Coordinated alkenes nucleophilic attack

The mechanism of the reaction in Figure 15.4 involves coordination of palladium to the alkene and nucleophilic attack of oxygen at the internal carbon atom to form the flve-membered ring. Palladium is bonded to the exocyclic carbon atom. (3-hydride elimination gives the exocyclic methylene,... 

Doyle has put forward arguments against the intermediacy of such complexes in catalytic cyclopropanation . Firstly, metal coordination activates the alkene to nucleophilic attack. Hence, an electrophilic metal carbene would add only reluctantly or not at all. Secondly, the stable PdCl2 complexes of dienes 8 and 428 do not react with ethyl diazoacetate, even if Rh fOAc) or PdCljfPhCbOj is added. The diazoester is decomposed only when it is added to a mixture of the Pd complex and excess diene. These results exclude the metal-carbene-olefin intermediate, but they leave open the possibility of metal carbene interaction with an uncomplexed olefin molecule. The preferred formation of exo-cyclopropanes in the PdCyPhCN) -catalyzed reactions between 8 and N2CHCOOEt or N2CPh2, with exo. endo ratios virtually identical to those observed upon cyclopropanation of monoolefin 429, also rule out coordination of a palladium carbene to the exocyclic double bond of 8 prior to cyclopropanation of the endocyclic double bond. 

As outlined in 5.8.2.3.4, coordination of an alkene to a neutral or cationic transition metal activates the alkene toward nucleophilic attack, leading to an alkylmetal product. Development of the analogous synthesis of alkenylmetal complexes starting with /y -alkyne-metal complexes is more recent. An early reaction of this type is ... 

Coordination of paUadium(n) compounds to alkenes, to form Tr-alkene palladium(II) complexes, activates the alkene to nucleophilic attack. In general, nucleophilic attack by carbon nucleophiles occurs on the face opposite the metal atom, resulting in the formation of a cr-bond paUadium(ll) alkyl complex. A wide variety of mechanisms are possible for the liberation of the carbon Ugand, the nature of the product obtained being dependent on the conditions employed and the nature of the complex. 

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. 

Alkenes coordinated by Pd(II) are attacked by carbon nucleophiles, and carbon-carbon bond formation takes place. The reaction of alkenes with carbon nucleophiles via 7r-allylpalladium complexes is treated in Section 3.1. 

Figure 22.13 shows the scheme used to describe the hydroformylation process. The active catalyst is HCo(CO)3, which is a 16-electron species that is coordinatively unsaturated. After that species is generated, the first step of the catalyzed process involves the addition of the alkene to the catalyst. In the next step, an insertion reaction occurs in which the alkene is inserted in the Co-H bond (nucleophilic attack by H on the alkene would accomplish the same result as described... 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

J. E. Backvall, Nucleophilic Attack on Coordinated Alkenes , in Reaction of Coordinated Ligands (Ed. P. S. Braterman), Plenum Press, London 1986, pp. 679-731. 

The NBO analysis characterizes the 7tcc— nnf interaction as relatively modest (25.0kcal mol-1), with 0.09e charge transfer from ethylene to HfH4. Accordingly, the coordinated alkene is somewhat activated toward nucleophilic attack, but the chemical effects are minor compared with those for other donor-acceptor motifs to be discussed below. 

The reaction is highly exothermic as one might expect for an oxidation reaction. The mechanism is shown in Figure 15.1. Palladium chloride is the catalyst, which occurs as the tetrachloropalladate in solution, the resting state of the catalyst. Two chloride ions are replaced by water and ethene. Then the key-step occurs, the attack of a second water molecule (or hydroxide) to the ethene molecule activated towards a nucleophilic attack by co-ordination to the electrophilic palladium ion. The nucleophilic attack of a nucleophile on an alkene coordinated to palladium is typical of Wacker type reactions. 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

Palladium-catalyzed allylic oxidations, in contrast, are synthetically useful reactions. Palladium compounds are known to give rise to carbonyl compounds or products of vinylic oxidation via nucleophilic attack on a palladium alkene complex followed by p-hydride elimination (Scheme 9.16, path a see also Section 9.2.4). Allylic oxidation, however, can be expected if C—H bond cleavage precedes nucleophilic attack 694 A poorly coordinating weak base, for instance, may remove a proton, allowing the formation of a palladium rr-allyl complex intermediate (89, path by694-696 Under such conditions, oxidative allylic substitution can compete... 

Alkenes bonded to platinum(II) can be displaced by strongly coordinating ligands such as cyanide ion or tertiary phosphines. The displacement of ethylene from Zeise s salt by phosphines is a useful method of preparation of complexes trawa-PtCl2(PR3)2.711 Amines will also displace alkenes from coordination to platinum(II), but this reaction can compete with nucleophilic attack at the coordinated alkenic carbon. The stability of platinum(II) alkene complexes follows the sequence C2H4 > PhCH=CH2 > Ph2C=CH2 555 Ph(Me)C=CH2.712... 

The more basic and less hindered pyridines undergo nucleophilic attack at an ethylene coordinated to platinum(II). Pyridine substitution reactions at platinum also occur, and in the presence of excess ethylene, alkene replacement is observed.72 ... 

Coordination of an alkene to platinum(O) differs from complexation to platmum(Il) Zerovalent platinum is an electron-rich metal center, whereas platinum(II) is electron poor. A a consequence alkenes coordinated to platinum(0) became more electron rich than in their fret state, and therefore susceptible to electrophilic attack. For alkenes complexed to platinum(II) their primary mode of reactivity is by attack from an external nucleophile. 

The formation of (63) is also a nice example of the influence of the nature of R in the Pt(l,5-cod)-(R2N4) complexes on the sensitivity of the coordinated cod ligand for nucleophilic attack. Only if R contains the strongly electron withdrawing N02 group is reaction of the alkenic bonds with nucleophiles observed.190... 

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