ABSTRACT
Abstract Bidentate phosphorus ligands bearing an O-acyl phosphite moiety show superior modifying properties to the rhodium catalyst used in the hydroformylation of internal olefins. Results obtained for the hydroformylation of internal octenes and 2-pentene, respectively, are presented. The new ligands do markedly enhance the isomerization activity of the rhodium center. Internal hydroformylation is clearly disfavoured. At 120 °C/ 20 bar CO/H2, a predominant terminal reaction is achieved. Thus, a 0.65...0.8 molar fraction of the desired terminal product is obtained with an aldehyde chemoselectivity exceeding 99.7%. Depending on the ligand structure and the olefinic substrate used, excellent turn over frequencies between 3000 and 7000 h-1 have been estimated. Further results concerning the coordination behaviour of the new ligands towards the precatalyst [acacRh(COD)] itself, as well as high pressure NMR investigations in the formation of O-acylphosphite-phosphite hydrido rhodium complexes, are presented. Introduction The regioselective synthesis of n-aldehydes is an important goal of the rhodium catalyzed alkene hydroformylation reaction. Considerable progress has been achieved within this field by the development of bidentate phosphines and phosphites as modifying ligands for the hydroformylation of terminal olefins.1 However, for industry the use of less reactive internal olefins as substrates for the production of terminal aldehydes is of interest also. Internal olefins are available on an a technical scale by low-cost routes and are hydroformylated mainly with the aid of modified/unmodified cobalt catalysts.2,3 It is still a greater challenge to develop rhodium catalysts able to convert these substrates with the desired good n-regioselectivity and high activity. Thus, diphosphine modified rhodium(I) allows for selectivities of ~90% to n-nonanal starting from 2-octene but turn over frequencies are below 400 h-1, better rates are obtained with the shorter 2-butene as a
substrate.4,5 Interesting even from the more practical point of view were results obtained for a hydroxy phosphonite which in the presence of rhodium forms a ligand mixture and finally leads to ~50% nonanal from internal octenes at a TOF = 2840 h-1, comparable to that of the unmodified cobalt counterpart used in industry.6,7 High activity for 1-olefin hydroformylation and isomerization is known for several phosphite derived catalysts, therefore encouraging to conduct further research in this field.8,9 Herein we report on the outstanding performance of O-acylphosphite modified rhodium catalysts used in the hydroformylation of internal octenes and 2-pentene. Results and Discussion A catalyst used for the n-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1alkenes. Additionally, β-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise iso-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of naldehyde (reaction d) as result of a dynamic kinetic control.
