Alkenes palladium-alkene complex

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... 

The effect of the nature of the electrophile on the stereoselectivity of reactions with substrates containing a terminal alkene and an allylic substituent is dramatically illustrated by some recent results with palladium electrophiles.124 Cyclizations of 3-methyl- or 3-phenyl-5-hydroxyalkenes with palladium catalysts proceed with high selectivity (>9 1) for the 2,3-trans isomer (equation 41).50-124 It is suggested that the steric interactions of the palladium-alkene complex affects the stereochemistry of these cyclizations. In some related cyclizations to form tetrahydropyran products (equation 42 and Table 10), reaction with iodine in the presence of sodium bicarbonate gives a different major diastereomer from cyclization with mercury(II) trifluoroacetate or palladium chloride.123... 

Figure 22-4 Mechanism for the hydrosilylation and dehydrogenative silylation of 1-alkenes catalyzed by cationic palladium complexes Pd represents [(phen)Pd]+. The palladium alkene complex A is the resting state of the cycle. Cycle I denotes the hydrosilylation cycle, Cycle II describes the dehydrogenative silylation reaction.

To account for the differences in reactivity and enantioselectivity observed in Heck reactions of unsaturated triflates and halides, two distinct mechanistic pathways have been proposed (as shown in the margin). The "cationic" pathway is generally invoked to describe asymmetric Heck reactions of unsaturated triflates or halides in the presence of Ag(I) or T1(I) additives. In the absence of such additives the Heck reaction is expected to proceed through a "neutral" reaction pathway. The modest enantioselectivity often observed in Heck reactions of this type has been attributed to the formation of a neutral palladium-alkene complex by partial ligand dissociation. ... 

Figure 5. Effect of added cosolvent on the results of immobilization of a palladium alkene complex. [Adapted from (81).]...

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. 

Nucleophilic attack on alkene-palladium(ll) complexes became commercially important when the Wacker process was developed. In this process, ethylene is converted to acetaldehyde (Scheme 6.4). This involves coordination of ethylene to palladium. The oxygen atom, which comes from water, may then become attached to carbon in one of two different ways. Direct nucleophilic attack on one carbon atom of the t -ethylene complex 6.1 by water can form an t -complex 63. Alternatively, ligand exchange at palladium, with water replacing chloride, can be followed by insertion of the coordinated ethylene into the palladium-oxygen bond to give the same t -complex 63, via the hydroxy complex 6.2. Which of these two variants of the mechanism... 

Heck reaction, palladium-catalyzed cross-coupling reactions between organohalides or triflates with olefins (72JOC2320), can take place inter- or intra-molecularly. It is a powerful carbon-carbon bond forming reaction for the preparation of alkenyl- and aryl-substituted alkenes in which only a catalytic amount of a palladium(O) complex is required. 

A catalytic amount of a reactive palladium(0)-complex 3 (i.e. PdLa in the catalytic cycle scheme shown below) is likely to be formed when the palladium(ll) acetate 2 oxidizes a small amount of the alkene ... 

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. 

Nickel(II) complexes of ligands 38 (R=H,Me R =H,Me,Et,Tr,CH30 R =H, CH3O R =H, F, CH3O) are highly active catalysts for ethylene polymerization [86,159], whereas palladium(II) complexes possess catalytic properties in the copolymerization of CO and alkenes [160] (Scheme 36). 

The hydration of propylene with sulfuric acid catalyst in high-temperature water was investigated using a flow reaction system.31 The major product is isopropanol. A biopolymer-metal complex, wool-supported palladium-iron complex (wool-Pd-Fe), has been found to be a highly active catalyst for the hydration of some alkenes to the corresponding alcohols. The yield is greatly affected by the Pd/Fe molar ratio in the wool-Pd-Fe complex catalyst and the catalyst can be reused several times without remarkable change in the catalytic activity.32... 

A similar involvement of palladium hydride, palladium alkyl, and palladium acyl complexes as intermediates in the catalytic cycle of the Pd-catalyzed hydroxycarbonylation of alkenes was reported for the aqueous-phase analogs. The cationic hydride PdH(TPPTS)3]+ was formed via the reduction of the Pd11 complex with CO and H20 to [Pd(TPPTS)3] and subsequent protonation in the acidic medium. The reaction of the hydride complex with ethene produced two new compounds, [Pd(Et)(TPPTS)3]+ and Pd(Et)(solvent)(TPPTS)2]+. The sample containing the mixture of palladium alkyl complexes reacted readily with CO to afford trans-[Pd(C(Q)Et)(TPPTS)2]+.665... 

A number of examples have been reported documenting the use of palladium phosphine complexes as catalysts. The dialkyl species [PtL2R2] (L2 = dmpe, dppe, (PMe3)2 R = Me, CH2SiMe3) catalyze the reaction of [PhNH3]+ with activated alkenes (acrylonitrile, methyl acrylate, acrolein).176 Unfunctionalized alkenes prove unreactive. The reaction mechanism is believed to proceed via protonation of Pt-R by the ammonium salt (generating PhNH2 in turn) and the subsequent release of alkane to afford a vacant coordination site on the metal. Coordination of alkene then allows access into route A of the mechanism shown in Scheme 34. Protonation is also... 

Probably the first metal alkene complex was Zeise s salt, K[Pt(C2H4)Cl3] or the bridged compound [PtCl2(C2H4)]2. These compounds were first prepared by Zeise in about 1825. The palladium analogs of these compounds are also now known. A large number of metal alkene complexes are known, and some of the chemistry of these materials will be described here. 

Addition of distannane to alkenes has been achieved only with strained cyclopropenes (Equation (60)).158 3,3-Disubstituted cyclopropenes undergo highly face-selective distannation in the presence of the palladium-isocyanide complex to afford m-adducts. 

Addition of a phosphorus-sulfur bond to a carbon-carbon triple bond is catalyzed by a palladium(O) complex (Equation (130)).298 Terminal aliphatic alkynes having various functional groups undergo the addition with PhS-P(0)(OPh)2 to afford (Z)-adducts in high yield. In contrast to aliphatic alkynes, phenylacetylene gives a mixture of E Z adducts. Internal alkynes and alkenes are unreactive. 

It was concluded that the high selectivity observed in the hydrogenation experiments using 26 b is explained by the relatively strong coordination of the alkyne to the palladium center, which only allows for the presence of small amounts of alkene complexes. Only the latter are responsible for the observed minor amounts of ( )-alkene, which was shown to be a secondary reaction product formed by a subsequent palladium-catalyzed, hydrogen-assisted isomerization reaction. Since no n-octane was detected in the reaction mixture, only a tiny... 

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