Theoretical olefin metathesis

Sakaki S (2005) Theoretical Studies of C-H s-Bond Activation and Related by Transition-Metal Complexes. 12 31-78 Satoh T, see Miura M (2005) 14 1-20 Satoh T, see Miura M (2005) 14 55-84 Savoia D (2005) Progress in the Asymmetric Synthesis of 1,2-Diamines from Azomethine Compounds. 15 1-58 Schmalz HG, Gotov B, Bbttcher A (2004) Natural Product Synthesis. 7 157-180 Schmidt B, Hermanns J (2004) Olefin Metathesis Directed to Organic Synthesis Principles and Applications. 13 223-267... 

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. 

Cundari, T. R., Gordon, M. S. Theoretical investigations of olefin metathesis catalysts. Organometallics 902,11, 55-63. 

Cavallo, L. Mechanism of Ruthenium-Catalyzed Olefin Metathesis Reactions from a Theoretical Perspective. J. Am. Chem. Soc. 2002, 124, 8965-8973. 

A number of theoretical studies support the results of the experimental work by Grubbs and Wagener. Chen performed a quantum mechanical/molecular mechanics (QM/MM) study on the olefin metathesis reaction comparing (2a) and (4a). The... 

There have been several theoretical studies on the olefin metathesis reaction and the structures of homogeneous catalysts [1-5]. However, we have not found examples of theoretical modelling of olefin metathesis active sites on heterogeneous catalysts. In the case of the heterogeneous molybdena catalysts, the active centres contain probably Mo [6,7], but other Mo valences are also possible (e.g., Mo [8]). 

Handzlik, J. and Ogonowski, J. (2000) Theoretical study on active sites of molybdena-alumina catalyst for olefin metathesis, in Corma, A., Melo, F.V., Mendioroz, S. and Fierro, J.L.G. (eds.). Studies in Surface Science and Catalysis 130, pp. 1181-1186. 

A special volume is dedicated to the papers from the 9th International Symposium on Olefin Metathesis.274 xhe development of well-defined metathesis catalysts is the subject of a review s and some theoretical calculations. 276 However, the development of mixed component systems continues with molybdenum and rhenium carbonyls and nitrosyl precursors enjoying topical popularity. 

In the last decade, impressive advances have been made in the synthesis of alkylidene and alkylidyne complexes that contain metals from groups 4 [5] and 5 [5i,j,m, 6], especially Ti and V, but - except for the ROMP of norbornene by vanadium complexes - group 4 and 5 metals have not shown wide-ranging activity for olefin metathesis. Well-characterized rhenium(VII) complexes are known to be active for metathesis, and many rhenium(Vll) alkylidene and alkylidyne complexes have been isolated [2a], but little attention has been paid to the syntheses of rhenium alkylidene or alkylidyne complexes in the last decade. Theoretical calculations have been carried out on M(X)(CHR )(Y)(Z) complexes... 

Oxo-alkyhdene complexes (e.g., W(0)(CHR)X2, where X is a chloride, alkoxide, etc.) are likely to be the catalysts in at least some of the classical olefin metathesis systems [65]. However, few tungsten oxo-alkylidene complexes have been isolated and characterized [66]. Of note, several have now been prepared that contain one or more terphenoxide ligands [41, 42]. Recent theoretical studies have focused on the subtle differences between imido alkylidene complexes and oxo-alkylidene complexes in olefin metathesis reactions [7f]. 

With the general mechanism of olefin metathesis established by experimental work, early theoretical studies focused on the details of several of the steps outlined above. Ligand exchange to form the initial olefin complex could occur by either an associative or dissociative mechanism. Experimental evidence from Grubbs and coworkers [2] pointed to a dissociative process. The structure of the active olefin complex was also a matter of uncertainty, as both bottom-bound (trans to L) and side-bound (cis to L) complexes have been reported (Chapter 8). Finally, the detailed structure and reactivity of the metallacyclobutane have been the focus of several theoretical investigations, as this intermediate was not initially experimentally observed (Chapter 8). 

Theoretical studies have been carried out on various ruthenium indenyhdene and benzyhdene complexes in order to estimate the impact of these ligands on olefin metathesis catalytic activity. A comparative study of the key steps associated with the activation of complexes bearing NHCs revealed only minor differences between the benzyhdene and the indenyhdene complexes [77]. It was recently shown that, due to the different steric and electronic properties of these hgands, not all ruthenium indenyhdene complexes foUow the same initiation mechanism... 

The mechanism of olefin metathesis reaction catalyzed by ruthenium (Ru) carbenoids was intensively studied over last 8 years both experimentally [1] and theoretically involving calculation method [2]. It has been assumed for this reaction to proceed with participant of 14-electix)n-Ru complexes and... 

Theoretical calculations have shown that aromatic N substituents on NHC ligands can exhibit a through space interaction with the metal via a donation from the C l m of the N substituent. This interaction can either be direct or transmitted by an additional group bonded to the metal. The effects on Ru-catalysed olefin metathesis and Pd-catalysed cross-coupling have been computed and were consistent with experimental observations. 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

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