Influence of remote substituents on the equatorial/axial selectivity in the monooxygenation of methylene C--H bonds of substituted cyclohexanes

The reactivity of individual C--H bonds in the methyl(trifluoromethyl)dioxirane TFDO oxygenation of stereogenic methylene groups in conformationally homogeneous monosubstituted cyclohexanes (2) has been determined. The unexpectedly high occurrence of O-atom insertion into C--H(ax) bonds suggests an in plane trajectory attack in the oxygenation while the diastereoselectivity of the reaction is qualitatively interpreted on the basis of the distinct hyperconjugative stabilization by the substituent of diastereomeric transition states due to long-range through bond interactions.     

Raman-spectrum investigation of some acetylenic alcohols and their acetic esters

Summary The formation of the acetic esters of tri- and di-bromo ketols by the action of HOBr on the acetic esters of secondary and tertiary acetylenic alcohols is due to the particularly strongly protonized (acidic) hydrogen atom of the methylidyne group=C-H, which results from profound frequency modulation of the vibrations of the latter by the low-frequency intermolecular vibrations at the hydrogen bonds.     

Conformational preferences of some substituted methyl groups in cyclohexanes as studied by carbon-13 nuclear magnetic resonance

Low temperature ¹³C NMR spectra of 80:20 mixtures of cis and trans-4? CH₃? CH₃? C₆H₁₀CH₂X, where ? C₆H₁₀-is 1, 4-disubtituted cyclohexyl and X=Br, CN, OH, OCH₃, Si(CH₃)₃, Sn(CH₃)₃, Pb(CH₃)₃ and HgOCOCH₃ have been recorded. The signals of the trans (e, e) components were assigned from the ambient temperature spectra of C₆H₁₁CH₂X and the established substituent effects of an equatorial methyl group in cyclohexane. Conformational equilibria of the cis (e, a?a, e) components were then computed from the intensities of the (remaining) signals (～180 K) of the two conformational isomers. From these equilibria A values of CH₂X were calculated, assuming additivity of conformational energies of CH₃ and CH₂X (the counter-poise approach). In general, these values are very similar to the value of the CH₃, although some trends do emerge. This study provides α, β, γ, and δ effects for a wide range of axial and equatorial ? CH₂X groups.     

Theoretic heat sink simulation and experimental investigation of the pyrolysis of substituted cyclohexanes

Reaxgen program for the pyrolysis mechanism of cycloalkanes was adopted to simulate the heat sink of substituted cyclohexanes. Thermal cracking of cyclohexanes was performed to examine the cracking performance, wherein the substituent effects were detailedly discussed under supercritical condition. It was found that Reaxgen program played a good part in the screening and optimization of cyclohexanes. A good agreement with the experimental data for the mono-substituted and bi-substituted cyclohexanes was demonstrated, however, some deviation for the tri-substituted cyclohexanes were observed. The experiment results indicated that the gaseous product yield increased sharply for monosubstituted cyclohexanes with short substituents containing no more than two carbons. Nevertheless, continuous increase in the alkyl chain depressed the gaseous product yield smoothly. The cyclic substituent dramatically inhibited the pyrolysis of cyclohexanes. All the substituents but cyclohexyl had no obvious effect on the yield of hydrogen and olefins(≤C4). For bi-substituted cyclohexanes, the more close the distance between the two substituents, the higher the gaseous product yield was obtained. However, opposite result on the selectivity to hydrogen and olefins(≤C4) was generally obtained except 1,3-dimethylcyclohexane. The position of tri-substituents acted little significance on the gaseous product yield, as well as the selectivity to hydrogen and olefins(≤C4).     

Kinetic studies of some substituted hexarhodium carbonyl clusters

Reactions of the halides X- (X- = chloride, bromide or iodide) with the substituted cluster Rh6(CO)15(PPh3) in oxygen-free chloroform lead to [Rh5(CO)14(PPh3)]-, Rh(CO)2(PPh3)2X and [Rh(CO)2X2]- in the molar ratios 2:1: approximately 13. Oxidation by the solvent is assumed to lead to most of the Rh(I) product, and the stoichiometry for reactions with I- can be defined as 4Rh6(CO)15(PPh3) + 27I- + 12CHCl3 --> 2[Rh5(CO)14(PPh3)]- + Rh(CO)2(PPh3)2I + 13[Rh(CO)2I2]- + 6C2H2Cl4 + 4CO + 12Cl-. This can be rationalized quite simply with the aid of a few generally justifiable assumptions. Rate constants for reactions with bromide increase to a limiting value with increasing [Br-] in a way that shows that breaking of one Rh-Rh bond, with an unusual closo to nido structural change, is rate determining. This opening of the cluster might be spontaneous or solvent induced. To complete the reaction, the bromide has to compete with the reverse nido to closo change. The same closo to nido change is also a major rate determining step for reactions with P(OPh)3 in oxygen-free solutions, and for reactions with bromide in oxygenated solutions in the presence of trifluoroacetic and some other acids. The limiting rates increase slightly with increasing basicity of the ligands P(p-XC6H4)3 along the series X = F3C, Cl, F, H and MeO. Activation parameters for these reactions are reported.     

Raman-spectrum investigation of the conformations of cis-and trans-1,3-dimethylcyclohexanols

Summary When configurations are named with respect to the actually largest substituents, the preferred conformation for the compound (I) in the liquid phase is trans-1e3a, and for compound (II) in the liquid phase cis-1e3e with equatorial hydrogen-bonded hydroxyls; when this nomenclature, based on the actually largest substituents is used, the Auwers-Sktta rule is not reversed for the compounds (I) and (II) in the liquid state.     

A carbon-13 cp/mas NMR investigation of the conformation of substituted cyclohexanes in thiourea inclusion compounds

Inclusion compounds of several monosubstituted cyclohexanes and thiourea have been studied using high-resolution ¹³C CP/MAS NMR spectroscopy. The ¹³C NMR chemical shifts of the substituted cyclohexane ring are sensitive to the conformation of the substituent and allow one to predict qualitatively the relative populations of the axial and equatorial conformers. For methylcyclohexane trapped in thiourea the methyl substituent prefers the equatorial conformation while for the cyclohexyl halides (Cl, Br and I) the axial conformer is preferred. In the case of fluorocyclohexane the equatorial conformer appears to predominate; however, this conformer is in rapid equilibrium with the axial conformer.     

Characterization of equatorial and axial six-membered-ring peroxyl radicals

Spectroscopic data are consistent with computations that show that, in their most stable conformations, the peroxyl moiety is equatorial in cyclohexylperoxyl radicals and axial in oxa- and most polyoxacyclohexyl-2-peroxyl radicals.     

Cyclic acetals as carbonyl blocking groups in the photo-fries rearrangement of acyl substituted aryl esters.

The ethylene acetals of $\text{o}$ and $\text{p}$-acetoxyacetophenone 3a,b, prepared from the corresponding hydroxyacetophenones by treatment with acetic anhydride in pyridine and subsequently with ethylene glycol and $\text{p}$-toluenesulphonic acid, give upon irradiation the expected photo-Fries products 4a,b (from 3a) or 6 (from 3b). These compounds are converted in part into diacetylphenols 5a,b as a consequence of a deacetalization, occurring to some extent during the irradiation and further in the chromatographic workup. The yields of the photoproducts and the $\text{ortho}$/$\text{para}$ ratio (in the case of 3a) are very similar to those given in the literature for phenyl acetate. By contrast, the acetoxyacetophenones 2a,b do not undergo any appreciable change upon irradiation,showing the deactivating effect of the acetyl side chain. Therefore, it is concluded that cyclic acetals are suitable carbonyl blocking groups in order to circumvent the deactivating effect of acyl substituents on the photo-Fries rearrangement of aryl esters.     

Synthesis and investigation of the electronic spectra of perchlorates of some substituted pyridinium ions

1,2,4,6-Substituted pyridinium perchlorates were obtained, and their electronic absorption and emission spectra were studied. When electron-donor substituents are introduced into the para position of the N-phenyl group, an intramolecular charge-transfer band (l _a transition), with, which the capacity for fluorescence of these compounds in the crystalline state is associated, appears in the long-wave portion of the absorption spectrum. Replacement of the methyl groups in the heteroring by phenyl groups is accompanied by hyperchromic and bathochromic effects and by the appearance of a capacity for fluorescence in solutions at room temperature. These phenomena are explained by an increase in the polarization of the -electron system of the molecules when phenyl groups are introduced.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 359–362, March, 1973.     

13C NMR chemical shifts of carbonyl groups in substituted benzaldehydes and acetophenones: substituent chemical shift increments

13C NMR Substituent chemical shift (SCS) increments have been determined for the carbonyl carbon of a variety of substituted benzaldehydes and acetophenones. The 13C NMR chemical shift of the carbonyl carbon can be predicted for many di- and trisubstituted benzaldehydes and acetophenones through simple additivity of the SCS increments. The magnitude and sign of the SCS increments have been explored using Hartree-Fock 6-31G* calculations to determine the natural atomic charges of the carbonyl carbon. When a substituent capable of intermolecular hydrogen bonding is present, deviations from additivity on the order of 2 ppm are observed in dilution experiments; deviations of up to 6 ppm can result from intramolecular hydrogen bonding.     

Some substituted thiourea complexes of molybdenum carbonyl

Preparation of monosubstituted thiourea complexes of molybdenum carbonyl [Mo(CO)₅L] both by thermal and UV irradiation methods have been described. The ligands (L) used were N,N′-dimethylthiourea (DMTU), N,N′-diphenylthiourea (DPTU), tetramethylthiourea (TMTU) and tetraethylthiourea (TETU). The complexes [Mo(CO)₅(L)] were characterised by NMR, IR, UV-visible and elemental analyses.     

Stereochemistry of the oxidation of imines derived from substituted cyclohexanones: axial vs equatorial attack and evidence for delivery by an adjacent hydroxyl group

A set of conformationally biased imines derived from substituted cyclohexanones and benzylamine or diphenylmethylamine, respectively, were oxidized to the corresponding oxaziridines. The structures of the oxaziridines were determined via NMR comparison of two series of differently N-substituted oxaziridines. Thus, those compounds having an axially disposed nitrogen substituent displayed an upfield-shifted axial proton in a 1,3-relationship to the oxaziridine nitrogen in the N-diphenylmethyl series relative to the N-benzyl compounds. Analysis of the products obtained from these reactions suggests that (1) adjacent hydroxyl groups favor syn oxidant addition and (2) imines containing adjacent methoxy groups preferentially undergo attack anti to the resident alkoxy substituent.     

NMR investigation of the oxyethylene unit ordering in some related dialkoxy laterally substituted nematogens

Laterally dialkylated nematogens containing a short terminal polyoxyethylene chain present a wide nematic range starting from room temperature. The ordering of the oxyethylene (OE) unit has been studied by 13C NMR in the liquid crystalline phase. The values of the order parameters were derived from the transient oscillations observed during variable contact-time cross-polarization experiments. Along the oxyethylene chain, the order parameters decrease monotonically. These order parameter values do not present the usual odd-even effect and are noticeably smaller than those found for terminal alkoxy chains. Entering the nematic phase, the 13C thermal evolution of the field-induced chemical shift in the OE unit is far smaller than the one usually observed for terminal alkyl or alkoxy chains, regardless of their non-negligible order parameters. In addition,no linear correlation between the order parameter and the field-induced chemical shift is observed for the carbons in the OE unit. This is an indication that the proportion between gauche - and trans -conformations in the OE unit is temperature dependent.