Carbonyl(chloro)bis(triphenylphosphine)rhodium(I)1

(trans/cis)

[13938-94-8]  · C37H30ClOP2Rh  · Carbonyl(chloro)bis(triphenylphosphine)rhodium(I)  · (MW 690.96) (trans)

[15318-33-9] (cis)

[16353-77-8]

(catalyst for carbonylation,2 decarbonylation,3 carbonyl exchange,4 reduction,5 hydrometalation,6 and C-H activation reactions;7 reagent for alkylation of acid chlorides8)

Physical Data: mp 209-210 °C (trans isomer), 204-205 °C (cis isomer), 195-197 °C (trans/cis mixture); IR carbonyl stretching 1970 cm-1 (nujol mull), 1977 cm-1 (CHCl3).

Solubility: sol chloroform and dichloromethane; moderately sol aromatic hydrocarbons and CCl4; insol ether, alcohols, and aliphatic hydrocarbons.

Form Supplied in: bright yellow crystalline solid, widely available.

Preparative Methods: to a solution of RhCl3.3H2O (2 g, 7.6 mmol) in 70 mL of absolute ethanol is slowly added triphenylphosphine (7.2 g, 2.75 mmol) in 300 mL of boiling absolute ethanol. The solution becomes clear within 5 min, at which point sufficient (10-20 mL) 37% formaldehyde solution is added, causing the red solution to become pale yellow. After several minutes, yellow microcrystals precipitate. Upon cooling, the resulting crystals are washed with ethanol and diethyl ether, dried (4.5 g, 85%) and recrystallized from hot toluene.1c

Handling, Storage, and Precautions: the complex is air-stable as a crystalline solid and can be stored without special precautions. In solution it readily reacts with oxygen, which therefore must be excluded during preparation and use of this reagent.

Carbonylation Reactions.

Rhodium is the most widely used catalyst for the carbonylation of alkenes with CO and H2.1,2 Although RhH(CO)2L2 (L = PPh3, PBu3, etc.) is recognized as the actual species that reacts with alkenes in the ligand modified rhodium catalyst system, many other rhodium complexes can be utilized as precursors in the presence of suitable ligands (L), CO, and H2. RhCl(CO)(PPh3)2 has been used as a catalyst precursor in the hydroformylation of alkenes and formaldehyde.1b,2b

Hydroformylation of terminal alkenes produces a mixture of regioisomeric linear and branched aldehydes. Triphenylphosphine-stabilized rhodium catalysts can be used to form linear, unbranched aldehydes from simple alkenes such as propene. The presence of triphenylphosphine, however, gives poor results in the hydroformylation of RfCH=CH2 (Rf = perfluoroalkyl group), which with Dodecacarbonyltetrarhodium or Hexadecacarbonylhexarhodium produces the branched aldehydes RfCH(CHO)Me regioselectively and in good yields.9

Heterocyclic compounds have been obtained through rhodium-catalyzed hydroformylation of functionalized alkenes such as allylamines (eq 1),10 3-butenamides (eq 2)11 and allyl alcohols.12

Hydroformylation of formaldehyde has been extensively investigated because of its importance to the commercial production of ethylene glycol (eq 3).1b,13 The RhCl(CO)(PPh3)2-Triethylamine catalyst system produced a mixture of straight chain carbohydrates, from trioses to hexoses.14 The hydroformylation in the presence of 3-ethylbenzothiazolinium bromide gives trioses selectively.

RhCl(CO)(PPh3)2 catalyzes the reaction of CO with epoxides (eq 4),15 amines (eqs 5 and 6),16,17 and azides (eq 7).17

Decarbonylation Reactions.

RhCl(CO)(PPh3)2 is a product of the stoichiometric decarbonylation of aldehydes with Chlorotris(triphenylphosphine)rhodium(I) (eq 8).3b,18 However, at elevated temperatures RhCl(CO)(PPh3)2 is an effective catalyst for the decarbonylation of aroyl chlorides and aroyl cyanides (eq 9).3a,19

Carbonyl Exchange (Isotope Labelling).

The carbonyl carbon of aroyl chlorides can be 13C or 14C labelled through acyl carbonyl exchange reactions with CO catalyzed by RhCl(CO)(PPh3)2 without formation of aryl chloride (ArCl) decarbonylation products (eqs 10 and 11).4 Statistical distribution of 13C is easily attained. The carbonyl group of aliphatic acid chlorides is also easily exchanged, but small amounts of decarbonylated products are formed.4 Alkene migration or isomerization does not occur in the carbonyl exchange reactions of 3-butenoyl chlorides, although 1,3-scrambling of chlorine has been observed in the RhCl(PPh3)3-catalyzed decarbonylation of 3-butenoyl chlorides to allyl chlorides.20

Reduction Reactions.

The most widely studied homogeneous hydrogenation catalysts are all rhodium complexes. RhCl(CO)(PPh3)2 has been rarely used because of its low reactivity as a hydrogenation catalyst. The low reactivity, however, allows it to be a potentially useful catalyst for selective hydrogenation of terminal alkenes.21 RhCl(CO)(PPh3)2-catalyzed reduction of diphenylacetylene with Sodium Borohydride gives (Z)-stilbene, whereas (E)-stilbene is formed with RhCl(PPh3)3 as the catalyst.22

Aromatic nitro compounds are reduced in good yields to aromatic amines with secondary alcohols as the reducing agent using the RhCl(CO)(PPh3)2-KOAc catalyst system (eq 12).23

Hydrometalation Reactions.

For the catalytic hydrometalation of alkenes, RhCl(CO)(PPh3)2 has no advantages over RhCl(PPh3)3 and its congeners, chloroplatinic acid (Hydrogen Hexachloroplatinate(IV)),24 or radical catalysts.25 The regioselectivity of hydrometalation of terminal alkynes depends on the ligands. For example, the hydrosilation and hydrogermylation of phenylacetylene in the presence of RhCl(CO)(PPh3)2 gives (E)-1-silyl- or 1-germyl-substituted 2-phenylethylene, whereas with Rh[(CF3CO)2CH](C2H4)2 the isomeric 2-metalated 2-phenylethylenes are produced (eq 13).26 In the hydrostannation of terminal alkynes, RhCl(CO)(PPh3)2 gives 2-stannyl-1-alkenes as the major product.6

Hydrosilanes easily react with acylmetal complexes (metal = Fe, Mo, Mn) to form silyloxyalkyl metal complexes by using RhCl(CO)(PPh3)2 or Rh(CO)(PPh3)3 as catalysts.27

Hydrocarbon C-H Activation.

Under photochemical conditions, RhCl(CO)(PPh3)2 activates C-H bonds of aromatic and aliphatic hydrocarbons, leading to carbonylated products (eq 14)7,28 and/or dehydrogenated products (eq 15).29 UV irradiation of a solution of RhCl(CO)(PPh3)2 leads to the extrusion of CO to form a coordinatively unsaturated complex, RhCl(PPh3)2 (eq 16), which then inserts into the C-H bond of the hydrocarbon.30 This photochemical reaction can be used for isotope labelling of aldehydes with 13CO (eq 17).28

Alkylation of Acid Chlorides.

A number of rhodium complexes, including trans-RhCl(CO)(PPh3)2, facilitate the alkylation of acid chlorides, affording ketones in moderate to high yields.8 This reaction proceeds by initial pretreatment of the catalyst precursor with an appropriate Grignard, organolithium, or organoytterbium31 reagent to give an active alkylrhodium(I) species (eq 18). This reaction is specific for acid chlorides. Alkylation of ketones32 and amines33 can also be effected by trans-RhCl(CO)(PPh3)2 in a carbon monoxide-water system. In a more recent study,34 nucleophilic addition of allylic stannanes to aromatic aldehydes has been accomplished by using derivatives of trans-RhCl(CO)(PPh3)2 in high yields (eq 19).

Oxidation Reactions.

The liquid phase oxidation of benzaldehyde to benzoic acid proceeds readily in the presence of a catalytic amount of trans-RhCl(CO)(PPh3)2 in benzene and under an atmosphere of oxygen (conversion is 78%).35 Oxidation of terminal alkenes to methyl ketones also employs the use of a rhodium catalyst.36

Miscellaneous.

trans-RhCl(CO)(PPh3)2 is a moderately active catalyst precursor for the hydrosilation37 and transfer hydrogenation38 of imines. This rhodium complex is also used in the photochemical dehydrogenation of hydrocarbons,39 and the thermal carbonylation of alkyl halides.40

The reaction of 1,5-dienes with hydrosilanes using trans-RhCl(CO)(PPh3)2 gives dehydrogenative silylation products, 1-silyl-1,5-dienes, in high yields (eq 20).41

Related Reagents.

Carbonylhydridotris(triphenylphosphine)rhodium(I); Chlorotris(triphenylphosphine)rhodium(I).


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Kiyoshi Kikukawa

Kinki University in Kyushu, Iizuka, Japan

Stephen A. Westcott

University of North Carolina, Chapel Hill, NC, USA



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