In modern organic chemistry, palladium-catalyzed reactions, e.g. cross couplings, are broadly used as a powerful synthetic method. These processes mostly proceed through a palladium(0)/palladium(II) redox couple, which rests upon the smooth interconversion between palladium(0) and palladium(II) species. Recently, catalytic reactions involving a palladium(IV) intermediate that is generated in situ by the oxidation of a palladium(II) precursor have attracted exceptional interest in organic synthesis. Such palladium(II)/palladium(IV) catalysis can realize unprecedented transformations complementary to the conventional palladium(0)/palladium(II) catalysis because of the unique reactivity of high-valent palladium(IV) complexes. In this review article, we focus on the difunctionalization of alkenes and alkynes based on the palladium(II)/palladium(IV) catalytic cycle.
This review describes the development of novel dearomatization reactions of phenols using transition metal catalysis. We found that an intramolecular ipso-Friedel-Crafts allylic alkylation of phenols proceeded smoothly in the presence of Pd catalyst, producing various spiro[4.5]cyclohexadienone derivatives. The present methods could be applied to a catalytic asymmetric reaction. An asymmetric intramolecular Friedel-Crafts allylic alkylation of phenols to produce chiral 10-vinyl 9,10-dihydrophenanthrenes was also investigated. The developed process was successfully applied to the enantioselective synthesis of bioactive natural products. We also developed a novel method for synthesizing spirocycles based on a Pd-catalyzed intramolecular ipso-Friedel-Crafts alkylation of phenols to η3-propargylpalladium(II) complexes. Mechanistic studies revealed that the present reaction proceeds through a rearomatization-assisited oxidative addition. An Au-catalyzed carbocyclization of phenols with a terminal alkyne via an intramolecular ipso-Friedel-Crafts alkenylation is also discussed.
A copper-catalyzed sulfonylation of alkenes or alkynes was performed using sodium sulfinates under oxidative conditions. These reactions afforded numerous alkenyl sulfones stereoselectively. For instance, the reaction using alkenes efficiently gave (E)-alkenyl sulfones in the presence of potassium iodide. The sulfonylation of alkynes produced (E)-alkenyl sulfones or (E)-β-haloalkenyl sulfones in good yields. Furthermore, a palladium-catalyzed arylation of terminal alkenes with sodium sulfinates could synthesize (E)-alkenes via the desulfonylation in air.
This review is comprised of seven topics: (1) Reactivity of O-acylTEMPOs towards nucleophiles and application to chemoselective transformations, (2) reactivity of anionic species derived from O-acylTEMPOs, (3) E-selective Knoevenagel condensation of acetoacetylTEMPOs and furan synthesis, (4) electrocyclization of 2,4-dienones derived from acetoacetic derivatives and 2-substituted enals, (5) diastereoselective addition of amide to O-(2-alkenoyl)TEMPOs and β-amino acids synthesis, (6) thermal stability of O-acylTEMPOs, and (7) reactivity of TEMPO anion and use of O-alkylTEMPOs as a latent carbonyl function. O-AcylTEMPOs were stable towards hydride-transferring and metallic alkylating reagents, allowing chemoselective transformation of diacid mixed alkyl/TEMP-1-yl esters. The lithium anionic species of O-acylTEMPOs were employed for alkylation, Michael addition, direct aldol reaction, and others. Knoevenagel condensation of O-acetoacetylTEMPOs with aldehydes substituted with an electron-withdrawing group led preferentially to E-adducts. The AlMe3-treated O-(2-alkenoyl)TEMPOs underwent smooth addition of the Mg amide. O-Alkylated TEMPOs were oxidized with mCPBA to afford the corresponding aldehydes, thus accomplishing a new protocol for the halide-carbonyl conversion.
Insulated conductive molecules, in which the π-conjugated compounds are covered by a cyclic protective sheath, have attracted considerable attention because of their potential applicability in next-generation molecular electronic devices. We have developed new methods of synthesizing insulated π-conjugated molecules involving the self-inclusion of rotaxane precursors linking π-conjugated units as a guest and permethylated cyclodextrin (PM CD) as a macrocyclic host. A new method for synthesizing insulated molecular wire (IMW) through the polymerization of insulated π-conjugated molecules as monomers was also developed. The obtained IMWs are highly soluble in organic solvents and have a high covering ratio, rigidity, and photoluminescence efficiency; further, they show high charge mobility, even in the solid state. This paper also highlights a new type of molecular wiring method based on the polymerization or co-polymerization of an insulated π-conjugated monomer between nanosized electrodes.
This review describes the sp2 carbon-halogen bond formations by reductive elimination from palladium (II) species and its recent developments in sp2 carbon-halogen reductive elimination after constructions of carbon-carbon bonds.