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Oxygen atom transfer reactions involving dioxomolybdenum(VI) complexes are of current interest because of their relevance in some important biological processes (1). Most synthetic complexes that mimic oxotransferases contain sterically demanding sulfur ligands because two of the criteria for relevant models are the sulfur environment and hindrance to irreversible mu-oxo dimer formation. However, some molybdenum complexes lacking these requisites show a remarkable ability to act as oxotransfer agents (2). Here we describe the facile synthesis of a simple, very active, catalyst (MoO sub 2 Cl sub 2 (DMSO) sub 2 , (DMSO = dimethylsulfode)) and its application in the oxidation of triphenylphosphine (PPh sub 3 ) to triphenylphosphine oxide (OPPh sub 3 ) by DMSO.
This oxidation of PPh sub 3 provides a good example of the catalytic oxotransfer process because PPh sub 3 is inexpensive, resistant to aerial oxidation, widely used in oxotransfer studies, and has a low volatility. Conducting the experiment in an aprotic medium with simple reagents minimizes the number of species involved in the overall process, which facilitates the interpretation of the results at the time that dramatically evidences the role of the catalyst (cleavage of strong multiple X-O bonds).
Preparing the Catalyst
The procedure presented here is related to that recently reported for the multigram scale synthesis of MoO sub 2 Cl sub 2 -(DMSO) sub 2 from aqueous hydrochloric solutions of MoO sub 3 (3) although the use of (NH sub 4 ) sub 6 M sub 07 O sub 24 4H sub 2 O, in place of MoO sub 3 , saves time and gives a product that is not contaminated appreciably with NH sub 4 C1. A typical run is described:
In a 5-mL vial are placed 0.25 g (0.2 mmole) of (NH sub 4 ) sub 6 M sub 07 O sub 24 *4H sub 2 O, 1 g H sub 2 O and 2 g (20 mmole) 37% HCl. The mixture is heated for 10 min at == 100...