Reliable determination of amidicity in acyclic amides and lactams

Two independent computational methods have been used for determination of amide resonance stabilization and amidicities relative to N,N-dimethylacetamide for a wide range of acyclic and cyclic amides. The first method utilizes carbonyl substitution nitrogen atom replacement (COSNAR). The second, new approach involves determination of the difference in amide resonance between N,N-dimethylacetamide and the target amide using an isodesmic trans-amidation process and is calibrated relative to 1-aza-2-adamantanone with zero amidicity and N,N-dimethylacetamide with 100% amidicity. Results indicate excellent coherence between the methods, which must be regarded as more reliable than a recently reported approach to amidicities based upon enthalpies of hydrogenation. Data for acyclic planar and twisted amides are predictable on the basis of the degrees of pyramidalization at nitrogen and twisting about the C-N bonds. Monocyclic lactams are predicted to have amidicities at least as high as N,N-dimethylacetamide, and the β-lactam system is planar with greater amide resonance than that of N,N-dimethylacetamide. Bicyclic penam/em and cepham/em scaffolds lose some amidicity in line with the degree of strain-induced pyramidalization at the bridgehead nitrogen and twist about the amide bond, but the most puckered penem system still retains substantial amidicity equivalent to 73% that of N,N-dimethylacetamide.     

A simple copper-catalyzed synthesis of tertiary acyclic amides

The N-arylation of aromatic and aliphatic secondary acyclic amides, known to be poor nucleophiles, has been accomplished using a simple and cheap copper catalytic system. The corresponding tertiary acyclic amides, which can be found in numerous biologically active compounds, have been obtained in good to excellent yields.     

Crystal structures and certain properties of hapnium-beryllium compounds

Summary The system Hf-Be was investigated by x-ray structural and microstructural analysis and by determinations of microhardness. The existence of four compounds was established: {ie402-1}Translated from Zhurnal Strukturnol Khimil, Vol. 2, No. 4, pp. 424–433, July–August 1961     

One-step RhCl3-catalyzed deprotection of acyclic N-allyl amides

A convenient one-step RhCl3-catalyzed deprotection of acyclic N-allyl amides is described. Preliminary mechanistic studies reveal that the key to the success of the one-step deprotection process is the dual function of RhCl3 in alcohol solvents. Reaction of RhCl3 with n-PrOH not only provides an active rhodium hydride species to catalyze isomerization of N-allyl amides to corresponding enamides but also generates a crucial catalytic amount of HCl to convert the enamides to deallylated amides through N,O-acetal exchange.     

The versatile conversion of acyclic amides to alpha-alkylated amines

A general and efficient method for the versatile functionalization of acyclic amide via N,O-acetal TMS ether, an excellent precursor for the N-acyliminium ion, has been developed.     

Titanium and zirconium benzofuranoxides. Crystal structures and catalytic properties

Reactions of Ti(OiPr)4 or Zr(OEt)4 with 4 equivalents of 2,3-dihydro-2,2-dimethyl-7-benzofuranol (ddbfoH) in toluene gave neutral complexes that in the solid state are dimers of [Ti(micro-ddbfo)2(ddbfo)6] and [Zr(ddbfo)3(EtOH)(micro-EtO)]2 composition. The former could also be conveniently synthesized in a direct reaction of TiCl4 with ddbfoH. This air-stable aryloxo compound was found to initiate living ring-opening polymerization of lactides affording polyesters with narrow molecular weight distribution. It also catalyzed addition of terminal acetylenes to aryl aldehydes.     

Reactions of phosphoric anhydride with acyclic amides. Synthesis of amidines and imides

Acyclic secondary amides such as benzanilides 1a–g , acetanilides 2a–d , and benzamide 1h undergo self-condensation in the presence of phosphoric anhydride to produce, in one step, N-substituted aroyl benzamidines 3a–g , acyl acetamidines 4a–d , and imide 5 , respectively. Mechanistic evidence is presented for nucleophilic attack by the iminol tautomers on the O-phosphorylated amides to give amidines and on the N-phosphorylated imidates to give imides. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:261–263, 1998     

Synthesis and properties of phenylogous amides

Vinylogy is a widely accepted principle involving the transfer of electronic chemical character through a double bond, and many reactions have been developed by applying this concept. In contrast, phenylogy, which involves the connection of two substituents through a benzene ring, is rarely recognized as a related idea. In this article, we present synthesis and physical properties, including their structure and reactivity of phenylogous amides. This amide mimetic unit is relatively stable and easily prepared by the Hartwig–Buchwald amination reaction. The effect of the resonance was examined by means of crystallography, reactivity and UV–vis spectroscopy.     

Structures of N,N-dialkoxyamides: pyramidal anomeric amides with low amidicity

The first X-ray structures of two anomeric N,N-dialkoxyamides (2 and 3) have been obtained, which confirm that they are highly pyramidalized at nitrogen and have long N-CO bonds, a characteristic of other anomeric amides and a consequence of drastically reduced amidicity. The crystals also demonstrate chirality at the amide nitrogen in the solid state. The structures are well-predicted by density functional calculations using N,N-dimethoxyacetamide as a model. The amidicity of N,N-dimethoxyacetamide has been estimated by two independent methods, COSNAR and a new transamidation method, which give almost identical resonance stabilization energies of -8.6 kcal mol(-1) and only 47% that of N,N-dimethylacetamide computed at the same level. The total destabilization is composed of a resonance and an inductive contribution, which we have evaluated separately. The electronegative oxygens at nitrogen are responsible for localization of the nitrogen lone pair on the amide nitrogen, a factor that contributes to a loss of resonance over and above the impact of pyramidalization at nitrogen, as well as the fact that N,N-dimethoxyacetamide is predicted to protonate on the carbonyl oxygen in preference to nitrogen.     

Crystal structures and physicochemical properties of mixed salts of ammonium nitrate and sulfate

Mixed salts (NH₄)₂SO₄·2NH₄NO₃ (1) and (NH₄)₂SO₄·3NH₄NO₃ (2) were synthesized and studied by X-ray diffraction analysis. The unit cell parameters of these salts were determined and their crystal structures were solved. The thermal stability of the salts was studied by differential scanning calorimetry and thermogravimetric analysis. The temperatures and enthalpies of incongruent melting of compounds 1 and 2 were determined. The enthalpies of formation from the constituent salts were estimated.     

Crystal structures and dielectric properties of ordered double perovskites containing Mg and Ta

The ordered double perovskites ALaMgTaO₆ (A=Ba, Sr, Ca) and La₂Mg(Mg_1/3Ta_2/3)O₆ have been prepared and characterized. Synchrotron X-ray powder diffraction analyses show that all four compounds exhibit a rock-salt type ordering of the B-site cations (Mg²⁺/Ta⁵⁺) and a random distribution of A-site cations (A²⁺/La³⁺). The space group symmetries are determined to be for BaLaMgTaO₆, and P2₁/n for SrLaMgTaO₆, CaLaMgTaO₆, and La₂Mg(Mg_1/3Ta_2/3)O₆. Diffuse-reflectance spectroscopy shows these ordered perovskites have optical band gaps in the range of 4.6−4.8 eV. These values are roughly 1 eV wider than the ternary perovskite oxides of Ta⁵⁺ such as KTaO₃, due to narrowing of the conduction bandwidth which results from Mg²⁺/Ta⁵⁺ ordering. These compounds are insulators with dielectric permittivities of κ=18-23, dielectric losses of tan δ=0.004-0.007, and small temperature coefficients of capacitance <100 ppm/K over the temperature range 20-150 °C. BaLaMgTaO₆ is of particular interest because it possesses a near-zero temperature dependence of capacitance.     

Recognition properties of acyclic glycoluril oligomers

The fragmentation reaction of bis-nor-seco-CB[10] with 3,5-dimethylphenol (3) delivers methylene bridged glycoluril pentamer 5 in 81% yield. The host-guest recognition properties of the previously known tetramer 4 and those of pentamer 5 and hexamer 6 toward cationic guests in water are used to delineate some important features of the binding of acyclic CB[n]-type receptors.     

Crystal structures,optical properties and theoretical calculation of novel two-photon polymerization initiators

Three novel A–π–D–π–A type compounds 3,6-bis[2-(4-pyridyl)ethenyl]-9-ethylcarbazole (1), 3,6-bis[2-(2-pyridyl)ethenyl]-9-ethylcarbazole (2), 3,6-bis[2-(4-pyridyl)ethenyl]-9-ethylpyridylcarbazole (3) were conveniently synthesized by Pd-catalyzed Heck coupling methodology and characterized by single crystal X-ray diffraction determination. One-photon fluorescence, one-photon fluorescence quantum yields, one-photon fluorescence lifetime, and two-photon fluorescence have been investigated. The calculated two-photon absorption cross-sections for the three initiators by quantum chemical method are as high as 947, 943 and 815 × 10⁻⁵⁰ cm⁴ s photon⁻¹, respectively. Two-photon initiating polymerization microfabrication experiments have been carried out and the possible polymerization mechanism was also discussed. The results show that they are good two-photon absorbing chromophores and effective two-photon photopolymerization initiators.     

Crystal structures and properties of four coordination polymers constructed from flexible pamoic acid

Four novel coordination polymers constructed from flexible pamoic acid, namely [Co(pam)(4,4′-bipy)]_n·nH₂O (1), [Ni(pam)(4,4′-bipy)(H₂O)₂]_n·2nCH₃CN (2), [Cd(pam)(py)₂]_n·npy (3) and [Mn₂(pam)₂(py)₆(H₂O)₂]_n·2npy (4), (H₂pam = pamoic acid, 4,4′-bipy = 4,4′-bipyridine, py = pyridine), have been synthesized and characterized by elemental analysis, infrared spectra and X-ray crystallography. Complex 1 is a 2-D coordination polymer constructed from chelating bis-bidentate pam and 4,4′-bipyridine bridging ligands. Complex 2 is a 2-D coordination polymer assembled by bis-monodentate pam and 4,4′-bipyridine, where acetonitrile is filled in the rectangle channels. Both 2-D coordination polymers display undulated (4,4) grid layers as sql topology. Complex 3 displays a 1-D polymeric chain using chelating bis-bidentate pam as bridging ligand. Complex 4 exhibits an interesting bis-monodentate pam-Mn(II) 1-D polymeric chain, in which exist two-type six-coordinated manganese centers. Mn(1) is bound to four pyridine ligands, whereas Mn(2) is combined to two pyridine and two H₂O molecules. Their thermal stabilities have been investigated. Cadmium complex 3 displays strong green luminescence with emission maximum at 543 nm.     

1-烷基-3-甲基咪唑溴化盐离子液体的晶体结构及性能

以不同链长溴代烷烃和N-甲基咪唑反应得到1-烷基-3-甲基咪唑溴化盐,用元素分析和核磁共振对化合物进行了表征.室温下用溶剂蒸发法得到了单晶,并用X射线单晶衍射法测定了晶体结构,该晶体属于三斜晶系,空间群为P-1.化合物采用双分子层结构,水分子参与结构的形成,整个化合物由交叉的线性烷基链、咪唑头基、溴离子和水分子组成,溴离子和水分子之间较强的氢键作用在(010)方向上形成了一个无限的O-H···Br氢键链.用偏光显微镜、差示扫描量热(DSC)技术研究了其液晶行为,证明其一水合物为近晶相热致液晶.液晶区域的温度范围较宽说明水分子起到稳定作用.     

Chair-boat interconversions in cyclic seven-membered amides and related structures

The transition state to ring reversal has been examined in a series of 7-membered rings related to caprolactam. The structural changes associated with introduction of Me groups, with alteration of the nature of the CN double bond, with ring fusion, or with introduction of a second, nonconjugated N atom perturb the transition state to ring reversal. From ¹H and ¹³C NMR studies, we find that the lactam bond itself favors a different transition state from that present in cycloheptene. With some exceptions, the observed barriers are consistent with TS II, in which ring reversal begins with movement of the C₅ end of the ring, or with TS III, in which ring reversal begins with C₃-C₄ bond torsion. The conclusions are consistent with the structural changes that result from the functional and steric alterations.     

"离子晶体、分子晶体和原子晶体"研究性教学初探

1　前言新编高中教材第三册 (限选 )第一章“晶体的类型与性质” ,是高中教材中重要的知识环节。而第一节“离子晶体、分子晶体和原子晶体”又是本章中的关键一节 ,它对高中化学所涉及到的晶体及典型物质的晶体结构进行了阐述。若仍像以往一样 ,仅泛泛的讲授 ,学生收获不大 ,如果作为研究课来学习 ,它却是开发学生智力 ,发展学生能力的好内容。因为它对于培养学生的三维空间想象能力、慎密的逻辑思维和综合分析问题的能力 ,是再好不过的教材。正是基于这种想法 ,我们开展了以网络课方式教学的研究性学习的尝试。2　教学重点1.掌握 3种典型晶…