Alkene rr-complex

Regeneration of an alkeneic rr-complex is likely to occur smoothly at the high temperatures often employed with the palladacycles. Insertion into the alkene and a j8-hydride elimination ultimately delivers the coupled product. The paUadacycle is recovered after reaction with abase (e.g., acetate). ... 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

The formation of the carbocationic intermediate (37), either directly or via an initial rr complex, appears to be rate-limiting, and the overall orientation of addition is Markownikov. There is evidence of some ANTI stereoselectivity, but this is not very marked and is dependent on the alkene and on the reaction conditions. 

When the Pd bears chiral ligands, these reactions can be enantioselective.1448 ir-Allylmo-lybdenum compounds behave similarly.1449 Because palladium compounds are expensive, a catalytic synthesis, which uses much smaller amounts of the complex, was developed. That is, a substrate such as an allylic acetate, alcohol, amine, or nitro compound1450 is treated with the nucleophile, and a catalytic amount of a palladium salt is added. The rr-allylpal-ladium complex is generated in situ. Alkene-palladium complexes (introducing the nucleophile at a vinylic rather than an allylic carbon) can also be used.1451... 

Complexes with alkenes and arenes are formed when the hydrocarbons are shaken with aqueous solutions of silver(I) salts. Di- or polyalkenes often give crystalline compounds with Ag+ bound to one to three double bonds. The formation of alkene complexes of varying stability may be used for the purification of alkenes, or for the separation of isomeric mixtures (e.g., 1,3-, 1,4-, and 1,5-cyclooctadienes), or of the optical isomers of a- and /3-pinene. There is very little back-bonding contribution in the formation of Ag1 rr-complexes. For example, the planar complex (hfa)Ag(Ph-C= C-Ph) contains an almost linear acetylene ligand with a C=C... 

What is more, the asymmetric structure of the surface dimers in silicon and germanium causes a polarization of the double bond. Hence may occur a nucleophilic attack on the Ji -orbital and the formation of a rr-complex from the attacking, electron rich alkene and the electron-deficient end of the surface dimer. The subsequent addition leading to the final product is easy to take place then (Figure 6.43). Obviously this is not the concerted and symmetric mechanism typical of pericyclic reactions, which is also why the prohibition of a thermal reaction is by-passed. 

The mechanism begins the same way as the Hg-mediated nucleophilic addition to alkenes. In the first step, an electrophilic tt complex forms between the alkyne and Hg(II). Water attacks one of the C s of the rr complex in Markovnikov fashion to give a 2-hydroxy-1-alkenylmercury(II) compound, an enol, which is protonated to give a carbocation. Fragmentative loss of Hg(II) then occurs to give a metal-free enol, which tautomerizes to give the ketone product. 

The addition of nucleophiles to alkenes is mediated hy Hg(II) salts and catalyzed hy Pd(II) salts. The difference between the two reactions is the fate of the alkylmetal(II) intermediate obtained after addition of the nucleophile to the rr complex. The alkylmercury(II) intermediate is stable and isolable, whereas the alkylpalladium(Il) intermediate undergoes rapid /3-hydride elimination. 

A number of enantiomerically pure complexes have been made, and this chemistry has been used in several natural product syntheses. Enantiopure complexes are readily available from the corresponding vinylic epoxides, and in cases where diastereoselective complexation is possible, diastereoselectivities tend to be moderate (typically 3 1 -4 1). The rationale for the origin of this diastereoselectivity has been proposed to derive from a preferential complexation of a Fe(CO)4 fragment to the alkene anti to the epoxide. Since the initial vinyl epoxide is conformationally flexible, four diastereomeric rr-complexes would be produced as a consequence of anti or syn complexation to the s-trans or 5-C/5 conformers. Isomerization of these initial rr-complexes to alkoxy-TT-allyl species would then enable interception of an iron-bound carbonyl ligand by the alkoxide to afford diastereomeric lactone complexes. Fortunately, equilibria between the two possible trans Tr-allyl complexes and their more stable cis Tr-allyl analogs simplifies the outcome significantly. Thus, for trans vinyl epoxides, the major diastereomer typically is the one designated as endo cis (the C-1 substituent points toward the iron atom) the minor diastereomer corresponds to the exo cis isomer (the C-1 substituent points away from the iron atom) (Scheme 51). For cis vinyl epoxides, this outcome is reversed - the exo cis isomer is the major product. 

As the term itself implies, formal oxidation state is a formalism with which the oxidation state of an atom or species in question is expressed in a round number and often somewhat arbitrarily. Thus, for example, the formal oxidation state (FOS) of Pd in a rr-complex obtained by tr-complexation of an alkene with Pd(0) species is considered to be 0, whereas the same Pd atom in the same complex may be assigned an FOS of + 2, if the process of formation is regarded as oxidative complexation and if the product is viewed as a palladacyclopropane, as indicated in Table 3. This sort of seeming ambiguity is not a concern, at least in the great majority of cases, as long as this formalism is dealt with in an internally consistent and logically sound manner. After all, most of the practical and synthetic matters in chemistry are dealt with in terms of formalism. 

Palladium forms complexes with a variety of neutral carbon ligands. While some of them, such as CO and isonitriles, bind to palladium in a ct-- or -fashion, others, such as alkenes, dienes, and alkynes, form rr-complexes. 

The ease with which nucleophiles add to alkenes and alkynes coordinated to Pd(II) combined with the tendency of Pd to form rr-allyl complexes accounts for the limited number of Pd(II)-alkene, Pd(II)-diene, or Pd(II)-alkyne complexes. Indeed, as detailed in the next section, depending on the conditions or their structural features, alkenes, dienes, and alkynes often lead to the formation of 7r-allylpalladium complexes instead of rf or rf rr-complexes. This is particularly true in the case of alkenes bearing hydrogens a to the double bond, 1,3-dienes, such as 1,3-butadiene, and alkynes of low steric hindrance, hi marked contrast, Pd(l,5-hexadiene)Cl2 is obtained from the reaction of Cl2Pd(PhCN)2 with allyl chloride. ... 

Finally, the C—C bond formation by the reaction of 7r-complexes of Pd derived from alkenes, dienes, and other 7r-compounds with enolates and related carbon nucleophiles a la Wacker reaction (Method VI in Scheme 1) provides yet another alternative, as exemplified by the results shown in Scheme For a more general discussion of the C—C bond formation via Wacker-type reaction of Pd rr-complexes with carbanions, the reader is referred to Sect. V.3.4. 

If the counterion (X) in the oxidative addition complex is iodide or bromide (and no thallium or silver salts are present) the dissociation of one of the phosphorus atoms in the bidentate ligand from the metal is probably attributed to the relatively high trans effect exerted by the halidesJ This reversible displacement facilitates formation of a neutral rr-complex, in which the rr-system of the electron-rich alkene is only weakly polarized. Therefore, after insertion and hydridopalladium halide elimination, a larger fraction of /3-arylated product is formed, since steric factors always favor terminal ary-lation. 

In principle, a palladacyclopropane might be generated starting from palladium(O) or palladium(II) 1 and an alkene 2 (Scheme 1) after coordination to form a rr-complex 3, oxidative addition would yield a o--complex 4, a pallada(II)- or pallada(IV)cyclopropane. Likewise, the analogous reaction with an alkyne 5 would lead to a palladacyclopropene 7. 

Unsaturated hydrocarbons such as alkenes, 1,5-cycIooctadiene, allyl compounds and cyclopentadienyl compounds easily react with palladium compounds to afford their 7r-compIexes. These rr-complexes are used as the raw materials for the syntheses of organopalladium compounds. For example, reactions are shown in eqs. (20.9)-(20.12) [26-31]. 

The Prins reaction has been modelled using DFT (density functional theory), using an alkene (RCH=CH2, R = Me or Ph), a formaldehyde dimer, and a proton-water cluster, H30" (H20)i3. Both alkenes feature a concerted path to give the 1,3-diols. An unprecedented hemiacetal intermediate, H0-CH2-0-CH(R)-CH2CH2-0H, was then identified it undergoes ring closure to the 1,3-dioxane product. Gas-phase Prins reaction of formaldehyde dimer with alkene has been studied computationally it proceeds via a r-complex (without formation of any intermediate rr-complex). ... 

Clearly, the mechanism of the addition reaction is not completely understood. For now, then, we will assume that the reaction forms a rr-complex. It is more stable than a vinylic cation would be, but it is not as stable as an aUcyl cation. Therefore, the transition state leading to its formation is less stable than the transition state leading to formation of an alkyl cation, agreeing with the observation that alkynes are less reactive than alkenes (Figure 1.2). 

As shown in Figure 8.5, hydrogenation begins with adsorption of H2 onto the catalyst surface. Complexation between catalyst and alkene then occurs as a vacant orbital on the metal interacts with the filled alkene rr orbital. In the final steps, hydrogen is inserted into the double bond and the saturated product diffuses away from the catalyst. The stereochemistry of hydrogenation is syn because both hydrogens add to the double bond from the same catalyst surface. 

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