Exceptionally Strong Electron‐Donating Ability of Bora‐Ylide Substituent vis‐à‐vis Silylene and Silylium Ion

Electropositive boron‐based substituent (phosphonium bora‐ylide) with an exceptionally strong π‐ and σ‐electron donating character dramatically increases the stability of a new type of N ‐heterocyclic silylene 2 featuring amino‐ and bora‐ylide‐substituents. Moreover, the related silylium ion 4 and transition‐metal–silylene complexes, with trigonal‐planar geometries around the silicon center, are also well stabilized. Therefore, the N,B‐heterocyclic silylene 2 can be used as a strongly electron‐donating innocent ligand in coordination chemistry similarly to N ‐heterocyclic carbenes.     

A Stable N‐Hetero‐Rh‐Metallacyclic Silylene

A cyclic (amino)metal‐substituted dicoordinated silylene derivative has been synthesized and fully characterized. Of particular interest is that the N‐hetero‐Rh^I‐metallacyclic silylene exhibits a distorted tetrahedral geometry around the rhodium atom and a considerably shortened Si?Rh bond (2.138 Å) compared to classical Si?Rh single bonds (ca. 2.30–2.35 Å). A theoretical investigation reveals that the geometrical deviation around the rhodium center from the classical square‐planar to a tetrahedral geometry increases the π‐donating and σ‐accepting character of the rhodium atom, thereby efficiently stabilizing the silylene moiety.     

Theoretical Study of the Mechanism of Cycloaddition Reaction between Silylene Silylene (H2SiSi:) and Acetone

The mechanism of the cycloaddition reaction between singlet silylene silylene (H₂Si?Si:) and acetone has been investigated with the CCSD (T)//MP2/6‐31G?? method. According to the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction channels. The present rule of this reaction is that the [2+2] cycloaddition reaction of the two π‐bonds in silylene silylene (H₂Si?Si:) and acetone leads to the formation of a four‐membered ring silylene (E3). Because of the unsaturated property of Si: atom in E3, it further reacts with acetone to form a silicic bis‐heterocyclic compound (P7). Simultaneously, the ring strain of the four‐membered ring silylene (E3) makes it isomerize to a twisted four‐membered ring product (P4).     

Synthesis and Rearrangement of Stable NHC→Silylene Adducts and Their Unique Reactivity towards Cyclohexylisocyanide

The syntheses and reactivity of the two N‐heterocyclic carbene (NHC)→ silylene complexes 2 and 4 have been investigated. The latter are easily accessible by reaction of the zwitterionic, N‐heterocyclic silylene LSi: 1 [L=Ar‐N‐C(=CH₂)CH?C(Me)‐N‐Ar, Ar=2,6‐iPr₂C₆H₃] with 1,3,4,5‐tetramethylimidazol‐2‐ylidene and 1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene, respectively. While compound 2 undergoes facile rearrangement above ?20 °C to give the unsymmetrical N‐heterocyclic silylcarbene 3 , the derivative 4 remains unchanged even after boiling in benzene. The remarkable reactivity of 3 and 4 towards cyclohexylisocyanide has been examined which leads in a unique series of C? H, Si? H, and C? N bond activations to the new triaminosilanes 5 and 6 , respectively. The novel compounds 3 , 4 , 5 , and 6 were fully characterized by ¹H, ¹³C, and ²⁹Si NMR spectroscopy, EI‐MS, elemental analysis, and single‐crystal X‐ray diffraction.     

Study of the Air‐Tolerant 1,3‐Diphosphacyclobutane‐2,4‐diyl through the Direct Arylation

Installing π‐functional substituents on the skeletal phosphorus atoms of the air‐tolerant 1,3‐diphosphacyclobutane‐2,4‐diyl unit are promising for tuning the open‐shell singlet P‐heterocyclic chromophore. The sterically encumbered 1,3‐diphosphaCycloButen‐4‐yl Anion ( CBA ), generated from the phosphorus‐carbon triple bond, was available for the regioselective arylation via nucleophilic aromatic substitution (S_NAr) reaction, addition to arynes, and single‐electron transfer (SET) process affording the corresponding P‐arylated 1,3‐diphosphacyclobutane‐2,4‐diyls. The photo‐absorption and redox properties correlated with the effects of the aryl substituents on the 1,3‐diphosphacyclobutane‐2,4‐diyl unit. The X‐ray analyses enabled not only to discuss the metric parameters but also to visualize the radicalic electrons via the electron‐density distribution analysis. The electron‐donating character of the P‐heterocyclic chromophores induced the p‐type semiconductor behavior. Detection of hydrogen fluoride via formation of the 1λ⁵,3λ⁵‐diphosphete derivative was also developed.     

The Striking Stabilization of Ni0(η6‐Arene) Complexes by an Ylide‐Like Silylene Ligand

The right mix does the trick : Elusive {Ni⁰(η⁶‐arene)} moieties can be dramatically stabilized by the N‐heterocyclic silylene ligand 1 , which has a zwitterionic mesomeric structure. The σ, π‐acid–base synergism between nickel and 1 explains the unexpectedly high stability of the new silylene complexes 2 , which enables arene exchange studies at a Ni⁰ center. Addition of B(C₆F₅)₃ to 2 affords the zwitterionic silylene complex 3 (see scheme, R=2,6‐iPr₂C₆H₃).

     

A Bis(silylene)‐Substituted ortho‐Carborane as a Superior Ligand in the Nickel‐Catalyzed Amination of Arenes

The synthesis and structure of the first 1,2‐bis(NHSi)‐substituted ortho‐carborane [(LSi:)C]₂B₁₀H₁₀ (termed SiCCSi) is reported (NHSi=N‐heterocyclic silylene; L=PhC(NtBu)₂). Its suitability to serve as a reliable bis(silylene) chelating ligand for transition metals is demonstrated by the formation of [SiCCSi]NiBr₂ and [SiCCSi]Ni(CO)₂ complexes. The CO stretching vibration modes of the latter indicate that the Si^II atoms in the SiCCSi ligand are even stronger σ donors than the P^III atoms in phosphines and C^II atoms in N‐heterocyclic carbene (NHC) ligands. Moreover, the strong donor character of the [SiCCSi] ligand enables [SiCCSi]NiBr₂ to act as an outstanding precatalyst (0.5 mol % loading) in the catalytic aminations of arenes, surpassing the activity of previously known molecular Ni‐based precatalysts (1–10 mol %).     

Substituent effects in trans‐p,p′‐disubstituted azobenzenes: X‐ray structures at 100 K and DFT‐calculated structures

The crystal and molecular structures of two para‐substituted azobenzenes with π‐electron‐donating –NEt₂ and π‐electron‐withdrawing –COOEt groups are reported, along with the effects of the substituents on the aromaticity of the benzene ring. The deformation of the aromatic ring around the –NEt₂ group in N,N,N′,N′‐tetraethyl‐4,4′‐(diazenediyl)dianiline, C₂₀H₂₈N₄, (I), may be caused by steric hindrance and the π‐electron‐donating effects of the amine group. In this structure, one of the amine N atoms demonstrates clear sp²‐hybridization and the other is slightly shifted from the plane of the surrounding atoms. The molecule of the second azobenzene, diethyl 4,4′‐(diazenediyl)dibenzoate, C₁₈H₁₈N₂O₄, (II), lies on a crystallographic inversion centre. Its geometry is normal and comparable with homologous compounds. Density functional theory (DFT) calculations were performed to analyse the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecules. The most significant changes are observed in the values of the N=N—C—C torsion angles, which for the isolated molecules are close to 0.0°. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates a slight decrease of the aromaticity in (I) and no substantial changes in (II).     

An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules,Including Metal‐Free Dehydrogenation of Water

The first single‐component N‐heterocyclic silylene borane 1 (LSi‐R‐BMes₂; L=PhC(N^t Bu)₂; R=1,12‐xanthendiyl spacer; Mes=2,4,6‐Me₃C₆H₂), acting as a frustrated Lewis pair (FLP) in small‐molecule activation, can be synthesized in 65 % yields. Its HOMO is largely localized at the silicon(II) atom and the LUMO has mainly boron 2p character. In small‐molecule activation 1 allows access to the intramolecular silanone–borane 3 featuring a Si=O→B interaction through reaction with O₂, N₂O, or CO₂, and formation of silanethione borane 4 from reaction with S₈. The Si^II center in 1 undergoes immediate hydrogenation if exposed to H₂ at 1 atm pressure in benzene, affording the silane borane 5‐H₂ , L(H₂)Si‐R‐BMes₂. Remarkably, no H₂ activation occurs if the single silylene LSiPh and Mes₃B intermolecularly separated are exposed to dihydrogen. Unexpectedly, the pre‐organized Si–B separation in 1 enables a metal‐free dehydrogenation of H₂O to give the silanone–borane 3 as reactive intermediate.     

An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules,Including Metal‐Free Dehydrogenation of Water

The first single‐component N‐heterocyclic silylene borane 1 (LSi‐R‐BMes₂; L=PhC(N^t Bu)₂; R=1,12‐xanthendiyl spacer; Mes=2,4,6‐Me₃C₆H₂), acting as a frustrated Lewis pair (FLP) in small‐molecule activation, can be synthesized in 65 % yields. Its HOMO is largely localized at the silicon(II) atom and the LUMO has mainly boron 2p character. In small‐molecule activation 1 allows access to the intramolecular silanone–borane 3 featuring a Si=O→B interaction through reaction with O₂, N₂O, or CO₂, and formation of silanethione borane 4 from reaction with S₈. The Si^II center in 1 undergoes immediate hydrogenation if exposed to H₂ at 1 atm pressure in benzene, affording the silane borane 5‐H₂ , L(H₂)Si‐R‐BMes₂. Remarkably, no H₂ activation occurs if the single silylene LSiPh and Mes₃B intermolecularly separated are exposed to dihydrogen. Unexpectedly, the pre‐organized Si–B separation in 1 enables a metal‐free dehydrogenation of H₂O to give the silanone–borane 3 as reactive intermediate.     

Disilenyl Silylene Reactivity of a Cyclotrisilene

The highly reactive silicon congeners of cyclopropene, cyclotrisilenes (c‐Si₃R₄), typically undergo either π‐addition to the Si=Si double bond or σ‐insertion into the Si?Si single bond. In contrast, treatment of c‐Si₃Tip₄ (Tip=2,4,6‐ⁱPr₃C₆H₂) with styrene and benzil results in ring opening of the three‐membered ring to formally yield the [1+2]‐ and [1+4] cycloaddition product of the isomeric disilenyl silylene to the C=C bond and the 1,2‐diketone π system, respectively. At elevated temperature, styrene is released from the [1+2]‐addition product leading to the thermodynamically favored housane species after [2+2] cycloaddition of styrene and c‐Si₃Tip₄.     

A Zwitterionic Silylene as Reactive Intermediate and Its Unusual Dimerization to an N‐Heterobicyclic Disilane

A way to synthesize the transient zwitterionic silylene L′Si : 8 {L’=CH[(C=CH₂)CMe(N(tBu))₂]} and achieve its facile dimerization to the remarkable N‐heterobicyclic disilane 8 ₂ is described. At first, employing the β‐diketiminate ligand L [L=CH(CMeN(tBu))₂], both starting materials LH ( 2 ) and its N‐lithium salt LLi ( 3 ) can react with SiBr₄ to yield the silylene precursor L′SiBr₂ ( 4 ) by silicon‐induced C? H activation at an exocyclic methyl group on the backbone of the ligand. Compound 4 reacts with SiBr₄ above room temperature to afford the unexpected terminal CH(SiBr₃)‐substituted dibromosilane 6 along with the unique tricyclic trisilane 7 . Reduction of 4 with KC₈ at 0 °C furnishes the novel N‐heterobicyclic disilane 8 ₂, which is a formal dimer of the desired zwitterionic silylene L′Si : ( 8 ). It has been reasoned that compound 8 ₂ may results from [4+1] cycloaddition of two molecules of 8 to give the transient dimer 8 ₂ ′ , which subsequently undergoes hydrogen transfer from a terminal methyl group on the backbone of the C₃N₂Si ligand to the low‐coordinate Si atom. The latter dimerization can be rationalized by the intrinsic zwitterionic character of 8 and insufficient steric protection through the tBu groups at the nitrogen atoms. The novel compounds 3 , 4 , 6 , 7 , and 8 ₂ have been characterized by ¹H, ¹³C, and ²⁹Si NMR spectroscopy, mass spectrometry, and elemental analysis. Additionally, the structures of 3 , 6 , 7 , and 8 ₂ were also established by single‐crystal X‐ray diffraction analyses.     

Substitution effects in the 15N NMR chemical shifts of heterocyclic azines evaluated at the GIAO‐DFT level

A systematic study of the accuracy factors for the computation of ¹⁵N NMR chemical shifts in comparison with available experiment in the series of 72 diverse heterocyclic azines substituted with a classical series of substituents (CH₃, F, Cl, Br, NH₂, OCH₃, SCH₃, COCH₃, CONH₂, COOH, and CN) providing marked electronic σ‐ and π‐electronic effects and strongly affecting ¹⁵N NMR chemical shifts is performed. The best computational scheme for heterocyclic azines at the DFT level was found to be KT3/pcS‐3//pc‐2 (IEF‐PCM). A vast amount of unknown ¹⁵N NMR chemical shifts was predicted using the best computational protocol for substituted heterocyclic azines, especially for trizine, tetrazine, and pentazine where experimental ¹⁵N NMR chemical shifts are almost totally unknown throughout the series. It was found that substitution effects in the classical series of substituents providing typical σ‐ and π‐electronic effects followed the expected trends, as derived from the correlations of experimental and calculated ¹⁵N NMR chemical shifts with Swain–Lupton's F and R constants.     

Building Molecular Complexity from Quinizarin: Conjoined Coumarins and Coronene Analogs

The double Knoevenagel condensation of 1,4‐dibenzoyloxyanthraquinone with methyl esters of arylacetic acids affords a series of compounds based upon a previously unknown 1,8‐dioxa‐benzo[e]pyrene‐2,7‐dione heterocyclic core. The aryl groups incorporated in the 3‐ and 6‐positions can be oxidatively coupled to the π‐expanded backbone to produce a further new heterocyclic core: 1,10‐dioxa‐dibenzo[dj]coronene‐2,9‐dione. The intriguing optical properties of these π‐expanded coumarin derivatives are discussed and rationalized through quantum chemical calculations. The broad absorption bands of 1,8‐dioxa‐benzo[e]pyrene‐2,7‐dione‐based dyes are attributed to both HOMO?1→LUMO and HOMO→LUMO transitions, which have a similar energy. Weakly coupled electron‐donating aryl substituents result in a moderate bathochromic shift of both the absorption and emission by 30–60 nm in toluene. The emissive properties of these compounds are in part determined by the oscillator strength of the main transition, lifetimes of the excited state, and by the energy match of the excited state with a triplet state of a similar energy. The 1,10‐dioxa‐dibenzo[dj]coronene‐2,9‐dione displays a much smaller Stokes shift, yet a markedly increased fluorescence quantum yield of 90 % owing to the increased rigidity compared with the 1,8‐dioxa‐benzo[e]pyrene‐2,7‐dione core.     

Optical,Redox, and DNA‐Binding Properties of Phenanthridinium Chromophores: Elucidating the Role of the Phenyl Substituent for Fluorescence Enhancement of Ethidium in the Presence of DNA

The phenanthridinium chromophores 5‐ethyl‐6‐phenylphenanthridinium ( 1 ), 5‐ethyl‐6‐methylphenanthridinium ( 2 ), 3,8‐diamino‐5‐ethyl‐6‐methylphenanthridinium ( 3 ), and 3,8‐diamino‐5‐ethyl‐6‐(4‐N,N‐diethylaminophenyl)phenanthridinium ( 4 ) were characterized by their optical and redox properties. All dyes were applied in titration experiments with a random‐sequence 17mer DNA duplex and their binding affinities were determined. The results were compared to well‐known ethidium bromide ( E ). In general, this set of data allows the influence of substituents in positions 3, 6, and 8 on the optical properties of E to be elucidated. Especially, compound 4 was used to compare the weak electron‐donating character of the phenyl substituent at position 6 of E with the more electron‐donating 4‐N,N‐diethylaminophenyl group. Analysis of all of the measurements revealed two pairs of chromophores. The first pair, consisting of 1 and 2 , lacks the amino groups in positions 3 and 8, and, as a result, these dyes exhibit clearly altered optical and electrochemical properties compared with E . In the presence of DNA, a significant fluorescence quenching was observed. Their binding affinity to DNA is reduced by nearly one order of magnitude. The electronic effect of the phenyl group in position 6 on this type of dye is rather small. The properties of the second set, 3 and 4 , are similar to E due to the presence of the two strongly electron‐donating amino groups in positions 3 and 8. However, in contrast to 1 and 2 , the electron‐donating character of the substituent in position 6 of 3 and 4 is critical. The binding, as well as the fluorescence enhancement, is clearly related to the electron‐donating effect of this substituent. Accordingly, compound 4 shows the strongest binding affinity and the strongest fluorescence enhancement. Quantum chemical calculations reveal a general mechanism related to the twisted intramolecular charge transfer (TICT) model. Accordingly, an increase of the twist angle between the phenyl ring in position 6 and the phenanthridinium core opens a nonradiative channel in the excited state that depends on the electron‐donating character of the phenyl group. Access to this channel is hindered upon binding to DNA.     

Facile Access to Silicon‐Functionalized Bis‐Silylene Titanium(II) Complexes

A series of unprecedented bis‐silylene titanium(II) complexes of the type [(η⁵‐C₅H₅)₂Ti(LSiX)₂] (L=PhC(NtBu)₂; X=Cl, CH_3, H) has been prepared using a phosphane elimination strategy. Treatment of the [(η⁵‐C₅H₅)₂Ti(PMe₃)₂] precursor ( 1 ) with two molar equivalents of the N‐heterocyclic chlorosilylene LSiCl ( 2 ), results in [(η⁵‐C₅H₅)₂Ti(LSiCl)₂] ( 3 ) with concomitant PMe₃ elimination. The presence of a Si? Cl bond in 3 enabled further functionalization at the silicon(II) center. Accordingly, a salt metathesis reaction of 3 with two equivalents of MeLi results in [(η⁵‐C₅H₅)₂Ti(LSiMe)₂] ( 4 ). Similarly, the reaction of 3 with two equivalents of LiBHEt₃ results in [(η⁵‐C₅H₅)₂Ti(LSiH)₂] ( 5 ), which represents the first example of a bis‐(hydridosilylene) metal complex. All complexes were fully characterized and the structures of 3 and 4 elucidated by single‐crystal X‐ray diffraction analysis. DFT calculations of complexes 3 – 5 were also carried out to assess the nature of the titanium–silicon bonds. Two σ and one π‐type molecular orbital, delocalized over the Si‐Ti‐Si framework, are observed.     

Thermally Induced Denitrogenative Annulation for the Synthesis of Dihydroquinolinimines and Chroman‐4‐imines

A rapid growth in synthetic methods for the preparation of diverse organic molecules using N‐sulfonyl‐1,2,3‐triazoles is of great interest in organic synthesis. Transition metals are generally used to activate the α‐imino diazo intermediates. Metal‐free methods have not been studied in detail, but can be a good complement to transition metal catalysis in the mild reaction conditions. We herein report a novel method for the preparation of 2,3‐dihydroquinolin‐4‐imine and chroman‐4‐imine analogs from their corresponding N‐sulfonyl‐1,2,3‐triazoles in the absence of metal catalysts. To achieve intramolecular annulation, the introduction of an electron‐donating group is required at the meta position of N‐sulfonyl‐1,2,3‐triazole methyl anilines. The inclusion of tailored substituents on the aniline moieties and nitrogen atoms enhances the nucleophilicity of the phenyl π‐electrons, thus allowing them to undergo a Friedel–Crafts‐type reaction with the highly electrophilic ketenimines. This metal‐free method was carefully optimized to generate a variety of dihydroquinolin‐4‐imines and chroman‐4‐imines in moderate‐to‐good yields.     

Design and Synthesis of Piezochromic Materials Based on Push–Pull Chromophores: A Mechanistic Perspective

Computational analysis predicts that intramolecular charge transfer (ICT) exists in anthraquinone imide (AQI) derivatives with electron‐donating substituents at the 6‐position, such as 4‐methoxylphenyl, 4‐N,N‐dimethylaminophenyl, and thiophene. However, for those with electron‐withdrawing ones, no clear ICT interaction could be observed. We predicted, on the basis of the simulation results, that AQI derivatives with electron‐donating substituents would be piezochromic. To verify this hypothesis, the corresponding AQI derivatives with various substituents were synthesized. Absorption spectra recorded with a diffuse reflectance method on powders revealed that 4‐methoxylphenyl‐, 4‐N,N‐dimethylaminophenyl‐, and thiophene‐substituted AQI exhibited piezochromism, but not 4‐nitrophenyl‐substituted AQI, which is in good agreement with the simulation results. Interestingly, redshifts of both the lower and higher energy absorption bands were observed along with redshifts of the emission spectra. However, XRD patterns before and after being pressed presented no significant changes, which was different from known piezochromic molecules described in the literature. An unprecedented mechanism in which enhanced ICT from better conjugation between the donor and acceptor segments induced by the decrease of θ under pressure could be responsible for the piezochromism of aryl‐substituted AQIs is proposed.     

Kinetic and mechanistic investigation into the influence of chelate substituents on the substitution reactions of platinum(II) terpyridine complexes

The substitution kinetics of the complexes [Pt{4′‐(o‐CH₃‐Ph)‐terpy} Cl]SbF₆ (CH₃PhPtCl(Sb)), [Pt{4′‐(o‐CH₃‐Ph)‐terpy}Cl]CF₃SO₃ (CH₃PhPtCl(CF)), [Pt(4′‐Ph‐terpy)Cl]SbF₆ (PhPtCl), [Pt(terpy)Cl]Cl·2H₂O (PtCl), [Pt{4′‐(o‐Cl‐Ph)‐terpy}Cl]SbF₆ (ClPhPtCl), and [Pt{4′‐(o‐CF₃‐Ph)‐terpy}Cl]SbF₆ (CF₃PhPtCl), where terpy is 2,2′:6′,2″‐terpyridine, with the nucleophiles thiourea (TU), N,N′‐dimethylthiourea (DMTU), and N,N,N′,N′‐tetramethylthiourea (TMTU) were investigated in methanol as a solvent. The substitution reactions of the chloride displacement from the metal complexes by the nucleophiles were investigated as a function of nucleophile concentration and temperature under pseudo‐first‐order conditions using the stopped‐flow technique. The reactions followed the simple rate law k_obs = k₂[Nu]. The results indicate that the introduction of substituents in the ortho position of the phenyl group on the ancillary ring of the terpy unit does influence the extent of π‐backbonding in the terpy ring. This controls the electrophilicity of the platinum center, which in turn controls the lability of the chloro‐leaving group. The strength of the electron‐donating or ‐withdrawing ability of the substituents correlates with the reactivity of the complexes. Electron‐donating substituents decrease the rate of substitution, whereas electron‐withdrawing substituents increase the rate of substitution. This was supported by DFT calculations at the B3LYP/LACVP+** level of theory, which showed that most of the electron density of the HOMO is concentrated on the phenyl ligand rather than on the metal center in the case of the strongest electron‐withdrawing substituent in CF₃PhPtCl. The opposite was found to be true with the strongest electron‐donating substituent in CH₃PhPtCl. Thiourea was found to be the best nucleophile with N,N,N′,N′‐tetramethylthiourea being the weakest due to steric effects. The temperature dependence studies support an associative mode of activation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 808–818, 2008     

Ab initio Study of Mechanism of Cycloaddition Reaction between Germylene Silylene (H2GeSi:) and Acetone

The mechanism of the cycloaddition reaction between singlet germylene silylene (H₂GeSi:) and acetone has been investigated with CCSD(T)/6‐31G*//MP2/6‐31G* method. From the potential energy profile, we could predict that the reaction has two competitive dominant reaction channels. The present rule of this reaction is that the [2+2] cycloaddition reaction of the two (‐bonds in germylene silylene and acetone generates a four‐membered ring silylene with Ge. Because of the unsaturated property of Si atom in the four‐membered ring silylene with Ge, it could further react with acetone, resulting in the generation of a bis‐heterocyclic compound with Si and Ge. Simultaneously, the ring strain of the four‐membered ring silylene with Ge makes it isomerize to a twisted four‐membered ring product.