Bis-phosphites and bis-phosphinites based on distally-functionalised calix[4]arenes: coordination chemistry and use in rhodium-catalysed, low-pressure olefin hydroformylation

Six calix4]arenes each bearing two non-cyclic PR2 units attached at distal phenolic oxygen atoms, p-Bu t-calix4]arene-25,27-(OPR2)2-26,28-(OR')2(R = OPh; R'= Prn, L1; R = OPh; R'= CH2CO2Et, L2; R= OPh; R'= CO2 cholesteryl, L3; R = Ph; R'= Prn, 4; R = Ph; R'= CH2CO2Et, L5; R = Ph; R'= CO2cholesteryl, L6) have been synthesized and their coordinative properties investigated. The diphosphites L1-L3, where the P centres are separated by 12 bonds, readily form chelate complexes provided the complexation reaction is achieved either by using a starting complex that possesses good leaving groups or by operating under high dilution in order to avoid oligomer formation. Thus, the cationic complexes Rh(COD)L1]BF4 and Rh(COD)L3]BF4 were both formed in high yield by reacting the appropriate diphosphite with either Rh(COD)(THF)2]BF4 or Rh(COD)2]BF4. At high dilution, reaction of L3 with the neutral complex PdCl2(COD)] afforded the chelate complex PdCl2L3] in 90% yield. The reaction of one equiv. of L1 with Rh(acac)(CO)2] resulted in the formation of Rh(acac)L1] without requiring high dilution conditions. When the latter reaction was carried out with 0.5 equiv. of L1, the bimetallic complex {Rh(acac)(CO)}2(eta]1-P,eta1-P'-L1)] was formed instead. Reaction at high dilution of with the cyclometallated complex Pd(o-C6H4CH2NMe2)(THF)2]BF4 gave the expected chelate complex Pd(o-C6H4CH2NMe2)]BF4. The latter slowly converts in solution to an oligomer in which the ligand behaves as a (eta1-P,eta1-P') bridging ligand, thus leading to a less strained structure. All six ligands, when mixed with Rh(acac)CO2], effectively catalyse the hydroformylation of octene and styrene. In the hydroformylation of octene, the linear aldehyde selectivities observed with L2 and L3 are significantly higher (linear : branched =ca. 10) than those obtained with the other 4 ligands of this study and also with respect to PPh3. Molecular modelling shows that the lower rim substituents of and form tighter pockets about the metal centre than do the other ligands and so sterically favour the formation of Rh(n-alkyl) intermediates over that of Rh(i-alkyl) ones. In styrene hydroformylation, all ligands result in the formation of unusually high amounts of the linear aldehyde, the b : l ratios being all close to 65 : 35. The highest activities were found when using an L/Rh ratio of 1/1.