Bis(triphenylphosphine)(maleic anhydride) palladium(0)

[17830-50-1]  · C40H32O3P2Pd  · (MW 729.05)

(bis(triphenylphosphine)(maleic anhydride)palladium(0) (1) is an isolable, moderately air-stable, palladium(0) complex that can be used in a variety of reactions where a preformed, organic-soluble palladium(0) complex is required as a catalyst, a catalyst precursor or a stoichiometric reagent. To date, however, almost all of the reported uses of 1 have been in palladium-catalyzed dimerization of 1,2- or 1,3-dienes.)

Alternate Name: bis(triphenylphosphine)(maleic anhydride)palladium; (maleic anhydride)bis(triphenylphosphine)palladium; [(3,4-h)-2,5-furandione]bis(triphenylphosphine)palladium; (2,5-furandione)bis(triphenylphosphine)palladium.

Form Supplied in: yellow crystalline solid.

Preparative Methods:1,2 maleic anhydride (98 mg, 1 mmol) was dissolved in THF (15 mL) and added dropwise over 10-20 min via a pressure-equalization funnel to a nitrogen blanketed solution of tetrakis(triphenylphosphine)palladium(0) (1.155 g, 1 mmol) in benzene (30 mL). The resulting mixture was stirred for additional 30 min at room temperature, after which the solvent was removed under reduced pressure. The resultant yellow oil was redissolved in a mixture of benzene and methanol (2:1, 15 mL) and filtered. Benzene (40 mL) was added to the filtrate and the resulting mixture was cooled in a refrigerator. Yellow crystals were separated from solution overnight. The crystals were isolated by filtration, washed with a mixture of benzene, methanol, and hexane (2:1:5), and dried in vacuo to yield 630 mg of 1 (86%).

Purification: recrystallized from benzene, washed with a mixture of benzene, methanol, and hexane (2:1:5), and dried in vacuo.2

Handling, Storage, and Precautions: limited stability in air, reported to be of the order of hours.1 Disposal: use typical precautions for heavy metals.

Introduction

Bis(triphenylphosphine)(maleic anhydride)palladium(0) (1) is an isolable palladium(0) complex that has been used primarily for the palladium-catalyzed linear dimerization of 1,3-dienes, especially 1,3-butadiene, and for the dimerization of 1,2-dienes, especially the simplest of the 1,2-dienes, allene. Three miscellaneous reactions (i.e., the exchange of allyl groups, a disilane metathesis reaction, and a novel addition reaction of an amine to norbornene) have also been reported. For the dimerization reactions of 1,3-dienes, four related reaction modes have been reported: (1) the linear dimerization without trapping to afford a 1,3,7-octatriene derivative; (2) the linear dimerization with incorporation of a protic trapping reagent to afford predominantly 1-substituted-2,7-octadiene derivatives; (3) the linear dimerization with incorporation of a silane trapping reagent to afford predominantly 1-trialkylsilyl-2,6-octadiene derivatives; and (4) diene dimerization with incorporation of an isocyanate trapping reagent to afford a mixture of cis- and trans-divinyl piperidones. For the dimerization of allene, most modes lead to products containing a 2-methyl-3-methylene-1-butene subunit.

Linear Dimerization of 1,3-Dienes in the Absence of Trapping Reagents

Bis(triphenylphosphine)(maleic anhydride)palladium(0) (1) catalyzes dimerization of 1,3-butadiene in aprotic solvents such as benzene (120 °C, 7 h, 64%), THF (115 °C, 7 h, 82%), and acetone (115 °C, 7 h, 86%) to give predominantly (E)-1,3,7-octatriene (2, R = H) (1).3 Surprisingly, it is reported that bis(triphenylphosphine)(dimethylfumarate)palladium(0), bis(triphenylphosphine)(p-benzoquinone)palladium(0), and tetrakis (triphenylphosphine)palladium(0), are almost completely ineffective under similar reaction conditions.3 However, 1,3-butadiene can also be efficiently dimerized by a variety of other catalyst systems,4 including via complementary reaction modes; for example, an aminophosphinite-modified nickel(0) catalyst gives the isomeric 1,3,6-octatriene in 95% yield.5

The dimerization of isoprene is complicated as dimerization can afford a number of isomeric octatrienes depending on whether the two isoprene units couple in a head-to-head, head-to-tail or tail-to-tail fashion. The reaction of isoprene in acetone catalyzed by 1 (100-110 °C, 6-8 h) is reported to afford exclusively the tail-to-tail coupling product, 2,7-dimethyl-1,3,7-octatriene (2, R = Me, 75% yield) (1).6 2-Ethyl-buta-1,3-diene reacts similarly to give 2,7-diethyl-1,3,7-octatriene (2, R = Et, 75% yield). The reaction with 2,3-dimethyl-buta-1,3-diene catalyzed by 1 is not as efficient and gives a 2:3 mixture of 2,3,6,7-tetramethyl-1,3,7-octatriene and 2,3,7-tetramethyl-6-methylene-1,7-octadiene in 75% combined yield but at low conversion (10%).6 Other palladium catalyst systems dimerize isoprene in a manner quite similar to 1. For example, good yields of 2,7-dimethyl-1,3,7-octatriene are obtained from the dimerization of isoprene using dibromo(diphos)palladium(II)/sodium phenoxide in the presence of phenol (89%),7 or alternatively, using di(acetylacetonate)palladium(II)/triphenylphosphine system in the presence of m-methoxybenzaldehyde (89%).8 In contrast, the dimerization of isoprene by the cationic (methallyl)palladium(II) complex, [Pd(cod)(methallyl)]PF6, in the presence of 1 equiv of tricyclohexylphosphine affords predominantly the tail-to-head isoprene dimer, (2E,4E) 2,6-dimethyl-1,3,6-octatriene (72% yield).9

Linear Dimerization of 1,3-Dienes with Incorporation of a Protic Trapping Reagent: Alcohols, Phenols, Carboxylates, Formate, and Amines

The palladium-catalyzed reaction of 1,3-dienes with pronucleophiles provides a useful pathway for the preparation of 2:1 diene:pronucleophile adducts. For example, the reaction of 1,3-butadiene with methanol catalyzed by 1 (0.04 mol %) and carried out in methanol (70 °C, 1 h) gives predominantly trans-1-methoxy-2,7-octadiene (3, 85%) along with minor amounts of 3-methoxy-1,7-octadiene (4, 5%) and 1,3,7-octatriene (3%) (2).3 This reaction has been extensively examined and a number of other palladium catalyst systems, comparable to 1 in effecting this dimerization-trapping reaction, have been found. Attractive alternative catalytic systems include bis(triphenylphosphine)(p-benzoquinone)palladium(0),3 bis(triphenylphosphine) (dimethyl fumarate) palladium(0),3 tetrakis(triphenylphosphine) palladium(0),3 the coordinately unsaturated (diallyl ether)(triphenylphosphine)palladium(0),10 the cationic complex [Pd(h3-allyl) {Ph2P(o-C6H4NMe2)}]PF6,11 the Pd2(dba)3 complex in combination with certain mono and diphosphine ligands,12 and the Pd2(dba)3 complex in combination with the P,N-bidentate ligand, N,N-dimethyl-2-diethylphosphinoaniline.13

In general, the distribution of products obtained from the palladium-catalyzed dimerization of butadiene with trapping by alcohols depends on the nature of alcohol used.3 In contrast to the reaction carried out in methanol, the reaction catalyzed by 1 in ethanol (110-115 °C, 6 h) gives only a 35% combined yield of a mixture of 1-ethoxy-2,7-octadiene and 3-ethoxy-1,7-octadiene along with a 49% yield of 1,3,7-octatriene. The corresponding reaction in isopropanol (100 °C, 4 h) yields exclusively 1,3,7-octatriene (75% yield). Complex 1 also catalyzes the dimerization/trapping of butadiene with phenol. When the reaction is carried out in benzene (80 °C), a 57% yield of a 4:1 mixture of 1-phenoxy-2,7-octadiene and 3-phenoxy-1,7-octadiene is obtained.3

The dimerization of butadiene in acetic acid catalyzed by 1 (120 °C) gives ca. 30% combined yield of 1-acetoxy-2,7-octadiene and 3-acetoxy-1,7-octadiene. In addition, several low boiling butadiene dimers are formed in this reaction.3 Other more efficient catalyst systems have been discovered for the synthesis of 1-acetoxy-2,7-octadiene from butadiene and acetic acid. For example, bis(acetylacetonate)palladium(II) in the presence of tertiary amines such as N,N,N,N-tetramethyl-1,3-butanediamine and a phosphite ligand, trimethylolpropanephosphite, catalyzes the formation of 1-acetoxy-2,7-octadiene in 81% yield, with only minor amounts of the isomer 3-acetoxy-1,7-octadiene (9%).14

Secondary amines, for example, morpholine, diisopropylamine and, piperidine, have been used as trapping reagents in the dimerization of butadiene catalyzed by complex 1. The reaction proceeds in the presence or absence of added solvent to give the octadienyl product 5 (R1, R2 = alkyl), generally in greater than 60% yield (3).3 For example, the reaction of 1,3-butadiene with morpholine (0.13 mol % 1, acetone, 80-90 °C, 1 h) affords a 97% yield of the morpholine derivative 5 (R1, R2 = (CH2CH2)2O). The complex 1-catalyzed reaction of butadiene with a primary amine such as aniline or n-butylamine generally gives a mixture of products 5 (R1 = alkyl, R2 = H) and 6 (R1 = alkyl), reflecting the addition of one or two octadienyl chains to the primary amine. The combined yields are generally good, 85% and 71% for aniline and n-butylamine, respectively. The relative rates and mono versusdialkylation selectivity of a series of p-substituted anilines (H2NC6H4X) were examined. It was reported that the relative rates of reaction follow the order: X = CO2CH3 < Cl < H < CH3 < OCH3, roughly similar to the basicity of the p-substituted anilines. However, while requiring somewhat longer reaction times, the two anilines bearing electron-withdrawing para-substituents gave exclusively the monoalkylation product 5 (R1 = H, R2 = 4-XC6H4), 48% yield for X = CO2CH3 and 58% yield for X = Cl. In contrast, p-methoxyaniline gave a 1.6:1 mixture of 5:6 (R2 = 4-MeOC6H4) in 87% combined yield. The attempted dimerization with trapping by carbazole and by acetamide gave only the nontrapped product, 1,3,7-octatriene. It was found that complex 1 is more effective than tetrakis(triphenylphosphine) palladium in the dimerization-trapping with amines.3

Linear Dimerization of 1,3-Dienes with Incorporation of a Silane Trapping Reagent

The reaction of butadiene with trimethylsilane in the presence of complex 1 (0.5 mol % 1, benzene, 85 °C, 4.5 h) gives the 2:1 butadiene:silane adduct, 1-trimethylsilyl-2,6-octadiene (7), in 98% yield (4).15 Under similar conditions, triethylsilane reacts with butadiene to give 1-triethylsilyl-2,6-octadiene in 84% yield, but trichlorosilane and dimethylphenylsilane give only 1:1 butadiene:silane adducts, typical diene hydrosilylation products.16 Bis(triphenylphosphine)(p-benzoquinone)palladium,16 dichloro(1,5-cyclooctadiene)palladium(II), bis (benzonitrile)dichloropalladium(II), and allylpalladium(II) chloride dimer have also been found to be effective catalysts for the reaction of butadiene with trimethylsilane, but it is reported that dichlorobis(triphenylphosphine)palladium(II) does not catalyze the reaction in 4.16 Tetrakis(triphenylphosphine)palladium(0) is reported to be an excellent catalyst for the reaction, but only in the absence of the solvent.17

Dimerization of 1,3-Dienes with Incorporation of Phenyl isocyanate as the Trapping Reagent

Complex 1 catalyzes the formation of a 1:1 mixture of divinyl piperidones (8 and 9) (82% combined yield) from the reaction of phenyl isocyanate with isoprene (benzene, 100 °C, 20 h) (5). When butadiene is used in the place of isoprene, a 1:1 mixture (E)- and (Z)-a,b-unsaturated piperidones (10) is formed (75%) (6).18

Dimerization of 1,2-Dienes in the Presence of Amines, Enamines, and Active Methylene Compounds

N-Butyl methylamine undergoes efficient coupling with allene catalyzed by complex 1 (0.1 mol %, THF, 110 °C, 3 h) to afford 2:1 allene:amine adduct N-butyl-N-methyl-N-(3-methyl-2-methylene-3-butenyl)amine (11, R1 = methyl, R2 = butyl) in 98% yield (7).19 Other secondary amines react similarly, although the yields vary somewhat. For example, the following secondary amines undergo complex 1-catalyzed coupling under the conditions described above: dimethylamine (70%, 11, R1 = R2 = Me), pyrrolidine (78%, 11, R1, R2 = -(CH2)4-), morpholine (41%, 11, R1, R2 = (CH2CH2)2O), N-ethyl cyclohexylamine (84%, 11, R1 = Et, R2 = C6H11), and N-methylaniline (63%, 11, R1 = Me, R2 = Ph). Other palladium catalyst systems have been examined, and under the conditions described above, the reaction of N-butyl methylamine with allene catalyzed by dichlorobis(triphenylphosphine)palladium(II) and palladium(II) dichloride afford 11 (R1 = methyl, R2 = butyl) in 71%, and 41% yields, respectively. The latter yield can be improved by running the palladium(II)dichloride-catalyzed reaction (110 °C, 3 h) in benzene (83% yield), hexamethylphosphoramide (81%) or acetonitrile (68%).19

Primary amines react with allene in the presence of complex 1 to give a mixture of products 11 and 12, the products of mono and dialkylation of the amine, respectively (7). The ratio of 11:12 can be influenced by varying the initial allene:amine ratio. Thus, using a 1:1.2 allene:methylamine ratio (i.e., allene as the limiting reagent) yields a roughly 2:1 mixture of 11 (R1 = Me, R2 = H, 42%) and 12 (R1 = Me) (20%). Using a 4:1 allene:methylamine ratio (i.e., methylamine is the limiting reagent), the reaction is reported to afford only product 11 (R1 = H, R2 = Me) in 58% yield. Other primary amines, such as ethylamine, isopropylamine, t-butylamine, aniline, and adamantylamine afford similar results. It should be noted that no appreciable formation of 1:1 adducts has been detected in the reactions with the primary or secondary amines described above, and tertiary amines are unreactive towards allene.19

The complex 1-catalyzed reactions of allene with active methylene compounds parallels the reaction with amines described above, with the exception that no reaction occurs with the palladium(II) catalyst systems cited above.19 The ratio of 13:14 (i.e., mono:dialkylation products) can be controlled by the ratio of allene:active methylene compound. For example, the complex 1-catalyzed reaction of a 1:1.2 mixture of allene:2,4-pentanedione (1 mol % 1, THF, 120 °C, 6 h) gives the monoalkylation product 13 (R1 = R2 = acetyl) in 60% yield with only 5% of the competing di-alkylation product 14 being formed (8). When the allene:2,4-pentanedione ratio is increased to 1:2.5, formation of the dialkylation product predominates and 13 and 14 are formed in < 5% and 47% yields, respectively. Other carbon acids, such as malonitrile, ethyl acetoacetate, diethyl malonate, and ethyl cyanoacetate are also active, although the reaction conditions employed for each varies slightly.19 In addition, complex 1-catalyzes the reaction of 1-(1-cyclohexenyl)pyrrolidine with allene (0.2 mol % 1, THF, 120 °C, 6 h) to afford the corresponding 1:2 enamine:allene adduct in 34% yield.

Dimerization of 1,2-Dienes with Dienes and Alkenes

The complex 1-catalyzed reaction of allene with an excess of 1,3-butadiene (0.4 mol % 1, THF, 120 °C, 5 h) forms the 2:1 allene:butadiene adduct, 2-methyl-3-methylene-1,5,7-octatriene (15) in fair yield (39%) as a 3:1 mixture of (E) and (Z)-isomers (9).20 Under similar conditions, bicyclo[2.2.1]hepta-2,5-diene is reported to react with allene to give a [2 + 2]-cycloadduct in 25% yield accompanied by substantial amounts of unidentified by-product whose elemental analysis suggests it is a 2:1 allene:bicyclo[2.2.1]hepta-2,5-diene adduct.20

Dimerization of 1,2-Dienes in the Presence of Silanes

Complex 1 is reported to catalyze the reaction of allene with triethylsilane (10). The 1:1 allene:triethylsilane adduct, triethylallylsilane (16), is obtained in 48% yield, and it is noteworthy that none of the 2:1 allene:triethylsilane adduct is reported to be found.19

Miscellaneous Reactions

Complex 1 catalyzes the quantitative formation of allyldiethylamine from allyl phenyl ether and diethyl amine as is typical of many palladium(0) catalysts systems.21,22 Complex 1 catalyzes disilane metathesis reaction between 1,1, 1,2,2-pentamethyl-2-vinyl-disilane (18) and 1,1,2,2-tetramethyl-1,2-disilacyclopentane (17) (1 mol % 1, benzene, reflux, 36 h) to give 3,3,4,4,8,8,9,9-octamethyl-3,4,8,9-tetrasila-1-decene (19) as the only isolated product, albeit in low yield (13%) (11). Under the same reaction conditions, dichlorobis(triphenylphosphine) palladium(II) and di(1,5-cyclooctadiene)nickel gave 19 in even lower amounts, 7.5% and 2.4%, respectively.23

Norbornadiene and piperidine react to form the tricyclic compound, 1-tricyclo[2.2.1.02,6]hept-3-yl-piperidine (20), when treated with complex 1 and trifluoroacetic acid (0.75 mol % 1, 30 mol % TFA, 110 °C, 5 h) (12). The role of complex 1 in this reaction is not clear. However, in the absence of trifluoroacetic acid, no apparent reaction occurs, and the acid-to-catalyst ratio is thought to be important. At an acid-to-catalyst ratio of approximately 40:1, the 1:1 norbornadiene:piperidine adduct 20 is obtained in ca. 70% yield.24

Related Reagents.

Bis(triphenylphosphine)(dimethylfumarate)palladium(0); bis(triphenylphosphine)(p-benzoquinone)palladium(0); tetrakis(triphenylphosphine)palladium(0).


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James M. Takacs & Alexei P. Leonov

University of Nebraska-Lincoln, Lincoln, NE, USA



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