Rhodium complexes hexafluoro

Based on the precedent of Van Leeuwen and Roobeek, livinghouse and co-workers screened a variety of electron-deficient phosphine/phosphite ligands for the rhodium-catalyzed [4-1-2] reaction, which provided an alternative catalyst system for the formation of 5,6- and 6,6-ring systems [13]. The most notable of these was the tris-(hexafluoro-2-propyl) phosphite-modified rhodium complex, which was applicable to both carbon- and oxygen-tethered substrates, and also provided the first example of a facial-directed diastereoselective intramolecular rhodium-catalyzed [4-i-2] reaction (Eq. 4). 

Recently, the air- and water-stable combinations of l-n-butyl-3-methyl-imidazolium chloride with sodium tetrafluoroborate or sodium hexafluoro-phosphate have been prepared. The rhodium complexes [RhCl(PPh3)3] and [Rh(COD)2](BF4) are completely soluble in these ionic liquids and catalyze the hydrogenation of cyclohexene in a typical two-phase reaction with numbers of turnovers of up to 6000 (131). 

The rhodium complex 42, containing the first example of an 2-hexafluoro-benzene ligand, has been prepared and crystallographically characterized (92). 

Beyond the 3+ oxidation state, rhodium forms a limited number of complexes in the 4+, 5+ and 6+ oxidation states. A recent review1221 gives an excellent summary of the chemistry of the higher oxidation state chemistry of Rh (as well as Ru, Os, Ir, Pd and Pt). For the 4+ oxidation state, the hexafluoro, hexachloro and trioxo dianions are well characterized. The known neutral species include RhF4 and some oxides. There are also scattered reports of Rhlv complexes containing substituted biguanides and Schiff base chelates. 

The complex has enjoyed relatively little use in organic synthesis. For iridium-catalyzed homogeneous hydrogenation of alkenes, Crabtree s iridium complex ((1,5-Cycloocta-diene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluoro-phosphate) is generally preferred, although this readily prepared Ir complex is active. It is more reactive than its rhodium counterpart in the catalytic isomerization of butenyl- to allylsilanes. ... 

The cationic complex (VII, X = Cl) forms a symmetrical ( Pnmr i.r.) complex with diphenylacetylene, which is very labile. The complexes [Rh2Cl2(/Lt-CO)(/Lc-acet)(dppm)2], where acet is hexafluoro-2-butyne or acetylenedicarboxylic acid dimethyl ester, are also symmetrical, and the acet ligand is coplanar with the rhodium atoms. The C C bond length is increased to 1.32 A, and so the complex is truly described as a cis-dimetallated alkene. Comparison with the reactions with CO suggests that the activation of an alkyne (or alkene) to hydrogenation may occur when the alkyne is initially coordinated to the catalyst at a terminal position, e.g., (XII), with subsequent oxidative addition and hydrogen transfer steps. A complex of the type (XII) may instead be only a minor but active component of a solution of which the alkyne-bridged complex is the major but less active component. ... 

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