· (MW 918.79)
(catalyst for hydroformylation, hydrogenation, isomerization, and hydrosilylation of alkenes. It has also been used as a carbonylation catalyst and as a catalyst for the conjugate addition of activated Michael donors)
Alternate Names: hydridocarbonyltris(triphenylphosphine)rhodium(I); carbonyltris(triphenylphosphine)rhodium(I) hydride.
Physical Data: mp 172-174 °C (under N2), 120-122 °C (in air).2
Solubility: moderately sol CHCl3, CH2Cl2, and C6H6; sparingly sol cyclohexane; insol light petroleum and EtOH.1,3
Form Supplied in: yellow solid, widely available.
Preparative Methods: can be prepared from RhCl3.3H2O, PPh3, and formaldehyde (see cautionary note below).2,4 It can also be prepared from other rhodium complexes,1 for example by the reaction of hydrazine with trans-carbonylchlorobis(triphenylphosphine)rhodium(I) in ethanolic solution.
Handling, Storage, and Precautions: is moisture sensitive and decomposes in many solvents above 80 °C. The preparation of the complex from RhCl3.3H2O, PPh3, and formaldehyde (see above) results in the liberation of HCl which can react with formaldehyde to form the potent carcinogen bis(chloromethyl) ether.
Hydroformylation (Oxo Reaction).1,5
RhH(CO)(PPh3)3 has largely displaced cobalt-based catalysts in industrial hydroformylations. The aldehyde products are reduced to the corresponding alcohols for use in the plastics industry.1 Whereas the earlier process involved operating at high temperatures and pressures, the use of RhH(CO)(PPh3)3 allows the reaction to occur at or below atmospheric pressure and at room temperature.6,7 The catalyst is often added to reactions as a solid. However, it can also be generated in situ from other rhodium complexes under typical hydroformylation conditions. In all these cases RhH(CO)(PPh3)3 is the actual precursor to the active catalysts, RhH(CO)(PPh3)2 and RhH(CO)2PPh3 (eq 1).8
For aliphatic alkenes, much emphasis has been placed on optimization of hydroformylation in favor of the linear product (eq 2) (R = alkyl). The best results are obtained by employing low gas pressures and a 1:1 ratio of H2 to CO, in the presence of added Triphenylphosphine.1,9 Added diphosphines with bite angles near 120° also increase selectivity for the linear product.10 Excess triphenylphosphine retards the rate of reaction and lower pressures of CO lead to competing hydrogenation and isomerization.11
Steric hindrance in the alkene can lead to excellent regioselectivity as well as selectivity for a terminal alkene in preference to a trisubstituted alkene (eq 3).12 A bulky silyl substituent also controls hydroformylation regioselectivity.13 The use of a polymer-supported catalyst also leads to a high ratio of linear to branched product.14
Hydroformylation of styrene is quite selective for the branched isomer. Reaction of the corresponding tricarbonylchromium complex, tricarbonyl(h6-styrene)chromium, improves this selectivity dramatically (from 87:13 to >98:2, in favor of the branched isomer),15 as does reaction with 1,2,3,4,5-pentafluorostyrene.16 Addition of diphenylpyridinylphosphine also results in improved regioselectivity (as high as 94:6 in favor of the branched isomer) if the reaction is carried out under high pressure.17
The influence of chelation control on regioselectivity has been examined using phosphorus,18,19 nitrogen,20,21 and sulfur22 substituents. In the case of nitrogen this has led to the synthesis of a variety of heterocycles, although often as isomeric mixtures (eq 4).21
Carbonylation is sometimes a competing process. In reactions with alkenylamines, carbonylation is generally favored where the amine is unhindered and nucleophilic.23,24 Double hydroformylations are also possible.25,26 Alkynes are usually less reactive than alkenes under typical hydroformylation conditions.23,27
Many studies of asymmetric hydroformylations have been carried out using chiral rhodium-based catalysts. Rather than preparing pure, chiral catalysts it is often simpler to generate such catalysts in situ by adding chiral diphosphine ligands to RhH(CO)(PPh3)3 in solution.28-32 Naturally, RhH(CO)(PPh3)3 is no longer the active catalyst in these reactions. Asymmetric hydroformylation of methyl 2-(acetylamino)acrylate using (-)-DIOP as the added chiral ligand provided the branched product with excellent regioselectivity and reasonable enantioselectivity (eq 5).33
If an alkene such as ethylene is treated with RhH(CO)(PPh3)3 and CO only (i.e. no H2) then co-oligomerization can occur, leading to polymers of the type H-(CH2CH2CO)
n-Et or H-(CH2CH2CO) n-OR.34 RhH(CO)(PPh3)3 also catalyzes hydroacylation35 and silylformylation36 of alkenes.
RhH(CO)(PPh3)3 is a hydrogenation catalyst selective for terminal alkenes. Under standard conditions, RhH(CO)(PPh3)3 is less efficient than the widely used Wilkinson's catalyst, Chlorotris(triphenylphosphine)rhodium(I). However, the selectivity of Wilkinson's catalyst derives from differences in double bond stereochemistry, with little difference in rate of hydrogenation of terminal, internal, or cyclic alkenes.38 RhH(CO)(PPh3)3, on the other hand, can catalyze the hydrogenation of terminal alkenes at ambient temperature and pressure in the presence of internal, conjugated, or cyclic double bonds (eq 6).38,39 At higher temperatures and pressures, internal40 and cyclic alkenes41 can also be hydrogenated.
Functional groups such as keto, aldehyde, hydroxy, cyano, chloro, and carboxy are not reduced by the complex. RhH(CO)(PPh3)3 will not catalyze the hydrogenation of alkynes. However, it will react stoichiometrically with alkynes to give stable complexes.42,43 Complexes generated from RhH(CO)(PPh3)3 (or Rh6(CO)16 or Rh4(CO)12) and diphosphines will catalyze the hydrogenation of a,b-unsaturated aldehydes.44 In the presence of chiral diphosphines this reaction is enantioselective, as in the hydrogenation of neral to produce (+)-citronellal (eq 7).44
RhH(CO)(PPh3)3 is a homogeneous catalyst for the isomerization of terminal alkenes to give the more stable internal alkene. Initially, the (Z)-2-alkene is formed and this is then slowly isomerized to the (E)-2-alkene.45,46 The isomerization of 3-phenoxyprop-1-ene has been achieved with RhH(CO)(PPh3)3; however, the yields and selectivity are not as good as those obtained using Dichlorotris(triphenylphosphine)ruthenium(II).47 The exo-methylene of a fused five-membered lactone has been isomerized in the presence of RhH(CO)(PPh3)3 to internalize the double bond.48 Double bond migration has also been observed in nitrogen heterocycles, such that cyclic N-allylamides are isomerized to the cyclic enamide.49 Allylic alcohols
are isomerized to aldehydes in the presence of RhH(CO)(PPh3)350 and when chiral diphosphines are added a modest degree of enantioselectivity is observed.51 b-Trimethylsilyl allylic alcohols have been isomerized to provide a stereoselective synthesis of trimethylsilyl enol ethers.52 This reaction is believed to proceed via the a-trimethylsilyl ketone as the direct transformation of a-trimethylsilyl ketones into trimethylsilyl enol ethers has been demonstrated.53 The stereoselective synthesis of a,b-unsaturated ketones has been achieved by the RhH(CO)(PPh3)3-catalyzed isomerization of vinyl epoxides.54 Other RhI catalysts were also shown to catalyze this isomerization and the best yields and selectivities were obtained with Hydridotetrakis(triphenylphosphine)rhodium (eq
An example of a catalytic isomerization in the presence of RhH(CO)(PPh3)3 that does not involve a double bond isomerization is the conversion of cis-2,3-epoxycyclohexan-1-ol to the trans isomer.55
As with many hydrogenation catalysts, the efficacy of RhH(CO)(PPh3)3 as a hydrosilylation catalyst has been investigated. RhH(CO)(PPh3)3 has been used in the hydrosilylation of a number of alkenic substrates, including 1-hexene,56 styrene,56,57 1,3-butadiene,58 and acrylonitrile.59 In the first two examples, RhCl(PPh3)3 and RhCl(CO)(PPh3)3, respectively, were found to be superior catalysts. In the last two examples, under certain conditions, RhH(CO)(PPh3)3 proved to be the most effective of the catalysts that were investigated. Alkynes have also been hydrosilylated to produce both (Z)- and (E)-alkenes.60
In the presence of RhH(CO)(PPh3)3, activated Michael donors such as ethyl cyanoacetate, malononitrile, and benzyl cyanide undergo conjugate addition to a,b-unsaturated carbonyl compounds in high yield.61 In the presence of the chiral trans chelating diphosphine ligand 2,2´´-bis(1-diphenylphosphinoethyl)-1,1´´-biferrocene (TRAP), and using a-substituted a-cyanocarboxylate as donor, this reaction proceeded with high enantioselectivity (eq 9).62
RhH(CO)(PPh3)3 has been used in carbonylation reactions of amines63 and alkenes64 and in cross-carbonylation of alkynes and alkenes.65 The complex has also been used as a decarbonylation catalyst,66 although it does not perform as well as Wilkinson's catalyst in such reactions. RhH(CO)(PPh3)3 has been used, albeit less efficiently than Pd/Al2O3, to effect the rearrangement of tetrahydrofurylhydrosilane to give the corresponding oxasilacyclohexane in which the silicon had migrated into the ring.67 The reduction of nitrobenzene by cyclohexanol in the presence of RhH(CO)(PPh3)3 has been reported,68 although Rh(CO)2Cl2 was found to be
a superior catalyst for this reaction. RhH(CO)(PPh3)3 also undergoes many inorganic ligand exchange reactions,1 which are not detailed here as they are presently of little value to the synthetic chemist.
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Maree P. Collis & Patrick Perlmutter
Monash University, Clayton, Victoria, Australia
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