Hoveyda - Grubbs type complex

These studies were extended to the Hoveyda-Grubbs type complexes [59]. Here, Verpoort and coworkers compared the catalytic activity of unsymmetrical complexes 74-79 with the classical Hoveyda-Grubbs complexes 3 and 11 (Figure 11.15). However, no improvement in activity was observed in any of the tested metathesis reactions. In this study, the increase of steric interactions led to a significant decrease in activity thus, 79, the most sterically hindered complex, was the least active catalyst for all olefin metathesis reactions tested. 

Scheme 10.13 RCM of JV-tosyl diallylamine catalyzed by a biotinylated Hoveyda-Grubbs type in complex with avidin [55].

As a practical example of application of this method, the Miyano reaction proved useful for the preparation of chiral binaphthyl Hoveyda-Grubbs-type catalyst 47, which finds utility in asymmetric metathesis (Scheme 8.10) [37]. This chiral complex possessing a stereogenic Ru center can be prepared in greater than 98% enantiomeric purity without resolution from optically pure binaphthalene 46 prepared by the Miyano method. Howevo", in contrast to the Meyers reaction, the Miyano reaction has so far not been employed for the total synthesis of natural products. 

A Grubbs-type ruthenium complex and a Hoveyda ruthenium complex were compared under similar conditions for recycled activity. Both the reference catalysts showed a large drop in metathesis activity in the subsequent tests. For example, a Grubbs-type ruthenium alkylidene catalyst showed a drop of nearly 50% conversion in the second run. 

The mechanisms in operation for some important classes of alkene metathesis catalyst have been studied density functional theory (DFT) studies have also provided useful insights into reaction mechanisms. In particular, the initiation mechanisms of Grubbs-type and Hoveyda-type complexes have been explored. In each case, insights into the effects of structure on initiation rate have been achieved. 

Vorfalt T, Wannowius KJ, Plenio H. Probing the Mechanism of Olefin Metathesis in Grubbs—Hoveyda and Grela Type Complexes. Angetv Chem Int Ed. 2010 49(32) 5533-5536. 

Since the discovery of the above-mentioned complexes several improvements have been obtained by a fine-tuning of the electronic and steric properties of the ligands. We will here present the recent modifications made to the Grubbs- and Hoveyda-type complexes. 

Nonhalide hgands were also used for the synthesis of polymer supported (PS-DVB) and nonsupported Grubbs- and Hoveyda-type complexes [30]. For instance, catalyst 18 bearing fluorinated carboxylate was prepared and exhibited a catalytic activity sometimes higher than that of the chlorinated 3 (Figure 3). 

Also, novel solid-phase bound Hoveyda-type catalysts were successfully applied in RCM reactions conducted in SCCO2 for which very low level of leaching of Ru into the product were observed performance of the catalysts highly depended on the nature of the support. Second-generation bis-silylated FIoveyda-Grubbs-type Ru-alkylidene complexes have also been covalently bonded to the sihca matrix of MCM-41. ... 

Olefin metathesis reaction that reorganizes carbon-carbon double bonds provides fundamentally new strategies for natural product synthesis and polymer chemistry. Hilvert and coworkers built up an artificial metalloenzyme by covalently tethering a Grubbs-Hoveyda-type Ru complex to a protein scaffold [78]. An /V-heterocyclic carbene (NHC) ligand, which has been reported as a suitable ligand for a number of water-soluble ruthenium-based metathesis catalysts, was derivatized with an electrophilic bromoacetamide. The Ru carbene complex (27 in Figure 10.16) was then attached by site-selective alkylation of the cysteine... 

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. 

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