Hoveyda-type complexes

There is a catalyst (13) in which the phosphine ligand is ionophilic. The Hoveyda-type complexes 14 possessing a hindered pyrrolidine carbene ligand show comparable activities to the standard catalysts. 

There have been other reports of immobilization of Hoveyda-type complexes on polystyrene supports. Blechert and co-workers have extended their methodology for immobilization of NHC complexes to the preparation of 172 (Scheme 70). The supported NHC ligand was prepared in the same manner as for 166, the metal complex then being attached to the support via a ligand exchange reaction. Metal loadings of 0.7mmolg resin were obtained. The authors also report a metal complex 174 bound to the support via... 

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. 

The second of the two most popular classes of metathesis precatalysts is the so-called Hoveyda-type complexes that bear a chelating alkoxystyrene ligand, which was discovered serendipitously during metathesis reactions of 2-alkoxystyrene compounds. 

Table 2.5 Influence of substrate and precatalyst structure on initiation rate of a series of Hoveyda-type complexes (NB entries 3 and 4 are GH2, while 10 is Gr2) " ...

Table 2.6 Initiation rates for Hoveyda-type complexes (at 298 K in DCM), as determined by UV/visible studies with EVE as substrate (25-200 mmol...

At around same time that the initiation mechanism of Hoveyda-type complexes was probed in detail, doubt was cast on another aspect of their... 

In this section, the initiation mechanisms of some common metathesis precatalysts have been discussed. Benzylidene, indenylidene, and Hoveyda-type complexes are the most widely used classes of metathesis precatalyst, and therefore the understanding of their properties is highly important. 

Plenio has developed an alternative way in which to treat concentration/ time data from RCM reactions mediated by Hoveyda-type complexes. The authors conducted a series of kinetic experiments (at 303—323 K in toluene-dg) using a range of precatalysts, at different concentrations, and with different precatalyst loadings. Using a detailed and a priori mathematical approach, the authors successfully separated values for rate constants for activation (kaci), catalysis (fe t) and decomposition (fejec), showing that each was concentration-independent (Eqn (2.21)). 

Scheme 2.61 Release of alkene ligands during initiation of Hoveyda-type complexes energies are relative to precatalyst plus A/rA/-diallylmethanesulfonamide starting...

The study of the initiation of Hoveyda-type complexes has been hampered by the issues involved in treating the entropy of intermolecular reactions. Opinions differ on the best way to treat this issue, with some researchers preferring to consider a pre-activated complex in which the two reactants are present in the same initial DFT calculation, while others prefer to treat the two reagents at infinite separation.Calculations of the thermodynamics of ring-closing have met similar issues, with a correction being required to calibrate DFT data (using experimental EM data) to obtain sensible EM values. " When no such correction is applied, DFT... 

Recently, Plenio et al. [63] described the synthesis of ferrocenyl-substituted Hoveyda-type complexes 101. These complexes exhibited latency toward the ROMP of COE at 20 C in toluene/CH2Cl2 (12 1) solution. Activation was achieved by the addition of acetylferrocenium tetrafluoroborate which oxidized the complexes, converting the electron-donating ferrocene into an electron-withdrawing ferrocenium unit (Scheme 12.15). The oxidized complexes showed significant ROMP activity, both by chemical and electrochemical oxidation. 

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). 

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