Ruthenium Catalysts in Alkene Metathesis

Indenylidene-Ruthenium Catalysts in Alkene Metathesis 265 Table 8.5 Diene and enyne RCM reactions with 2 mol% of complex XXb at room temperature. 

Indenyli dene-Ruthenium Catalysts in Alkene Metathesis 271... 

Ruthenium Allenylidenes and Indenylidenes as Catalysts in Alkene Metathesis... 

In 1998 it was revealed that allenylidene-ruthenium complexes, arising simply from propargylic alcohols, were efficient precursors for alkene metathesis [12], This discovery first initiated a renaissance in allenylidene metal complexes as possible alkene metathesis precursors, then it was observed and demonstrated that allenylidene-ruthenium complexes rearranged into indenylidene-ruthenium intermediates that are actually the real catalyst precursors. The synthesis of indenylidene-metal complexes and their efficient use in alkene metathesis are now under development. The interest in finding a convenient source of easy to make alkene metathesis initiators is currently leading to investigation of other routes to initiators from propargylic derivatives. 

Other closely related ruthenium-allenylidene were made and evaluated in alkene metathesis [32]. Werner et al. [49] also produced allenylidene complexes of analogous structure to that of the Grubbs catalyst, but containing hemilabile phosphine such as complex X (Scheme 8.9). However, the Ru—O bond may be too stable to initiate the rearrangement into indenylidene, the coordination of alkene and to become a catalyst. 

Two observations initiated a strong motivation for the preparation of indenylidene-ruthenium complexes via activation of propargyl alcohols and the synthesis of allenylidene-ruthenium intermediates. The first results from the synthesis of the first indenylidene complexes VIII and IX without observation of the expected allenylidene intermediate [42-44] (Schemes 8.7 and 8.8), and the initial evidence that the well-defined complex IX was an efficient catalyst for alkene metathesis reactions [43-44]. The second observation concerned the direct evidence that the well-defined stable allenylidene ruthenium(arene) complex Ib rearranged intramo-lecularly into the indenylidene-ruthenium complex XV via an acid-promoted process [22, 23] (Scheme 8.11) and that the in situ prepared [33] or isolated [34] derivatives XV behaved as efficient catalysts for ROMP and RCM reactions. 

NMR studies of degenerate ligand exchange in generation I and generation II ruthenium alkylidene pro-catalysts for alkene metathesis... 

Abstract Ruthenium holds a prominent position among the efficient transition metals involved in catalytic processes. Molecular ruthenium catalysts are able to perform unique transformations based on a variety of reaction mechanisms. They arise from easy to make complexes with versatile catalytic properties, and are ideal precursors for the performance of successive chemical transformations and catalytic reactions. This review provides examples of catalytic cascade reactions and sequential transformations initiated by ruthenium precursors present from the outset of the reaction and involving a common mechanism, such as in alkene metathesis, or in which the compound formed during the first step is used as a substrate for the second ruthenium-catalyzed reaction. Multimetallic sequential catalytic transformations promoted by ruthenium complexes first, and then by another metal precursor will also be illustrated. 

Previous studies on allenylidene-ruthenium complexes as alkene metathesis catalysts revealed that on thermal reaetion they produced a new active species that was also evidenced by kinetic studies and spectroscopic observations [44]. This species was identified arising from another observation the profitable influence of strong acid addition [11], Thus the RCM of A,A-diallyltosylamide led to a TOF of 10.5/h with complex 7a (80 C, 3 h, 70 %) and to a TOF of 53/h (room temperature, 1 h, 75 %) when five equivalent of TfOH or HBF4 were added to complex 7a. More drammatically, the ROMP of eyclooctene with 7a was achieved at room temperature in 15 h (TOF = 63/h), whereas only 1 min was necessary when five equivalent of TfOH were added to 7a (TOF=57.200/h) (Table 5). It was clear that strong acid addition promoted the generation of a new very active species. 

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