Issue 7, 2005

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

Abstract

Six calix[4]arenes each bearing two non-cyclic PR2 units attached at distal phenolic oxygen atoms, p-But-calix[4]arene-25,27-(OPR2)2-26,28-(OR′)2 (R = OPh; R′ = Prn, L1111; R = OPh; R′ = CH2CO2Et, L222; R = OPh; R′ = CO2cholesteryl, L333; R = Ph; R′ = Prn, L444; R = Ph; R′ = CH2CO2Et, L555; R = Ph; R′ = CO2cholesteryl, L666) have been synthesized and their coordinative properties investigated. The diphosphites L1111L333, 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)L1111]BF4 and [Rh(COD)L333]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 L333 with the neutral complex [PdCl2(COD)] afforded the chelate complex [PdCl2L333] in 90% yield. The reaction of one equiv. of L1111 with [Rh(acac)(CO)2] resulted in the formation of [Rh(acac)L1111] without requiring high dilution conditions. When the latter reaction was carried out with 0.5 equiv. of L1111, the bimetallic complex [{Rh(acac)(CO)}21-P1-P′-L1111)] was formed instead. Reaction at high dilution of L222 with the cyclometallated complex [Pd(o-C6H4CH2NMe2)(THF)2]BF4 gave the expected chelate complex [Pd(o-C6H4CH2NMe2)L222]BF4. The latter slowly converts in solution to an oligomer in which the ligand behaves as a (η1-P1-P′) bridging ligand, thus leading to a less strained structure. All six ligands, when mixed with [Rh(acac)CO)2], effectively catalyse the hydroformylation of octene and styrene. In the hydroformylation of octene, the linear aldehyde selectivities observed with L222 and L333 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 L222 and L333 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.

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

Article information

Article type
Paper
Submitted
04 Jan 2005
Accepted
17 Feb 2005
First published
03 Mar 2005

Dalton Trans., 2005, 1301-1309

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

S. Steyer, C. Jeunesse, J. Harrowfield and D. Matt, Dalton Trans., 2005, 1301 DOI: 10.1039/B419284B

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