Palladium complexes hydride

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... 

Green used the cationic phosphine palladium hydride complex for the reaction of methanol (44). 

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. 

When carbon monoxide is bubbled through a methanol solution of (dppp)Pd(triflate)2 a carbomethoxy-palladium species is formed, which can undergo insertion of alkenes and hence this is a feasible alternative initiation route to chain-growth polymerisation (Figure 12.4) [13], To ensure a clean formation of the carbomethoxy species, however, exclusion of water is a prerequisite. If during the preparation water was present the formation of a palladium hydride complex (dppp)PdFT was observed (reaction (1), Figure 12.2). 

Ref. (234) reported a theoretical study of the solvent effects on various isomers of the palladium hydride complex PdH3Cl(NH3)2 in dichloromethane. The influence of the solvent was investigated by discrete MP2 and SAPT, and continuum SCRF calculations. The theoretical relation between SCRF and SAPT, Eq. (1-177), was fully confirmed by the numerical results from the discrete SAPT and continuum SCRF calculations, cf. Table 1-7 and Figure 1-4. Interestingly, both the discrete MP2 and continuum SCRF models predicted the same relative stabilities for the isomers of PdH3Cl(NH3)2 in dichloromethane. Small energetic differences between the results of the discrete and continuum calculations could be explained by the entropy effects, neglected in the discrete model. 

The thermally stable nickel and palladium hydride complexes, trans-[MHX(PR3)2], where M = Ni or Pd, R = cyclohexyl or isopropyl, and X = halogen, have been prepared by various methods.1-5 Hydrido[tetra-hydroborato( 1 - )] complexes can be prepared from them by metathet-ical reactions.3 The hydrido[tetrahydroborato( 1 — )]bis(tricyclohexylphos-... 

Palladium hydride complexes are of considerable interest in the catalytic chemistry of palladium because of their postulated occurrence as intermediates in a number of reactions such as hydrogenations, isomerizations, and oxidation reactions. In contrast to Pt(II), which forms stable hydrides, most Pd(II) hydrides are unstable. Although there are earlier reports of unstable Pd(II) hydride complexes being formed, the first stable hydride was prepared by Brooks and Clocking by the following reaction (20, 21) ... 

The similarity in the mechanisms of the two carbonylations is apparent. In the / -chloroacyl chloride formation, the first step is the insertion of olefins into the palladium-chlorine bond. For the catalytic carbonylations, the insertion of olefins takes place to the palladium hydride complex formed in situ. In both cases, divalent palladiums react with olefins. 

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. 

Dihydrogen reacts with [(Ph3P)2Pd(CF3COO)2l complex at 70 °C, in the presence of excess triphenylphosphine and aqueous CF3COOH, to give the cationic palladium hydride complex [(Ph3P)3Pd(H)]. This complex could also be prepared by the addition of the acid to [Pd(PPh3)4l at 25 °C. The structure was confirmed by and P NMR studies. ... 

Synthesis and Reaction ofii - Si-H)Pd 0) Complex as an Equivalent to PSiP-Palladium Hydride Complexes... 

The q -(Si-H)Pd(0) 43 was found to catalyze the hydrocarboxylation reaction of allenes, indicating that 43 worked as the palladium hydride complex 13 in solution via reversible oxidative addition/reductive elimination of the Si-H bond after dissociation of PPhj (Scheme 9.12) [21]. This result prompted us to investigate a... 

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... 

A palladium hydride complex is a catalyst in the isomerization of epoxides and a hydride mechanism via two distinct enantio-determining steps (Scheme 145) is suggested. ... 

NMR studies revealed the presence of palladium hydride complexes as well as complexes with and without phosphine. They furthermore showed rapid release of -cymene from palladium indicating that the active catalyst most likely does not include p-cymene. 

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