Initiators metathesis

W(CCMe3)(0CMe3)3 reacts rapidly with unsymmetric acetylenes to give the initial metathesis products, RC=CCMe3 and/or R C CCMe3, and the symmetric acetylenes catalytically. The most impressive is the metathesis of 3-heptyne where the value for k (M-1 sec-1) 1s between 1 and 10. Therefore, in neat 3-heptyne ( 1 M) at 25° the number of turnovers 1s of the order of several per second. If we assume that W(VI) or Mo(VI) alkylidyne sites or complexes are responsible for the relatively slow metathesis in the known heterogeneous (30) and homogeneous (31) systems, then 1t becomes clear that the concentration of active species on the surface or 1n solution must be extremely small. 

Ring closing and cross metathesis allow the rapid synthesis of simple cyclic and acyclic systems. The metathesis activity that is now possible using well-defined catalysts allows for the rapid generation of complexity from simple starting materials by relay processes and combinations of metathesis steps. Many of these reactions have been recognized only recently, are now beginning to be used in complex synthetic transformations. A few of these types of reactions will be outlined here to demonstrate the power of these multistep, relay processes. In these processes, an initial metathesis step leads to a new carbene that results in further transformations of the substrate. 

The same neopentylidene-alkoxo complexes react with cis-2-pentene to give the two initial metathesis products (4,4-dimethyl-2-pentene and 5,5-dimethyl-3-hexene) and catalyze the metathesis of cis-2-pentene to 2-butenes and 3-hexenes. Furthermore, propylene and ethylene appear in the reaction medium as the catalyst deactivates. These latter olefins are formed by rearrangement of the ethylidene and propylidene intermediates, providing the mechanism for the metathesis chain termination step ... 

Studies with CH3CD=CH2 (Mol 1970), CH3 CH-CH2, CH3CH= CH2, and CH3CH=CH2 (Mol 1968) on Re207/Al203 (activated in dry air at 580°C) show that the initial metathesis reaction occurs cleanly at 85°C, but the product but-2-ene... 

A novel class of metathesis catalysts derives from methyltrioxorhenium, CH3Re03 (MTO) [22]. Combined with certain solid supports such as Si02/Al203 or Nb205, highly active catalysts are generated [23, 24]. They even tolerate fimctional groups such as ketones, esters, and carboxylic acids. Based upon NMR spectroscopic evidence [25] and in accord with theoretical studies, surface-attached Re=CH2 species initiate metathesis via a novel type of tautomerism [Eq. (3)]. 

By 1992, Grubbs and co-workers had discovered an alternative catalyst that overcame many of these shortcomings. Indeed, although ruthenium alkylidene 12 (Scheme 5) displays a lower metathesis activity than Schrock s molybdenum systems, it importantly demonstrated air stability and the ability to initiate metathesis in the presence of alcohols, water, and carboxylic acids. Thus, 12 represents the first true catalyst for general bench top olefin metathesis reactions, and over time has been optimized to 13 (Scheme 6), which has proven far easier to prepare than the parent structure 12 and constitutes the current gold standard with which all new catalyst systems are compared. Without question, this... 

Equally significant are the last two catalyst systems shown in Scheme 6. As mentioned above, strongly basic ligands promote the activity of ruthenium-based metathesis systems of the type defined by 13 as such, the discovery by Herrmann and co-workers in 1998 that a modified variant of 13 bearing more strongly a-donat-ing A(-heterocyclic carbene ligands (15, Scheme 6) could initiate metathesis is quite important. " Despite this potentially beneficial feature, however, the overall metathesis activity of 15 is only a... 

Equally important as recycling, problems involved with the use of certain reagents can often be completely avoided if they are attached to a resin. For instance, the ruthenium alkylidene catalysts typically employed to initiate metathesis reactions often decompose into darkly colored, metal-containing by-products, unwanted materials that sometimes require special protocols to remove and which are... 

In some instances, the subsequent chemistry can actually assist ruthenium removal. An example of this phenomenon was observed during a later hydrogenation step of an initial metathesis product using a palladium-carbon catalyst... 

Initial metathesis of the substrate C=C bond gives MeCH=CR(OR) and a C=W carbene intermediate. This forms a metalacycle with the nearby alkyne and metathesis-like steps lead to product. 

The propene flow (purity 99.95%) was admitted (8 ml.min ) sequentially in each reactor for one horn in order to measure the initial metathesis activity of the catalyst. 

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