Catalysis of esterification reactions by oxo-molybdenum-alkoxides: X-ray structures of Cl-2(O)Mo(mu-Pr-n)(2)(mu-(HOPr)-Pr-n)Mo(O)Cl-2 and Cl-2(O)Mo(mu-OEt)(2)(mu(2)-eta(1)eta(1)-HOCH2CH2OH)Mo(O)Cl-2

The compound Cl-2(O)Mo(mu-OEt)(2)(mu-HOEt)Mo(O)Cl-2, 1, has been shown to c atalyse esterification reactions. The reaction rates increase with decreasi ng acidity of the carboxylic acids employed. The steric effects observed ar e the same as in catalysis by Bronsted acids, but catalysis by 1 can also b e applied in esterifications of protected amino acids. 1 is modified under the catalytic conditions yielding a species which coordinates and thereby a ctivates the carboxylic acids in the rate-determining step. Subsequently an alkoxide ligand, probably originating in the coordination sphere of Mo cen tres, attacks the carboxylic centre producing the corresponding ester. Othe r oxo molybdenum(V) compounds do catalyse esterifications, too, but 1 is si gnificantly more efficient as a precatalyst, even in comparison with closel y related compounds, like Cl-2(O)Mo(mu-OEt)(2)(mu(2)-eta(1)eta(1)-HOCH2CH2O H)Mo(O)Cl-2 4, in which the labile EtOH bridge had been replaced by HOCH2CH 2OH. The crystal structures of the compounds 4 and Cl-2(O)Mo(mu-O " Pr)(2)( mu-HO " Pr)Mo(O)Cl-2, 2, which possesses a propanol bridge, are discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.