σ‐Alkylpalladium Intermediates in Intramolecular Heck Reactions:Isolation and Catalytic Activity

The isolation of σ‐alkylpalladium Heck intermediates, possible when β‐hydride elimination is inhibited, is a rather rare event. Performing intramolecular Heck reactions on N‐allyl‐2‐halobenzylamines in the presence of [Pd(PPh₃)₄], we isolated and characterized a series of stable bridged palladacycles containing an iodine or bromine atom on the palladium atom. Indolyl substrates were also tested for isolation of the corresponding complexes. X‐ray crystallographic analysis of one of the indolyl derivatives revealed the presence of a five‐membered palladacycle with the metal center bearing a PPh₃ ligand and an iodine atom in a cis position with respect to the nitrogen atom. The stability of the σ‐alkylpalladium complexes is probably a consequence of the strong constraint resulting from the bridged junction that hampers the cisoid conformation essential for β‐hydride elimination. Subsequently, the thus obtained bridged five‐membered palladacycles were proven to be effective precatalysts in Heck reactions as well as in cross‐coupling processes such as Suzuki and Stille reactions.     

Insertion of Isolated Alkynes into Carbon–Carbon σ‐Bonds of Unstrained Cyclic β‐Ketoesters via Transition‐Metal‐Free Tandem Reactions: Synthesis of Medium‐Sized Ring Compounds

The transition‐metal‐free insertion of isolated alkynes into carbon–carbon σ‐bonds of unstrained cyclic β‐dicarbonyl compounds has been reported. These tandem reactions offer an efficient synthesis of medium‐sized ring or fused‐ring compounds through ring expansion. The methodology has the potential to be widely used throughout organic synthesis due to the easily accessible starting materials and mild reaction conditions.     

Manganese(IV) Corroles with σ‐Aryl Ligands

Different pathways for the preparation of organometallic manganese(IV) corroles with σ‐aryl ligands have been evaluated. The treatment of a manganese(III) corrole with Grignard reagents PhMgX (X = Cl, Br), followed by aerial oxidation yields oxidized halogenido complexes [(cor)Mn^IVX] instead of the anticipated organometallic compounds. Reaction of these halogenido species, especially the bromido compound, with excess Grignard reagents or with lithium aryls results in the formation of the desired σ‐aryl compounds via salt metatheses. Three examples of this class of rare complexes have been characterized by means of optical and ¹H NMR spectroscopy, and in two cases single crystal X‐ray diffraction studies have been carried out. In the crystal, the molecular structures of the σ‐phenyl‐ and the σ‐(p‐bromophenyl) derivatives were observed to be very similar, albeit both species pack in different pattern.     

Mechanism of Gold(I)‐Catalyzed Conia‐ene Reaction of β‐Ketoesters with Alkynes: A DFT Study

Density functional calculations have been performed to comparatively investigate two possible pathways of Au(I)‐catalyzed Conia‐ene reaction of β‐ketoesters with alkynes. Our studies find that, under the assistance of trifluoromethanesulfonate (TfO), the β‐ketoester is the most likely to undergo Model II to isomerize into its enol form, in which TfO plays a proton transfer role through a 6‐membered ring transition state. The coordination of the Au(I) catalyst to the alkynes triple bond can enhance the eletrophilic capability and reaction activity of the alkynes moiety, which triggers the nucleophilic addition of the enol moiety on the alkynes moiety to give a vinyl‐Au intermediate. This cycloisomerizaion step is exothermal by 21.3 kJ/mol with an energy barrier of 56.0 kJ/mol. In the whole catalytic process, the protonation of vinyl‐Au is almost spontaneous, and the formation of enol is a rate‐limiting step. The generation of enol and the activation of Au(I) catalyst on the alkynes are the key reasons why the Conia‐ene reaction can occur in mild condition. These calculations support that Au(I)‐catalyzed Conia‐ene reactions of β‐ketoesters with alkynes go through the pathway 2 proposed by Toste.     

Synthesis and Characterization of Cyclopropaosmanaphthalenes Containing a Fused σ‐Aromatic Metallacyclopropene Unit

Metalla‐aromatics are important complexes that show unique properties owing to their highly conjugated systems, which show Hückel or Möbius aromaticity. Recently, several metalla‐aromatics showing spiro‐aromaticity or σ‐aromaticity have been reported. Herein, we report the isolation of the first cyclopropametallanaphthalenes, in which the metallacyclopropene ring shows σ‐aromaticity and weak hyperconjugative aromaticity. The reaction of OsCl₂(PPh₃)₃ with o‐ethynylphenyl alkynes in the presence of PPh₃ followed by protonation with HCl yielded the first cyclopropametallanaphthalenes. The reaction mechanism and the aromaticity were also investigated by density functional theory studies.     

Coupling of an N‐Heterocyclic Carbene on Iron with Alkynes to Form η5‐Cyclopentadienyl‐Diimine Ligands

A cyclometalated N‐heterocyclic carbene ligand in a half‐sandwich iron complex was found to couple with alkynes, leading to a unique type of ring opening of the carbene ligand and the formation of ferrocenyl–diimine complexes. An intermediary iron complex obtained from the reaction with phenylacetylene reveals that the ring opening follows the formation of a fused heterocycle consisting of an imidazole ring and two alkynes.     

Pd‐Catalyzed Enantioselective Ring Opening/Cross‐Coupling and Cyclopropanation of Cyclobutanones

A palladium‐catalyzed enantioselective sequential ring‐opening/cross‐coupling of cyclobutanones is disclosed that provides chiral indanones bearing C3‐quaternary stereocenters. The reaction process involves palladium‐catalyzed nucleophilic addition of cyclobutanones and aryl halides, enantioselective β‐carbon elimination, and intermolecular trapping of a transient σ‐alkylpalladium complex with boronic acids. Alternatively, an intramolecular cyclopropanation is realized through C?H bond functionalization in the absence of external coupling reagents, affording chiral cyclopropane‐fused‐indanones in good yields and enantioselectivity.     

Construction of Hexahydrophenanthrenes By Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Substituted Internal Alkynes through C−H Activation

The treatment of benzylallene‐substituted internal alkynes with [RhCl(CO)₂]₂ effects a novel cycloisomerization by C(sp²)?H bond activation to produce hexahydrophenanthrene derivatives. The reaction likely proceeds through consecutive formation of a rhodabicyclo[4.3.0] intermediate, σ‐bond metathesis between the C(sp²)?H bond on the benzene ring and the C(sp²)?Rh^III bond, and isomerization between three σ‐, π‐, and σ‐allylrhodium(III) species, which was proposed based on experiments with deuterated substrates.     

Comparative studies on group III σ‐hole and π‐hole interactions

The σ‐hole of M₂H₆ (M = Al, Ga, In) and π‐hole of MH₃ (M = Al, Ga, In) were discovered and analyzed, the bimolecular complexes M₂H₆···NH₃ and MH₃···N₂P₂F₄ (M = Al, Ga, In) were constructed to carry out comparative studies on the group III σ‐hole interactions and π‐hole interactions. The two types of interactions are all partial‐covalent interactions; the π‐hole interactions are stronger than σ‐hole interactions. The electrostatic energy is the largest contribution for forming the σ‐hole and π‐hole interaction, the polarization energy is also an important factor to form the M···N interaction. The electrostatic energy contributions to the interaction energy of the σ‐hole interactions are somewhat greater than those of the π‐hole interactions. However, the polarization contributions for the π‐hole interactions are somewhat greater than those for the σ‐hole interactions. © 2016 Wiley Periodicals, Inc.     

An Efficient Copper‐Catalyzed One‐Pot Synthesis of 1‐Aryl‐1,2,3‐triazoles from Arylboronic Acids in Water under Mild Conditions

A convenient method for one‐pot two‐step 1,3‐dipolar cycloadditon reaction of arylboronic acid, sodium azide followed with terminal alkynes in the presence of 2‐pyrrolecarbaldiminato‐Cu(II) complexes catalyst is reported. Various 1‐aryl‐1,2,3‐triazoles were prepared in 63%–97% yields in water at 30°C without any additives and avoiding the isolation of unstable aryl azides.     

Characterisation of Alkane σ‐Complexes

Alkane σ‐complexes have evolved from a curious phenomenon to an intermediate of intense interest, fuelling research into the area. Over the last fifteen years, metal alkane complex characterisation has evolved to incorporate reports employing UV/Vis, IR and NMR spectroscopy, and X‐ray and neutron diffractometry. Previously, due to the sparse geometric characterisation of alkane σ‐complexes, assumptions regarding bonding geometries and selectivities were made by comparison to related σ‐complexes, or by analysis of C?H activation products. This minireview assembles relevant literature that illuminates the metrics of alkane–metal bonding, and critically analyses the binding mode, selectivity and stability of alkane complexes.     

Palladium‐Catalyzed Oxidative Difunctionalization of Alkenes with α‐Carbonyl Alkyl Bromides Initiated through a Heck‐type Insertion: A Route to Indolin‐2‐ones

The oxidative interception of various σ‐alkyl palladium(II) intermediates with additional reagents for the difunctionalization of alkenes is an important research area. A new palladium‐catalyzed oxidative difunctionalization reaction of alkenes with α‐carbonyl alkyl bromides is described, in which the σ‐alkyl palladium(II) intermediate is generated through a Heck insertion and trapped using an aryl C(sp²)? H bond. This method can be applied to various α‐carbonyl alkyl bromides, including primary, secondary, and tertiary α‐bromoalkyl esters, ketones, and amides.     

FeCl3-Mediated Reaction of 1,4-Dilithio-1,3-dienes with Alkynes Affording Benzene Derivatives

Unactivated alkynes reacted with 1,4-dilithio-1,3-diene derivatives in the presence of FeCl3 affording substituted benzene derivatives via a formal[4 2] cycloaddition.     

Copper‐Catalyzed 1,2‐Addition of α‐Carbonyl Iodides to Alkynes

β,γ‐Unsaturated ketones are an important class of organic molecules. Herein, copper catalysis has been developed for the synthesis of β‐γ‐unsaturated ketones through 1,2‐addition of α‐carbonyl iodides to alkynes. The reactions exhibit wide substrate scope and high functional group tolerance. The reaction products are versatile synthetic intermediates to complex small molecules. The method was applied for the formal synthesis of (±)‐trichostatin A, a histone deacetylase inhibitor.     

Synthesis,characterization, anti‐inflammatory and analgesic activity of transition metal complexes of 3‐[1‐(2‐hydroxyphenyl) ethylideamino]‐2‐phenyl‐3,4‐dihydroquinazolin‐4(3H)‐one

A new quinazolinone derivative, 3‐[1‐(2‐hydroxyphenyl)ethylideamino]‐2‐phenyl‐3,4‐dihydroquinazolin‐4(3H)‐one ( LH ) was synthesized by the condensation of 2‐hydroxyacetophenone‐2‐aminobenzoylhydrazone and benzaldehyde. The cyclization to form 1,2‐dihydroquinazolinone was confirmed by IR, 1D and 2D HETCOR studies. Coordination compounds of Co(II), Ni(II), Cu(II) and Zn(II) of LH were synthesized and characterized using various physico‐chemical studies like stoichiometric, conductivity, magnetic moment measurements and spectral techniques such as IR, NMR, UV‐vis and EPR spectroscopy. The elemental analysis and thermal studies suggested a general stoichiometry [M(HEPDQ)Cl] for all the complexes. A four‐coordinate geometry was assigned to all the complexes. The complexes along with the parent ligand were screened for their anti‐inflammatory activity, using carrageenan‐induced rat paw edema, and for their analgesic activity by Eddy's hot plate method. The activity of the ligand was enhanced on complexation with metal ions. This enhanced activity was attributed to the increased lipophilic nature of the complexes. Notable anti‐inflammatory activity was observed for Ni(II), Cu(II) and Zn(II) complexes. The analgesic activity of the ligand was greater than the standard at 60 min. and at a 10 mg kg⁻¹ dose, whereas the activity of Ni(II) and Cu(II) complexes at 10 mg kg⁻¹ dose was comparable with the standard used. Copyright © 2011 John Wiley & Sons, Ltd.     

Diastereo‐ and Enantioselective Synthesis of (E)‐δ‐Boryl‐Substituted anti‐Homoallylic Alcohols in Two Steps from Terminal Alkynes

We report the highly diastereo‐ and enantioselective preparation of (E)‐δ‐boryl‐substituted anti‐homoallylic alcohols in two steps from terminal alkynes. This method consists of a cobalt(II)‐catalyzed 1,1‐diboration reaction of terminal alkynes with B₂pin₂ and a palladium(I)‐mediated asymmetric allylation reaction of the resulting 1,1‐di(boryl)alk‐1‐enes with aldehydes in the presence of a chiral phosphoric acid. Propyne, which is produced as the byproduct during petroleum refining, could be used as the starting material to construct homoallylic alcohols that are otherwise difficult to synthesize with high stereocontrol.     

Direct Synthesis of Chiral Allenoates from the Asymmetric CH Insertion of α‐Diazoesters into Terminal Alkynes

The asymmetric C? H insertion of α‐diazoesters into 1‐alkynes was achieved using chiral cationic guanidinium salts and copper(I) complexes. Optically active 2,4‐disubstituted allenoates were generated under mild reaction conditions from various α‐diazoesters and 1‐alkynes in high yield (up to 99 %) and enantioselectivity (up to 97:3 e.r.). Control experiments excluded the possibility of an asymmetric isomerization of alkynoates.     

Iron‐Induced Regio‐ and Stereoselective Addition of Sulfenyl Chlorides to Alkynes by a Radical Pathway

The radical addition of the Cl? S σ‐bond in sulfenyl chlorides to various C? C triple bonds has been achieved with excellent regio‐ and stereoselectivity in the presence of a catalytic amount of a common iron salt. The reaction is compatible with a variety of functional groups and can be scaled up to the gram‐scale with no loss in yield. As well as terminal alkynes, internal alkynes underwent stereodefined chlorothiolation to provide tetrasubstituted alkynes. Preliminary mechanistic investigations revealed a plausible radical process involving a sulfur‐centered radical intermediate via iron‐mediated homolysis of the Cl? S bond. The resulting chlorothiolation adducts can be readily transformed to the structurally complex alkenyl sulfides by cross‐coupling reactions. The present reaction can also be applied to the complementary synthesis of the potentially useful bis‐sulfoxide ligands for transition‐metal‐catalyzed reactions.     

Metal‐Free σ‐Bond Metathesis in 1,3,2‐Diazaphospholene‐Catalyzed Hydroboration of Carbonyl Compounds

The first metal‐free catalytic hydroboration of carbonyl derivatives has been developed in which a catalytic amount of 1,3,2‐diazaphospholene effectively promotes a hydroboration reaction of aliphatic and aromatic aldehydes and ketones. The reaction mechanism involves the cleavage of both the P? O bond of the alkoxyphosphine intermediate and the B? H bond of pinacolborane as well as the formation of P? H and B? O bonds. Thus, the reaction proceeds through a non‐metal σ‐bond metathesis. Kinetic and computational studies suggest that the σ‐bond metathesis occurred in a stepwise but nearly concerted manner.     

Organomagnesium‐Catalyzed Isomerization of Terminal Alkynes to Allenes and Internal Alkynes

Organomagnesium complexes 2 were synthesized from N,N‐dialkylamineimine ligands 1 and dibenzylmagnesium by benzylation of the imine moiety. 3‐Aryl‐1‐propynes reacted with 2 to form the corresponding tetraalkynyl complexes, which acted as catalysts for the transformation of these terminal alkynes into allenes and further to internal alkynes under mild conditions. To the best of our knowledge, this example is the first of an organomagnesium‐catalyzed isomerization of alkynes. Notably, the reactions proceeded through temporally separated autotandem catalysis, thus allowing the isolation of the allene or internal alkyne species in good yields. Mechanistic experiments suggested that the catalytically active tetraalkynyl complexes consist of a tautomeric mixture of alkynyl‐, allenyl‐, and propargylmagnesium species.