Quantum semiempirical studies on the formation of aromatic polyamides, 1. Effect of structural parameters of diamines on the polyamidation reaction

A semiempirical quantum-mechanical study of structural and electronic parameters of aromatic diamines was performed using the method AM1. The diamines have been considered as condensation monomers, and the reactivities of both amino groups against acyl chlorides have been computed. The study shows that the amidation reaction is controlled mainly by the global charge on the amino group and, in a much smaller extent, by the frontier orbitals. This behaviour is consistent with the large energy gap between the HOMO 1

1 HOMO: highest occupied molecular orbital.

of the diamine and the LUMO 2

2 LUMO: lowest unoccupied molecular orbital.

of the acyl chloride. It has been found also that the electronic nature of the substituents, affecting the charge of the amino groups, greatly affects the carbons directly joined to these groups. This permits to correlate the reactivities with the calculated charges on these carbons:     

A theoretical study of the addition of silyl radicals to olefinic monomers

In this paper, the high reactivity of silyl macroradicals toward double bonds of olefinic compounds has been explained by means of quantum‐mechanical calculations through their frontier orbital characteristics. In this way, the main orbital interaction corresponds to the overlapping between the SOMO of the disilyl radical and the LUMO of the olefin. In order to obtain more accurate results of differential reactivity, an orbitalic SOMO‐HOMO interaction should be included in addition to the main SOMO‐LUMO one. Also, we theoretically studied the regioselectivity of the addition of silyl radicals to double bonds obtaining similar results as for carbon centered radicals where the reaction takes place on the less hindered carbon of the olefin. Regarding to the geometrical and electronic parameters, it has been shown that carbon radicals have a sp² geometry and a negative charge on the radical center whilst silyl radicals have a sp³ goemetry and a positive charge. Both factors contribute to the enhanced reactivity of silyl radicals with respect to carbon ones.     

Quantum chemical studies of propene,ethylene, acetylene and dihydrogen reactivity in the insertion reaction into the titanium-alkyl bond

Using the ab initio method of SCF MO LCAO

1 SCF MO LCAO: Self-consistent field molecular orbital linear combination of atomic orbitals.

in a valency-splitted basis of the Gaussian functions we have studied the addition of various monomers (C₃H₈, C₂H₄, C₂H₂) and dihydrogen to the titanium-alkyl bond in the complex H₂TiCH₃. The structure of transition states in the insertion reaction, heats of π-complex formation and activation energies for the insertion of the coordinated monomers have been calculated. The calculation results show that the reactivity decreases in the order C₂H₂ > C₂H₄ > C₃H₈ > H₂. According to the results obtained, the energy of the π^*-antibonding orbital of monomers can serve as an index of relative reactivity in the insertion reaction into the metal-alkyl bond.     

The very low-pressure pyrolysis of phenyl ethyl ether,phenyl allyl ether,and benzyl methyl ether and the enthalpy of formation of the phenoxy radical

A value of the enthalpy of formation of the phenoxy radical in the gas phase, ΔH°_,298K (?O·, g) = 11.4 ± 2.0 kcal/mol, has been obtained from the kinetic study of the unimolecular decompositions of phenyl ethyl ether, phenyl allyl ether, and benzyl methyl ether

1 Trivial names for ethoxy benzene, 2-propenoxy (allyloxy) benzene, and α-methoxytoluene, respectively

at very low pressures. Bond fission, producing phenoxy or benzyl radicals, respectively, is the only mode of decomposition in each case. The present value leads to a bond dissociation energy BDE(?O—H) = 86.5 ± 2 kcal/mol,

2 1 kcal = 4.18674 kJ (absolute)

in good agreement with recent estimates made on the basis of competitive oxidation steps in the liquid phase. A comparison with bond dissociation energies of aliphatic alcohols, BDE(RO—H) = 104 kcal/mol, reveals that the stabilization energy of the phenoxy radical (17.5 kcal/mol) is considerably greater than the one observed for the isoelectronic benzyl radical (13.2 kcal/mol). Decomposition of phenoxy radicals into cyclopentadienyl radicals and CO has been observed at temperatures above 1000°K, and a mechanism for this reaction is proposed.     

On Cardioactive Steroids. VIII. The Synthesis of Bufalin and Resibufogenin Isomers

α-Isobufalin

1 Systematic names are given at the titles of the Exper. Part.

( 1 ) and β-isoresibufogenin ( 3 ) have been synthesized from testosterone by a method which features a novel oxidative furan to pyrone transformation.     

Organic radicals with inversion of SOMO and HOMO energies and potential applications in optoelectronics

Organic radicals possessing an electronic configuration in which the energy of the singly occupied molecular orbital (SOMO) is below the highest doubly occupied molecular orbital (HOMO) level have recently attracted significant interest, both theoretically and experimentally. The peculiar orbital energetics of these SOMO–HOMO inversion (SHI) organic radicals set their electronic properties apart from the more common situation where the SOMO is the highest occupied orbital of the system. This review gives a general perspective on SHI, with key fundamental aspects regarding the electronic and structural factors that govern this particular electronic configuration in organic radicals. Selected examples of reported compounds with SHI are highlighted to establish molecular guidelines for designing this type of radical, and to showcase the potential of SHI radicals in organic spintronics as well as for the development of more stable luminescent radicals for OLED applications.

Recent experimental and theoretical results on SHI organic radicals are reported with electronic and structural insights regarding the key parameters leading to this specific (non-aufbau) electronic configuration.     

Stereochemistry of the coupling step in photo-S_(RN) 1 reaction

The photo-S_(RN) 1 reaction of (+)-camphor and aryl halides was investigated in order toestimate the stereochemistry of the coupling step of aryl radicals with a nucleophile. The ratio of endo toexo products determined by ~1H NMR and CD spectra was found to be 99:1. MNDO calculationsof the orbital parameters showed a favorable one-sided overlap of the enolate ion with the SOMO of arylradical. In addition, fragmentation of (ArX)~- was found to be related to the energy of SOMO of(ArX)~-.     

A PM3 study on intrinsic decarboxylation process of methyl–ethyl-α–pyridylacetic acid

To probe the decarboxylation process of methyl–ethyl–α pyridylacetic acid (MEPA), molecular orbital calculations on the optimized geometry, transition-state geometry, and intrinsic reaction coordinate were performed by the MNDO –PM 3 method. The salient features of the optimized structure of MEPA are that the carboxyl anion is nearly on the plane of the pyridine ring (the dihedral angle of C8? C7? C2? N1 is 14.7°) and that the interatomic distance

1 …? is used for a noncovalent bond, such as N⁺ 1 …? O^?9.

of O^?9 …? H1′ is 1.6 Å (exchange of electrons exists between their atoms). The transition-state geometry of the decarboxylation process has the following features: (1) the activation enthalpy is 6.0 kcal/mol, (2) the dihedral angle of C8? C7? C2? N1 is ?50.2°, and (3) the interatomic distance of O^?9? H1′ and C7? C8 increase by 111 and 124%, respectively, as compared with the optimized geometry. From the extreme beginning of the intrinsic decarboxylation process, the exchange of electrons between O^?9 …? H1′ begins to decrease. This decrease, which is considered to be induced by the rotation of C2? C7, seems to initiate the dissociation of C7? C8. © 1995 John Wiley & Sons, Inc.     

Simulation of free radical addition reactions modified by the presence of cobalt macrocycles

Free radical addition reactions in the presence of cobaloximes and related compounds have been modelled. Several mechanisms are presented and similarities with the “persistent radical effect” noted by Daikh and Finke are discussed. Cobaloximes and salophen

1 System. name: N-acetyl-p-aminophenyl salicylate.

derivatives are widely used in organic synthesis to build carbon-carbon bonds, whereas in polymer synthesis they are used as catalytic chain transfer agents in the production of oligomers. This work shows that these reactions are closely related and also demonstrates the influence that an external radical source has on the overall reaction kinetics.     

A theoretical study of the decomposition of halogenated alkoxy radicals. II. Fluorine extrusion

The extrusion of a fluorine atom from alkoxy radicals is analyzed theoretically and compared with experiment. Although the MNDO/CI method gives qualitatively correct information about the nature of the wavefunction, the MNDO method with or without CI is inadequate to describe the dissociation reaction path, presumably due to parameterization problems. On the contrary, ab initio SCF coupled with extended CI seems to correctly mimic the barrier for fluorine extrusion. Using modified Marcus theory in conjunction with the ab initio results for CH₂FO, we evaluate the barriers for the other radicals investigated.     

Reaction of 1,5-diphenyl-3-arylverdazyles with СН-acids

The presence of electron-donor substituents in the phenyl ring at the atom С³ of the tetrazinyl ring increases, and of electron-acceptor substituents reduces the reaction rate of 1,5-diphenyl-3-arylverdazyl radicals with СН-acids. The reaction is described with a kinetic equation of the second order with respect to verdazyl, the process rate is determined by the energy of the electron transfer from the SOMO (singly occupied molecular orbital) of the radical to the LUMO of the СН-acid.     

Structure et synthèse de la damascénone (triméthyl-2,6,6-trans-crotonoyl-1-cyclohexadiène-1,3), constituant odorant de l'essence de rose bulgare (rosa damascena Mill.

Damascenone

1 DORICENONE (trade mark applied for by Firmenich & Cie, Geneva).

an odoriferous ketone isolated in minute amounts from Bulgarian rose oil (Rosa damascena Mill.) is shown to be trans-2,6,6-trimethyl-1-crotonoyl-cyclohexa-1, 3-diene (I). A synthesis starting from beta;-cyclocitral is described for this conspicuous constituent of rose oil, which displays a powerful fragrance. The synthesis of four related, more saturated ketones, damascones

2 DORINONE (trade mark applied for by Firmenich & Cie, Geneva).

, is also described.     

Conformational analysis of 7,7-dichloro-2,5-dioxabicyclo[4.1.0] heptane by NMR

The ¹H NMR spectra of 7,7-dichloro-2,5-dioxabicyclo-[4.1.0]heptane ( 1 ) were analyzed through the application of programs NMRIT and NMREN2.

1 See Ref. 1.

The half-chair conformation of 1 was obtained from the vicinal coupling constants by the Buys R method.

2 See Ref. 2.

     

Synthesis of new derivatives of insulin sequence B1-8 with improved solubility

The synthesis of the protected octapeptide N-t-butoxycarbonyl-phenylalanyl-valylasparaginyl-glutaminyl-N^im-tritylhistidyl-leucyl-S-benzhydrylcysteinyl-glycine-hydrazide

1 L

, corresponding to the amino-terminal sequence of the insulin B-chain, is described. Up to the protected octapeptide all intermediates have greatly improved solubility properties in neutral organic solvents in comparison with other synthetic derivatives of the same sequence.     

Etude dans la série des radicaux hétérocycliques. Partie XV. Décomposition aprotique de l'amino-6-éthyl-2-benzothiazole dans des substrats aromatiques et hétéroaromatiques: préparation des mésityl-6- et furyl-6-éthyl-2-benzothiazoles,des sels quaternaires et des spiropyrannes correspondants

Study of heteroaromatic radicals. Part XV .

1 Partie XIV: cf. [1].

Aprotic decomposition of 6-amino-2-ethylbenzothiazole in aromatic and heteroaromatic substrates: preparation of 6-mesityl- and 6-furyl-2-ethylbenzothiazoles and of the corresponding quaternary salts and spyropyrans In the presence of excess isopentyl nitrite in aprotic solvent, the 6-amino-2-ethyl-benzothiazole leads to 6-aryl- and 6-heteroaryl-2-ethylbenzothiazoles 1 to 14 in ca. 20–50% yield (Scheme 1, Table 1). The 2-ethyl-6-benzothiazolyl radicals generated during these reactions exhibit a weak electrophilic character (Table 2). Preparatively this method has been applied to the synthesis of 6-mesityl- and 6-furyl-2-ethylbenzothiazoles 6 and 7 , which were used to prepare the corresponding quaternary salts 15 , 16 and the spiropyrans 19, 20 (Scheme 4).     

Carbon-13 nuclear magnetic resonance spectroscopy IX—monosubstituted ethylenes

Carbon-13 chemical shifts of sixteen monosubstituted ethylenes were obtained. In order to explain the chemical shifts, σ and π electron densities of these compounds are calculated by the σ-included ω-HMO method.

1 See Ref. 8.

A linear relationship exists between carbon-13 chemical shifts and the calculated electron densities, and also between substituent constants and electron densities. A slope of unity is obtained between the chemical shifts of α carbons of monosubstituted ethylenes and those of carbons adjacent to the substituents in monosubstituted benzenes. On the other hand, a plot of chemical shifts of C_ortho of benzene derivatives against that of the β carbon in ethylene derivatives gives a slope of 3. These slopes can be explained by the calculated electron densities. A slope of 4/3 is obtained between the direct coupling constant ¹J(C? H) of the α carbon in monosubstituted ethylenes and that in the corresponding substituted methanes.     

Heterocyclic Spiro-naphthalenones. Part V. Synthesis of Some 2-Benzazepines from 3,4-Dihydrospiro [furan-2-(5H), 1′(2′H)-naphthalene]-2,5-dione

Compound 2 was rearranged on treatment with 2N NaOH to the 2-benzazepine-3-propanoic

1 See footnote (5) of the preceding paper.

acid 3 . Some aspects of the chemistry of this acid were studied.     

Electronic, Energetic, and Geometric Properties of Methylene-Functionalized C60

Chemical functionalization of C₆₀ fullerene with one to six carbene (CH₂) molecule(s) has been investigated using density functional theory. We have found that the reaction is regioselective so that a CH₂ molecule prefers to be adsorbed atop a C–C bond which is shared between two hexagonal rings of the C₆₀, releasing energy of ?3.95 eV. Singly occupied molecular orbital (SOMO) of the CH₂ interacts with LUMO of the C₆₀ via a [2 + 1] cycloaddition reaction. Energy of the reaction and work function of the system are decreased by increasing the number of adsorbed CH₂ molecules. The HOMO/LUMO energy gap of C₆₀ is slightly changed and the electron emission from its surface is facilitated upon the functionalization.     

Three chlorinated bipyridine compounds: Quantum calculations and their relationship to the molecular structure and physicochemical properties of the molecules considered

The Cl³⁵ nuclear quadrupole resonances were measured for three chlorinated bipyridine compounds. The number of resonances obtained is explained in terms of the position of one ring of the compound with respect to the other. Quantum chemical calculations

1 Details of the computer programs used are available from the author on request.

confirm the explanation given.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solution. Part XII. Solvent effects in competitive heterolytic and homolytic dediazoniations

In heterolytic dediazoniations arenediazonium salts form aryl cations. The reaction rates are relatively slow; they depend only to a small extent on the solvent. It is shown that the solvents in which the heterolytic dediazoniation mechanism is predominant have a low nucleophilicity, whereas in solvents of high nucleophilicity homolysis of arenediazonium salts, i.e. the formation of aryl radicals and related intermediates, is favoured. Under comparable conditions, homolytic rates are faster than the corresponding rates of heterolysis. Homolysis is strongly enhanced by addition of nucleophiles which form relatively stable radicals by electron transfer. The ability of additives to catalyze homolysis of arenediazonium salts can be explained using the concept of a nucleofugic

1 In the original proposal [32] we used the word nucleofugal. In keeping with a forthcoming proposal on nomenclature in physical organic chemistry by Commission III.2 (Physical Organic Chemistry) of IUPAC we now use the word nucleofugic.

homolytic leaving group.