Olefin complexes electrophilicity

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. 

Such J-mctals as Cu(I) [but not Cu(II)], form a variety of compounds with ethenes, for example [Cu(C2H4)(H20)2]C104 (from Cu, Cu2+, and C2H4) or Cu(C2H4)(bipy)+. It is necessary to mention that, of all the metals involved in biological systems, only copper reacts with ethylene [74b]. Such homoleptic alkene complexes can be useful intermediates for the synthesis of other complexes. The olefin complexes of the metals in high formal oxidation states are electron deficient and therefore inert toward electrophilic reagents. By contrast, the olefin complexes of the metals in low formal oxidation states are attacked by electrophiles such as protons at the electron-rich metal-carbon a-bonds [74c]. 

The conjugate addition/cycloaddition manifold is highly influenced by a number of factors, including the pyrrole complex, electrophile, solvent, temperature, and, in some cases, concentration. The key for predicting the course of the reaction between the olefin and an Ti2-pyrrole complex is the coordination site of the metal at the time of electrophilic attack (Figure 19). Although the intermediate azomethine ylide, where... 

Polymeric phosphin-Ni complex also has a selectivity in its catalytic activity (154). Such a sterically selectivity is shown in an intrapolymer electrophilic reaction of Fe-carbonyl-olefin complex (155,156). 

The electrophilic activation of a C—C multiple bond as a result of coordination to an electron-deficient metal ion is fundamental to much of organometallic chemistry, both conceptually and in synthetic applications (11). The Wacker process, a classic example of an efficient catalytic oxidation, is an important industrial reaction, used for the conversion of ethylene into acetaldehyde. The catalytic reaction begins with the coordination of ethylene to a Pd(ll) center, leading to activation of the ethylene moiety. The key step is the reaction of the metal-olefin complex with a nucleophile to give substituted metal-alkyl species (12). The integration of this reaction into a productive catalytic cycle requires the eventual cleavage of the newly generated M—C bond. 

The electrophilic methylene ligand undergoes nucleophilic additions with PPhs, py, and CN-f-Bu. It reacts with O and S donors to give [ReCp2( -H2CE)]+ (E = O, S) and with N2CHTMS to form the olefin complex [ReCp2( ] -H2C=CHTMS)]+. 

Interestingly, Widenhoefer reported a similar palladium(II) catalyzed cycliza-tion of indoles onto alkenes (Scheme 58) [72]. This mild protocol for cyclization/ carboxylation of 2-alkenyl indoles makes possible catalytic addition of a carbon-nucleophile and carbonyl group across a C-C bond. The mechanism, however, is thought to involve outer-sphere attack of indole onto a palladium-olefin complex rather than the electrophilic C-H activation of the indole C(3)-H bond, exhibited by the Stoltz carbocyclization. 

This is the rate-determining step involving heterolytic splitting of the hydrogen molecule and formation of an hydridoruthenium(II) complex. The next step involves rearrangement of the hydrido-7r-olefin complex to a (T-alkyl complex via insertion of the olefin into the metal hydride bond. Finally, electrophilic attack occurs on the metal-bonded carbon... 

The chiral anisole derivative 119 has been used in the synthesis of several asymmetric functionalized cyclohexenes (Fig. 27) [55]. In a reaction sequence similar to that employed with racemic anisole complexes (see above), 119 adds an electrophile and a nucleophile across C4 and C3, respectively, to form the cy-clohexadiene complex 120. The vinyl ether group of 120 can then be reduced by the tandem addition of a proton and hydride to C2 and Cl respectively, affording the olefin complex 121. Direct oxidation of 121 liberates the cyclohexenes of the type 122 having the initial asymmetric auxiliary still intact. Alternatively, the auxiliary may be cleaved under acidic conditions to afford q -allyl complexes, which can undergo reactions with nucleophiles regioselectively at Cl. Oxidative decomplexation liberates the cyclohexenes 123-127. 

Cationic olefin complexes of dicarbonyl(> -cyclopentadienyl) iron have been of wide interest in syntheses for a number of years. These complexes, generally isolated as their tetrafluoroborate or hexafluorophosphate salts, have been prepared by the reaction of Fe(f -CsHs)(CO)2Br with simple olefins in the presence of Lewis acid catalysts, by protonation of allyl ligands in Fe(t/ -CsHjXCO)2[(allyl)KC ] complexes, or by treatment of these with cationic electrophiles, by hydride abstraction from Fe(f) -CsHjXCO)2(alkyI) complexes, through reaction of epoxides with Fe(fi -CsHjXCO)2 anion followed by protonation, or by thermally induced ligand exchange between [Fe( i -CjHsXCO)2(ij -2-methyH-propene)][BF4] " or [Fe( -C,HsXCO)2(tetrahydrofuran)][BF4] and excess olefin. 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

In contrast to the increase in reactivity of hydrocarbons with nucleophiles after coordination to electron-accepting metal centers, an increase in reactivity of unsaturated hydrocarbons with electrophiles is observed upon coordination to particularly electron-ridi metal centers. This contrasting reactivity is shown schematically in Figure 11.3. Olefin complexes of very electron-rich metal centers are best described as metallacyclopropane complexes, as noted in Chapters 1 and 2. As such, the olefin ligands in these complexes contain a large degree of M-C cr-bond character and react with electrophiles. Reactions of electrophiles with coordinated ligands are described in Chapter 12. 

Electrophilic attack has also been shown to occur at the 7-position of several t) es of organometallic ligands. Electrophilic attack at the 7-position of T -allyls may be the most common. These reactions form substituted olefin complexes, as shown generally in Equations 12.53 and 12.54. Although fewer examples of the reactions of electrophiles with allyl complexes have been reported than the reactions of nucleophiles with allyl complexes, a number of catalytic processes have been developed that are likely to occur by electrophilic attack at the 7-position of allyl complexes, and a few examples of the reactions of electrophiles with isolated T -allyl complexes have been reported. " ... 

Electrophilic attack on olefin ligands coordinated to electron-rich, strongly backbonding metals is illustrated by the reactions of (P group 4 olefin and alkyne complexes, as well as some electron-rich olefin complexes. Zirconocene- and and hafnocene-olefin complexes generated by reaction of zirconocene dichloride with two equivalents of alkyl lithium and isolated upon addition of a phosphine ligand react with carbonyl compounds and weak protic acids to form insertion products and alkyl complexes. Several examples of the reactions of these complexes with electrophiles are shown in Equations 12.65-12.66. Zirconocene-alkyne complexes prepared by thermolysis of vinyl alkyl complexes and titanium-alkyne complexes generated by the reduction of Ti(OPr ) also react with electrophiles, such as aldehydes and acid, to form products from insertion into the M-C bond and protonation of the M-C bond respectively. 

Electrophilic attack by trityl cation on olefin complexes cani also occur to abstract a hydride from the allylic position. This process converts a neutral olefin complex into a cationic allyl complex and has been conducted with complexes that are less strongly backbonding than the complexes described in the preceding two paragraphs. One example of such an allylic hydride abstraction from a rhenium-propene complex is shown in Equation 12.69. An early example to form a homoconjugated system is shown in Equation 12.70. ... 

The proposed catalytic cycle for the above-described conjugate reduction is outlined in Scheme 17. Initial coordination of the nucleophilic Pd(0)-phosphine complex to the electron-deficient olefin to form complex I is a reversible process that occurs rapidly at room temperature. Oxidative addition of the sihcon hydride moiety to complex I would result in the hydrido olefin complex II. Migratory insertion of the hydride ligand into the electrophilic /S-carbon of the coordinated olefin can result in the palladium enolate intermediate in. Reductive elimination of the silicon moiety and the enolate completes the catalytic cycle and forms the silyl enol ether IV. The latter is prone to acid-catalyzed hydrolysis to produce the saturated ketone. "" ... 

C-C bond formation resulting from electrophilic attack on coordinated double-bond systems is a useful synthetic reaction (Section lll,B). The formation of C-X bonds by electrophilic attack of X, however, is restricted essentially to or D. Electrophiles such as Br or NOj usually lead to oxidative cleavage of the metal-olefin complex, although NBS (iV-bromosuccinimide) has been reported to act as a hydride acceptor in some cases (Khand et al., 1968, 1969). 

Mimoun [209] assumes a mechanism based on the formation of a complex TT with charge transfer between nucleophilic olefin and electrophilic carbon atom of the peroximethylenic group ... 

The competitive alkylidene transfer to olefins and alkylidene to olefin interconversion of the electrophilic carbene complexes [CpFe(CO)L(=CHR)] (where L = CO or PH3 R = Me or Et) has been studied theoretically. These transformations are believed to involve cationic olefin complexes of the type [GpEe(CO)L(olefin)] . ... 

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