Leaving group effects in gas-phase substitutions and eliminations

Using a methodology recently developed for studying the product distributions of gas-phase S(N)2 and E2 reactions, the effect of the leaving group on the reaction rate and branching ratio was investigated. Using a dianion as the nucleophile, reactions with a series of alkyl bromides, iodides, and trifluoroacetates were examined. The alkyl groups in the study are ethyl, n-propyl, n-butyl, isobutyl, isopropyl, sec-butyl, and tert-butyl. The data indicate that leaving group abilities are directly related to the exothermicities of the reaction processes in both the gas phase and the condensed phase. Gas-phase data give a reactivity order of iodide > trifluoroacetate > bromide for S(N)2 and E2 reactions. Previous condensed phase data indicate a reactivity order of iodide > bromide > trifluoroacetate for substitution reactions; however, the basicities of bromide and trifluoroacetate are reversed in the condensed phase so this reactivity pattern does reflect the relative reaction exothermicities. Aside from this variation, the gas-phase data parallel condensed phase data indicating that the substituent effects are rooted in the nature of the alkyl substrate rather than in differences in solvation. The experimental data are supported by calculations at the MP2/6-311+G(d,p)//MP2/6-31+(d) level.     

Structural and thermodynamic properties of group 13 imidometallanes and their heavier analogues

Systematic theoretical studies of the [XMYH](n) inorganic rings and clusters (M = Al, Ga, In; Y = N, P, As; X = H, F, Cl, Br, I; n = 1-6) have been carried out using hybrid Hartree-Fock density functional theory. A consistent set of the structural and thermodynamic properties has been obtained. The stability of the MY bond decreases in the order Al > Ga >or= In; N > P > As. Terminal groups X have a minor influence on the subsequent elimination enthalpies of the clusters. In the case of X = H, hydrogen elimination makes formation of the [HMYH](6) oligomers from MH(3) and YH(3) thermodynamically favorable; while in the case of halide substituents, formation of [XMYH](6) is thermodynamically unfavorable, except for the system with the strongest MY bond (AlN). Substitution of the acidic hydrogen by X is favorable energetically for all [HMYH](6) clusters, but is complicated by the processes of cluster destruction to form the [X(2)MYH(2)](2) dimers. The high stability of the [HMNH](6) clusters makes them attractive single-source precursors for the production of 13-15 composites.     

Arylmethylenemalonaldehydes and their heterocyclic analogues: A novel group of organic lewis acids

The first general synthesis of arylmethylenemalonaldehydes and their heterocyclic analogues has been developed; some physico-chemical properties and reactions of these compounds are briefly mentioned.     

Theoretical investigations of the reactivities of cationic six-membered carbene analogues of group 14 elements

The potential energy surfaces for the chemical reactions of cationic six-membered group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ) and CCSD (CCSD/LANL2DZ//B3LYP/LANL2DZ) methods. Five six-membered group 14 cationic heavy carbene species, [HC(CMeNPh)2E:](+), where E = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reaction, C-H bond insertion, multiple bond cycloaddition, dimerization, and O-H bond insertion, have been used to study the chemical reactivities of these group 14 cationic carbene species. Basically, our present theoretical work predicts that the larger the angle NEN bond angle and the smaller the singlet-triplet splitting of the carbene, the lower its activation barriers will be and, in turn, the more rapid are its chemical reactions with other species. Moreover, the theoretical investigations suggest that the relative carbenic reactivity decreases in the order C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is its cationic carbene toward chemical reaction. As a result, we predict that the cationic six-membered group 14 carbene species (E = C, Si, Ge, Sn, and Pb) should be stable, readily synthesized, and isolated at room temperature. Our computational results are in good agreement with the available experimental observations. Furthermore, the singlet-triplet energy splitting of the carbene, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict its reactivities. The results obtained allow a number of predictions to be made.     

Charge redistribution and photoacidity: neutral versus cationic photoacids

A series of pyrene photoacids is used to investigate excited-state proton transfer with time-dependent pump-probe spectroscopy. The deprotonation dynamics of a cationic photoacid, 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), shows single exponential dynamics( approximately 30 ps) in water. This is in contrast to what is observed for the neutral photoacids 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and 8-hydroxy-N,N,N',N',N",N"-hexamethylpyrene-1,3,6-trisulfonamide, which display biexponential dynamics. For the cationic photoacid, the vast majority of the intramolecular charge redistribution does not occur in the protonated state. Instead, the charge redistribution, which is responsible for the photoacidity and the observed spectroscopic changes, occurs primarily following the excited-state proton transfer. The lack of charge redistribution prior to proton transfer causes APTS to display single exponential kinetics. In contrast, the dynamics for the neutral photoacids are multiexponential because major charge redistribution precedes proton transfer followed by additional charge redistribution that accompanies proton transfer. Previous studies of HPTS in water are discussed in terms of the results presented here.     

Charge influence in semi-empirical calculations of heteroatomic ionic and neutral molecules

Iterative extended Hükel calculations for all valence electrons and iterative PPP calculations in the variable electronegativity formalism for -electrons were performed on benzene, pyridine, fluorobenzene, and the pyrylium ion. The charge distributions for all compounds were found more uniform and plausible with the iteration procedures than without. Polarization effects from the-electrons were found to be of importance for the -electrons. The lone-pair picture of the highest occupied MO in pyridine is preserved in the iterative extended Hückel method, and two lone-pairs were obtained on the fluorine atom of fluorobenzene. The results indicate that this atom is not hybridized.

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Zusammenfassung Benzol, Pyridin, Fluorbenzol und Pyriliumion wurden mit einer iterativen EH-Methode und einer iterativen PPP-VE-Methode berechnet. Es zeigte sich, daß Ladungsverteilungen bei Benutzung des Iterationsverfahrens besser beschrieben werden. Polarisationseffekte der-Elektronen auf die -Elektronen stellen sich als wichtig heraus. Auch in der IEH-Methode bleibt das oberste besetzte MO des Pyridins ein einsames Elektronenpaar. Am Fluoratom im Fluorbenzol werden zwei einsame-Elektronenpaare erhalten. Die Resultate führen zu der Annahme, daß dieses Atom nicht hybridisiert ist.

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Resumé Des calculs par les méthodes itératives de Hückel étendu pour tous les électrons de valence et de PPP avec électronégativité variable pour les électrons ont été effectués pour le benzène, la pyridine, le fluorobenzène et l'ion pyrylium. Les distributions de charge pour tons ces composés sont plus uniformes et plus plausibles avec les procédés itératis que sans. Les électrons ont un effet de polarisation important sur les électrons . La plus haute orbitale occupée dans la pyridine reste identifiable comme la paire libre dans la méthode de Hückel étendue, et l'on obtient deux paires libres sur l'atome de fluor du fluorobenzène. Cet atome n'est pas hybride.

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Sponsored in part by King Gustaf VI Adolfs 70-Years Fund for Swedish Culture, Knut and Alice Wallenberg's Foundation, and in part by the Swedish National Research Council.     

Theoretical designs for neutral five-membered carbene analogues of the group 13 elements: a new target for synthesis

Potential energy surfaces for the chemical reactions of neutral five-membered group 13 carbenoids have been studied using density functional theory (B3LYP/LANL2DZ). Five five-membered group 13 carbenoid species, HCMeP(PhN)2X, where X = B, Al, Ga, In, and Tl, have been chosen as model reactants in this work. Also, three kinds of chemical reaction, C-H bond insertion, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these group 13 carbenoids. Our present theoretical work predicts that the larger the angleNXN bond angle in the neutral five-membered group 13 carbenoid, the smaller the singlet-triplet splitting, the lower the activation barrier, and, in turn, the more rapid are its various chemical reactions. Moreover, the theoretical investigations suggest that the relative carbenoidic reactivity decreases in the order B > Al > Ga > In > Tl. That is, the heavier the group 13 atom (X), the more stable is its carbenoid with respect to chemical reactions. As a result, we predict that the neutral five-membered group 13 carbenoids (X = Al, Ga, In, and Tl) should be stable, readily synthesized, and isolated at room temperature. Furthermore, the neutral five-membered group 13 carbenoid singlet-triplet energy splitting, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.     

Insect pheromones and their analogues

A scheme has been developed for the synthesis of (R)-(–)-10-methyltridecan-2-one, the sex pheromone of the southern corn rootworm (Diabrotica undecimpunctata) using as the chiral synthon (R)-(+)-methylheptenal obtained by the hydroperoxide oxidation of the readily available (S)-(+)-dihydromyrcene.Institute of Chemistry, Bashkir Scientific Center, Urals Branch, Russian Academy of Sciences, Ufa. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 125–129, January–February, 1992.     

Insect pheromones and their analogues

The synthesis has been effected of octa-2E,6E-diene-1,8-diol diisovalerate — the sex pheromone of the Crimean click beetleAgriotes tauricus — by the carboxymethylenation of the readily available 6-methylhept-5-en-2-one, the allyl oxidation of a terminal methyl group, reduction of the ester fraction, and interaction of the diol so obtained with isovaleryl chloride. The required pheromone can also be obtained from available cyclic oligomers of isoprene.Institute of Chemistry, Bashkir Scientific Center, Urals Branch, Academy of Sciences of the USSR, Ufa. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 565–568, July–August, 1991.     

Insect pheromones and their analogues

A stereospecific synthesis has been achieved of tetradec-11E-enal and of tetradec-11E-en-l-ol and its acetate — components of the pheromones of many species of insects of the order Lepidoptera — from the readily available undecenoic acid, using the Knoevenagel reaction and catalytic hydroalumination.Institute of Chemistry, Bashkir Scientific Center, Urals Branch, Russian Academy of Sciences, Ufa. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 272–276, March–April, 1992.     

Insect pheromones and their analogues

Pheromone components of insects of the generaHeliothis andMamestra have been synthesized with the use of functionally differentiated ozonolysis of cyclododecene.Institute of Chemistry, Bashkir Scientific Center, Urals Branch, Academy of Sciences of the USSR, Ufa. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 132–135, January–February, 1991.     

Insect pheromones and their analogues

The synthesis has been effected of octa-2E,6E-diene-1,8-diol diisovalerate — the sex pheromone of the Crimean click beetleAgriotes tauricus — by the carboxymethylenation of the readily available 6-methylhept-5-en-2-one, the allyl oxidation of a terminal methyl group, reduction of the ester fraction, and interaction of the diol so obtained with isovaleryl chloride. The required pheromone can also be obtained from available cyclic oligomers of isoprene.     

Selective substitutions in the C10-methyl group in erythromycin derivatives

A method for chemical modifications of the relatively unreactive C10-methyl group in the erythromycin macrolactone ring has been developed. Erythralosamine was protected as an N-oxide in the N,N-dimethylamino group and reacted with NBS in acetic acid to provide two regioisomeric allylic bromides. The same amine was formed from both isomers on nucleophilic substitution. Both regioisomeric bromides in cross-coupling reactions under Stille conditions provided the same product from substitution in the 10-methyl group via a common π-allylic palladium complex. Under Negishi conditions with trimethylalane, the Pd-catalysed cross-coupling provided the 10-ethyl homologue. X-ray analyses were used to confirm the structure of erythralosamine, and to determine the structures of the allylic bromides from erythralosamine N-oxide.     

Multiple bonding in Groups 13 and 14 homonuclear ethyne analogues

Hybrid density functional theory calculations have been carried out on model systems of Groups 13 and 14 alkyne analogues [M₂R₂]²⁻, Li₂[M₂R₂] and X₂R₂ (M=B, Al, Ga, In and Tl, X=C, Si, Ge, Sn and Pb and R=H, Me and Ph). The paper addresses both the nature of the bonding in these systems and the origin of the structural variations. Analysis of the orbitals and calculations of bond orders has identified the factors determining the adoption of a ‘classical’ linear, ethyne-like structure or a trans-bent geometry. The structural trends down each group and role of the substituent R group and the counter-ion have been also been investigated. It is found that all the elements below the first period of these groups adopt ‘non-classical’ trans-bent configurations in these molecules to minimise the steric repulsions between the two ends of the molecule whilst enhancing the bonding to the substituent even though this results in a weakened metal–metal bond as evidenced by bond lengthening and a reduction in the bond order. Complexation of the M₂R₂ ²⁻ ions by lithium reduces the electrostatic repulsions in the M₂R₂ ²⁻ complexes of the Group 13 elements by stabilisation of the π-like orbitals. Whilst the presence of Li⁺ is not responsible for the trans-bending, interactions between the Li atoms and the substituents are important in determining the extent of the distortion.     

DFT assessment of the spectroscopic constants and absorption spectra of neutral and charged diatomic species of group 11 and 14 elements

The spectroscopic constants and absorption spectra of neutral and charged diatomic molecules of group 11 and 14 elements formulated as [M₂]^+/0/? (M = Cu, Ag, Au), and [E₂]^+/0/? (E = C, Si, Ge, Sn, Pb) have been calculated at the PBE0/Def2‐QZVPP level of theory. The electronic and bonding properties of the diatomics have been analyzed by natural bond orbital analysis approach and topology analysis by the atoms in molecules method. Particular emphasis was given on the absorption spectra of the diatomic species, which were simulated by time‐dependent density functional theory calculations employing the hybrid Coulomb‐attenuating CAM‐B3LYP density functional. The simulated absorption spectra of the [M₂]^+/0/? (M = Cu, Ag, Au) and [E₂]^+/0/? (E = C, Si, Ge, Sn, Pb) species are in close resemblance with the experimentally observed spectra whenever available. The neutral M₂ and E₂ diatomics strongly absorb in the ultraviolet region, given rise to UVC, UVA and in a few cases UVB absorptions. In a few cases, weak absorbion bands also occur in the visible region. The absorption bands have thoroughly been analyzed and assignments of the contributing principal electronic transitions associated to individual excitations have been made. © 2014 Wiley Periodicals, Inc.     

Total synthesis of luotonin A and 14-substituted analogues

A new and concise synthesis of luotonin A was achieved from the previously described compounds ethyl 4-oxo-3,4-dihydroquinazoline-2-carboxylate and 1-(2-nitrophenyl)prop-2-en-1-one. New 14-substituted luotonin A analogues were prepared using 14-chloroluotonin A as the key intermediate.     

Synthesis of muramoyltripeptides and their isotope-labeled analogues

The synthesis has been effected of the methyl ester of N-acetylmuramoyl-L-alanyl-D-isoglutamylglycine and its hexadecyl -glycoside and the corresponding [1-¹⁴C]glycine analogues.M. V. Frunze Simferapol' State University. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 245–248, March–April, 1990.     

Bismuth-doped tin clusters: experimental and theoretical studies of neutral Zintl analogues

The electron count of gas-phase clusters is increased gradually by element substitution in order to mimic the total number of electrons of known stable closo-clusters. A combination of elements from the fourth and fifth group of the periodic table such as Sn and Bi is well-suited for this approach. Hence, these small Sn-Bi clusters are investigated by employing the electric field deflection method. For clusters in the series Sn(M-N)Bi(N) (M = 5-13, N = 1-2), the beam profiles obtained in cryogenic experiments are dominated by beam broadening, indicating the presence of a permanent electric dipole moment that is sensitive to the (rigid) cluster structure. An intensive search for the global minimum structure employing a density functional theory/genetic algorithm method is performed. Dielectric properties for the identified low-energy isomers are computed. The structural and dielectric properties are used in beam profile simulations in order to discuss the experimental data. Comparison of theoretical and experimental results enables identification of the growing pattern of these small bimetallic clusters. For multiply doped clusters, it is concluded that the dopant atoms do not form direct Bi-Bi bonds, but more interestingly, a rearrangement of the cluster skeleton becomes apparent. The structural motifs are different from pure tin clusters but rather are rationalized using the corresponding structures of tin anions or are based on the Wade-Mingos concept. Further evidence for this idea is deduced from nuclear independent chemical shift calculations, which show nearly identical behavior for negatively charged pure and neutral bimetallic clusters. All of these findings are consistent with the idea of neutral Zintl analogues in the gas phase.