Piers catalysts

Eyring equation, this affords a calculated = 0.0792 s at 35 C and at a (2-isopropoxystyrene) =0.414M. The calculated rate constant at this temperature allows it to be directly compared to that of the Grubbs catalyst 2 at 35 C, which is 4.6 E-04s (IM styrene). This reveals that precatalyst 5 initiates about 172 times faster than 2. 

Modification of the Piers ruthenium carbene complex was performed through anion exchange as well as different phosphine substituents to evaluate their effects on initiation and stabihty. For RCM performed at 0 C, these complexes were found to exhibit similar initiation rates, indicating that the anion is non-coordinating and plays no role in the initiation step [40]. 

By tracking the dimerization process over a range of temperatures, the equi-Hbrium constant and forward (k ijn) and reverse (/Tdedim) constants were 

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The Piers catalyst has played a leading role in the identification of ruthenacy-clobutane intermediates. This precatalyst is distinctive from the others because it is a stable, 14-electron complex, and can be considered to be pre-initiated since no ligand dissociation is needed to enter the first turn of the catalytic cycle. 

Bergius-Pier An improved version of the Bergius (1) process in which the activity of the catalyst was increased by treatment with hydrofluoric acid. Invented by H. Pier and others in the 1930s and used in Germany during World War II. 

Racemic diquinane enone rac-6 was prepared by Piers and Orellana starting from cyclopentenone (Scheme 6) [11]. After the preparation of the heterocuprate from stannane 20, conjugate addition to cyclopentenone in the presence of BF3 Et20 provided carbonyl compound 21. It was expected that conversion of 21 by intramolecular alkylation and subsequent hydrogenation should provide the desired endo-substituted diquinane rac-13. While other hydrogenation methods proved to be rather unselective, reduction in the presence of Wilkinson s catalyst finally resulted in the formation of rac-13 with good facial diastereoselectivity [11]. 

Because of the importance of the promotion effect and because many of the central questions surrounding TMS catalysis are about promotion, it is valuable to review a history of the effect. The first reference to a catalyst based on molybdenum and cobalt sulfides capable of desulfurizing coal oils in the presence of hydrogen was a patent from I. G. Farben Industrie dated May 24, 1928 (5). Before this, M. Pier and his team at BASF (1924-1925)... 

The accepted mechanism for olefin metathesis proceeds through formation of a metallacyclobutane after olefin coordination to the 14e species. Piers et al. have collected the first evidence for the metallacyclobutane intermediate 19 in the condensed phase [52], The proposed C2V symmetry of this key structure has been predicted by calculations [53] (for related theoretical investigations on olefin metathesis, see [54-57]). Metallacyclobutane formation is likely to determine the regio- and stereochemical outcome of the metathesis reaction, and insight into its geometry is therefore critical in the development of new, selective catalysts. Cycloreversion and olefin dissociation complete the catalytic cycle to re-form the catalytically active species ([Ru] = CH2) which can bind phosphine to re-form the precatalyst or olefin for a subsequent metathesis transformation. 

In 2005, Piers et al. prepared the 14-electron (14e) phosphonium alkyh-dene ruthenium complex 24. This catalyst displays higher activity in the RCM of diethyl diallylmalonate at 0 °C when compared to the second generation catalyst 3 (> 90% conversion after 2 h for 24 versus 25% conversion after 4 h for 3 and > 90% after 5 h for the Schrock molybdenum-based catalyst) (Eq. 27). RCM reactions of trisubstituted, six-membered ring, or seven-membered ring substrates are catalyzed at room temperature affording good... 

P. E. Romero, W. E. Piers, and R. McDonald, Rapidly Initiating Ruthenium Olefin-Metathesis Catalysts, Angew. Chem. Int. Ed. 43, 6161-6165 (2004). 

Tris(pentafluorophenyl)borane [B(CgFj)3], is a powerful and selective Lewis acid catalyst used in many reactions in organic chemistry [1 ]. Parks and Piers [5] found that B(CgFj)3 catalyzes the hydrosilylation of carbonyl compounds. The silylation of alcohols with the formation of Ft as the only by-product [6] and the cleavage of silyl ether and ether bonds catalyzed by B(C F5)3 [7, 8] provide an... 

Ozawa, R Yamamoto, S. Kayagishi, S. Hiraoka, M. Ideda, S. Minami, T. Ito, S. Yoshifuji, M. Chem. Lett. 2001, 972. For the conversion of a conjugated ketone to a silyl enol ether with R3SiH and a triarylborane catalyst, see Blackwell, J.M. Morrison, D.J. Piers, W.E. Tetahedron 2002,58, 8247. For the conversion of a conjugated ketone to a silyl enol ether with RsSiH and a rhodium catalyst, see Mori, A. Kato, T. Synlett2002, 1167. 

A recent NMR study characterized a ruthenacyclobutane anolog related to structure 27 starting with the methylidene analog of Grubbs second-generation catalyst. See E. F. van der Eide, R E. Romero, and W. E. Piers, J. Am. Chem. Soc., 2008,130, 4484. 

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