Metathesis Hoveyda-Grubbs catalyst

The remaining lone pair on carbon (which is not delocalized) can coordinate to Ru, just like the phosphine lone pair, giving a catalyst known as Grubbs IT (the second generation of the Grubbs T we made use of in the metathesis described above). In another widely used catalyst (known as the Hoveyda-Grubbs catalyst ) the second phosphine is also replaced by intramolecular coordination. 

E. de Brito Sa , J.M.E. de Matos, Ring closing metathesis by Hoveyda-Grubbs catalysts a theoretical approach of some aspects of the initiation mechanism and the influence of solvent. Inorg. Chim. Acta 426, 20-28 (2015)... 

More recently, Solans-Monfort and coworkers [54] have studied the ringclosing enyne metathesis of l-allyloxy-2-propyne with the second-generation Hoveyda-Grubbs catalyst (Scheme 7.9). The ene-then-yne and yne-then-ene pathways were almost equally favorable in the reactions using unsubstituted enynes. The exo/endo selectivity of the diene product was also investigated. 

Fukuyama and coworkers [48] have also utilized an olefin additive in the optimization of a crucial ring-rearrangement metathesis sequence to form the key 5,6-ring system in the synthesis of (-)-isoschizogamine (Scheme 12.27). The tandem metathesis reaction of the norbornene 87 was difficult due to the substitution pattern of norbornene skeleton, which restricted the approach of the Ru catalyst. For example, the treatment of norbornene 87 with the second-generation Hoveyda - Grubbs catalyst 4 at reflux in benzene provided lactone 89 in only 24% isolated yield. To optimize this reaction, the use ofthe modified Hoveyda-Grubbs catalyst 88 [49] and the addition of 20 equiv of 1,6-heptadiene as an additive were quired. The desired lactone 89 was then obtained in 73% yield. 

In this study we focused on the ethylene-promoted Hoveyda-Grubbs catalyst decomposition. The formation of large amount of propylene, butenes, and ethane was observed as well as different isopropoxystyrene derivatives. Future investigations of substrate role in olefin metathesis catalj decomposition and the mechanism of etiiane formation are in progress. 

In this contribution, we will overview reactivity modes and efficient synthetic routes based on metathesis transformations of vinylsilanes in the presence of Grubbs catalysts. The efficient cross-metathesis of vinylsilanes, carrying a large spectrum of different substituents at silicon, with various olefins in the presence of the first- and second-generation Grubbs catalyst and Hoveyda-Grubbs catalyst is described (Eq. 1). 

In 2008, Butenschon and co-workers reported an unusual variation on the second-generation Hoveyda-Grubbs catalyst, in which the 2-isopropoxybenzy-lidene ligand was coordinated to a highly electron-withdrawing tricarbonylchro-mium moiety (19). In a series of RCM, CM and enyne metathesis reactions carried out to screen its catalytic properties, complex 19 matched or outperformed the results obtained with its parent 15 and related initiators. 

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

Scheme 172). " Cross-metathesis of the terminal alkene with 3,4-dime-thoxystyrene, induced in the presence of the Hoveyda—Grubbs catalyst (7), was followed by oxidation of the alcohol to give the unsaturated ketone (—)-1369 in 85% yield. Alternatively, 1367 could be oxidized to the cyclo-pentenone (- -)-1370 initially, after which a one-pot metathesis cascade with... 

In the same year the Crowe group reported the first example of Z-selective cross metathesis of acrylonitrile and various terminal olefins catalyzed by Schrock catalyst shown in Fig. 21 [48]. Acrylonitrile was required to achieve moderate to good Z-selectivities. A model that minimizes steric interaction between imido ligand of the catalyst and the substituent on the aUcene was proposed to account for the Z-selectivity. In 2001 the Blechert group reported that second-generation Hoveyda-Grubbs catalyst promoted similar reactions with essentially the same level of Z-selectivity (typically 4 1 Z E) [49]. 

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