Hoveyda first-generation catalyst

Scheme 4.2 (a) Carbenes 4 and 5 were reacted with bispyridyl complex 7 to yield complexes 8 and 9 and (b) Grubbs-Hoveyda, first-generation catalyst (10) was reacted with 4-6... 

These complexes showed higher thermal stabihty in toluene at 80 °C than the Hoveyda first-generation catalyst, with half-lives ranging from 3 to 6 h, depending on the nature of the Schiff base-derived hgand. They also showed latent catalyst behavior, as only moderate-to-low olefin metathesis activity was observed at room temperature in CM and RCM [44]. On the other hand, these complexes were active in the ROMP of cyclooctene and cyclopentene. The NHC-containing catalyst was found to be especially efficient, leading to a TOP of 667 min at room temperature [43]. 

Scheme 1.7 Synthesis of natural product-like molecules with unprecedented scaffold diversity. Initially, building blocks were added iteratively to a fluorous-tagged linker, with intermediates purified by fluorous-solid phase extraction. Metathesis cascades were used to reprogramme the scaffolds and to release final products from the fluorous-tagged linker. Reagents and conditions. (1) Grubbs first-generation catalyst, 21a 23% 21b 56% (2) fluorous-tagged Hoveyda-Grubbs second-generation eatalyst, 21c 33%.

The most commonly used Ru catalysts are described in Scheme 1, the Gmbbs first-generation catalyst 1 [9], the Nolan 2 [10], and Gmbbs second-generation catalyst 3 [11], the Hoveyda catalyst 4 [12], the phosphine-free Hoveyda catalyst 5 [13], the allenylidene precursor 6 [14] leading to active indenyhdene catalyst 7 [15], and the indenylidene complex 8 [16]. 

Figure 2 Metathesis catalysts Grubbs first-generation catalyst (5), Grubbs second-generation catalyst (6), Hoveyda—Grubbs catalyst (7), and Grubbs—Nolan catalyst (8).

It is noteworthy that these five syntheses used four different metathesis catalysts in the key alkene forming step. For the cyclization of 7, the use of the Grubbs first generation catalyst Gl, that couples terminal alkenes but tends not to interact with internal alkenes, was probably critical to success. The Hoveyda catalyst H2 is more expensive than the Grubbs second generation catalyst G2, but can often be effective in appHcations in which G2 is sluggish. The Schrock Mo catalyst is less user fiiendly than the (relatively) air and moisture stable Ru catalysts, but is very reactive. 

Hoveyda and coworkers [227] used a domino process to give chromanes 6/3-8 by treatment of 6/3-7 in the presence of ethylene. One of the first-generation Grubbs catalyst 6/3-9 and one of Blechert s [228] early examples allowed the synthesis of bicyclic compounds of different sizes, depending on the length of the tether thus, the reaction of 6/3-10 led to 6/3-11 using 30 mol% of the Schrock Mo complex 6/3-12. 

Fig. 3 Olefin metathesis catalysts Schrock tungsten (Cl) and molybdenum (C2) alkylidene complexes, Grubbs first- (C3) and second-generation (C4) catalysts, Hoveyda-Grubbs second-generation catalyst (C5), and Grubbs third-generation catalyst (C6)...

The ene-yne CM of fatty acid-derived terminal alkenes with several alkyne derivatives was shown by Bruneau et al. [75], These reactions, which led to renewable conjugated dienes, were performed in a one-pot two-step procedure. In the first step, the ethenolysis of methyl oleate was performed in the presence of the first-generation Hoveyda-Grubbs catalyst (2.5 mol%) using dimethyl carbonate as solvent at room temperature. After completion of the ethenolysis (90% conversion), C4 (1 mol%) and the corresponding alkyne (0.5 equivalents with respect to olefins) were added and the reaction was run at 40°C for 2 h (Scheme 9). The desired dienes were thus obtained in high yields close to the maximum theoretical value (50%). Moreover, in order to maximize the formation of functional dienes, the same reaction sequence was applied to the diester obtained by SM of methyl oleate. In this way, the yield of functional dienes was increased up to 90% depending on the... 

In the articles on metathesis reactions, ruthenium compounds of organometallic intramolecular-coordination five-membered ring compounds are used for what are generally referred to as Hoveyda-Grubbs first-generation 12 and second-generation 1.3 catalysts (Fig. 1.1) [68, 69]. 

II) first-generation Grubbs catalyst (III) second-generation Grubbs catalyst and (IV) second-generation Hoveyda-Grubbs catalyst (HG-II) [5]. 

The first-generation Hoveyda-Grubbs catalyst decays completely within 4 h in the presence of ethylene in CD2CI2 solutions at 55 C, but its mode of decomposition is not well understood [49]. These types of catalysts are not susceptible to unimolecular degradation involving phosphine attack on the methyiidene, since these systems do not dissociate a phosphine ligand. It is conceivable that... 

The ruthenium indenylidene complex 6 has also been used as precursor for the first-generation Grubbs- and Hoveyda-type catalysts, as shown in Scheme 14.13. It was shown by Nolan [39] that the reaction of styrene with 6 afforded the first-generation Grubbs catalyst 21 in excellent yield. This procedure benefits from the use of commercially available starting materials and avoids the use of diazo compounds. 

The thermal stabihty of complex 57a was found to be improved relative to the corresponding first-generation ruthenium-indenylidene complex 6 and Hoveyda catalysts 56 [70]. At 110 °C, complex 57a showed only 50% decomposition after 6 d, whereas 50% decomposition ofthe Hoveyda-I complex 56 was reached within 2 d. The non-chelated ruthenium-indenylidene 6 survived only a few hours at 80 C (Scheme 14.29) [70]. 

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