Reaction of 1-aryl-2-methylenecyclopropanes with rhodium(I) complexes leading to ring opening isomerization and π co-ordination of the CC double bond

Abstract

1-Aryl-2-methylenecyclopropanes reacted with [RhCl(PPh₃)₃] at 50 °C to give [RhCl(η⁴-CH₂CArCHCH₂)(PPh₃)₂] (Ar = C₆H₅ 1a, C₆H₄F-p 1b, C₆H₄Me-p 1c or C₆H₄OMe-p 1d) via ring opening isomerization of the substrate and its subsequent co-ordination to Rh. The diene-co-ordinated rhodium complexes have been characterized by X-ray crystallography and NMR spectroscopy. Similar reaction at 0 °C afforded the rhodium(I) complexes with π-co-ordinated 1-aryl-2-methylenecyclopropane, [RhCl(η²-CH₂CCH₂CHAr)(PPh₃)₂] (Ar = C₆H₅ 2a, C₆H₄F-p 2b, C₆H₄Me-p 2c or C₆H₄OMe-p 2d). Exchange of the ligand of 2a with added 1-aryl-2-methylenecyclopropanes occurs reversibly at 30–45 °C with the thermodynamic parameters of the reactions 2a + CH₂CCH₂CHC₆H₄X-p ⇄ 2b (or 2c) + CH₂CCH₂CHC₆H₅ being ΔH° = –10.3 kJ mol^–1 and ΔS° = –32 J K^–1 mol^–1 for X = F and ΔH° = 2.2 kJ mol^–1 and ΔS° = –2.6 J K^–1 mol^–1 for X = Me, respectively, at 298 K. The structure of a PEt₃ co-ordinated analog, [RhCl(η²-CH₂CCH₂CHC₆H₄Me-p)(PEt₃)₂] 3c, has been determined by X-ray crystallography. The reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh₃)₃] at 25 °C gave a mixture of 1a and 2a. Heating of a benzene solution of 2a at 50 °C turned it into 1a in low yield (<7%), while the reactions of 1-methylene-2-phenylcyclopropane with 2a at the same temperature gave 1a (10%) and 2-phenylbuta-1,3-diene (14%). The amounts of the products formed via ring opening isomerization in these reactions are much smaller than those in the reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh₃)₃] at 50 °C.