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Basicity of five-membered cyclic allenes: Proton and cation affinity evaluation using density functional theory calculations
Abstract
The proton affinity (PA) of stable five-membered cyclic allenes 1–13 in the gas phase was studied by DFT/B3LYP computational method in conjunction with the 6-311+G(d,p) basis set. The PAs of the title compounds are large. Their protonation at the central carbon introduces super- and hyperbases as evidenced by their enthalpies of protonation ΔH = 272–315 kcal/mol, which are above the threshold of superbasicity and in some cases close to the hyperbasicity. The trends of PA correlate quite well with the energy level of the HOMO orbitals and the molecular electrostatic potential maps of the studied allenes. The Wiberg bond order analysis gives a reasonable explanation of the exocyclic delocalization involving the π-donor substituents with the allenic unit of the studied compounds. Protonation of central carbon in the cyclic allenes leads to a change in the structural arrangement and an increment in the aromaticity of the pyrazole ring, which was evaluated by the HOMA index. Furthermore, the B3LYP/6-311+G(d,p) calculated cation affinities for the cyclic allene 6 increases as K⁺<Na⁺<Li⁺<Cu⁺<Mg²⁺<Fe²⁺<Cu²⁺. Cation affinity is strongly dependent on the charge-to-size ratio of the Mⁿ⁺.
The gas-phase basicity of nitriles can be enhanced by a push–pull effect. The role of the intercalated scaffold between the pushing group (electron-donor) and the pulling (electron-acceptor) nitrile group is crucial in the basicity enhancement, simultaneously having a transmission function and an intrinsic contribution to the basicity. In this study, we examine the methylenecyclopropene and the N-analog, cyclopropenimine, as the smallest cyclic π systems that can be considered for resonance propagation in a push–pull system, as well as their derivatives possessing two strong pushing groups (X) attached symmetrically to the cyclopropene scaffold. For basicity and push–pull effect investigations, we apply theoretical methods (DFT and G2). The effects of geometrical and rotational isomerism on the basicity are explored. We establish that the protonation of the cyano group is always favored. The push–pull effect of strong electron donor X substituents is very similar and the two π-systems appear to be good relays for this effect. The effects of groups in the two cyclopropene series are found to be proportional to the effects in the directly substituted nitrile series X–C≡N. In parallel to the basicity, changes in electron delocalization caused by protonation are also assessed on the basis of aromaticity indices. The calculated proton affinities of the nitrile series reported in this study enrich the gas-phase basicity scale of nitriles to around 1000 kJ mol−1.
The geometry-based HOMA (Harmonic Oscillator Model of Aromaticity) descriptor, based on the reference compounds of different delocalizations of n- and π-electrons, can be applied to molecules possessing analogous bonds, e.g., only CC, only CN, only CO, etc. For compounds with different heteroatoms and a different number of CC, CX, XX, and XY bonds, its application leads to some discrepancies. For this reason, the structural descriptor was modified and the HOMED (Harmonic Oscillator Model of Electron Delocalization) index defined. In 2010, the HOMED index was parameterized for compounds with C, N and O atoms. For parametrization, the reference molecules of similar delocalizations of n- and π-electrons were employed. In this paper, the HOMED index was extended to compounds containing the CP, CS, NN, NP, PP, NO, NS, PO, and PS bonds. For geometrical optimization of all reference molecules and of all investigated heterocompounds, the same quantum–chemical method {B3LYP/6-311+G(d,p)} was used to eliminate errors of the HOMED estimation. For some tautomeric systems, the Gn methods were also employed to confirm tautomeric preferences. The extended HOMED index was applied to five-membered heterocycles, simple furan and thiophene, and their N and P derivatives as well as for tautomeric pyrrole and phosphole and their N and P derivatives. The effects of additional heteroatom(s) in the ring on the HOMED values for furan are parallel to those for thiophene. For pyrroles, aromaticity dictates the tautomeric preferences. An additional N atom in the ring only slightly affects the HOMED values for the favored and well delocalized NH tautomers. Significant changes take place for their rare CH forms. When intramolecular proton-transfer is considered for phosphole and its P derivatives, the PH tautomers seem to be favored only for 1,2,3-triphosphole/1,2,5-triphosphole and for 1,2,3,5-tetraphosphole. For other phospholes, the CH forms have smaller Gibbs energies than the PH isomers. For phosphazoles, the labile proton in the favored form is linked to the N atom. The PH forms have smaller HOMED indices than the NH tautomers but higher than the CH ones.
For over a century, the structures and reactivities of strained organic compounds have captivated the chemical community. Whereas triple-bond-containing strained intermediates have been well studied, cyclic allenes have received far less attention. Additionally, studies of cyclic allenes that bear heteroatoms in the ring are scarce. We report an experimental and computational study of azacyclic allenes, which features syntheses of stable allene precursors, the mild generation and Diels-Alder trapping of the desired cyclic allenes, and explanations of the observed regio- and diastereoselectivities. Furthermore, we show that stereochemical information can be transferred from an enantioenriched silyl triflate starting material to a Diels-Alder cycloadduct by way of a stereochemically defined azacyclic allene intermediate. These studies demonstrate that heteroatom-containing cyclic allenes, despite previously being overlooked as valuable synthetic intermediates, may be harnessed for the construction of complex molecular scaffolds bearing multiple stereogenic centres.
Density functional theory (DFT) calculations using the B3LYP, M05-2X and M06-2X functionals were employed in the study of basicity of allenes incorporated in seven-membered rings. These highly unstable species upon protonation give thermodynamically very stable tropylium cations, which are the origin of their extremely high basicity. An endocyclic allene → tropylium cation sequence was used for the design of novel hydrocarbon superbases, with calculated gas phase proton affinities exceeding well above 300 kcal mol⁻¹. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016.
Theoretical reactivity indices based on the conceptual Density Functional Theory (DFT) have become a powerful tool for the semiquantitative study of organic reactivity. A large number of reactivity indices have been proposed in the literature. Herein, global quantities like the electronic chemical potential, the electrophilicity and the nucleophilicity indices, and local condensed indices like the electrophilic and nucleophilic Parr functions, as the most relevant indices for the study of the organic reactivity, are discussed.
We have reported for the first time designed silylene superbases involving intramolecular H(+)π interaction using density functional theory (DFT) calculations. The non-covalent interactions augment the proton affinity values by ∼13 kcal mol(-1) in the designed superbase () compared to the acyclic silylene, [:Si(NMe2)2]. These divalent Si(ii) compounds can act as powerful neutral organic superbases in gas and solvent phases. The DFT calculations performed at the B3LYP/6-311+G**//B3LYP/6-31+G* level of theory showed that the gas phase proton affinity of the paracyclophane based silylene superbase () reaches up to ∼271.0 kcal mol(-1), which is the highest in paracyclophane Si(ii) compounds. In tetrahydrofuran solvent medium, the calculated proton affinity of was found to be 301.4 kcal mol(-1). The paracyclophane-based silylene systems are used for binding with small alkali metal ions. The calculated results showed that these systems selectively bind to lithium ions over sodium ions due to the small size of lithium ions which is well fitted in the space between the silicon atom and the phenyl ring. Furthermore, we have used the lithiated silylene and (which exhibits bis-protonation) for gas storage (CO and CO2). The calculated results showed both the lithiated silylene and bind preferentially to CO2 than CO. The calculated gravimetric density of CO2 is found to be 26.97 wt% for -Li2-(CO2)4. The energy decomposition analysis (EDA) has been performed to investigate the role of various contributing factors to the total binding strength of the CO2 or CO molecules with lithiated silylene superbases. EDA reveals that the electrostatic energy and polarization energy are the major driving force for higher total interaction energy of the lithiated-silylene-CO2 complex than the lithiated-silylene-CO complex. The lithiated silylene systems showed a higher binding energy with CO2 than the previously reported imidazopyridamine at the same level of theory. These results suggest that the lithiated silylene systems can be used as a more efficient CO2 storage material than the aforementioned system. The calculated desorption energies per CO2 and CO (ΔEDE) also indicate the recyclable property of the materials.
It is shown by a reliable DFT method that bicyclic structures 7–10, constructed using 1,3-diaminopropane as an essential building block, can act as powerful neutral organic superbases in the gas phase, aqueous solution and in acetonitrile. This new molecular framework is versatile in terms of anchoring different functional groups to achieve higher basicities. The buttressing effect of substituents on the basicity of these organic bases was also observed. The proton affinities calculated at the B3LYP/6-311+G**//B3LYP/6-31+G* level for 1,2-diaminoethane and 1,3-diaminopropane were found to be in good agreement with the experimental results. Barriers for proton transfer between the N atoms of the diamine and diimine cations are also reported.
The theoretical and experimental research on carbodiphosphoranes C(PR3)2 and related compounds CL2, both as free molecules and as ligands in transition metal complexes, is reviewed. Carbodiphosphoranes are examples of divalent
carbon(0) compounds CL2 which have peculiar donor properties that are due to the fact that the central carbon atom has two lone electron pairs. The
bonding situation is best described in terms of L→C←L donor acceptor interactions which distinguishes CL2 compounds (carbones) from divalent carbon(II) compounds (carbenes) through the number of lone electron pairs. The structures
and stabilities of transition metal complexes with ligands CL2 can be understood and predictions can be made considering the double donor ability of the carbone compounds.
KeywordsCarbodicarbene–Carbodiphosphorane–Divalent carbon(0)–DFT calculations–Transition metal complex
The HOMA (Harmonic Oscillator Model of Aromaticity) index, reformulated in 1993, has been very often applied to describe π-electron delocalization for mono- and polycyclic π-electron systems. However, different measures of π-electron delocalization were employed for the CC, CX, and XY bonds, and this index seems to be inappropriate for compounds containing heteroatoms. In order to describe properly various resonance effects (σ-π hyperconjugation, n-π conjugation, π-π conjugation, and aromaticity) possible for heteroatomic π-electron systems, some modifications, based on the original HOMA idea, were proposed and tested for simple DFT structures containing C, N, and O atoms. An abbreviation HOMED was used for the modified index.
Calculated proton affinities (PAs) and gas phase basicities (GPBs) are reported for diamantane (C14H20), triamantane (C18H24), 'globular and planar' isomers of tetramantane (C22H28) and pentamantane (C26H32), and for one 'globular' isomer of each of the larger diamondoid molecules: C51H58, C78H72, C102H90, and C131H116. Assuming CxHy as the parent diamondoid molecule, we calculated PA and GPB values for a variety of CxHy+1+ isomers, as well as for the reaction CxHy + H+ yielding CxHy-1+ + H2(g); the latter is slightly favored based on GPB values for diamantane through pentamantane, but less favored compared to certain CxHy+1+ isomers of C51H58, C102H90, and C131H116. Indeed, the GPB values of C51H58, C102H90, and C131H116 classifiy them as 'superbases'. Calculations that had the initial location of the proton in an interstitial site inside the diamondoid molecule always showed the H having moved to the outside of the diamondoid molecule; for this reason, we focused on testing a variety of initial configurations with the proton placed in an initial position on the surface. Additional protons were added to determine the limiting number that could be, per these calculations, taken up by the diamondoid molecules and the maximum number of protons are shown in parentheses: C14H20(2), C18H24(3), C22H28(3), C26H32(3), C51H58(4). Bader charge distributions obtained for CxHy+1+ isomers (for diamantane through pentamantane) suggest that the positive charge is essentially completely delocalized over all the H atoms. NMR spectra were calculated for different isomers of C14H19+, and compared to the published NMR spectrum for when diamantane was mixed with magic acid and H2(g) was produced.
The hydride affinity (HA) of 20 allene and heteroallene derivatives containing unsaturated rings such as cyclopentadiene (CP) ring has been computationally investigated using the B3LYP/6‐311++G(3df, 2p) method. The interaction of hydride ion (H−) with the central sp1 carbon of the designed molecules containing electron‐withdrawing groups on the CP ring has been highly desirable, and their HA values were obtained in the range of 369–788 kJ/mol. The aromaticity of CP anion has the key role in the HA increasing and its stability. The harmonic oscillator model of aromaticity (HOMA) and nucleus‐independent chemical shifts (NICS) values associated with the CP ring were calculated and compared before and after the interaction with H−. The calculated data indicated a sharp increase in HOMA and NICS indices for the anions. In the following, the protonation of three designated allenes containing a cyclopropene or cycloheptatriene and a CP ring was also investigated using the density functional theory calculations. Electrostatic potential maps for some key structures were also used for charge distribution analysis. The hydride affinity (HA) of the allene derivatives containing unsaturated rings such as cyclopentadiene (CP) ring has been investigated using B3LYP/6‐311++G(3df,2p) method. Interaction of H− with central carbon of the molecules has been highly desirable, and their HA values were obtained in the range of 369–788 kJ/mol. The aromaticity of CP has the key role in the HA increasing. The data indicated a sharp increase in aromatic indices for the anions. Molecular electrostatic potentials (MEPs) were also used for charge distribution analysis.
The gas phase proton affinities (PAs) of substituted allenes in the gas phase were examined with the B3LYP/6-311+G(d,p) computational method. The calculations revealed that the designed allenes bearing a group having the potential for aromaticity offer scaffolds suitable for engineering robust non-ionic organic bases. Their protonation at C(sp) site introduce super- and hyperbases as evidenced by their enthalpies of protonation ΔH (13) = 1243 and ΔH (24) = 1305 kJ mol-1, which are close to the threshold of hyperbasicity of 1255 kJ mol-1. It was conclusively shown that basicity of allenes was dramatically amplified by substitution of electron releasing groups such as methyl and dimethylamino on the molecular framework. The allene disubstitued with two methylenecyclopropene groups, gave the smallest superbase possessing only hydrocarbon structure 23. Furthermore, interactions of some of the designed allene derivatives, with the cations Li+, Na+, K+ and Cu+ were investigated using the mentioned computational method. The calculated gas-phase cation affinities and cation basicities for the molecules decrease as H+>Cu+>Li+>Na+>K+. Nucleus-independent chemical shift and harmonic oscillator model of aromaticity as two indices of aromaticity were used as a measure of molecule/cation interaction.
Carbocyclic carbenes (CCCs) are a class of nucleophilic carbenes which are very similar to N‐heterocyclic carbenes (NHCs) in terms of their reactivity, but they do not contain a stabilizing heteroatom in their cyclic ring system. In this study, 17 representative known CCCs and 34 newly designed CCCs are evaluated using quantum chemical methods, and the results are compared in terms of their stability, nucleophilicity, and proton affinity (PA) parameters. The results are divided on the basis of ring size of the known and reported CCCs. The stability, nucleophilicity, PA, complexation energy, and bond strength–related parameters were estimated using M06/6‐311++G(d,p) method. The results indicated that the CCCs known in the literature are strong σ‐electron donating species and have considerable π‐accepting properties. This study led to the design and identification of a few new CCCs with dimethylamine and diaminomethynyl substituents which can be singlet stable and are substantially nucleophilic. © 2018 Wiley Periodicals, Inc. Carbocyclic carbenes are interesting species but still lack thorough investigation based on their stability and reactivity. This is a systematic effort to understand different properties of CCCs with variable ring sizes. This work also exploits some promising substitutions to establish a balance between the nucleophilic properties and stability.
Density functional theory (DFT) calculations at B3LYP/6-311++G(d,p) level have been carried out on organic fluorosulfuric acids, in order to examine their acidities in gas-phase. Strong acidity effect is predicted for a novel series of fluorosulfuric acids in which an oxygen is substituted by an 1,3-cyclohexanedione or 1,3-cyclopentadiene group. The acidity of these acids without any electron withdrawing groups on the ring, was more than fluorosulfuric acid. It is shown that substitution of hydrogen with electron withdrawing groups such as –CN, –F, and –C[dbnd]O on the ring increases the acidity to superacidic values, 216–306 kcal/mol. Substitution of –CN groups enhanced the acidity to 216.0 kcal/mol. Some prototropic tautomers of proposed fluorosulfuric acids are also investigated by DFT calculations.
DFT B3LYP calculations convincingly showed that aromatic pnictogen-oxides offer scaffolds suitable for tailoring powerful organic superbases exhibiting exceptional oxygen basicity in both gas-phase and polar aprotic acetonitrile solution. With their protonation enthalpies and pKa values, they surpass the basicity of classical proton sponges and related nitrogen bases. The most potent system is provided with two arsenic-oxide moieties on the phenanthrene framework assisted with the two phosphazeno groups in the para-position to both basic centers. With its proton affinity PA = 300.5 kcal mol-1, the latter system breaks the gas-phase hyperbasicity threshold of 300 kcal mol-1, while its pKa = 54.8 promotes it as an unprecedented superbase in acetonitrile. The origin of such a dramatic basicity enhancement is traced to a fine interplay between (a) steric repulsions of the two negatively charged oxygens destabilizing a neutral base, (b) a favorable intramolecular [O–H.....O]- hydrogen bonding in conjugate acids, and (c) an efficient cationic resonance upon protonation supported with the electron-donating substituents. Given the growing interest in highly basic compounds together with related basic catalysts and metal complexing agents, we hope that the results presented here would open a new avenue of research in these fields and direct the attention towards utilizing aromatic pnictogen-oxides in designing improved organic materials.
It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term "smart" is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.
Molecular geometry is of fundamental importance for any profound interpretation of molecular properties. This was elegantly pointed out by Roald Hoffmann and triggered intensive research in this direction. Experimental and theoretical methods enabling the determination of molecular geometry, developed since the mid-20th century, have provided numerical confirmation of the above-mentioned statement. Geometry-based aspects of aromaticity are the main subject of this Review. The theoretical method and the basis set used for estimation of HOMA constants should be the same as that used for geometry optimization of a given system. Their analysis was carried out for 25 systems and various advanced quantum-chemical models. The reference systems for the double CC, CN, and CO bond lengths are ethene, methylimine, and formaldehyde, respectively.
The B3LYP/6-311++G (d,p) density functional approach was used to study the gas-phase metal affinities of Guanosine (ribonucleoside) for the Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Cu+ cations. In this study we determine coordination geometries, binding strength, absolute metal ion affinities, and free energies for the most stable products. We have also compared the results for Guanosine, with our previously reported results for 2′-Deoxyguanosine. Based on the results, it is obvious that MIA is strongly dependent on the charge-to-size ratio of the cation. Guanosine interacts more strongly with Zn2+ than do with Mg2+, Ca2+, and Cu+ and therefore stronger interactions lead to higher MIA. In both free molecules and their complexes, the Syn orientation of the base is stabilized by an intramolecular O5′–H···N3 hydrogen bond and the anti orientation of the base is stabilized by an intramolecular C–H···O hydrogen bond formed between the (C8-H8) and the O5′ atom of the sugar moiety. It is also interesting to mention that linear correlation between calculated MIA values and the atomic numbers (Z) of the metal ions of Li+, Na+, and K+ were found. Furthermore, the influences of metal cationization on the strength of the N-glycosidic bond, torsion angles, angle of pseudorotation (P), and intramolecular C–H···O and O–H···O hydrogen bonds have been studied. Natural bond orbital (NBO) analysis was performed to calculate the charge transfer and natural population analysis of the complexes. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of interactions.
Recently, allenes have been widely used as starting materials to synthesize various organic compounds and polymeric materials, especially through reactions catalyzed by transition metal catalysts. In many of the catalytic reactions, insertion of allenes is one of the most important elementary steps. In this review, stoichiometric insertion reactions of transition metal complexes with allenes affording well-defined inserted products are summarized, which may help chemists to understand the mechanisms of catalytic reactions of allenes and to design new catalytic reactions of allenes.
Ein einzelner Donorsubstituent an jedem äußeren C-Atom genügt, um die CCC-Einheit von Allenen sehr flexibel zu machen und dem zentralen Kohlenstoffatom Kohlenstoff(0)-Charakter zu verleihen. Das so zugängliche viergliedrige carbocyclische Allen kann doppelt protoniert werden und verhält sich gegenüber Übergangsmetallen als sehr starker η1-Donorligand (siehe Schema).
The available data on gas-phase basicities and proton affinities of approximately 1700 molecular, radical and atomic neutral species are evaluated and compiled. Tables of the data are sorted (1) according to empirical formula and (2) according to evaluated gas basicity. This publication constitutes an update of a similar evaluation published in 1984.
The CNDO method has been applied to the cyclopropylcarbinyl and cyclobutyl cations, and has given results which are in very good accord with experimental data. A cross-ring interaction is calculated to be of importance with cyclobutyl derivatives, and agrees with the large difference in rate observed with equatorial and axial leaving groups. Some properties of bicyclobutane as well as the relative energies for some models of the activated complex for the thermal rearrangement of bicyclobutane have also been calculated and compared with experimental data. The CNDO method appears to have considerable promise in the investigation of the organic chemical phenomena.
A study was conducted to demonstrate advancements in determining the absolute proton affinities of neutral organic molecules in the gas phase (GP) and their interpretation. The study provided information about some of the main achievements and advancements in theoretical and computational studies of proton affinities, avoiding a taxonomic account of the literature. It focused on the results obtained by computational chemistry in studying the GP acid/base facets of predominantly organic compounds in their ground states (GS). Significant advancements were made in these areas due to advancements in powerful computer hardware and the development of new methods of quantum mechanics for tackling many-body problems, implemented as efficient algorithms in versatile software. Computational natural sciences also eliminated the distinctions between traditional disciplines of physics, chemistry, and molecular biology, providing an important link between rigorous theory and experiment.
The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.
A comprehensive review is presented on nucleus-independent chem. shift as a criterion for aromaticity. [on SciFinder (R)]
Thermal cyclization of 1-[2-(arylethynyl)phenyl]-3-trimethylsilylpropynones affords a mixture of benzo[b]fluorenones and benzo[c]fluorenones. The ratio of the two isomers can be efficiently varied between 100:0 and 0:100 by introducing substituents with appropriate electronic and steric properties on the aryl rings and using an appropriate solvent.
Isodesmic and homodesmic equations at the B3LYP/6-311+G(d,p)+ZPVE level of theory have been used to estimate strain for the homologous series of cyclic allenes and cyclic butatrienes. A simple fragment deformation approach also has been applied and appears to work better for the larger rings. For the cyclic allene series, estimates for allene functional group strain (kcal/mol) include: 1,2-cyclobutadiene, 65; 1,2-cyclopentadiene, 51; 1,2-cyclohexadiene, 32; 1,2-cycloheptadiene, 14; 1,2-cyclooctadiene, 5; 1,2-cyclononadiene, 2; 1,2,4-cyclohexatriene, 34; and bicyclo[3.2.1]octa-2,3-diene, 39. For cyclic butatrienes, functional group strain estimates include: 1,2,3-cyclobutatriene, >100; 1,2,3-cyclopentatriene, 80; 1,2,3-cyclohexatriene, 50; 1,2,3-cycloheptatriene, 26; 1,2,3-cyclooctatriene, 17; and 1,2,3-cyclononatriene, 4. Barriers to interconversion of enantiomers in cyclic allenes are reduced with increasing strain. Newly predicted values include: 1,2-cyclopentadiene <1 kcal/mol and bicyclo[3.2.1]octa-2,3-diene, 7.4 kcal/mol. Estimated levels of strain parallel the known reactivity of these substances.
Nowadays, about 150 natural products comprising an allenic or cumulenic structure are known. The chemistry of these compounds has turned out to be a very attractive and prolific area of interest: advances in the isolation and characterization of new allenic natural products have led to the establishment of efficient synthetic procedures which in many cases also open up an access to enantiomerically pure target molecules. Inspired by the intriguing biological activities of many allenic natural products, allene moieties are now systematically introduced in pharmacologically active classes of compounds (steroids, prostaglandins, amino acids, nucleosides). The functionalized allenes thus obtained often exhibit impressive activities as mechanism-based enzyme inhibitors, cytotoxic, or antiviral agents. A prerequisite for further developments in this field is the efficient stereoselective synthesis of allene derivatives.
To develop a molecular mechanics force field for modeling complexes of transition metals and organic ligands, the electrostatic and covalent contributions in the coordination bonds were investigated using quantum mechanical density functional theory and model complexes of glyoxal diimine and the 2+ cations of the first row transition metals. The VDD and Hirshfeld charges are found to be closely correlated with the extent of the electron transfer between the ligands and the cations. Assuming the electrostatic contribution can be represented by the atomic partial charges, the covalent contributions in the coordination bonds are estimated to be in a range of 54-92% for the systems calculated. A simple force field was parametrized to validate the partial charge representation.
(Chemical Equation Presented) Pushed to the limit: Pushing C=C p bonds to the breaking point by using a push-push substitution pattern forces allenes to bend (see structure; C light blue, N dark blue). An acyclic allene featuring a C=C=C bond angle of 134.8° has been isolated in which the typically sp-hybridized central carbon atom approaches a configuration that has two lone pairs of electrons, and acts as a very strong ?1-donor ligand for transition metals. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.
Quantum-chemical calculations with DFT (BP86) and ab initio methods (MP2, SCS-MP2) were carried out for protonated and diprotonated compounds N-H(+) and N-(H(+))(2) and for the complexes N-BH(3), N-(BH(3))(2), N-CO(2), N-(CO(2))(2), N-W(CO)(5), N-Ni(CO)(3) and N-Ni(CO)(2) where N=C(PH(3))(2) (1), C(PMe(3))(2) (2), C(PPh(3))(2) (3), C(PPh(3))(CO) (4), C(CO)(2) (5), C(NHC(H))(2) (6), C(NHC(Me))(2) (7) (Me(2)N)(2)C==C==C(NMe(2))(2) (8) and NHC (9) (NHC(H)=N-heterocyclic carbene, NHC(Me)=N-substituted N-heterocyclic carbene). Compounds 1-4 and 6-9 are very strong electron donors, and this is manifested in calculated protonation energies that reach values of up to 300 kcal mol(-1) for 7 and in very high bond strengths of the donor-acceptor complexes. The electronic structure of the compounds was analyzed with charge- and energy-partitioning methods. The calculations show that the experimentally known compounds 2-5 and 8 chemically behave like molecules L(2)C which have two L-->C donor-acceptor bonds and a carbon atom with two electron lone pairs. The behavior is not directly obvious when the linear structures of carbon suboxide and tetraaminoallenes are considered. They only come to the fore on reaction with strong electron-pair acceptors. The calculations predict that single and double protonation of 5 and 8 take place at the central carbon atom, where the negative charge increases upon subsequent protonation. The hitherto experimentally unknown carbodicarbenes 6 and 7 are predicted to be even stronger Lewis bases than the carbodiphosphoranes 1-3.
The molecular electrostatic potentials of divalent carbon(0) and divalent carbon(ii) compounds are calculated and the results are compared with theoretically predicted proton affinities and complexation energies with BH(3).
(Chemical Equation Presented) Polarization of C=C bonds is the trick! A "push-push" polarization of the double bonds stabilized the nonlinear allene 1a (spherical angleC-C-C 134.8(2)°), which can be described as carbodicarbene 1b. A similar behavior has been observed for 1,3-dimethyl-2-methyleneimidazoline, which can be seen as an olefin 2a or an ylide 2b. Both compounds 1 and 2 act as interesting electron-rich h1 ligands for transition metals.
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