Synthesis of Annelated Phospholes through Intramolecular C?H Activation by Monovalent Phosphorus

Electrophilic terminal phosphinidene complexes [Ar‐Ar‐P‐W(CO)₅] (Ar‐Ar: biaryl or an analogue thereof) undergo a spontaneous insertion of the phosphorus atom into the vicinal C H bonds to give annelated phospholes. Twelve examples are described, including biphenyl, thienyl, pyrrolyl, and benzofuryl groups as biaryl moieties. The activation energy of the insertion reaction is quite low (about 2 kcal mol⁻¹).     

Non‐biaryl atropisomers. part 1. configurationally stable 1,2‐diaryl‐3‐methyl‐1,4,5,6‐tetrahydropyrimidinium salts

In the present communication we describe two examples of a new kind of configurationally stable non‐biaryl atropisomers in which the Ar‐N bond is the chiral axis, namely 1‐(o‐nitrophenyl)‐2‐aryl‐3‐methyl‐1,4,5,6‐tetrahydropyrimidinium iodides 1. Stereochemical features of such compounds are analyzed on the basis of their ¹H and ¹³C one‐ and two‐dimensional nmr spectra. A comparison is made with the corresponding amidines 2 .     

Nitric Oxide Insertion Reactivity with the Bismuth–Carbon Bond: Formation of the Oximate Anion, [ON(C6H2tBu2O)]−, from the Oxyaryl Dianion, (C6H2tBu2O)2−

The first example of NO insertion into a Bi?C bond has been found in the direct reaction of NO with a Bi³⁺ complex of the unusual (C₆H₂tBu₂‐3,5‐O‐4)^2? oxyaryl dianionic ligand, namely, Ar′Bi(C₆H₂tBu₂‐3,5‐O‐4) [Ar′=2,6‐(Me₂NCH₂)₂C₆H₃] ( 1 ). The oximate complexes [Ar′Bi(ONC₆H₂‐3,5‐tBu₂‐4‐O)]₂(μ‐O) ( 3 ) and Ar′Bi(ONC₆H₂‐3,5‐tBu₂‐4‐O)₂ ( 4 ) were formed as a mixture, but can be isolated in pure form by reaction of NO with a Bi³⁺ complex of the [O₂C(C₆H₂tBu₂‐3‐5‐O‐4]^2? oxyarylcarboxy dianion, namely, Ar′Bi[O₂C(C₆H₂tBu₂‐3‐5‐O‐4)‐κ²O,O’]. Reaction of 1 with Ph₃CSNO gave an oximate product with (Ph₃CS)^1? as an ancillary ligand, (Ph₃CS)(Ar′)Bi(ONC₆H₂‐3,5‐tBu₂‐4‐O) ( 5 ).     

Y,Lu, and Gd Complexes of NCO/NCS Pincer Ligands: Synthesis,Characterization, and Catalysis in the cis‐1,4‐Selective Polymerization of Isoprene

A series of NCO/NCS pincer precursors, 3‐(Ar²OCH₂)‐2‐Br‐(Ar¹N?CH)C₆H₃ ((^Ar1NCO^Ar2)Br, 3a , 3b , 3c , 3d ) and 3‐(2,6‐Me₂C₆H₃SCH₂)‐2‐Br‐(Ar¹N?CH)C₆H₃ ((^Ar1NCS^Me)Br, 4a and 4b ) were synthesized and characterized. The reactions of [^Ar1NCO^Ar2]Br/ [^Ar1NCS^Me]Br with nBuLi and the subsequent addition of the rare‐earth‐metal chlorides afforded their corresponding rare‐earth‐metal–pincer complexes, that is, [(^Ar1NCO^Ar2)YCl₂(thf)₂] ( 5a , 5b , 5c , 5d ), [(^Ar1NCO^Ar2)LuCl₂(thf)₂] ( 6a , 6d ), [(^Ar1NCO^Ar2)GdCl₂(thf)₂] ( 7 ), [{(^Ar1NCS^Me)Y(μ‐Cl)}₂{(μ‐Cl)Li(thf)₂(μ‐Cl)}₂] ( 8 , 9 ), and [{(^Ar1NCS^Me)Gd(μ‐Cl)}₂{(μ‐Cl)Li(thf)₂(μ‐Cl)}₂] ( 10 , 11 ). These diamagnetic complexes were characterized by ¹H and ¹³C NMR spectroscopy and the molecular structures of compounds 5a , 6a , 7 , and 10 were well‐established by X‐ray diffraction analysis. In compounds 5a , 6a , and 7 , all of the metal centers adopted distorted pentagonal bipyramidal geometries with the NCO donors and two oxygen atoms from the coordinated THF molecules in equatorial positions and the two chlorine atoms in apical positions. Complex 10 is a dimer in which the two equal moieties are linked by two chlorine atoms and two Cl? Li? Cl bridges. In each part, the gadolinium atom adopts a distorted pentagonal bipyramidal geometry. Activated with alkylaluminum and borate, the gadolinium and yttrium complexes showed various activities towards the polymerization of isoprene, thereby affording highly cis‐1,4‐selective polyisoprene, whilst the NCO? lutetium complexes were inert under the same conditions.     

Mechanism of Palladium‐Catalyzed Suzuki–Miyaura Reactions: Multiple and Antagonistic Roles of Anionic “Bases” and Their Countercations

In Suzuki–Miyaura reactions, anionic bases F^? and OH^? (used as is or generated from CO₃^2? in water) play multiple antagonistic roles. Two are positive: 1) formation of trans‐[Pd(Ar)F(L)₂] or trans‐[Pd(Ar)‐ (L)₂(OH)] (L=PPh₃) that react with Ar′B(OH)₂ in the rate‐determining step (rds) transmetallation and 2) catalysis of the reductive elimination from intermediate trans‐[Pd(Ar)(Ar′)(L)₂]. Two roles are negative: 1) formation of unreactive arylborates (or fluoroborates) and 2) complexation of the OH group of [Pd(Ar)(L)₂(OH)] by the countercation of the base (Na⁺, Cs⁺, K⁺).     

Simultaneous Induction of Axial and Planar Chirality in Arene–Chromium Complexes by Molybdenum‐Catalyzed Enantioselective Ring‐Closing Metathesis

The molybdenum‐catalyzed asymmetric ring‐closing metathesis of the various C_s‐symmetric (π‐arene)chromium substrates provides the corresponding bridged planar‐chiral (π‐arene)chromium complexes in excellent yields with up to >99 % ee. With a bulky and unsymmetrical substituent, such as N‐indolyl or 1‐naphthyl, at the 2‐positions of the η⁶‐1,3‐diisopropenylbenzene ligands, both biaryl‐based axial chirality and π‐arene‐based planar chirality are simultaneously induced in the products. The axial chirality is retained even after the removal of the dicarbonylchromium fragment, and the chiral biaryl/heterobiaryl compounds are obtained with complete retention of the enantiopurity.     

Highly symmetric single nickel catalysts bearing bulky bis(α‐diimine) ligand: Synthesis,characterization, and electron‐effects on copolymerization of norbornene with 1‐alkene at elevated temperarure

Three new three‐dimensional geometry bulky α‐diimine ligands ( L ) containing electron‐donating and electron‐withdrawing groups, 9,10‐dihydro‐9,10‐ethanoanthracene‐11,12‐di(Ar)imine (Ar = p‐PhCH₃, L1 ; Ar=p‐PhCl, L2 ; Ar=p‐PhCF₃, L3 .), and their corresponding single Ni(II) catalysts, NiL₂Br₂ ( Ni(L1)₂Br₂ , Ni(L2)₂Br₂ , and Ni(L3)₂Br₂ , were synthesized and the molecular structure were determined by X‐ray crystallography. All NiL₂Br₂ catalysts were tested for norbornene polymerization and copolymerization of norbornene with 1‐alkene after activation with B(C₆F₅)₃. The results that the polymerization catalytic activities for norbornene up to 10⁵ g_polymer/mol_Ni·h even at 140 °C, shown that NiL₂Br₂ catalysts have high thermal stability. Meanwhile, catalysts with electron‐withdrawing groups could achieve higher reactivity. The obtained poly(NB‐co‐1‐alkene)s were confirmed to be vinyl‐addition copolymers and noncrystalline. All copolymers exhibited high 1‐alkenes insertion ratio, good thermal stability (T_d > 375 °C), high molecular weight (up to 10⁵ g/mol), good solubility in common organic solvents and could be processed into films with good transparency in the visible region. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3495–3505     

Pd‐Catalyzed Carbonylative α‐Arylation of Aryl Bromides: Scope and Mechanistic Studies

Reaction conditions for the three‐component synthesis of aryl 1,3‐diketones are reported applying the palladium‐catalyzed carbonylative α‐arylation of ketones with aryl bromides. The optimal conditions were found by using a catalytic system derived from [Pd(dba)₂] (dba=dibenzylideneacetone) as the palladium source and 1,3‐bis(diphenylphosphino)propane (DPPP) as the bidentate ligand. These transformations were run in the two‐chamber reactor, COware, applying only 1.5 equivalents of carbon monoxide generated from the CO‐releasing compound, 9‐methylfluorene‐9‐carbonyl chloride (COgen). The methodology proved adaptable to a wide variety of aryl and heteroaryl bromides leading to a diverse range of aryl 1,3‐diketones. A mechanistic investigation of this transformation relying on ³¹P and ¹³C NMR spectroscopy was undertaken to determine the possible catalytic pathway. Our results revealed that the combination of [Pd(dba)₂] and DPPP was only reactive towards 4‐bromoanisole in the presence of the sodium enolate of propiophenone suggesting that a [Pd(dppp)(enolate)] anion was initially generated before the oxidative‐addition step. Subsequent CO insertion into an [Pd(Ar)(dppp)(enolate)] species provided the 1,3‐diketone. These results indicate that a catalytic cycle, different from the classical carbonylation mechanism proposed by Heck, is operating. To investigate the effect of the dba ligand, the Pd⁰ precursor, [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)], was examined. In the presence of DPPP, and in contrast to [Pd(dba)₂], its oxidative addition with 4‐bromoanisole occurred smoothly providing the [PdBr(Ar)(dppp)] complex. After treatment with CO, the acyl complex [Pd(CO)Br(Ar)(dppp)] was generated, however, its treatment with the sodium enolate led exclusively to the acylated enol in high yield. Nevertheless, the carbonylative α‐arylation of 4‐bromoanisole with either catalytic or stoichiometric [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)] over a short reaction time, led to the 1,3‐diketone product. Because none of the acylated enol was detected, this implied that a similar mechanistic pathway is operating as that observed for the same transformation with [Pd(dba)₂] as the Pd source.     

Synthesis of 2‐Aryl‐ and 2‐Heteroaryl‐3,5‐dimethoxy‐1,4‐benzoquinones Involving Pd‐Catalyzed Cross‐Coupling of (2,3,4,6‐Tetramethoxyphenyl)boronic Acid

A series of 2‐aryl‐ and 2‐heteroaryl‐substituted 3,5‐dimethoxy‐1,4‐benzoquinones (compounds 27 – 36 ) have been synthesized by cross‐coupling of (2,3,4,6‐tetramethoxyphenyl)boronic acid ( 2 ) with aromatic bromides or iodides in the presence of [Pd⁰(Ph₃)₄] and Na₂CO₃, followed by AgO‐promoted oxidation of the resulting biaryl compounds 17 – 26 .     

Buchwald–Hartwig Amination of Nitroarenes

The Buchwald–Hartwig amination of nitroarenes was achieved for the first time by using palladium catalysts bearing dialkyl(biaryl)phosphine ligands. These cross‐coupling reactions of nitroarenes with diarylamines, arylamines, and alkylamines afforded the corresponding substituted arylamines. A catalytic cycle involving the oxidative addition of the Ar−NO₂ bond to palladium(0) followed by nitrite/amine exchange is proposed based on a stoichiometric reaction.     

Three Roles for the Fluoride Ion in Palladium‐Catalyzed Hiyama Reactions: Transmetalation of [ArPdFL2] by Ar′Si(OR)3

From the kinetic data on the transmetalation/reductive elimination in fluoride‐promoted Hiyama reactions, obtained using electrochemical techniques, it has been established that fluoride ions play three roles. F^? reacts with trans‐[ArPdBrL₂] (L=PPh₃) to form trans‐[ArPdFL₂], which reacts with Ar′Si(OMe)₃ in the rate‐determining transmetalation, whereas trans‐[ArPdBrL₂] does not react with Ar′Si(OMe)₃. F^? reacts with Ar′Si(OMe)₃ to deliver the unreactive silicate Ar′SiF(OMe)₃^?, thus leading to two antagonistic kinetic effects. In addition, F^? catalyzes the reductive elimination from intermediate trans‐[ArPdAr′L₂].     

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate

Hydration has a drastic impact on the structure and function of flexible biomolecules, such as aromatic ethylamino neurotransmitters. The structure of monohydrated protonated phenylethylamine (H⁺PEA?H₂O) is investigated by infrared photodissociation (IRPD) spectroscopy of cold cluster ions by using rare‐gas (Rg=Ne and Ar) tagging and dispersion‐corrected density functional theory calculations at the B3LYP‐D3/aug‐cc‐pVTZ level. Monohydration of this prototypical neurotransmitter gives an insight into the first step of the formation of its solvation shell, especially regarding the competition between intra‐ and intermolecular interactions. The spectra of Rg‐tagged H⁺PEA?H₂O reveal the presence of a stable insertion structure in which the water molecule is located between the positively charged ammonium group and the phenyl ring of H⁺PEA, acting both as a hydrogen bond acceptor (NH⁺???O) and donor (OH???π). Two other nearly equivalent isomers, in which water is externally H bonded to one of the free NH groups, are also identified. The balance between insertion and external hydration strongly depends on temperature.     

On the Triple Role of Fluoride Ions in Palladium‐Catalyzed Stille Reactions

The mechanism of Stille reactions (cross‐coupling of ArX with Ar′SnnBu₃) performed in the presence of fluoride ions is established. A triple role for fluoride ions is identified from kinetic data on the rate of the reactions of trans‐[ArPdBr(PPh₃)₂] (Ar=Ph, p‐(CN)C₆H₄) with Ar′SnBu₃ (Ar′=2‐thiophenyl) in the presence of fluoride ions. Fluoride ions promote the rate‐determining transmetallation by formation of trans‐[ArPdF(PPh₃)₂], which reacts with Ar′SnBu₃ (Ar′=Ph, 2‐thiophenyl) at room temperature, in contrast to trans‐[ArPdBr(PPh₃)₂], which is unreactive. However, the concentration ratio [F^?]/[Ar′SnBu₃] must not be too high, because of the formation of unreactive anionic stannate [Ar′Sn(F)Bu₃]^?. This rationalises the two kinetically antagonistic roles exerted by the fluoride ions that are observed experimentally, and is found to be in agreement with the kinetic law. In addition, fluoride ions promote reductive elimination from trans‐[ArPdAr′(PPh₃)₂] generated in the transmetallation step.     

Synthesis and characterization of aluminum(III) and tin(II) complexes supported by diiminophosphinate ligands and their application in ring‐opening polymerization catalysis of ε‐caprolactone

A series of Al(III) and Sn(II) diiminophosphinate complexes have been synthesized. Reaction of Ph(ArCH₂)P(?NBu^t)NHBu^t (Ar = Ph, 3 ; Ar = 8‐quinolyl, 4 ) with AlR₃ (R = Me, Et) gave aluminum complexes [R₂Al{(NBu^t)₂P(Ph)(CH₂Ar)}] (R = Me, Ar = Ph, 5 ; R = Me, Ar = 8‐quinolyl, 6 ; R = Et, Ar = Ph, 7 ; R = Et, Ar = quinolyl, 8 ). Lithiated 3 and 4 were treated with SnCl₂ to afford tin(II) complexes [ClSn{(NBu^t)₂P(Ph)(CH₂Ar)}] (Ar = Ph, 9 ; Ar = 8‐quinolyl, 10 ). Complex 9 was converted to [(Me₃Si)₂NSn{(NBu^t)₂P(Ph)(CH₂Ph)}] ( 11 ) by treatment with LiN(SiMe₃)₂. Complex 11 was also obtained by reaction of 3 with [Sn{N(SiMe₃)₂}₂]. Complex 9 reacted with [LiOC₆H₄Bu^t‐4] to yield [4‐Bu^tC₆H₄OSn{(NBu^t)₂P(Ph)(CH₂Ph)}] ( 12 ). Compounds 3–12 were characterized by NMR spectroscopy and elemental analysis. The structures of complexes 6 , 10 , and 11 were further characterized by single crystal X‐ray diffraction techniques. The catalytic activity of complexes 5–8 , 11 , and 12 toward the ring‐opening polymerization of ε‐caprolactone (CL) was studied. In the presence of BzOH, the complexes catalyzed the ring‐opening polymerization of ε‐CL in the activity order of 5 > 7 ≈ 8 > 6 ? 11 > 12 , giving polymers with narrow molecular weight distributions. The kinetic studies showed a first‐order dependency on the monomer concentration in each case. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4621–4631, 2006     

Halogenomercury Salts of Sterically Crowded Triazenides – Convenient Starting Materials for Redox‐Transmetallation Reactions

Diaryl‐substituted triazenides Ar(Ar′)N₃HgX [Ar/Ar′ = Dmp/Mph, X = Cl ( 2a ), Br ( 3a ), I ( 4a ); Ar/Ar′ = Dmp/Tph, X = Cl ( 2b ), I ( 4b ) with Mph = 2‐MesC₆H₄, Mes = 2,4,6‐Me₃C₆H₂, Tph = 2′,4′,6′‐triisopropylbiphenyl‐2‐yl and Dmp = 2,6‐Mes₂C₆H₃] were synthesized by salt‐metathesis reactions in ethyl ether from the readily available starting materials Ar(Ar′)N₃Li and HgX₂. These compounds may be used for redox‐transmetallation reactions with rare‐earth or alkaline earth metals. Thus, reaction of 4b or 2b with magnesium or ytterbium in tetrahydrofuran afforded the triazenides Dmp(Tph)N₃MX(thf) ( 5b : M = Mg, X = I; 6b : M = Yb, X = Cl) in good yield. All new compounds were characterized by melting point, ¹H and ¹³C NMR spectroscopy and for selected species by IR spectroscopy or mass spectrometry. In addition, the solid‐state structures of triazenides 2a , 2b , 3a , 4b , 5b and 6b were investigated by single‐crystal X‐ray diffraction.     

Crystalline Radicals Derived from Classical N‐Heterocyclic Carbenes

One‐electron reduction of C2‐arylated 1,3‐imidazoli(ni)um salts (IPr^Ar)Br (Ar=Ph, 3 a ; 4‐DMP, 3 b ; 4‐DMP=4‐Me₂NC₆H₄) and (SIPr^Ar)I (Ar=Ph, 4 a ; 4‐Tol, 4 b ) derived from classical NHCs (IPr=:C{N(2,6‐iPr₂C₆H₃)}₂CHCH, 1 ; SIPr=:C{N(2,6‐iPr₂C₆H₃)}₂CH₂CH₂, 2 ) gave radicals [(IPr^Ar)]^. (Ar=Ph, 5 a ; 4‐DMP, 5 b ) and [(SIPr^Ar)]^. (Ar=Ph, 6 a ; 4‐Tol, 6 b ). Each of 5 a , b and 6 a , b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid‐state characterization of NHC‐derived carbon‐centered radicals 6 a , b by single‐crystal X‐ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group.     

Donor‐Induced Helical Inversion of 1,1′‐Binaphthyl Connecting with Two Molybdenum Complexes

An atropisomeric biaryl molecule with a given absolute configuration could present two opposite helical conformations through the rotation around C? C single bond. To the best of our knowledge, the biaryl system is the simplest helical inversion model apart from stereomutation between two enantiomers. Herein, we first report such true helical inversion phenomena of biaryl compounds. Two [Mo^VIO₂(L)]‐type complexes, in which L is a tridentate dioxoanionic pyridine O,N,O‐ligand, are coalesced on the 2,2′,3,3′‐positions of an (R)‐1,1′‐binaphthyl unit and an intramolecular dioxo bridge is formed by two Mo?O???Mo interactions. Exterior strong donors can coordinate to molybdenum to interrupt this dioxo bridge and inversions from negative to positive chirality are explicitly observed by circular dichroism spectroscopy, consistent with single‐crystal X‐ray diffraction analyses.     

Synthesis of Elusive Chloropnictenium Ions

This work describes the synthesis and full characterization of elusive chloropnictenium ion salts of the type [^RAr*N(SiMe)ECl][A] (^RAr*=2,6‐(CHPh₂)‐4‐R‐C₆H₂, R=Me, tBu; E=Sb, Bi; A^?=GaCl₄, Al(OCH(CF₃)₂)₄). In these species the cation is significantly stabilized by weak arene interactions to flanking phenyl groups of the ^RAr* moiety. In this context the bonding situation has been studied by computational means and the reactivity towards the Lewis base 4‐dimethylaminopyridine (dmap) was investigated.     

Reductive Dehydrogenation of a Stannane via Multiple Sn−H Activation by Frustrated Lewis Pairs

A bulky substituted stannane Ar*SnH₃ (Ar*=2,6‐(2′,4′,6′‐triisopropylphenyl)phenyl) was treated with the well‐known frustrated Lewis pair (FLP) Pt Bu₃/B(C₆F₅)₃ in varying stoichiometries. To some degree, hydride abstraction and adduct formation is observed, leading to [Ar*SnH₂(Pt Bu₃)]⁺ which is rather unreactive toward further dehydrogenation. In a competing process, the FLP proved to be capable of completely striping‐off hydrogen and hydrides to generate the first cationic phosphonio‐stannylene [Ar*Sn(Pt Bu₃)]⁺. This behavior provides insight into the activation/abstraction mechanism processes involved in these Group 14 hydride derivatives.     

Redox Behaviour of Cymantrene Fischer Carbene Complexes in Designing Organometallic Multi‐tags

A series of Group 7 Fischer carbene complexes, [Cp(CO)₂Mn^I=C(OEt)Ar] (Cp=cyclopentadienyl, Ar=Th=thienyl ( 1 a ), Ar=Fu=furyl ( 2 a ), Ar=Fc=ferrocenyl ( 3 a )) and biscarbene complexes, [Cp(CO)₂Mn?C(OEt)?Ar′?(OEt)C?Mn(CO)₂Cp] (Ar′=Th′=2,5‐thienylene ( 1 b ), Ar′=Fu′=2,5‐furylene ( 2 b ), Ar′=Fc′=1,1′‐ferrocendiyl ( 3 b )) was synthesized and characterized. Chemical oxidation of [Cp(CO)₂Mn?C(OEt)Fc] ( 3 a ) and isolation of the oxidised species [3 a][PF₆] possessing a Mn^II centre proved possible below ?30 °C in dichloromethane solution. The ESR spectrum of the transiently stable radical cation, [3 a][PF₆] , confirmed the presence of a low‐spin Mn^II centre characterized by a rhombic g tensor (g_x=1.975, g_y=2.007 and g_z=2.130) in frozen dichloromethane at 77 K with ⁵⁵ Mn hyperfine coupling constants A₁, A₂ and A₃ of 115, 33 and 43 G, respectively. Electrochemical studies demonstrated the influence of the Ar substituent on the oxidation potential. All complexes showed that the redox potentials of carbene double bond reduction and Mn^I oxidation were dependent on the type of Ar group, but only 3 b showed resolved oxidations for the two Mn^I centres. Surprisingly, Mn^I oxidation occurs at lower potentials than ferrocenyl oxidation. Density functional theory (DFT) calculations were carried out to delineate the nature of the species involved in the oxidation and reduction processes and clearly confirm that oxidation of Mn^I is favoured over that of ferrocene.