Palladium hydride complex reaction with

The catalytic system studied by Rennard and Kokes was in fact very complex. It can be expected that the satisfactory prolongation of the reaction should, however, result in a deviation from the formulated kinetics. Unfortunately no investigation comparable to that of Scholten and Kon-valinka has been done in the case of olefin hydrogenation. Such a study of the catalytic activity of the pure /3-phase of palladium hydride in comparison with the a- or (a + /3)-phases would supplement our knowledge concerning catalytic hydrogenation on 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... 

The mechanism of this reaction was considered on the basis of hydropalladation (Scheme 14). To minimize steric repulsions, the palladium hydride complex approaches the C=CH2 moiety of the allene in the anti-Markovnikov mode from the opposite side of the substituent. This addition gives a 7t—allyl palladium complex with the (Z)-configuration,18 which is converted to the (Z)-product by C-P bond formation, with regeneration of the Pd(0) catalyst. 

In a new and significant development, it was found that chlorocyclohexadienyl manganese tricarbonyl complexes undergo palladium-catalyzed coupling reactions with a variety of carbon nucleophiles to afford products from which hydride may be abstracted with trityl cation, thus generating new [( -arene)Mn(CO)3]+ complexes containing substituents that cannot be introduced in any other manner. This is particularly true of electron-withdrawing substituents. Scheme 16 illustrates the chemistry involved. 

Trost proposed the following mechanism to explain formation of n -allyl complexes from alkenes.271.216 Initial reaction with PdCl4 generates a n -alkene complex (352), which is in equilibrium with the n-allyl palladium hydride complex 353. The presence of a base (such as an amine or a phosphine) or CuCl2 led to removal of the hydrogen from Pd—H to give the n -allyl palladium dimer 354. The CuCl2 oxidative procedure is more efficient than addition of a base. 

Insertions of alkynes into palladium-hydride complexes have been reported to take place as part of catalytic cycles, resulting in circumstantial yet credible evidence for the occurrence of alkenylpalladium complexes. The first example of palladium-catalyzed hydroarylation of alkynes with organoboronic acids B(OH)2R has been given. The mechanistic interpretation based on labeling studies involves hydropalladation of the alkyne to give an intermediate alkenylpalladium(ii) species that reacts with the organoboronic acid (Scheme 11). Similar direct coupling reactions... 

An asymmetric variant of this reaction was developed using chiral Pd complex 111 with either silanes or disiloxanes [66-68]. Both relative and absolute stereochemistries were controlled in this system and good yields (60-85%) were obtained after oxidation (Eq. 18). Formation of the silane-containing product was inhibited by the presence of water due to competitive formation of the palladium hydrides and silanols [68]. The use of disiloxanes as reductants, however, provided expedient oxidation to the alcohol products without decreasing the isolated yields enantioselectivity was 5-15% lower in this more robust system [66]. Benzhydryldimethylsilane proved to be a good compromise between high yield and facile oxidation [66]. Palladium com-... 

Two competing chain-transfer mechanisms in copolymerization of CO and ethene catalyzed by Pd11 acetate/dppp complexes were found. One involves termination via an isomerization into the enolate followed by protonation with methanol the rate of this reaction should be independent of the concentration of the protic species. The second chain-transfer mechanism comprises termination via methanolysis of the acylpalladium species, and subsequent initiation by insertion of ethene into the palladium hydride bond.501... 

A similar involvement of palladium hydride, palladium alkyl, and palladium acyl complexes as intermediates in the catalytic cycle of the Pd-catalyzed hydroxycarbonylation of alkenes was reported for the aqueous-phase analogs. The cationic hydride PdH(TPPTS)3]+ was formed via the reduction of the Pd11 complex with CO and H20 to [Pd(TPPTS)3] and subsequent protonation in the acidic medium. The reaction of the hydride complex with ethene produced two new compounds, [Pd(Et)(TPPTS)3]+ and Pd(Et)(solvent)(TPPTS)2]+. The sample containing the mixture of palladium alkyl complexes reacted readily with CO to afford trans-[Pd(C(Q)Et)(TPPTS)2]+.665... 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

Disubstituted silole derivatives are synthesized by the palladium-catalyzed reaction of (trialkylstannyl)di-methylsilane with terminal alkynes (Equation (107)).266 The mechanism is supposed to involve a palladium silylene complex, which is generated via /3-hydride elimination from LJ3d(SiMe2H)(SnBu3) (Scheme 62). Successive incorporation of two alkyne molecules into the complex followed by reductive elimination gives rise to the silole products. 

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