Alkene complexes nucleophilic attack

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

Palladium(II) salts, in the form of organic solvent soluble complexes such as PdCl2(RCN)2, Pd(OAc>2 or Li2PdCU, are by far the most extensively utilized transition metal complexes to activate simple (unactivated) alkenes towards nucleophilic attack (Scheme 1). Alkenes rapidly and reversibly complex to pal-ladium(II) species in solution, readily generating alkenepalladium(II) species (1) in situ. Terminal monoalkenes are most strongly complexed, followed by internal cis and trans (respectively) alkenes. Geminally disubstituted, trisubstituted and tetrasubstituted alkenes are only weakly bound, if at all, and intermolecular nucleophilic additions to these alkenes are rare. 

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 ... 

Hydroaminations occurring by nucleophilic attack on ir-ligands are the oldest class of hydroamination and are discussed first. A mechanism for the hydroamination of alk-enes and alkynes catalyzed by palladium(II) complexes is shown in Scheme 16.16. By this pathway, coordination of the alkene or alkyne through the -ir-system occurs to generate a cationic or electron-poor, neutral metal-olefin or metal-alkyne complex. Nucleophilic attack of the amine on the coordinated olefin or alkyne then occurs. Nucleophilic attack on coordinated olefins and alkynes is presented in detail in Chapter 11. As noted in Chapter 11, this nucleophilic attack occurs at the internal position of an alkene or alkyne. 

Cydopentadlenyl Iron Reagent In the case of simple alkenes, the best-studied system is Rosenblum s Fp reagent (see also Section 14.3), (CpFe(CO)2(alkene)). Thanks to its positive ionic charge, it activates even simple alkenes for nucleophilic attack. The sequence shown in Eq. 14.77 illustrates how the alkene complex may be synthesized from a p-alkoxy alkyl... 

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. 

K. F. McDaniel, Comp. Organomet. Chem. II, 1995,12, 601-622. Transition Metal Alkene, Diene, and Dienyl Complexes Nucleophilic Attack on Alkene Complexes. R. W. Bates, Comp. Organomet. Chem. II, 1995, 12, 349-386. Transition Metal Carhonyl Complexes. 

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. 

Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

No reaction of soft carbon nucleophiles takes place with propargylic acet-ates[37], but soft carbon nucleophiles, such as / -keto esters and malonates, react with propargylic carbonates under neutral conditions using dppe as a ligand. The carbon nucleophile attacks the central carbon of the cr-allenylpal-ladium complex 81 to form the rr-allylpalladium complex 82, which reacts further with the carbon nucleophile to give the alkene 83. Thus two molecules of the a-monosubstituted /3-keto ester 84, which has one active proton, are... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

The stereochemistry of addition depends on the details of the mechanism. The addition can proceed through an ion pair intermediate formed by an initial protonation step. Most alkenes, however, react via a complex that involves the alkene, hydrogen halide, and a third species that delivers the nucleophilic halide. This termolecular mechanism is generally pictured as a nucleophilic attack on an alkene-hydrogen halide complex. This mechanism bypasses a discrete carbocation and exhibits a preference for anti addition. 

Amouri and coworkers also demonstrated that the nucleophilic reactivity of the exocyclic carbon of Cp Ir(T 4-QM) complex 24 could be utilized to form carbon -carbon bonds with electron-poor alkenes and alkynes serving as electrophiles or cycloaddition partners (Scheme 3.17).29 For example, when complex 24 was treated with the electron-poor methyl propynoate, a new o-quinone methide complex 28 was formed. The authors suggest that the reaction could be initiated by nucleophilic attack of the terminal carbon of the exocyclic methylene group on the terminal carbon of the alkyne, generating a zwitterionic oxo-dienyl intermediate, followed by proton transfer... 

Waymouth and coworkers used chiral zirconocene complexes such as 56 with Et3Al as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicyclic compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by (i-alkoxidc ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12]. 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

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. 

Recently, Ohe and IJemura reported a novel approach to the catalytic cyclopropanation of alkenes via 2-furyl178 179 or 2-pyrrolyl carbenoids180 that originate from the intramolecular nucleophilic attack of a carbonyl oxygen or an imine nitrogen (ene-yne-ketone and ene-yne-imine precursor, respectively) on a 7t-alkyne complex or a cationic cr-vinyl complex. Initially, the group 6 complexes like Cr(CO)s were used. Soon it was found that a series of late transition... 

In the envisaged titanium oxo complex, the Ti atom is side-bound to the peroxy moiety (02H), consistent with all the spectroscopic results mentioned in Section III in Scheme 27, between the two O atoms that are side-bound to Ti4+, the O atom attached to both the Ti and H atoms is expected to be more electrophilic than the O atom attached to only the Ti atom and is likely to be the site of nucleophilic attack by the alkene double bond. The formation of the Ti-OH group (and not the titanyl, Ti=0, as proposed by Khouw et al. (221)) after the epoxidation and its subsequent condensation with Si-OH to regenerate the Ti-O-Si links had been observed (Section III.B) by FTIR spectroscopy by Lin and Frei (133). Because this is a concerted heterolytic cleavage of the 0-0 bond, high epoxide selectivity and retention of stereochemistry may be expected, as indeed has been observed experimentally (204). 

All these features have been initially interpreted102-104 in terms of a molecular mechanism involving two successive alkene-iodine complexes of 1 1 and 1 2 stochiometries the second of which evolves by internal nucleophilic attack of the uncomplexed double bond to the diiodo derivative (equation 87). The intramolecular attack of the second double bond has been regarded as rate determining, owing to the fact that the overall rate law is second order in iodine rather than the usual third order. Nevertheless more... 

Co-ordination of an alkene to an electronegative metal (often it may carry a positive charge) activates the alkene toward attack of nucleophiles. After the nucleophilic attack the alkene complex has been converted into a c-bonded alkyl complex with the nucleophile at the (3-position. With respect to the alkene (in the "organic" terminology) the alkene has undergone anti addition of M and the nucleophile Nu, see Figure 2.25. 

As indicated under section 2.2. the overall result is the same as that of an insertion reaction, the difference being that insertion gives rise to a yw-addition and nucleophilic attack to an anri-addition. Sometimes the two reaction types are called inner sphere and outer sphere attack. There is ample proof for the anti fashion the organic fragment can be freed from the complex by treatment with protic acids and the organic product can be analysed [19], Appropriately substituted alkenes will show the syn or anti fashion of the addition. The addition reaction of this type is the key-step in the Wacker-type processes catalysed by palladium. 

We have already reviewed the activation of alkenes, alkynes, and carbon monoxide towards nucleophilic attack. The heterolytic splitting of dihydrogen is also an example of this activation it will be discussed in Section 2.10. The reaction of nucleophiles with silanes co-ordinated to an electrophilic metal can be regarded as an example of activation towards nucleophilic attack (Figure 2.28). Complexes of Ir(III) and Pd(II) give t.o.f. for this reaction as high as 300,000 mol.mol. fh"1. 

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