Chlorotris(triphenylphosphine)rhodium(I)1,2

RhCl(PPh3)3

[14694-95-2]  · C54H45ClRh  · Chlorotris(triphenylphosphine)rhodium(I)  · (MW 925.24)

(catalyst precursor for many reactions involving alkenes, alkynes, halogenated organics, and organometallic reagents; notably hydrogenations, hydrosilylations, hydroformylations, hydroborations, isomerizations, oxidations, and cross-coupling processes)

Alternate Name: Wilkinson's catalyst.

Physical Data: mp 157 °C. It exists in burgundy-red and orange polymeric forms, which have identical chemical properties (as far as is known).

Solubility: about 20 g L-1 in CHCl3 or CH2Cl2, about 2 g L-1 in benzene or toluene; much less in acetic acid, acetone, methanol, and other aliphatic alcohols. Virtually insol in alkanes and cyclohexane. Reacts with donor solvents like DMSO, pyridine, and acetonitrile.

Form Supplied in: burgundy-red powder, possibly containing excess triphenylphosphine, triphenylphosphine oxide, and traces of rhodium(II) and -(III) complexes.

Analysis of Reagent Purity: 31P NMR displays resonances for the complex in equilibrium with dissociated triphenylphosphine (CH2Cl2, approximate d ppm: 31.5 and 48.0 {J values: Rh-P1 -142 Hz; Rh-P2 -189 Hz; P1-P2 -38 Hz} shifted in the presence of excess PPh3).3 Triphenylphosphine oxide contaminant can also be observed (CH2Cl2, d ppm: 29.2) but paramagnetic impurities are generally not evident. In rhodium NMR a signal is observed at -1291 ppm.

Preparative Method: good quality material can be obtained using the latest Inorganic Syntheses procedure,4 with careful exclusion of air. Recrystallization is not recommended.

Handling, Storage, and Precautions: the complex should be stored at reduced temperature under dinitrogen or argon. It oxidizes slowly when exposed to air in the solid state, and faster in solution. Such partial oxidation can influence the catalytic efficacy. Consequently, the necessary precautions are governed by the reaction in question. For mechanistic and kinetic studies, reproducible results may only be obtained if the catalyst is freshly prepared and manipulated in an inert atmosphere; even the substrate should be treated to remove peroxides. For hydrogenations of alkenes on a preparative scale, complex that has been handled in the air for very brief periods should be active, but competing isomerization processes may be enhanced as a result of partial oxidation of the catalyst. At the other extreme, exposure to air just before use is clearly acceptable for oxidations in the presence of O2 and t-BuOOH.

Background.1

In solution, Wilkinson's catalyst is in equilibrium with the 14e species RhCl(PPh3)2 (1) and triphenylphosphine. The 14e complex is far more reactive than the parent material; consequently it is the reactive entity most likely to coordinate with the substrate and/or the reagents. Generally, the catalytic cycles involving this material then proceed via a cascade of oxidative addition, migratory insertion, and reductive elimination reactions. The postulated mechanism for the hydrogenation of alkenes illustrates these features (Scheme 1), and is typical of the rationales frequently applied to comprehend the reactivity of RhCl(PPh3)3. Other types of transformations may be important (e.g. transmetalations), and the actual mechanisms are certainly more complicated in many cases; nevertheless, the underlying concepts are similar.

Two important conclusions emerge from these mechanistic considerations. First, RhCl(PPh3)3 is not a catalyst in the most rigorous sense, but a catalyst precursor. This distinction is critical to the experimentalist because it implies that there are other ways to generate catalytically active rhodium(I) phosphine complexes in solution. Wilkinson's catalyst is a convenient source of homogeneous rhodium(I); it has been extensively investigated because it is easily obtained, and because it was discovered early in the development of homogeneous transition metal catalysts. However, for any transformation there always may be better catalyst precursors than RhCl(PPh3)3. Secondly, reactions involving a catalytic cycle such as the one shown in Scheme 1 are inherently more complicated than most in organic chemistry. Equilibria and rates for each of the steps involved can be influenced by solvent, temperature, additives, and functional groups on the substrate. Competing reactions are likely to be involved and, if they are, the performance of the catalytic systems therefore is likely to be sensitive to these parameters. Consequently, the purity of the Wilkinson's catalyst used is an important factor. Indeed, less pure catalyst occasionally gives superior results because removal of a fraction of the triphenylphosphine in solution by oxidation to triphenylphosphine oxide gives more of the dissociation product (1).

In summary, practitioners of organometallic catalysis should consider the possible mechanistic pathways for the desired transformation, then screen likely catalyst systems and conditions until satisfactory results are obtained. Wilkinson's catalyst is one of the many possible sources of homogeneous rhodium(I).

Hydrogenations.

Wilkinson's catalyst is highly active for hydrogenations of unconjugated alkenes at ambient temperatures and pressures. Steric effects are important insofar as less hindered alkenes react relatively quickly, whereas highly encumbered ones are not reduced (eq 1).5 Hydrogen in the presence of RhCl(PPh3)3 under mild conditions does not reduce aromatic compounds, ketones, carboxylic acids, amides, or esters, nitriles, or nitro (eq 2) functionalities. Moreover, hydrogenations mediated by Wilkinson's catalyst are stereospecifically cis (eq 1). These characteristics have been successfully exploited to effect chemo-, regio-, and stereoselective alkene reductions in many organic syntheses (eqs 1-3). For instance, steric effects force delivery of dihydrogen to the least hindered face of the alkene in (eq 3).6 Eq 4 illustrates that 1,4-cyclohexadienes can be reduced with little competing isomerization/aromatization,7 unlike many other common hydrogenation catalysts.5

Strongly coordinating ligands can suppress or completely inhibit hydrogenations mediated by Wilkinson's catalyst; examples include 1,3-butadiene, many phosphorus(III) compounds, sulfides, pyridine, and acetonitrile. Similarly, strongly coordinating substrates are not hydrogenated in the presence of Wilkinson's catalyst, presumably because they bind too well. Compounds in this category include maleic anhydride, ethylene, some 1,3-dienes, and some alkynes. Conversely, transient coordination of functional groups on the substrate can be useful with respect to directing RhCl(PPh3)3 to particular regions of the molecule for stereoselective reactions. However, in directed hydrogenations Wilkinson's catalyst is generally inferior to more Lewis acidic cationic rhodium(I) and iridium(I) complexes.8 The activity of Wilkinson's catalyst towards hydrogenation of alkenes has been reported to be enhanced by trace quantities of oxygen.9

Hydrogenations of alkynes mediated by Wilkinson's catalyst generally give alkanes. Cis-alkene intermediates formed in such reactions tend to be more reactive than the alkyne substrate, so this is usually not a viable route to alkenes. Some alkynes suppress the catalytic reactions of RhCl(PPh3)3 by coordination. Nevertheless, hydrogenation of alkynes mediated by RhCl(PPh3)3 can be useful in some cases, as in eq 5 in which the catalyst tolerates sulfoxide functionalities and gives significantly higher yields than the corresponding reduction catalyzed by Palladium on Barium Sulfate.10

Wilkinson's catalyst can mediate the hydrogenation of allenes to isolated alkenes via reduction of the least hindered bond.11 Di-t-butyl hydroperoxide is hydrogenated to t-BuOH in the presence of RhCl(PPh3)3, though this transformation could occur via a radical process.12

Hydrogen Transfer Reactions.

Wilkinson's catalyst should lower the energy barrier for dehydrogenations of alkanes to alkenes since it catalyzes the reverse process, but no useful transformation of this kind have been discovered. Presumably, the activation energy for this reaction is too great since alkanes have no coordinating groups. Alcohols and amines, however, do have ligating centers, and can dehydrogenate in the presence of Wilkinson's catalyst. These reactions have been used quite often, mostly from the perspective of hydrogen transfer from an alcohol or amine to an alkene substrate, although occasionally to dehydrogenate alcohols or amines.

2-Propanol solvent under basic conditions has been extensively used to transfer hydrogen to alkenes and other substrates. Elevated temperatures are usually required and under these conditions RhCl(PPh3)3 may be extensively modified prior to the catalysis. Ketones, alkenes (eq 6), aldimines (eq 7),13 nitrobenzene, and some quinones are reduced in this way.

Wilkinson's catalyst mediates a Cannizzaro-like process with benzaldehyde in ethanol; the aldehyde serves as a dihydrogen source to reduce itself, and the benzoic acid formed is esterified by the solvent (eq 8).14 Pyrrolidine is N-methylated by methanol in the presence of RhCl(PPh3)3, a reaction that presumably occurs via hydrogen transfer from methanol, condensation of the formaldehyde formed with pyrrolidine, then hydrogen transfer to the iminium intermediate (eq 9).15

Hydrosilylations.16

Wilkinson's catalyst is one of several complexes which promote hydrosilylation reactions, and it often seems to be among the best identified.17 However, hydrosilylations with RhCl(PPh3)3 tend to be slower than those mediated by H2PtCl6. Good turnover numbers are observed, the catalyst eventually being inactivated by P-C bond cleavage reactions at the phosphine,18 and other unidentified processes. Catalysts without phosphine ligands may be even more robust than RhCl(PPh3)3 because they are unable to decompose via P-C bond cleavage.19 Wilkinson's catalyst is relatively efficient with respect to converting silanes to disilanes.20 The latter reaction could be useful in its own right but in the context of hydrosilylation processes it means that the product yields based on the silane are less than quantitative.

For hydrosilylation of alkenes, the reaction rate increases with temperature and hence many of these reactions have been performed at 100 °C. Higher reaction rates are obtained for silanes with very electronegative substituents and low steric requirements (e.g. HSi(OEt)3 > HSi(i-Pr)3). Terminal alkenes usually are hydrosilylated in an anti-Markovnikov sense to give terminal silanes. Internal alkenes tend not to react (e.g. cyclohexene), or isomerize to the terminal alkene which is then hydrosilylated (eq 10). Conversely, terminal alkenes may be partially isomerized to unreactive internal alkenes before the addition of silane can occur. 1,4-Additions to dienes are frequently observed, and the product distributions are extremely sensitive to the silane used (eq 11).

a,b-Unsaturated nitriles are hydrosilylated, even g-substituted ones, to give 2-silyl nitriles with good regioselectivity (eq 12).21 Secondary alkyl silanes are also formed in the hydrosilylation of phenylethylene. In fact, the latter reaction has been studied in some detail, and primary alkyl silanes, hydrogenation product (i.e. ethylbenzene), and E-2-silylphenylethylenes are also formed (eq 13).22 Equimolar amounts of ethylbenzene (2) and E-2-silylphenylethylene (4) are produced, implying these products arise from the same reaction pathway. It has been suggested that this involves dimeric rhodium species because the relative amounts of these products increase with the rhodium:silane ratio; however, competing radical pathways cannot be ruled out. Certainly, product distributions are governed by the proportions of all the components in the reaction (i.e. catalyst, silane, and alkene), and the reaction temperature. Side products in the hydrosilylation of 1-octene include vinylsilanes and allylsilanes (eq 14).23,24

Hydrosilylation of alkynes gives both trans products (i.e. formally from cis addition), and cis products (from either isomerization or trans addition); H2PtCl6, however, gives almost completely cis addition to trans products.25 Moreover, CC-H to CC-SiR3 exchange processes can occur for terminal alkynes giving (6) (eq 15).25-27 The product distribution in these reactions is temperature dependent, and other factors may be equally important. Nonstereospecific transition metal catalyzed hydrosilylations of alkynes are not confined to Wilkinson's catalyst, and the origin of the trans addition product has been investigated in detail for other homogeneous rhodium and iridium complexes.19

Hydrosilylation of terminal alkenes has been used in a polymerization process to form new polymeric organic materials.28

Hydrosilylation of a,b-unsaturated aldehydes and ketones gives silylenol ethers via 1,4-addition, even when the 4-position is relatively hindered.29 Hydrolysis of the silyl enol ethers so formed gives saturated aldehydes. Combination of these reduction and hydrolysis steps gives overall reduction of alkenes conjugated to aldehydes, in selectivities which are generally superior to those obtained using hydridic reducing agents (eqs 16 and 17). Dihydrosilanes tend to reduce a,b-unsaturated carbonyl compounds to the corresponding alcohols, also with good regioselectivity (eq 18).

Similar hydrosilylations of a,b-unsaturated esters are useful for obtaining silyl ketene acetals with over 98:2 (Z) selectivity (eq 19);30 this transformation is complementary to the reaction of a-bromo esters with zinc and chlorotrialkylsilanes, which favors the formation of the corresponding (E) products.30 In cases where (E):(Z) stereoselectivity is not an issue, Rhodium(III) Chloride (RhCl3.6H2O) may be superior to Wilkinson's catalyst.31 Unconjugated aldehydes and ketones are reduced by silanes in the presence of RhCl(PPh3)3; trihydrosilanes react quicker than di- than monohydrosilanes.32,33

Alcohols (eq 20)34 and amines (eq 21)35 react with silanes in the presence of Wilkinson's catalyst to give the silylated compounds and, presumably, hydrogen. These reactions are useful in protecting group strategies.

N,N-Dimethylacrylamide and triethylsilane combine in the presence of Wilkinson's catalyst (50 °C) to give a O,N-silylketene acetal as the pure (Z) isomer after distillation; this reaction can be conveniently performed on a gram scale (eq 22). The products have been used in new aldol methodology.36

Hydrostannylations.

Hydrostannanes add to alkynes in uncatalyzed reactions at 60 °C. Phenylacetylene, for instance gives a mixture of (E)- and (Z)-vinylstannanes, wherein the tin atom has added to the terminal carbons. In the presence of Wilkinson's catalyst, however, the hydrostannylation proceeds at 0 °C to give mostly the regioisomeric vinylstannanes (eq 23).37 Terminal stannanes in the latter process seem to result from competing free radical additions. This may not be a complication with some other catalysts; the complexes PdCl2(PPh3)2 and Mo(h3-allyl)(CO)2(NCMe)2 also mediate hydrostannylations of alkynes, and they are reported to be 100% cis selective.38 Hydrostannanes and thiols react in a similar way to silanes and alcohols (eq 24).39

Hydroacylations.

Alkenes with aldehyde functionality in the same molecule, but displaced by two carbon atoms, can cyclize via intramolecular hydroacylation reactions. Substituent effects can have a profound influence on these transformations. For instance, 3,4-disubstituted 4-pentenals cyclize to cyclopentanones without serious complications,40 but 2,3-disubstituted 4-pentenals give a cyclopropane as a competing product (eqs 25 and 26).41 Formation of the latter material illustrates two features which restrict the applicability of this type of reaction. First decarbonylation of the aldehyde can occur, in this case presumably giving a rhodium alkyl complex which then inserts the pendant alkene functionality. Secondly, decarbonylation reactions convert the catalyst into RhCl(CO)(PPh3)2, which tends to be inactive. Moreover, the reaction is only generally applicable to the formation of five-membered rings, and it is apparently necessary to use quite large amounts of Wilkinson's catalyst to ensure good yields (eq 27).42 Rhodium(I) complexes other than RhCl(PPh3)3 can give better results in some cases.43

Lactols can be cyclized under the typical hydroacylation conditions (eq 28), presumably via equilibrium amounts of the corresponding aldehyde.40 Finally, intermolecular hydroacylation has been formally achieved in the reaction of a pyridyl aldimine with ethylene under pressure at 160 °C; here the pyridine functionality anchors the aldimine to the rhodium, and decarbonylation is impossible (eq 29).

Decarbonylations.

Wilkinson's catalyst has been known for some time to decarbonylate aldehydes, even heavily functionalized ones, to the corresponding hydrocarbons.44 Some examples are shown in eqs 30-33, illustrating high stereochemical retention in the decarbonylation of chiral, cyclopropyl, and unsaturated aldehydes.45,46 Acid chlorides are also decarbonylated by RhCl(PPh3)3.

The problem with all these reactions is that stoichiometric amounts of the catalyst are required, and the process is inordinately expensive. Consequently, it has only been used by those wishing to illustrate a decarbonylation occurs for some special reason, or in the closing stages of small scale syntheses of complex organic molecules. Very recently, however, it has been shown that the reaction can be made catalytic by adding Diphenyl Phosphorazidate.47 The role of the latter is to decarbonylate the catalytically inactive RhCl(CO)(PPh3)2, regenerating rhodium(I) without carbonyl ligands. Examples of this catalytic process are shown in eqs 34 and 35. The path is now clear for extensive use of RhCl(PPh3)3 for catalytic decarbonylation reactions in organic synthesis.

Catalytic decarbonylations of a few substrates other than aldehydes have been known for some time, e.g. conversion of benzoic anhydrides to fluorenones at high temperatures (ca. 225 °C).48

Hydroformylations.49

Carbon monoxide reacts rapidly with RhCl(PPh3)3 to give RhCl(CO)(PPh3)2. With hydrogen, in the presence of triphenylphosphine, the latter carbonyl complex affords some Carbonylhydridotris(triphenylphosphine)rhodium(I), and this very actively mediates hydroformylations.50 Reactions wherein RhCl(PPh3)3 is used as a hydroformylation catalyst probably proceed via this route. A more direct means of hydroformylation is to use RhH(CO)(PPh3)3. Nevertheless, Wilkinson's catalyst (an unfortunate term here because Wilkinson also pioneered hydroformylations using RhH(CO)(PPh3)3) has been used to effect hydroformylations of some substrates. Eq 36 is one example and illustrates that transient coordination of the acyl group with rhodium apparently leads to predominant formation of a branched chain aldehyde, whereas straight chain aldehydes are usually formed in these reactions.51 Other hydroformylation catalysts that have been studied include cobalt and iridium based systems.49

Hydroborations.52

Addition of Catecholborane to alkenes is accelerated by Wilkinson's catalyst, and other sources of rhodium(I) complexes.53 Unfortunately, the reaction of Wilkinson's catalyst with catecholborane is complex; hence if the conditions for these reactions are not carefully controlled, competing processes result. In the hydroboration of styrene, for instance, the secondary alcohol is formed almost exclusively (after oxidation of the intermediate boronate ester, eq 37); however, the primary alcohol also is formed if the catalyst is partially oxidized and this can be the major product in extreme cases.54,55 Conversely, hydroboration of the allylic ether (12) catalyzed by pure Wilkinson's catalyst gives the expected alcohol (13), hydrogenation product (14), and aldehyde (15), but alcohol (13) is the exclusive (>95%) product if the RhCl(PPh3)3 is briefly exposed to air before use.54 The syn-alcohol is generally the favored diastereomer in these and related reactions (eq 38), and the catalyzed reaction is therefore stereocomplementary to uncatalyzed hydroborations of allylic ether derivatives.56-58

Other sources of rhodium(I) are equally viable catalysts for hydroborations, notably Rh(h3-CH2CMeCH2)(i-Pr2PCH2CH2P-i-Pr2) which gives a much cleaner reaction with catecholborane than Wilkinson's catalyst.59 Other catalysts for hydroborations are also emerging.60-62

Catecholborane hydroborations of carbonyl and related functionalities are also accelerated by RhCl(PPh3)3 (eqs 39-41); however, several related reactions proceed with similar selectivities in the absence of rhodium.63-65

Cyclization, Isomerization, and Coupling Reactions.

Inter- (eq 42)66 and intramolecular (eq 43)67 cyclotrimerizations of alkynes are mediated by Wilkinson's catalyst. This is an extremely efficient route to ring fused systems. Similarly, Diels-Alder-like [4 + 2] cyclization processes are promoted by RhCl(PPh3)3;68 dienophile components in these reactions need not be electron deficient, and they can be an alkene or alkyne (eqs 44 and 45). Allenes oligomerize in pathways determined by their substituents. For instance, four molecules of allene combine to give a spirocyclic system (eq 46), but tetraphenylallene isomerizes to give an indene (eq 47).69

Wilkinson's catalyst is also capable of mediating the formation of C-C bonds in reactions which apparently proceed via oxidative addition of an unsaturated organohalide across the metal (eq 48),70 or via transmetalation from an organometallic (eq 49).71 These two transformation types are very similar to couplings developed by Heck so, predictably, some palladium complexes also mediate these reactions (see Tetrakis(triphenylphosphine)palladium(0) and Palladium(II) Acetate).

Intermolecular reactions of dienes, allenes, and methylenecyclopropanes with alkenes are mediated by RhCl(PPh3)3, although mixtures of products are usually formed (eqs 50, 51).72-75

Wilkinson's catalyst mediates hydrogenation of 1,4-cyclohexadienes without double bond isomerization (see above), but at elevated temperatures in the absence of hydrogen it promotes isomerization to conjugated dienes (eq 52).76 Isomerization of allylamines to imines followed by hydrolysis has also been performed using RhCl(PPh3)3 (eq 53),77 although RhH(PPh3)4 and other catalysts are more frequently used for this reaction type.78

Oxidations.

Cleavage of alkenes to aldehydes and ketones is promoted by Wilkinson's catalyst under pressures of air or oxygen,79 but these reactions are inferior to ozonolysis because they tend to form a mixture of products. More useful are the oxidations of anthracene derivatives to anthraquinones in the presence of oxygen/t-Butyl Hydroperoxide and catalytic RhCl(PPh3)3 (eq 54).80,81 Wilkinson's catalyst reacts with oxygen to form an adduct so RhCl(PPh3)3 is clearly quite different from the true catalyst in all the reactions mentioned in this section.

Other Transformations.

At high temperatures (>200 °C) aromatic sulfonyl chlorides are desulfonated to the corresponding aryl halides in the presence of Wilkinson's catalyst (eq 55).82 Benzamides and malonamide also decompose under similar conditions, giving benzonitrile and acetamide, respectively.83

Diazonium fluoroborates are reduced to the corresponding unsubstituted aryl compounds by Wilkinson's catalyst in DMF; the solvent is apparently the hydride source in this reaction (eq 56).84

Finally, aryl group interchange between triarylphosphines is mediated by Wilkinson's catalyst at 120 °C, but a near statistical mixture of the exchanged materials is formed along with some byproducts.85

Related Reagents.

Bis(bicyclo[2.2.1]hepta-2,5-diene)rhodium Perchlorate; [1,4-bis(diphenylphosphino)-butane](norboradiene)rhodium tetrafluroborate Catecholborane; (1,5-Cyclooctadiene)[1,4-bis(diphenylphosphino)butane]iridium(I) Tetrafluoroborate; (1,5-Cyclooctadiene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluorophosphate; Octacarbonyldicobalt; Palladium(II) Chloride; Tetrakis(triphenylphosphine)palladium(0).


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Kevin Burgess & Wilfred A. van der Donk

Texas A & M University, College Station, TX, USA



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