Halides palladium

Syntheses of palladium halides often involve direct reaction with a halogen Pd 

Pdp2 is that rare substance, a paramagnetic palladium compound, explicable in terms of (distorted) octahedral coordination of palladium with octahedra sharing comers [15]. It exists in two forms, both having figg 2.0/ig, rather below the spin only value for two unpaired electrons. Bond lengths are Pd-F 2.172 A (two) and 2.143 A (four) in the tetragonal form (rutile stracture). 

The other palladium(II) halides are all diamagnetic. PdCl2 exists in well defined a- and /0-forms [16] (as well as a 7-form) the former has a PdCl4/2 

MXj Pale violet solid Red solid, dec. Brown solid 600°C Black (a), deep red (/3), black (7), dec. 350°C Black-brown solid Brown solid Black solid, dec. 500°C 

MX3 Black solid Green-black solid Black-green crystal Black solid 

PdF4 is the only stable palladium(IV) halide [18] (testimony to the oxidizing nature of palladium(IV)) and is a very moisture-sensitive diamagnetic red solid the structure is based on Pd6F24 hexameric units linked three-dimensionally. It has octahedrally coordinated palladium with two terminal (cis) fluorines and four bridging ones. Despite the absence of other tetra-halides, the complete series of PdX - exist (cf. Ir). 

Syntheses of palladium halides often involve direct reaction with a halogen Pd------------------------------— PdF3 SeF4°rSF4 PdF2 

PdF6 has been reported [19] (but not confirmed) to result from the reaction of powdered palladium with atomic fluorine under pressure (900-1700 Pa) as a dark red solid, unstable at 0°C that oxidizes both oxygen and water. An IR band at 711 cm-1 has been assigned as u(Pd-F). There are unsubstantiated claims for PdF5. 

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Diphenylacetylene and methylphenylacetylene react with palladium halides to give a very interesting series of compounds some of which are almost certainly ir-allylicpalladium complexes. Thus diphenylacetylene (ChHjo) reacts with palladium(II) chloride in ethanol to give hexaphenyl-benzene and a complex of the composition [PdCl(Ci4Hi0)2(OC2H6)] 148, 149) which we formulate as the ir-allylic complex (XLVI) (see Section... 

Recently, we [53] and others [54] simultaneously reported an example of a complex in which the transition metal dictates the coordination mode, viz. urea-functionalized phosphine 20, which forms a trans palladium complex, complemented by hydrogen bonding ofthe urea fragments (Figure 10.6). Bear in mind that any monophosphine, and even wide bite angle diphosphines, give trans complexes with a hydrocarbyl palladium halide. More interestingly, the urea moieties can function as a host for another halide ion. 

Complex Compounds of Platinum and Palladium Halides with Dialkyl Selenides.6... 

Extensive studies by Amatore, Jutand, and co-workers have shed light on the structure and oxidative addition chemistry of a number of synthetically important palladium complexes [42], In particular, these workers have shown that the major species in a solution of Pd(dba)2 and BINAP is Pd(dba)BINAP and that oxidative addition of Phi to this complex generates (Bl-NAP)Pd(Ph)I [42d,43], In addition, it has been demonstrated that palladium halide complexes such as (PhjP jaryljPdCl do not dissociate the halide ligand in DMF solution [44], whereas the corresponding triflate complex is completely dissociated [44,45], As noted earlier, the nature of the oxidative addition intermediates defines two mechanistic pathways for the Heck reaction the neutral pathway for unsaturated halide substrates and the cationic pathway for unsaturated triflate substrates [2c-g,3,7-9]. Further, it is possible for halide substrates to be diverted to the cationic pathway by addition of Ag(I) orTh(I) salts [3], and it is possible to divert some triflate substrates to the neutral pathway by addition of halide additives [38]. Individual steps of these two pathways have recently received some scrutiny. 

For the C-C bond-forming step coordination of an electrophilic aryl palladium halide to a cyclopentadienyl anion is assumed, followed by reductive elimination. Presumably the Pd catalyst is not involved in the C-H bond-breaking step, which is interpreted as an apparently simple deprotonation with cesium carbonate as base. The overall process is similar to the arylation of other soft nucleophiles [9]. 

The subsequent. mi-elimination, yielding a 1,2-substituted alkene and hydrido-palladium halide, is, however, reversible, and therefore the thermodynamically more stable (E )-alkene, ( )-RCH=CHZ, is generally produced when the coupling reaction is performed with a terminal olefin. It is worth mentioning that the formation of this 1,2-disubstituted alkene is accompanied with the formation of 1,1-disubstituted alkene, CH2 = C(Z)R, the amount of which depends on the kind of catalyst and the reaction conditions, a lower reaction... 

The facility of arene reductive elimination underpins numerous C-C, C-O and C-N bond-forming reactions, which may be catalysed by late transition metals, in particular palladium (Figure 4.10). Although there are many variants, the general reaction scheme involves introduction of the aryl in electrophilic form via oxidative addition of an aryl halide (or sulfonate), substitution of the palladium halide by a nucleophile (which may also be carbon based) followed by reductive elimination. It is noteworthy that nucleophilic aromatic substitution in the absence of such catalysts can be difficult. 

Most of the mechanistic work on this reaction has been devoted to determining the role of the base. Its most obvious function would be to complex the Lewis-acidic boron reagent, rendering it nucleophihc and thus activating it toward transmetallation. However, Miyaura, Suzuki, and coworkers noted that an electron-rich tetracoordinate boronate complex was less reactive than a bivalent boronic ester. From this, they surmised that the role of the base was not to activate the boron toward transmetallation, but rather to transform the palladium halide intermediate to the hydroxide or alkoxide species, which would then be more reactive toward boron. However, in a mass spectrometry study of a reaction between a pyridyl halide substrate and an aryl boroiuc acid, Aliprantis and Canary saw no evidence of palladium hydroxide or alkoxide intermediates, despite observing signals in the mass spectra assignable to every other palladium intermediate of the proposed catalytic cycle. ... 

The cross-coupling reaction of organoboron compounds with organic halides or triflates proceeds selectively in the presence of a base, such as sodium or potassium carbonate, phosphate, hydroxide, and alkoxide [11, 45], The bases can be used in aqueous solution, or in suspension in dioxane or DMF. In contrast, the cross-coupling reaction with certain electrophiles, such as allylic acetates [45], 1,3-butadiene monoxide [49], and propargyl carbonates [50], occurs under neutral conditions without any assistance from a base. The transmetallation of organoboron compounds with palladium halides under basic or neutral conditions can be considered to involve three routes, 1, 2, and 3 (Schemes 2-18, 2-20, and 2-23, below). 

Wang, Y.H., Yang, F.H., Yang R.T., Heinzel, J.M., and Nickens, A.D. Desulfurization of high-sulfur jet fuel by pi-complexation with copper and palladium halide sorbents. Industrial Engineering Chemistry Research, 2006, 45 (22),7649. 

Aryl ethyl ethers are deethylai exposure to microwaves in the pres of selectivity by the addition of eth Palladium halide complexed to haloarenes with powdered NaCN ii Olefiturtions. Vinylogation 1,3-dioxolan-2-ylmethyltriphen> Ipl feasible. Similarly, asymmetric Ej been induced by RbOH in the pte Oxidations and reductions. benzylamines to benzamides with aldehydes can be carried out in tolu Epoxidation of various alken phase-transfer conditions using so economical and environmentally I a,P-epoxy ketones are deoxj H,NC(=NH)SO,H under phase-tnu... 

Palladium halide complexed to a crown-phosphine effectively promotes substitution of haloarenes with powdered NaCN in refluxing dioxane. ... 

In the reaction, an organic halide first forms an organopalladium halide complex with the catalyst, by oxidative addition. This complex then adds to an olefin and the adduct decomposes, by elimination of a hydrido-palladium halide, to form a new olefin in which a vinylic position is substituted by the organic moiety of the substrate halide. 

Rearrangement of the initial insertion product occurs in the reaction of various vinylic palladium halides with alkenes . The vinylic palladium halides are prepared in situ from vinyl mercurials and palladium halides. The initial insertion products undergo a jS-metal hydride elimination followed by a reverse readdition to form relatively stable n-allylic Pd complexes ... 

Insertions of vinylic palladium halides with olefins not conjugated with carbonyl groups is more complex. Rearrangement of the initial insertion product to a i-allyl-palladium derivative usually occurs as in Eq. (p). Since the a -allylic complexes are relatively stable, catalytic reactions to form dienes with tertiary amines as bases are slow and inefficient . A useful catalytic reaction occurs, however, if a nucleophilic secondary amine is used as base instead of the tertiary amine. The a-allylic palladium intermediates are attacked by the amine to form tertiary allylic amines and regenerate the catalyst ... 

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