Résonance magnétique nucléaire de 13C et 17O d'éthers aliphatiques. Effets γ entre les atomes d'oxygène et de carbone

The ¹³C- and ¹⁷O-chemical shifts of 31 aliphatic ethers are measured and discussed. The ¹⁷O-chemical shifts of the ethers ROR′ correlate with chemical shifts for the methylene groups of the corresponding alkanes RCH₂R′. The constant of proportionality can be related to the orbital expansion term 〈r^?3〉_2p. The δ_c for carbon atoms can also be correlated with δ_c for the corresponding alkanes. The origin of the correlation is discussed taking into account the conformational modifications resulting from introduction of an oxygen atom in an alkyl chain.     

Lineare Korrelation der chemischen Verschiebung der Vinylprotonen mit den LCAO-MO-π-Elektronendichten in meta- und para-substituierten Styrolen und Zimtsäuren

A linear correlation was found between the proton chemical shifts of the vinyl protons in m/p-substituted styrenes and cinnamic acids and the π-electron densities of the corresponding carbon atom, calculated by the simple LCAO-MO procedure, using standard values for the perturbation parameters.     

Cationic polymerization of α,β-disubstituted olefins. V. Reactivity of propenyl alkyl ethers in cationic polymerization

To elucidate the effect of the introduction of a methyl group in the β-position of a vinyl monomer, propenyl alkyl ethers were copolymerized with vinyl ethers having the same alkoxy group. Propenyl alkyl ethers with an unbranched alkoxy group (ethyl or n-butyl propenyl ether) were more reactive than the corresponding vinyl ethers. This behavior is quite different from that of β-methylstyrene derivatives. However, propenyl alkyl ethers with branched alkoxy groups at the α carbon atom (isopropyl or tert-butyl propenyl ether) were less reactive than the corresponding vinyl ethers. Also, cis- isomers were more reactive than the trans isomers, regardless of the kind of alkoxy group and the polarity of the solvent.     

Chemistry of trifluorostyrenes and their dimers: 2. Synthesis of substituted α, β, β-trifluorostyrenes and α, β, β-trifluoroethenylnaphthalenes. Hammett correlations of their NMR parameters and the concept of “distorted π-electron clouds”

Six α, β, β-trifluorostyrenes with the following substituents, viz., p-MeO, p-Me, m-Me, p-Cl, m-Cl, and m-CF₃, were synthesized by the reaction of the corresponding Grignard reagents with tetrafluoroethylene in tetrahydrofuran. Similarly, α-and β-trifluoroethenylnaphthalenes were prepared. The substituent electronic effects on the ¹⁹F-NMR parameters were investigated for the trifluorostyrenes (I). Linear correlations between the Hammett σ constants and the following ¹⁹F-NMR parameters were established, namely, chemical shifts δ. (F¹) and δ (F²), coupling constants J₁₂, differences of chemical shifts Δδ_3-1 (δ (F³)—δ(f¹) or Δδ_3-2. The results are consistent with previous expectations based on the simple concept of “distorted π-electron clouds”. Facts are presented which indicate that the Δδ_3-1 (or Δδ_3-2) values may serve as empirical measures of the degree of polarization of the π bonds of these fluoroolefins.     

Thermodynamics of vinyl ethers—XXI: Evaluation of some cis interaction energies

Thermodynamics of isomerization reactions on some substituted vinyl methyl ethers have been studied for evaluating the magnitudes of the interaction energies S[R¹... R²] between the substituents R¹ and R², juxtaposed in a cis position across the CC bond of vinyl ethers. The results obtained are (values given in kJ mol^-1): S[Me... t-Bu]=18.2±1.0, S[Ph...i-Pr]=11±2, S[i-Pr...Et] =6.1±0.6, S[i-Pr... i-Pr] = 6.0 ± 0.6, S[0...t-Bu]=2.9±0.5, S[O...i-Pr]= -0.7±0.5, S[O...Et] = ?1.5±0.5, S[O...Ph]= ?2.1+-0.6, and S[O... Me] =?2.9 ±0.2 (the symbol O stands for the ethereal oxygen atom of vinyl ethers). The negative interaction energy values reveal that the cis interaction between the ethereal oxygen and the alkyl (aryl) group concerned is stabilizing.     

Structural aspects of two α‐dihydrazones displaying a complete survey of intermolecular interactions

The compounds (2′E,2′E)‐2,2′‐(propane‐1,2‐diylidene)bis[1‐(2‐nitrophenyl)hydrazine], C₁₅H₁₄N₆O₄, (I), and (2Z,3Z)‐ethyl 3‐[2‐(2‐nitrophenyl)hydrazinylidene]‐2‐[2‐(4‐nitrophenyl)hydrazinylidene]butanoate tetrahydrofuran hemisolvate, C₁₈H₁₈N₆O₆·0.5C₄H₈O, (II), are puzzling outliers deviating from a general synthetic route aimed at the preparation of substituted pyrazoles. Possible reasons for this outcome, which is exceptional in an otherwise firmly established synthetic procedure, are analyzed. Compound (I) is unsolvated, while compound (II) crystallizes with a tetrahydrofuran solvent molecule lying on an inversion centre. The ethoxycarbonyl chain of (II), in turn, appears disordered into two equally populated (50%) moieties. In both structures, a plethora of different commonly occurring weak intermolecular interactions [viz. π(phenyl)...π(phenyl), π(C=N)...π(C=N), π(phenyl)...π(C=N), N—H...O and C—H...O] appear responsible for the crystal stability. Much less common are the short O(nitro)...O(nitro) contacts which are observed in the structure of (I), an example of unusual `electron donor–acceptor' (EDA) interactions.     

Struktur-Reaktivitätsuntersuchungen mit heterosubstituierten Nitrilen—VII: 13C-, 14N- und 19F-NMR-Spektroskopische Untersuchungen an Verbindungen vom Typ Ar?X?CN

The chemical shifts of aromatic nitriles of the general structure para-Y? C₆H₄? X? CN with X = O, S, Se and N(CH₃) have been investigated by the ¹³C NMR technique. For cyanates (X = O) the ¹⁴N shifts and for Y = F the ¹⁹F shifts were likewise measured. The chemical shifts and the corresponding ¹³C shift increments Δ_n have been found to correlate with the appropriate substituent constants σ_R⁰, σ_p⁰ and σ_I, as well as with the π-electron densities calculated in the PPP approximation.     

Photochemische Reaktionen 115. Mitteilung [1]. Zur Photochemie konjugierter γ,δ-Epoxyenone: Der Einfluss eines Hydroxylsubstituenten in ϵ-Stellung

Photochemistry of Conjugated γ,δ-Epoxyenones: The Influence of a Hydroxy Substituent in ?-Position On ¹n, π*- or ¹π,π*-excitation (λ ≥ 347 or λ=254 nm), the ?-hydroxy-γ;,δ-epoxyenone 8 undergoes fission of the C(γ)? O bond followed by the cleavage of the C(δ)-C(?) bond. This hitherto unknown sequence of reactions is evidenced by the structure determination of the new type products 10–17 and 25 , including a synthetic proof for 12 and the X-ray analysis of 11 (X-ray data: triclinic P1; a=7,386(2), b=8,904(4), c=9,684(5)Å; α=82,29(4)°, β=74,46(3)°, γ=82,29(3)°; Z=2). The selective ¹π,π*-excitation also induces competitive C(γ)-C(δ) bond cleavage to yield the bicyclic acetal 18 and a ketonium-ylide intermediate a , which photochemically forms a carbene b giving the allene 19 and the cyclopropene 20 . On ¹n,π*-excitation of the acetate 9 the initial C(γ)-O bond fission is, in contrast to the behaviour of the corresponding alcohol 8 , followed by a 1,2-methyl shift affording (E/Z)- 28 or by a cyclization-autoxidation process yielding the lactone 29 .     

Quantum chemical analysis of the structure and spectroscopic properties of aryl vinyl ethers

A series of aromatic vinyl ethers and some compounds close to them in structure are studied by DFT (B3LYP/6-311+G(2d,p)) and MP2(full)/6-311+G(2d,p) methods. Measurements of Raman spectra are also used. The calculation of vibrational spectra of aryl vinyl ether (AVE) isomers shows that stretching vibrations ν(C=C) are most conformation sensitive. The calculated value of I(C=C) for vinyl phenyl ether more than twice exceeds the corresponding value for vinyl methyl ether. The calculated and experimental values of I(C=C) are consistent with the hypothesis about the presence of a common conjugated π-system in the molecules of substituted AVEs. Here the bridging oxygen atom provides the π,p,π-interaction.     

Carbon-13 n.m.r. characterization of indenyllithium in solution

¹³C N.m.r. chemical shifts obtained for indenyl- and cyclopentadienyllithium in several, mostly ethereal, solvents are presented. The chemical shift changes induced by varying the solvent are assumed to probe the actual π-electron density distribution, thus providing an insight into the structure of the ion-pair. The increase in the estimated π-electron densities obtained for the protonated aromatic carbons of indenyllithium by increasing the coordinating ability of the solvent gives evidence of a change of the ion-pair equilibrium towards solvent-separated species. The average chemical shifts for indenyl- and cyclopentadienyllithium were independent of the choice of solvent, a fact which supports the ion-pair model in all studied media.     

Fluorescence à 77 K,rotation interne et structure electronique du styréne

A vibronic analysis of the fluorescence spectrum of styrene in crystalline solution at 77 K corroborates the assignment of the O.O band at 34758 cm^?1 for gas-phase spectra. The torsional frequency of the vinyl group with respect to the phenyl ring is used to evaluate the internal rotation barrier of styrene (5.7 kcal mol^?1). Configuration interaction MO calculations including all the mono and di-excilations within the π-electron system of styrene suggest that the first π—π^* transition has a moment tilted by 10° with respect to the long axis of the molecule; this is in agreement with the rotational structure of the gas-phase band at 34758 cm^?1, the analysis of which results in an A-type band.     

Structure and reactivity of α,β-unsaturated ethers. XIII. Cationic copolymerization and acid-catalyzed hydrolysis of 1,2-dialkoxyethylenes

The cationic copolymerizations of geometrical isomers of 1,2-dimethoxy- and 1,2-diethoxyethylenes with vinyl isobutyl ether as a reference monomer have been carried out in methylene chloride at ?70° using boron trifluoride etherate as catalyst. The kinetics of the acid-catalyzed hydrolysis of these ethers has also been investigated in 80% aqueous dioxane, in order to compare the results with the polymerizabilities. It has been found that the cis ethers are ca. four times as reactive as their trans isomers in both reactions. On the other hand, it has been proved that a β-alkoxyl substitution reduces the hydrolysis reactivity of vinyl alkyl ethers by a factor of ca. 10^?3 while it even enchances the cationic polymerizability. These contrasting results are interpretable from the nature of the transition states which are different for the two reactions.     

Carbon-13 nuclear magnetic resonance spectroscopy—VII:para-substituted fluorobenzenes

C-13 and F-19 NMR spectra of seventeen para-substituted fluorobenzenes were measured and the chemical shifts as well as coupling constants with respect to substituents were analysed. The chemical shifts of the fluorine, the C₁ and the C₂ atoms were found to depend on the total electron densities. In the case of the C₃ atom, the chemical shifts seem to depend on π-electron densities rather than the total electron densities. The present calculations also indicate that the chemical shift of the C₄ atom depends mainly on σ-electron densities due to the inductive effects of substituents. The strongest factor influencing the coupling constant, ⁿJ(C? F), is also considered to be the π-electron densities on the carbon atoms. In the case of the direct couplings, ¹J(C? F), the π-bond orders are important.     

Photochemische Reaktionen. 68. Mitteilung. Die Photoisomerisierung von α,β-ungesättigten γ,δ-Epoxyketonen: 9α, 10α- und 9β, 10β-Oxido-3-oxo-17β-acetoxy-Δ4-östren

Irradiation in the n→π* absorption band of the α,β-unsaturated γ,δ-epoxyketone 5 in ethanol at ?65° exclusively afforded the rearranged ene-dione 13 , whereas at + 24° under otherwise unchanged reaction conditions or upon triplet sensitization with Michler's ketone and with acetophenone at + 24° essentially identical mixtures of 13 (major product), 14 , and 15 were obtained. Selective π→π* excitation of 5 at ?78° and + 24° led to similar product patterns. The 9β,10β-epimeric epoxyketone 7 selectively isomerized to 14 and 15 at + 24° and n → π* or π → π* excitation. Neither the epoxyketones 5 and 7 nor the photoproducts 13–15 were photochemically interconverted. In separate photolyses each of the latter gave the double bond isomers 16 , 18 , and 19 , respectively. Cleavage of 13 to the dienone aldehyde 17 competed with the double bond shift ( → 16 ) when photolyzed in alcoholic solvents instead of benzene. The selective transformations 5 → 13 (at ?65° and n → π* excitation) and 7 → 14 + 15 are attributed to stereoelectronic factors facilitating the skeletal rearrangements of the diradicals 53 and 55 , the likely primary photoproducts resulting from epoxide cleavage in the triplet-excited compounds 5 and 7 , via the transition states 54 , 56 , and 57 . The loss of selectivity in product formation from 5 at higher temperature and n → π* excitation or triplet sensitization is explicable in terms of radical dissociation into 58 and 59 increasingly participating at the secondary thermal transformations of 53 . The similar effect of π → π* excitation even at ?78° indicates that some of the π,π* singlet energy may become available as thermal activation energy. It is further suggested that the considerably lesser ring strain in 14 and 15 , as compared with 13 , is responsible that selectivity in product formation from 7 is maintained also at +24° and at π → π* excitation.     

Photochemische Reaktionen. 103. Mitteilung [1]. Zur Photochemie konjugierter Epoxy-en-carbonylverbindungen der Jonon-Reihe: UV.-Bestrahlung α,β-ungesättigter ε-Oxo-γ, δ-epoxy-carbonylverbindungen und Untersuchung des Mechanismus der Epoxy-enon/Furan-Isomerisierung

The Photochemistry of Conjugated γ,δ-Epoxy-ene-carbonyl Compounds of the Ionone Series: UV.-Irradiation of α,β-Unsaturated ε-Oxo-γ,δ-epoxy Compounds and Investigation of the Mechanism of the Isomerization of Epoxy-enones to Furanes On ¹n, π*-excitation (λ ≥ 347 nm; pentane) of the enonechromophore of 3 , three different reactions are induced: (E/Z)-isomerization to give 13 (7%), isomerization by cleavage of the C(γ)–C(δ) bond to yield the bicyclic ether 14 (36%) and isomerization by cleavage of the C(γ)? O bond to give the cyclopentanones 15 (13%) and 16 (11%; s. Scheme 2). On ¹π, π*-excitation (λ = 254 nm; acetonitrile) 13 (14%), 15 (6%), and 16 (6%) are formed, but no 14 is detected. In contrast, isomerization by cleavage of the C(δ)? O bond to give the cyclopentanone 17 (23%) is observed. The reaction 3 → 17 appears to be the consequence of an energy transfer from the excited enone chromophore to the cyclohexanone chromophore, which then undergoes β-cleavage. Irradiation of 4 with light of λ = 254 nm (pentane) yields the analogous products 20 (18%), 21 (9%), 22 (7%), and 24 (7%; s. Scheme 2). Selective ¹n, π*-excitation (λ ≥ 280 nm) of the cyclohexanone chromophore of 4 induces isomerization by cleavage of the C(δ)? O bond to give the cyclopentanones 23 (9%) and 24 (44%). Triplet-sensitization of 4 by excited acetophenone induces (E/Z)-isomerization to provide 20 (12%) and isomerization by cleavage of the C(δ)? O bond to yield 21 (26%) and 22 (20%), but no isomerization via cleavage of the C(δ)? O bond. It has been shown, that the presence of the ε;-keto group facilitates C(γ)? C(δ) bond cleavage to give a bicyclic ether 14 , but hinders the epoxy-en-carbonyl compounds 3 and 4 from undergoing cycloeliminations. The activation parameters of the valence isomerization 13 → 18 , a thermal process, have been determined in polar and non-polar solvents by analysing the ¹H-NMR. signal intensities. The rearrangement proceeds faster in polar solvents, where the entropy of activation is about ?20 e.u. Opening of the epoxide ring and formation fo the furan ring are probably concerted.     

Linear Korrelation der 13C-Kernresonanzfrequenzen mit den LCAO-MO-π-Elektronendichten in substituierten Benzonitrilen

The ¹³C NMR. spectra of seventeen m- and p- subsituted benzonitriles were examined and partially anlayzed. The C-1 carbon shieldings can be expressed in terms of a linear correlation with the LCAO-MO-π-electron density at C-1. No such correlation was found for the cyanocarbon, whose electron density shows only slight varriations for the different substitutents. This fact is consistent with the relatively small range of the chemical shifts at this center.     

1H NMR spectra. Part 29§: proton chemical shifts and couplings in esters—the conformational analysis of methyl γ‐butyrolactones

The ¹H NMR spectra of 35 cyclic and acyclic esters are analysed to give the ¹H chemical shifts and couplings. The substituent chemical shifts of the ester group were analysed using three‐bond (γ) effects for near protons and the electric field, magnetic anisotropy and steric effect of the ester group for more distant protons. The electric field is calculated from the partial atomic charges on the O?C = O atoms, and the asymmetric magnetic anisotropy of the carbonyl group acts at the midpoint of the C = O bond. The values of the anisotropies Δχ_parl and Δχ_perp were for the aliphatic esters 10.35 and ?18.84 and for the conjugated esters 7.33 and ?15.75 (×10^?6 ų/molecule). The oxygen steric coefficients found were 104.4 (aliphatic C = O), 45.5 (aromatic C = O) and 16.0 (C–O) (×10^?6 Å⁶/molecule). After parameterisation, the overall RMS error for the data set of 280 entries was 0.079 ppm. The strongly coupled ¹H NMR spectra of the 2‐methyl, 3‐methyl and 4‐methyl γ‐butyrolactones were analysed and the methyl conformational equilibrium obtained from the observed couplings. The observed versus calculated density functional theory (DFT) ΔG(ax‐eq) was 1.0 (1.01), 0.34 (0.54) and 0.65 (0.71) kcal/mol res. The shielding effect of a methyl cis to a proton in the five‐membered lactone rings is ?0.40 ±0.05 ppm and deshielding trans effect 0.12 ±0.05 ppm, which is common to both five and six membered rings. The cis/trans isomerism in the vinyl esters methyl acrylate, crotonate and methacrylate and methyl furoate was examined using the ¹H chemical shifts. The calculated shifts of both the cis and trans isomers were in good agreement with the observed shifts. Copyright © 2012 John Wiley & Sons, Ltd.     

Structure and reactivity of α,β-unsaturated ethers. III. Cationic copolymerizations of alkenyl alkyl ethers

The relative cationic polymerizabilities of the geometrical isomers of various alkenyl alkyl ethers were studied both in copolymerizations with each other and in their respective copolymerizations with vinyl isobutyl ether as standard. Copolymerizations were carried out in methylene dichloride at ?78°C. with boron trifluoride etherate as catalyst. The cis isomers have been found to be more reactive than the corresponding trans isomers. A primary alkyl substituent on the β-cis position of vinyl ethyl ether enhances the reactivity. Yet the steric effect is noticeable when the substituents are bulky. Compounds substituted with cis-β-isobutyl and with β-dimethyl showed little tendency to homopolymerization. It was proved that the polymer ends derived from cis and from trans monomers are respectively different in character because of the restricted rotation of the end unit around the terminal carbon–carbon bond. The alternation tendency, remarkable in the copolymerization of cis monomers with vinyl ether, was explained in terms of the cis-opening mechanism.     

A Novel Chemoselective Cleavage of (tert‐Butyl)(dimethyl)silyl (TBS) Ethers Catalyzed by Ce(SO4)2⋅4 H2O

(tert‐Butyl)(dimethyl)silyl (^tBuMe₂Si; TBS) phenyl/alkyl ethers were efficiently cleaved to the corresponding parent hydroxy compounds in good yields using catalytic amounts of Ce(SO₄)₂?4 H₂O by microwave‐assisted or conventional heating in MeOH. Intramolecular and competitive experiments demonstrated the chemoselective deprotection of TBS ethers in the presence of triisopropylsilyl (ⁱPr₃Si; TIPS) and (tert‐butyl)(diphenyl)silyl (^tBuPh₂Si; TBDPS) ethers.     

Decarboxylative C(sp3)−O Cross‐Coupling

Alkyl aryl ethers are an important class of compounds in medicinal and agricultural chemistry. Catalytic C(sp³)?O cross‐coupling of alkyl electrophiles with phenols is an unexplored disconnection strategy to the synthesis of alkyl aryl ethers, with the potential to overcome some of the major limitations of existing methods such as C(sp²)?O cross‐coupling and S_N2 reactions. Reported here is a tandem photoredox and copper catalysis to achieve decarboxylative C(sp³)?O coupling of alkyl N‐hydroxyphthalimide (NHPI) esters with phenols under mild reaction conditions. This method was used to synthesize a diverse set of alkyl aryl ethers using readily available alkyl carboxylic acids, including many natural products and drug molecules. Complementarity in scope and functional‐group tolerance to existing methods was demonstrated.