Macrocyclization by Nickel‐Catalyzed,Ester‐Promoted,Epoxide–Alkyne Reductive Coupling: Total Synthesis of (−)‐Gloeosporone

Ringing the changes : The total synthesis of the title compound centers around a novel strategy that employs a nickel(0)–phosphine complex and triethyl borane in an efficient closure of a 14‐membered ring through C? C bond formation (see scheme; cod=cyclooctadiene). The synthesis was accomplished in 10 steps and in approximately 9 % overall yield.

     

Scope and Limitations of the Base‐Free Copper(I) Oxide Catalyzed N‐Heteroarylation of 1H‐(Benz)imidazoles with B‐Heteroarylboronic Acids or 2‐Heteroaryl‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolanes

The known, very efficient base‐free copper(I) oxide catalyzed N‐arylation reaction performed in MeOH at room temperature for the synthesis of N‐substituted azoles and amines was extended to the heterocyclic series, i.e., we report herein the base‐free copper(I) oxide catalyzed N‐heteroarylation of 1H‐(benz)imidazole, by means of electron‐rich or electron‐deficient B‐heteroarylboronic acids or 2‐heteroaryl‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolanes (Schemes 1 and 2). Under these conditions, N‐heteroarylated 1H‐(benz)imidazoles were obtained in good to excellent yields (Tables 1 and 2). This is the first time that 2‐heteroaryl‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolanes were used in this type of reaction.     

Cover Picture: Palladium‐Catalyzed Carbon–Carbon Bond Formation and Cleavage of Organo(hydro)fullerenes (Chem. Eur. J. 19/2009)

An unexpected C? H bond dimerization reaction and C? C bond‐cleavage reaction in organo(hydro)fullerenes have been discovered. In their Communication on page 4760 ff. , K. Itami and M. Nambo describe the use of Pd catalysts for a number of interesting reactions of such fullerenes.

     

Single‐Electron‐Transfer (SET)‐Induced Oxidative Biaryl Coupling by Polyalkoxybenzene‐Derived Diaryliodonium(III) Salts

Metal‐free oxidative C? C coupling by using polyalkoxybenzene‐derived diaryliodonium(III) salts as both the oxidant and aryl source has been developed. These salts can induce single‐electron‐transfer (SET) oxidation to yield electron‐rich arenes and subsequently transfer the polyalkoxyphenyl group into in situ generated aromatic radical cations to produce biaryl products. The reaction is promoted by a Lewis acid that activates the iodonium salts. It has been revealed that the reactivity of the salts under acidic conditions is quite different to their known behavior under basic conditions. The reactivity preference of a series of iodonium salts in the SET oxidation and their ligand transfer abilities have been systematically investigated and the results are summarized in this report.     

Aerobic and Electrochemical Oxidative Cross‐Dehydrogenative‐Coupling (CDC) Reaction in an Imidazolium‐Based Ionic Liquid

The ionic liquid 1‐butyl‐3‐methylimidazolium tetrafluoroborate [BMIm][BF₄] has demonstrated high efficiency when applied as a solvent in the oxidative nitro‐Mannich carbon? carbon bond formation. The copper‐catalyzed cross‐dehydrogenative coupling (CDC) between N‐phenyltetrahydroisoquinoline and nitromethane in [BMIm][BF₄] occurred with high yield under the described reaction conditions. Both the ionic liquid and copper catalyst were recycled nine times with almost no lost of activity. The electrochemical behavior of the tertiary amine substrate and β‐nitroamine product was investigated employing [BMIm][BF₄] as electrolyte solvent. The potentiostatic electrolysis in ionic liquid afforded the desired product with a high yield. This result and the cyclic voltammetric investigation provide a better understanding of the reaction mechanism, which involves radical and iminium cation intermediates.     

Pyridine‐Enhanced Head‐to‐Tail Dimerization of Terminal Alkynes by a Rhodium–N‐Heterocyclic‐Carbene Catalyst

A general regioselective rhodium‐catalyzed head‐to‐tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh–N‐heterocyclic‐carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π‐alkyne‐coordinated Rh^I species [RhCl(NHC)(η²‐HC?CCH₂Ph)(py)] ( 3 ) and [RhCl(NHC){η²‐C(tBu)?C(E)CH?CHtBu}(py)] ( 4 ) and the Rh^III–hydride–alkynyl species [RhClH{? C?CSi(Me)₃}(IPr)(py)₂] ( 5 ). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne C? H oxidative addition, alkyne insertion, and reductive elimination. A 2,1‐hydrometalation of the alkyne is the more favorable pathway in accordance with a head‐to‐tail selectivity.     

Three‐Coordinate Nickel(I) Complexes Stabilised by Six‐, Seven‐ and Eight‐Membered Ring N‐Heterocyclic Carbenes: Synthesis,EPR/DFT Studies and Catalytic Activity

Comproportionation of [Ni(cod)₂] (cod=cyclooctadiene) and [Ni(PPh₃)₂X₂] (X=Br, Cl) in the presence of six‐, seven‐ and eight‐membered ring N‐aryl‐substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three‐coordinate Ni^I complexes [Ni(NHC)(PPh₃)X] (X=Br, Cl; NHC=6‐Mes 1 , 6‐Anis 2 , 6‐AnisMes 3 , 7‐o‐Tol 4 , 8‐Mes 5 , 8‐o‐Tol 6 , O‐8‐o‐Tol 7 ). Continuous wave (CW) and pulsed EPR measurements on 1 , 4 , 5 , 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ∣3d〉 and ∣3d〉 character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced ³¹P superhyperfine coupling to the PPh₃ group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1 , 4 , 5 and 6 as pre‐catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar=Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 1 > 4 ≈ 6 ? 5 ).     

“Design” of Boron‐Based Compounds as Pro‐Nucleophiles and Co‐Catalysts for Indium(I)‐Catalyzed Allyl Transfer to Various Csp3‐Type Electrophiles

We have recently uncovered a general indium(I)‐catalyzed method for allylations and propargylation of acetals and ketals with a water‐ and air‐stable allyl boronate. By using a more reactive allyl borane, we have successfully extended this methodology to the more challenging C C coupling with ethers. Herein, we report an improved methodology for the indium(I)‐catalyzed allylation of acetals and ethers, through combination of the allyl boronate with a commercially available “hard” Lewis acid, B‐methoxy‐9‐BBN (BBN=borabicyclo[3.3.1]nonane), as an effective co‐catalyst. Significantly, our work highlights for the first time the correlation between the Lewis acidity of “electrophilic” boron‐based compounds and their “nucleophilic” reactivity in Csp³–Csp³ couplings, catalyzed by a “soft” low‐oxidation main group metal. In addition, we also report several applications of these methodologies to the selective synthesis of various carbohydrate derivatives.