Ring-closing metathesis reactions

Fig. la—d Typical alkene metathesis reactions ring-closing (RCM) and ring-opening (ROM) metathesis (a), diene cross metathesis (CM, b), ROM-RCM (c), and ROM-double RCM (d) sequences (ring-rearrangement reactions, RRM)... 

Fig. 2a-c Typical enyne metathesis reactions ring-closing enyne metathesis (a) enyne cross metathesis (b and c)... 

Fig. 3a,b Typical diyne metathesis reactions ring-closing alkyne metathesis (RCAM, a) diyne cross metathesis (ACM, b)... 

Fig. 4.12. Another variation of the olefin metathesis reaction ring-closing metathesis (RCM).

Among the various types of olefin metathesis reactions, ring-closing metathesis (RCM) attracted the most attention due to its excellent efficiency for the synthesis of large rings. 

Banfi L, Basso A, Guanti G, Riva R (2003) Application of tandem Ugi reaction/ring-closing metathesis in multicomponent synthesis of unsaturated nine-memhered lactams. Tetrahedron Lett 44 7655-7658... 

Catalytic processes using diazo chemistry and stoichiometric methods with Fischer carbenes are complimentary for the introduction of a substituted carbene into a molecule. For methylene addition, however, there is no viable alternative to the modified Simmons-Smith reaction. Ring-closing metathesis, and its ROMP counterpart, have matured so fast that even now they rank among the most useful synthetic methodologies in organic chemistry. 

SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage TBC, triple bond cleavage ADMET, acyclic diene metathesis RCM ring-closing metathesis ROMP ring-opening metathesis polymerization... 

Olefin metathesis is a unique carbon skeleton redistribution in which unsaturated carbon-carbon bonds are rearranged in the presence of metal carbene complexes. With the advent of efficient catalysts, this reaction has emerged as a powerful tool for the formation of C-C bonds in chemistry [1]. Olefin metathesis can be utilized in five types of reactions ring-closing metathesis (RCM), ring-opening metathesis (ROM), respective ringopening metathesis polymerization (ROMP), cross-metathesis (CM), and acyclic diene metathesis polymerization (ADMET). 

Scheme 1.34 summarizes the most characteristic metathesis reactions. The following abbreviations are used CM (cross metathesis), RCM (ring-closing metathesis), ROM (ring-opening metathesis), ROMP (ring-opening metathesis polymerization) and ADMET (acyclic diene metathesis polymerization). 

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. 

Pauson-Khand reaction ring-closing metathesis ring-opening metathesis reticulated vitreous carbon anode transannular Diels-Alder reaction terf-butyldiisopropylsilyl terf-butyldimethylsilyl tert-butyllithium triethylsilyl... 

Olefin metathesis has been of interest both industrially and academically because of its great synthetic utility (4). Mechanistically related metathesis reactions include ring opening metathesis (ROM), ring-closing metathesis (ROM), cross-metathesis (CM), acyclic diene metathesis (ADMET) polymerization, and ring-opening metathesis polymerization (ROMP) (Fig. 12) (5-8,390,391). 

However, other than molybdenum-based complexes, none of the Ru complexes were active enough to perform important metathesis reactions such as difficult cross-metathesis and ring-closing metathesis (RCM) of substituted double bonds. 

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]. 

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