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1.
α‐Cyclopropyl stability impacts on singlet and triplet heterocyclic carbenes with acyclic, cyclic, and cyclic‐unsaturated structures are compared and contrasted to di‐t‐butyl as well as t‐butylcyclopropylcarbenes through appropriate isodesmic reactions at B3LYP/AUG‐cc‐pVTZ level. Substitution of one of the t‐butyl groups of di‐t‐butylcarbene with a cyclopropyl alters the ground state multiplicity from triplet to singlet with a singlet–triplet energy separation (ΔEs–t) of 7.2 kcal/mol. Additional heteroatom substitution increases ΔEs–t values for the resulting α‐heteroatom cyclopropylcarbenes in the following order: amino > oxy > thio > phophino. α‐Cyclopropyl group stabilizes singlet states of all our carbenes two to three times more than their corresponding triplet states. The ΔEs–t values of all the carbenes are increased through cyclization, while the introduction of unsaturation in the rings causes small and rather random changes. To probe the kinetic stability of the species, we calculated the transition states for the opening of cyclopropyl through 1,2‐C shift. Interestingly, the 4.1 kcal/mol energy barrier in cyclopropylcarbene is significantly increased in the presence of heteroatoms to 31.2 kcal/mol for aminocyclopropylcarbene. The reactivity of the species is discussed in terms of nucleophilicity and electrophilicity issues showing our carbenes, especially acyclic ones, more nucleophilic than the common N‐heterocyclic carbenes. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

2.
Nine boat‐shaped cyclonona‐3,5,7‐trienylidenes are compared and contrasted with respect to their multiplicity, nucleophilicity, electrophilicity, band gap (ΔEHOMO ? LUMO), Natural bond orbital (NBO) atomic charge, force constant, as well as the aptitude for dimerization, and rearrangement through proper isodesmic reactions at B3LYP/AUG‐cc‐pVTZ and B3LYP/6‐311++G**//B3LYP/6‐31+G* levels of theory. The nine cyclic carbenes include unsubstituted (1CH2) plus eight α‐cyclopropylcyclonona‐3,5,7‐trienylidenes, which are substituted with ?‐SiMe2, ?‐NMe, ?‐PMe, ?‐O, ?‐S, ?‐CH2, ?‐cyclopropyl, and ?‐CMe2 (2SiMe2, 2NMe, 2PMe, 2O, 2S, 2CH2, 2cyclopropyl, and 2CMe2, respectively). The latter eight species enjoy the stabilizing interaction of the occupied Walsh orbital of cyclopropyl with the vacant pπ orbital of the carbene center (Walshcyclopropyl → pπ carbene). Among them, the singlet closed shell 2NMe appears the most promising for exhibiting the highest relative singlet–triplet energy gap (ΔEs ? t = 27.1 kcal mol?1). In contrast, the least stable derivative is triplet 2SiMe2, which exhibits the lowest relative ΔEs ? t of ?5.5 kcal mol?1. The overall trend of ΔEs‐t is 2NMe > 2PMe > 2S > 2O > 2cyclopropyl > 2CMe2 > 2CH2 > 1CH2 > 2SiMe2. With one negative force constant, the unsubstituted 1CH2 turns out to be a transition state, whereas the rest emerge as minima. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

3.
Replacement of α‐methylenes with BH, AlH, CMe2, SiH2, NH, NMe, NtButyl, NPh, PH, O, and S in non‐planar cyclonona‐3,5,7‐trienylidene (CH2) alters its status from an unstable transition state to rather stable minima, at B3LYP/6‐311++G**//B3LYP/6‐31 + G* levels of theory. All species appear with singlet closed shell (Scs) global minima, except for SiH2 and CH2 which exhibit triplet electronic ground states. The order of stability based on singlet–triplet energy gap (ΔEs–t / kcalmol?1) is: CMe2 (45.8) > NH (35.8) > NMe (32.3) > O (31.5) > NtButyl (27.7) ≥ NPh (27.5) ≥ BH (27.4) > S (21.9) > PH (17.0) > CH2 (?4.4) > SiH2 (?12.5). In contrast to many reports on N‐heterocyclic carbenes, here alkyl groups appear to exert a higher stabilizing effect than heteroatoms, making CMe2 the most stable. In addition bulky NMe, NtButyl, and NPh appear more nucleophilic than their synthesized imidazol‐2‐ylidene congeners. Excluding SiH2, isodesmic reactions reveal that all substituents stabilize singlet state considerably more than the corresponding triplet. Finally, this work is hoped to pave the path for future matrix isolations and IR studies of these rather stable cyclic non‐planar carbenes. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

4.
High levels of ab initio and DFT calculations (B3LYP/6‐311++G**, B3LYP/AUG‐cc‐pVTZ, and CCSD(T)/6‐311++G** levels) coupled with isodesmic reactions are used to compare and contrast the multiplicities and relative stabilities of singlet (s) and triplet (t) acyclic carbenes, including: dimethylcarbene, diaminocarbene, and diphosphinocarbene along with their saturated and unsaturated cyclic ones. Cyclization is unfavorable for all acyclic carbenes while unsaturation of cyclic analogs appears favorable. The simultaneous cyclization and unsaturation of dimethylcarbene increases the singlet–triplet energy gap (ΔEs–t), while for diphosphinocarbene the situation is reversed. For diaminocarbene the increase of ΔEs–t is encountered only during cyclization. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

5.
Density functional theory (DFT) calculations introduced triplet ground states for [6]n SiC‐cyclacenes and ‐acenes with alternate silabenzene rings including silicon atoms in 2 opposite edges (n = 6, 8, 10, 12). The singlet‐triplet energy gap (ΔE(S‐T)), binding energy per atom (BE/n), and NBO calculation with very small band gap (ΔELUMO‐HOMO) confirmed the triplet ground states. In contrast to polyacenes, the singlet [6]n SiC‐cyclacenes displayed more stability improvement than triplets, through n increasing. This may open the way for synthesis of larger stable [6]n SiC‐cyclacenes. The ΔE(S‐T), BE/n, and the strain energy through homodesmic equations indicated more stability for larger [6]n SiC‐cyclacenes, which was more noticeable in singlet states. Cyclacenes and acenes with high conductivity and full point charge were introduced as suitable candidates for hydrogen storage.  相似文献   

6.
High‐level Density Functional Theory calculations, coupled with appropriate isodesmic reaction, are employed to investigate the effects of α‐carbon, ammonium, phosphorus, and sulfur ylides, cyclization, and unsaturation on the stability, multiplicity, and reactivity of novel singlet (S) and triplet (T) carbenes. Among them the highly π‐donating α‐ammonium ylide is found to exert the highest stabilizing effect on the carbenic center. α‐Ammonium ylides resist dimerization and hydrogenation. They show wider singlet–triplet energy gap (ΔΕS–T), broader band gap (ΔΕHOMO–LUMO), and higher nucleophilicity compared to the reported stable N‐heterocyclic carbenes. Aromatic cyclic unsaturated ammonium, phosphorus, and sulfur ylide carbenes appear more stable than their saturated cyclic analogs which are in turn more viable than their acyclic counterparts. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

7.
In regard to the worldwide interest in synthesis and application of stable carbenes, ab initio and DFT calculations (CCSD(T)/6‐311G*//QCISD/6‐31G* and B3LYP/AUG‐cc‐pVTZ//B3LYP/6‐31 + G* levels) are employed to reach at a series of symmetrically substituted diheteroatom carbenes. In this way the multiplicity, stability, and reactivity of diethylcarbene and its saturated and unsaturated cyclic congeners are compared and contrasted to the corresponding disilyl‐, diamino‐, diphosphino‐, dioxy‐, and dithiocarbenes. The investigations reveal the enlargement of the singlet–triplet energy gaps (ΔES‐T) in the order of (amino ≈ oxy) > thio > phosphino > alkyl > silyl. The observed trend is thoroughly analyzed applying appropriate isodesmic reactions which differentiate the substituent effects on each of the singlet and triplet states of all the carbenes. In contrast to previous reports, it is found that π‐donor/σ‐acceptor amino substituents stabilize not only the singlet but also the triplet states. The effects of cyclization and unsaturation are also probed in each series of the symmetrically substituted carbenes. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity, and proton affinity. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

8.
The conformational behaviors of trans‐2,3‐dihalo‐1,4‐dithiane [halo = F ( 1 ), Cl ( 2 ), Br ( 3 )] and trans‐2,5‐dihalo‐1,4‐dithiane [halo = F ( 4 ), Cl ( 5 ), Br ( 6 )] have been analyzed by means of complete basis set CBS‐4, hybrid‐density functional theory (B3LYP/6‐311 + G**//B3LYP/6‐311 + G**) based methods, and natural bond orbital (NBO) interpretation. Both methods showed that the axial conformations of compounds 1–5 are more stable than their equatorial conformations but CBS‐4 resulted in an equatorial preference for compound 6 . The Gibbs free energy difference (Geq?Gax) values (i.e., ΔGeq–ax) at 298.15 K and 1 atm between the axial and equatorial conformations decrease from compound 1 to compound 2 but increase from compound 2 to compound 3 . Also, the calculated ΔGeq–ax values decrease from compound 4 to compound 6 . The NBO analysis of donor–acceptor (LP → σ*) interactions showed that the anomeric effect (AE) increase from compound 1 to compound 3 and also from compound 4 to compound 6 . On the other hand, the calculated dipole moment values between the axial and equatorial conformations [Δ(µeq?µax)] decrease from compound 1 to compound 3 . The conflict between the increase of AE and the decrease of Δ(µeq?µax) values could explain the variation of the calculated ΔGeq–ax for compounds 1–3 . The Gibbs free energy difference values between the axial and equatorial conformations (i.e., ΔGax–ax and ΔGeq–eq) of compounds 1 and 4 , 2 and 5 and also 3 and 6 have been calculated. The correlations between the AE, bond orders, pairwise steric exchange energies (PSEE), ΔGeq–ax, ΔGax–ax, ΔGeq–eq, dipole–dipole interactions, structural parameters, and conformational behaviors of compounds 1–6 have been investigated. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

9.
The kinetics of nucleophilic bimolecular substitution reactions of γ‐functionalized allyl bromides with non‐substituted and p‐substituted sodium arenesulfinates has been studied. Both the structure of allyl bromides and nucleophilicity of arenesulfinate ions exerted a significant effect on the values of the kinetic parameters such as the second‐order rate constants k, activation energy EA, and changes in the entropy ΔS, enthalpy ΔH, and free energy ΔG of the formation of the activated complex from reactants. Based on the evaluation of kinetic parameters, the reactants could be arranged, according to their decreasing reactivity in the SN2‐reactions as follows: p‐toluenesulfinate ion > benzenesulfinate ion > p‐chlorobenzenesulfinate ion and 4‐bromo‐2‐butenenitrile > 1,3‐ dibromopropene, respectively. Comparison was also made between the kinetic data obtained and some delocalization reactivity indexes for both the substrates and nucleophiles. The enthalpy–entropy compensation effect was observed for the reactions of sodium arenesulfinates with γ‐functionalized allyl bromides. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
Aryl‐substituted polyfluorinated carbanions, ArCHRf? where Rf = CF3 ( 1 ), C2F5 ( 2 ), i‐C3F7 ( 3 ), and t‐C4F9 ( 4 ), were analyzed by means of the natural bond orbital (NBO) theory at the B3LYP/6‐311+G(d,p) computational level. A lone pair NBO at the formal anionic center carbon (Cα) was not found in the Lewis structure. Instead, significant donor/acceptor NBO interactions between π(Cα‐C1) and σ*(Cβ‐F) or σ*(Cβ‐Cγ) were observed for 1 , 2 , 3a (strong electron‐withdrawing substituent, from p‐CF3 to p‐NO2), and 4 . Their second‐order donor/acceptor perturbation interaction energy, E(2), values decreased with the increase of the stability of carbanions. A larger E(2) value corresponds to longer Cβ‐F and Cβ‐Cγ bonds and a shorter Cα‐Cβ bond, indicating that the E(2) values can be associated with the negative hyperconjugation of the Cβ‐F and Cβ‐Cγ bonds. In accordance with this, the E(2) values for π(Cα‐C1) → σ*(Cβ‐F) were linearly correlated with the ΔGoβ‐F values (an empirical measure of β‐fluorine negative hyperconjugation obtained from an increased acidity). In 3b (weak electron‐withdrawing substituents, from H to m‐NO2) very large E(2) values for LP(Fβ) → π*(Cα‐Cβ) were obtained. This was attributed to the Cβ‐F bond cleavage and the Cα‐Cβ double bond formation in the Lewis structure that is caused by the extremely strong negative hyperconjugation of the Cβ‐F bond.  相似文献   

11.
The conformational equilibria of 3‐methyl‐3‐silathiane 5 , 3‐fluoro‐3‐methyl‐3‐silathiane 6 and 1‐fluoro‐1‐methyl‐1‐silacyclohexane 7 have been studied using low temperature 13C NMR spectroscopy and theoretical calculations. The conformer ratio at 103 K was measured to be about 5 ax: 5 eq = 15:85, 6 ax: 6 eq = 50:50 and 7 ax: 7 eq = 25:75. The equatorial preference of the methyl group in 5 (0.35 kcal mol?1) is much less than in 3‐methylthiane 9 (1.40 kcal mol?1) but somewhat greater than in 1‐methyl‐1‐silacyclohexane 1 (0.23 kcal mol?1). Compounds 5–7 have low barriers to ring inversion: 5.65 (ax → eq) and 6.0 (eq → ax) kcal mol?1 ( 5 ), 4.6 ( 6 ), 5.1 (Meax → Meeq) and 5.4 (Meeq → Meax) kcal mol?1 ( 7 ). Steric effects cannot explain the observed conformational preferences, like equal population of the two conformers of 6 , or different conformer ratio for 5 and 7 . Actually, by employing the NBO analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions between the Si–X and adjacent C–H, C–S, and C–C bonds proved responsible. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

12.
The effects of phosphorous atom on the stability, multiplicity, and reactivity of six‐member cyclic silylenes are investigated at B3LYP/AUG‐cc‐pVTZ//B3LYP/6‐31+G* and MP2/6‐311++G**//B3LYP/6‐31+G* coupled with appropriate isodesmic reactions. From a thermodynamic point of view, 1H‐2‐silaphosphinine‐2‐ylidene ( 1a ) and 1H‐4‐silaphosphinine‐4‐ylidene ( 2a ) are relatively the most stable with singlet–triplet energy gaps (ΔES–T) of 37.0 and 28.1 kcal/mol, respectively. The calculated energy barrier for the 1,2‐H shift of 1a to the corresponding 2‐silapyridine ( 1 ) is 26.5 kcal/mol, which is lower than the 28.8 kcal/mol required for the 1,4‐H shift of 2a to the corresponding 4‐silapyridine ( 2 ). In contrast to the previous reports, isodesmic reactions indicate that π‐donor/σ‐donor phosphorous destabilizes the singlet while stabilizes the triplet state. Both 1a and 2a silylenes appear invulnerable to the head‐to‐head as well as the head‐to‐tail dimerization, inviting experimental explorations. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

13.
Dyes derived from the biradicalic oxyallyl and cyclopentadienylium were calculated by time‐dependent density functional theory (TDDFT) and the characteristics of the prominent low‐energy transitions revealed by graphical means. According to theoretical and experimental studies, 4‐aminophenyl‐substituted dyes absorb intensely at long wavelengths up to the near infrared. If the amino groups are removed the absorption wavelengths are changed little. As found in the previous studies on the squaraine and croconaine oxyallyl dyes, the substituents play only a minor role in the spectral excitation. Charge‐transfer‐type excitations do not occur between the donor aryl substituents and the central oxyallyl or cyclopentadienylium acceptor group. This behaviour is exceptional since donor–acceptor compounds tend to produce charge‐transfer‐ or polymethine‐type electronic transitions. The hitherto rarely used electron density difference (EDD) maps clearly unveiled the spectral excitation features. The spectral excitation of the title compounds is predominantly localized at the oxyallyl and cyclopentadienylium groups, respectively. Characteristics of simple chromophoric compounds and of conventional CT‐ and polymethine dyes are given for comparison. The biradicaloid character of these dyes is supported by the calculated low singlet–triplet splitting energies. Spin properties were characterized in terms of expectation values of the S2‐operator and antiaromatic properties in terms of nucleus‐independent chemical shifts (NICS). According to the ΔES/T and <S2> criteria compounds with the acyclic oxyallyl fragment are more biradicaloid. The parent compounds oxyallyl, thioxyallyl and cyclopentadienylium, display extremely large <S2> values. These compounds are triplets in the ground state. The absorption wavelengths of selected biradicaloid species were also calculated by the multi‐reference SORCI method. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

14.
The first conformational analysis of 3‐silathiane and its C‐substituted derivatives, namely, 3,3‐dimethyl‐3‐silathiane 1 , 2,3,3‐trimethyl‐3‐silathiane 2 , and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane 3 was performed by using dynamic NMR spectroscopy and B3LYP/6‐311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion barriers ΔG for 1 and 2 were estimated to be 6.3 and 6.8 kcal/mol, respectively. These values are considerably lower than that of thiacyclohexane (9.4 kcal/mol) but slightly higher than the one of 1,1‐dimethylsilacyclohexane (5.5 kcal/mol). The conformational free energy for the methyl group in 2 (?ΔG° = 0.35 kcal/mol) derived from low‐temperature 13C NMR data is fairly consistent with the calculated value. For compound 2 , theoretical calculations give ΔE value close to zero for the equilibrium between the 2 ‐Meax and 2 ‐Meeq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (?ΔG° = 1.8 kcal/mol) and the predominance of the 3 ‐SiMe3 eq conformer at room temperature was confirmed by the simulated 1H NMR and 2D NOESY spectra. The effect of the 2‐substituent on the structural parameters of 2 and 3 is discussed. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

15.
A simple linear regression (Q equation) is devised to position solvolyses within the established SN2‐SN1 spectrum of solvolysis mechanisms. Using 2‐adamantyl tosylate as the SN1 model and methyl tosylate as the SN2 model, the equation is applied to solvolyses of ethyl, allyl, secondary alkyl and a range of substituted benzyl and benzoyl tosylates. Using 1‐adamantyl chloride as the SN1 model and methyl tosylate as the SN2 model, the equation is applied to solvolyses of substituted benzoyl chlorides in weakly nucleophilic media. In some instances, direct correlations with methyl tosylate were employed. Grunwald–Winstein l values and kinetic solvent isotope effects are also used to locate solvolyses within the spectrum of mechanisms. Product selectivities (S) for solvolyses at 50 °C of p‐nitrobenzyl tosylate in binary mixtures of alcohol–water and of alcohol–ethanol for five alcohols (methanol, ethanol, 1‐propanol and 2‐propanol and t‐butanol) are reported and show the expected order of solvent nucleophilicity (RCH2OH > R2CHOH > R3COH). The data support the original assignments establishing the NOTs scale of solvent nucleophilicity. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

16.
Trends between the Hammett's σp and related normal , inductive σI, resonance σR, negative and positive polar conjugation and Taft's σp° substituent constants and the distance, δN? H NMR chemical shift, oxidation potential (Ep/2°x, measured in this study by cyclic voltammetry (CV)) and thermodynamic parameters (pK, ΔG0, ΔH0 and ΔS0) of the dissociation process of unsubstituted 3‐(phenylhydrazo)pentane‐2,4‐dione (HL1) and its para‐substituted chloro (HL2), carboxy (HL3), fluoro (HL4) and nitro (HL5) derivatives were recognized. The best fits were found for σp and/or in the cases of , δN? H and Ep/2°x, showing the importance of resonance and conjugation effects in such properties, whereas for the above thermodynamic properties the inductive effects (σI) are dominant. HL2 exists in the hydrazo form in DMSO solution and in the solid state and contains an intramolecular H‐bond with the distance of 2.588(3) Å. It was also established that the dissociation process of HL1–5 is non‐spontaneous, endothermic and entropically unfavourable, and that the increase in the inductive effect (σI) of para‐substitutents (? H < ? Cl < ? COOH < ? F < ? NO2) leads to the corresponding growth of the distance and decrease of the pK and of the changes of Gibbs free energy, of enthalpy and of entropy for the HL1–5 acid dissociation process. The electrochemical behaviour of HL1–5 was interpreted using theoretical calculations at the DFT/HF hybrid level, namely in terms of HOMO and LUMO compositions, and of reactivities induced by anodic and cathodic electron‐transfers. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

17.
The gas‐phase elimination kinetics of the title compounds were carried out in a static reaction system and seasoned with allyl bromide. The working temperature and pressure ranges were 200–280 °C and 22–201.5 Torr, respectively. The reactions are homogeneous, unimolecular, and follow a first‐order rate law. These substrates produce isobutene and corresponding carbamic acid in the rate‐determining step. The unstable carbamic acid intermediate rapidly decarboxylates through a four‐membered cyclic transition state (TS) to give the corresponding organic nitrogen compound. The temperature dependence of the rate coefficients is expressed by the following Arrhenius equations: for tert‐butyl carbamate logk1 (s?1) = (13.02 ± 0.46) – (161.6 ± 4.7) kJ/mol(2.303 RT)?1, for tert‐butyl N‐hydroxycarbamate logk1 (s?1) = (12.52 ± 0.11) – (147.8 ± 1.1) kJ/mol(2.303 RT)?1, and for 1‐(tert‐butoxycarbonyl)‐imidazole logk1 (s?1) = (11.63 ± 0.21)–(134.9 ± 2.0) kJ/mol(2.303 RT)?1. Theoretical studies of these elimination were performed at Møller–Plesset MP2/6‐31G and DFT B3LYP/6‐31G(d), B3LYP/6‐31G(d,p) levels of theory. The calculated bond orders, NBO charges, and synchronicity (Sy) indicate that these reactions are concerted, slightly asynchronous, and proceed through a six‐membered cyclic TS type. Results for estimated kinetic and thermodynamic parameters are discussed in terms of the proposed reaction mechanism and TS structure. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

18.
The effects of substituents on the stability of 4‐substituted(X) cub‐1‐yl cations ( 2 ), as well as the benchmark 4‐substituted(X) bicyclo[2.2.2]oct‐1‐yl cation systems ( 7 ), for a set of substituents (X = H, NO2, CN, NC, CF3, COOH , F, Cl, HO, NH2, CH3, SiH3, Si(CH3)3, Li, O?, and NH) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) level of theory. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 2 / 7 and polar field/group electronegativity substituent constants (σF and σχ, respectively). The analysis reveals that the ΔE values of both systems are best described by a combination of both substituent constants. This highlights the distinction between through‐space and through‐bond electronic influences characterized by σF and σχ, respectively. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

19.
The competitive 1,5‐electrocyclization versus intramolecular 1,5‐proton shift in imidazolium allylides and imidazolium 2‐phosphaallylides has been investigated theoretically at the DFT (B3LYP/6‐311 + +G**//B3LYP/6‐31G**) level. 1,5‐Electrocyclization follows pericyclic mechanism and its activation barrier is lower than that for the pseudopericyclic mechanism by ~5–6 kcal mol?1. The activation barriers for 1,5‐electrocyclization of imidazolium 2‐phosphaallylides are found to be smaller than those for their nonphosphorus analogues by ~3–5 kcal mol?1. There appears to be a good correlation between the activation barrier for intramolecular 1,5‐proton shift and the density of the negative charge at C8, except for the ylides having fluorine substituent at this position ( 7b and 8b ). The presence of fluorine atom reduces the density of the negative charge at C8 (in 7b it becomes positively charged) and thus raises the activation barrier. The ylides 7f and 8f having CF3 group at C8, in preference to the 1,5‐proton shift, follow an alternative route leading to different carbenes which is accompanied by the loss of HF. The carbenes Pr 7 , 8b – e resulting from intramolecular 1,5‐proton shift have a strong tendency to undergo intramolecular SN2 type reaction, the activation barrier being 7–28 kcal mol?1. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

20.
Metal–ligand bond enthalpy data can afford invaluable insights into important reaction patterns in organometallic chemistry and catalysis. In this paper, the Fe–O and Fe–S homolytic bond dissociation energies [ΔHhomo(Fe–O)'s and ΔHhomo(Fe–S)'s] of two series of para‐substituted phenoxydicarbonyl(η5‐cyclopentadienyl) iron [p‐G‐C6H4OFp ( 1 )] and (para‐substituted benzenethiolato)dicarbonyl(η5‐cyclopentadienyl) iron [p‐G‐C6H4SFp ( 2 )] were studied using Hartree–Fock and density functional theory (DFT) methods with large basis sets. In this study, Fp is (η5‐C5H5)Fe(CO)2, and G are NO2, CN, COMe, CO2Me, CF3, Br, Cl, F, H, Me, MeO, and NMe2. The results show that DFT methods can provide the best price/performance ratio and accurate predictions of ΔHhomo(Fe–O)'s and ΔHhomo(Fe–S)'s. The remote substituent effects on ΔHhomo(Fe–O)'s and ΔHhomo(Fe–S)'s [ΔΔHhomo(Fe–O)'s and ΔΔHhomo(Fe–S)'s] can also be satisfactorily predicted. The good correlations [r = 0.98 (g, 1), 0.98 (g, 2)] of ΔΔHhomo(Fe–O)'s and ΔΔHhomo(Fe–S)'s in series 1 and 2 with the substituent σp+ constants imply that the para‐substituent effects on ΔHhomo(Fe–O)'s and ΔHhomo(Fe–S)'s originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. ΔΔHhomo(Fe–O)'s ( 1 ) and ΔΔHhomo(Fe–S)'s ( 2 ) conform to the captodative principle. Insight from this work may help the design of more effective catalytic processes. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

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