Configuration and conformation of substituted azetidines

Studies of compounds such as 1-cyclohexyl-2-carbomethoxy-4-methylazetidine by H-1 nmr coupling constants, nuclear Overhauser effects, C-13 steric shifts, and N-15 nmr support the revised assignment of configuration for cis/trans isomers in a series of N-alkyl substituted azetidines. The trans isomer displayed spectral characteristics consistent with a planar or nearly planar ring. The cis isomer favors a puckered ring, with major substituents at C-2 and C-4 equatorial. Compounds lacking the C-4 methyl are also puckered. Puckering angles are estimated. In contrast, trans-1-cyclohexyl-2-carboxy-4-methylazetidine (a zwitterion) prefers a puckered ring, possibly implicating the time averaged effect of nitrogen inversion on ring shape. The N-15 spectra show a large difference in chemical shift between cis and trans isomers of the azetidines, although analogous aziridines show an even larger difference.     

12H-dibenzo[d,g][1,3,2]dioxaphosphocins: Synthesis,structures and “through-space” spin-spin coupling in their NMR spectra

A series of 12H-dibenzo[d,g][1,3,2]dioxaphosphocins has been prepared by the reactions of bisphenols with either ethyl phosphorodichloridate or phosphorus pentasulfide. The structures of a pair of cis and trans isomers in this series were elucidated by X-ray crystallography. Both isomers adopt the boat-chair conformation in the solid state with the bulky group at C-12 in the pseudo-equatorial position. Some flattening of the heterocyclic ring due to the pseudo-axial ethoxy group was observed in the cis isomer. A novel transannular cyclisation reaction was observed in the mass spectra of the cis isomers and this has allowed us to assign the configurations of all the isomers in the series. The pmr spectra of the compounds have been explained in terms of the rigid boat-chair conformation; however, as bulky groups were introduced at C-12, signs of mobility were observed for the cis isomers and an equilibrium was established with mobile boat forms. A stereospecific long-range coupling between P and the proton at C-12 could be transmitted “through space” by the antiperiplanar lone-pair electrons on the ring oxygen atoms.     

The heavier pnictogen and chalcogen analogues of isocyanic and cyanic acids and their dimers: A high level ab initio study

The CCSD(T)/cc-pVTZ//CCSD/cc-pVTZ method is used to determine the geometries and energetics of the isomers HXCY vs HY─CX (XN, P, As; YO, S) and their dimers from chain dimerizations and head-to-head or head-to-tail [2 + 2] cyclodimerizations. The HO─CX structures with CX triple bonds lie at energies at least 18.5 kcal/mol above their HXCO isomers. However, the energy differences between the HXCS and HS─CX isomers are found to be particularly small, especially in the [H,P,C,S] and [H,As,C,S] systems. For (HNCY)₂, the lowest energy dimers are the chain isomers, which lie ~11 kcal/mol below the lowest energy cyclic dimers aNO containing a NCNC ring and cNS containing a NCSC ring. Formation of the remaining dimers through dimerization from two monomers is predicted to be endothermic and thus thermodynamically disfavored. However, the energies of the chain isomers in the other (HXCY)₂ (XP, As; YO, S) series are higher than those of the corresponding isomeric lowest energy cyclodimers. For (HXCO)₂ (XP, As), the lowest energy structures are the head-to-head dimers hPO and hAsO containing a C─C─XX ring. For (HXCS)₂ (XP, As), the lowest energy structures are the head-to-head dimers gPS and gAsS with a CCXS ring.     

Synthesis and stereochemistry of rac.-trans-tetrahydro-6-hydroxy-7-(4-methoxyphenyl)-1,4-thiazepin-5(2H)-one

The base catalyzed reaction of 2-aminoethanethiol ( 3 ) with trans-3-(p-methoxyphenyl)glycidate ( 4 ) gave a mixture of isomeric lactams 5 and 6 , and in addition, a by-product 7 . The structures of these isomers were proven by X-ray crystallography. The data revealed that both isomers adopt the chair conformation in the solid state and the size of the heterocyclic ring in compound 5 is a six- and in compound 6 is a seven-membered ring.     

NMR Study of the Steric and Electronic Structure of 2-(2-Acylethenyl)pyrroles

According to the ¹H and ¹³C NMR data, 2-(2-acylethenyl)- and 2-(2-acyl-1-phenylethenyl)pyrroles in chloroform exist exclusively in the keto form. The Z isomers of these compounds are characterized by coplanar arrangement of the olefinic fragment, carbonyl group, and pyrrole ring. The strong intramolecular hydrogen bond NÄH···O is responsible for the presence of only one rotamer. The corresponding E isomers give rise to equilibrium between conformers with syn and anti arrangement of the olefinic fragment with respect to the pyrrole ring. A very strong conjugation between the ketovinyl group and the pyrrole ring is likely to arise from an appreciable contribution of the zwitterionic structure.     

Stereochemical studies—XXXVII: Saturated heterocycles—XIII Configuration and conformation of Z- and E-N-methyl- and -N- benzyl-2-p-nitrophenyl-4,5- and -5,6-tetramethylenetetrahydro-1,3-oxazines

The configuration and conformation of Z- and E-N-methyl- and -N-benzyl-2-p-nitrophenyl-4,5-, and -5,6-tetramethylenetetrahydro-1,3-oxazines were determined by ¹H and ¹³C NMR spectroscopy. The Z isomers were proved to be conformationally homogeneous, having the heteroatom in axial and equatorial position, respectively, in the case of the 5,6- and 4,5-tetramethylene compounds. Consequently, the p-nitrophenyl group and the anellated cyclohexyl ring are all-cis arranged in the Z-5,6-tetramethylene compounds while in the case of the Z-4,5-tetramethylene isomers the p-nitrophenyl group and the cyclohexyl ring are trans arranged to the hetero ring.     

Isomerizations of CH3SO2: Quantum Chemistry and Topological Study

The structures, stabilities and the isomerization reactions of CH₃SO₂ isomers in a doublet electronic state have been studied at B3LYP/6‐311+ +G (d,p), MP2/6‐311++G (d,p) and CCSD(T)/6‐311++G (d,p) levels. The three different levels of calculation give the similar results: thirteen minimum isomers were located and they were connected by eleven transition states. Among the thirteen isomers, cis‐CH₃OSO, trans‐CH₃OSO and CH₃SO₂ are the most stable species, and they should be detected easily in experiment. This is well consistent with the experimental result. These isomers could isomerize to each other by chemical bond vibration, chemical bond rotation and atom migration. The non‐planar ring structure transition state (STS), which was found in this paper, extended the concept of ring STS to the non‐planar systems.     

Photochemistry of the vitamin D series. II. Wavelength dependence of photoisomerization of precalciferol

In addition to the known ring opening at shorter wavelengths it was found that ring opening in Δ^5,7-steroids occurs also at λ > 300 nm. Ring closure of the corresponding seco- steroids in this region was confirmed. The existence of three rotational isomers with respect to the 5,6-bond is assumed to be responsible for the observed wavelength dependency of the photoisomerizations of previtamin D. In this case the 5,6-s-trans isomer should absorb uv light at shorter wavelengths yielding tachysterol whereas the two 5,6-rotational isomers with skew conformation should absorb at longer wavelengths yielding lumisterol and ergosterol, respectively.     

Ring opening of pyridines: the pseudo‐cis and pseudo‐trans isomers of tetra‐n‐butylammonium 4‐nitro‐5‐oxo‐2‐pentenenitrilate

The reaction of 2‐chloro‐5‐nitropyridine with two equivalents of base produces the title carbanion as an intermediate in a ring‐opening/ring‐closing reaction. The crystal structures of the tetra‐n‐butylammonium salts of the intermediates, C₁₆H₃₆N⁺·C₅H₃N₂O₃⁻, revealed that pseudo‐cis and pseudo‐trans isomers are possible. One crystal structure displayed a mixture of the two isomers with approximately 90% pseudo‐cis geometry and confirms the structure predicted by the S_N(ANRORC) mechanism. The pseudo‐cis intermediate undergoes a slow isomerization over a period of months to the pseudo‐trans isomer, which does not have the appropriate geometry for the subsequent ring‐closing reaction. The structure of the pure pseudo‐trans isomer is also reported. In both isomers, the negative charge is highly delocalized, but relatively small differences in C—C bond distances indicate a system of conjugated double bonds with the nitro group bearing the negative charge. The packing of the two unit cells is very similar and largely determined by the interactions between the planar carbanion and the bulky tetrahedral cation.     

CH3NO2异构化反应的理论研究

利用密度泛函和电子密度拓扑分析方法对CH3NO2 (NM)的异构化反应进行了研究。 找到了九种可能的异构体和八个反应通道。通过内禀反应坐标(IRC)分析确认了过渡态与异构体之间的连接关系。计算结果表明,在CH3NO2→CH3ONOt反应过程中,过渡态为紧密结构(在整个反应过程中CH3NO2没有分解为CH3 和NO2 ),与Arenass等人的结论一致。在CH3NOOc→CH2NOOH反应过程中,存在有一个含有四元环→五元环→四元环→五元环变化过程的结构过渡区,这也是在反应过程中首次发现五元环状过渡结构。     

Bonding of acetylene to copper atom,dimer, and trimer

The bonding of acetylene to copper atom, dimer, and trimer was investigated with a Kohn–Sham density functional approach. Full geometry optimization yielded the equilibrium structures of various Cu_nC₂H₂ species. Gradient corrections were included in the calculation of binding energies (BE ). The Cu—C₂H₂ complex was found to have a C_s structure and a BE of 10 kcal/mol. Three isomers of Cu₂C₂H₂ have similar total energies: a C_2v end-bonded structure with a BE of 18 kcal/mol, and two 1,2-dicupro ethylene isomers—a cis form with a BE of 12 kcal/mol and a trans form with a BE of 15 kcal/mol. Two stable C_2v isomers of Cu₃C₂H₂ were found. In both isomers, the Cu₃ ring relaxes from its isosceles structure, with two short bonds (2.247 Å) and one long bond (2.478 Å), and adopts a nearly equilateral geometry. In one isomer of Cu₃C₂H₂, the acetylene is bonded to one apex of the Cu₃ ring with a BE of 29 kcal/mol. In the other, it is bonded to two copper atoms of one side of the Cu₃ ring with a BE of 33 kcal/mol. © 1994 John Wiley & Sons, Inc.     

Relative Stabilities of the Geometrical and Conformational Isomers of 2-Alkylideneoxacycloalkanes. A DFT Study

The relative stabilities of the geometrical and rotational isomers of 2-alkylideneoxacycloalkanes (-oxiranes, -oxetanes, -tetrahydrofurans, and -tetrahydropyrans; alkyl = Et, Pr, i-Bu, 2,2-dimethylpropyl) have been studied by DFT calculations. Independent of the size of the alkyl group, the E and Z isomers of alkylideneoxiranes have almost comparable stabilities (the Z form, however, being slightly favored), but, with increasing size of the heterocyclic ring, the relative stability of the E isomer decreases. This is particularly prominent for the tetrahydropyran derivatives with alkyl = 2,2-dimethylpropyl, in which marked repulsive interactions between the t-Bu group and the 3-CH₂ group of the tetrahydropyran ring make the E form, ca. 13 kJ mol^–1 less stable than the Z isomer. On the other hand, for alkyl = Et, Pr, and i-Bu, the relative stabilities of the geometrical isomers are almost independent of the alkyl group. Besides the relative stabilities of the geometrical isomers, energetics of the rotational isomerism of the alkyl group about the C(sp ³)—C(sp ²) bond is also surveyed.     

A Quantum Chemical Study of N14 Cluster

Ab initio molecular orbital theory and density functional theory have been employed to study N₁₄ cluster with low spin at the HF/6-31G*, B3LYP/6-31G*, B3PW91/6-31G*, BP86/6-31G*, and BHLYP/6-31G* levels of theory. Twelve isomers were studied, including one previously investigated cage molecule. The most stable isomer of N₁₄ is a C _2h -symmetric molecule that contains two separated five-membered nitrogen rings connected by a —N=N—N=N— bridge. The second, third, and fifth most stable isomers each have one five-membered nitrogen ring. The theoretical results suggest that the five-membered nitrogen ring gives rise to a particularly stable structural unit, and the more side chains that the five-membered nitrogen ring links with, the less stable the structure will become.     

Synthesis and configuration of trans-1-amino-4-benzyl-2,6-dimethylpiperazine as an intermediate of semi-synthetic rifamycins

The synthesis of trans-1-amino-4-benzyl-2,6-dimethylpiperazine (1b), trans-4-benzyl-2,6-dimethylpiperazine (VIIb) and trans-2,6-dimethylpiperazine (IIb) are described, by condensation of N-benzyl-1,2-propanediamine with α-bromopropionate and successive thermal cyclization and reduction with lithium aluminum hydride. The assignment of the cis, trans configuration to the isomers was based on a thorough examination of the ring proton nmr signals of the two isomers of 4-benzyl-2,6-dimethylpiperazine (VIIa and VIIb) interpreted in terms of conformational considerations.     

High‐Resolution Photoelectron Imaging of IrB3−: Observation of a π‐Aromatic B3+ Ring Coordinated to a Transition Metal

In a high‐resolution photoelectron imaging and theoretical study of the IrB₃^? cluster, two isomers were observed experimentally with electron affinities (EAs) of 1.3147(8) and 1.937(4) eV. Quantum calculations revealed two nearly degenerate isomers competing for the global minimum, both with a B₃ ring coordinated with the Ir atom. The isomer with the higher EA consists of a B₃ ring with a bridge‐bonded Ir atom (C_s , ²A′), and the second isomer features a tetrahedral structure (C_3v , ²A₁). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C_3v isomer involves significant covalent Ir?B bonding and weak ionic bonding with charge transfer from B₃ to Ir, and can be viewed as an Ir–(η³‐B₃⁺) complex. This study provides the first example of a boron‐to‐metal charge‐transfer complex and evidence of a π‐aromatic B₃⁺ ring coordinated to a transition metal.     

Catalytic Desymmetric Cycloaddition of Diaziridines with Metalloenolcarbenes: The Role of Donor–Acceptor Cyclopropenes

A chiral copper(I) complex catalyzes reactions of symmetric diaziridines with enol diazo compounds, which react through N?N bond ring opening in a formal [3+3] cycloaddition to form four chiral centers with high stereocontrol. A broad spectrum of bridged dinitrogen heterocycles were obtained in high yields and excellent diastereo‐ and enantioselectivities from γ‐substituted enol diazoacetates, while their geometrical isomers gave different enantioselectivities. Donor–acceptor cyclopropenes formed from the geometrical isomers of the γ‐substituted enol diazoacetates underwent catalytic ring opening to give only the Z isomer of the metalloenolcarbene intermediate, provided excellent yields and selectivities for the 1,5‐diazabicyclo[n.3.1]non‐2‐ene derivatives.     

Stereochemical studies. Part 67. Saturated heterocycles. Part 52. The mass spectra of some condensed skeleton 1,3-oxazin-4-one derivatives

The mass spectrometric behaviour of twenty saturated heterocyclic compounds with a 1,3-oxazin-4-one moiety fused with cis or trans anellation to a cycloalkane ring (C₅-C₈) was studied. The roles of the C-2 and N-3 substituent(s) were found to be characteristic, while the size of the cycloalkane ring seemed to be unimportant. Some fragmentation processes involving breakdown of the oxazinone ring of the cis or trans isomers displayed significant stereoselectivity. A striking new decomposition process involving significant chlorine elimination from the molecular ion of some 2-p-chlorophenyl derivatives was observed and was studied in some detail.     

Cationic Closo Carboranes—Promising Weakly Coordinating Ions

The unexplored carbon rich cationic closo carboranes, C₃B_n?3H_n⁺¹ (n=5, 6, 7, 10, 12) are investigated theoretically. The position isomers were calculated at the B3LYP/6‐31G* level, and the charge distribution in the cluster is estimated by NBO analysis. The criterion of ring‐cap orbital overlap compatibility along with the number of B? C, C? C, and B? B bonds help in explaining the stability order in each category. The most stable isomer is the one with maximum ring‐cap orbital overlap and largest number of B? C bonds. The order of relative stability among the trigonal bipyramid is 1c > 1b > 1a ′, where the stability is proportional to the number of CH caps over the small three‐membered ring. The C₃B₃H₆⁺ isomer with the one allyl C3 group ( 2b ) is more favorable than the one with a cyclopropenyl group ( 2a ). Among the C₃B₄H₇⁺ isomers the stability order is 3e > 3d > 3c > 3b > 3a , which mostly depends on the ring‐cap orbital overlap. In the bicapped square antiprism (4) where there is large number of isomers, the order follows the rule of ring cap compatibility and the number of B? C bonds. The order of 5e > 5d > 5c > 5b > 5a obtained from the calculations is in perfect agreement with the above sited rules. Equations (1) – (5) devised for estimating the stability of isomers of C₃B_n?3H_n⁺ indicate an increase in stability with cage size. The mono‐positive charge of the isomers is distributed throughout the cage, making them suitable candidates as weakly electrophillic cations. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1542–1551, 2001     

Electronic structures of the S2O and S3 isomers: an ab initio CI study

The electronic structures of the S₂O and S₃ isomers have been dealt with by the multireference double-excitation (MRD) configuration-interaction (CI) calculations, using contracted [5s3p1d] and [4s2p1d] basis functions for the S and O atoms, respectively. The ground-state geometries for the SOS (symmetric chain), S₂O (symmetric ring) and SSO (unsymmetric chain) are optimized, and their vertical singlet excitation energies are calculated. It is found that SSO is the most stable of the three isomers and that the ground state (¹A₁) of the S₂O (ring) is correlated with the excited states of SOS (2¹A₁) and SSO (3¹A). The chain and ring isomers of S₃ have been treated in a similar manner. Energetics for the ring closure of the O₃, SO₂, SSO and S₃ chain molecules are discussed on a unified ground.Dedicated to Professor J. Koutecký on the occasion of his 65th birthdayPresented at the 5th International Congress on Quantum Chemistry, Montréal, August 1985     

Two conformers of 10,11‐dihydro‐5H‐dibenzo[a,d]cyclo­heptene spiro‐linked with homobenzoquinone epoxide

The crystal structures of the two thermally equilibrated conformational isomers of the epoxide 1′,5′‐dimethyl­spiro[10,11‐dihydro‐5H‐dibenzo[a,d]cyclo­heptene‐5,8′‐4′‐oxatricyclo[5.1.0.0^3,5]octane]‐2′,6′‐dione, C₂₃H₂₀O₃, have been determined by X‐ray diffraction. In the tricyclic dione skeleton, the oxirane and cyclo­propane rings adopt an anti structure with respect to the conjunct quinone frame. The spiro‐linked 10,11‐dihydro‐5H‐dibenzo[a,d]cyclo­heptene ring of the major isomer has a fairly twisted boat form, folding opposite to the adjoining cyclo­propane methyl substituent, whereas the seven‐membered ring of the minor isomer has an almost ideal twist–boat form, inversely folding to the side of the relevant methyl group. The conformational structures of these isomers have been compared with those of the corresponding isomers of the unepoxidized homobenzoquinone.