Alkylidene metal catalyst derivatives

Alkene metathesis is a catalytic reaction that has brought revolutions during the last 15 years, not only in catalysis and organic synthesis but also in polymer and material science. This is due to the discovery of the catalytic mechanism based on metal-caibene by Chauvin [1] and of well-defined, efficient catalysts from 1990 based on coordinatively unsaturated alkylidene-metal complexes mainly derivatives of molybdenum by Schrock [2] and of ruthenium (Ru) by Gmbbs [3]. The increasing importance of alkene metathesis and its catalysts by the scientific community has led to the award of 2005 chemistry Nobel Prize to the main pioneers in this field, Chauvin, Gmbbs, and Schrock [1-3]. 

The catalytic desymmetrization shown in Scheme 5 involves a meso-tetraene substrate optically pure unsaturated siloxane 23 is obtained in >99% ee and 76% yield [16], The unreacted siloxy ether moiety is removed to deliver optically pure 24. Mo-alkylidenes derived from both enantiotopic terminal alkenes in 22 are likely formed. Since metal-alkylidene formation is reversible, the major product arises from the rapid RCM of the matched segment of the tetraene. If any of the mismatched RCM takes place, a subsequent and more facile matched RCM leads to the formation of the meso-bicyclized product. Such a byproduct is absent from the unpurified mixture containing 23, indicating the exceptionally high degree of stereodifferentiation induced by the chiral Mo complex. As before, catalyst 4a is not effective in promoting ARCM of 22. 

Klabunde et al have found that alkylidene derivatives of phosphorus catalysed methylene exchange among olefins. In contrast to metal-based metathesis catalysts, the phosphoms-based catalysts are effective with olefins in which the C=C bond is conjugated with a functional group such as CN or COOR. 

Cyclopentadienyl derivatives of ruthenium were first complexes of this metal which were found to be able to catalyze ATRP 1, 7-10). Subsequently carborane (11-12) and alkylidene (13) rathenium complexes were employed as ATRP catalysts. 

More recently, a Pd(II) salt was shown to catalyze the 1,2-insertion polymerization of a 7-oxanorbornadiene derivative (Fig. 10-16) [50]. The resulting saturated polymer, when heated, gives polyacetylene via a retro-Diels-Alder reaction. (This reaction is reminiscent of the Durham route to polyacetylene discussed below). One advantage of this technique over other routes is that it employs a late transition metal polymerization catalyst. Catalysts using later transition metals tend to be less oxophilic than the d° early transition metal complexes typically used for alkene and alkyne polymerizations [109,110]. Whereas tungsten alkylidene catalysts must be handled under dry anaerobic conditions, the Pd(II)-catalyzed reaction of water-insoluble monomers may be run as an aqueous emulson polymerization. 

Computational studies have provided a valuable working hypothesis for the Z-selectivity exhibited by cyclometalated catalyst 6 [31, 71, 72], In contrast to the bottom-bound metallacycles observed with previous generaticMis of ruthenium metathesis catalysts (cf. Sect. 2.1), it is proposed that ruthenacycles derived from 4 and 6 adopt a side-bound conformation (7a). The ratimiale for the preferential formation of side-bound ruthenacycles is twofold (Fig. 1) first, there are significant steric interactions present between the developing metaUacylobutane and the adamantyl moiety in the bottom-bound conformation (7b) that are alleviated in the side-bound conformation. Moreover, the bottom-bound conformation is destabilized as it requires back-donation from the same ruthenium d-orbital that is back-donating into the NHC this competition is alleviated in the side-bound conformation, as two separate metal d-orbitals are now available for back-donation into both the NHC and alkylidene carbon p-orbitals (Fig. 1) [71]. 

Covalently attaching the alkylidene to the NHC ensured the proximity of this polymer end to the necessary metal center and enabled the ROMP of COD or dodecatriene to afford cyclic polybutadiene (Figure 20). Later, the same catalyst was also employed with dendritic norbomene derivatives to afford the corresponding cyclic polymer. However, this area still remains challenging and a field for future improvements. 

Metathesis pre-catalysts typically bear NHCs with ortho-tolyl, mesityl, or 2,6-di(isopropyl)phenyl substituents. Complexes bearing At-phenyl substituents underwent extensive decomposition via C-H activation under relatively mild conditions. Although the NHC remained attached to the metal throughout this process, the alkylidene was compromised. Similarly, Blechert reported that derivatives of the Hoveyda catalyst bearing aryl ligands without substitution in the ortho position underwent C-H activation in air, resulting in new, unreactive all lidenes. Restricting the rotation of the N-aryl... 

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