Palladium complexes catalytic reactions

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

In their zerovalent compounds, all three metals (Ni, Pd, Pt) undergo oxidative addition of alkyl, aryl, and acyl halides. For palladium, in particular, such reactions are key steps in a wide range of catalytic reactions. Palladium(II) and platinum(II) complexes also add C—X bonds to generate Pd(IV) and Pt(IV) species. Since C—C or C—H bond formation by reductive elimination often occurs readily, a common reaction sequence involves C—X addition followed by coupling of two alkyl groups, or an alkyl and a hydride ligand. 

Masked formyl cyanides such as cyanohydrin alkyl ethers of formyl cyanide are also applicable to the aldol reaction. Palladium complexes, especially Pd2(dba)3-CHCl3, show high catalytic activities for the additions to aldehydes (Eq. 66) [136]. 

Enolate Arylation Reactions. The direct coupling of aryl halides with enolates (or enolate equivalents) of ketones, esters, and amides is now well established. Malonic esters, cyanoacetates, and malononitrile can be arylated upon treatment with aryl halides in the presence of Pd(dba)2 and electron-rich phosphines or N-heterocyclic carbenes. Carbene ligands have also proven effective in promoting the a-arylation of protected amino acids. As a caveat to the use of Pd(dba)2, the arylation of azlactones in the presence of this palladium source and phosphines was less efficient than that with Pd(OAc)2. The dba ligands were found to react with azlactone substrates to form catalytically inactive palladium complexes. Diastereoselective enolate arylation has been achieved through the use of chiral auxiliaries appended to preformed enol silyl ethers (eq 23). The role of the zinc additive is not clear, however, it appears that discrete zinc enolates are not involved. 

When 72 was treated with AgBF4 and butyl acrylate under stoichiometric conditions, decomposition products resulting from reductive elimination from all of the key catalytic intermediates were observed. The stability of the catalyst under the actual reaction conditions was much higher, and was attributed to faster rates of p-hydride elimination from the arylated acrylate at the elevated reaction temperatures, which removed at least two of the decomposition pathways. However, it should be noted that these decomposition pathways could be responsible for the generation of catalytically active palladium complexes containing one or zero NHC ligands in this or other systems. 

Acetyl chlotide is reduced by vatious organometaUic compounds, eg, LiAlH (18). / fZ-Butyl alcohol lessens the activity of LiAlH to form lithium tti-/-butoxyalumium hydtide [17476-04-9] C22H2gA102Li, which can convert acetyl chlotide to acetaldehyde [75-07-0] (19). Triphenyl tin hydtide also reduces acetyl chlotide (20). Acetyl chlotide in the presence of Pt(II) or Rh(I) complexes, can cleave tetrahydrofuran [109-99-9] C HgO, to form chlorobutyl acetate [13398-04-4] in about 72% yield (21). Although catalytic hydrogenation of acetyl chlotide in the Rosenmund reaction is not very satisfactory, it is catalyticaHy possible to reduce acetic anhydride to ethylidene diacetate [542-10-9] in the presence of acetyl chlotide over palladium complexes (22). Rhodium trichloride, methyl iodide, and ttiphenylphosphine combine into a complex that is active in reducing acetyl chlotide (23). 

Succinic anhydride is manufactured by catalytic hydrogenation of maleic anhydride [108-31-6]. In the most widely used commercial process this reaction is performed in the Hquid phase, at temperatures of 120—180°C and at moderate pressures, in the range of 500—4000 kPa (72—580 psi). Catalysts mentioned in the patent Hterature include nickel (124), Raney nickel (125,126), palladium on different carriers (127,128), and palladium complexes (129). The hydrogenation of the double bond is exothermic Ai/ = —133.89 kJ/mol (—32 kcal/mol) (130). 

Heck reaction, palladium-catalyzed cross-coupling reactions between organohalides or triflates with olefins (72JOC2320), can take place inter- or intra-molecularly. It is a powerful carbon-carbon bond forming reaction for the preparation of alkenyl- and aryl-substituted alkenes in which only a catalytic amount of a palladium(O) complex is required. 

The original Sonogashira reaction uses copper(l) iodide as a co-catalyst, which converts the alkyne in situ into a copper acetylide. In a subsequent transmeta-lation reaction, the copper is replaced by the palladium complex. The reaction mechanism, with respect to the catalytic cycle, largely corresponds to the Heck reaction.Besides the usual aryl and vinyl halides, i.e. bromides and iodides, trifluoromethanesulfonates (triflates) may be employed. The Sonogashira reaction is well-suited for the synthesis of unsymmetrical bis-2xy ethynes, e.g. 23, which can be prepared as outlined in the following scheme, in a one-pot reaction by applying the so-called sila-Sonogashira reaction ... 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

A series of new palladium complexes 26 bearing a phosphine-functionalised NHC ligand has been shown to be more efficient than typical catalytic systems (i.e. Pd(OAc), PPhj) for the reaction depicted in Scheme 7.10 [55], In fact, these conditions afforded the 9-benzylidene-97f-fluorene in higher yields when iodobenzene was used, whereas the reaction of the less reactive phenyl bromide with diphenylacetylene was achieved for the first time affording the desired product in moderate to good yields. 

The intramolecular addition of the O-H bond to alkynes catalyzed by palladium complexes has been developed by K. Utimoto et al. (Eq. 6.59) [104]. An alkynyl alcohol can be converted to a cyclic alkenyl ether in the presence of a catalytic amount of [PdCl2(PhCN)2 or [PdCl2(MeCN)2] in ether or THE at room temperature. When the reaction was carried out in MeCN-H20 under reflux in the presence of a catalytic amount of PdCl2, hydration of the acetylenic alcohol occurred and the ketoalcohol was obtained in good yield instead. 

At the beginning of the 1970s a convenient procedure was described for converting olefins into substituted butanedioates, namely through a Pd(II)-cata-lysed bisalkoxycarbonylation reaction. So far various catalytic systems have been applied to this process, but it took twenty years before the first examples of an enantioselective bisalkoxycarbonylation of olefins were reported. Ever since, the asymmetric bisalkoxycarbonylation of alkenes catalysed by palladium complexes bearing chiral ligands has attracted much attention. The products of these reactions are important intermediates in the syntheses of pharmaceuticals such as 2-arylpropionic acids, the most important class of... 

Nucleophilic Substitution of xi-Allyl Palladium Complexes. TT-Allyl palladium species are subject to a number of useful reactions that result in allylation of nucleophiles.114 The reaction can be applied to carbon-carbon bond formation using relatively stable carbanions, such as those derived from malonate esters and (3-sulfonyl esters.115 The TT-allyl complexes are usually generated in situ by reaction of an allylic acetate with a catalytic amount of fefrafcz s-(triphenylphosphine)palladium... 

It is not clear whether the X anion remains ligated to the palladium(II) center. For example, for acetic acid, the palladium hydride was initially postulated as being HPd(OAc)L ,377,378 but more recently as HPdL +.367 To date, none of these complexes has been characterized.367 Oxidative addition of acetic acid or formic acid to a palladium(O) complex in DMF affords a cationic palladium hydride /ruw.v-I IPd(PPh3)2(DMF)+, with an acetate or a formate counter-anion. Both reactions are reversible and involve an unfavorable equilibrium so that a large excess of acid is required for the quantitative formation of the palladium hydride complex.379 This allows us to conclude that the catalytic reactions initiated by reaction of palladium(O) and acetic acid (or formic acid) proceed via a cationic palladium hydride trans-HPdfPPtHWDMF)"1", when they are performed in DMF.379... 

In summary, these results demonstrate that air-stable POPd, POPdl and POPd2 complexes can be directly employed to mediate the rate-limiting oxidative addition of unactivated aryl chlorides in the presence of bases, and that such processes can be incorporated into efficient catalytic cycles for a variety of cross-coupling reactions. Noteworthy are the efficiency for unactivated aryl chlorides simplicity of use, low cost, air- and moisture-stability, and ready accessibility of these complexes. Additional applications of these air-stable palladium complexes for catalysis are currently under investigation. 

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