Monophosphite ligands rhodium complex

The selectivity to the linear product nonanal vas strongly dependent on the CO pressure (see Table 6.4). The linear to branched ratio drops from 29 at Pco = 5 bar to 4 at Pco = 40 bar. Part of this selectivity change can be ascribed to enhanced isomerization at lower CO pressure, but faster P-hydride elimination cannot account completely for the increased formation of the branched aldehyde. The reduced selectivity was attributed to partial ligand dissociation resulting in less selective rhodium monophosphites and/or ligand-free complexes. 

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

Rhodium complexes, formed in situ with [Rh(COD)2]BF4 and monodentate chiral spiro phosphite and phosphine ligands, catalyse the AH of both (Z)- and (E)-P-arylenamides with up to 97% ee. A library of 19 chiral binol-monophosphite ligands containing a phthalic acid secondary bis-amide group has been synthesized and screened for use in stereocontrol of rhodium-catalysed hydrogenation of several prochiral dehydroamino esters and enamides. Spectroscopic and computational studies... 

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