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Co-oxidative epoxidation of cyclohexene with molecular oxygen, isobutyraldehyde reductant, and the polyoxoanion-supported catalyst precursor [(n-C4H9)4N]5Na3[(1,5-COD)Ir · P2W15Nb3O62]. The importance of key control experiments including omitting the catalyst and adding radical-chain initiators

1996; Elsevier BV; Volume: 114; Issue: 1-3 Linguagem: Inglês

10.1016/s1381-1169(96)00301-9

1873-314X

Noritaka Mizuno, Heiko Weiner, Richard G. Finke,

Vanadium and Halogenation Chemistry

To reveal the underlying problems and misleading results that can be obtained from uninitiated O2/aldehyde/olefin co-oxidative epoxidations that proceed by a radical-chain mechanism, a series of nearly identical experiments were set up at the University of Tokyo and Colorado State University. The main catalyst precursor studied is the oxidatively resistant, polyoxoanion-supported organometallic complex [(n-C4H9)4N]5Na3[(1,5-COD)Ir · P2W15Nb3O62], 1. Three other LnM · P2W15Nb3On−62 complexes were also examined (LnM = Re(CO)+3, Ru(C6H6)2+, and Rh(1,5-COD)+], as well as the framework-incorporated cobalt(II) complex, [(n-C4H9)4N]4H[PW11COIIO3−39]. Several types of important but often omitted control reactions were also performed, again to expose the pitfalls in studies of O2/aldehyde/olefin co-oxidative epoxidations, specifically: (i) the control of leaving out the catalyst completely, (ii) controls for O2-mass-transfer limitations, (iii) controls examining a range of different solvents (CH3CN, CH2Cl2, ClCH2CH2Cl, and Cl2CHCHCl2), and (iv) controls comparing uninitiated versus deliberately peroxide-initiated reactions. The resultant reproducibilities, product conversions, selectivities, and yields are presented and discussed, as are stereochemical results using cis- and trans-stilbene. Several important insights are generated for the area of co-oxidative epoxidations proceeding by a radical-chain mechanism, most notably that the catalyzed results are inferior to ROOH-initiated, uncatalyzed co-oxidative epoxidations examined under otherwise identical conditions.