13C NMR study of the structures of some acyclic and cyclic ketene acetals

A ¹³C NMR study of the spatial structures of some acyclic and cyclic ketene acetals (for example, ketene dimethyl acetal and 2-methylene-1,3-dioxolane) has been carried out. The conclusions obtained are based on observation of the effect of structural changes on the ¹³C NMR chemical shift of the β carbon on the ketene moity. Since the extent of p-π conjugation and hence the ¹³C chemical shift of this carbon depend on the spatial orientation of the alkoxy groups about the O-C(sp²) bonds, the shift concerned may be used as a measure of the planarity of the system. The most stable retamers of ketene dimethyl acetal are s-cis,s-cis (planar) and s-cis,gauche (slightly nonplanar), in the order of decreasing stability. For ketene dialkyl acetals, the relative stability of the planar s-cis,s-cis form decreases with increasing bulkiness of the alkyl groups, but at least for primary and secondary alkyl groups, the s-cis,s-cis rotamer appears to be the most favored species. The conformations of 5- to 8-membered cyclic ketene acetals are discussed and compared with those of the corresponding cyclic vinyl ethers and hydrocarbons.     

Reactions of some cyclic ketene acetals with maleic anhydride and acrylonitrile

The reactions of 9-methyl-4-methylidene-3,5-dioxabicyclo[5.4.0]undecane with maleic anhydride and acrylonitrile were studied by NMR spectroscopy and were found to give 1:1 molecular complexes with equilibrium constants of 0.24±0.06 and 0.14±0.04 l mol^?1, respectively. Analysis of the calculated (AM1) heats of formation ΔH, differences in the dipole moments Δμ, frequencies and intensities of stretching vibrations of the donor and acceptors, and charge distribution showed formation of π-π, mixed π-π-H, and H complexes with partial charge transfer. These complexes can be used in copolymerization of the above monomers.     

A dipole moment study of organo-chalcogen compoun

Analysis of the dipole moments of chalcogenoanisoles, the directions of which are given by those of their p-bromo derivatives, shows that the mesomeric moment decreases on passing from anisole to thioanisole, selenoanisole and telluroanisole (m = 1.1, 0.5, 0.25 and 0.18 D, respectively). In p-nitrochalcogenoanisoles and 1-chalcogenochroman-4-ones the interaction moment follows the reverse order, which is ascribed to the increasing sensitivity of the chalcogen mesomeric moment to the chalcogen atomic number and polarizability. The less-hindered (Te, O)-cis conformation is preferred for 2-acyl-3-methyltellurothiophenes and 3-acyl-2-methyltellurothiophenes (acyl: formyl or acetyl), and 3-formyl-4-methyltellurothiophene. Rotational isomerism in bis(2-furyl), bis(2-thienyl), bis(3-thienyl) and bis(2-selenienyl) ditelluride is also examined, and the dipole moments of 1-chalcogenochroman-4-ones, 2-chalcogenochroman-1-ones and 2-chalco-genochrom-1-ones analyzed.     

Theoretical studies of five-membered ring ketene acetals

Cyclic ketene acetals are a class of organic molecules characterized by a nucleophilic exo-methylene carbon attached to a carbon with two adjacent O, N, or S atoms. We have carried out a systematic computational study of a series of five-membered cyclic acetals like 2-methylene-1,3-dioxolane and its OS, SS, NO, NS, and NN analogs as well as all the protonated species. The calculations were performed at the MP2 level using a triple zeta plus polarization basis set. The nucleophilicity was discussed in terms of geometrical factors, calculated atomic charges, calculated chemical shifts, and proton affinities. All the six neutral species were strong nucleophiles. The NN analog was predicted to be the strongest and the SS analog the weakest nucleophile.     

Reaction of nitrones with silyl ketene acetals: a DFT study

Theoretical calculations at the DFT level indicate that the reaction of a nitrone with a silyl ketene acetal proceeds, contrary to previous suggestions, through a classical 1,3-dipolar cycloaddition, followed by a silyl group transfer step to give the open-chain product. Introduction of a diffuse basis set is necessary to describe properly nitrones. The influence of a Lewis acid has been studied.     

Isothiourea-catalysed asymmetric C-acylation of silyl ketene acetals

Screening of a range of chiral isothioureas and acyl donors to promote the asymmetric C-acylation of silyl ketene acetals indicates that C(2)-aryl-dihydropyrimidobenzothiazole-derived isothioureas and propionic anhydride give optimal reactivity and enantioselectivity in this process. Under optimised conditions 3-acyl-3-aryl or 3-acyl-3-alkylfuranones are prepared in good yields and moderate to excellent enantioselectivities (up to 98% ee; ee=enantiomeric excess).     

A dipole moment study of annular tautomerism in 5-p-tolyltetrazole

The electric dipole moment of 5-(p-tolyl)tetrazole in dioxane at 30.0° (4.99 D) suggests that the compound is a mixture of the 1H-tautomer (60 ± 10%) and the 2H-tautomer (40 ± 10%) in this medium. This determination was made by taking μ (2H) as μ (2Me) = 2.41 D in benzene, and by taking μ (1H) as μ (1Me) = 6.03 D in benzene with due allowance being made for the presence of interannular conjugation in the 1H-tautomer but not in the 1-methyl isomer. The dipole moments of 1-methyl-5-(p-tolyl)tetrazole and 2-methyl-5-(p-tolyl)tetrazole were also measured in dioxane; that of 1,5-trimethylenetetrazole was measured in benzene. Comparison with relevant data for unsubstituted tetrazole has also been made.     

Acid catalyzed rearrangement of vinyl and ketene acetals

Substituted vinyl and ketene acetals undergo smooth oxygen-to-carbon rearrangement with a catalytic amount of TMSOTf to afford chain-extended ketones or esters, respectively. The developed procedure has been applied to the stereoselective synthesis of C-glycosides from the corresponding anomeric vinyl ethers.     

The study of dipole moment of some substituted acetic acids in an electronically excited state

The electronic absorption spectra of eight substituted acetic acids have been measured at room temperature in several solvents. The ground state dipole moments are evaluated experimentally for these molecules. These ground state values are used in conjunction with the spectral results to evaluate their first electronically excited state dipole moments. For all the molecules investigated here the dipole moments in the excited state are higher than their ground state values.     

A dipole moment study of n-trimethyl-ammoniobenzamidates and n-benzoyliminodimethylsulphur(iv)

Analysis of the dipole moments of N-trimethylammoniobenzamidates Me₃N⁺-N^?COC₆H₄X, with X = H, p-F, p-Cl or p-NO₂, and of N-aroyliminodimethylsulphur(IV) Me₂SNCOC₆H₄X (X = H and p-NO₂) shows that, as solutes, these compounds exist in the syn conformation. Models are proposed for N-trimethylammonio-orthochloroben-zamidate and N-orthocyanobenzoyliminodimethylsulphur(IV). The (Me₃N⁺-N^t-) and (SN) dipole moments, and the (N-CO) and (SN-CO) mesomeric moments, are derived and discussed.     

A CASPT2 study of the dipole moment surfaces of hydrogen sulphide molecule

The ability of the CASPT2 method to yield accurate H₂S dipole moment surfaces that could be further used for conclusive predictions about rovibrational calculations, has been evaluated using ANO-L basis set. The optimised geometry, permanent dipole moment, linearity barrier as well as general features of the μ_x and μ_z dipole moment components in the vicinity of the equilibrium configuration agree favourably with available empirical determination and with recent accurate ab initio calculations. Dipole moment functions behaviour is also well reproduced for those geometrical configurations which are far from equilibrium in dissociation path. Computational technical aspects are discussed.     

The first synthesis of cyclopropanone acetals from the reaction of Fischer carbene complexes with ketene acetals

The reaction of iso-propoxy stabilized Fischer carbene complexes with ketene acetals gives moderate to excellent yields of cyclopropanone acetals when carried out under a carbon monoxide atmosphere. This is in contrast to the known reaction of methoxy substituted complexes which give cyclic ortho esters under the same conditions. A mechanism is proposed which involves a branch point between the two products as the zwitterionic intermediate resulting from nucleophilic addition of the ketene acetal to the carbene carbon. A 1,3-migration of the methoxyl group to the cationic center leads to the ortho ester and a ring closure by backside attack leads to the cyclopropanone acetal. A double-labeling experiment shows that the 1,3-migration occurs by an intramolecular process that is proposed to involve a bridging oxonium ion. The effect of the isopropoxy group is thus interpreted to be to sterically hinder the formation of a bridged oxonium ion.     

A novel reaction of cyclic ketene acetals with phenyl isocyanate through zwitterion

A novel reaction of cyclic ketene acetals with phenyl isocyanate through zwitterion was found to yield the corresponding spiro compounds in good yields.     

Rhenium-catalyzed reaction of carbonyl compounds with ketene silyl acetals

It was found that rhenium complex is an effective catalyst for the reaction of carbonyl compounds with ketene silyl acetals. A wide range of β-silyloxy esters is obtained by the treatment of carbonyl compounds with ketene silyl acetals in the presence of a catalytic amount of ReBr(CO)₅ in moderate to good yields.     

A dipole moment study of the conformational properties of diaryl diselenides and related compounds

The electric dipole moments of the diaryl diselenides (RC₆H₄)₂Se₂ (R  H, 4-F, 4-Br, 4-CH₃, 3-F) were measured in benzene solution at 25 and 45°C. The conformations of these compounds were deduced by matching experimental moments with values calculated for a variety of possible conformations. In the dissolved state the diselenides exist at 25°C in fixed “skew” conformations characterized by dihedral angles of 75–106° between the CSeSe planes, corresponding to the conformational energy minima. At 45°C oscillations about the SeSe bonds are excited in the diphenyl and bis(4-methylphenyl) diselenides, whereas the 4-bromophenyl derivative exhibits free rotation. The fluoro compounds have temperature-independent dipole moments, suggesting “rigid conformations” with dihedral angles of 106° (4-F) and 74.4° (3-F). An analysis of the dipole moments at 25 and 45°C obtained for the compounds (RC₆H₄)₂X₂ (R  H, 3-F, 4-F, 4-Br, 4-CH₃; X  S, Se, Te) showed that the conformational properties of these derivatives change on passing from X  S to X  Te. The observed variations are explicable in terms of a decreasing repulsion between the lone electron pairs of the chalcogen atoms on going from the disulfides to the ditellurides and a concomitant reduction of the energy barrier to rotations about the XX bonds.     

Enantioselective acylation of silyl ketene acetals through fluoride anion-binding catalysis

A highly enantioselective acylation of silyl ketene acetals with acyl fluorides has been developed to generate useful α,α-disubstituted butyrolactone products. This transformation is promoted by a new thiourea catalyst and 4-pyrrolidinopyridine and represents the first example of enantioselective thiourea anion-binding catalysis with fluoride.     

A comparative study of the dipole polarizability of some Zn clusters

We have employed a hierarchy of basis sets and computational techniques in order to approach the polarizability (alpha) of Zn(m), m = 2-20, in a systematic way. This procedure allows the proper approximate results to be selected and validated. More specifically, we have developed in a systematic way a series of basis sets that have been employed for the computation of the polarizability of the Zn atom. Comparison of the computed with the experimental and the best theoretical results allows us to comment on the quality of the basis sets, and to select some of the more successful ones in order to compute the polarizabilities of Zn(m), m = 2-20. We have employed a series of methods to take into account the correlation contribution. These include the following techniques: MP2, CC2, CCSD, CCSD(T), and DFT(B3LYP). We have used two effective core potentials, one small (3s2 3p6 3d10 4s2) and one large (4s2) core. The relativistic contribution to the properties is found to be significant. Thus we have studied in detail the relativistic effects on the polarizability of some small zinc clusters, by employing the Douglas-Kroll approximation in connection with the polarized basis sets developed by Kell? and Sadlej and the methods HF, CC2, and CCSD. Most of the polarizability values are static, but some frequency-dependent properties have also been computed in order to find out the difference between the dynamic and static ones. It is considered that the sharp changes of alpha(Zn(m))/m vs m may be correlated with the change of bonding, from van der Waals to covalent and finally to metallic bonding, in Zn(m). The present comprehensive study of the polarizabilities of Zn(m) includes a limited number of results for the first and second hyperpolarizability of some of the considered zinc clusters.     

Correlation between molecular dipole moment and centrosymmetry in some crystalline diphenyl ethers

The presence of a large molecular dipole moment in diphenyl ethers leads unequivocally to a centrosymmetric crystal structure.