Observation of X1A1 vinylidene by photoelectron spectroscopy of the C2H2− ion

Photoelectron spectra of the vinylidene anion (C₂H₂^?) show vibrational structure in X¹A₁ vinylidene up 12 kcal/ mol above the vibrational ground state. Analysis yields an EA(C₂H₂$\text{X}$¹ A₁) of 0.47 ± 0.02 eV, and frequencies for the CC stretch and HCH bend. Absence of the ³B₂ state in the photoelectron spectra indicates the ¹A₁-³B₂ splitting in vinylidene is ? 1.7 eV.     

Synthesis of 3,3,3-trifluoropropene from vinylidene fluoride

3,3,3-Trifluoropropene is obtained under mild conditions via 3,3,3-trifluoropropyl acetate or 3,3,3-trifluoropropyl methyl ether as key intermediates. The intermediates were prepared by conjugate addition of FCH₂OR (R = COCH₃, CH₃) generated $\text{in situ}$ from HF, paraformaldehyde and HOR to vinylidene fluoride.     

The influence of electron-withdrawing substituents on the geometries and barriers to inversion of vinyl anions

Whereas $\text{ab}$$\text{initio}$ calculations predict that the vinyl anion is bent and has a high barrier to inversion, substitution of cyano, methoxycarbonyl, and formyl substituents at C-1 lower this barrier. Nevertheless, the first two substituents produce significantly bent anions, while the bent and linear 1-formylvinyl anions are essentially isoenergetic.     

Zur konformation und intramolekularen beweglichkeit von [6]paracyclophan-8,9-dicarbonsäurediethylester)

The title compound 1 prepers the conformations $\text{1a}$/$\text{1b}$ predicted for [6]paracyclophane; the barrier for the intramolecular process $\text{la}$ ? $\text{1b}$ was found to be Δ G⁺ = 13.9 kcal/mol at +4°C.     

Resolution of N-benzyl-N-(1,2-dihydro-2-oxoquinolin-1-yl)glycine

The $\text{N}$$\text{N}$ chiral axis in the title glycine derivative has allowed its resolution into enantiomers: the barrier Δ G^≠ for racemisation (rotation around the $\text{N}$$\text{N}$ bond) is 26.2 Kcal mol^?1.     

Solvent dependence of the barrier to the Ph-N internal rotation in N-methylaniline

The barrier to Ph-N internal rotation in $\text{N}$-Methylaniline in CF₂Cl₂ (from ¹H NMR) is 6.9 kcal/mol. Solvents which are hydrogen bond acceptors raise the barrier, while hydrogen bond donors lower the barrier.     

Conformational behaviour and inclusion compound forming properties of 5,18-disubstituted derivatives of 5, 11, 12, 18-tetrahydrotribenzo[b,f,j][1,4]diazacyclododecine-6, 17-dione

An X-ray structure analysis shows that the 5, 18-dimethyl derivative ($\text{5}$) of the title compound ($\text{4}$) crystallises from xylene as a 1:1 inclusion compound in which the host molecules adopt propeller-like conformations ($\text{7}$ and $\text{7}$*) with almost perfect $\text{C}$₂ symmetry. Dynamic ¹H n.m.r. spectroscopy shows that the 5, 18-dibenzyl derivative ($\text{6}$) of ($\text{4}$) forms a 1:1 inclusion compound with ethanol in the solid state and undergoes ring inversion ($\text{7}$ ? $\text{7}$*) between enantiomeric propeller-like conformations in solution against a barrier of 21.1 kcal mol^?1     

Structure and conformational isomerism of the major dimer of 2,3-naphthoquinodimethane

The major product from 2,3-naphthoquinodimethane formed by cyclisation of $\text{o}$-dipropadienylbenzene was found to be the dimer 5 containing an eight-membered ring, for which the inversion barrier was determined by dynamic ¹H NMR spectrometry, ΔG3 = 18 kcal/mole.     

N-nitrosothialdine. Synthesis,X-ray crystallography,and N-N rotational barrier

Crystalline N-nitrosothialdine ($\text{2}$) has been prepared in 38% yield by treating thialdine ($\text{1}$) with $\text{n}$-butyl nitrite and acetic acid in hexane. X-Ray crystallography of $\text{2}$ revealed that its three methyl groups are a11 $\text{cis}$ and, rather surprisingly, all equatorial; this finding contrasts sharply with results for other nitrosamine heterocycles which have been investigated, whose bulky alpha substituents are forced into a primarily axial orientation by the large steric requirement of the N-N-0 system. The equatorial methyl group both displaces the nitroso group from the C-N-C plane and twists it somewhat about the N-N bond. N-Nitrosothialdine's unusually low barrier to rotation about the N-N bond (72 kJ/mole) is attributed to this steric crowding, combined with inductive electron withdrawal by the two sulfur atoms.     

Kinetically and thermodynamically controlled synthesis of 2,6-disubstituted cyclohexanone semicarbazones. A molecular mechanics study of a1,3-strai

Reasonable values for Me-Me, H-Me and Me-H A^1,3-strain have been evaluated by molecular mechanics calculations on differently substituted methylenecyclohexanes (5.46–6.75, 1.03 and 0.71 $\text{kcal}\text{mol}$, respectively). The chair-to-chair inversion barrier of methylenecyclohexanes has also been calculated. Methods are described for the thermodynamic and for the kinetic preparation of acylhydrazones of 2,6-dialkylcyclohexanones. A PMR analysis of the $\text{cis}$ and $\text{trans}$-2,6-dimethylcyclohexanone semicarbazones has allowed the determination of the preferred conformers. The results agree with MM results on model hydrocarbons. Finally, a MM study of hydrocarbon models for the antibiotics isocycloheximide, cycloheximide and neocycloheximide points out to a decrease in stability on going from the former to the later, in agreement with the reported experimental results.     

A molecular orbital study of the cis-trans energy difference in glyoxal

The barrier to internal rotation around the C-C bond in glyoxal, and the energy difference between the cis and the trans form of the molecule have been estimated using (7,$\text{3}\text{4}$) and (9,$\text{5}\text{4}$) Gaussian basis sets including polarization functions. The predicted energy of the cis form relative the ground state trans form was found to be substantially higher (4.9–6.3 kcal mol^?1) than the experimental value of 3.2 kcal mol^?1.     

A theoretical investigation of the rotational barriers of peroxyformic acid

The internal rotational barriers in peroxyformic acid have been studied employing ab initio MO calculations. The C-O and O-O rotational barriers were calculated to be 7.68 and 1.04 $\text{kcal}\text{mol}$, respectively. The relatively low O-O rotational barrier is attributed to a balance between electron repulsion and hydrogen bonding in the syn chelated conformer.     

Synthesis and NMR spectroscopy of 2′, 3′, 4′, 5′, 6′ - pentafluoroformanilide

Pentafluoroformanilide was prepared by a new synthetical route. The ¹H, ¹³C, and ¹⁹F chemical shifts for the compound indicated the presence of $\text{cis}$ and $\text{trans}$ isomers in chloroform with a rotational energy barrier of 13.4 Kcal mol^-1 for the N-C(O) bond.N, N′-Bis(2,3,4,5,6-pentafluorophenyl) formamidine was obtained from pentafluoroformanilide.     

Conformational studies by dynamic NMR. Part XV. The rotational barrier of N-methyl aniline

Low temperatures C-13 NMR spectra allowed the determination of the rotational barrier of N-methyl aniline in solution. The shifts of the anisochronous ortho and meta carbons were assigned by $\text{ab initio}$ calculations.     

Conformation and barrier to inversion of a 12-membered heterocycle containing two disulfide bonds

Raman and ¹H NMR spectroscopic data obtained for a dimeric product 4c formed on iodine oxidation of N-pnenyl-N-thiomethyl-thioglycolic acid amide (3c) indicate that this product is 4,11-diphenyl-4,11-diaza-1,2,7,8-tetrathia-cyclododecane-5,10-dione. The barrier to interconversion between the conformers of 4c has been determined by DNMR. The height of the barrier (ΔG^≠ = 17.3 ±0.1 $\text{kcal}\text{mol}$) is in agreement with the proposed structure. This investigation thus gives a support to the previously proposed mechanism of regiospecific dimeric oxidations of unsymmetrical dithols of type 3c.     

Synthetic studies on β-lactam antibiotics. Part 16. synthesis of 1-carba-2-penem-3-carboxylic acid esters from penicillins utilizing a carbon-carbon coupling reaction

Allylazetidinones $\text{9}$, $\text{10}$, prepared by coupling of allylcoppers $\text{8}$ with chloroazetidinones $\text{6}$, $\text{7}$, were converted into carbapenem esters $\text{16}$, $\text{28}$–$\text{31}$ using an Emmons-Horner reaction to introduce the 6-side chain and an intramolecular Wittig reaction to form the carbapenem ring system.     

Ylures sans sel,evolution en phosphoranes et equilibre ylure ⇌ phosphorane : I Synthese et structure

Synthesis of new salt-free ylids $\text{9}$,$\text{12}$ to $\text{16}$, $\text{19}$, $\text{20}$, $\text{22}$, $\text{27}$ and $\text{29}$ and phoiphoranes $\text{10}$, $\text{17}$, $\text{18}$, $\text{21}$, $\text{23}$, $\text{30}$ to $\text{33}$ by addition of a trivalent phosphorus compound (phosphites and amino-phosphines ) $\text{1}$ to $\text{7}$ with dimethyl acetylencedicartboxylate in presence of a protic trapping reagent are described. The results are consistent with trapping of carbanionic species. In relation with the. cyclic of acyclic structure of the triivalent phosphorus compound and the protic trapping reagent ie : methanol, phenol, carboxylic acid, etc.., several pathways are involved. Clearly), three phenomena are shown : one can obtain an ylid via a phosphorane or conversely a phosphorane via an ylid or an equilibrium Phoiphorane ? ylid. Results are dealing with thermodynamic or kinetically controlled reactions.     

Ab initio study of the rotational energy barrier in carbonylylide

The rotational energy barrier in carbonylylide CH₂OCH₂ is studied using RHF CI calculations. Depending on the size of the CI and the basis set (STO-3G and 4–31G), values in the range 13–17 $\text{kcal}\text{mol}$ are found. At this level of calculation, the mid-point of the isomerization process can be mainly described by the diradical rather than the zwitterion.     

The reaction of 2-azaallenium salts with carbodiimides and tert-butyl isocyanide

2-Azaallenium salts $\text{1}$ react with diisopropyl carbodiimide to afford the formal dihydro-1,3-diazetium salts $\text{5}$. The structure of $\text{5c}$ has been confirmed by an X-ray diffraction analysis. With the carbodiimide $\text{2e}$ the tetraphenyl-2-azaallenium salt $\text{1a}$ reacts to give the guanidinium compound $\text{6}$.The diazetium salts $\text{5}$ react with further carbodiimide to afford the formal 2-azaallenium salts $\text{11}$. With tert-butyl isocyanide the 2-azaallenium salts $\text{1}$ undergo an Ugi reaction to yield the nitriles $\text{14}$.     

The mechanism of biomolecular methyl transfer in a methyl aminoalkanesulfonate. application of 18o isotope shifts in 13cmr spectra of sulfonic esters.

¹³Cmr spectra show complete absence of scrambling of an ¹⁸O label in $\text{1}$$\text{a}$ recovered after partial conversion to $\text{2}$$\text{a}$ thereby precluding an $\text{0}$-methylation mechanism for the bimolecular formation of $\text{2}$$\text{a}$ from $\text{1}$$\text{a}$.