CpMo(CO) 3 Cl as a precatalyst for the epoxidation of olefins
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Catalysis Letters Vol. 101, Nos. 1–2, May 2005 ( 2005) DOI: 10.1007/s10562-004-3760-y
CpMo(CO)3Cl as a precatalyst for the epoxidation of olefins Anabela A. Valentea, Joa˜o D. Seixasb,c, Isabel S. Gonc¸alvesa,*, Marta Abrantesb, Martyn Pillingera, and Carlos C. Roma˜ob b
a Department of Chemistry, CICECO, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal Instituto de Tecnologia Quı´mica e Biolo´gica da Universidade Nova de Lisboa, Quinta do Marqueˆs, EAN, Apt, 127, 2781-901 Oeiras, Portugal c Alfama, IBET, Apartado 12, 2781–901 Oeiras, Portugal
Received 15 June 2004; accepted 15 October 2004
The tricarbonyl complex CpMo(CO)3C1 was found to act as a precatalyst for the reaction of tert-butyl hydroperoxide and olefins to yield the corresponding epoxides and diols. Under the reaction conditions, oxidative decarbonylation leads to the formation of the dioxo complex CpMoO2Cl. The catalytic recyclability and the effect of temperature on the catalytic results have been examined, and a reaction mechanism proposed, supported by kinetic modelling. KEY WORDS: molybdenum; organometallic oxides; oxidative decarbonylation; olefin epoxidation; tert-butyl hydroperoxide.
1. Introduction High oxidation state organometallic oxides have found numerous applications as catalysts for oxidation reactions [1]. Perhaps the most famous example, methyltrioxorhenium(VII) (MTO), is an efficient catalyst for olefin epoxidation (using H2O2 as the oxidant), Baeyer Villiger oxidation, aromatic oxidation and metal carbonyl oxidation [2–4]. Dioxomolybdenum(VI) complexes of the type MoO2R2L2 (R = Me, Et; L = Lewis base ligand) have also been shown to be active catalysts for olefin epoxidation using tert-butyl hydroperoxide (TBHP) as the oxidising agent [4–7]. Surprisingly, complexes of the type (g5-C5R5)MoO2X (X = Cl, Br) have been less well studied as oxidation catalysts, despite the fact that they are among the earliest examples of organometallic oxides [8–10]. Nevertheless it has been known for some time that the pentamethylcyclopentadienyl analogue Cp*MoO2Cl is able to catalyse the olefin epoxidation reaction with TBHP [11]. More detailed studies have perhaps been held back by the lack of a convenient synthetic route to (g5-C5R5)MoO2X complexes. The Cp* derivative was prepared in 61% yield by irradiation of Cp*Mo(CO)3Cl under oxygen [11]. However, a similar approach for the synthesis of the cyclopentadienyl derivative CpMoO2Cl (starting from the dimer [CpMo(CO)3]2) only results in a 25% yield [9]. A step forward in this chemistry came with the discovery that the complexes (g5-C5R5)MoO2Cl (R = H, CH3, CH2Ph) are easily prepared from their corresponding and readily available (g5-C5R5)Mo (CO)3Cl precursors by reaction with TBHP in n-decane [12]. It was found that the dioxomolybdenum(VI) * To whom correspondence should be addressed. E-mail: [email protected]
complexes catalyse the epoxidation of cyclooctene, styrene, and 1-octene with TBHP as the oxidising agent. Turnover frequencies (TOF) of up to 21,000 mol 1 were found that even
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