Alkyl sulphonic acid surface-functionalised silica as heterogeneous acid catalyst in the solvent-free liquid-phase addition of acetic acid to camphene

In search for heterogeneous Bronsted acid catalysts as alternatives for corrosive liquid acid catalysts, silica's were surface-functionalised with linear alkyl chains containing thiol and thioacetic ester end-groups, which were subsequently converted into sulphonic acids by oxidation with H2O2/HOAc. FT-IR, CP/MAS C-13 NMR, XPS, TGA and acid-base titrations established the formation of a surface-bound monolayer of alkyl sulphonic acid groups. The catalytic properties of the surface-functionalised silica's were evaluated in the solvent-free liquid-phase hydro-acyloxy-addition of acetic acid (1) to camphene (2), yielding the pine-fragrance isobornyl acetate (3). Reaction rates (normalised on the concentration of sulphonic acid groups) were identical to those found using the homogeneous CH3SO3H reference catalyst. The equal rates suggest that a similar reaction mechanism is operative. The reaction rate with the solid surface-functionalised silica catalysts increased by two orders of magnitude upon addition of a small amount of water. In the presence of water the alcohol analogue of 3, i.e. isobomeol (15), is an important intermediate. Generation of homogeneous alkyl sulphonic acids via hydrolysis followed by leaching from the silica surface does not occur; the formation of 3 stopped after removal of the solid catalyst from the reaction mixture by filtration. Thus, the alkyl sulphonic acid functionalised silica's act as proper heterogeneous Bronsted acid catalysts. (C) 2002 Elsevier Science B.V. All rights reserved.