Palladium complexes insertion into metal carbon bonds

Radicals and carbenes ch37 ch38 metal to carbon are key steps Carbon monoxide inserts into metal-carbon bonds Palladium is the most important metal C-C, C-0, and C—N bonds can be made with Pd catalysis Cross-coupling of two ligands is common Allyl cation complexes are useful electrophiles ... 

In many palladium-catalyzed reactions, generation of cationic intermediates has been often found to enhance the catalytic activities [60]. A number of reports have demonstrated that the CO insertion into metal-carbon a-bonds can be accelerated by formation of cationic complexes from neutral complexes [50,61-63]. 

In general, carbonylation proceeds via activation of a C-H or a C-X bond in the olefins and halides or alcohols, respectively, followed by CO-insertion into the metal-carbon bond. In order to form the final product there is a need for a nucleophile, Nu". Reaction of an R-X compound leads to production of equivalent amounts of X", the accumulation of which can be a serious problem in case of halides. In many cases the catalyst is based on palladium but cobalt, nickel, rhodium and mthenium complexes are also widely used. 

In the process of carbonyl insertion the 1,1 migratory insertion of the coordinated CO ligand into the metal-carbon bond results in the formation of a metal-acyl complex (Figure 1-7). This process, as nearly all elementary steps discussed so far, is reversible, but even when using atmospheric CO pressure the equilibrium is mostly shifted towards insertion. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Over 35 years ago, Richard F. Heck found that olefins can insert into the metal-carbon bond of arylpalladium species generated from organomercury compounds [1], The carbopalladation of olefins, stoichiometric at first, was made catalytic by Tsutomu Mizoroki, who coupled aryl iodides with ethylene under high pressure, in the presence of palladium chloride and sodium carbonate to neutralize the hydroiodic acid formed (Scheme 1) [2], Shortly thereafter, Heck disclosed a more general and practical procedure for this transformation, using palladium acetate as the catalyst and tri-w-butyl amine as the base [3], After investigations on stoichiometric reactions by Fitton et al. [4], it was also Heck who introduced palladium phosphine complexes as catalysts, enabling the decisive extension of the ole-fination reaction to inexpensive aryl bromides [5],... 

The efficiency of transmetalation from boron to palladium in the catalytic 1,4-addition of aryl or 1-alkenylboronic acids to enones encouraged us to extend the protocol to the addition of aryl- and 1-alkenylboronic adds to aldehydes in an aqueous solution (Eq. 4). The insertion of carbonyl groups into transition metal-carbon bonds has not received much attention, but the catalytic use of transition metals may allow such addition of various organometallics which are inert without a catalyst, the asymmetric addition using a chiral phosphine complex, or the reaction in an aqueous phase. 

The formation of ortho palladium products from a-aryl nitrogen derivatives and palladium salts is well known. Complexes formed from azobenzene, Schiff bases, tertiary benzylamines and oximes readily undergo insertion of CO into the metal-carbon bond to give, after work-up, a variety of heterocyclic compounds. Unfortunately, such reactions use expensive palladium salts in stoichiometric quantities. However, a number of related reactions have been shown to proceed in the presence of only catalytic quantities of palladium. Isoindolinones, for example, can be synthesized in good yield by reaction of o-bromoaminoalkylbenzenes with CO (100 C, 1 bar) in the presence of catalytic amounts of Pd(OAc)2, PPha and Bu"3N (equation 56). °... 

Polymerization of isocyanides is a thermodynamically feasible process, in agreement with the stoichiometric multiple insertion observed in reactions between metal-alkyl complexes and isocyanides. The entropy loss in the case of isocyanides is lower than for insertion of CO. Isocyanide insertions into palladium-alkyl a bonds are faster than those for the platinum(II) analogues. The latter, on the other hand, usually lead to more stable and better defined products. Insertion of isocyanides into platinum-carbon bonds has been studied extensively Reaction (j) is typical the ionic product was strongly suggested by observation that the compounds isolated under mild conditions are 1 1 electrolytes. 

It has been suggested that the number of cation palladium(u) also causes the isomerization of cubane to cuneane, like the silver(i) but unlike d rhodium(i) complexes, eliminates this idea. The redox properties of the respective complexes may be relevant - rhodium(i) is much readier to undergo oxidative addition than silver(i) or palladium(ii). One factor which may be of considerable importance is the relative strengths of metal-carbon bonds. It is energetically much more favourable to insert rhodium(i) into a carbon-carbon bond than to insert silver(i) into such a bond. Though the different courses of isomerizations catalysed by rhodium(i) and silver(i) complexes may be ascribed to the operation of a non-concerted mechanism for the former but... 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

Metal-Halogen Counpounds. One of the few examples of an olefin insertion into a metal-halogen compound has been reported by Tsuji. The reaction, which also supports the idea that sigma-bonded metal-carbon compounds are intermediates in the palladium chloride-olefin oxidation reaction, was the addition of carbon monoxide to the ethylene palladium chloride 7r-complex in nonaqueous solvents to produce a moderate yield of 3-chloropropionyl chloride (96). 

Olefinic compounds will often insert into carbon-transition metal bonds as CO does, and this reaction is an important step in many catalytic syntheses. When this step is combined with an oxidative addition of an organic halide to a palladium(O) complex in the presence of a base, a very useful, catalytic olefinic substitution reaction results (26-29). The oxidative addition produces an organopalladium(II) halide, which then adds 1,2 to the olefinic reactant (insertion reaction). The adduct is unstable if there are hydrogens beta to the palladium group and elimination of a hydridopalladium salt occurs, forming a substituted olefinic product. The hydridopalladium salt then reforms the... 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

The resulting extraordinary stability of NHC-metal complexes has been utilized in many challenging applications. However, an increasing number of publications report that the metal-carbene bond is not inert [30-38]. For example, the migratory insertion of an NHC into a ruthenium-carbon double bond [30], the reductive elimination of alkylimidazolium salts from NHC alkyl complexes [37] or the ligand substitution of NHC ligands by phosphines [36,38] was described. In addition, the formation of palladium black is frequently observed in applications of palladium NHC complexes, also pointing at decomposition pathways. 

This reaction constitutes a special type of process in which a hydrogen atom and a nucleophile are added across the diene with fonnation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20] amination [21,22], and addition of active methylene compounds [21 a,23,24], These reactions are initiated by an oxidative addition of H-Nu to the palladium(0) catalyst, which produces a palladium hydride species 1 where the nucleophile is coordinated to the metal (Scheme 8-1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a (jr-allyl)palladium intermediate. Now depending on the nature of the nucleophile (Nu) the attack on the jr-allyl complex may occur either by external trans-aVtBck (path A) or via a cw-migration from palladium to carbon (path B). 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

The following mechanism was proposed for the carbonylation of olefin-palladium chloride complex (10). The first step is coordination of carbon monoxide to the complex. Insertion of the coordinated olefin into the palladium-chlorine bond then forms a -chloroalkylpalladium complex (IV). This complex undergoes carbon monoxide insertion to form an acylpalladium complex (V), as has been assumed for many metal carbonyl-catalyzed carbonylation reactions. The final step is formation of a )8-chloroacyl chloride and zero-valent palladium by combination of the acyl group with the coordinated chlorine. 

Metal carbon insertions of C02 occur in both main group and transition metal complexes.5,19 The reactions result in a strong M—O bond at the expense of a weak M—C bond (together with a C C a bond at the expense of a C O n bond). Insertion into t/weo-W(CO)5 CHDCHDPh gave the threo carboxylate, indicating retention of configuration at the o-carbon, as with CO insertion.29 Cis- RW(CO)4L (R = Me, Et or Ph) showed second-order kinetics toward C02 insertion, first order in anion and in C02. Replacement of CO by a phosphine or phosphite increased the insertion rate.30 Palladium catalysis of C02 insertion into unreactive Sn C bonds in allyl stannanes provided three isomeric tetra-carboxylates from tetraallyl tin. Attempted reactions of tetraalkyl, vinyl or aryl tin complexes did not proceed. Insertion into an intermediate rf -allyl palladium complex was suggested to lead to carboxylate products.31,32... 

The 5-exo carbopaUadative construction of oxindoles has been widely investigated. One intriguing example comes from an approach toward gelsemine 93 (Scheme 14). Cychza-tion of aryl bromide 89 produces diastereomeric oxindoles 90 and 91. Conditions were developed that favor the synthesis of either isomer. Exposure of 89 to the palladium catalyst Pd2(dba)3 under hgandless conditions provides spirocychc oxindole 90 as the major product in good yield. In this reaction, insertion of the transition metal into the carbon-bromine bond and complexation to the eneamide alkene leads to complex 87. Migratory insertion then gives intermediate 88, which after /S-hydride elimination provides 90. 

Intramolecular Insertion.—This occurs in metal insertion into phenyl-ort/ru-carbon to hydrogen bonds during the intramolecular oxidative addition of triphenylphosphine complexes of iridium(i). Intramolecular insertion also takes place in palladium(n)-catalysed decomposition of vinyl... 

A few final comments should be made on the insertions of substrates containing C-C multiple bonds into the bonds between a transition metal and an electronegative heteroatom. First, insertions of olefins into related thiolate and phosphide complexes are as rare as insertions into alkoxo and amido complexes. Reactions of acrylonitrile into the metal-phosphorus bonds of palladium- and platinum-phosphido complexes to give products from formal insertions have been observed, and one example is showm in Equation 9.90. However, these reactions are more likely to occur by direct attack of the phosphorus on the electrophilic carbon of acrylonitrile than by migratory insertion. Second, the insertions of alkynes into metal-oxygen or metal-nitrogen covalent bonds are rare, even though the C-C ir-bond in an alkyne is weaker than the ir-bond in an alkene. 

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