Zwitterionic palladium intermediate

Vinyl epoxides and allylic carbonates are especially useful electrophiles because under the influence of palladium(O) they produce a catalytic amount of base since X- is an alkoxide anion. This is sufficiently basic to deprotonate most nucleophiles that participate in allylic alkylations and thus no added base is required with these substrates. The overall reaction proceeds under almost neutral conditions, which is ideal for complex substrates. The relief of strain in the three-niembered ring is responsible for the epoxide reacting with the palladium(O) to produce the zwitterionic intermediate. Attack of the negatively charged nucleophile at the less hindered end of the ic-allyl palladium intermediate preferentially leads to overall 1,4-addition of the neutral nucleophile to vinyl epoxides. 

While simple unactivated cyclopropanes have yet to be used for [3 + 2] cycloaddition, Tsuji and coworkers have developed a palladium-catalyzed cycloaddition reaction using electron-deficient vinylcy-clopropanes. Thus, vinylcyclopropane (43) undergoes smooth cyclization with methyl acrylate in the presence of a palladium catalyst to give vinylcyclopentane (44) as a mixture of diasteroisomers (equation 35). The cycloaddition probably proceeds through the zwitterionic ( ir-allyl)palladium intermediate (45) and its stepwise reaction with the acrylate (equation 36). Enones such as cyclopentenone and methyl vinyl ketone will also react. Reaction of the same vinylcyclopropane with phenyl isocyanate produces vi-nyllactam (46) (equation 37).Some cycloaddition reactions with (cyclopropyl)Fp complexes have also been reported. However, the substrates are limited to SO2 and TCNE and the yields have not been disclosed (equation 38). ... 

Addition of PhPdl to the allene triggers cyclopropyl ring opening to generate a cr-palladium species, which readily leads to a 1,3,5-triene through /3-elimination. From the observed diastereoselectivities, the reaction seemingly proceeds stepwise via the well-stabilized zwitterionic intermediate. 

Theoretical studies 225>226> as well as preparative work strongly indicate that the reactive palladium organic intermediate in Reaction 115b and 115c is an unsym-metrical, zwitterionic trimethylenemethane-palladium (TMM-Pd) complex, as formulated in Eq. 117. Moreover, cycloaddition with a cyclic TMM-Pd-precursor revealed that the electron-deficient olefin attacks the TMM-Pd unit from the side away from the metal. This demonstrates that complexation of the olefin with the metal does not occur prior to C—C bond formation 183>. 

Beside carbenes and carbenoids, metal-catalyzed cyclopropanation using other types of intermediate is also feasible. Treatment of 5-methyl-5-phenyl-4-methylene-l,3-dioxolan-2-one with palladium(O) catalyst in the presence of norbomene produced the cyclopropyl ketone in excellent yield (Scheme 8) (60). The active intermediate was proposed to be a Zwitterion... 

An elaborate strategy was needed for the synthesis of a palladium(O) complex with a quinonemethide ligand coordinated by the exocyclic double bond (complex 12 in Scheme 13), which involves aiming at the synthesis of the unstable palladium(n) complex of the zwitterionic form of the molecule 13. This non-observed intermediate spontaneously affords the desired complex. Treatment with dba produces the unstable free quinonemethide, which is a biologically relevant metabolite. The crystal structure of 12 was solved and showed a notably elongated exocyclic double bond (1.437 A) and a displacement of this bond out of the ring plane by 10.78° away from the palladium atom. 

Methylenecyclopentanes. Many lines of evidence indicate that in the presence of catalytic Pd°, (1) forms the reactive intermediate palladium-TMM complex (3). This complex appears to be zwitterionic in character, but its reactivity is governed mainly by its nucleophilicity/basicity. Thus the TMM... 

A novel 1,3-0 to C-alkyl shift occurs in the presence of bis[l,2-bis(diphenylphosphino)ethane]palladium such that a number of alkylidene-tetrahydrofurans undergo rearrangement to cyclopentanones (Scheme 7). The mechanism of the reaction is thought to involve oxidative addition of the allyl ether to Pd to form a zwitterionic intermediate, which then collapses by C-alkylation to the cyclic product. Related rearrangement reactions have also been reported. ... 

A mechanism that involves a zwitterionic intermediate 25 that is featured by hydrogen bonding interaction between its P(0)R2 moiety and a Brpnsted acid (X-OH) is proposed (Scheme 36). Formation of such a complex has not been substantiated with palladium due to the difficulty of isolation of species 24. However, platinum analogues HPt[P(0)R2](dmpe) have been found to react... 

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