Hydroboration of Arynes with N‐Heterocyclic Carbene Boranes

Arynes were generated in situ from ortho‐silyl aryl triflates and fluoride ions in the presence of stable N‐heterocyclic carbene boranes (NHC? BH₃). Spontaneous hydroboration ensued to provide stable B‐aryl‐substituted NHC‐boranes (NHC? BH₂Ar). The reaction shows good scope in terms of both the NHC‐borane and aryne components and provides direct access to mono‐ and disubstituted NHC‐boranes. The formation of unusual ortho regioisomers in the hydroboration of arynes with an electron‐withdrawing group supports a hydroboration process with hydride‐transfer character.     

The psuedo‐michael reaction of 2‐aminoimidazolines 2. Part 1. Synthesis and structure assignment of isomeric 5(1H)‐Oxo and 7(1H)‐Oxo‐2,3‐dihydroimidazo[1,2‐a]pyrimidine‐6‐carboxylates

The pseudo‐Michael reaction of 1‐aryl‐2‐aminoimidazolines‐2 with diethyl ethoxymethylenemalonate (DEEM) was investigated. Extensive structural studies were performed to confirm the reaction course. For derivatives with N1 aromatic substituents, it was found that the reaction course was temperature dependent. When the reaction temperature was held at ?10 °C only the formation of 1‐aryl‐7(1H)‐oxo‐2,3‐dihydroimi‐dazo[1,2‐a]pyrimidine‐6‐carboxylates ( 4 ) was observed in contrast to earlier suggestions. Under the room temperature conditions, the same reaction yielded mixtures, with varying ratio, of isomeric 1‐aryl‐7(1H)‐oxo‐ ( 4a‐4f ) and 1‐aryl‐5(1H)‐oxo‐2,3‐dihydroimidazo[1,2‐a]pyrimidine‐6‐carboxylates ( 5a‐5f ). The molecular structure of selected isomers, 4b and 5c , was confirmed by X‐ray crystallography. Frontal chro‐matography with delivery from the edge was applied for the separation of the isomeric esters. The isomer ratio of the reaction products depended on the character of the substituents on the phenyl ring. The 1‐aryl‐7(1H)‐oxo‐carboxylates ( 4a‐4f ) were preferably when the phenyl ring contained H, 4‐CH₃, 4‐OCH₃ and 3,4‐Cl₂ substituents. Chloro substitution at either position 3 or 4 in the phenyl ring favored the formation of isomers 5a‐5f . The isomer ratios were confirmed both by ¹H NMR and chromatography. The reaction of the respective hydrobromides of 1‐aryl‐2‐aminoimidazoline‐2 with DEEM, in the presence of triethylamine, gave selectively 5(1H)‐oxo‐esters ( 5a‐5f ).     

One‐Pot Synthesis of 1,4‐Oxathiino[2,3‐b]quinoxalines or ‐pyrazines from 2,3‐Dichloroquinoxaline or ‐pyrazine and 1‐Aryl‐2‐bromoalkan‐1‐ones

An efficient one‐pot synthesis of novel heterocyclic derivatives, 2‐aryl‐1,4‐oxathiino[2,3‐b]quinoxalines or ‐pyrazines 5 , via the reaction of 2,3‐dichloroquinoxaline or ‐pyrazine with Na₂S?9 H₂O, and subsequent treatment of the resulting 2‐chloro‐3‐sodiosulfanylquinoxaline or ‐pyrazine 2 with 1‐aryl‐2‐bromo‐1‐alkanones and then NaH under mild conditions is described.     

Porphyrinylboranes Synthesized via Porphyrinyllithiums

As the most nucleophilic porphyrins, meso‐ or β‐lithiated porphyrins were generated by iodine–lithium exchange reactions of the corresponding iodoporphyrins with n‐butyllithium at ?98 °C. Porphyrinyllithiums thus prepared were used for synthesis of dimesitylporphyrinylboranes through reactions with fluorodimesitylborane. The boryl groups proved to serve as an electron‐accepting unit to alter the photophysical and electrochemical properties. In addition, 5‐diarylamino‐15‐dimesitylboryl‐substituted donor–accepter porphyrins showed increased intramolecular charge‐transfer character in the S₁ state. Furthermore, the reaction of β‐lithiated porphyrin with dichloromesitylborane provided a boron‐bridged porphyrin dimer, which exhibited a conjugative interaction between two porphyrin units through the vacant p‐orbital on the boron center.     

Palladium‐Catalyzed Cascade CH Trifluoroethylation of Aryl Iodides and Heck Reaction: Efficient Synthesis of ortho‐Trifluoroethylstyrenes

A palladium‐catalyzed selective C? H bond trifluoroethylation of aryl iodides has been explored. The reaction allows for the efficient synthesis of a variety of ortho‐trifluoroethyl‐substituted styrenes. Preliminary mechanistic studies indicate that the reaction might involve a key Pd^IV intermediate, which is generated through the rate‐determining oxidative addition of CF₃CH₂I to a palladacycle; the bulky nature of CF₃CH₂I influences the reactivity. Reductive elimination from the Pd^IV complex then leads to the formation of the aryl–CH₂CF₃ bond.     

A Series of Isolable Silanoic Thio‐, Seleno‐, and Telluroesters (LSi(X)OR) with Donor‐Supported SiX Double Bonds (L=β‐Diketiminate; X=S,Se, Te)

The first silicon analogues of carbonic (carboxylic) esters, the silanoic thio‐, seleno‐, and tellurosilylesters 3 (Si?S), 4 (Si?Se), and 5 (Si?Te), were prepared and isolated in crystalline form in high yield. These thermally robust compounds are easily accessible by direct reaction of the stable siloxysilylene L(Si:)OSi(H)L′ 2 (L=HC(CMe)₂[N(aryl)₂], L′=CH[(C?CH₂)‐CMe][N(aryl)]₂; aryl=2,6‐iPr₂C₆H₃) with the respective elemental chalcogen. The novel compounds were fully characterized by methods including multinuclear NMR spectroscopy and single‐crystal X‐ray diffraction analysis. Owing to intramolecular N→Si donor–acceptor support of the Si?X moieties (X=S, Se, Te), these compounds have a classical valence‐bond N⁺–Si–X^? resonance betaine structure. At the same time, they also display a relatively strong nonclassical Si?X π‐bonding interaction between the chalcogen lone‐pair electrons (n_π donor orbitals) and two antibonding Si? N orbitals (σ*_π acceptor orbitals mainly located at silicon), which was shown by IR and UV/Vis spectroscopy. Accordingly, the Si?X bonds in the chalcogenoesters are 7.4 ( 3 ), 6.7 ( 4 ), and 6.9 % ( 5 ) shorter than the corresponding Si? X single bonds and, thus, only a little longer than those in electronically less disturbed Si?X systems (“heavier” ketones).     

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.     

σ‐Noninnocence: Masked Phenyl‐Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of ?2 at a transition‐metal center. For a series of formal high‐valent Ni^IV complexes, aryl–CF₃ bond‐forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015 , 137, 8034–8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal Ni^IV centers are better described as approaching a +II oxidation state, originating from highly covalent metal–ligand bonds, a phenomenon attributable to σ‐noninnocence. A direct consequence is that the elimination of aryl–CF₃ products occurs in an essentially redox‐neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF₃ group is transferred to an electrophilic aryl group. The uncovered role of σ‐noninnocence in metal–ligand bonding, and of an essentially redox‐neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.     

Regioselective N‐Arylation of Some Pyrimidine and Purine Nucleobases

N‐Arylation of some pyrimidine and purine nucleobases in the presence of SiO₂ and Cs₂CO₃ under solvent‐free as well as in solution conditions is described. In these conditions, N1‐aryl pyrimidines and N9‐aryl purines have been obtained regioselectively in moderate to high yields with relatively short reaction times.     

Stereospecific synthesis of a family of novel (E)‐2‐aryl‐1‐silylalka‐1,4‐dienes or (E)‐4‐aryl‐5‐silylpenta‐1,2,4‐trienes via a cross‐coupling of (Z)‐silyl(stannyl)ethenes with allyl halides or propargyl bromide

Stereospecific synthesis of a family of novel (E)‐2‐aryl‐1‐silylalka‐1,4‐dienes or (E)‐4‐aryl‐5‐silylpenta‐1,2,4‐trienes via a cross‐coupling of (Z)‐silyl(stannyl)ethenes with allyl halides or propargyl bromide is described. In the reaction with allyl bromide, either a Pd(dba)₂? CuI combination (dba, dibenzylideneacetone) in DMF or copper(I) iodide in DMSO–THF readily catalyzes or mediates the coupling reaction of (Z)‐silyl(stannyl)ethenes at room temperature, producing novel vinylsilanes bearing an allyl group β to silicon with cis ‐disposition in good yields. Allyl chlorides as halides can be used in the CuI‐mediated reaction. CuI alone much more effectively mediates the cross‐coupling reaction with propargyl bromide in DMSO–THF at room temperature compared with a Pd(dba)₂? CuI combination catalysis in DMF, providing novel stereodefined vinylsilanes bearing an allenyl group β to silicon with cis ‐disposition in good yields. Copyright © 2008 John Wiley & Sons, Ltd.     

Difunctionalisation of arenes and heteroarenes by directed metallation and sulfoxide-magnesium exchange

The aryl sulfoxide moiety allows an expedient two‐step difunctionalisation of readily available diaryl sulfoxides. Highly functionalised 1,2,4‐trisubstituted arenes and difunctionalised heteroarenes (furans, thiophenes, benzofurans and pyridines) were prepared in a two‐step sequence, triggered by an aryl sulfoxide group. In the first step, the sulfoxide moiety acts as a metallation‐directing group, allowing smooth ortho‐magnesiation with TMPMgCl ? LiCl (TMP=tetramethylpiperidine). After a quenching reaction with an electrophile, the resulting sulfoxide is converted into a second magnesium reagent with iPrMgCl ? LiCl (sulfoxide–magnesium exchange), which can be trapped with various electrophiles. Highly chemoselective TMPMgCl ? LiCl and iPrMgCl ? LiCl are compatible with a broad range of functional groups (e.g., F, Cl, CF₃, CN, CO₂tBu, alkynyl, ethers, thioethers). Large‐scale reactions (25–40 mmol) and the preparation of fully functionalised furans and thiophenes are also reported.     

Fluorinated Phenanthrenes as Aryne Precursors: PAH Synthesis Based on Domino Ring Assembly Using 1,1‐Difluoroallenes

On treatment with the catalyst InBr₃, 1,1‐difluoroallenes that bear a cyclopentene moiety and an aryl group underwent domino ring assembly in the presence or absence of N‐bromosuccinimide or N‐iodosuccinimide to afford aryne precursors such as three‐ringed ortho‐fluoro(halo)phenanthrenes, four‐ringed ortho‐fluoro(halo)tetraphenes, ortho‐fluoro(halo)chrysenes and fluoro[4]helicenes. Metalation of the aryne precursors followed by elimination of the fluoride resulted in the unprecedented systematic generation of arynes bearing π‐extended systems. Diels?Alder reactions of these arynes with isobenzofurans afforded the corresponding cycloadducts whose reductive aromatisation in an SnCl₂/HBr system furnished fully aromatised benzotriphenylenes. In addition, oxidative aryl?aryl coupling (the Scholl reaction) of these benzotriphenylenes facilitated the synthesis of ‘half HBCs’ (hexabenzocoronenes).     

Photochemical Intramolecular C−H Addition of Dimesityl(hetero)arylboranes through a [1,6]‐Sigmatropic Rearrangement

A new reaction mode for triarylboranes under photochemical conditions was discovered. Photoirradiation of dimesitylboryl‐substituted (hetero)arenes produced spirocyclic boraindanes, where one of the C−H bonds in the ortho ‐methyl groups of the mesityl substituents was formally added in a syn fashion to a C−C double bond of the (hetero)aryl group. Quantum chemical calculations and laser flash photolysis measurements indicated that the reaction proceeds through a [1,6]‐sigmatropic rearrangement. This behavior is reminiscent of the photochemical reaction mode of arylalkenylketones, thus demonstrating the isosteric relation between tricoordinate organoboron compounds and the corresponding pseudo‐carbocationic species in terms of pericyclic reactions. Despite the disrupted π‐conjugation, the resulting spirocyclic boraindanes exhibited a characteristic absorption band at relatively long wavelengths (λ =370—400 nm).     

Oxygenation of Ruthenium Carbene Complexes Containing Naphthothiophene or Naphthofuran: Spectroscopic and DFT Studies

The aryl propargylic alcohol 1‐[2‐(thiophen‐3‐yl)phenyl]prop‐2‐yn‐1‐ol ( 1a ) is readily prepared from 2‐(thiophen‐3‐yl)benzaldehyde. In the presence of visible light, treatment of 1a with one‐half mole equivalent of [Ru]Cl ([Ru]?Cp(dppe)Ru) (dppe=1,2‐bis(diphenylphosphino)ethane) and NH₄PF₆ in O₂ affords the naphtha[2,1‐b]thiophene‐4‐carbaldehyde ( 4a ) in high yields. The cyclization reaction of 1a proceeds through the formation of the carbene complex 2a that contains the naphtha[2,1‐b]thiophene ring, which is isolated in a 1:1 stoichiometric reaction. The C? C bond formation between the inner carbon of the terminal triple bond and the heterocyclic ring is confirmed by structure determination of 2a using single‐crystal X‐ray diffraction analysis. Facile oxygenation of 2a by O₂ yields the aldehyde product 4a accompanied by the formation of phosphine oxide of dppe. Oxygen is most likely activated by coordination to the ruthenium center when one PPh₂ unit of the dppe ligand dissociates. This dissociated PPh₂ unit then reacts with the coordinated oxygen nearby to generate half‐oxidized dppe ligand and an unobserved oxo–carbene intermediate. Coupling of the oxo/carbene ligands followed by demetalation then yields 4a . Presumably the resulting complex with the half‐oxidized dppe ligand continuously promotes cyclization/oxygenation of 1a to yield the second aldehyde molecule. In alcohol such as MeOH or EtOH, the oxygenation reaction affords a mixture of 4a and the corresponding esters 5a or 5a' . Four other aryl propargylic alcohols 1b , 1c , 1d , 1e , which contain thiophen‐2‐yl, isopropenyl, fur‐3‐yl, and fur‐2‐yl, respectively, on the aryl ring are also prepared. Analogous aldehydes 4b , 4c , 4d , 4e are similarly prepared from 1b , 1c , 1d , 1e , respectively. For oxygenations of 1b , 1d , and 1e in alcohol, mixtures of aldehyde 4 , ester 5 , and acetal 8 are obtained. The carbene complex 2b obtained from 1b was also characterized by single‐crystal X‐ray diffraction analysis. The UV/Vis spectra of 2a and 2b consist of absorption bands with a high extinction coefficient. From DFT calculations on 2a and 2b , the visible light is found to populate the LUMO antibonding orbital of mainly Ru?C bonds, thereby weakening the Ru?C bond and promoting the oxygenation/demetalation reactions of 2 .     

Theory of the Formation and Decomposition of N‐Heterocyclic Aminooxycarbenes through Metal‐Assisted [2+3]‐Dipolar Cycloaddition/Retro‐Cycloaddition

The theoretical background of the formation of N‐heterocyclic oxadiazoline carbenes through a metal‐assisted [2+3]‐dipolar cycloaddition (CA) reaction of nitrones R¹CH?N(R²)O to isocyanides C?NR and the decomposition of these carbenes to imines R¹CH?NR² and isocyanates O?C?NR is discussed. Furthermore, the reaction mechanisms and factors that govern these processes are analyzed in detail. In the absence of a metal, oxadiazoline carbenes should not be accessible due to the high activation energy of their formation and their low thermodynamic stability. The most efficient promotors that could assist the synthesis of these species should be “carbenophilic” metals that form a strong bond with the oxadiazoline heterocycle, but without significant involvement of π‐back donation, namely, Au^I, Au^III, Pt^II, Pt^IV, Re^V, and Pd^II metal centers. These metals, on the one hand, significantly facilitate the coupling of nitrones with isocyanides and, on the other hand, stabilize the derived carbene heterocycles toward decomposition. The energy of the LUMO_CNR and the charge on the N atom of the C?N group are principal factors that control the cycloaddition of nitrones to isocyanides. The alkyl‐substituted nitrones and isocyanides are predicted to be more active in the CA reaction than the aryl‐substituted species, and the N,N,C‐alkyloxadiazolines are more stable toward decomposition relative to the aryl derivatives.     

Palladium‐Catalyzed Enantioselective Cacchi Reaction: Asymmetric Synthesis of Axially Chiral 2,3‐Disubstituted Indoles

We report herein the first examples of a palladium‐catalyzed enantioselective Cacchi reaction for the synthesis of indoles bearing a chiral C2‐aryl axis. In the presence of a catalytic amount of Pd(OAc)₂ and (R,R)‐QuinoxP* ligand, reaction of N‐aryl(alkyl)sulfonyl‐2‐alkynylanilides with arylboronic acids under oxygen atmosphere afforded enantioenriched 2,3‐disubstituted indoles in high yields and enantioselectivity. The indole ring is constructed de novo in this process and a complexation‐induced chirality transfer is proposed to account for the observed enantioselectivity.     

CBr4‐Mediated Cross‐Dehydrogenative Coupling Reaction of Amines

A novel CBr₄‐mediated dehydrogenative Povarov/aromatization tandem reaction of glycine derivatives with alkenes, leading to complex quinoline derivatives, and a CBr₄‐mediated dehydrogenative C?H functionalization of N‐aryl tetrahydroisoquinolines with nucleophiles to form C?C and C?P bonds are reported. The reactions were performed under very simple and mild reaction conditions; only CBr₄ was used as a promoter. A plausible mechanism involving a radical process is proposed.     

Practical Enantioselective Arylation and Heteroarylation of Aldehydes with In Situ Prepared Organotitanium Reagents Catalyzed by 3‐Aryl‐H8‐BINOL‐Derived Titanium Complexes

A highly efficient and practical method for the catalytic enantioselective arylation and heteroarylation of aldehydes with organotitanium reagents, prepared in situ by the reaction of aryl‐ and heteroaryllithium reagents with ClTi(OiPr)₃, is described. Titanium complexes derived from DPP‐H₈‐BINOL ( 3 d ; DPP=3,5‐diphenylphenyl) and DTBP‐H₈‐BINOL ( 3 e ; DTBP=3,5‐di‐tert‐butylphenyl) exhibit excellent catalytic activity in terms of enantioselectivity and turnover efficiency for the transformation, providing diaryl‐, aryl heteroaryl‐, and diheteroarylmethanol derivatives in high enantioselectivity at low catalyst loading (0.2–2 mol %). The reaction begins with a variety of aryl and heteroaryl bromides through their conversion into organolithium intermediates by Br/Li exchange with nBuLi, thus providing straightforward access to a range of enantioenriched alcohols from commercially available starting materials. Various 2‐thienylmethanols can be synthesized enantioselectively by using commercially available 2‐thienyllithium in THF. The reaction can be carried out on a 10 mmol scale at 0.5 mol % catalyst loading, demonstrating its preparative utility.     

Anilinolysis of Picryl Benzoate Derivatives in Methanol: Reactivity,Regioselectivity, Kinetics,and Mechanism

The reaction of picryl benzoate derivatives 1a–g with aniline in methanol proceeds through CO? O and Ar? O bond cleavage pathways. Furthermore, the reactivity of these esters toward anilinolysis is correlated to the energy gap between highest occupied molecular orbital aniline and lowest unoccupied molecular orbital of each ester. The regioselectivity of acyl? oxygen versus aryl? oxygen cleavage is also discussed. The overall rate constants k_tot split into k_{CO? O} (the rate constant of acyl‐oxygen cleavage) and k_{Ar? O} (rate constant of aryl‐oxygen cleavage). The CO? O bond cleavage advances through a stepwise mechanism in which the formation of the tetrahedral intermediate is the rate‐determining step. The Ar? O bond cleavage continues through a S_NAr mechanism in which the departure of the leaving group from the Meisenheimer complex occurs rapidly after its formation in the rate‐determining step.     

Facile Synthesis of N‐Carboranyl Amines through an ortho‐Carboryne Intermediate

The efficient o‐carboryne precursor 1‐Li‐2‐OTf‐o‐C₂B₁₀H₁₀ reacts with lithium amides at room temperature to give a series of N‐carboranyl amines in moderate to high isolated yields. This reaction is compatible with a broad substrate scope from primary to secondary, alkyl to aryl amines. The reaction mechanism is also proposed on the basis of experimental results and DFT calculations. This represents the first general and efficient method for the synthesis of 1‐NR¹R²‐o‐carboranes.