Furans protonated, ring opening

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). 

Furans and some of its derivatives have been cyclopropanated with the ketocarbenoids derived from ethyl diazoacetate and copper catalysts. The 2-oxabicyclo[3.1.0]hex-3-enes thus formed are easily ring-opened to 1,4-diacylbutadienes thermally, thermo-catalytically or by proton catalysis 14,136). The method has been put to good use by Rh2(OAc)4-catalyzed cyclopropanation of furan with diazoketones 275 to bicyclic products 276. Even at room temperature, they undergo electrocyclic ring-opening and cis, trans-dienes 277a are obtained with fair selectivity 257,258). These compounds served as starting materials in the total syntheses 257 259) of some HETE s (mono-... 

Protonation of pyrrole, furan and thiophene derivatives generates reactive electrophilic intermediates which participate in polymerization, rearrangement and ring-opening reactions. Pyrrole itself gives a mixture of polymers (pyrrole red) on treatment with mineral acid and a trimer (146) under carefully controlled conditions. Trimer formation involves attack on the neutral pyrrole molecule by the less thermodynamically favored, but more reactive, (3-protonated pyrrole (145). The trimer (147) formed on treatment of thiophene with phosphoric acid also involves the generation of an a-protonated species. 

No pA aH value is available for O-protonation of furan, but it is probably much less basic at oxygen than an aliphatic ether. Acid-catalysed deuteration occurs at an a-positionf 3/4-deuterio-furans are not obtained because, although P-protonation probably occurs, the cation produced is more susceptible to water, leading to hydrolytic ring opening. An estimate of p/faH -10.0 was made for the 2-protonation of 2,5-di-t-butylfuran, which implies a value of about -13 for furan itself. An a-protonated cation, stable in solution, is produced on treatment of 2,5-di-t-butylfuran with concentrated sulfuric acid. ... 

Under conditions of irreversible deprotonation (BuLi in THF or Et20), the 2-position of the hetero ring is metallated [1, 2, 9]. The pK values of benzofuran and benzothiophene [124] are slightly lower than those of the non-condensed heterocycles, so that complete metallation in the 2-position presumably can be effected with LDA in THF. However, the difference in acidity between the 2- and 3-protons is probably less than that in furan and thiophene. Ring opening therefore seems relatively easy under suitable basic reaction conditions (compare [141]), e.g.) ... 

The first mention of polythiophenes as potential conducting polymers can be found in the literature in 1967. A. G. Davies and coauthors investigated the acid catalyzed polymerization of furan, pyrrole, and thiophene hetero-cycles. Although this polymerization reaction was known before, several new and interesting facts could be elucidated. Besides the confirmation of the cyclic structure of the polymer units (instead of ring opened), the electric conductivity was investigated. In contrast to the already known types of electronically conducting polymers (polyaniline and polypyrrole, see Chapter 1), the conductivity of all polyheterocycles in Armour s paper, isolated as trichloro- or trifluoroacetates (triflate), is ionic. In the case of the polythiophene, ion pairs formed by the reaction of polythiophene with trifluoro acetic acid dissociated in methylene chloride solution to the protonated polymer and triflate anions. The polythiophene triflate decomposed at relatively low temperatures (60°C-80°C). ... 

Protonation is the simplest form of electrophilic attack in furan it leads to polymerization since the ring can open to polyfunctional products or the protonated species can act as an electrophile and attack another furan molecule, so most work has to be on substituted furans. Even so, the naturally occurring trialkyl furan, petasalbin, is cleaved by so mild a reagent as silica164 ... 

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