First example of ionic hydrogenation in the bicyclo[1.1.0]butane series

Abstract

Bicyclo[1.1.0]butanes having an electron-withdrawing sulfonyl group at the bridgehead position (“activated bicyclobutanes”) are known [1] to undergo reduction to cyclobutyl sulfones by the action of LiAlH4 as hydride ion carrier in a way similar to the reduction of activated alkenes (α,β-unsaturated sulfones) to alkyl sulfones with the same reducing agent [2]. Taking into account the olefin-like character of the central bicyclobutane C–C bond [3], it might be expected that bicyclobutanes having an electron-donor group at the bridgehead position, like nucleophilic alkenes (e.g., 2-methylpropene or styrene), will undergo ionic hydrogenation [4, 5] by the action of strong acids in the presence of a hydride ion source (Et3SiH, NaBH4, etc.). However, we have found no published data on such version of hydrogenation of bicyclobutane derivatives. In the present communication we report on the first example of successful ionic hydrogenation in the bicyclobutane series. As initial compound we selected a bridged bicyclobutane derivative, 1-phenyl-7-phenylsulfonyltricyclo[4.1.0.0]heptane (I) [6]. Treatment of I with trifluoroacetic acid and NaBH4 at –10°C under dry argon (i.e., under the conditions of ionic hydrogenation reported in [5]) gave norpinane II in 41% yield. Despite moderate yield, the product can be readily isolated from the reaction mixture by crystallization. Its structure and steric configuration were reliably determined on the basis of the H and C NMR data. As expected, compound I was also reduced with LiAlH4 in THF at 0°C, but the product was compound III (yield 91%) which is epimeric to norpinane II at C. By special experiment we showed that sterically hindered norpinane II is converted almost completely into more stable diastereoisomer III in 5 h on heating with potassium tert-butoxide in boiling THF. We also found that even trace amounts of norpinane III were not formed in the ionic hydrogenation of tricycloheptane I with LiAlH4. 6-exo-Phenyl-7-anti-phenylsulfonylbicyclo[3.1.1]heptane (II). mp 134–135°C (from hexane–ethyl acetate). H NMR spectrum, δ, ppm: 1.80–2.04 m (2H, 3-H); 2.05–2.22 m and 2.23–2.40 m (2H each, 2-H, 4-H); 3.06 d (1H, 6-H, J = 3.1 Hz); 3.23 d (1H, 7-H, J = 3.1 Hz); 3.30 br.s (2H, 1-H, 5-H); 7.22–7.32 m (3H), 7.32–7.43 m (2H), 7.44–7.53 m (2H), 7.56– 7.66 m (1H), and 7.71 d (2H, J = 7.6 Hz) (Harom). C NMR spectrum, δC, ppm: 14.8 (C); 33.0 (C, C); 40.0 (C, C); 47.5 (C); 70.2 (C); 125.7, 126.9 (3C), 127.7 (2C), 127.9 (2C), 128.9 (2C), 133.0, 140.5 (Carom). Found, %: C 72.87; H 6.37. C19H20SO2. Calculated, %: C 73.04; H 6.45.