Ring-closing metathesis reaction transition metal-catalyzed

Recent advances on the synthesis of piperidines through ruthenium-catalyzed ring-closing metathesis reactions 12H(84)75. Rhodium-catalyzed [2+2+2] cycloaddition for the synthesis of substituted pyridines, pyridones, and thiopyranimines 12H(85)1017. Synthesis of 2,2 -bipyridines by transition metal-catalyzed alkyne/nitrile [2+2+2] cycloaddition reactions 12H(85)1579. 

Metallocyclobutanes from cyclopropanes have been frequently invoked in transition metal-catalyzed rearrangements of strained ring hydrocarbons, and this body of chemistry is quite rich and diverse, as evidenced in the excellent review by Bishop (72). Because of this diversity, the significance of isolated observations should not be overstated nevertheless, certain reactions outlined by Bishop are closely related to the carbene retroadditions reported by Gassman and co-workers using metathesis catalysts. 

Since the alkene formed in this reaction can further react with other alkenes, many products should be formed in the cross-metathesis (CM). Therefore, in the early days, only ring-closing metathesis (RCM) of diene was investigated. It is known that the reaction is catalyzed by a transition metal. Pioneering work on olefin metathesis was undertaken by Villemin and Tsuji, who reported the synthesis of lactones using alkene metathesis ... 

During the present decade, a wide variety of polycyclic carbacephem derivatives have been reported starting from readily available monocyclic /3-lactams, which after transformation in more functionalized compounds and further cyclization yielded different fused carbacephems. Several approaches for the preparation of fused carbacephem derivatives including cycloaddition reactions such as the [2+2], 1,3-dipolar, and Diels-Alder reactions, as well as transition metal-catalyzed reactions such as the Pauson-Khand and ring-closing metathesis (RCM) reactions have been reported in the literature. 

Homogeneous transition metal-catalyzed olefin metathesis—encompassing reactions such as ROMP, acyclic diene metathesis (ADMET), ring-closing metathesis (RGM), and metathesis of acyclic olefins—is a focal issue in both organic synthesis and polymer chemistry, and such reactions have been performed in ILs. 

The transition-metal-catalyzed olefin metathesis is a still recent, but already well-established, development, which has evolved toward a highly useful synthetic tool [27]. The ring-closing variant of this reaction ]ring-closing metathesis (RCM)] has proved to be very suitable for the preparation of carbo- and heterocycles of any ring size, except for those that are strained ]28, 29]. In the case of 5,6-dihydropyran-2-ones, RCM of homoaUyl acrylates has very often been used for the direct creation of this heterocyclic system, most particularly in the past decade (Figure 2.4). Allyl esters of fi,y-unsaturated acids may also be subjected to RCM with the formation of P,y-unsaturated 8-lactones, which can be easily isomerized by acid or base catalysis to their a,fi-unsaturated counterparts ]6, 30-39]. 

Several transition metal complexes can catalyze the exchange of partners of two double bonds. Known as the olefin metathesis reaction, this process can be used to close or open rings, as well to interchange double-bond components. 

In this section, we have concentrated on the use of transition-metal nucleophilic-carbene catalysts to bring about ring-closing alkene metathesis reactions. These same types of compounds can also be used to catalyze a remarkable reaction called ringopening alkene metathesis polymerization (ROMP). A special value of ROMP is that it can be used to prepare highly unsaturated polymers. For a discussion of ROMP techniques, see Section 29.6E. 

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