Grubbs-Hoveyda complex

Chiral silver complexes bearing bidentate NHC ligands (24) have been synthesized. They are used in alkene metathesis and allylic alkylation reactions high diastereos- (g) electivity is observed induced by the chiral backbone on the prochiral biphenyl.27 Ruthenium-based complexes obtained from transmetalation with a Grubbs-Hoveyda complex exhibited high activities and enantioselectivities in ring-opening metathesis/ ... 

A computational study of ring-closing enyne metathesis of l-allyloxy-2-propyne with the Grubbs-Hoveyda complex has revealed that ene-then-yne and yne-then-ene pathways proceed through equivalent steps, the difference being the order in which they take place there is no clear energetic preference for either. 

Mechanistic studies have been used to attempt to explain the rapid initiation rate of the 18-electron pyridine solvates. Using the reported initiation rate of 4a [6], the upper limit of free energy of activation was determined to be 15.45kcalmol" at 5°C using the Eyring equation [39], Similar to the Grubbs-Hoveyda complexes, these precatalysts have multiple potential pathways by which they can initiate (e.g., interchange or dissociative). An associative mechanism can be ruled out due to the coordinatively saturated, six-coordinate nature of these complexes. 

Further evaluation of these new Z-selective catalysts showed that, for the bidentate anionic hgand complexes, initiation rates similar to the second-generation Grubbs-Hoveyda complex 3a could be achieved through modification of the complex s electronic and steric environment. Solvent was shown to play a role in overall reaction yields, but its effects were shown to be related to catalyst decomposition, not the precatalyst initiation. 

Ashworth IW, Hilher IH, Nelson DJ, Percy JM, Vincent MA. Olefin metathesis by grubbs-hoveyda complexes computational and experimental studies of the mechanism and substrate-dependent kinetics. CS Catal. 2013 3(9) 1929—1939. 

Thiel V, Hendann M, Wannowius K-J, Plenio H. On the Mechanism of the Initiation Reaction in Grubbs—Hoveyda Complexes. J Am Chem Soc. 2011 134(2) 1104-1114. 

The simpler architecture is the 1,1 -biphenyl scaffold, likewise introduced by Hoveyda and coworkers [19]. The synthesis of the imidazolium salt starts with a chiral diamine and a substituted, achiral biphenyl [82-84], Subsequent introduction of a Mes substituent on the remaining primary amino end and ring closure reaction yields the chiral saturated imidazolium salt after hydrolysation of the methoxy group to liberate the phenolic hydroxy group (see Figure 4.22). Reaction with silver(I) oxide and carbene transfer to a Grubbs (Hoveyda) catalyst sets up the ruthenium catalyst complex. 

General aspects and new metathesis catalysts. For alkene metathesis Grubbs I (1) and Grubbs II (2, 3) complexes, and the Grubbs-Hoveyda catalyst (4A) and Grela catalyst (4B) remain the workhorses. 

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... 

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 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. 

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. 

It is noteworthy that the Grubbs-Hoveyda catalyst can polymerize monosub-stituted and diphenylacetylenes [67]. The Grubbs-Hoveyda catalyst and a series of Ru carbene complexes catalyze the polymerization of o-substituted PhAs, as represented by (o-isopropoxy)phenylacetylene [68]. The substituents at the ortho position of monomers are assumed to serve as supportive ligands that maintain and prolong the life of unstable propagating carbene species. 

The sequential combination of RCM and double-bond isomerization was recently employed by You s group [42] for the design of an RCM-isomerization-Pictet-Spengler cascade for a highly efficient enantioselective synthesis of tetrahydro-P-carbolines 21, where a Hoveyda-Grubbs II complex and a chiral phosphoric acid (CPA-H) cooperatively acted as a bicatalytic system (Scheme 12.13). 

---