Reactions of 1,1,3,3-tetramethylcyclobutane on evaporated metal films

Abstract

Reactions of 1,1,3,3-tetramethylcyclobutane (TMCB)–hydrogen mixtures on evaporated metal films have shown that both ring scission to 2,2,4-trimethylpentane (TMP) and ring enlargement to 1,1,3-trimethylcyclopentane (TMCP) are dominant on sintered platinum films, while ring scission to TMP predominates on sintered palladium and on unsintered molybdenum films. Unsintered tantalum and molybdenum films gave a large production of iC₄ hydrocarbon, particularly above ca. 500 K: ring enlargement was a minor reaction found particularly with tantalum. The homogeneous reaction of TMCB giving a large iC₄ production sets in at 600 K. The possibility, suggested by the product distribution, that the reaction of TMCB on Mo and Ta is a metal-assisted free-radical reaction is examined. While it may not be completely excluded that the catalysed ring enlargement on Pt and Pd is a free-radical reaction it is argued that only the previously proposed Rooney–Samman bond-shift mechanism accommodates without added qualifications published facts on bond shifts, including those at quaternary carbon atoms, and also ring enlargements.