Molybdenum precatalyst

The metathesis step is effected with the molybdenum precatalyst [MoN(Ar)3] (Scheme 2.19). In the presence of dichloromethane, the active catalyst (the exact structure of which is unknown) reacts with 48 to give alkyne 49 in 80% yield. Hydrogenation of 49 exclusively leads to Z-olefin 50. It is interesting to note that the olefin metathesis reported by Nicolaou with the diene analog of 48 gives a 1 1 ratio of E/Z isomers [33]. 

Abstract For many years after its discovery, olefin metathesis was hardly used as a synthetic tool. This situation changed when well-defined and stable carbene complexes of molybdenum and ruthenium were discovered as efficient precatalysts in the early 1990s. In particular, the high activity and selectivity in ring-closure reactions stimulated further research in this area and led to numerous applications in organic synthesis. Today, olefin metathesis is one of the... 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

It was found by Trost that the low reactivity could be circumvented by the employment of labile ligands, such as the propionitrile in the Mo(CO)3(EtCN)3 precatalyst [57]. Instead of directly transferring this procedure to microwave heating applications, a useful and easily handled microwave procedure was developed for rapid and selective molybdenum-catalyzed allylic alkylations under noninert conditions (Eq. 11.39) [12]. The former, more sensitive, two-step reaction was fine-tuned into a robust one-step procedure employing the inexpensive and stable precatalyst Mo(CO)6, used in low concentrations. The alkylations were conducted in air and resulted in complete conversions, high yields, and an impressive enantiomeric excess (98%) in only 5-6 min. Despite the daunting temperatures, up to 250°C with THF... 

The proposed catalytic cycle is shown in Fig. 8.5. A variety of molybdenum compounds may be used as the precatalyst, and Mo(CO)6 is shown as a representative one. Under the strong oxidizing conditions the precatalyst is oxidized to 8.21, a species that has molybdenum in a 6+ oxidation state and a di-MoOf unit. The other ligands are two solvent molecules and hydroxo and/ or alkoxo groups. In the absence of a solvent, positions occupied by S are occupied by /-butanol, the decomposition product of /-butyl hydroperoxide. The important points to note are that molybdenum is in its highest oxidation state (6+), and there are weakly bound solvent molecules. 

In the molybdenum-catalyzed epoxidation of alkenes with /-butyl hydroperoxide what would be the effect of (a) Addition of external /-butanol in the reaction mixture (b) use of [Mo02(EG)2]2+ (EG = ethylene glycol) as the precatalyst rather than Mo(CO)6. 

Bindl, M., Stade, R., Heilmann, E. K., Picot, A., Goddard, R., and Fiirstner, A. (2009) Molybdenum nitride complexes with PhsSiO ligands are exceedingly practical and tolerant precatalysts for alkyne metathesis and efficient nitrogen transfer agents. J. Am. Chem. Soc., 131, 9468-9470. 

Heppekausen J, Piirstner A. Rendering schrock-type molybdenum alkylidene complexes air stable user-friendly precatalysts for alkene metathesis. Angew Chem Int Ed. 2011 50(34) 7829-7832. 

Indeed, organometallic precatalysts can be transformed during an induction period into catalyticaUy active species that do not contain metal-carbon bonds. For example, molybdenum [7a] and tungsten [7b] carbonyls catalyze aerobic photooxygenation of cyclohexane to cyclohexyl hydroperoxide (primary product) and cyclohexanol and cyclohexanone (Fig. 1.4). The proposed mechanism is shown in Fig. 1.5. It includes the formation during the induction period of an oxo derivative. Complexes CpFe( r-PhH)BF4 and ( 7r-durene)2Fe(BF4)2 also catalyzed the aerobic alkane photooxygenation [7c]. The mechanism has not been studied. 

Sketch (a) the transition state for a concerted metal atom-assisted 3,9 hydride shift (b) two PNP ligands (c) the ligand used for selective dimerization of butadiene (d) a general structure for molybdenum- and tungsten-based metathesis precatalyst (e) a six-coordinate rathenium precatalyst for metathesis (f) a solid isolated from the reaction between Pd(OAc)j plus PRj (R = o-tolyl) (g) a T-shaped palladium complex and a two-coordinate palladium complex with a monodentate phosphine (h) an iron complex with a seven-membered metallacycle (i) the transition state for metal-catalyzed cyclopropanation (j) a rhodium and a copper precatalyst used in cyclopropanation reactions. 

Several molybdenum complexes that are invoked as intermediates of the N2 fixation cycle can be used as the initial catalyst precursors, instead of the starting N2 complex. That these species provide similar overall results with respect to yields of reduced N2 is consistent with the cyde proposed. Therefore, they are either (i) spedes generated along the N2 fixation path or possibly (ii) precatalysts that can be chemically converted to the relevant active catalyst... 

---