C−C Bond‐Forming Strategy by Manganese‐Catalyzed Oxidative Ring‐Opening Cyanation and Ethynylation of Cyclobutanol Derivatives

A novel C?C bond‐forming strategy employing manganese‐catalyzed ring‐opening of cyclobutanol substrates, followed by cyanation or ethynylation, is described. A cyano C1 unit and ethynyl C2 unit are regiospecifically introduced to the γ‐position of ketones at room temperature, providing a mild yet powerful method for production of elusive aliphatic nitriles and alkynes. All transformations described are based on a common sequence: 1) oxidative ring‐opening of cyclobutanol substrates by C?C bond cleavage; 2) radical addition to triple bonds bearing an arylsulfonyl group; and 3) radical‐mediated C?S bond cleavage.     

Cross‐Coupling of Organolithium with Ethers or Aryl Ammonium Salts by C−O or C−N Bond Cleavage

Various aryl‐, alkenyl‐, and/or alkyllithium species reacted smoothly with aryl and/or benzyl ethers with cleavage of the inert C?O bond to afford cross‐coupled products, catalyzed by commercially available [Ni(cod)₂] (cod=1,5‐cyclooctadiene) catalysts with N‐heterocyclic carbene (NHC) ligands. Furthermore, the coupling reaction between the aryllithium compounds and aryl ammonium salts proceeded under mild conditions with C?N bond cleavage in the presence of a [Pd(PPh₃)₂Cl₂] catalyst. These methods enable selective sequential functionalizations of arenes having both C?N and C?O bonds in one pot.     

A DFT Study on the Conversion of Aryl Iodides to Alkyl Iodides: Reductive Elimination of R−I from Alkylpalladium Iodide Complexes with Accessible β‐Hydrogens

DFT calculations have been performed on the palladium‐catalyzed carboiodination reaction. The reaction involves oxidative addition, alkyne insertion, C?N bond cleavage, and reductive elimination. For the alkylpalladium iodide intermediate, LiOtBu stabilizes the intermediate in non‐polar solvents, thus promoting reductive elimination and preventing β‐hydride elimination. The C?N bond cleavage process was explored and the computations show that PPh₃ is not bound to the Pd center during this step. Experimentally, it was demonstrated that LiOtBu is not necessary for the oxidative addition, alkyne insertion, or C?N bond cleavage steps, lending support to the conclusions from the DFT calculations. The turnover‐limiting steps were found to be C?N bond cleavage and reductive elimination, whereas oxidative addition, alkyne insertion, and formation of the indole ring provide the driving force for the reaction.     

Substituent effects on the ring opening of 2-aziridinylmethyl radicals

[reaction: see text] Substituent effects on the ring-opening reactions of 2-aziridinylmethyl radicals were studied systematically for the first time utilizing the ONIOM(QCISD(T)/6-311+G(2d,2p):B3LYP/6-311+G(3df,2p)) method. It was found that various substituents on the nitrogen atom had a relatively small effect on the ring opening of the 2-aziridinylmethyl radical. A pi-acceptor substituent at the C(1) position reduced the energy barrier for C-C cleavage dramatically, but it increased the energy barrier for C-N cleavage significantly at the same time. When the C(1) substituent is alkyl, the ring opening should always strongly favor the C-N cleavage pathway, regardless of whether the N substituent is alkyl, aryl, or COR. When the C(1) substituent is CHO (or CO-alkyl, CO-aryl, or CO-OR but not CO-NR(2)), the ring opening strongly favors the C-C cleavage pathway, regardless of whether the N substituent is alkyl, aryl, or COR. When the C(1) substituent is aryl (or alkenyl or alkynyl), the ring opening should favor the C-C cleavage pathway if the N substituent is alkyl or COR. If both the C(1) substituent and the N substituent are aryl, the ring opening should proceed via both the C-C and C-N cleavage pathways. The solvent effect on the regioselectivity of the ring opening of the 2-aziridinylmethyl radicals was found to be very small. The substituent effects on C-C cleavage could be explained successfully by the spin-delocalization mechanism. For the substituent effects on C-N cleavage, an extraordinary through-bond pi-acceptor effect must be taken into account. Furthermore, studies on bicyclic 2-aziridinylmethyl radicals showed that the ring strain could also affect the regiochemistry of the ring-opening reactions.     

Cycloaddition of benzothiete and electron-deficient nitriles

The o-quinoid 8π electron system 2 , generated by thermal ring opening of benzothiete ( 1 ), enters regio-specific [8π + 2π] cycloaddition reactions with electron-deficient nitriles 3a-d , yielding the 4H-1,3-benzothiazines 4a-d. A competitive dimerization of 1 leads to 1,5-dibenzo[b,f]dithiocin (5). Depending on the nitrile further competitive or subsequent reactions (2 + 3b → 7b, 2 + 3d → 4d → 8d) can occur. The cycloadducts 10e and 11e gained from 3e anticipate a primary cleavage of 3e to methylisothiocyanate 9e which reacts at the C?N double bond as well as at the C?S double bond.     

The impact of protonation and deprotonation of 3-methyl-2'-deoxyadenosine on N-glycosidic bond cleavage

The enzyme-substrate contacts that are believed to be involved in depurination by proton transfer have been modelled by protonation and deprotonation of 3-methyl-2'-deoxyadenosine (3-MDA) using quantum mechanical calculations in the gas-phase and solution media. The change in the charge distribution on the sugar ring and nucleobase that is introduced by the protonation and deprotonation strongly affects the N-glycosidic bond length. The unimolecular cleavage and hydrolysis of the N-glycosidic bond, involving D(N)*A(N) and A(N)D(N) pathways, have been considered at several levels of theory. The trend in the energy barriers is A(N)D(N) > cleavage > D(N)*A(N). All probable proton transfer reactions resulting from enzyme-substrate contacts do not facilitate the N-glycosidic bond cleavage of 3-MDA. The deprotonation of 3-MDA that may result from the interaction between H6 and enzyme do not facilitate bond cleavage. The protonation at N7 induces more positive charge on the sugar ring and further facilitates the depurination relative to the protonation at N1. The changes in the charges calculated on the ribose and nucleobase are in good relationship with the C1'-C2', C1'-O4', and N-glycosidic bond lengths along the cleavage. The change in energy barrier ΔE of glycosidic bond cleavage from the gas-phase to solution media strongly depends on the charge of the species.     

Hydride Ligands Make the Difference: Density Functional Study of the Mechanism of the Murai Reaction Catalyzed by [Ru(H)(2) (H(2) )(2) (PR(3) )(2) ] (R=cyclohexyl)

The catalytic cycle for the Murai reaction at room temperature between ethylene and acetophenone catalyzed by [Ru(H)(2) (H(2) )(2) (PMe(3) )(2) ] has been studied computationally at the B3PW91 level. The active species is the ruthenium dihydride complex [Ru(H)(2) (PMe(3) )(2) ]. Coordination of the ketone group to Ru induces very easy C?H bond cleavage. Coordination of ethylene after ketone de-coordination, followed by ethylene insertion into a Ru?H bond, creates the Ru?ethyl bond. Isomerization of the complex to a Ru(IV) intermediate creates the geometry adapted to C?C bond formation. Re-coordination of the ketone before the C?C coupling lowers the energy of the corresponding TS. The highest point on the potential energy surface (PES) is the TS for the isomerization to the Ru(IV) intermediate, which prepares the catalyst geometry for the C?C coupling step. Inclusion of dispersion corrections significantly lowers the height of the overall activation barrier. The actual bond cleavage and bond forming processes are associated to low activation barriers because of the presence of hydrogen atoms around the Ru center. They act as redox buffers through formation and breaking of H?H bonds in the coordination sphere. This flexibility allows optimal repartition of the various ligands according to the change in stereoelectronic demands along the catalytic cycle.     

Cleavage of the CN Bond in Carbodiimides via Release of High Ring Strain: A New Strategy for the Selective Synthesis of 2‐Aminoaryl Alkynyl Imines

A novel pattern of the cleavage and reorganization of C?N bond in the multicomponent reaction (MCR) of terminal alkynes or haloalkynes, carbodiimides, and benzynes is achieved for the first time to construct efficiently 2‐aminoaryl alkynyl imines. The selective formation and ring‐opening of the azetine intermediate with the high ring strain is essential for this reaction. Further transformation of 2‐aminoaryl alkynyl imines via the Cu‐catalyzed cycloisomerization is explored to provide steroselectively the bi‐, tri‐, and tetracyclic fused pyrrolines.     

STUDIES ON ORGANOPHOSPHORUS COMPOUNDS XXX. MASS SPECTROSCOPIC INVESTIGATION OF 2-ALKYL-2-OXO-1,3,2-DIOXA-PHOSPHORINANE AND-PHOSPHEPANE

Abstract

The mass spectra of 2-alkyl-2-oxo-1,3,2-dioxa-phosphorinane and-phosphepane showed that the ring opening was in competition with the cleavage of the P[sbnd]C bond. According to the fragmentation pathway, which was dependent on the structure of exocyclic substituents on phosphorus, the 2-alkyl-2-oxo-1,3,2-di-oxa-phosphorinanes can be classified in two categories. The main process in category A was the ring opening and/or C[sbnd]C bond cleavage. While in category B the cleavage of P[sbnd]C bond was predominant. However, for 2-alkyl-2-oxo-1,3,2-dioxa-phosphepane. no matter how the structure of 2-alkyl group was, the ring opening was a dominant process.     

Theoretical Elucidation of the Mechanism of Cleavage of the Aromatic C?C Bond in Quinoxaline by a Tungsten‐Based Complex [W(PMe3)4(η2‐CH2PMe2)H]

The aromatic C? C bond cleavage by a tungsten complex reported recently by Sattler and Parkin 15 offers fresh opportunities for the functionalization of organic molecules. The mechanism of such a process has not yet been determined, which appeals to computational assistance to understand how the unstrained C? C bond is activated at the molecular level. 16 , 17 In this work, by performing density functional theory calculations, we studied various possible mechanisms of cleavage of the aromatic C? C bond in quinoxaline (QoxH) by the W‐based complex [W(PMe₃)₄(η²‐CH₂PMe₂)H]. The calculated results show that the mechanism proposed by Sattler and Parkin involves an overall barrier of as high as 42.0 kcal mol^?1 and thus does not seem to be consistent with the experimental observation. Alternatively, an improved mechanism has been presented in detail, which involves the removal and recoordination of a second PMe₃ ligand on the tungsten center. In our new mechanism, it is proposed that the C? C cleavage occurs prior to the second C? H bond addition, in contrast to Sattler and Parkin’s mechanism in which the C? C bond is broken after the second C? H bond addition. We find that the rate‐determining step of the reaction is the ring‐opening process of the tungsten complex with an activation barrier of 28.5 kcal mol^?1 after the first PMe₃ ligand dissociation from the metal center. The mono‐hydrido species is located as the global minimum on the potential‐energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide new insight into the mechanism of the remarkable C? C bond cleavage.     

银/氧化铝催化乙醇选择性还原氮氧化物过程中典型中间体的吸附态研究

机动车污染物排放是我国大气复合污染形成的重要原因之一.尽管柴油车在我国机动车保有量中所占比例不到20%,但其排放的颗粒污染物(PM)和氮氧化物(NOx)分担率均超过60%.因此,控制柴油车尾气排放成为我国亟待解决的大气污染问题.目前,氨选择性催化还原NOx技术(NH3-SCR)已规模化应用于柴油车污染排放控制,出于安全性考虑,以尿素水溶液作为氨的来源.但NH3-SCR技术应用于柴油车尾气净化存在如下缺点：需要布建庞大的尿素添加基础设施、后处理系统复杂等.与此相反,以车载燃油为还原剂来源的HC-SCR技术可有效规避上述难题,展现了较好的应用前景.但是,直接以柴油为还原剂时, HC-SCR对NOx净化的效率还难以满足日益严格的排放法规的要求,因此需要深入研究HC选择性还原NOx的微观机制与构效关系,并以此为指导,发展以车载燃料为还原剂来源的高效净化NOx的新原理和新方法.已有的研究表明,银/氧化铝(Ag/Al2O3)具有优异的催化乙醇选择性还原NOx的能力,是最有希望应用于柴油车尾气NOx净化的催化剂-还原剂组合体系.鉴于此,本论文以Ag/Al2O3催化剂上乙醇-SCR反应为研究对象,以密度泛函理论计算方法(DFT)搭建了Ag/Al2O3催化剂的理论模型,考察了反应物乙醇(CH3CH2OH)、关键中间体(烯醇式物种CH2=CHO?和?NCO)在Ag/Al2O3催化剂上的吸附特征,采用电子态密度分析(DOS)研究了以上物种被活化的电子机制,以期甄别Ag/Al2O3催化乙醇选择性还原NOx的活性位结构,为高性能的HC-SCR催化剂设计提供指导.
　　依据化学态的不同, Ag/Al2O3催化剂上活性组分银可分为：高度分散的离子态(Ag+、在催化剂表面以Ag?O形式存在)、部分氧化团簇(Agnδ+)和金属颗粒银(Agn0),其中氧化态的银是催化乙醇选择性还原NOx的活性组分. Al2O3载体的主要暴露晶面为(110)和(100),在上述晶面上Al的配位状态存在明显差异,显著影响了银物种的锚定与分散,形成了具有不同键合特征的Ag?O?Al结构.基于对Al2O3暴露晶面上Al配位状态的分析,搭建了6种Ag?O?Al结构模型.结合Al MAS NMR对Ag/Al2O3实际催化剂的表征结果和理论模型吸附能的分析,获得了最为可能的两种Ag?O?Al结构： Ag?O?Altetra(AlO4)和Ag?O?Alocta(AlO6);前者为AgO与Al2O3(110)面Altrip位键合形成的特征结构(Al最终为四配位),后者系AgO锚定于Al2O3(100)面Alpenta位的能量最优结构(Al最终为六配位).
　　在Ag?O?Altetra上, Altetra位具有较强的酸性, Ag、Al原子轨道的杂化融合有利于电子转移;以上特性促进CH3CH2OH、CH2=CHO?、?NCO的吸附活化.在HC-SCR反应中,关键中间体?NCO通过与NOx直接反应可形成最终产物N2和CO2.可见,?NCO中N=C键的拉伸活化、断裂对上述反应的发生至关重要.由电子态密度分析可知, N=Cσ键能向Ag?O?Altetra中Altetra位转移电子,而Ag与Al的轨道融合能反馈电子到N=C π键;在这两种电子转移机制作用下,?NCO中的N=C键被最大程度弱化,有利其断裂,转化为最终产物N2和CO2.而Ag?O?Alocta上,并没有N=C键的活化拉伸,反而呈现出N=C键收缩趋势,不利于N=C键的断裂与最终产物的形成.由此推定, Ag?O?Altetra是Ag/Al2O3催化剂上HC-SCR反应的活性中心.     

Mechanism of the primary stages of decomposition of aliphatic nitro- and fluoronitronitramines

The primary stage of the decomposition of compounds RN(NO₂)CH₂C(NO₂)₂X is the homolytic cleavage of the C?NO₂ bond, at X=NO₂ and N?NO₂ bond at X=F. The inductive effect of substituents decreases the dissociation energies of the C?N and N?N bonds by 1–2 kcal mol^?1. Kinetic effects caused by the spatial interaction of groups and by stepwise decomposition of polyfunctional compounds are described.     

Cuprous Oxide Catalyzed Oxidative CC Bond Cleavage for CN Bond Formation: Synthesis of Cyclic Imides from Ketones and Amines

Selective oxidative cleavage of a C? C bond offers a straightforward method to functionalize organic skeletons. Reported herein is the oxidative C? C bond cleavage of ketone for C? N bond formation over a cuprous oxide catalyst with molecular oxygen as the oxidant. A wide range of ketones and amines are converted into cyclic imides with moderate to excellent yields. In‐depth studies show that both α‐C? H and β‐C? H bonds adjacent to the carbonyl groups are indispensable for the C? C bond cleavage. DFT calculations indicate the reaction is initiated with the oxidation of the α‐C? H bond. Amines lower the activation energy of the C? C bond cleavage, and thus promote the reaction. New insight into the C? C bond cleavage mechanism is presented.     

Mechanistic Study on the Cleavage and Reorganization of C(sp3)?H and CN Bonds in Carbodiimides: Synthesis of 1,2‐Dihydrothiopyrimidines and 2,3‐Dihydropyrimidinthiones through Four‐Component Coupling

This study sheds light on the cleavage and reorganization of C(sp³)? H and C?N bonds of carbodiimides in a three‐component reaction of terminal alkynes, sulfur, and carbodiimides by a combination of methods including 1) isolation and X‐ray analysis of six‐membered‐ring lithium species 2‐S , 2) trapping of the oxygen‐analogues ( B‐O and D‐O ) of both four‐membered‐ring intermediate B‐S and ring‐opening intermediate D‐S , 3) deuterium labeling studies, and 4) theoretical studies. These results show that 1) the reaction rate‐determining step is [2+2] cycloaddition, 2) the C?N bond cleavage takes place before C(sp³)? H bond cleavage, 3) the hydrogen attached to C6 in 2‐S originates from the carbodiimide, and 4) three types of new aza‐heterocycles, such as 1,2‐dihydrothiopyrimidines, N‐acyl 2,3‐dihydropyrimidinthiones, and 1,2‐dihydropyrimidinamino acids are constructed efficiently based on 2‐S . All results strongly support the idea that the reaction proceeds through [2+2] cycloaddition/4π electrocyclic ring‐opening/1,5‐H shift/6π electrocyclic ring‐closing as key steps. The research strategy on the synthesis, isolation, and reactivity investigation of important intermediates in metal‐mediated reactions not only helps achieve an in‐depth understanding of reaction mechanisms but also leads to the discovery of new synthetically useful reactions based on the important intermediates.     

Beitrag zum Mechanismus der stossinduzierten Fragmentierungen von quaternären Stickstoffverbindungen. 2. Mitteilung über stossinduzierte Fragmentierungen von felddesorbierten Kationen

Mechanistic studies on the collision-induced fragmentations of quaternary ammonium ions The collision-induced fragmentation behaviour of field desorbed quarternary ammonium ions has been investigated. A main reaction of these ions is the cleavage of the N? C bond accompanied by hydrogen rearrangement, i.e. alkane loss from the tetraalkyl substituted ammonium ions of the iodides 1, 2 and 3 , respectively, Deuterium labelling indicates that the hydrogen transfer to the leaving group occurs to the extent of about 80% from the α-position and about 20% from the other positions of an alkyl group. Pronounced heterolytic cleavage of the N? C bond is observed in the benzyl substituted ammonium ion of 4 . The β-phenylethyl substituted ammonium ion of 5 shows a homobenzylic heterolysis, possibly yielding the phenonium ion j.     

C?H and N?H bond activation in reactions of vinyl acetate,allyl cyanide,allylamine and other amines with (η-C5H5)2Rh2(CO)(CF3C2CF3)

There is activation of olefinic C? H bonds when (η-C₅H₅)₂Rh₂(CO)(CF₃C₂CF₃) is treated with vinyl acetate or allyl cyanide. These reactions are initiated by exposure to sunlight. In the vinyl acetate reaction, each of the three vinylic C? H bonds can be broken, but there is strong preference for cleavage at the substituted carbon. The products formed in these reactions are bisalkenyl complexes of the type (η-C₅H₅)₂Rh₂{μ-C(CF₃)C(CF₃)H}(μ-CR?CR′R″), and all isomers have been thoroughly characterized by NMR analysis. Similar reactions with allylamine and other amines (NH₂R, NHMe₂) occur in the dark and proceed by N? H bond cleavage. Near-quantitative amounts of the products, (η-C₅H₅)Rh₂{C(CF₃)C(CF₃)H}(C(O)NRR′) are isolated. Spectroscopic data indicate a bridging carboxamide ligand attached to the Rh? Rh bond from oxygen and nitrogen donor sites. It is proposed that coordination of O or N to rhodium has a strong influence on all of the reactions studied.     

Gold‐Catalyzed [3+2]/Retro‐[3+2]/[3+2] Cycloaddition Cascade Reaction of N‐Alkoxyazomethine Ylides

A novel cascade reaction has been developed for the synthesis of 2,6‐methanopyrrolo[1,2‐b]isoxazoles based on the gold‐catalyzed generation of an N‐allyloxyazomethine ylide. This reaction involves sequential [3+2]/retro‐[3+2]/[3+2] cycloaddition reactions, thus providing facile access to fused and bridged heterocycles which would be otherwise difficult to prepare using existing synthetic methods. Notably, this reaction allows the efficient construction of three C−C bonds, one C−O bond, one C−N bond and one C−H bond, as well as the cleavage of one C−C bond, one C−O bond and one C−H bond in a single operation. The intermolecular cycloaddition of an N‐allyloxyazomethine ylide and the subsequent application of the product to the synthesis of tropenol is also described.     

Breakdown of cyclic dithiohemiorthoamide tetrahedral intermediates - RC(SR′)(NR″2)SH - at sub-zero temperatures 1,2

The uslfhydrolyses of thioimidate salts $\text{2a}$–$\text{2d}$, in the presence of anh. NASH in dry 2-butanone at ?82°C, proceed by the preferential cleavage of the CN bond rather than the CS bond.     

Comprehensive Analysis of DNA Strand Breaks at the Guanosine Site Induced by Low‐Energy Electron Attachment

To elucidate the role of guanosine in DNA strand breaks caused by low‐energy electrons (LEEs), theoretical investigations of the LEE attachment‐induced C? O σ‐bonds and N‐glycosidic bond breaking of 2′‐deoxyguanosine‐3′,5′‐diphosphate (3′,5′‐dGMP) were performed using the B3LYP/DZP++ approach. The results reveal possible reaction pathways in the gas phase and in aqueous solutions. In the gas phase LEEs could attach to the phosphate group adjacent to the guanosine to form a radical anion. However, the small vertical detachment energy (VDE) of the radical anion of guanosine 3′,5′‐diphosphate in the gas phase excludes either C? O bond cleavage or N‐glycosidic bond breaking. In the presence of the polarizable surroundings, the solvent effects dramatically increase the electron affinities of the 3′,5′‐dGDP and the VDE of 3′,5′‐dGDP^?. Furthermore, the solvent–solute interactions greatly reduce the activation barriers of the C? O bond cleavage to 1.06–3.56 kcal mol^?1. These low‐energy barriers ensure that either C_5′? O_5′ or C_3′? O_3′ bond rupture takes place at the guanosine site in DNA single strands. On the other hand, the comparatively high energy barrier of the N‐glycosidic bond rupture implies that this reaction pathway is inferior to C? O bond cleavage. Qualitative agreement was found between the theoretical sequence of the bond breaking reaction pathways in the PCM model and the ratio for the corresponding bond breaks observed in the experiment of LEE‐induced damage in oligonucleotide tetramer CGTA. This concord suggests that the influence of the surroundings in the thin solid film on the LEE‐induced DNA damage resembles that of the solvent.     

Computational studies on azaphosphiridines, or how to effect ring-opening processes through selective bond activation

The relative energies of azaphosphiridine and its isomers, the ring stability towards valence isomerization, and the ring strain, as well as the kinetics and thermodynamics of possible ring‐opening reactions of P^III derivatives ( 1 – 5 ) and P^V chalcogenides ( 6 – 9 ; O to Te), were studied at high levels of theory (up to CCSD(T)). The barrier to inversion at the nitrogen atom in the trimethyl‐substituted P^III derivative 5 increases from 12.11 to 15.25 kcal mol^?1 in the P‐oxide derivative 6 (P^V); the relatively high barrier to inversion at the phosphorus in 5 (75.38 kcal mol^?1) points to a configurationally stable center (MP2/def2‐TZVPP//BP86/def2‐TZVP). The ring strain for azaphosphiridine 5 (av. 22.6 kcal mol^?1) was found to increase upon P‐oxidation ( 6 ) (30.8 kcal mol^?1; same level of theory). Various ring‐bond‐activation processes were studied: N‐protonation of P^III ( 5 ) and P^V ( 6 , 7 ) derivatives leads to highly activated species that readily undergo P? N bond cleavage. In contrast, metal chlorides such as LiCl, CuCl, CuCl₂, BeCl₂, BCl₃, AlCl₃, TiCl₃, and TiCl₄ show little P? N bond activation in 5 and 7 . Remarkably, TiCl₃ selectively activates the C? N bond, and induces stronger bond activation for P^V ( 6, 7 ) than for P^III azaphosphiridines ( 5 ). The ring‐expanding rearrangement of P^V azaphosphiridines 6 – 9 to yield P^III 1,3,2‐chalcogena‐azaphosphetidines 32 a – d is predicted to be preferred for the heavier chalcogenides 7 – 9 , but not for the P‐oxide 6 . The first comparative analysis of three bond strength parameters is presented: 1) the electron density at bond critical points, 2) Wiberg’s bond index, and 3) the relaxed force constant. This reveals the usefulness of these parameters in assessing the degree of ring bond activation.