Heterogeneous and homogeneous hydrogenation of hexamethyl-Dewar-benzene

The heterogeneous hydrogenation of hexamethyl-Dewar-benzene (1) over various transition metal catalysts has been studied. At 25° selective (> 95%) cis,exo addition of hydrogen was observed yielding 1,2,3,4,5 endo, 6 endo-hexamethylbicyclo[2.2.0]hex-2-ene (2). Hydrogenation at 50°, of 2 and of 1,2,3,4,5 endo, 6 exo-hexamethylbicyclo[2.2.0]hex-2-ene (3) was found to be less stereospecific; the extent of trans addition increased in the order Ru < Rh < Pd < Pt. Homogeneous hydrogenation of 1 has been accomplished using platinum-tin complexes; a 3 : 1 mixture of 3 and 2 was formed. The mechanism of this homogeneous hydrogenation is discussed on the basis of deuteration experiments. The three 1,2,3,4,5,6-hexamethylbicyclo[2.2.0]hex-2-enes and five 1,2,3,-4,5,6-hexamethylbicyclo[2.2.0]hexanes have been isolated. Configurations have been determined by means of NMR spectroscopy. The thermal isomerization of 1,2 endo, 3 endo, 4, 5 endo, 6 endo-hexamethylbicyclo[2.2.0]hexane into 1,2 exo, 3 exo, 4, 5 exo, 6 exo-hexamethylbicyclo[2.2.0]-hexane is described.

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