Solvent effects on barrier to rotation of enaminonitriles using inversion transfer (1)H NMR spectroscopy and FTIR spectroscopy

The barrier to rotation and hydrogen bonding interactions of 2,2-dicyano-1-(N,N'-dimethylamino)vinylbenzene (1) were studied in a range of solvents. The barrier to rotation of 1 in chloroform was 14.8 kcal/mol and increased by 1.7 kcal/mol in a protic solvent, trifluoroethanol. FTIR studies showed a shift in the cyano stretch of 1 to a higher wavenumber in trifluoroethanol, which is consistent with 1 participating in a hydrogen bonding interaction at the vinyl carbon (C(v)) of the enaminonitrile group.     

Triphenylphosphine-Catalyzed Simple Synthesis of Dimethyl 1-Aryl-4-ethoxy-5-oxo-4,5-dihydro-1 H -pyrrole-2,3-dicarboxylates

Ethyl 2-arylamino-2-oxo-acetates undergo a complex reaction with dimethyl acetylenedicarboxylate in the presence of triphenylphosphine to produce dimethyl 1-aryl-4-ethoxy-5-oxo-4,5-dihydro-1 H -pyrrole-2,3-dicarboxylates in good yields. Dynamic NMR study of dimethyl 1-(2-methylphenyl)-4-ethoxy-5-oxo-4,5-dihydro-1 H -pyrrole-2,3-dicarboxylate shows a fairly high energy barrier ( j G p = 53.2 kJmol m 1 ) for rotation around the N-aryl single bond, which leads to an observable atropisomerism.     

Origin of methyl torsional barrier in 1-methyl-2-(1H)-pyridone

The laser induced fluorescence excitation and single vibronic excitation dispersed fluorescence spectra have been studied for supersonic jet cooled 1-methyl-2(1h)-pyridone. The methyl torsional bands and some low frequency vibrational transitions were assigned for both ground and excited states. The torsional parameters V(3)=244 cm(-1) and V(6)=15 cm(-1) for the ground state and V(3)=164 cm(-1) and V(6)=40 cm(-1) for the excited state were obtained. To get the insight into the methyl torsional barrier, ab initio calculations were performed and compared with the experimental results. Origin of potential barrier was traced by partitioning the barrier energy into changes in bond-antibond interaction, structural, and steric energies accompanying methyl rotation using natural bond orbital analysis. The role of local interactions in ascertaining the barrier potential reveals that its nature cannot be understood without considering the molecular flexing. The hyperconjugation between CHsigma(*) and ring pi(*) observed in lowest unoccupied molecular orbital (LUMO) stabilizes the methyl group conformer that undergoes a 60 degrees rotation in the excited state with respect to that of the ground state, and it is the change in LUMO that plays important role in the excited state barrier formation.     

Origin of methyl torsional barrier in 1-methyl-2(1H)-pyridinimine and 3-methyl-2(1H)-pyridone: II. Ground state

To get the insight into the electronic structure-methyl torsion correlation in nitrogen heterocyclic molecules, a comparative study on torsion of the methyl group in 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) was carried out using ab initio calculations. To understand the barrier forming mechanism in the ground state and its consequence on the molecular structure, the ground state torsional potential has been investigated by partitioning the barrier energy using the natural bond orbital (NBO) theoretical framework. The NBO analysis reveals that the delocalization energy is the barrier forming term whereas the Lewis energy is always antibarrier for all these molecules. To get further insight into the effect of local electronic structure on the methyl torsional barrier, the individual bond-antibond interactions and structural energy contributions have been investigated. It was found that when the bond order difference between the vicinal bonds does not change appreciably during the course of methyl rotation, the local electronic interactions with the methyl group do not play any decisive role in barrier formation as observed in the case of 1MPY and 1MPI. In these cases, it is the skeletal relaxation during methyl rotation that plays an important role in determining the barrier. On the other hand, if the bond order change is appreciable as is the case for 3MPY, the local interactions alone suffice to describe the origin of the torsional barrier of the methyl group.     

Conformational studies by dynamic NMR. 74.(1) stereomutations of the conformational enantiomers in peri-substituted 1-acylnaphthalenes

Naphthalenes bearing an acyl and a phenyl group in a peri relationship give rise to a pair of enantiomers in the temperature range where the rotations of the acyl group are slow. Such enantiomers were observed by means of low temperature NMR spectra in chiral environments. The barrier to rotation for the acyl substituents, that causes the interconversion of the enantiomers, was demonstrated to be lower than that for the phenyl group. In an appropriately synthesized derivative it was possible to measure the two barriers that were found equal to 10.4 and 15.9 kcal mol(-)(1), respectively. The barriers for the acyl group rotation increase regularly (from 9.5 to 13.2 kcal mol(-)(1)) with the increasing dimension of the RCO groups (R = Me, Et, Pr(i), Bu(t)). When a bromine atom replaces the phenyl group, the enantiomerization barrier for the corresponding acyl derivatives increases significantly.     

A theoretical investigation on the potential energy surface of CN2H rotation of the encapsulated 1-bicyclo[2.2.1]heptyldiazirine

The potential energy surface (PES) of CN₂H rotation of the encapsulated 1-bicyclo[2.2.1]heptyldiazirine (BHD) inside a molecular container: Cram’s hemicarcerand (CH) was explored using two different DFT involved ONIOM methods: B3LYP/6-31G^**//ONIOM(B3LYP/6-31G^*: AM1) and B971/6-31G^**//ONIOM(B971/6-31G^*: AM1). The free-state PES of CN₂H rotation was also calculated, respectively by B3LYP/6-31G^**//B3LYP/6-31G^* and B971/6-31G^**//B971/6-31G^* methods for comparison. The findings in this study have shown that the PES profiles differ from each other notably in the two states. In the encapsulated state the rotation barrier corresponding to the free-state conversion with the largest rotation barrier increases by about 2 kcal/mol, which has exceeded the largest rotation barrier in the free-state. The conformational preference behavior towards certain BHD isomers, which might be in better conformational compatibility with the container, has been demonstrated.     

1H dynamic nuclear magnetic resonance study of hindered rotations in N-aryl-N-benzyl alkyl carbamates

The NMR variable temperature behaviour of a series of N-aryl-N-benzyl alkyl carbamates was investigated. The barrier to rotation about the N-aryl bond was determined for all the compounds studied. The values obtained, which are in good agreement with those found for structurally related N-aryl-N-benzylamides, are in the range ΔG≠ = 60.7-89.6kJ mol^?1. For some carbamates another conformational phenomenon has been observed, namely the hindered rotation about the carbonyl carbon–nitrogen bond, with a barrier to rotation corresponding to reported values for similar systems.     

Nickel-catalyzed, cascade cycloadditions of 1-ethynyl-8-halonaphthalenes with nitriles: synthesis, structure, and physical properties of new pyrroloarenes

The reaction of 1-ethynyl-8-halonaphthalenes 1 with nitriles in the presence of the catalytic system [NiBr(2)(dppe)]/Zn (dppe=1,2-bis(diphenylphosphino)ethane) is found to produce unusual pyrroloarenes 2. The carbon-nitrogen triple bond in nitrile is activated twice, and five new bonds are formed in a one-pot transformation, which causes a pyrrole and two six-membered rings to be generated simultaneously. The scope and limitations of this reaction are examined. Similarly, alkyl-bridged diynes also furnish the corresponding polycycles. Diaryl-substituted cycloadducts 2 (R(1)=Ar) are fluxional, because of the restriction in rotation of the aryl groups. The rotational barrier is studied by performing (1)H NMR experiments at various temperatures. The structures of several compounds are determined by X-ray crystallographic analysis. The photophysical and electrochemical properties of the pyrroloarenes are also investigated.     

Microwave spectra and barriers to internal rotation of z- and e-1-propenyl isocyanide

A synthetic procedure yielding a mixture of Z- and E-1-propenyl isocyanide (CH(3)CH═CHNC) is described. The microwave spectrum of this mixture has been recorded in the 12-100 GHz spectral range, and the spectra of the Z and E isomers have been assigned for the first time. Most transitions of the Z form were split into two components of equal intensity due to tunneling of the methyl group, which allowed the barrier to internal rotation of this group to be determined as 4.0124(12) kJ/mol by fitting 568 transitions with a maximum value of J = 46 using the computer program Xiam. This fit had a root-mean-square deviation as large as 4.325. The same transitions were therefore fitted anew using the more sophisticated program Erham. This fit has a rms deviation marginally better (4.136) than the Xiam fit. No split MW lines were found for E-1-propenyl isocyanide. The absence of splittings is ascribed to a barrier to internal rotation of the methyl group that is significantly higher than the barrier of the Z isomer. It is concluded that the barrier must be larger than 6 kJ/mol for the E form. The experimental work was augmented by quantum chemical calculations at CCSD/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVTZ levels of theory. The CCSD method predicts rotational constants of the Z and E forms well. The B3LYP barriers to internal rotation of a series of substituted propenes were calculated and found to be in good agreement with experiments. Calculations of the quartic centrifugal distortion constants of the two 1-propenyl isocyanides by the B3LYP and MP2 methods were less successful.     

Ab initio study on the mechanism of cycloaddition reaction of ketene with methylenimine: A new reaction scheme

The cycloaddition reaction of ketene and methylenimine, leading to 2-azetidinone, has been studied theoretically by RHF /3-21G and IRC. This reaction is believed to be nonsynchronous and concerted, taking place through a twisted transition state. Four π orbitals are involved in this reaction, which is a “2 × [1 + 1]”-type cycloaddition. In the course of the reaction, rotation of the methylene group instead of oxygen in ketene was ascertained. The activated barrier is calculated to be 33.9 kcal/mol. © 1992 John Wiley & Sons, Inc.     

Facile ketene-ketene and ketene-ketenimine rearrangements: a study of the 1,3-migration of alpha-substituents interconverting alpha-imidoylketenes and alpha-oxoketenimines, a pseudopericyclic reaction

Theoretical calculations (B3LYP/6-311+G(3df,2p)//B3LYP/6-31G) of the 1,3 migration of NR(2) transforming alpha-oxoketenimines 1 to alpha-imidoylketenes 3 and vice versa indicate that this process is a pseudo-pericyclic reaction with a low activation energy (NH(2) 97 kJ mol(-1), N(CH3)(2) 62 kJ mol(-1)). The oxoketenimines were found to be more stable (by 18-35 kJ mol(-1)) which is in line with experimental observations. The hindered amine rotation in the amide and amidine moieties adjacent to the cumulenes are important in the migration of the NR(2) group, as one of the rotation transition states is close to the 1,3 migration pathway. This gives an interesting potential energy surface with a valley-ridge inflection (VRI) between the orthogonal hindered amine rotation and 1,3 migration transition states. The imidoylketene may also undergo ring closure to an azetinone 5; however, this is metastable, and under the conditions that allow the 1,3-migration, the oxoketenimine 1 will be favored. The imine NH E/Z-interconversion of the ketenimine group takes place by inversion and has a low activation barrier ( approximately 40 kJ mol(-1)). In all the amidines examined the E/Z-interconversion of the imine function was predicted to be by rotation with a high barrier (>80 kJ mol(-1)), in contrast to all other reported imine E/Z-interconversions which are by inversion.     

Materials for hydrogen storage: structure and dynamics of borane ammonia complex

The activation energies for rotations in low-temperature orthorhombic ammonia borane were analyzed and characterized in terms of electronic structure theory. The perdeuterated (11)B-enriched ammonia borane, (11)BD(3)ND(3), sample was synthesized, and the structure was refined from neutron powder diffraction data at 175 K. This temperature has been chosen as median of the range of previously reported nuclear magnetic resonance spectroscopy measurements of these rotations. A representative molecular cluster model was assembled from the refined geometry, and the activation energies were calculated and characterized by analysis of the environmental factors that control the rotational dynamics. The barrier for independent NH(3) rotation, E(a) = 12.7 kJ mol(-1), largely depends on the molecular conformational torsion in the solid-state geometry. The barrier for independent BH(3) rotation, E(a) = 38.3 kJ mol(-1), results from the summation of the effect of molecular torsion and large repulsive intermolecular hydrogen-hydrogen interactions. However, a barrier of E(a) = 31.1 kJ mol(-1) was calculated for internally correlated rotation with preserved molecular conformation. Analysis of the barrier heights and the corresponding rotational pathways shows that rotation of the BH(3) group involves strongly correlated rotation of the NH(3) end of the molecule. This observation suggests that the barrier from previously reported measurement of BH(3) rotation corresponds to H(3)B-NH(3) correlated rotation.     

Determination of Internal Rotation Barrier

The interrelation between the experimental heights of the potential barrier to internal rotation about the C-C bond and those calculated by the semiempirical and nonempirical quantum-chemical methods for 25 compounds was studied. The height of the potential barrier to internal rotation about the C-C bond can be determined by the AM1 semiempirical method with correction introduced according to the correlation dependence established.     

Spectroscopic studies on methyl torsional behavior in 1-methyl-2(1H)-pyridone, 1-methyl-2(1H)-pyridinimine, and 3-methyl-2(1H)-pyridone. I. Excited state

The laser induced fluorescence excitation and dispersed fluorescence spectra of three nitrogen heterocyclic molecules 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) have been studied under supersonic jet cooled condition. The methyl torsional and some low frequency vibrational transitions in the fluorescence excitation spectrum were assigned for 1MPY. These new assignments modify the potential parameters to the methyl torsion reported earlier. Some striking similarities exist between the torsional and vibrational transitions in the fluorescence excitation spectra of 1MPY and 1MPI. Apart from pure torsional transitions, a progression of vibration-torsion combination bands was observed for both these molecules. The excitation spectrum of 3MPY resembles the spectrum of its parent molecule, 2-pyridone. The barrier height of the methyl torsion in the excited state of 3MPY is highest amongst all these molecules, whereas the barrier in 1MPI is higher than that of 1MPY. To get an insight into the methyl torsional barrier for these molecules, results of the ab initio calculations were compared with the experimental results. It was found that the conformation of the methyl group undergoes a 60 degrees rotation in the excited state in all these molecules with respect to their ground state conformation. This phase shift of the excited state potential is attributed to the pi*-sigma* hyperconjugation between the out-of-plane hydrogen of the methyl group and the molecular frame. It has been inferred that the change in lowest unoccupied molecular orbital energy plays the dominant role in the excited state barrier formation.     

Racemisation dynamics of torsion angle restricted biphenyl push-pull cyclophanes

The thermodynamics of the atropisomerisation of torsion angle restricted, axial chiral biphenyl-based push-pull cyclophanes were studied. Using (1)H NMR coalescence measurements the rotation barrier around the central C-C bond was determined to be 50 kJ mol(-1) for the propyl-bridged biphenyl derivative 1b, displaying only a negligible solvent dependence. By protonation of the piperidinyl nitrogen as electron donor, the free energy ΔG(?)(T) of the rotation barrier increased, indicating that the tendency of the push-pull system to planarise may be considered as a driving force for the atropisomerisation. For the more restricted butyl-bridged cyclophane 1c a rotation barrier of ΔG(?)(T) = 90 kJ mol(-1) was measured using dynamic chromatography. The difference in the free energy of rotation around the central C-C bond probably reflects the crowdedness of the transition states.     

Synthesis and optical resolution of aminophosphines with axially chiral C(aryl)-N(amine) bonds for use as ligands in asymmetric catalysis

N-Aryl indoline-type aminophosphines 1a-c were obtained in good yields by a nucleophilic aromatic substitution (S(N)Ar) reaction followed by silane reduction. Aminophosphine 1d was also prepared from 2,3-difluorobenzaldehyde (4) via dimethylhydrazone. Optical resolution of C(aryl)-N(amine) bond atropisomers was achieved using (S)-(+)-di-mu-chlorobis[2-[(dimethylamino)ethyl]phenyl-C(2),N]dipalladium(II) ((S)-10). The determination of absolute configuration and the investigation of the rotation barrier for C(aryl)-N(amine) bond axial stability of an aminophosphine 1 are described. Finally, the ability of the chiral phosphine ligand 1 is demonstrated in a catalytic asymmetric reaction, such as a palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate (up to 95% ee).     

电子效应调控丁烯二腈光分子开关的CASSCF和MS-CASPT2理论研究

采用多组态CASSCF方法和MS-CASPT2方法研究了丁烯二腈中性分子及阳离子和阴离子的顺-反异构化机理.结果表明,中性分子和离子态的光顺-反异构化反应经历不同的非绝热跃迁途径:中性丁烯二腈受光激发至S1态后,需克服一个不低于19.7 k J/mol的能垒才有机会到达基态和激发态间的圆锥交叉(S_1/S_0-CI),随后经非辐射跃迁回到基态,S_1/S_0-CI在结构上偏离C=C双键旋转路径,且能量较高,因此会降低旋转速度,阻碍旋转的单向性;丁烯二腈阳离子和阴离子自由基的D_0态和D_1态旋转势能面在90°处相交,优化的D_1/D_0-CI与D_1态中间体的结构和能量均相近,因此从D1态经由D_1/D_0-C_I无辐射跃迁到D_0态的过程无势垒,在此过程中C=C旋转方向性得到最大限度的保持.研究结果证实了电子诱导不仅能降低基态热旋转势垒,而且能够调控光旋转的非绝热跃迁机理.     

MINDO-SCF-MO-berechnung der cis-trans-isomerisierung von N-methyl- und N-phenylazomethin

MINDO/2-SCF-MO calculations for the ground state properties of N-methyl- and N-phenyl-azomethin have been carried out. The calculated rotation barrier for the methyl group in N-methyl-azomethin was 0·8 kcal/mol, the eclipsed conformation being most stable. The calculated rotation barrier about the CN bond in the protonated methylazomethin was 27·9 kcal/mol. MINDO/1-SCF-MO treatment for the N-inversion barrier of the unprotonated species yielded 13·00 kcal/mol. Similar MINDO/2 calculations for N-phenylazomethin yielded 4·0 kcal/mol for the rotation barrier of the phenyl ring around the CN= bond, the perpendicular conformation of the ring to the CNC plane being most stable. For the corresponding N protonated derivative the value 27·3 kcal/mol was calculated for the rotation barrier around the CN bond. MINDO/1 treatment yielded an inversion barrier of 14·0 kcal/mol for N-phenylazomethin.     

Cycloreversion of cyclobutanes

Semiempirical MO calculations with the method SINDO1 were performed to study the potential energy surface of cyclobutane and several substituted cyclobutanes with substituents F, OCH₃ and CN. The reaction pathway with the lowest activation energy leading to two ethylenic fragments is nonconcerted. One carbon bond is broken after symmetric opening of two adjacent bond angles and twisting of the carbon framework. The first transition state is asymmetric and diradicaloid. The reaction proceeds to a diradicaloid, non-zwitterionic intermediate. The second transition state is characterized by bond breaking of the inner carbon-carbon bond. For the unsubstituted case, the barrier for free rotation of the outer methylenic groups was also calculated. In comparison, the unsubstituted reaction is characterized by transition states of almost equal energy, whereas in the substituted reactions the barriers for the second bond breaking are much higher than for the first bond breaking step.     

Rotational spectra and conformational structures of 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone

Microwave spectra have been recorded for 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone from 11 to 24 GHz using a Fourier-transform microwave spectrometer. Only one spectrum from a single conformational isomer was observed for each species. The rotational transitions in the spectrum of 1-phenyl-2-propanone were split into separate transitions arising from the A- and E-torsional levels of the methyl rotor. The fit of the E-state transitions to a "high-barrier" internal rotation Hamiltonian determines V3 = 238(1) cm-1 and rotor-axis angles of thetaa = 87.7(5) degrees, thetab = 50.0(5) degrees, and thetac = 40.0(5) degrees. Ab initio optimizations (MP2/6-31G**) and single-point calculations (MP2/6-311++G**) were used to model the structures of 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone. The lowest energy conformations of these species were found to be stabilized by weak OH-pi, NH-pi, and CH-pi hydrogen-bonding interactions. Moments of inertia, derived from the model structures, were used to assign the spectra to the lowest energy conformation of each species. A series of MP2/6-31G* partial optimizations along the internal rotation pathway were used to estimate the barrier to methyl rotation to be 355 cm-1 for 1-phenyl-2-propanone.