Phosphine-Catalyzed [4+1] Cycloadditions of Allenes with Methyl Ketimines,Enamines, and a Primary Amine

Unprecedented phosphine-catalyzed [4+1] cycloadditions of allenyl imides have been discovered using various N-based substrates including methyl ketimines, enamines, and a primary amine. These transformations provide a one-pot access to cyclopentenoyl enamines and imines, or (chiral) γ-lactams through two geminal C−C bond or two C−N bond formations, respectively. Several P-based key intermediates including a 1,4-(bis)electrophilic α,β-unsaturated ketenyl phosphonium species have been detected by ³¹P NMR and HRMS analyses, which shed light on the postulated catalytic cycle. The synthetic utility of this new chemistry has been demonstrated through a gram-scaling up of the catalytic reaction as well as regioselective hydrogenation and double condensation to form cyclopentanoyl enamines and fused pyrazole building blocks, respectively.     

The diiodine basicity scale: toward a general halogen-bond basicity scale

The new diiodine basicity scale pK_BI2 is quasi‐orthogonal to most known Lewis basicity scales (hydrogen‐bond, dative‐bond and cation basicity scales). The diiodine basicity falls in the sequence N>P≈Se>S>I≈O>Br>Cl>F for the iodine‐bond acceptor atomic site and SbO≈NO≈AsO>SeO>PO>SO>C?O>? O? >SO₂ or PS?? S? >C?S?N?C?S for the functionality of oxygen or sulfur bases. Substituent effects are quantified through linear free energy relationships, which allow the calculation of individual complexation constants for each site of polybases and thus the classification of aromatic ethers as carbon π bases and of aromatic amines, thioethers and selenoethers as N, S and Se bases, respectively. The pK_BI2 values of nBu₃N⁺‐N^?C≡N, 2‐aminopyridine and 1,10‐phenanthroline reveal a superbasic nitrile, a hydrogen‐bond‐assisted iodine bond and a two‐centre iodine bond, respectively. The diiodine basicity scale is a general inorganic but family‐dependent organic halogen‐bond basicity scale because organic halogen‐bond donors such as IC≡N and ICF₃ have a stronger electrostatic character than I₂. The family independence can be restored by the addition of an electrostatic parameter, either the experimental pK_BHX hydrogen‐bond basicity scale or the computed minimum electrostatic potential.     

Synthesis of Substituted 2‐Amino‐1,3‐oxazoles via Copper‐Catalyzed Oxidative Cyclization of Enamines and N,N‐Dialkyl Formamides

A facile synthesis of 2‐amino‐1,3‐oxazoles via Cu^I‐catalyzed oxidative cyclization of enamines and N ,N ‐dialkyl formamides has been developed. The reaction proceeds through an oxidative C−N bond formation, followed by an intramolecular C(sp²)−H bond functionalization/C−O cyclization in one pot. This protocol provides direct access to useful 2‐amino‐1,3‐oxazoles and features protecting‐group‐free nitrogen sources, readily available starting materials, a broad substrate scope and mild reaction conditions.     

Study of E/Z isomerization of (arylamino)methylidenefuran‐2(3H)‐ones by 1H, 13C, 15N spectroscopy and DFT calculations in different solvents

The structure and configuration of the series of previously unknown arylaminomethylidenefuran‐2(3H)‐ones have been determined in solution by ¹H, ¹³C, ¹⁵N nuclear magnetic resonance spectroscopy including two‐dimensional experiments such as ¹H─¹H COSY, dqCOSY, ¹H─¹³C HSQC, ¹H─¹³C HMBC. It was found that synthesized substances exist as an equilibrium mixture of E‐ and Z‐enamines in solution. It was established on the basis of density functional theory calculations that the exchange between the two push–pull enamines is a simple rotation around an exocyclic partial double bond that depends on the effect of the solvents. Copyright © 2017 John Wiley & Sons, Ltd.     

NMR investigation of chlorophosphorylation products of N‐vinylazoles

The structure of novel phosphorus‐containing N‐vinylazoles prepared by action of phosphorus pentachloride has been studied by multinuclear ¹H, ¹³C, ¹⁵N, ³¹P and two‐dimensional (2D) NMR spectroscopy. N‐vinyl‐substituted 1,2‐diazoles and 1,2,3‐triazoles have undergone phosphorylation, exclusively, on double bond. N‐vinylazoles‐based hexa‐coordinated phosphorus compounds have been synthesized for the first time. ³¹P NMR spectroscopy provides the most convenient and unambiguous method for the investigation of E—Z‐isomeric structures of phosphorylated enamines. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrogen‐Bond Cooperativity in Formamide2–Water: A Model for Water‐Mediated Interactions

The rotational spectrum of formamide₂–H₂O formed in a supersonic jet has been characterized by Fourier‐transform microwave spectroscopy. This adduct provides a simple model of water‐mediated interaction involving the amide linkages, as occur in protein folding or amide‐association processes, showing the interplay between self‐association and solvation. Mono‐substituted ¹³C, ¹⁵N, ¹⁸O, and ²H isotopologues have been observed and their data used to investigate the structure. The adduct forms an almost planar three‐body sequential cycle. The two formamide molecules link on one side through an N?H???O hydrogen bond and on the other side through a water‐mediated interaction with the formation of C=O???H?O and O???H?N hydrogen bonds. The analysis of the quadrupole coupling effects of two ¹⁴N‐nuclei reveals the subtle inductive forces associated to cooperative hydrogen bonding. These forces are involved in the changes in the C=O and C?N bond lengths with respect to pure formamide.     

Application de la Résonance Magnétique de l'Azote 15 à l'Etude de la Délocalisation Electronique dans les Anilines,Aminopyridines et Enamines

¹⁵N spectroscopy at natural abundance has been applied to the study of electronic delocalization in the N? C bond of a series of substituted anilines. The relationships which offer a good way for evaluating the extent of electronic delocalization from ¹⁵N chemical shift data in anilines have been extended to aminopyridines, aminopyrimidines and nucleosides. The results obtained for C? N bonds in the case of anilines and aliphatic enamines are consistent with those concerning amides and analogous compounds. The validity of ΔG^≠-δ¹⁵N relationships for predicting π delocalization is discussed in terms of the relative contribution of δ- or π-density and non-bonded atom interactions to the total barrier height.     

Vacancy‐Rich Ni(OH)2 Drives the Electrooxidation of Amino C−N Bonds to Nitrile C≡N Bonds

Electrochemical synthesis based on electrons as reagents provides a broad prospect for commodity chemical manufacturing. A direct one‐step route for the electrooxidation of amino C?N bonds to nitrile C≡N bonds offers an alternative pathway for nitrile production. However, this route has not been fully explored with respect to either the chemical bond reforming process or the performance optimization. Proposed here is a model of vacancy‐rich Ni(OH)₂ atomic layers for studying the performance relationship with respect to structure. Theoretical calculations show the vacancy‐induced local electropositive sites chemisorb the N atom with a lone pair of electrons and then attack the corresponding N(sp³)?H, thus accelerating amino C?N bond activation for dehydrogenation directly into the C≡N bond. Vacancy‐rich nanosheets exhibit up to 96.5 % propionitrile selectivity at a moderate potential of 1.38 V. These findings can lead to a new pathway for facilitating catalytic reactions in the chemicals industry.     

Unprecedented Ring–Ring Interconversion of N,P,C‐Cage Ligands

The novel N,P,C‐cage complexes 5 a – f and 6 a – f have been obtained by the reaction of the P‐pentamethylcyclopentadienylphosphinidene complex 2 , generated thermally from 2H‐azaphosphirene complex 1 , with N‐methyl‐C‐arylcarbaldimines 3 a – f . Li/Cl phosphinidenoid complex 8 reacted with 3 a , b to give N,P,C‐cage complexes 6 a , b , whereas with 3 c – f , complexes 6 c – f were obtained in negligible amounts only. Both types of ligand N,P,C‐cage structures 5 and 6 were found to be in an unprecedented equilibrium, with 5 a , f as the predominant species. Transient electrophilic terminal phosphinidene complexes 10 a – f serve as intermediates in both ligand interconversions ( 5 a , f ? 6 a , f ), as evidenced through trapping reactions with phenylacetylene and N‐methyl‐C‐phenylcarbaldimine, thus leading to the novel N,P,C‐cage complexes 13 b and 15 . DFT calculations predicted a small difference in the relative energies of the two types of N,P,C‐cage ligands, and a remarkable stabilisation of the aminophosphinidene complex 10 as the common precursor, thereby providing an insight into this surprising 5‐ring–3‐ring interconversion. In depth analysis of intermediate 10 revealed the occurrence of both through‐bond (conventional inductive/mesomeric effects) and through‐space (non‐covalent interactions) mechanisms, which amount to 67.8 and 14.4 kcal mol^?1, respectively, and account for the remarkable stabilisation of this intermediate.     

Catalytic Reduction of Cyclic Ethers with Hydrosilanes

Catalytic reductive transformations of ethers as a synthetic building block are an important class of chemical reactions because a range of essential chemical feedstocks and fuels in contemporary life can be prepared through the key step of ethereal C?O bond cleavage of cellulosic biomass. Although conventional stoichiometric and catalytic methods for sp²‐ and sp³‐C?O bond cleavage of linear ethers and alcohols with hydrosilanes are well established, silylative ring opening of cyclic ethers has been less highlighted in this context. This review outlines catalytic systems for the silylative reduction of a range of cyclic ethers, including epoxides and sugars, leading to the corresponding alcohols and/or hydrocarbons. The chemical reactivity and selectivity of these ring‐opening catalytic processes are discussed with respect to the type of substrates; the representative catalytic working modes are also described.     

C−N,C−S and S−S Bond Cleavage by Rhodium PCcarbeneP Pincer Complexes

Rhodium PC_carbeneP complexes 1‐L {L=PPh₃, PPh₂(C₆F₅)} react with isothiocyanate, carbodiimide and disulphide to enable C?S, C?N and S?S bond cleavage. The cleaved molecules are sequestered by the metal center and the pincer alkylidene linkage, forming η²‐coordinated sulfide or imide centered pincer complexes. When a C?S or S?S bond is cleaved, the resulting complexes can bridge two rhodium centers through sulphur forming dimeric complexes and eliminating a monodentate phosphine ligand.     

n-π-Interaction in Enamines and Enaminoketones. A 15N-NMR. Study

¹⁵N-Chemical shifts of 32 enamines, 11 enaminoketones and 28 closely related amines have been determined with the isotope in natural abundance. In order to eliminate substituent effects, differential chemical shifts Δδ(N) are defined as δ_N(amine)-δ_N(enamine). This parameter is shown to correlate well with the free enthalpy of activation ΔG# for restricted rotation about the N? C(α) bond in enamines with extended conjugation. Δδ(N) values of substituted anilinostyrenes correlate also with ¹³C-chemical shifts of the β-carbon in the enamine system and with Hammett σ-constants of the aniline substituents. The experimental results suggest that differential ¹⁵N shifts are a useful probe to study n, π-interaction in enamines.     

Catalytic Alkyne Arylation Using Traceless Directing Groups

By using Pd⁰/Mandyphos, we achieved a three‐component aminoarylation of alkynes to generate enamines, which are then hydrolyzed to either α‐arylphenones or α,α‐diarylketones. This Pd‐catalyzed method overcomes established known pathways to enable the use of amines as traceless directing groups for C?C bond formation.     

Palladium‐Catalyzed Intermolecular Oxidative Coupling Reactions of (Z)‐Enamines with Isocyanides through Selective β‐C(sp2)‐H and/or C=C Bond Cleavage

Herein, two efficient palladium‐catalyzed intermolecular oxidative coupling reactions of (Z)‐enamines with isocyanides via selective β‐C(sp²)‐H and/or C=C bond cleavage have been developed, leading to controllable chemodivergent and stereoselective construction of a wide range of (E)‐β‐carbamoylenamine derivatives containing strong intramolecular hydrogen bonds. Furthermore, possible reaction pathways for these transformations are proposed on the basis of preliminary mechanism studies.     

A Two‐Coordinate Cobalt(II) Imido Complex with NHC Ligation: Synthesis,Structure, and Reactivity

The synthesis, structural characterization, and reactivity of the first two‐coordinate cobalt complex featuring a metal–element multiple bond [(IPr)Co(NDmp)] ( 4 ; IPr=1,3‐bis(2′,6′‐diisopropylphenyl)imidazole‐2‐ylidene; Dmp=2,6‐dimesitylphenyl) is reported. Complex 4 was prepared from the reaction of [(IPr)Co(η²‐vtms)₂] (vtms=vinyltrimethylsilane) with DmpN₃. An X‐ray diffraction study revealed its linear C? Co? N core and a short Co? N distance (1.691(6) Å). Spectroscopic characterization and calculation studies indicated the high‐spin nature of 4 and the multiple‐bond character of the Co? N bond. Complex 4 effected group‐transfer reactions to CO and ethylene to form isocyanide and imine, respectively. It also facilitated E? H (E=C, Si) σ‐bond activation of terminal alkyne and hydrosilanes to produce the corresponding cobalt(II) alkynyl and cobalt(II) hydride complexes as 1,2‐addition products.     

Palladium‐Catalyzed Cross‐Coupling of Alkenyl Carboxylates

Carboxylate esters have many desirable features as electrophiles for catalytic cross‐coupling: they are easy to access, robust during multistep synthesis, and mass‐efficient in coupling reactions. Alkenyl carboxylates, a class of readily prepared non‐aromatic electrophiles, remain difficult to functionalize through cross‐coupling. We demonstrate that Pd catalysis is effective for coupling electron‐deficient alkenyl carboxylates with arylboronic acids in the absence of base or oxidants. Furthermore, these reactions can proceed by two distinct mechanisms for C?O bond activation. A Pd^0/II catalytic cycle is viable when using a Pd⁰ precatalyst, with turnover‐limiting C?O oxidative addition; however, an alternative pathway that involves alkene carbopalladation and β‐carboxyl elimination is proposed for Pd^II precatalysts. This work provides a clear path toward engaging myriad oxygen‐based electrophiles in Pd‐catalyzed cross‐coupling.     

Ruthenium Catalyzed C−H Acylmethylation of N‐Arylphthalazine‐1,4‐diones with α‐Carbonyl Sulfoxonium Ylides: Highway to Diversely Functionalized Phthalazino‐fused Cinnolines

A direct ortho‐Csp²‐H acylmethylation of 2‐aryl‐2,3‐dihydrophthalazine‐1,4‐diones with α‐carbonyl sulfoxonium ylides is achieved through a Ru^II‐catalyzed C?H bond activation process. The protocol featured high functional group tolerance on the two substrates, including aryl‐, heteroaryl‐, and alkyl‐substituted α‐carbonyl sulfoxonium ylides. Thereafter, 2‐(ortho‐acylmethylaryl)‐2,3‐dihydrophthalazine‐1,4‐diones were used as potential starting materials for the expeditious synthesis of 6‐arylphthalazino[2,3‐a]cinnoline‐8,13‐diones and 5‐acyl‐5,6‐dihydrophthalazino[2,3‐a]cinnoline‐8,13‐diones under Lawesson's reagent and BF₃?OEt₂ mediated conditions, respectively. Of these, the BF₃?OEt₂‐mediated cyclization proceeded in DMSO as a solvent and a methylene source via dual C?C and C?N bond formations.     

Asymmetric Alkynylation/Lactamization Cascade: An Expeditious Entry to Enantiomerically Enriched Isoindolinones

An unprecedented Cu^I–pybox‐diPh‐catalyzed highly enantioselective (up to >99 % ee) alkynylation/lactamization cascade has been developed as a general catalytic system for the synthesis of diversely substituted isoindolinones of immense biological importance. The cascade effects one C? C and two C? N bond‐forming events in one reaction vessel under operationally simple, additive‐free reaction conditions in good to excellent yields. The methodology was further extended to the synthesis of tetrahydroisoquinoline scaffolds common to several biologically active natural products in a two‐step sequence with remarkable selectivity (up to 94 % ee).     

Synthesis and Structure of Hypervalent Iodine(III) Reagents Containing Phthalimidate and Application to Oxidative Amination Reactions

A new class of hypervalent iodine reagents containing phthalimidate was synthesized, and structurally characterized by X‐ray analysis. The benziodoxole‐based reagent displays satisfactory solubility in common organic solvents and is reasonably stable in solution as well as in the solid state. The reagent was used for the oxidative amination of the C(sp³)? H bond of N,N‐dimethylanilines. In addition, the reagent was also applicable to oxidative amination with rearrangement of trialkylamines as well as enamines that were prepared in situ from secondary amines and aldehydes.