Schrock s molybdenum catalysts

The difference in reactivity is perfectly revealed in Metz s total synthesis of the molluscicidal furanosesquiterpene lactones ricciocarpin A (50) and B (51) (Scheme 9) [32]. Attempts to convert acrylate 43 to lactone 44 using Grubbs5 catalyst A or Schrock s molybdenum catalyst B resulted in very low yields of the... 

Diene 265, substituted by a bulky silyl ether to prevent cycloaddition before the metathesis process, produced in the presence of catalyst C the undesired furanophane 266 with a (Z) double bond as the sole reaction product in high yield. The same compound was obtained with Schrock s molybdenum catalyst B, while first-generation catalyst A led even under very high dilution only to an isomeric mixture of dimerized products. The (Z)-configured furanophane 266 after desilylation did not, in accordance with earlier observations, produce any TADA product. On the other hand, dienone 267 furnished the desired macrocycle (E)-268, though as minor component in a 2 1 isomeric mixture with (Z)-268. Alcohol 269 derived from E-268 then underwent the projected TADA reaction selectively to produce cycloadduct 270 (70% conversion) in a reversible process after 3 days. The final Lewis acid-mediated conversion to 272 however did not occur, delivering anhydrochatancin 271 instead. 

Alkene cross-metathesis has also been recently used for the modification of silsesquioxanes and spherosilicates, by Feher and co-workers [46]. Reaction of vinylsilsesquioxane 28 with a variety of simple functionalised alkenes, in the presence of Schrock s molybdenum catalyst 3, gave complete conversion of the starting material and very good isolated yields of the desired products (75— 100%) (for example Eq. 28). 

Using 1,5-hexadiene, it was shown that, depending upon whether molybdenum- or ruthenium-based catalysts are employed, a change in mechanism appears to occur. In the presence of Schrock s molybdenum catalyst, 1,5-hexadiene produces principally linear poly(l-butenylene) [scheme (24)] [33], but with Grubbs s ruthenium catalyst the primary product is the cyclic dimer 1,5-cyclooctadiene [scheme (25)] [25,33] ... 

Most of the recent synthetic applications of M-RCM involve one of the above catalysts, particularly G1 or G2, chosen as a function of its own reactivity profile, generally after preliminary reaction assays on the genuine substrate or specific model compounds. The sensitivity of the RCM reaction to steric hindrance is well established. These ruthenium catalysts exhibit high affinity for carbon-carbon double bonds and are compatible with the presence of many functional groups, even the presence of free polar hydroxyl or amino groups. Their use does not require special conditions such as glove boxes, which are required when using Schrock s molybdenum catalyst. 

Facing the decisive ring-closure step, Schrock s molybdenum catalyst was used to promote the stereocontrolled annulative metathesis of diene 67, which ultimately yielded carbaffuctofuranose 64 after exhaustive debenzylation. 

This approach would neither be possible, nor conceivable, without the advent of modern olefin metathesis catalysts. Figure 3 shows a few of the most commonly used catalysts. In this work, we initially relied upon Schrock s Molybdenum catalyst 6 (7) to effect the ring closures, but now exclusively rely upon the second generation Grubbs ruthenium catalyst 7 (8). 

RCM has attracted much attention and has seen a tremendous increase in synthetic applications over the last decade <2000CR2963, 2006JOM(691)5129>. In this reaction, two C-C multiple bonds, such as double and double, or double and triple in the same molecule, are converted to unsaturated carbocycles or heterocycles in the presence of a metal carbene complex. The versatility of Schrock s molybdenum catalyst and Grubbs ruthenium complexes 68 and 69 (Scheme 10) in carbo- and heterocyclizations, respectively, of very different ring sizes were demonstrated <2000CR2963, 2006JOM(691)5129>. 

Hoveyda and co-workers have developed chiral catalysts for asymmetric alkene metathesis. They have demonstrated that with their chiral molybdenum catalyst asymmetric syntheses of dihydrofurans through catalytic kinetic resolution by RCM and enantioselective desymmetrization by RCM are feasible processes (Scheme 40) <1998JA9720>. The use of Schrock s molybdenum catalysts for asymmetric alkene metathesis has been reviewed <2001CEJ945>. 

To a degassed solution of the diene (130 mg, 0.34 mmol) in benzene (4.0 mL) under nitrogen atmosphere at 20 °C was added Schrock s molybdenum catalyst in benzene (0.13 mL of 100 mg of catalyst in 1.0 mL of benzene, 0.02 imnol). (Note it is recommended that this catalyst is handled in a glove box or glove bag under an inert atmosphere.) The solution was heated to 60 °C for 1 h, at which point TLC analysis indicated that the reaction was complete. The reaction mixture was concentrated. The crude product was purified via chromatography eluting with 20% ether in hexanes to afford 105 mg (88%) of the hzi-olefin as a colorless oil. 

In the presence of Schrock s molybdenum catalyst Mo(CHCMe2Ph)(NAr)[OCMe(CF3)2] yields of 18-90% and total turnover numbers of 4-25 were achieved. As a matter of feet, the ubiquitious availability of terminal olefins combined with their low prices makes this methodology potentially usefiil for industrial applications. 

More recently, the pincer iridium catalysts for alkane dehydrogenation, in combination with Schrock s molybdenum catalyst for olefin metathesis (Chapter 21), has been shown to catalyze alkane metathesis (Equation 18.47). This combination of catalysts provides... 

The overall scheme to generate the supported catalysts is shown in Equation 21.3a. Partially dehydrated silica is treated with a solution of Schrock s tungsten catalyst in pentane. The acidic hydroxyl groups protonate an alkyl group to form the siloxide-supported catalyst. In a similar fashion, partially dehydrated silica is treated with a solution of a bispyr-rolyl version of Schrock s molybdenum catalyst to release pyrrole and generate a catalyst... 

The synthesis of manzamine A by Martin and co-workers using olefin metathesis - was one of the earliest demonstrations - of how this reaction could be applied as a strategy in natural product synthesis. The ring-closing metathesis in this synthesis was conducted with Schrock s molybdenum catalyst, as shown in Equation 21.8. 

Schrock s molybdenum catalyst [Mo] 2, however, produced a low molecular weight polymer when exposed to 2-methyl-l,5-hexadiene [61]. Interestingly, it was shown that the monomer initially dimerized to 2,9-dimethyl-l,5,9-decatriene through metathesis of the unsubstituted vinyl group. Over time, however, the substituted vinyl groups underwent CM with the internal olefins to produce 1,4-polyisoprene. This was the first example of the condensation of... 

A series of polycarbonates has been synthesized via ADMET with Schrock s molybdenum catalyst [90]. A diene containing a bisphenol-A unit was polymerized to a polymer with an of 1.5 x lO gmol and a PDI of 1.9 (Figure 13.11). 

Schenk S, Kirchner B, Reiher M. A stable six-coordinate intermediate in ammonia— dinitrogen exchange at Schrock s molybdenum catalyst. Chem Eur J. 2009 15 5073-5082. 

For example, diallylmethylchlorosilane has been synthesized using techniques described by Bums and has been condensed to form oligomers at room temperature using Schrock s molybdenum catalyst system (75). Further, bis-4-pentenyl-dichlorosilane has been produced and condensed as well by this chemistiy 16), These reactions are shown in Figure 16. The NMR data for the dichloro-... 

Only 16 years have elapsed since Grubbs and Fu reported that Schrock s molybdenum catalyst ([Mo-I[) could be used to induce efficient cyclization of functionaHzed a,a)-diene-amines. Since then, a wide range of RCM reactions to form the nitrogen heterocyclic products have been reported, including applications in the synthesis of natural products. These applications have been greatly facihtated by the advent... 

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