Ligands isopropoxy

The search for even more active and recyclable ruthenium-based metathesis catalysts has recently led to the development of phosphine-free complexes by combining the concept of ligation with N-heterocyclic carbenes and benzyli-denes bearing a coordinating isopropoxy ligand. The latter was exemplified for Hoveyda s monophosphine complex 13 in Scheme 5 [12]. Pioneering studies in this field have been conducted by the groups of Hoveyda [49a] and Blechert [49b], who described the phosphine-free precatalyst 71a. Compound 71a is prepared either from 56d [49a] or from 13 [49b], as illustrated in Scheme 16. 

The overall catalytic cycle is believed to involve various titanium complexes which all have at least one isopropoxy ligand attached to the metal (Scheme 1). Given this fact, it is evident that kind and structure of the alkoxide can influence the catalysis, in particular the chirality transfer step (4—>5, via 2) and the displacement of the product sulfoxide from 5 to regenerate 3. Evidence for this assumption was obtained in studies with both other titanium alkoxides and alcohols such as methanol. In all cases less efficient catalyst systems resulted. 

The second strategy, implemented to increase the efficiency of Hoveyda-type catalyst, uses electronic effects to weaken the ruthenium-oxygen bond in 6. Thus, Grela reported the synthesis of catalyst 20 bearing a nitro group in para position to the isopropoxy ligand [32] (Scheme 12). 

The six coordinated titanium(IV) compounds, Ti(acac)2(X)2, where X is methoxy, ethoxy, isopropoxy, -butoxy, or chloro, all adopt the cis-configuration. This is beheved to result from the ligand-to-metal TT-electron donation (88,89). 

The ligands N-[bis(isopropoxy)thiophosphoryI]thiobenzamide and N-[bis(isopro-poxy)thiophosphoryl]-N -phenylthiourea can be deprotonated with the acetylacetonate gold(III) derivative [Au(C6F5)2(acac)] [26] giving the corresponding pentafluorophenyl complexes with the ligand acting as a chelate one [192]. 

Palladium(II)-copper(I) cocatalyzed couplings of several (/ ,/ -dialkylvinyl)phenyliodo-nium triflates with 4-isopropoxy-3-tri- -butylstannyl-3-cyclobutene-l,2-dione (71) in DMF at room temperature have recently been described (equation 259)157. As observed in earlier studies of Cu(I)- or Pd(II)-promoted reactions of vinyliodonium salts, the vinyl ligands are introduced with retention of configuration. Since iodobenzene is a byproduct of ligand coupling, the production of 3-phenyl-4-isopropoxy-3-cyclobutene-1,2-dione might also be expected. However, the (/ ,/ -dialkylvinyl)iodonium ions are much more reactive... 

This problem can be overcome by tuning the ligand sphere of catalyst Ru-4. For example, both the isopropoxy-tethered ruthenium complex (H2lMes)(Cl)2Ru=CH-... 

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