Selective Oxygen Atom Insertion into an Aryl–Palladium Bond

Abstract

The chemistry of a palladium­(II) complex containing both an alkyl– and an aryl–palladium bond is reported. The reaction of [Pd­(CH2CMe2C6H4)­(MesNCHCHNMes)] with bromine or iodine leads to reductive elimination of 1,1-dimethylcyclobutabenzene with formation of [PdX2(MesNCHCHNMes)] (X = Br, I). However, the reaction with hydrogen peroxide gives [Pd­(CH2CMe2C6H4O)­(MesNCHCHNMes)] by overall oxygen atom insertion into the aryl–palladium rather than the alkyl–palladium bond. This complex [Pd­(CH2CMe2C6H4O)­(MesNCHCHNMes)] reacts with bromine, iodine, or hydrogen peroxide to give 3,3-dimethyl-2,3-dihydrobenzofuran and the corresponding complex [PdX2(MesNCHCHNMes)]. The mechanisms of reaction and basis for selectivity are discussed. The results support the view that oxygen atom insertion is a mechanistically viable pathway for selective catalytic oxidation of hydrocarbons by the green oxidant hydrogen peroxide.

Cycloneophylpalladium(IV) Complexes: Formation by Oxidative Addition and Selectivity of Their Reductive Elimination Reactions

“…Previous studies of reductive elimination from cycloneophylpalladium­(IV) complexes have shown a high degree of selectivity, though the reactions may occur by C–C coupling to give the cyclobutane derivative CB or by CH 2 –X or Ar–X coupling to give organopalladium­(II) products (Schemes and ). − , However, the reductive elimination reactions from the palladium­(IV) complexes 3 – 6 were not very selective (Scheme and Table ). The decomposition of 3 – 6 in CDCl 3 solution gave the cyclobutane derivative CB by C–C coupling in yields ranging from 5 to 30% (Table ).…”

“…25 On the other hand, the diimine complex [Pd(CH 2 CMe 2 C 6 H 4 )(MesNCHCHNMes)] (G) reacts with bromine or iodine to give the palladium(II) dihalide complex I and the cyclobutane derivative CB, probably by way of a palladium(IV) intermediate H that could not be detected (Scheme 2). 28 With hydrogen peroxide G gives the oxygen atom insertion product J. Further reaction with bromine or iodine gave the benzofuran derivative BF and the palladium-(II) complexes I, but again no palladium(IV) intermediate could be detected.…”

“…It is now well established that Pd­(II)/Pd­(IV) cycles are important in catalysis by palladium complexes, especially in reactions involving strong oxidants such as halogens, dioxygen, and peroxides. , For example, the catalytic reaction of iodine with alkanes or arenes to form iodo-containing hydrocarbons is thought to involve alkane C–H bond activation to form an alkylpalladium­(II) complex, followed by oxidative addition of iodine and then reductive elimination from palladium­(IV). − These impressive advances have stimulated further efforts to understand the factors that affect reactivity and selectivity in oxidative addition at palladium­(II) and reductive elimination from palladium­(IV) complexes. − In studies of selectivity in reductive elimination, the cycloneophylpalladium­(IV) complexes have played an important role as a model to distinguish between reactivity at C­(sp 2 ) or C­(sp 3 ) centers. ,,,,− When the palladium­(IV) complexes ( A ) are sufficiently stable to be isolated, the mechanisms of reductive elimination have been studied and, in most cases, have been shown to involve five-coordinate intermediates such as B (Scheme ). − , External nucleophilic attack on such intermediates occurs selectively at the CH 2 group and can lead, for example, to C–O or C–N bond formation (Scheme a).…”

“…With cycloneophylpalladium complexes, there have been few studies of oxidative addition–reductive elimination by reaction with bromine, iodine, or methyl iodide. , The complex D , where X = I, formed by oxidative addition of iodine to the Pd­(II) precursor, but this derivative is stable to reductive elimination . On the other hand, the diimine complex [Pd­(CH 2 CMe 2 C 6 H 4 )­(MesNCHCHNMes)] ( G ) reacts with bromine or iodine to give the palladium­(II) dihalide complex I and the cyclobutane derivative CB , probably by way of a palladium­(IV) intermediate H that could not be detected (Scheme ).…”

Nickel and Palladium Complexes with Reactive σ‐Metal‐Oxygen Covalent Bonds

“…[55] CÀ O reductive couplings from organo-Ni or À Pd alkoxides can be induced by oxidation, of from high-valent precursors in the less common oxidation states + 3 or + 4. [56] In addition, bulky or wide-bite angle coligands can be tuned to favor CÀ O coupling from Pd(II). [32,57] The difficulty of CÀ O reductive elimination is the cause of the lesser degree of development attained by palladiumcatalyzed coupling of aryls and alcohols to ethers [13,58] in contrast with the analogous CÀ N coupling in Buchwald-Hartwig aryl amination.…”