Cationic products from O-protonation reaction of 2-acylnorbornadiene complexes of rhodium with hydrogen chloride in ether: structure, reactivity and nature of hydrogen bond

Abstract

The cationic hydroxyallylolefin complexes of rhodium, [RH(η 2,3-C 7H 7-2 CRH  OH) η 5C 5H 4R 1 +Cl − (II–IV), were prepared as stable salts from the reaction of Rh(η 4-C 7H 7-2-COR)(η 5-C 5H 4R 1) (Ia, b, d) (where R = H, Me, R 1 = H; R = H, R 1 = Me) with HCl in absolute ether. The structure of these cations was deduced from their IR, 1H NMR and 13C NMR spectra. The dimeric dichlorocyclopentadienylrhodium complex, [(η 5-C 5H 5)RhCL 2] 2, was isolated from Ic (R = Ph, R 1 = H) under the same conditions. In view of the IR and 1H NMR spectral data of cations II-IV, the presence of an interionic hydrogen bond of the OHCl type was postulated. The likelihood that a similar hydrogen bond is present in O-protonated acylferrocene and in some θ-enol iron complexes is discussed. Certain reactions of the hydroxyallylolefin cations were studied. Intramolecular oxidative addition of Cl − to the cationic complexes in CH 2Cl 2 solution gave endo/exo isomeric 2-acyl- 5-norbornenes and the dimeric complexes [(η 5-C 5H 4R 1)RhCl 2] 2 (where R 1 = H, Me) as the main products. Hydrogenation of protonated carbonyl group of complex II to give methyl group proceeds under the action of Et 3SiH in CH 2Cl 2 solution. The mechanisms of these reactions are described.