The C-N rotation barrier of the lithium enolate of acetamide: an ab initio and density functional theory investigation

Semiempirical (PM3), ab initio (HF/6-31+G(d) and MP2/6-31+G(d)), and density functional (pBP/DN) calculations are used to investigate the rotation barrier of the carbon-nitrogen bond in a simple enolate anion: lithium acetamide, 1. For comparison, the amidate anion 2, vinylamine 3, and a simulated dimer 4 were also calculated. In all systems, the barrier to rotation was found to be less than 10 kcal x mol(-1) in agreement with experiment. The correlated calculations show the barrier to be lowest for the anion 2. The results show conjugation effects in 1 and 2 comparable to that in vinylamine 3 and imply that polarization effects are more important than charge transfer in amine conjugation.     

Computational analysis of the solvent effect on the barrier to rotation about the conjugated C-N bond in methyl N, N-dimethylcarbamate

It is known experimentally that, in contrast to the case of amides, barriers to rotation about the conjugated C-N bonds of carbamates show very little solvent dependence. Calculations of the relative solvation energies of the equilibrium and transition state structures of methyl N,N-dimethylcarbamate (MDMC) and N, N-dimethylacetamide (DMA) were carried out using a continuum reaction field model in order to investigate the reason that bulk solvent polarity raises the barrier for DMA but leaves the barrier for MDMC unchanged. The results confirmed that MDMC is insensitive to bulk solvent polarity, probably as a result of the relatively small molecular dipole moment. Calculations of proton affinities and of the strength of association with a single water molecule were then performed in order to investigate why hydrogen-bond-donating solvents affect DMA but not MDMC. These calculations showed that MDMC is a less capable hydrogen-bond acceptor than DMA, and that the rotational barrier of MDMC does not increase in response to protonation or hydrogen-bonding nearly as much as the barrier of DMA does. Both of these factors contribute to making the rotational barrier of MDMC insensitive to solvent hydrogen-bond donor ability.     

Substituent effects on the barrier to carbamate C-N rotation

Seven aryl-substituted t-butyl N-methyl-N-aryl carbamates were prepared, and in each case, the barrier to C-N bond rotation, , was determined in CDCl₃ solution using variable temperature NMR. A linear free energy relationship is observed between and the electronic stabilization effect of the substituent on the N-aryl ring. More specifically, electron donating groups increase , whereas withdrawing groups decrease . A plot against σ- was more linear (r²=0.96), than a plot against σ (r²=0.90) or σ+ (r²=0.88) and a value of ρ=1.76 was obtained at 243 K. Thus, rotation about the carbamate C-N bond in weakly polar chloroform involves a decrease in positive charge on the nitrogen.     

Study of rotation barriers in N,N-dimethylnicotinamide and quaternary salt derivatives by proton NMR. A correlation of rotation barriers of N,N-dimethylamides with carbon-13 chemical shifts

The rotational barrier in the N,N-dimethylamide group is studied by proton NMR for N,N-dimethylnicotinamide and two quaternary salts in aqueous solution. The carbon-13 NMR spectra of a number of aromatic N,N-dimethylamides are discussed. A roughly linear correlation is found between the ΔG^≠ values of the rotation barriers and the chemical shifts of the carbon atom substituted by the amide group.     

Gas chromatography-mass spectrometry-Fourier transform infrared spectrometry and high-performance liquid chromatographic characterization of mononitro- and dinitro-isomers from the nitration of N,N-dimethyldiphenylacetamide

Gas chromatographic retention data, Fourier transform infrared and mass spectrometric (electron impact, electron attachment and methane chemical ionization) profiles are reported for the products of mono- and dinitration of N,N-dimethyldiphenylacetamide. Differentiations of analytical importance among isomers could be gathered by the study which led to their complete identification.     

"Amide resonance" correlates with a breadth of C-N rotation barriers

Complete basis set calculations (CBS-QB3) were used to compute the CN rotation barriers for acetamide and eight related compounds, including acetamide enolate and O-protonated acetamide. Natural resonance theory analysis was employed to quantify the "amide resonance" contribution to ground-state electronic structures. A range of rotation barriers, spanning nearly 50 kcal/mol, correlates well to the ground-state resonance weights without the need to account for transition-state effects. Use of appropriate model compounds is crucial to gain an understanding of the structural and electronic changes taking place during rotation of the CN bond in acetamide. The disparate changes in bond length (DeltarCO < DeltarCN) are found to be consonant with the resonance model. Similarly, charge differences are consistent with donation from the nitrogen lone pair electrons into the carbonyl pi* orbital. Despite recent attacks on the resonance model, these findings demonstrate it to be a sophisticated and highly predictive tool in the chemist's arsenal.     

Barrier of rotation around C-N bond in 1-(N,N-dimethylcarbamoyl)-π-cyclopentadienyl-π-(3)-1, 2-dicarbollylcobalt

Conclusions The barriers of rotation around the C-N bond in the amides C₅H₅CoB₉H₉C₂HCON(CH₃)₂, 1-(CH₃)₂NCO-1, 2-B₁₀C₂H₁₁ and 1-(CH₃)₂NCO-1,7-B₁₀C₂H₁₁ are substantially lower than in ordinary amides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2119–2120, September, 1977.     

Stereochemical control of both C-C and C-N axial chirality in the synthesis of chiral N,O-biaryls

A Rh(I)-catalyzed asymmetric [2 + 2 + 2] cycloaddition of achiral ynamides is described here. This work demonstrates a unique concept of stereochemical control of both the C-C and C-N axial chirality and provides an approach to the synthesis of chiral N,O-biaryls as well as chiral anilides.     

Luminescent atropisomeric N,N-chelating ligands from copper-catalyzed one-pot C-N and C-C coupling reactions

New atropisomeric N,N-chelating ligands with a 3,3'-bis[2-(2'-py)-indolyl] unit have been achieved via one-pot reactions that involve the formation of multiple C-N bonds between an indolyl and a brominated benzene and the indolyl 3,3'-C-C coupling. The new ligands display distinct blue intramolecular excimer emission (lambda(max) = 445 nm). Cu(I) ions bind to the new N,N-chelate ligands with a homochiral selectivity. The complex [Cu(bpib)2][BF(4)] (bpib = bis{3,3'-[N-phenyl-2-(2'-py)-indolyl]}) crystallizes as chiral crystals, thus allowing enantiomeric separation.     

Hindered internal rotation about the C-N partially double bond in N-acyl derivatives of 1,2,3,4-tetrahydroquinoline

The inhibited internal rotation about the C-N partially double bond in N-acyl-1,2,3,4-tetrahydroquinolines was investigated by NMR spectroscopy. It is demonstrated that these compounds exist in the form of Z and E conformers, and their ratios are determined under the conditions of stereochemical rigidity. The activation parameters of internal rotation about the C-N bond, specifically the barrier to rotation, as a function of the form·of the acyl grouping were calculated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1092–1096, August, 1980.     

N-methylmonothiocarbamatopentamminecobalt(III): restricted C-N bond rotation and the acid-catalyzed O- to S-bonded rearrangement

High-resolution (1)H and (13)C NMR studies on the linkage isomers [(NH(3))(5)CoOC(S)NHCH(3)](2+) and [(NH(3))(5)CoSC(O)NHCH(3)](2+) reveal that the O-bonded form exists as a 5:1 mixture of Z and E isomers arising from restricted rotation about the C-N bond. Similarly, restricted rotation is observed (at 20 degrees C) for the S-bonded isomer (Z/E ca. 18:1), but not for the isoelectronic carbamate ion [(NH(3))(5)CoOC(O)NHCH(3)](2+), nor for the unsubstituted carbamato complex [(NH(3))(5)CoOC(O)NH(2)](2+). An analysis of the variable-temperature NMR data for the O-bonded carbamato and urea complexes has provided quantitative data on the rotational barriers, and these ions involve much faster C-N bond rotations than the thiocarbamato complexes. The acid-catalyzed reaction of [(NH(3))(5)CoOC(S)NHCH(3)](2+) is confirmed, but there is much less parallel hydrolysis (ca. 2%) than previously reported (40 +/- 10%) for 0.1 M HClO(4). In 1 M HClO(4), [(NH(3))(5)CoSC(O)NHCH(3)](2+) and [(NH(3))(5)CoOH(2)](3+) are formed in parallel as an 83:17 mixture. The kinetic data suggest that the protonated form is at least 20-fold more reactive than the free ion and that the linkage isomerization and hydrolysis pathways are both acid-catalyzed, the latter clearly more so than the rearrangement.     

The rotational barrier of an enaminonitrile. Charge and energy redistribution during rotation about the C-N bond

Ab initio quantum mechanical techniques were used together with PROAIM electron density partitioning and CHELPG electrostatic potential analysis to examine the charge density distribution of model enaminonitrile1 in its planar ground state and in its two rotational transition states. The barrier to rotation about the C-N bond was calculated to be 15.4 and 15.6 kcal/mole for the two rotational transition states at the HF/6-31G^** level of theory, and was found to originate from a redistribution of electronic kinetic energy between the amino group and the rest of the molecule in a manner similar to that found for formamide and sulfonamide. Similarly, the C-N bond length and amino group electron population were found to depend upon the C-N torsional angle. Electrostatically derived atomic point charges were also examined at each stationary point using the CHELPG program. CHELPG electrostatic potential results were found to represent the traditional external viewpoint of the charge density consistent with a resonance model, while the results from PROAIM calculations were found to describe the underlying charge density and kinetic energy density redistribution responsible for the rotational barrier.     

Electrocatalytic construction of the C–N bond from the derivates of CO2 and N2

Electrochemical synthesis of C-N bond-containing compounds(e.g.,urea,amino acid,amide,amine,and their derivates)from CO₂/N₂and their derivates is emerging as a promising sustainable strategy[1-7].CO₂and its derived products,CO,HCOOH,(COOH)₂,etc.,could serve as carbon sources(Figure 1)[8].N₂,making up 80%of air,is an appealing nitrogen source.However,the low solubility of N2 and the high dissociation energy for the N≡N bond limit its application.     

On thermal stability of polydiphenylenesulfophthalide lithium salt

Thermolysis of polytriarylcarbinol (PTAC-Li) (lithium salt of polydiphenylenesulfophthalide (PDSP)) was studied in the temperature range from 100 to 500 °С by thermogravimetric analysis (TG) and IR and electronic spectroscopy to check the available data on the higher thermal stability of PDSP salts over the initial polymer. The mass losses detected by the TG method in the polymer salt at 80—150 and 240—350 °С are mainly caused by the desorption of weakly and strongly bound water. According to the calculations in the B3LYP/6-311+G(d,p) approximation, the С—ОН and C—SO ₃ ^- Li⁺ bonds are weakest in the carbinol model for PTAC-Li (D(C—O) ~ D(C—S) ~ 72 kcal mol^–1). The thermolysis of PDSP is accompanied by SO₂ evolution, whereas hydroxy and sulfo groups detached from PTAC-Li macromolecules remain in the thermolyzate. Phenol fragments and an inorganic phase, the final form of which is lithium sulfate, are formed in this process. An analysis of the IR and UV spectra of the thermolyzates of PTAC-Li and PDSP confirmed that fluorenyl fragments are predominantly formed upon the thermolysis of these polymers. The data obtained do not confirm a higher stability of PTAC-Li compared to that of PDSP.     

Aggregation studies of complexes containing a chiral lithium amide and n-Butyllithium

A system consisting of a chiral lithium amide and n-BuLi in tol-d(8) solution was investigated with (1)H and (13)C INEPT DOSY, (6)Li and (15)N NMR, and other 2D NMR techniques. A mixed 2:1 trimeric complex was identified as the major species as the stoichiometry approached 1.5 equiv of n-BuLi to 1 equiv of amine compound. (1)H and (13)C INEPT DOSY spectra confirmed this lithium aggregate in the solution. The formula weight of the aggregate, correlated with diffusion coefficients of internal references, indicated the aggregation number of this complex. Plots of log D(rel) vs log FW are linear (r > 0.9900). (6)Li and (15)N NMR titration experiments also corroborated these results. These NMR experiments indicate that this mixed aggregate is the species that is responsible for asymmetric addition of n-BuLi to aldehydes.     

Plastic crystalline lithium salt with solid-state ionic conductivity and high lithium transport number

Plastic crystallinity of lithium salt, [LiB(OCH(2)CH(2)OCH(3))(4)] (1), and its solid-state ionic conductivity are disclosed. The addition of small amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to borate 1 led to the drastic increase of the ionic conductivity and lithium transport number of the electrolyte.