Table 2 - uploaded by Golam Rasul
Content may be subject to copyright.
Source publication
A comparative study of the 2,3-dimethyl-3-fluoro-2-butyl cation and its chloro analog was carried out by the ab initio/GIAO-CCSD(T) (gauge invariant atomic orbital-coupled cluster with single, double, and perturbative triple excitation) method. The structures and (13)C NMR chemical shifts of the cations were calculated at the GIAO-CCSD(T)/tzp/dz//M...
Contexts in source publication
Context 1
... ion 1, although a minimum on the potential energy surface, is 12.8 kcal/mol less stable than the structure 3 (Table 1). This clearly rules out a path involving equilibration of the ion 2 through intramolecular fluorine migration through in- termediate 1. Relative energies of the structures 1-4 were plot- ted in Fig. 2 The 13 C NMR chemical shifts of 1-3 were calculated by the GIAO-coupled cluster method at the GIAO-CCSD(T)/tzp/dz level using MP2/cc-pVTZ geometry (Table 2). For comparison, the 13 C NMR chemical shifts of the ions were also computed at the GIAO-MP2/tzp/dz and GIAO-SCF/tzp/dz levels ( Table 2). ...
Context 2
... clearly rules out a path involving equilibration of the ion 2 through intramolecular fluorine migration through in- termediate 1. Relative energies of the structures 1-4 were plot- ted in Fig. 2 The 13 C NMR chemical shifts of 1-3 were calculated by the GIAO-coupled cluster method at the GIAO-CCSD(T)/tzp/dz level using MP2/cc-pVTZ geometry (Table 2). For comparison, the 13 C NMR chemical shifts of the ions were also computed at the GIAO-MP2/tzp/dz and GIAO-SCF/tzp/dz levels ( Table 2). The calculated 13 C NMR spectrum of 2 shows absorption at δ 13 C 346.8 (C2 carbon or C + ), 108.6 (C3), and 25.5-45.3 ...
Context 3
... have also computed the 13 C chemical shifts of chloro ions 5-7 at the GIAO-MP2/tzp/dz level using MP2/cc-pVTZ geometries, and these are shown in Table 2. More accurate 13 C chemical shift calculations of 5-7 at the GIAO-CCSD(T)/tzp/dz level were not possible as the total number of electrons involved in chloro ions are considerably more than those of structurally similar fluoro ions. ...
Context 4
... accurate 13 C chemical shift calculations of 5-7 at the GIAO-CCSD(T)/tzp/dz level were not possible as the total number of electrons involved in chloro ions are considerably more than those of structurally similar fluoro ions. Calculated δ 13 C of C1 and methyl carbons of the chloro- nium ion 5 are 130.0 and 27.2, respectively (Table 2), compared with the reported experimental values of 151.7 and 28.0, re- spectively (16). Again, as the ions (5, 6, and 7) are energetically different by less than 10 kcal/mol, an equilibrium mixture (un- dergoing fast chlorine exchange on the NMR timescale) in- volving ions 5, 6, and 7 might best represent the structure of the 2,3-dimethyl-3-chloro-2-butyl cation (Scheme 3). ...
Similar publications
Complexity of the potential energy surface of the 9-homocubyl cation is revealed by Born-Oppenheimer molecular dynamics simulations and high ab initio levels. The stereospecific automerizations observed experimentally involve bridged ions, which have either an aromatic or an anti-aromatic character. New pathways leading to more stable isomers are u...
Citations
... By comparing the NPA charges of fumaryl fluoride with the NPA charges of the HF complex of the dication, the negative charges of the oxygen atoms increase slightly, whereas the negative charges of the fluorine atoms decrease considerably. The electron distribution shifts from the fluorine atom, which is inductively electron withdrawing but electron donating by resonance (+ M effect) despite its high electronegativity, [28][29][30][31][32] to the CÀ F bond in the diprotonated cation. This resonance effect, that opposes the inductive effect, generating a more electropositive carbon, is established in literature as + R effect, [33,34] which is illustrated in Scheme 1. ...
... The bond between carbon and fluorine reveals a low double bond character, which is described in the literature for several organofluorine compounds. [28][29][30][31][32][33][34] To draw comparisons of diprotonated fumaryl fluoride with organofluorine species, which are already related to resonance effects based on fluorine, the CÀ F bond lengths of CHF 3 [35] and CF 3 Cl [36] are consulted. The carbon fluorine bond distances in CHF 3 (1.326(13) ...
... In order to investigate the electron distribution and the charge-related properties of the diprotonated species, electrostatic potential (ESP) maps combined with natural population analysis (NPA) charges were calculated. The electron distribution shifts from the fluorine atom, which is inductively electron withdrawing, although it is electron donating by resonance, [28][29][30][31][32] to the CÀ F bond in the diprotonated cation, known in literature as the + R effect. [33] In the NMR spectra of diprotonated fumaryl fluoride, the enhanced resonance-electron-donating ability is observed in the increased protonfluorine coupling constants. ...
The reaction of fumaryl fluoride with the superacidic solutions XF/MF5 (X=H, D; M=As, Sb) results in the formation of the monoprotonated and diprotonated species, dependent on the stoichiometric ratio of the Lewis acid to fumaryl fluoride. The salts [C4H3F2O2]⁺[MF6]⁻ (M=As, Sb) and [C4H2X2F2O2]²⁺([MF6]⁻)2 (X=H, D; M=As, Sb) are the first examples with a protonated acyl fluoride moiety. They were characterized by low‐temperature vibrational spectroscopy. Low‐temperature NMR spectroscopy and single‐crystal X‐ray structure analyses were carried out for [C4H3F2O2]⁺[SbF6]⁻ as well as for [C4H4F2O2]²⁺([MF6]⁻)2 (M=As, Sb). The experimental results are discussed together with quantum chemical calculations of the cations [C4H4F2O2 ⋅ 2 HF]²⁺ and [C4H3F2O2 ⋅ HF]⁺ at the B3LYP/aug‐cc‐pVTZ level of theory. In addition, electrostatic potential (ESP) maps combined with natural population analysis (NPA) charges were calculated in order to investigate the electron distribution and the charge‐related properties of the diprotonated species. The C−F bond lengths in the protonated dication are considerably reduced on account of the +R effect.
... Irrespective of the electronic properties on the aromatic rings, high enantioselectivities were observed. However, the diastereoselectivities were almost 1 : 1. Coupled with the findings in the literature, [161][162][163][164] fluorocyclization proceeds via the formation of a benzylic carbocation intermediate, which undergoes less stereoselective intramolecular C-O bond formation. Thus, we anticipated that the stereoselectivity would be improved if a substituent was attached at the β-position of an allylic amide to restrict the conformation of the carbocation intermediate. ...
In spite of only a few naturally occurring products having one or more fluorine atoms, organofluorine compounds have been widely utilized in pharmaceutical, agrochemical, and functional material science fields due to the characteristic properties of the fluorine atom. Therefore, the development of new methods for the introduction of fluorine-containing functional groups has been a long-standing research topic. This article discusses our contributions to this area. The first topic is on the trifluoromethylations of C–C multiple bonds using Togni reagent based on our working hypothesis that hypervalent iodine could be activated by coordination of the carbonyl moiety to the Lewis acid catalyst. The second topic relates to asymmetric fluorofunctionalization of alkenes. A newly designed phase-transfer catalyst consisting of a carboxylate anion functioning as a phase-transfer agent and a primary hydroxyl group as a site that captures the anionic substrate was revealed to be an effective catalyst for asymmetric fluorolactonization. Inspired by the mechanistic studies of fluorolactonization, we produced a linked binaphthyl dicarboxylate catalyst, which catalyzes the 6-endo-fluorocyclization and the deprotonative fluorination of allylic amides in a highly enantioselective manner. The third topic is on C–H fluorofunctionalizations using either catalysis or photoactivation. Benzylic trifluoromethylation, which is still a rare reaction, using Togni reagent and aromatic C–H trifluoromethylation using Umemoto reagent under simple photoirradiation conditions were achieved. In addition, the Csp³–H fluorination of alkyl phthalimide derivatives is demonstrated.
Graphical Abstract Fullsize Image
... Theoretically, it has been calculated for (CH 3 ) 2 X + that the C-C distances are 2.66Å (X = F), 2.94Å (X = Cl), 3.10Å (X = Br), and 3.33 Å (X = I) and the corresponding CXC angels are 120.2˚ (X = F), 105.0˚ (X = Cl), 101.4˚ (X = Br), and 97.7˚ (X = I) [20] [21]. In excellent correspondence with these results is the recent chemical and theoretical evidence for a symmetrical fluoronium ion in solution [22]. ...
Herein, we disclose an approach to synthesize tert-alkyl cyclopropanes by leveraging C-F bond functionalization of gem-difluorocyclopropenes using tris(pentafluorophenyl)borane catalysis. The reaction proceeds through the intermediacy of a fluorocyclopropenium ion, which was confirmed by the isolation of [Ph2(C6D5)C3]+[(C6F5)3BF]-. We found that silylketene acetal nucleophiles were optimal reaction partners with fluorocyclopropenium ion intermediates yielding fully substituted cyclopropenes functionalized with two α-tert-alkyl centers (63-93% yield). The regioselectivity of the addition to cyclopropenium ions is controlled by their steric and electronic properties and enables access to 3,3-bis(difluoromethyl)cyclopropenes in short order. The resulting cyclopropene products are readily reduced to the corresponding orphaned cyclopropanes under hydrogenation conditions. Quantum chemical calculations reveal the nature of the C-F bond cleavage steps and provide evidence for catalysis by boron and not silylated oxonium ions, though Si-F bond formation is the enthalpic driving force for the reaction.
Fluorine is an attractive element in the field of pharmaceutical and agrochemical chemistry due to its unique properties. Considering the chiral environment in nature, where enantiomers often show different biological activities, the introduction of fluorine atom(s) into organic molecules to make chiral fluorinated compounds is an important subject. Herein, we describe the story of the development of our chiral carboxylate-based phase-transfer catalysts and their applications for asymmetric fluorocyclizations of alkenes bearing a carboxylic acid, an amide, and an oxime as an internal nucleophile with a dicationic fluorinating reagent, Selectfluor. We also describe dearomative fluorinations of indole derivatives, 2-naphthols, and resorcinols.
Introduction of fluorine atom(s) into an organic framework is one of the common strategies in life science field due to its unique properties. Therefore, fluorine atom(s) can be found in many bioactive compounds including medicinal drugs. Considering chiral environment in nature, the development of efficient fluorination to construct chiral fluorine-containing compounds is an important subject. In this context, we have studied the development of asymmetric fluorofunctionalization of alkenes and deraomative fluorination of aromatic compounds using a newly designed phase-transfer catalysts. In 2015, we reported the first successful example of asymmetric fluorolactonization using a hydroxy-carboxylate catalyst. Based on the mechanistic studies of the fluorolactonization, we were able to create a dianionic phase-transfer catalyst with more efficient catalytic activity. The dianionic phase-transfer catalyst was found to promote asymmetric 6-endo-fluorocyclization and deprotonative fluorination of simple cyclic and acyclic allylic amides, asymmetric fluorocyclization of ene-oximes, and dearomatizing fluorinations of indoles, 2-naphthols, and resorcinols. According to the mechanistic studies using allylic amides, the active catalytic species switches depending on the substitution pattern of allylic amides, which would result in the reversal of the face-selectivity. Thus, an active aggregate of the catalyst/Selectfluor ion pair promotes the reaction of γ,γ-disubstituted allylic amides, while a monomer complex would be active for γ-monosubstituted substrates.
Present review outlines the advances and perspectives of computational 1H and 13C NMR applied to the stereochemical studies of inorganic, organic, and bioorganic compounds, involving in particular natural products, carbohydrates, and carbonium ions. The first part of the review briefly outlines theoretical background of the modern computational methods applied to the calculation of chemical shifts and spin‐spin coupling constants at the DFT and the non‐empirical levels. The second part of the review deals with the achievements of the computational 1H and 13C NMR in the stereochemical investigation of a variety of inorganic, organic, and bioorganic compounds, providing in an abridged form the material partly discussed by the author in a series of parent reviews. Major attention is focused herewith on the publications of the recent years, which were not reviewed elsewhere.
Haliranium ions are intermediates often involved in complex cyclisations, where their structure allows for control over stereospecific outcomes. Extending previous studies into their structure and reactivity in the gas phase, this work focuses on the bimolecular reactivity of ethyl bromiranium and iodiranium ions with cyclic alkenes. The products observed via mass spectrometry were broadly attributed to either addition by cyclohexene at the iranium carbon or attack at the heteroatom to undergo associative π-ligand exchange. The model proposed was supported by both kinetic experiments and DFT calculations, where the rate of parent ion consumption proceeded at the collision rate (Br: k2 = 1.25 × 10-9 and I: k2 = 1.28 × 10-9 cm3 molecule-1 s-1) with the subsequent partitioning dependent on the relative stability of the initial intermediates and the relatively large barriers present in the addition pathway. Exploration of the effect of cycloalkene ring strain on the iodiranium ion reactivity was conducted with a series of crossover experiments with 50 : 50 mixtures of either cyclohexene or cis-cyclooctene and styrene, where the outcomes were dependent on the competing ring strain relief gained by reaction with each neutral. The nature of the exchange transition state was determined to be pseudocoarctate following both natural bond orbital (NBO) and anisotropy of the induced current density (ACID) analysis.
