Lewis Acid Mediated Tandem Reaction of Propargylic Alcohols to Tetrazoles Involving CO‐ and CC‐Bond Cleavage Reactions and a CN‐Bond Formation

A novel and direct synthesis of 1‐aryl‐5‐arylvinyl‐tetrazoles from easily prepared propargylic alcohols and TMSN₃ is developed in the presence of TMSCl under mild conditions (TMS=trimethylsilyl). The process involves an allenylazide intermediate, followed by a C?C‐bond cleavage and C?N‐bond formation to afford the desired products. Moreover, this method offers a good functional‐group applicability and can be scaled‐up to grams (yield up to 85 %).     

Reversible 1,1‐Hydroboration: Boryl Insertion into a CN Bond and Competitive Elimination of HBR2 or RH

Boranes with the general formula of HBR₂ have been found to undergo a facile 1,1‐hydroboration reaction with pyrido[1,2‐a]isoindole ( A ), resulting in insertion of a BR₂ unit into a C? N bond and the formation of a variety of BN heterocycles. Investigation on the thermal reactivity of the BN heterocycles revealed that these molecules have two distinct and competitive thermal elimination pathways: HBR₂ elimination (or retro‐hydroboration) versus R? H elimination, depending on the R group on the B atom and the chelate backbone. Mechanistic aspects of these highly unusual reactions have been established from both experimental and computational evidence. Adduct formation between HBR₂ and A was found to be the key intermediate in 1,1‐hydroboration of A .     

Direct Borylation of Primary CH Bonds in Functionalized Molecules by Palladium Catalysis

Organoborane compounds are among the most commonly employed intermediates in organic synthesis and serve as crucial precursors to alcohols, amines, and various functionalized molecules. A simple palladium‐based system catalyzes the conversion of primary C(sp³)? H bonds in functionalized complex organic molecules into alkyl boronate esters. Amino acids, amino alcohols, alkyl amines, and a series of bioactive molecules can be functionalized with the use of readily available and removable directing groups in the presence of commercially available additives, simple ligands, and oxygen (O₂) as the terminal oxidant. This approach represents an economic and environmentally friendly method that could find broad applications.     

Unusual Nitrile–Nitrile and Nitrile–Alkyne Coupling of FcCN and FcCCCN

The reactions of the Group 4 metallocene alkyne complexes, [Cp*₂M(η²‐Me₃SiC₂SiMe₃)] ( 1 a : M=Ti, 1 b : M=Zr, Cp*=η⁵‐pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc?C?N and Fc?C?C?C?N (Fc=Fe(η⁵‐C₅H₅)(η⁵‐C₅H₄)), is described. In case of Fc?C?N an unusual nitrile–nitrile C?C homocoupling was observed and 1‐metalla‐2,5‐diaza‐cyclopenta‐2,4‐dienes ( 3 a , b ) were obtained. As the first step of the reaction with 1 b , the nitrile was coordinated to give [Cp*₂Zr(η²‐Me₃SiC₂SiMe₃)(N?C‐Fc)] ( 2 b ). The reactions with the 3‐ferrocenyl‐2‐propyne‐nitrile Fc?C?C?C?N lead to an alkyne–nitrile C?C coupling of two substrates and the formation of 1‐metalla‐2‐aza‐cyclopenta‐2,4‐dienes ( 4 a , b ). For M=Zr, the compound is stabilized by dimerization as evidenced by single‐crystal X‐ray structure analysis. The electrochemical behavior of 3 a , b and 4 a , b was investigated, showing decomposition after oxidation, leading to different redox‐active products.     

Activation of Homolytic SiZn and SiHg Bond Cleavage,Mediated by a Pt0 Complex,via Novel PtZn and PtHg Compounds

The thermally stable [(tBuMe₂Si)₂M] (M=Zn, Hg) generate R₃Si^. radicals in the presence of [(dmpe)Pt(PEt₃)₂] at 60–80 °C. The reaction proceeds via hexacoordinate Pt complexes, (M=Zn ( 2 a and 2 b ), M=Hg ( 3 a and 3 b )) which were isolated and characterized. Mild warming or photolysis of 2 or 3 lead to homolytic dissociation of the Pt? MSiR₃ bond generating silyl radicals and novel unstable pentacoordinate platinum paramagnetic complexes (M=Zn ( 5 ), Hg ( 6 )) whose structures were determined by EPR spectroscopy and DFT calculations.     

Coordination and Activation of AlH and GaH Bonds

The modes of interaction of donor‐stabilized Group 13 hydrides (E=Al, Ga) were investigated towards 14‐ and 16‐electron transition‐metal fragments. More electron‐rich N‐heterocyclic carbene‐stabilized alanes/gallanes of the type NHC?EH₃ (E=Al or Ga) exclusively generate κ² complexes of the type [M(CO)₄(κ²‐H₃E?NHC)] with [M(CO)₄(COD)] (M=Cr, Mo), including the first κ² σ‐gallane complexes. β‐Diketiminato (′nacnac′)‐stabilized systems, {HC(MeCNDipp)₂}EH₂, show more diverse reactivity towards Group 6 carbonyl reagents. For {HC(MeCNDipp)₂}AlH₂, both κ¹ and κ² complexes were isolated, while [Cr(CO)₄(κ²‐H₂Ga{(NDippCMe)₂CH})] is the only simple κ² adduct of the nacnac‐stabilized gallane which can be trapped, albeit as a co‐crystallite with the (dehydrogenated) gallylene system [Cr(CO)₅(Ga{(NDippCMe)₂CH})]. Reaction of [Co₂(CO)₈] with {HC(MeCDippN)₂}AlH₂ generates [(OC)₃Co(μ‐H)₂Al{(NdippCme)₂CH}][Co(CO)₄] ( 12 ), which while retaining direct Al?H interactions, features a hitherto unprecedented degree of bond activation in a σ‐alane complex.     

Converting Ag2SCdS and Ag2SZnS into AgCdS and AgZnS Nanoheterostructures by Selective Extraction of Sulfur

A mild three‐step solution strategy is developed to prepare Ag? MS (M=Zn, Cd) nanoheterostructures composed of MS nanorods with silver tips. First, Ag₂S? MS heterostructures are synthesized by following a solution–liquid–solid mechanism with Ag₂S nanoparticles as catalysts, then the Ag₂S sections of the heterostructures are converted into silver nanoparticles by selective extraction of sulfur. Notably, for the prepared Ag? CdS heterostructures, the localized surface plasmon resonance of silver remarkably intensifies the photoluminescence of CdS by enhancing the excitation light absorption, which is beneficial for potential applications of CdS nanoparticles in the fields of biolabeling, light‐emitting diodes, and so forth. The strategy reported herein would be useful for designing and fabricating other metal–semiconductor hybrid nanostructures with desirable performances.     

Luminescent Palladium(II) Complexes with π‐Extended Cyclometalated [RC^N^NR′] and Pentafluorophenylacetylide Ligands: Spectroscopic,Photophysical, and Photochemical Properties

A series of cyclometalated Pd^II complexes that contain π‐extended R? C^N^N? R′ (R? C^N^N? R′=3‐(6′‐aryl‐2′‐pyridinyl)isoquinoline) and chloride/pentafluorophenylacetylide ligands have been synthesized and their photophysical and photochemical properties examined. The complexes with the chloride ligand are emissive only in the solid state and in glassy solutions at 77 K, whereas the ones with the pentafluorophenylacetylide ligand show phosphorescence in the solid state (λ_max=584–632 nm) and in solution (λ_max=533–602 nm) at room temperature. Some of the complexes with the pentafluorophenylacetylide ligand show emission with λ_max at 585–602 nm upon an increase in the complex concentration in solutions. These Pd^II complexes can act as photosensitizers for the light‐induced aerobic oxidation of amines. In the presence of 0.1 mol % Pd^II complex, secondary amines can be oxidized to the corresponding imines with substrate conversions and product yields up to 100 and 99 %, respectively. In the presence of 0.15 mol % Pd^II complex, the oxidative cyanation of tertiary amines could be performed with product yields up to 91 %. The Pd^II complexes have also been used to sensitize photochemical hydrogen production with a three‐component system that comprises the Pd^II complex, [Co(dmgH)₂(py)Cl] (dmgH=dimethylglyoxime; py=pyridine), and triethanolamine, and a maximum turnover of hydrogen production of 175 in 4 h was achieved. The excited‐state electron‐transfer properties of the Pd^II complexes have been examined.     

Understanding the Mechanisms of Unusually Fast HH,CH,and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon–carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co‐workers have demonstrated extremely rapid C?C reductive elimination from cis‐[AuPPh₃(4‐F‐C₆H₄)₂Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy‐atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis‐[Au(PPh₃)₂(4‐F‐C₆H₄)₂]⁺ was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for H?H, C?H, and C?C bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp³ carbon atoms. Metal–carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis‐[AuPPh₃(H)CH₃]⁺ predict that at ?52 °C, about 82 % of the reaction occurs by hydrogen‐atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.     

Nitrene Insertion into CC and CH Bonds of Diamide Diimine Ligands Ligated to Chromium and Iron

The impact of redox non‐innocence (RNI) on chemical reactivity is a forefront theme in coordination chemistry. A diamide diimine ligand, [{‐CH?N(1,2‐C₆H₄)NH(2,6‐iPr₂C₆H₃)}₂]ⁿ (n=0 to ?4), (dadi)ⁿ, chelates Cr and Fe to give [(dadi)M] ([ 1 Cr(thf)] and [ 1 Fe]). Calculations show [ 1 Cr(thf)] (and [ 1 Cr]) to have a d⁴ Cr configuration antiferromagnetically coupled to (dadi)^2?*, and [ 1 Fe] to be S=2. Treatment with RN₃ provides products where RN is formally inserted into the C? C bond of the diimine or into a C? H bond of the diimine. Calculations on the process support a mechanism in which a transient imide (imidyl) aziridinates the diimine, which subsequently ring opens.     

Carboxylate Assisted Ni‐Catalyzed CH Bond Allylation of Benzamides

A one‐step synthetic method was developed for allylation of benzamides using Ni(COD)₂/RCO₂H and [Ni(μ‐H₂O)(OOCCMe₃)₂(HOOCCMe₃)₂]₂ ( A′ ) catalytic system. Efficient, well‐defined, air and moisture‐stable Ni–pivalate complex was isolated and employed in catalytic allylation. The influence of solvent on product selectivity was also investigated.     

Chiral Discrimination in Dimers of Diphosphines PH2PH2 and PH2PHF

A theoretical study of the conformational profile of two diphosphines, PH₂?PH₂ and PH₂?PHF, is carried using second‐order Møller–Plesset perturbation theory (MP2) computational methods. The chiral minima found are used to build homo‐ and heterochiral dimers. Six minima are found for the (PH₂?PH₂)₂ dimers and 27 for the (PH₂?PHF)₂ dimers. Pnicogen and hydrogen bonds, the non‐covalent forces that stabilize the complexes, are characterized by Atoms in Molecules (AIM) and Natural Bond Orbital (NBO) methodologies. Those with several pnicogen bonds are more stable than those with hydrogen bonds. The chirodiastaltic energies amount to a total of 1.77 kJ mol^?1 for the R_a:R_a versus R_a:S_a (PH₂?PH₂)₂ dimers, 0.81 kJ mol^?1 for the RS_a:RS_a versus RS_a:SR_a (PH₂?PHF)₂ dimers, and 2.93 kJ mol^?1 for the RR_a:RR_a versus RR_a:SS_a (PH₂?PHF)₂ dimers. In the first and second cases, the heterochiral complex is favored whereas in the third case, the homochiral complex is favored.     

Cobalt‐Catalyzed C(sp2)H Alkoxylation of Aromatic and Olefinic Carboxamides

The cobalt‐catalyzed alkoxylation of C(sp²)? H bonds in aromatic and olefinic carboxamides has been developed. The reaction proceeded under mild conditions in the presence of Co(OAc)₂?4H₂O as the catalyst and tolerates a wide range of both alcohols and benzamide substrates, including even olefinic carboxamides. In addition, this reaction is the first example of the direct alkoxylation of alkenes through C? H bond activation.     

BH,CH,and BC Bond Activation: The Role of Two Adjacent Agostic Interactions

Tuning the nature of the linker in a L～BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ‐C? H and ε‐B_sp2? H, agostic interactions. Such a unique coordination mode stabilizes a 14‐electron “RuH₂P₂” fragment through connected σ‐bonds of different polarity, and affords selective B? H, C? H, and B? C bond activation as illustrated by reactivity studies with H₂ and boranes.     

BiZn Bond Formation in Liquid Ammonia Solution: [BiZnBi]4−, a Linear Polyanion that is Iso(valence)‐electronic to CO2

Reactions of the zinc(I) complex [Zn₂(Mesnacnac)₂] (Mesnacnac=[(2,4,6‐Me₃C₆H₂)NC(Me)]₂CH) with solid K₃Bi₂ dissolved in liquid ammonia yield crystals of the compound K₄[ZnBi₂]?(NH₃)₁₂ ( 1 ), which contains the molecular, linear heteroatomic [Bi? Zn? Bi]^4? polyanion ( 1 a ). This anion represents the first example of a three‐atomic molecular ion of metal atoms being iso(valence)‐electronic to CO₂ and being synthesized in solution. The analogy of the discrete [Bi? Zn? Bi]^4? anion and the polymeric [(ZnBi_4/2)^4?] unit to monomeric CO₂ and polymeric SiS₂ is rationalized.     

Contrasting electronic requirements for CH binding and CH activation in d6 half‐sandwich complexes of rhenium and tungsten

A computational study of the interaction half‐sandwich metal fragments (metal = Re/W, electron count = d⁶), containing linear nitrosyl (NO⁺), carbon monoxide (CO), trifluorophosphine (PF₃), N‐heterocyclic carbene (NHC) ligands with alkanes are conducted using density functional theory employing the hybrid meta‐GGA functional (M06). Electron deficiency on the metal increases with the ligand in the order NHC < CO < PF₃ < NO⁺. Electron‐withdrawing ligands like NO⁺ lead to more stable alkane complexes than NHC, a strong electron donor. Energy decomposition analysis shows that stabilization is due to orbital interaction involving charge transfer from the alkane to the metal. Reactivity and dynamics of the alkane fragment are facilitated by electron donors on the metal. These results match most of the experimental results known for CO and PF₃ complexes. The study suggests activation of alkane in metal complexes to be facile with strong donor ligands like NHC. © 2015 Wiley Periodicals, Inc.     

Terminal Phosphanido Rhodium Complexes Mediating Catalytic PP and PC Bond Formation

Complexes with terminal phosphanido (M? PR₂) functionalities are believed to be crucial intermediates in new catalytic processes involving the formation of P? P and P? C bonds. We showcase here the isolation and characterization of mononuclear phosphanide rhodium complexes ([RhTp(H)(PR₂)L]) that result from the oxidative addition of secondary phosphanes, a reaction that was also explored computationally. These compounds are active catalysts for the dehydrocoupling of PHPh₂ to Ph₂P? PPh₂. The hydrophosphination of dimethyl maleate and the unactivated olefin ethylene is also reported. Reliable evidence for the prominent role of mononuclear phosphanido rhodium species in these reactions is also provided.     

Intramolecular Aminocyanation of Alkenes by NCN Bond Cleavage

A metal‐free, Lewis acid promoted intramolecular aminocyanation of alkenes was developed. B(C₆F₅)₃ activates N‐sulfonyl cyanamides, thus leading to a formal cleavage of the N? CN bonds in conjunction with vicinal addition of sulfonamide and nitrile groups across an alkene. This method enables atom‐economical access to indolines and tetrahydroquinolines in excellent yields, and provides a complementary strategy for regioselective alkene difunctionalizations with sulfonamide and nitrile groups. Labeling experiments with ¹³C suggest a fully intramolecular cyclization pattern due to the lack of label scrambling in double crossover experiments. Catalysis with Lewis acid is realized and the reaction can be conducted under air.     

High Performance Pd,PdNi, PdSn and PdSnNi Nanocatalysts Supported on Carbon Nanotubes for Electrooxidation of C2C4 Alcohols

Nanocatalysts Pd, Pd₈Ni₂, Pd₈Sn₂ and Pd₈Sn₁Ni₁ supported on multi‐walled carbon nanotubes (MWCNTs) were successively synthesized by the chemical reduction method in the glycol‐water mixture solvent. Transmission electron microscopy results show that the prepared Pd, Pd₈Ni₂, Pd₈Sn₂ and Pd₈Sn₁Ni₁ nanoparticles are uniformly dispersed on the surface of MWCNTs. The average particle sizes of the nanocatalysts are 3.5–3.8 nm. Electroactivity of the prepared catalysts towards oxidation of ethanol, 1‐propanol, 2‐propanol, n‐butanol, iso‐butanol and sec‐butanol (C2? C4 alcohols) in alkaline medium was studied by cyclic voltammetry and chronoamperometry. The current density obtained for the electrooxidation of C2? C4 alcohols depends on the catalysts and the various structures of the alcohols. Addition of Sn or/and Ni to Pd nanoparticles enhances the electroactivity of the Pd/MWCNT catalyst. Furthermore, the ternary Pd₈Sn₁Ni₁/MWCNT catalyst presents the highest electroactivity for the oxidation of C2? C4 alcohols among the prepared catalysts. Electrocatalytic activity order among propanol isomers and butanol isomers is as follows respectively: 1‐propanol > 2‐propanol, and n‐butanol > iso‐butanol > sec‐butanol > tert‐butanol. This is consistent with the Mulliken charge value of the carbon atom bonded with hydroxyl group in the corresponding alcohol molecule.     

Mild and Selective Activation and Substitution of Strong Aliphatic CF Bonds

A procedure for chemoselectively manipulating the strong aliphatic C?F bond with direct transformation into a C?N bond under mild conditions is reported. The activation and subsequent substitution of primary alkyl fluorides is mediated by La[N(SiMe₃)₂]₃, and results in high to excellent yields of tertiary amines. The methodology displays high selectivity towards the C(sp³)?F bond, and a variety of secondary amines are applicable as nucleophiles. Mechanistic investigations reveal a reaction that is first order with respect to [La[N(SiMe₃)₂]₃], [R¹R²NH], and [alkyl fluoride], and a 6‐membered cyclic transition state is proposed. In addition, ¹H NMR spectroscopy shows that La[N(SiMe₃)₂]₃ is the active species involved in the substitution and that protonolysis of the amine, yielding La[NR¹R²]₃, lowers the reactivity.