Application of NMR spectroscopy of chiral association complexes. 8—Rotation about the C(sp2)–C(aryl) Bond in 2,6–disubstituted benzamides

The barrier to rotation about the C(sp²)? C(aryl) single bond in non-planar benzoyl compounds was investigated using N,N-dimethyl, N,N-tetramethylene and N,N-diisopropyl derivatives of 2,4,6-trimethylbenzamide and 2,6-dimethoxybenzamide and N,N-dimethyl derivatives of the corresponding thiobenzamides. Their ¹H and ¹³C NMR spectra were determined and assigned and the splittings in the ¹H spectra due to the addition of the optically active shift reagent (+)-Eu(hfbc)₃, are discussed. The free enthalpy of activation was calculated from the coalescence temperature of the ortho-methyl or -methoxyl signals and from a line-shape analysis. An exact equation for the determination of the rate constant at coalescence and a valid approximation which includes the line-width are presented. A distinct effect of the size of the N-substituent on the barrier to rotation in 2,6-dimethoxybenzamides was observed, while the π values for the 2,4,6-trimethylbenzamides studied are practically identical.     

Application of NMR spectroscopy of chiral association complexes 11—Rotation about the C(sp2)—C(aryl) Bond in 2,6-Difluorobenzamides Studied by 19F NMR

The energy barrier to rotation about the C(sp²) C(aryl) single bond in non-planar N,N-dimethyl-, N,N-tetramethylene-, N,N-diisopropyl-2,6-difluorobenzamides and in N,N-dimethyl-2-chloro-6-fluorobenzamide was investigated by ¹⁹F{¹H} NMR in the presence of an optically active shift reagent. The free energy of activation was calculated from the coalescence temperature of the fluorine signals; the calculation was accomplished using the simple, approximative equation and, also, by the graphical procedure including the (eigen) line width, i.e. without the broadening caused by two-site exchange. A distinct effect of the size of the N-substituents on the ΔG_c‡values was observed, in accordance with expectation.     

Application of NMR spectroscopy of chiral association complexes. 14—chiral recognition of terpene and cyclohexene hydrocarbons by 1H and 13C NMR

Chiral NMR recognition of terpenic and similar hydrocarbons by Ag(fod) and optically active lanthanide shift reagents is described. Twenty-four ¹H signals of eight compounds and, as a first report in the literature, eleven ¹³C signals of three compounds were split into signal pairs due to the respective enantiomers. Although the magnitudes of the splittings are not connected to the structure of the substrate molecules in an obvious manner, the configurations of the similar compounds camphene and epi-β-santalene were related empirically. All spectra were sufficiently resolved to allow separate integration of enantiomer signals; from the integrals the enantiomeric purities of seven non-racemic mixtures were determined with good accuracy. The shift curves made several hitherto unknown assignments feasible.     

Application of NMR spectroscopy of chiral association complexes. 13—electron donor–acceptor complexes between racemic carbazolo[3,4-c]carbazoles and an optically active acceptor

Strong electron donor–acceptor (EDA) association between carbazolo[3,4-c]carbazoles and an optically active tetranitrofluorenone derivative was detected by UV–visible spectroscopy and by ¹H NMR shifts. ¹H NMR splittings at low temperatures are due to diastereomeric association complexes and, thus, prove the chirality of carbazolocarbazoles.     

13C NMR spectroscopy of cyclopropane derivatives. 1—monocyclic compounds

The ¹³C NMR data of an appreciable number of cyclopropane derivatives have been collected. Most of the spectra were recorded by ourselvesand some were taken from the Literature. With a view to furthering the useof ¹³C NMR spectroscopy as a diagnostic tool in this field, we havemeasured not only the chemical shifts,δ, but also the coupling constants ¹J(CH). It is shown that both embody valuable structural information.     

Lactonization of C(sp2)—H Bonds in Enamides with CO2

Herein, we report a novel synthesis of 1,3‐oxazin‐6‐ones from enamides with CO₂ through C—H carboxylation and one‐pot cyclization. This transition‐metal‐free and redox‐neutral process features broad substrate scope, good functional group tolerance and facile product derivatization. The nucleophilic attack to CO₂ from the electron‐rich alkene is demonstrated for this reaction.     

Photoactive electron donor–acceptor complex platform for Ni-mediated C(sp3)–C(sp2) bond formation

A dual photochemical/nickel-mediated decarboxylative strategy for the assembly of C(sp³)–C(sp²) linkages is disclosed. Under light irradiation at 390 nm, commercially available and inexpensive Hantzsch ester (HE) functions as a potent organic photoreductant to deliver catalytically active Ni(0) species through single-electron transfer (SET) manifolds. As part of its dual role, the Hantzsch ester effects a decarboxylative-based radical generation through electron donor–acceptor (EDA) complex activation. This homogeneous, net-reductive platform bypasses the need for exogenous photocatalysts, stoichiometric metal reductants, and additives. Under this cross-electrophile paradigm, the coupling of diverse C(sp³)-centered radical architectures (including primary, secondary, stabilized benzylic, α-oxy, and α-amino systems) with (hetero)aryl bromides has been accomplished. The protocol proceeds under mild reaction conditions in the presence of sensitive functional groups and pharmaceutically relevant cores.

This works demonstrates the implementation of an electron donor–acceptor (EDA) complex platform toward Ni-catalyzed C(sp³)–C(sp²) bond formation, circumventing the need for exogenous photocatalysts, additives, and stoichiometric metal reductants.     

Application of NMR spectroscopy of chiral association complexes. 12—rate determination of an intramolecular motion in a free molecule by means of dynamic NMR measurements in the presence of an auxiliary compound

The rate constants of intramolecular processes can sometimes be measured only in the presence of optically active auxiliary compounds. The problem of obtaining the rate for the free molecule was solved by linear extrapolation of rate constant from the complexed to the uncomplexed molecule (LERCUM). This method is developed theoretically and checked experimentally. 2-Bromo-2-methyl-1-(3-bromo-2-methoxy-4,5,6-trimethylphenyl)-1-propanone has the advantage that rotation about its carhonyl-to-aryl bond can be measured both in the presence of (+)?2,2,2-trifluoro-1-phenylethanol as well as in the absence of an auxiliary compound. The novel method of extrapolation is confirmed by the agreement between both results.     

13C NMR spectra of natural products. 2—eudesmanolides

The ¹³C NMR spectra of eight eudesmanolides have been measured and the chemical shifts assigned. Among the compounds studied are the naturally occurring α-cyclocostunolide, santamarin, arbsculin A and reynosin.     

15N NMR spectroscopy. 19—spectroscopic characterization of cyclodipeptides (2,5-dioxopiperazines)

Various cyclodipeptides containing glycine, alanine, leucine, valine, phenylalanine, phenylglycine and sarcosine units were synthesized by cyclization of dipeptide pentachlorophenyl esters. The ¹³C and natural abundance ¹⁵N NMR spectra of these heterocycles were measured in trifluoroacetic acid and compared with the spectra of the corresponding amino acids and polypeptides. The ¹³C NMR carbonyl signals of all cyclodipeptides show a 1.5–4.0 ppm upfield shift relative to the corresponding polypeptides. The ¹⁵N NMR signals show no such consistent relationship. The substituent effects and the neighbouring residue effects observed in the ¹⁵N NMR spectra of the cyclodipeptides are different from those of polypeptides, while the one bond N? H coupling constant of cis and trans amide groups was almost identical. The nitrogen and the carbonyl signal of the Gly units in cyclo-Gly-Phe show an extraordinary downfield shift, reflecting the interaction of the phenyl group with the 2,5-dioxopiperazine ring.     

Highly stable magnetically separable copper nanocatalyst as an efficient catalyst for C(sp2)–C(sp) and C(sp2)–C(sp2) cross‐coupling reactions

A copper catalyst has been explored as an efficient and recyclable catalyst to effect Sonogashira and Suzuki cross‐coupling reactions. After modification of 2‐(((piperazin‐1‐ylmethyl)imino)methyl)phenol (PP) on the surface of amorphous silica‐coated iron oxide (Fe₃O₄@SiO₂@Cl) magnetic core–shell nanocomposite, copper(II) chloride was employed to synthesize the Fe₃O₄@SiO₂@PP‐Cu catalyst, affording a copper loading of 1.52 mmol g⁻¹. High yield, low reaction times, non‐toxicity and recyclability of the catalyst are the main merits of this protocol. The catalyst was characterized using Fourier transform infrared, X‐ray photoelectron, energy‐dispersive X‐ray and inductively coupled plasma optical emission spectroscopies, X‐ray diffraction, scanning and transmission electron microscopies, and vibrating sample magnetometry.     

Rhodium(I)‐Catalyzed [4 + 2] Cycloaddition Reactions of 2‐Alkylenecyclo‐butanols with Alkynes and (E)‐2‐Nitroethenylbenzene through C(sp2)—C(sp3) Bond Cleavage

An intermolecular [4 + 2] cycloaddition was realized through C—C bond cleavage in the presence of Rh(I) catalyst. The selective ring opening of 2‐alkylenecyclobutanols enables the generation of active alkenylrhodium species, which underwent smooth cross addition over alkynes and (E)‐2‐nitroethenylbenzene, leading to highly substituted all‐carbon six‐membered rings in a single step and in a complete atom economy.     

Enantioselective palladium-catalyzed C(sp2)–C(sp2) σ bond activation of cyclopropenones by merging desymmetrization and (3 + 2) spiroannulation with cyclic 1,3-diketones

Catalytic asymmetric variants for functional group transformations based on carbon–carbon bond activation still remain elusive. Herein we present an unprecedented palladium-catalyzed (3 + 2) spiro-annulation merging C(sp²)–C(sp²) σ bond activation and click desymmetrization to form synthetically versatile and value-added oxaspiro products. The operationally straightforward and enantioselective palladium-catalyzed atom-economic annulation process exploits a TADDOL-derived bulky P-ligand bearing a large cavity to control enantioselective spiro-annulation that converts cyclopropenones and cyclic 1,3-diketones into chiral oxaspiro cyclopentenone–lactone scaffolds with good diastereo- and enantio-selectivity. The click-like reaction is a successful methodology with a facile construction of two vicinal carbon quaternary stereocenters and can be used to deliver additional stereocenters during late-state functionalization for the synthesis of highly functionalized or more complex molecules.

An unprecedented palladium-catalyzed (3 + 2) spiro-annulation merging C–C bond activation and desymmetrization was developed for the enantioselective construction of synthetically versatile and value-added oxaspiro products with up to 95% ee.     

Synthesis and complete assignment of the 1H and 13C NMR spectra of 6-substituted and 2,6-disubstituted pyridazin-3(2H)-ones

Several pyridazin-3(2H)-one derivatives were synthesized starting from alkyl furans using oxidation with singlet oxygen to give 4-methoxy or 4-hydroxybutenolides, key intermediates of the synthetic strategy followed. For all pyridazinones reported, a complete assignment of the (1)H and (13)C NMR spectra using one- and two-dimensional NMR spectroscopic methods, which included NOE, DEPT, COSY, HSQC and HMBC experiments, was accomplished. Correlations between the chemical shifts of the heterocyclic ring atoms and substituents at N-2 and C-6 were analyzed.     

Applications of 95Mo NMR. 5—substituent effects in the 95Mo and 13C NMR spectra of benzyltricarbonyl(η5-cyclopentadienyl)molybdenum(II) derivatives

A systematic study has been made of the effects of substituent induced chemical shifts in [(η⁵-C₅H₅)(CO)₃Mo(CH₂C₆H₄R)] compounds. Both ⁹⁵Mo and ¹³C NMR shifts in the aromatic ring are reported. The (η⁵-C₅H₅)(CO)₃MoCH₂? group is a reasonably strong resonance donor (σ_R° = ?0.21) and weak inductive donor (σ_I = ?0.07). The molybdenum chemical shifts are extremely sensitive to the effects of distant substituents (range c. 40 ppm). Since the shift correlates well with substituent constants in this series, it is suggested that the chemical shift is controlled by the paramagnetic term for this spin 5/2 nucleus.     

13C NMR spectral study of polyacetoxyxanthones. IV—13C NMR spectroscopy of substituted xanthones

The ¹³C NMR chemical shifts of nine acetoxyxanthones are reported and identified. The acetoxy substituent effects have been evaluated and the corresponding shift increments proposed.