Palladium alkenyl species

This implies that the other elementary steps in cycle B (Scheme 1), i.e., Pd-carbomethoxy formation and protonolysis of the palladium-alkenyl species, must even be considerably faster than the observed overall high reaction rate. A high rate of Pd-carbomethoxy formation (at equilibrium) could be expected for the strongly electrophilic metal center. However, the latter step, protonolysis of the Pd-alkenyl bond in l-palladium-2-carbomethoxypropene and 2-palladium-1-car-bomethoxypropene, respectively, is expected to be a slow reaction, because the proton has to overcome a relatively high barrier of (electrostatic) repulsion by the cationic palladium center on its way to the palladium-carbon bond. 

The application of in situ-generated (alkoxy)palladium(II) species (Scheme 14.23) can be extended to reactions of a-carbonates with organoboron compounds. Crosscouplings of allenes 108 with aryl (or alkenyl) boron acids or their esters catalyzed by a palladium(O) complex afforded the 2-aryl(alkenyl)-l,3-butadienes 109 in excellent yields (Scheme 14.24) [53], The coupling reactions of 9-BBN-derived intermediates such as ester 111 can be accelerated by applying K3P04 as additive (Eq. 14.15). 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

Commercially available palladium compounds in the presence of various ligands have often been used as catalysts (Table 3-1). The first choice is often the air-stable and relatively inexpensive palladium acetate however, several of the other published variants can be preferable in certain applications. It is commonly assumed that the palladium(II) species is reduced in situ by the solvent, the alkene [11], the amine [12] or the added ligand (frequently a phosphane, which is oxidized to a phosphane oxide) [13]. In some cases, highly dispersed elemental palladium on charcoal can be applied. In the case of alkenyl or aryl bromides, phosphanes are necessary to avoid precipitation of palladium black (c.f., however. Section 3.2.4.), whereas iodides have been reported to be less reactive in the presence of phosphanes. Triflates have been found to be more reactive in the presence of chloride ions, as the chloride ligand is more easily removed from palladium than the tiiflate ion [14], However, this also has become questionable, because successful coupling reactions of alkenyl triflates have been performed in the absence of chloride ions [15]. 

Oxidative addition of palladium(O) species into unsaturated halides or triflates provides a popular method for the formation of the a-bound organopalladium(II) species. It is important to use an unsaturated (e.g. aryl or alkenyl) halide or tri-flate, as (3-hydride elimination of alkyl palladium species can take place readily. Oxidative addition of palladium(0) into alkenyl halides (or triflates) occurs stere-ospecifically with retention of configuration. The palladium is typically derived from tetrakis(triphenylphosphine)palladium(0), [Pd(PPh3)4], or tris(dibenzylidene-acetone)dipalladium(O), [Pd2(dba)3], or by in situ reduction of a palladium(II) species such as [Pd(OAc)2] or pd(PPh3)2Cl2]. 

Palladium acyl species can also undergo intramolecular acyl-palladation with alkenes to form five- and six-membered ring y-keto esters through exocyclic alkene insertion (eq 12). The carbonylative coupling of o-iodoaryl alkenyl ketones is also promoted by Pd(dba)2 to give bicyclic and polycyclic quinones through endocyclization followed by /3-H elimination. Sequential carbonylation and intramolecular insertion of propargylic and allylic alcohols provides a route to y-butyrolactones (eq 13). ... 

A cascade Heck reaction with termination by nucleophiles is considered to start with an oxidative addition of a heteroatom-carbon bond (starter) onto a palladium(O) species (startup reaction), followed by carbopalladation of a nonaromatic carbon-carbon double or triple bond without subsequent dehydropalladation (relay), a second and possibly further carbopalladation of a carbon-carbon double or triple bond (second etc. relay). The terminating step is a displacement of the palladium residue by an appropriate nucleophile. It is crucial for a successful cascade carbopalladation that no premature dehydropalladation takes place, and that can be prevented by using alkynes and 1,1-disubstituted alkenes (or certain cycloalkenes) as relay stations since they give kinetically stable alkenyl- or neopentylpalladium intermediates, respectively. In addition, reaction of haloalkenes with alkenes in certain cases may form rr-allyl complexes, which are then trapped by various nucleophiles. 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

B.i.b. Alkynes Containing Proximate 1,3-Dicarbonyl Groups. 2-Propargyl-1,3-dicarbonyl compounds react with alkenyl triflates or alkenyl/aryl/heteroaryl halides to give 2,3,5-trisubstituted-furans (Scheme 4). The process probably proceeds through an oxypalladation step that involves a nucleophilic attack of a stabilized enolate across the activated carbon-carbon triple bond, reductive elimination of a palladium(0) species from the resultant oxypalladation adduct, and isomerization of the initially formed alkylidene derivative. 

Addition of a Z-Pd-X species across a multiple bond is a very interesting way of generating an alkyl/alkenyl palladium(II) species that can then undergo carbonylation and then capture by an intramolecular hydroxyl group. Because two distinct regiochemistries are possible in an addition across a multiple bond, two pathways, (a) and (b) (Scheme 20), are possible, leading to isomeric products characterized by different ring sizes. 

Palladium-catalyzed trimerization of alkynes has been developed, " but simple terminal alkynes undergo dimerization to form enynes. A mechanism for the formation of head-o-tail enynes has been proposed that proceeds through palladium(iv) complexes 202 or 203. Probably, however, the acidic terminal alkyne will cleave the palladium-alkenyl bond to give the enyne product and an alkynylpalladium(ii) species that can enter a new catalytic cycle instead." ... 

A coordinatively unsaturated 14-electron paUadium(0) complex, usually containing two tertiary phosphanes as weakly electron-donating ligands, has been proved to be the catalyticaUy active species. It is commonly generated in situ from either from a palla-dium(O) complex or by reduction of relatively inexpensive paUadium(II) acetate or chlo-ride.f " In the first step of the catalytic cycle ( in Scheme 1), a haloalkene, a haloarene, or a similar substrate is oxidatively added to the coordinatively unsaturated palladium(O) species to generate a tr-alkenyl- or tr-arylpalladiumfll) complex. " This then coordinates an alkene molecule, and when the latter and the alkenyl (aryl) residue on the palladium are in a cis orientation, the cr-alkenyl- or cr-arylpalladium complex can... 

An analogous reaction mode is followed in the 1 2 cross-coupling of 2-bromostyrene with acenaphthylene, which yields a bisannelated tetrahydrofulvene (Schenae 48). Norbomene can favorably serve as a relay for cascade carbopalladations as the /3-hydride elimination is virtually impossible. The reaction always starts with an alkenyl-or arylpalladium starter, generated either by oxidative addition of an alkenyl or aryl halide to a palladium(O) species or by hydro- or carbopalladation of an alkyne, adding to the double bond. With /3-bromostyrene, norbomene can yield the same type of bisannelated tetrahydrofulvene derivativef" i as with acenaphthylene, but under different reaction conditions can also react with a 2 1 stoichiometry to give a cyclohexadiene-annelated norbomane derivative (Scheme 49). ... 

---