Palladium-alkyl-carbon monoxide complexes

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... 

Carbon monoxide rapidly inserts into the carbon—zirconium bond of alkyl- and alkenyl-zirconocene chlorides at low temperature with retention of configuration at carbon to give acylzirconocene chlorides 17 (Scheme 3.5). Acylzirconocene chlorides have found utility in synthesis, as described elsewhere in this volume [17]. Lewis acid catalyzed additions to enones, aldehydes, and imines, yielding a-keto allylic alcohols, a-hydroxy ketones, and a-amino ketones, respectively [18], and palladium-catalyzed addition to alkyl/aryl halides and a,[5-ynones [19] are examples. The acyl complex 18 formed by the insertion of carbon monoxide into dialkyl, alkylaryl, or diaryl zirconocenes may rearrange to a r 2-ketone complex 19 either thermally (particularly when R1 = R2 = Ph) or on addition of a Lewis acid [5,20,21]. The rearrangement proceeds through the less stable... 

Insertion of CO is therefore always kinetically controlled. When an alkyl palladium species has formed, the open site will be occupied by a coordinating CO molecule. Carbon monoxide coordinates more strongly to palladium than ethene, even when the palladium centre is cationic. The reason for this is steric the cone angle of ethene is much larger than that of CO and the steric hindrance in the ethene complex is therefore much larger. If the barriers of activation for the insertion processes of ethene and CO are of the same order of... 

Terminal monoalkenes were alkylated by stabilized carbanions (p a 10-18) in the presence of 1 equiv. of palladium chloride and 2 equiv. of triethylamine, at low temperatures (Scheme l).1 The resulting unstable hydride eliminate to give the alkene (path b), or treated with carbon monoxide and methanol to produce the ester (path c).2 As was the case with heteroatom nucleophiles, attack at the more substituted alkene position predominated, and internal alkenes underwent alkylation in much lower (=30%) yield. In the absence of triethylamine, the yields were very low (1-2%) and reduction of the metal by the carbanion became the major process. Presumably, the tertiary amine ligand prevented attack of the carbanion at the metal, directing it instead to the coordinated alkene. The regiochemistry (predominant attack at the more sub-... 

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. 

RX RCHO. Alkyl halides can be converted directly into aldehydes in moderate to high yield by reaction with carbon monoxide (1-3 atm.) and tri-n-butyltin hydride catalyzed by the palladium(O) complex. The reaction involves insertion of carbon monoxide to form an acyl halide, which is known to be reduced to an aldehyde under these conditions (10, 411). Direct reduction of the halide can be minimized by slow addition of the tin hydride to the reaction and by an increase in the carbon monoxide pressure. 

Vanhoye and coworkers [402] synthesized aldehydes by using the electrogenerated radical anion of iron pentacarbonyl to reduce iodoethane and benzyl bromide in the presence of carbon monoxide. Esters can be prepared catalytically from alkyl halides and alcohols in the presence of iron pentacarbonyl [403]. Yoshida and coworkers reduced mixtures of organic halides and iron pentacarbonyl and then introduced an electrophile to obtain carbonyl compounds [404] and converted alkyl halides into aldehydes by using iron pentacarbonyl as a catalyst [405,406]. Finally, a review by Torii [407] provides references to additional papers that deal with catalytic processes involving complexes of nickel, cobalt, iron, palladium, rhodium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, tin, lead, zinc, mercury, and titanium. 

L, used in this mechanism, is a ligand which can stabilize the intermediate palladium complexes and satisfy a coordination number of the palladium whatever it is. L, for example, can be carbon monoxide, phosphines, solvents, or another molecule of palladium. Formation of hydride complexes by the oxidative addition of hydrogen chloride or hydrogen to a metal complex is well known (9, 27), as is formation of alkyl metal complexes by addition of metal hydrides to olefins. 

Primary and secondary alkyl iodides can be formylated with the platinum complex PtCl2(PPh3)2, instead of palladium complexes as the catalyst in good to high yield at 120° and 100 atm of carbon monoxide and hydrogen (1 1) in the presence of potassium carbonate (Eq. 65). ... 

Interception of the Tr-allyl palladium complex by soft nucleophiles, particularly malonates, has been described above. Alkenes, alkynes and carbon monoxide can also insert into the Tr-allyl palladium complex, generating a u-alkyl palladium species. When an internal alkene is involved, a useful cycbzation reaction takes place (sometimes called a palladium-ene reaction).Addition of palladium(O) to the allylic acetate 225 gave the cyclic product 226 (1.225). The reaction proceeds via the -ir-allyl palladium complex (formed with inversion of configuration), followed by insertion of the alkene cis- to the palladium and p-hydride elimination. In some cases it is possible to trap the a-alkyl palladium species with, for example, carbon monoxide. 

The reactions of benzylic halides with carbon monoxide and alcohols form esters in good yields. However, the reactions of alkyl halides are more limited for two reasons. First, the oxidative addition of alkyl halides occurs less readily to palladium complexes than the oxidative addition of aryl halides. This difference was noted in Chapter 7. Second, the intermediate alkylpalladium halide can undergo P-hydrogen elimination. As noted in Chapters 9 and 10, these hurdles have been overcome in some cases, and cross-coupling... 

Vrieze and Elsevier and BrookharP - have studied insertions involved in the polymerization process. Brookhart and co-workers have quantified the origin of the nearly perfect alternation between the incorporation of carbon monoxide and ethyelene. The activation barriers for the various insertion steps involving palladium complexes of a bidentate phosphine are provided in Figure 17.10. - ° The activation energies for the insertion of carbon monoxide into the alkylpalladium complex to form an acylpalladium species and the insertion of ethylene into a cationic palladium acyl to form a y-keto alkylpalladium complex are both lower than that for insertion of ethylene into the palladium alkyl. These insertion reactions occur at -80 °C and -100 C with barriers of 13.4 and 12.3 kcal/mol, respectively. 

The reactions of some optically active benzyl halides have provided further evidence for the nucleophilicities of the [Pd(PR3)j] species (Lau et al., 1974, 1976 Wong et al., 1974 Stille and Lau, 1976). The salient features of these studies are summarised in Scheme 6. The stereochemistry of the addition is established by reaction of the palladium-alkyl complex with carbon monoxide (this insertion is known to take place by an intramolecular migration process, with retention of configuration in the migrating alkyl group), and subsequent formation of an ester from this acyl complex. 

The usual experimental procedure for CO insertion is to heat a solution of the alkyl or aryl complex with a high pressure of carbon monoxide. However, many of these reactions take place under mild conditions. A high pressure of carbon monoxide may only be necessary to ensure a high yield of acyl derivative in a reversible reaction. The following examples illustrate some of the conditions used. Note that the palladium compound carbonylates more readily than its platinum analog C2H5Mn(CO)s also reacts more easily than C2H5Re(CO)s. 

Unactivated olefins are inert toward attack of nucleophiles. When complexed to palla-dium(II) salts, stabilized carbanions (p/(j, = 10-17) react intermolecularly with these olefinpalladium(II) complexes to generate alkylpalladium complexes. Alkylation occurs predominantly at the 2-position after a reductive or /3-elimination procedure or an insertion reaction with carbon monoxide. Nonstabilized carbanions attack the palladium directly, forming aUcylpalladium complexes that lead to alkene alkylation products at the 1-position. All these reactions required stoichiometric amounts of palladium salts. 

The a-keto amides are less susceptible to hydrolysis and preparation of a-keto esters and acids are preferable for synthesizing various derivatives thereof. Various aryl iodides and bromides can be converted into a-keto esters on reactions with alcohols and carbon monoxide in the presence of a base such as tertiary amines or potassium acetate with catalytic amounts of tertiary phosphine-coordinated palladium complexes (Eq. 11).[42]-[46] jjjgjj yields of a-keto esters can be achieved only when iodide substrates are used. Double carbonylation of aryl bromides to a-keto esters can be accomplished with difficulty at much slower rates. Alkyl and benzyl iodides give no double carbonylation products. 

The role of palladium in organic synthesis continues to be explored and exploited. Enol stannanes are monoalkylated by allylic acetates in the presence of tetrakis(triphenylphosphine)palladium, Enol stannanes give higher selectivity for monoalkylation than enolate ions or silyl enol ethers. High regioselec-tivity is observed for alkylation at the less substituted end of the allyl moiety. Olefins, after complexation to palladium(ll), alkylate enolate anions. The organopalladium product may be converted into saturated ketones, or into enones by /3-elimination, or acylated with carbon monoxide (Scheme... 

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