Palladium catalysts alkoxycarbonylation

Very high regioselectivities (> 99.5% iso) were obtained, using PdCl2(PhCN)2 in combination with (+)-neomethyldiphenylphosphine and toluene-j9-sulfonic acid, under mild conditions (70 °C and 10 bar). More recently, the palladium-catalyzed alkoxycarbonylation and amido-carbonylation of aryl bromides and iodides in [bmim][BF4] and [bmim][PF6] has been described. Enhanced reaction rates were observed compared to conventional media and the ionic liquid-catalyst could be recycled. 

A certain disadvantage of this method is that stoichiometric amounts of the palladium catalyst have to be employed. While it is possible to reoxidize the in situ generated Pd(0) in the above-mentioned alkoxycarbonylations, the development of efficient catalytic procedures for the alkoxypalladation/y5-hydride elimination reactions still represents an unsolved problem. However, first successes [12] in special systems suggest that this problem should not be insoluble. 

Carbonylphosphine complexes of zero-valent palladium are considerably less stable and more reactive than their Ni counterparts. Most common triphenyl-phosphine complexes of Pd are excellent catalysts for various carbonylation reactions of aryl iodides and bromides [15,129-131 ]. It is conceivable that the palladium-catalyzed alkoxycarbonylation of ArCl proceeds via a mechanism similar to that proposed for the analogous reactions of bromo- and iodoarenes (Scheme 5) [45,159,160,161]. 

Alkylation of sodium 1-(alkoxycarbonyl)methyIphosphonates proceeds equally with acetates in THF from low to room temperature or in DME at reflux. The asymmetric allylic alkylation of the sodium diethyl l-(ethoxycarbonyl)methylphosphonate with 3-acetoxy-l,3-diphenyl-l-propene and cyclic allylic acetates in the presence of a chiral palladium catalyst, prepared from chiral phosphine and palladium acetate, in THF at room temperature proceeds in good yields (44-88%) and high ec s. ... 

The chelate-stabilized, electron-rich Pd(0) complex, (dippp)2Pd(0) 6 (Scheme 8) was an efficient catalyst for carbonylation of aryl chlorides. Alper reported that bis(tricyclo-hexylphosphine)paUadium dichloride is an active catalyst for the carbonylation of aryl chlorides to carboxylic acid. Neat aryl chlorides reacted with carbon monoxide and aqueous KOH in the presence of catalytic amounts of this catalyst to give the corresponding carboxylic acids upon subsequent acidification. The Pd-catalyzed amidalion of electron-deficient aryl chloride readily afforded a carbonylation prodnct in the presence of low CO pressure and a slight excess of an iodide salt. Alkoxycarbonylation of aryl chlorides using palladium catalyst Pd(PCy3)2(dba) has been reported. ... 

In 2013, Bhanage designed and developed an IL-immobilized palladium catalyst 183 and successfully employed in carbonylation reactions, including alkoxycarbonylation, phenoxycarbonylation, and aminocarbonylation ( heme 24) [67]. Alkoxycarbonylation reaction between aryl iodide 184 and alcohol or phenol or amine was performed in an autoclave pressurized with 0.5 MPa of carbon monoxide. Current process would be useful for the utilization of carbon monoxide for synthesis of valuable chemicals from simple starting materials with the use of recyclable catalyst. Furthermore, this catalytic system avoids use of phosphine ligands. The IL-supported palladium catalyst was recycled up to four successive recycles. 

Hydroxycarbonylation and alkoxycarbonylation of alkenes catalyzed by metal catalyst have been studied for the synthesis of acids, esters, and related derivatives. Palladium systems in particular have been popular and their use in hydroxycarbonylation and alkoxycarbonylation reactions has been reviewed.625,626 The catalysts were mainly designed for the carbonylation of alkenes in the presence of alcohols in order to prepare carboxylic esters, but they also work well for synthesizing carboxylic acids or anhydrides.137 627 They have also been used as catalysts in many other carbonyl-based processes that are of interest to industry. The hydroxycarbonylation of butadiene, the dicarboxylation of alkenes, the carbonylation of alkenes, the carbonylation of benzyl- and aryl-halide compounds, and oxidative carbonylations have been reviewed.6 8 The Pd-catalyzed hydroxycarbonylation of alkenes has attracted considerable interest in recent years as a way of obtaining carboxylic acids. In general, in acidic media, palladium salts in the presence of mono- or bidentate phosphines afford a mixture of linear and branched acids (see Scheme 9). 

Alkoxycarbonylation has been known for a long time, but the rates and selectivities of the new catalysts are outstanding. The mechanism of the alkoxycarbonylation reaction catalysed by palladium has been the topic of research for many years [55], Stepwise reactions had shown the feasibility of two mechanistic pathways, shown in Figure 12.20, but kinetic studies and in situ observations on catalytic systems were lacking. 

Highly selective transformation of terminal acetylenes to either linear or branched carboxylic acids or esters may be achieved by appropriately selected catalyst systems. Branched esters are formed with high selectivity when the acetylenes are reacted with 1-butanol by the catalyst system Pd(dba)2/PPh3/TsOH (dba = dibenzylideneacetone) or palladium complexes containing PPh3. Pd(acac)2 in combination with various N- and O-containing phosphines and methanesulfonic acid is also an efficient catalyst for the alkoxycarbonylation of 1-alkynes to yield the branched product with almost complete selectivity.307,308... 

Reduction of /V-alkoxycarbonyl-2-azabicyclo[2.2.0]hcx-5-cnc with hydrogen and palladium on charcoal catalyst gave the corresponding bicyclo[2.2.0]hexanes <2003JOC1626>. Reductive arylation occurred when /V-alkoxycarbo-nyl-2-azabicyclo[2.2.0]hexane was treated with 2-chloro-5-iodopyridine in the presence of palladium(m) acetate, triphenylphosphine, piperidine, formic acid, and dimethylformamide (DMF) to give mainly 175 and 176 but in moderate and variable yields (28-58%) <2000T9233>. 

Ryu and coworkers subsequently reported photochemically initiated radical alkoxycarbonylation reactions of alkyl iodides 177 using 5 mol% of Pd(PPh3)4 as the catalyst (Fig. 42) [210]. The reaction proceeds by light-stimulated reductive generation of an alkyl radical by the palladium(O) catalyst 179A. This radical has three options to react further. Direct addition to CO forms the acyl radical 177A. 

The most frequently used metallic catalysts for acyldiazo- and (alkoxycarbonyl)dia-zomethanes are complexes or salts of rhodium, palladium and copper. Alkenylboronic esters A-silylated allylamines and acetylenes are successfully cyclopropanat-ed with diazocarbonyl compounds under catalysis of one of those metal derivatives. Newly developed metallic catalysts for diazoacetic esters include polymer-bound, quantitatively recoverable Rh(II) carboxylate salts ", Cu(II) supported on NATION ion exchange poly-mer ruthenacarborane clusters, Rh2(NHCOCH3)4 which produces cyclopropanes with substantially enhanced trans (anti) selectivity as shown below and (rj -CsHs)... 

Alkoxycarbonylation of amines. Carbamates can be prepared by reaction of a primary amine with carbon monoxide, oxygen, and an alcohol catalyzed by either 5% rhodium on activated carbon or palladium black and an alkali metal halide, particularly an iodide such as Csl or K1 (equation 1). Essentially no reaction occurs in the absence of the salt. Dialkylureas are intermediates in the reaction, and can be isolated as the major product when less active catalysts such as IrCI, arc used. 

Palladium complexes bearing chiral monodentate phosphine ligands 592 (R = H, Ph) are effective catalysts for asymmetric alkoxycarbonylation of allyl phosphates <1997TL8227>. 

A useful synthesis of ( )-j -ethoxycarbonylvinylsilanes by palladium-catalyzed regio- and stereospecific hydroesterification (EtOH -I- CO) (or carboethoxylation) of trimethylsilylacetylenes has been reported recently [210]. Alkoxycarbonyl or carbonyl functionalization of vinylsilanes are useful synthetic intermediates [211, 212]. The use of PdCl2(dppf) as a catalyst (with SnCl2 2 H2O as cocatalyst) is found to be superior and gives excellent yields. A key step in the reaction is thought to involve hydropalladation to give 60 or 61. The preference for 60 to 61 is understood... 

Depending on the catalyst system and the reaction conditions, especially at elevated CO pressure it is possible to obtain selectively double carbonylation reactions to 1-keto carboxylic derivatives [25]. Recent mechanistic investigations have shown that double CO insertion into the palladium-carbon bond does not occur directly instead, the terminal step of double carbonylation is generally a coupling reaction between metal-bonded acyl, alkoxycarbonyl or amidocarbonyl groups and CO. 

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