Olefins, metathesis Z-selective

This review will focus on isolated and characterized high-oxidation state molybdenum and tungsten alkylidene and metallacyclobutane complexes. Attention will be directed largely toward monoalkoxide pyrrolide (MAP) complexes because they have yielded the majority of new results in the last several years. MAP species have been found to be especially efficient in several Z-selective olefin metathesis reactions, such as homocoupling, cross-coupling, ethenolysis, and ROMP (see Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7). Most of what is presented here has appeared since a review in 2009 [4]. 

Flook, M. (2011) Z-Selective Olefin Metathesis Processes and O s/Syndioselective ROMP with High Oxidation State Molybdenum Alkyli-denes. PhD thesis. Massachusetts Institute of Technology. 

Keitz BK, Endo K, Patel PR, Herbert MB, Grubbs RH. Improved Ruthenium Catalysts for Z-Selective Olefin Metathesis. J dm Chem Soc. 2011 134(l) 693-699. 

Scheme 5.3 General pathway for Z-selective olefin metathesis with molybdenum alkyli-dene catalysts.

Accordingly, considerable effort has been dedicated to the development of olefin metathesis catalysts exhibiting kinetic selectivity. As a result, a number of Z-selective tungsten-, molybdenum-, and ruthenium-based olefin metathesis catalysts have been recently developed (For Mo- and W-based Z-selective catalysts [24-41], For Ru-based Z-selective catalysts [42-45], For cyclometalated Ru-based Z-selective catalysts [46-58]). Many of these systems exhibit consistently high levels of activity and selectivity across a broad range of substrates. Herein, we will focus specifically on the cyclometalated ruthenium-based catalysts developed in our laboratory [46-58]. This chapter is intended to provide a comprehensive summary of the evolution of these cyclometalated ruthenium catalysts, from their initial serendipitous discovery to their recent applications in Z-selective olefin metathesis transformations. Current mechanistic hypotheses and limitations, as well as future directions, will also be discussed. 

Following the promising results obtained with 6 in preliminary Z-selective olefin metathesis assays [46, 47], a variety of new cyclometalated complexes were... 

Initial attempts to introduce other cyclometalated substituents or bulkier orthosubstituents on the A-aryl group only led to decomposition of the catalyst [48, 73]. Fortunately, replacement of silver pivalate with sodium pivalate allowed for a milder protocol to prepare previously inaccessible catalysts (e.g., 10 and 11) [52, 55]. With the exception of catalysts lacking ort/jo-substitution on the A-aryl ring (e.g., 13) [73], a variety of A-aryl and cyclometalated substituents were accommodated (Fig. 2). When these new catalysts were assayed in homo-CM reactions, a dramatic improvement was noted for catalyst 10, which exhibited TONs approaching 7,400 and near perfect Z-selectivity (>95%) [52]. This represents the highest catalytic efficiency exhibited by a Z-selective olefin metathesis catalyst reported to date. Catalyst 10 maintained remarkable activity and Z-selectivity in a variety of homodimerization reactions, as well as a selection of more complicated RCM and CM reactions (cf. Sect. 3). 

The abovementioned cyclometalated ruthenium catalysts have been applied in a number of Z-selective olefin metathesis reactions [46-58]. Of these catalysts, 10 remains the state of the art with respect to a general Z-selective catalyst for CM (Sect. 3.1) and mRCM (Sect. 3.3). The potential of 10 remains to be fully evaluated in more specialized transformations such as AROCM (Sect. 3.2), Z-selective ethenolysis (Sect. 3.5), or ROMP (Sect. 3.4). 

Cyclometalated Ruthenium Alkylidene Complexes A Powerful Family of Z-Selective Olefin Metathesis Catalysts. 1... 

Recent Development of Mo- and W-Alkylidene Monoaryloxide Pyrrolide Complexes for Highly Z-Selective Olefin Metathesis... 

Earlier this year, the Grubbs group reported the preparation of the Ru-based catalyst with a chelating iV-heterocyclic carbene (NHC) ligand that catalyzes highly Z-selective olefin metathesis (Fig. 31) [69, 70]. This catalytic system provided similar levels of efficiency and selectivity to the W-alkylidene complexes for homocoupling reactions. The reason for the Z-selectivity is not clear at this point. Extension of the substrate scope of this catalytic system is expected. 

Fig. 31 New Ru-carbene catalyst for highly Z-selective olefin metathesis reactions...

Fig. 39 Improved njthenium catalysts for Z-selective olefin metathesis by Grubbs et al.

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