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

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Abstract
1-Aryl-2-methylenecyclopropanes reacted with [RhCl(PPh3)3] at 50 °C to give [RhCl(η4-CH2 CArCHCH2)(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-CH2 CCH2CHAr)(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 + CH2 CCH2CHC6H4X-p ⇄ 2b (or 2c) + CH2 CCH2CHC6H5 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-CH2 CCH2CHC6H4Me-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 (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.

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