Rhodium‐catalysed hydroformylation of 1‐octene using aryl and ferrocenyl Schiff base‐derived ligands

Monometallic and heterobimetallic complexes of Rh(I) bearing chelating N ,O ‐bidentate aryl‐ and ferrocenyl‐derived ligands have been synthesised via Schiff base condensation reactions, and characterised fully using ¹H NMR, ¹³C{¹H} NMR and Fourier transform infrared spectroscopies, elemental analysis and mass spectrometry. The new monometallic and heterobimetallic complexes were evaluated as potential catalyst precursors in the hydroformylation of 1‐octene at 95°C and 40 bar. The ferrocenylimine mononuclear compounds were inactive in the hydroformylation experiments. The Rh(I) monometallic and the ferrocene–Rh(I) heterobimetallic pre‐catalysts displayed good activity and conversion of 1‐octene as well as outstanding chemoselectivity towards aldehydes in the hydroformylation reaction.     

Confining Phosphanes Derived from Cyclodextrins for Efficient Regio‐ and Enantioselective Hydroformylation

Two confining phosphane ligands derived from either α‐ or β‐cyclodextrin produce singly P^III‐ligated metal complexes with unusual coordination spheres. High‐pressure NMR studies have revealed that rhodium hydride complexes of the same type are also formed under hydroformylation conditions. This unique feature strongly favors the formation of the branched aldehyde at the expense of the linear one with high enantioselectivity in the rhodium‐catalyzed hydroformylation of styrene.     

A dinuclear phosphite rhodium(I) complex obtained by syngas treatment of a Rh/hydroxy phosphonite olefin hydroformylation precatalyst

A hydroxy phosphonite was found to be unstable during the catalyst preformation routine applied towards a rhodium olefin hydroformylation catalyst. C—P bond cleavage occurred when the phosphonite was reacted with [(acac)Rh(1,5‐COD)] (acac is acetyl acetate and 1,5‐COD is cycloocta‐1,5‐diene) at 80 °C and 20 bar of CO/H₂. As a result, a nearly planar six‐membered ring structure consisting of two rhodium(I) cations and two bridging phosphorous acid diester anions was formed, namely bis[μ‐(4,8‐di‐tert‐butyl‐2,10‐dimethoxydibenzo[d,f][1,3,2]dioxaphosphepin‐6‐yl)oxy]‐1:2κ²P:O;1:2κ²O:P‐bis{[6‐([1,1′‐biphenyl]‐2‐yloxy)‐4,8‐di‐tert‐butyl‐2,10‐dimethoxydibenzo[d,f][1,3,2]dioxaphosphepine‐κP]carbonylrhodium(I)} toluene tetrasolvate, [Rh₂(C₂₂H₂₈O₅P)₂(C₃₄H₃₇O₅P)₂(CO)₂]·4C₇H₈. Further coordination of phosphite and of carbonyl groups resulted in 16‐electron rhodium centres.     

Heteroatom-substituted secondary phosphine oxides (HASPOs) as decomposition products and preligands in rhodium-catalysed hydroformylation

O,O'-3,3'-Di-tert-butyl-5,5'-dimethoxy-1,1'-biphenyl-2,2'-diyl phosphonate (1) is the hydrolysis product of several mono- and bis-phosphites used as ligands in industrial hydroformylation and other catalytic reactions. As a result of a tautomeric equilibrium, this pentavalent heteroatom-substituted phosphine oxide (HASPO) can rearrange to the corresponding trivalent phosphorus compound. The latter is able to react with typical rhodium-containing precursors frequently used for the generation of catalysts. The resulting species were characterised by NMR spectroscopy and X-ray structure analysis. Proof is given that a rhodium complex of 1 forms an active hydroformylation catalyst. Moreover, 1 can add to aldehydes, which are generated as products in the hydroformylation. Thus a broad range of subsequent reactions can be associated with the degradation of the original phosphite ligands, which has a strong influence on the overall outcome of the hydroformylation reaction.     

Rh‐ and Pt‐catalyzed cycloisomerization of enynes derived from terpenes

The Rh‐catalyzed cycloisomerization of new terpenoid derivatives featuring an O‐tethered enyne unexpectedly leads to polycyclic derivatives containing an inner cyclopropane ring or a diene moiety, depending on the structure of the enyne, as observed in the PtCl₂‐catalyzed processes. Copyright © 2008 John Wiley & Sons, Ltd.     

Carbon Dioxide Induced Phase Switching for Homogeneous‐Catalyst Recycling

SwitchPhos : Rhodium complexes formed from PPh₃ ligands functionalized with weakly basic amidine groups are highly active catalysts for the hydroformylation of alkenes. On bubbling with CO₂ in the presence of water, the yellow rhodium complexes move into the water phase, whereas bubbling with N₂ at 60 °C causes them to switch back into the organic phase. The catalysts can be used for reactions in water or an organic phase.

     

Spiroketal‐Based Phosphorus Ligands for Highly Regioselective Hydroformylation of Terminal and Internal Olefins

A new class of bidentate phosphoramidite ligands, based on a spiroketal backbone, has been developed for the rhodium‐catalyzed hydroformylation reactions. A range of short‐ and long‐chain olefins, were found amenable to the protocol, affording high catalytic activity and excellent regioselectivity for the linear aldehydes. Under the optimized reaction conditions, a turnover number (TON) of up to 2.3×10⁴ and linear to branched ratio (l/b) of up to 174.4 were obtained in the Rh^I‐catalyzed hydroformylation of terminal olefins. Remarkably, the catalysts were also found to be efficient in the isomerization–hydroformylation of some internal olefins, to regioselectively afford the linear aldehydes with TON values of up to 2.0×10⁴ and l/b ratios in the range of 23.4–30.6. X‐ray crystallographic analysis revealed the cis coordination of the ligand in the precatalyst [Rh( 3 d )(acac)], whereas NMR and IR studies on the catalytically active hydride complex [HRh(CO)₂( 3 d )] suggested an eq–eq coordination of the ligand in the species.     

Highly regioselective hydroformylation with hemispherical chelators

The hemispherical diphosphites (R,R)- or (S,S)-5,11,17,23-tetra-tert-butyl-25,27-di(OR)-26,28-bis(1,1'-binaphthyl-2,2'-dioxyphosphanyloxy)calix[4]arene (R=OPr, OCH(2)Ph, OCH(2)-naphtyl, O-fluorenyl; R=H, R'=OPr) (L(R)), all with C(2) symmetry, have been synthesised starting from the appropriate di-O-alkylated calix[4]arene precursor. In the presence of [Rh(acac)(CO)(2)], these ligands straightforwardly provide chelate complexes in which the metal centre sits in a molecular pocket defined by two naphthyl planes related by the C(2) axis and the two apically situated R groups. Hydroformylation of octene with the L(Pr)/Rh system turned out to be highly regioselective, the linear-to-branched (l:b) aldehyde ratio reaching 58:1. The l:b ratio significantly increased when the propyl groups were replaced by -CH(2)Ph (l:b=80) or -CH(2)naphthyl (l:b=100) groups, that is, with substituents able to sterically interact with the apical metal sites, but without inducing an opening of the cleft nesting the catalytic centre. The trend to preferentially form the aldehyde the shape of which fits with the shape of the catalytic pocket was further confirmed in the hydroformylation of styrene, for which, in contrast to catalysis with conventional diphosphanes, the linear aldehyde was the major product (up to ca. 75 % linear aldehyde). In the hydroformylation of trans-2-octene with the L(benzyl)/Rh system, combined isomerisation/hydroformylation led to a remarkably high l:b aldehyde ratios of 25, thus showing that isomerisation is more effective than hydroformylation. Unusually large amounts of linear products were also observed with all the above diphosphites in the tandem hydroformylation/amination of styrene (l:b of ca. 3:1) as well as in the hydroformylation of allyl benzyl ether (l:b ratio up to 20).     

A new rhodium complex with a nitrogen‐containing bis(phosphine oxide) ligand as an efficient catalyst for the hydroformylation of styrene

A new rhodium complex with a nitrogen‐containing bis(phosphine oxide) ligand has been synthesized. The complex was applied to hydroformylation of styrene and displayed high activity and regioselectivity towards the branched aldehyde, which was found to be higher than those of the tertiary bis(phosphine) analogue. Copyright © 2006 John Wiley & Sons, Ltd.     

Rhodium complexes with a new chiral nitrogen‐containing BINOL‐based diphosphite or phosphonite ligand: synthesis and application to hydroformylation of styrene and/or hydrogenation of prochiral olefins

Two new cationic rhodium(I) complexes with a chiral nitrogen‐containing BINOL‐based diphosphite or phosphonite ligand have been synthesized. Chiral diphosphite was prepared by the reaction of N‐phenyldiethanolamine with two equivalents of [(R)‐(1,1′‐binaphthalene‐2,2′‐diyl)]chlorophosphite. In its rhodium complex the ligand is bound to the metal via both phosphorus atoms, and a Rh–N interaction is also possible. Synthesis of the chiral phosphonite was achieved by the reaction of 2‐(N,N‐dimethylaminophenyl)‐bis(diethylamino)phosphine with one equivalent of R‐BINOL. In its rhodium complex, the ligand is P,N‐bonded, forming a five‐membered chelate ring. The first complex was applied to hydroformylation of styrene and displayed high activity and chemo‐ and regioselectivity, but unfortunately no asymmetric induction was found. Both complexes were evaluated in the hydrogenation of prochiral olefins with moderate activities and low enantioselectivities. Copyright © 2005 John Wiley & Sons, Ltd.     

Rhodium/Phosphine catalysed selective hydroformylation of biorenewable olefins

This work reports rhodium catalyzed selective hydroformylation of natural olefins like eugenol, estragole, anethole, prenol and isoprenol using biphenyl based Buchwald phosphine ligands (S‐Phos ( L ₁ ), t‐Bu XPhos ( L ₂ ), Ru‐Phos ( L ₃ ), Johnphos ( L ₄ ) and DavePhos ( L ₅ ). Ru‐Phos ( L ₃ ) ligand exhibited high impact on the hydroformylation of eugenol providing high selectivity (90%) of linear aldehyde as major product. In addition, internal natural olefins like anethole and prenol provided moderate to high selectivity (65% and 85% respectively) of branched aldehydes as a major products. The various reaction parameters such as influence of ligands, P/Rh ratio, syngas pressure, temperature, time and solvents have been studied. A high activity and selectivity gained on the way to the linear aldehydes it may be due to the bulky, steric cyclohexyl and isopropoxy groups present in L ₃ phosphine ligand. Moreover, this catalytic system was smoothly converting natural olefins into corresponding linear and branched aldehydes with higher selectivity under the mild reaction conditions.     

Phosphabarrelenes as ligands in rhodium-catalyzed hydroformylation of internal alkenes essentially free of alkene isomerization

Despite significant research efforts in the past, one of the remaining problems to be solved in industrially important hydroformylation is the chemoselective low-pressure hydroformylation of internal alkenes. We report here on a new class of phosphabarrelene/rhodium catalysts 2 that display very high activity towards hydroformylation of internal alkenes with an unusually low tendency towards alkene isomerization. Preparation of new phosphabarrelene ligands, studies of their coordination properties, as well as results obtained in the rhodium-catalyzed hydroformylation of cyclic and acyclic internal alkenes are reported.     

Highly regioselective rhodium-catalysed hydroformylation of unsaturated esters: the first practical method for quaternary selective carbonylation

Highly regioselective hydroformylation of unsaturated esters can be achieved when a highly reactive, ligand-modified, rhodium catalyst is employed near ambient temperatures (15-50 degrees C) and pressures over 30 bar. The use of 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phosphaadamantane shows distinct advantages over other commonly applied phosphanes in terms of reaction rate, and regio- and chemoselectivity. Hydroformylation of a range 1,1-di- and 1,1,2-trisubstituted unsaturated esters yields quaternary aldehydes that are forbidden products according to Keulemans Rule. The aldehydes can be reductively aminated with molecular hydrogen to give beta-amino acid esters in high yield. The overall green chemical process involves converting terminal alkynes into unusual beta-amino acid esters with only water generated as an essential byproduct. This catalytic system has also been applied to the hydroformylation of simple 1,2-disubstitued unsaturated esters, which have been hydroformylated with excellent alpha-selectivity and good chemoselectivity for the first time.