Alkenes palladium

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... 

Balme G, Bouyssi D, Monteiro N (2002) Palladium-Catalyed Reactions Involving Attack on Palladium-Alkene, Palladium-Alkyne, and Related jr-Complexes by Carbon Nucleophiles. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 2245... 

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. ... 

A detailed discussion of the influence of pressure, temperature, catalyst variations, and the removal of water from the reaction mixture, as well as the influence of different solvents on selectivity and reaction rates, may be found in [12]. For more details about the reaction mechanism and the chemistry of palladium-alkene-CO complexes cf. [13, 14, 17]. 

Balme, G., Bouyssi, D., Monteiro, N. Palladium-catalyzed reactions involving attack on palladium-alkene, palladium-alkyne, and related ic-complexes by carbon nucleophiles. Handbook of Organopattadium Chemistry for Organic Synthesis 2002, 2, 2245-2265. 

P. M. Henry, Palladium-Catalyzed Reactions Involving Nucleophilic Attack on 7i-Ligands of Palladium-Alkene, Palladium-Alkyne, and Related Derivatives, in Handbook of Organopalladium Chemistry for Organic Synthesis, E.-i. Negishi, Ed., Wiley-Interscience New York, 2002, pp. 2119-2139. 

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

V 3 Palladium-Catalyzed Reactions Involving Nucleophilic Attack on 7T-Ligands of Palladium-Alkene, Palladium-Alkyne, and Related Derivatives... 

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