Ionic hydrogenation furans

The popular approach to tetrahydrofurans involves an electrophilic process and the commonly used electrophiles for the cyclization are acids, oxygen, halogen, mercury (see Section 3.11.2.2.9) and selenium. The ionic hydrogenation of furans with excess triethyl-silane in trifluoroacetic acid affords high yields, e.g. 2-methylfuran is reduced to 2-methyl-tetrahydrofuran and 2-ethylfuran to 2-ethyltetrahydrofuran (see Section 3.11.2.5). The synthesis of several dihydro and tetrahydrofurans containing natural products by chirality transfer from carbohydrates has been used successfully for total synthesis, e.g. (-)-nonactic acid. A reasonable yield of 2-alkyltetrahydrofuran was prepared from 4-alkylbut-l-en-4-ol by hydroboration followed by cyclization with p-toluenesulfonic acid. 

Ionic hydrogenation is also described for indole, carbazole and furan derivatives (79MI1). However, it is most important for thiophenes since the latter are well known as poisons for most hydrogenation catalysts. 

A very convenient and versatile new reduction method, which has been intensively studied for the reduction of thiophenes (vide infra), makes use of the combination of a trialkylsilane and an acid, usually trifluoroacetic acid (TFA). This so-called ionic hydrogenation provides a novel reduction of furans to tetrahydrofurans (Scheme 3). Although Bolestova et al. find that the conditions are critical in order to avoid polymerization, the yields of tetrahydrofurans can be quite good. The reduction is accelerated by the presence of boron trifluoride etherate but fails with carbonyl-substituted furans, such as methyl pyro-mucate. ... 

Similar to other five-membered-ring heterocycles, furans can be reduced by ionic hydrogenation. While the choice of reaction conditions is critical in order to avoid polymerization, the yields of tetrahydrofurans can be good26. Nevertheless, this method has not been frequently used and catalytic hydrogenation is generally preferred. 

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