[2+2+1+1] Cycloaddition for de novo Synthesis of Densely Functionalized Phenols.

A unique benzannulation strategy for regioselective de novo synthesis of densely functionalized phenols is described. Through metal-mediated formal [2+2+1+1] cycloaddition of two different alkynes and two molecules of CO, a series of densely functionalized phenols were obtained. The benzannulation strategy allows efficient regioselective installation up to five different substituents on a phenol ring. The resulting phenols have a substitution pattern different from those obtained from Dötz and Danheiser benzannulations.

Organometallic Chemistry of Transition Metal Alkylidyne Complexes Centered at Metathesis Reactions.

Transition metals form a variety of alkylidyne complexes with either a d0 metal center (high-valent) or a non-d0 metal center (low-valent). One of the most interesting properties of alkylidyne complexes is that they can undergo or mediate metathesis reactions. The most well-studied metathesis reactions are alkyne metathesis involving high-valent alkylidynes. High-valent alkylidynes can also undergo metathesis reactions with heterotriple bonded species such as N≡CR, P≡CR, and N≡NR+. Metathesis reactions involving low-valent alkylidynes are less known. Highly efficient alkyne metathesis catalysts have been developed based on Mo(VI) and W(VI) alkylidynes. Catalytic cross-metathesis of nitriles with alkynes has also been achieved with M(VI) (M = W, Mo) alkylidyne or nitrido complexes. The metathesis activity of alkylidyne complexes is sensitively dependent on metals, supporting ligands and substituents of alkylidynes. Beyond metathesis, metal alkylidynes can also promote other reactions including alkyne polymerization. The remaining shortcomings and opportunities in the field are assessed.

Alkyne Metathesis with d2 Re(V) Alkylidyne Complexes Supported by Phosphino-Phenolates: Ligand Effect on Catalytic Activity and Applications in Ring-Closing Alkyne Metathesis.

A family of d2 Re(V) alkylidyne complexes bearing two decorated phosphino-phenolates (POs) and a labile pyridine ligand were prepared that can efficiently promote alkyne metathesis reactions in toluene. The relative activity of these complexes varies with the PO ligands. Complexes with an electron-rich metal center have a higher activity. Ligand exchange experiments suggest that the pyridine ligands of the Re(V) alkylidynes with more electron-donating PO ligands are more labile and are more easily released to generate catalytically active species. However, complexes with electron-withdrawing PO ligands are more air-stable than those with electron-donating PO ligands. These Re(V) alkylidyne catalysts can promote the homometathesis of functionalized internal alkyl- and aryl-alkynes, as well as ring-closing alkyne metathesis (RCAM) of methyl-capped diynes, forming macrocycles with a ring size ≥12 efficiently for concentrations ≤5 mM. These reactions represent the first examples of RCAM mediated by non-d0 alkylidyne complexes. The Re(V) alkylidyne catalysts tolerate a wide range of functional groups including ethers, esters, ketones, aldehydes, alcohols, phenols, amines, amides, and heterocycles. Moreover, the catalytic RCAM reactions promoted by robust Re(V) alkylidyne catalysts could also proceed normally in wet toluene.

Dewar Metallabenzenes from Reactions of Metallacyclobutadienes with Alkynes.

Dewar benzenes, the metastable valence isomers of benzenes, have been extensively studied both experimentally and theoretically. In contrast, little is known about Dewar metallabenzenes having a transition metal in the skeletal framework, despite the recent intensive interest in the development of chemistry of metallabenzenes. Herein, we report the isolation and characterization of the first examples of structurally characterized Dewar metallabenzenes. These compounds were generated via unprecedented reactions of metallacyclobutadienes with aminoalkynes.

Robust Alkyne Metathesis Catalyzed by Air Stable d2 Re(V) Alkylidyne Complexes.

We report in this communication the first example of catalytic alkyne metathesis reactions mediated by well-defined non-d0 alkylidyne complexes. The air-stable d2 Re(V) alkylidyne complex Re4, bearing two PO-chelating ligands and a labile pyridine ligand, could catalyze homometathesis of internal alkynes with a broad substrate scope, including alcohols, amines, and even carboxylic acids. The catalyst can tolerate heating, air, and moisture in both solid and solution states, and the catalytic metathesis reactions could proceed normally in wet solvents.

Rhenium-Promoted C-C Bond-Cleavage Reactions of Internal Propargyl Alcohols.

The first examples of C-C bond cleavage reactions of internal propargyl alcohols to give vinylidene complexes are described. Treatment of [Re(dppm)3 ]I with RC≡CC(OH)R'R'' (R=aryl, alkyl; C(OH)R'R''=C(OH)Ph2, C(OH)Me2 , C(OH)HPh, C(OH)H2 ) produced the vinylidene complexes ReI(=C=CHR)(dppm)2 with the elimination of C(O)R'R''. Computational studies support that the reactions proceed through a ß-alkynyl elimination of alkoxide intermediates Re{OC(R')(R'')C≡CR}(dppm)2 .

Syntheses and Structures of Ruthenium Complexes Containing a Ru-H-Tl Three-Center-Two-Electron Bond.

Treatment of CpRuH(PP) (PP=dppm, dppe) with TlPF6 produced [CpRu(H)(Tl)(PP)]PF6 . X-ray diffraction and computational studies suggest that the complexes contain a Ru-H-Tl 3c-2e bond and can be viewed as the first σ-complexes of period 6 main-group hydrides [CpRu{η2 -(H-Tl)}(PP)]PF6 or [Tl{η2 -H-RuCp(PP)}]PF6 . The complexes can be stored as a solid at room temperature for days without appreciable decomposition, but are unstable in solution and evolved to the trimetallic complexes [{CpRu(PP)}2 (µ-Tl)]PF6 .

Synthesis and Characterization of Dirhenadehydro[12]annulenes.

The 12-membered-ring metallacycles [mer-Re{≡CCH=C(R)C≡C-}Cl(PMe2 Ph)3 )]2 (R=CMe3 , 1-adamantyl), which are organometallic analogues of antiaromatic octadehydro[12]annulene, are prepared by heating the methyl carbyne complexes mer-Re{≡CCH=C(R)C≡CH}(CH3 )Cl(PMe2 Ph)3 . An intermolecular σ-bond metathesis between the Re-CH3 bond and the acetylenic C-H bond is proposed for their formation.

Synthesis of rhenabenzenes from the reactions of rhenacyclobutadienes with ethoxyethyne.

Treatment of Na[Re(CO)5 ] with RCCCO2 Et (R=phenyl, naphthalen-1-yl, phenanthren-9-yl and pyren-1-yl) followed by reaction with acetyl chloride and ethanol afforded the rhenacyclobutadienes Re{-C(R)C(CO2 Et)C(OEt)}(CO)4 . Reactions of these rhenacyclobutadienes with HCCOEt produced rhenabenzenes Re{-C(R)C(CO2 Et)C(OEt)CHC(OEt)}(CO)4 . Except for R=Ph, new rhenacyclobutadienes with pendant alkenyl substituents Re{-C(R)C(C(OEt)CH(CO2 Et))C(OEt)}(CO)4 were also isolated from these reactions. The NMR spectroscopic and X-ray structural data, as well as the aromatic stabilization energy (ASE) values suggest that the rhenabenzenes are aromatic, with extensive delocalized π character.

Theoretical studies on the regioselectivity of iridium-catalyzed 1,3-dipolar azide-alkyne cycloaddition reactions.

Iridium-catalyzed cycloaddition of thioalkynes and bromoalkynes with azides have been investigated with the aid of density functional theory (DFT) calculations at the M06 level of theory. Our investigation focused on the different regioselectivity observed for the reactions of the two classes of alkynes. The DFT results have shown that the mechanisms of cycloaddition reactions using thioalkynes and bromoalkynes as substrates are similar yet different. The reactions of thioalkynes occur via a metallabicyclic Ir-carbene intermediate formed through alkyne-azide oxidative coupling via attack of the azide terminal nitrogen toward the ß alkyne carbon, whose carbene ligand is stabilized by an alkylthio/arylthio substituent. Reductive elimination from the intermediate leads to the formation of the experimentally observed 5-sulfenyltriazole. In the reactions of bromoalkynes RC≡CBr, the reaction mechanism involves the initial formation of a six-membered-ring metallacycle intermediate in the oxidative coupling step. The six-membered-ring intermediate then undergoes isomerization via migrating the terminal azide nitrogen from the ß carbon to the α carbon to form a much less stable metallabicyclic Ir-carbene species from which reductive elimination gives 4-bromotriazole.

Iridium-catalyzed intermolecular azide-alkyne cycloaddition of internal thioalkynes under mild conditions.

An iridium-catalyzed azide-alkyne cycloaddition reaction (IrAAC) of electron-rich internal alkynes is described. It is the first efficient intermolecular AAC of internal thioalkynes. The reaction exhibits remarkable features, such as high efficiency and regioselectivity, mild reaction conditions, easy operation, and excellent compatibility with air and a broad spectrum of organic and aqueous solvents. It complements the well-known CuAAC and RuAAC click reactions.

Aromatic chloroosmacyclopentatrienes.

Aromatic metallacycles are of considerable current interest. Reported aromatic metallacycles are mainly those with carbon, nitrogen, oxygen and sulfur. In this work, we report the synthesis and characterization of aromatic chloroosmacyclopentatrienes, which represent the first structurally confirmed metallaaromatic with a chlorine atom in its framework. Single-crystal X-ray diffraction studies show that these planar chloroosmacyclopentatrienes possess a very short Os-ClC distance suggesting M=ClC bond character.

Synthesis and characterization of rhenabenzyne complexes.

Reactions of [ReH(5)(PMe(2)Ph)(3)] with alkynols HC≡CC(OH)(R)C≡CSiMe(3) (R=tBu, iPr, 1-adamantyl) in the presence of HCl give the vinylcarbyne complexes [Re{≡CCH=C(R)C≡CSiMe(3)}Cl(2)(PMe(2)Ph)(3)], which react with tBuMgCl to give [Re{≡CCH=C(R)C≡CSiMe(3)}HCl(PMe(2)Ph)(3)]. Treatment of [Re{≡CCH=C(R)C≡CSiMe(3)}HCl(PMe(2)Ph)(3)] with nBu(4)NF gives [Re{≡CCH=C(R)C≡CH}HCl(PMe(2)Ph)(3)], which first isomerizes to the bicyclic complexes [Re{CH=CH-C(R)=CCH=}Cl(PMe(2)Ph)(3)], and then to the rhenabenzynes [Re{≡CCH=C(R)CH=CH}Cl(PMe(2)Ph)(3)]. The NMR spectroscopic and structural data as well as the aromatic stabilization energy (ASE) and nucleus-independent chemical-shift (NICS) values suggest that these rhenabenzynes have aromatic character.

RuH2(CO)(PPh3)3 catalyzed selective formation of 1,4-disubstituted triazoles from cycloaddition of alkynes and organic azides.

The ruthenium hydride complex RuH(2)(CO)(PPh(3))(3) was found to be an effective catalyst for the cycloaddition reactions of terminal alkynes and azides. In the presence of RuH(2)(CO)(PPh(3))(3), various azides reacted with a range of terminal alkynes to produce 1,4-disubstituted 1,2,3-triazoles with 100% selectivity and moderate to excellent yields.

Synthesis and characterization of bi(metallacycloprop-1-ene) complexes.

In all previously reported metallacycloprop-1-ene or η2-vinyl complexes, the metal center bears only one vinyl moiety. We have now successfully synthesized and structurally characterized the first complexes bearing two η2-vinyl moieties or spiro bi(metallacycloprop-1-ene) complexes from reactions of alkynes with rhenium phosphine complexes. Computational studies indicate that the metallacycloprop-1-ene rings are aromatic and the complexes represent a rare σ-type spirometalla-aromatic system.

Electrophilic substitution reactions of metallabenzynes.

The electrophilic substitution reactions of metallabenzynes Os(≡CC(R)═C(CH(3))C(R)═CH)Cl(2)(PPh(3))(2) (R = SiMe(3), H) were studied. These metallabenzynes react with electrophilic reagents, including Br(2), NO(2)BF(4), NOBF(4), HCl/H(2)O(2), and AlCl(3)/H(2)O(2) to afford the corresponding bromination, nitration, nitrosation, and chlorination products. The reactions usually occur at the C2 and C4 positions of the metallacycle. These observations support the notion that metallabenzynes exhibit aromatic properties.

Ruthenium-catalyzed regioselective deuteration of alcohols at the ß-carbon position with deuterium oxide.

A convenient method for regioselective H/D exchange between D(2)O and alcohols at the ß-carbon position using the catalytic system [(p-cymene)RuCl(2)]/ethanolamine/KOH is described. This method is applicable for deuteration of both primary and secondary alcohols. The H/D exchange reactions proceed through an oxidation/modification/reduction reaction sequence. Alcohols are first temporarily oxidized to carbonyl compounds by the hydrogen transfer catalyst. The carbonyl compounds then undergo deuteration at the carbon adjacent to the carbonyl group by keto-enol tautomerization in the presence of D(2)O and a catalytic amount of base. The deuterated carbonyl compounds are then reduced to produce deuterated alcohols. In support of the reaction mechanism, a well-defined bimetallic ruthenium complex was isolated from the reaction of [{(p-cymene)RuCl(2)}(2)] with ethanolamine. The activity of this complex is similar to that of [{(p-cymene)RuCl(2)}(2)]/ethanolamine.

Selective and efficient cycloisomerization of alkynols catalyzed by a new ruthenium complex with a tetradentate nitrogen-phosphorus mixed ligand.

The new ruthenium complex [Ru(N(3)P)(OAc)][BPh(4)] (4), in which N(3)P is the N,P mixed tetradentate ligand N,N-bis[(pyridin-2-yl)methyl]-[2-(diphenylphosphino)phenyl]methanamine was synthesized. The complex was found to be catalytically active for the endo cycloisomerization of alkynols. The catalytic reactions can be used to synthesize five-, six-, and seven-membered endo-cyclic enol ethers in good to excellent yields. A catalytic cycle involving a vinylidene intermediate was proposed for the catalytic reactions. Treatment of complex 4 with PhC[triple bond]CH and H(2)O gave the alkyl complex [Ru(CH(2)Ph)(CO)(N(3)P)][BPh(4)] (30), which supports the assumption that the catalytic reactions involve addition of a hydroxyl group to the C=C bond of vinylidene ligands.

Designing Cr complexes for a neutral Fe-Cr redox flow battery.

We lay out the design principles of Cr complexes to address issues of slow kinetics and parasitic reactions in the Fe-Cr redox flow battery (ICRFB). We identify theoretically and experimentally dipicolinic acid as a promising ligand, and synthesize its derivative to improve the solubility of the Cr complex to 0.7 M. We couple it with ferrocyanide for a neutral ICRFB delivering 120 stable cycles.

Palladium-catalyzed three-component cascade cyclization reaction of bisallenes with propargylic carbonates and organoboronic acids: efficient construction of cis-fused bicyclo[4.3.0]nonenes.

A bicycle built for two: The title reaction affords cis-fused bicyclo[4.3.0]nonenes from readily available 1,5-bisallenes with structurally diverse propargylic carbonates and arylboronic acids (see scheme; X = NTs, C(E(1))(2) with E(1) = CO(2)Bn, SO(2)Ph, dba = trans,trans-dibenzylidenacetone). The reaction may involve a sequential oxidative addition, two different types of three carbopalladations, and a Suzuki-type coupling.