.4 shows the miscibility of three ionic liquids with water and some organic solvents. [bmim][PFe] was found to be miscible with organic solvents whose dielectric constant is higher than 7, but was not soluble in less polar solvents or in water. Basic [bmim][AlCl4] was found to react with protic solvents, and the acidic form also reacted with acetone, tetrahydrofuran and toluene. 

Especially noteworthy is the following feature the oxygen donors, dimethyl-formamide, tetrahydrofuran, methanol, and ethanol seem to allow a stabilization of the CO ligand which is well comparable with that achieved by the much stronger a-donors, triethylamine or N-methylimidazole (Table 10, [50a-/]). It is suggested that these ligands also act as simultaneous a- and ir-donors, as do the imidazoles. With its low basicity, DMF seems to have a very favorable a/ff-donor balance for the trans fixation of rr-acceptor ligands, which are themselves weak o-donors, like CO, N2, NO , or 02, and indeed, the 7r-donor function of DMF is well documented (S). 

Since Lewis base additives and basic solvents such as tetrahydrofuran are known to deaggregate polymeric organolithium compounds, (21,23,26) it was postulated that ketone formation would be minimized in the presence of sufficient tetrahydrofuran to effect dissociation of the aggregates. In complete accord with these predictions, it was found that the carbonation of poly(styryl)lithium (eq. 9), poly(isoprenyl)-lithium, and poly(styrene-b-isoprenyl)lithium in a 75/25 mixture (by volume) of benzene and tetrahydrofuran occurs quantitatively to produce the carboxylic acid chain ends (8 ). 

The same basic strategy was applied to the synthesis of the smaller fragment benzyl ester 28 as well (Scheme 4). In this case, aldehyde 22 prepared from (S)-2-hydroxypentanoic acid [9] was allylated with ent-10 and tin(IV) chloride, and the resulting alcohol 23 was converted to epimer 24 via Mitsunobu inversion prior to phenylselenenyl-induced tetrahydrofuran formation. Reductive cleavage of the phenylselanyl group, hydrogenolysis of the benzyl ether, oxidation, carboxylate benzylation, and desilylation then furnished ester 28. 

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