The conformational analysis of tetrahydro-1,4,2-dioxazines

A conformational study of some tetrahydro-1,4,2-dioxazines by use of ¹H and ¹³C NMR spectroscopy is reported. The conformational characteristics of this ring are compared to those of the related systems, tetrahydro-1,2- and 1,3-oxazine. A study of model compounds allows the assignment of ring and nitrogen inversion processes in the variable temperature NMR spectra. Ring inversion is found to be a lower energy process than nitrogen inversion. The barriers to these processes are measured in several derivatives and the implications of the results for studies of nitrogen inversion in other 6-membered rings are pointed out. The conformational free energy differences of N-Me and N-Et groups are measured and discussed. It is somewhat easier to put an N-Et group axial than N-Me (ca.0.25 kcal mole^?1). It also appears than an Et group at C-6 goes axial more readily than does a Me.     

Novel monoaza- and diazacrown ethers incorporating sugar units and their extraction ability towards picrate salts

A number of new 15- and 1 8-membered ring aza-crown (4, 5, 8–13, 17, 18) and lariat ethers (6, 7) incorporating glucose or galactose units have been synthesized in good yields by a simple route. Their extracting abilities were measured with Li⁺, Na⁺, K⁺ and NH ₄ ⁺ cations. The substituents at the nitrogen atom and the type of monosaccharide affected this property significantly.     

Anionic lanthanide shift reagents. Application to the assignment of NH peaks of amidinium ions

Anionic EDTA complexes of Pr(III) and Eu(III) produce downfield and upfield shifts, respectively, of the NH resonances of amidinium ions, RC(NH₂)₂⁺. These LnEDTA^? -induced shifts arise through the formation of weak ion pairs. Results for the acetamidinium ion show that H_z, the high field NH, experiences a greater shift than H_E. This result confirms the expectation that in the ion pair the anionic shift reagent approaches closer to H_z. Thus LnEDTA^?-induced shifts can be used to assign NH peaks of amidinium ions. Such shifts are then used to assign the low field NH peak of the benzamidinium ion to H_E, whereas the low field NH peaks of the azoisobutyramidinium ion and formamidinesulfinic acid are assigned as H_z.     

Cuprous complexes and dioxygen. Part X. Influence of ring size on the autoxidation of Cu(I) complexes with cis-and trans-N2S2 macrocyclic ligands

The autoxidation of the Cu⁺ complexes with a series of six 12-, 14- and 16-membered macrocycles containing the N₂S₂ set of donor atoms has been studied with an oxygen electrode and with the stopped-flow technique. Inspite of the identical set of coordinating groups, the reactivity of the Cu⁼ complexes towards O₂ varies by more than 5 orders of magnitude, with rate constants between <0.1 and 2·10⁴ M^?1S^?1. Simple bimolecular rate laws are obtained from initial rates of autoxidation, but successive one-electron transfers with intermediate formation of superoxide are implied from the analysis of complete reaction curves for complexes with the 12-membered macrocycles. The kinetic parameters are compared with the redox potentials for the corresponding CuL²⁺/CuL⁺ couples. Only a rather rough correlation is found and steric factors must in addition be responsible for the observed reactivity pattern which shows a decrease of autoxidation rate with increasing size of the macrocyclic ring. No systematic effect was observed for the influence of cis- vs. trans-configuration of the donor atoms.     

Structures of 18-crown-6, 15-crown-5 and their metal complexes in methanol solution as studied by Raman spectroscopy

Raman spectra of 18-crown-6, 15-crown-5, and their complexes with Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺ and Ba²⁺ have been investigated in methanol solution. Normal coordinate calculations are presented for the D_3d⁻ and C_i-symmetric structures of 18-crown-6. Analysis of the Raman spectra gives information on the ring conformations of the crown ethers and the stoichiometry of complexation. The uncomplexed ethers adopt diverse conformational states in methanol at room temperature, the D_3d state being the most stable in 18-crown-6 and various states equally stable in 15-crown-5. Most of the cations form 1:1 or 2:1 crown—metal complexes depending on the cation size relative to the hole size of crown. Exceptionally, Cs⁺ forms both 1:1 and 2:1 complexes with 18-crown-6. The ring structures in complexes of 18-crown-6 are not much distorted from the D_3d one, though the distortion is rather large in the 2:1 Cs²⁺ and 1:1 Ca⁺ complexes. Complexes of 15-crown-5 exhibit for types of ring structure depending on the size and charge of the cation. These structures are likely to involve distortion from the gauche or gauche′ conformation about the CC bonds and various conformation about the CO bonds.     

Ring and nitrogen inversion in a flexible highly symmetrical molecule: The conformations of 1,3,7,9,13,15,19,21-octaazapentacyclo[19,3,1,1,3,7,19,13,115,19]

The possible conformations of the title compound and their modes of interconversion via ring and nitrogen inversion processes are delineated. At high temperatures (> +80°) the ¹H NMR spectra are consistent with time averaged D_4th symmetry and rapid ring and nitrogen inversion. At lower temperatures (ca. ?10°) the time-averaged symmetry if D_2d and inversion of the 6-membered rings is frozen out, nitrogen inversion remaining rapid. The free energy of activation for the total inversion of all four 6-membered rings is 13·5 kcal mole ^?1, higher than in similar monocyclic systems. This higher energy is a reflection of the multiple ring inversion pathway required for total inversion of all the 6-membered rings.     

Ionen : V. Das tetraphenyloborat-ion als kernresonanz-verschiebungsreagens bei arsonium- und stibonium-kationen

Arsonium and stibonium cations form contact ion pairs even with very large counterions in chlorohydrocarbons. In the tetraphenyloborates of these cations, the aromatic ring current of the anion phenyl rings causes the NMR signals of protons in α-position of As and Sb t move upfield 1 to 3 ppm. The ion pairs are short-lived so that the B(C₆H₅)^?₄ ion has the characteristic properties of an NMR shift reagent.     

NMR assignments and the acid–base characterization of the pomegranate ellagitannin punicalagin in the acidic pH-range

In exploring the capability of nuclear magnetic resonance (NMR) spectroscopy for pomegranate juice analysis, the eight aromatic singlet resonances of α- and β-punicalagin were clearly identified in the ¹H NMR spectra of juice samples. The four downfield resonances were found to be sensitive to small pH changes around pH 3.50 where the NMR spectra of the juice samples were recorded. To understand this unusual behavior, the ¹H and ¹³C resonance assignments of the punicalagin anomers were determined in aqueous solution and pH titrations with UV and ¹H NMR detection carried out to characterize the acid–base properties of punicalagin over the pH range 2–8. Simultaneous fitting of all of the pH-sensitive ¹H NMR signals produced similar but significantly different pK _a values for the first two deprotonation equilibria of the gallagic acid moiety of the punicalagin α- (pK _a1?=?4.57?±?0.02, pK _a2?=?5.63?±?0.03) and β- (pK _a1?=?4.36?±?0.01, pK _a2?=?5.47?±?0.02) anomers. Equivalent pK _a values, (α?:?6.64?±?0.01, β?:?6.63±?0.01) were measured for the third deprotonation step involving the ellagic acid group, in good agreement with a prior literature report. The punicalagin anomer equilibrium readjusts in parallel with the proton dissociation steps as the pH is raised such that β-punicalagin becomes the most abundant anomer at neutral pH. The unusual upfield shifts observed for the glucose H3 and H5 resonances with increasing pH along with the shift in the α/β anomer equilibrium are likely the consequence of a conformational rearrangement.

Formation and structure of [C8H8O]+˙ ions,generated from gas phase ions of phenyl-cyclopropylcarbinol and 1-phenyl-1-(hydroxymethyl)cyclopropane

The structure and formation of [C₈H₈O]^+. ions generated from phenylcyclopropylcarbinol and 1-phenyl-1-hydroxymethylcyclopropane upon electron impact, have been studied using kinetic energy release measurements, by determination of ionization and appearance energies and by collisional activation. It is shown that the non-decomposing [C₈H₈O]^+˙ ions have exclusively the structure of the enol ion of phenylacetaldehyde, although it is less stable than the enol ion of acetophenone by about 45 kJ mol^?1. This has been interpreted as an indication that the [C₈H₈O]^+˙ ions from phenylcyclopropylcarbinol are formed by an attack of either the phenyl ring or the hydroxyl group upon the C-1? C-2 (or C-1? C-3) bond of the cyclopropane ring under a simultaneous expulsion of ethene and migration of the attacking group to the C-1 position. The [C₈H₈O]^+˙ ion from 1-phenyl-1-(hydroxymethyl)cyclopropane is formed by opening of the cyclopropane ring via a benzylic cleavage. A kinetically controlled hydrogen shift in the resulting ring opened ion prior to or during ethene loss then leads to the formation of [C₈H₈O]^+˙ ions which have the structure of the enol ion of phenylacetaldehyde.     

Concerning some unusual trimethylsilyl proton chemical shifts

Proton and ¹³C NMR data are presented for six different compounds containing the fragment C₆H₅? C? CH₂SiMe₃. In a number of instances it was observed that, in the ¹H NMR spectrum, the SiMe₃ groups had a chemical shift significantly upfield from internal tetramethylsilane (δ = ?0·14 to ?0·36). These unexpected upfield chemical shifts of the SiMe₃ groups are suggested to result from the predominance, on a time averaged basis, of conformations which place the methyl groups attached to silicon in the face of an aromatic ring. The preference for such conformations is, in turn, the result of rotational preferences exhibited by the ‘flat’ aromatic ring. These results suggest that conformational analysis of systems containing a phenyl ring should take more explicit account of the fact that the preferred orientation of this phenyl ring can have a profound influence on the conformation adopted by the remainder of the molecule. In addition, the preferred conformation of the phenyl ring can have a significant effect upon the observed ¹H NMR chemical shifts, while the ¹³C chemical shifts are relatively insensitive to conformational factors and can be explained by well-known substituent effects previously delineated for all-carbon systems.     

Molecular orbital calculations on transition metal complexes : XXIII. 1H nmr shifts in sandwich,mixed sandwich,and related tricarbonyl complexes

INDO SCF MO calculations have been carried out for a variety of diamagnetic sandwich, mixed sandwich, and related tricarbonyl complexes of the 3d series, and for the free ligand systems of the cyclopentadienyl anion (Cp^?), the neutral benzene molecule (Bz), and the cycloheptatrienyl cation (Ch⁺). The π bond orders for the CC linkages of the ligand rings all show significant, but broadly comparable, reductions on complexation, and the ¹H NMR shift for a given ring proton, relative to that for the appropriate free ligand, Δτ, shows a good linear correlation with the corresponding change in the charge density at that proton, ΔPππ(H). The plot of Δτ against ΔPππ(H) shows a positive (upfield) intercept of about 2.5 ppm on the Δτ axis, and it is concluded that the results provide evidence for an appreciable diminution in the aromatic character of the ligand rings on complex formation.     

Synthesis, X-ray structures, and catalytic applications of palladium(II) complexes bearing N-heterocyclic iminocarbene ligands

Two new Pd(II) N-heterocyclic iminocarbene complexes (C-N)PdCl₂ that contain 5-membered chelate rings have been prepared by carbene transfer from a silver iminocarbene precursor to (COD)PdCl₂. The new Pd imonocarbene complexes, as well as two that have been previously reported (altogether three 5-membered and one 6-membered chelate ring complexes) have been evaluated as catalysts for the Suzuki-Miyaura coupling reaction. The complexes were found to be active in the reaction, but without exceptional catalytic performances. The 5-membered chelate ring complexes appeared to be more robust and remained active for a longer time than the 6-membered ring congener. The catalytic performance of the 5-membered chelate ring complexes appeared to be rather insensitive to the steric demands of the imine-N-aryl group. The X-ray structure of one of the Ag iminocarbene complexes reveals the κ¹(C) bonding of the iminocarbene moiety in a nearly linear Ag(I) complex; two monomeric units are associated through a weak Ag-Ag interaction. The X-ray structures of two new Pd iminocarbene complexes (C-N)PdCl₂ confirm the chelating κ²(C,N) nature of the iminocarbene moiety; in both complexes, the Pd-Cl distances trans to carbene-C are slightly longer than those trans to imine-N.     

Isomerisierung von Methoxy- und Carbomethoxysubstituierten Cycloalkyl- und Alken-radikalkationen

The very complex isomerization patterns of methoxy and carbomethoxy substituted cycloalkanes (3- to 7-membered rings) have been investigated using collisional activation, metastable ion characteristics and field ionization kinetics. The extent of isomerization depends on both the ring size and the substituent. Irrespective of the electronic properties of the substituent, ring opening involves exclusively the C-1? C-2 bond whereby linear alkene radical cations are formed. In the case of OCH₃- and COOCH₃ substituents the position of the resulting double bond (terminal or α,β-unsaturated) is determined more by the ring size of the precursor molecules and less by the electronic properties of the substituents. Contrary to these findings alklyl substituted cycloalkanes (3- to 5-membered rings) rearrange exclusively to terminal alkene radical cations. The barrier for double bond isomerization seems to be substantially influenced by substituents.     

Substituent Effects on 31P and 13C Chemical Shifts of Substituted Diphenyl 1-anilino-1-arylmethanephosphonates and Their Anions

Abstract

For a series of 31 novel diphenyl 1-anilino-1-arylmethanephosphonates, substituted in the meta and para position of the anilino and/or the aryl ring, ³¹P chemical shifts show good linear correlation with Taft's [sgrave]° parameters, the ³¹P nucleus appearing better shielded in the case of electron-withdrawing substituents. This inverse relationship is due to a field effect of the substituent dipole which polarizes π-electron clouds in the molecule, resulting in a higher P=O double bond order, and thence better ³¹P shielding. A corresponding shift of π-electron density is likewise observed for the ¹³C resonances of the two diastereotopic phenoxy and the anilino or aryl rings, respectively, where-M ands-I substituents cause a downfield shift of para and meta, and an upfield shift of ortho and ipso carbon resonances.     

Effects of Ring Size on Spectral Properties of Copper(II)-Diaminodiamide Complexes in Aqueous Solution

The electronic absorption spectra of aqueous solutions containing copper(II) ion and N, N′-bis(carbamoylmethyl)trimethylenediamine(L-1, 3, 1) at 25.0 ± 0.1°C and 0.10 mol L^?1 NaClO₄, were measured as a function of pH. The absorption maxima and the molar absorptivities of [Cu(L-l, 3, l)]²⁺, [Cu(H.₁L-1, 3, 1)]⁺, and [Cu(H_-2L-l, 3, l)] were calculated from these spectra. A comparison of the spectral parameters of these complexes with those of the copper(lI)-diaminodiamide complexes containing 6, 5, 6-and 6, 6, 6-membered ring systems indicates that the chelate ring size has significant effects on these spectral parameters.     

NMR studies of some second and third row transition metal complexes of monothio-β-diketones

A ¹³C and ¹⁹F NMR study of twenty-four ruthenium, rhodium, palladium and platinum complexes containing a difluoromethyl or a trifluoromethyl substitutent(R′) on the monothio-β-diketone, RCSCH₂COR′, is reported. The R-substituents are 2′-thienyl, 2′-naphthyl, phenyl, p-fluorophenyl or p-methylphenyl. The ¹³C NMR data show the chemical shift of the diketonate ring carbons to be geometry dependent. Similarly, the ¹⁹F NMR spectra show chemical shift data which are also metal dependent. The thiocarbonyl and methine carbon's shieldings are also dependent on the nature of the R-group. The rhodium and platinum complexes show carbon-metal and carbon-fluorine spin coupling. The paramagnetic ruthenium(III) complexes give ¹⁹F NMR spectral resonances which are broad and shifted upfield from the corresponding diamagnetic rhodium, palladium and platinum complexes. ¹³C and ¹⁹F NMR data supports a facial octahedral geometry for the rhodium(III) complexes.     

Conformational Behaviour of Substituted 2-Methoxy-2-Oxo-1,2-Oxaphospholan-3-Ols

Abstract

Conformational behaviour of about 30 2-methoxy-2-oxo-1,2- oxaphospho l an-3-0 1 s containing various substituents was examined by ¹H and ¹³C NMR. Vicinal coupling constants J(HCCH), J(HCCP), J(HCOP), J(CCOP) and J(CCCP) were employed in this study. Conformation of the 1,2-oxaphospholane ring is governed almost exclusively by substituents at C-3, C-4 and C-5, as we l l as by their orientation. The configuration of the P atom has little or no influence on conformation of the ring in diastsreomeric pairs. Strong preference of phenyl, methyl and substituted methyl groups to occupy the equatorial or pseudoequatoria l positions was observed for all but one compounds studied. In the cis-fused bicyclic syst ems conformat ionally rigid 6-membered rings forced the 1,2-oxaphospholane rings to adopt an enve l ope-l ike (E₄) conformation. No influence of the p=o……HO-C-3 hydrogen bond on conformation of the 1,2-oxaphospholane ring was found. Preferred conformations for (2R, 3R, 4R)-3-(hydroxymethyI)-2-methoxy-2-oxo-1,2-oxaphospho lane-3,4-diol and its triacetate are shown below.     

Temperature dependent NMR spectra of oxazepines,thiazepines, oxazocines and thiazocines

The temperature dependence of several 7-membered 5-substituted-5,11-dihydrodibenz[b,e] [1,4]oxazepines (Z = O) and thiazepines (Z = S), as well as of 8-membered 12-substituted -6,11-dihydro-12H-dibenz[b,f] [1,4]oxazocines (Z = O) and thiazocines (Z = S), has been studied by NMR spectroscopy. The average ΔF* (kcal/mol) values at coalescence temperatures are 14·0 (M = CHO), 18·2 (M = COCH₃) for oxazepines, 17·3 (M = CHO), >21 (COCH₃) for thiazepines, and 13·0 for oxazocines and 13·3 for thiazocines, respectively. These data show that replacement of oxygen by a sulfur atom raises the barrier height of ring inversion, but that this effect is greater with the 7-membered than with 8-membered heterocycles. Thus the 8-membered rings have a greater degree of flexibility than do the 7-membered ones. With the oxazepines and the thiazepines, it was also possible to measure the thermodynamics (ΔF*, ΔH* and ΔS*) of the hindered rotation around the N? CO bond by studying the temperature dependence of the formyl and acetyl proton resonances. The results suggest that the processes of conformational inversion and hindered rotation in the same molecule are independent of each other. The rate of hindered rotation is slower than the rate of conformational inversion. The conformational preferences of the compounds are also discussed.     

Liquid-liquid extraction of copper(I) by cyclic tetrathio ethers

The liquid-liquid extraction of copper(I) with 12-, 13-, 15- and 16-membered cyclic tetrathio ethers ([n]aneS₄, where n represents the total number of carbon and sulphur atoms in the cyclic ligand ring) was examined stoichiometrically using picrate ion (Pic^?) for the formation of the ion pair. Copper(I) was extracted with four ligands (L) into 1,2-dichloroethane as the ion-pair compound, [Cu(I)L]⁺Pic^?. The extraction constant, K_ex, with each ligand was determined. As the ring size of cyclic tetrathio ethers increases, the log K_ex values, including that previously reported for [14]aneS₄, increase from 7.7 to 9.4. The value of Δ log K_ex, which represents the increase in log K_ex due to the addition of one carbon atom to the macrocyclic ring, was large between [13]aneS₄ and [14]aneS₄ (Δ log K_ex=1.0) and small between [14]aneS₄ and [15]aneS₄ (Δ log K_ex=0.1).     

Stabilities in Water of Alkali Metal Ion Complexes with Dibenzo-24-crown-8 and Dibenzo-18-crown-6 and Their Transfer Activity Coefficients from Water to Nonaqueous Solvents

Stability constants K _ML for the 1:1 complexes of Na⁺, K⁺, Rb⁺, and Cs⁺ with dibenzo-24-crown-8 (DB24C8) and dibenzo-18-crown-6 (DB18C6) in water have been determined by a capillary electrophoretic technique at 25°C. The K _ML sequence is Na⁺ < K⁺ < Rb⁺ < Cs⁺ for DB24C8 and Na⁺ < K⁺ > Rb⁺ > Cs⁺ for DB18C6. Compared with DB18C6, DB24C8 exhibits higher selectivity for K⁺ over Na⁺, but lower selectivity for K⁺, Rb⁺, and Cs⁺. To evaluate the solvation of the complexes in water, their transfer activity coefficients sH₂O between polar nonaqueous solvents and water have been calculated. The sH₂O values provide the following information: interactions with water of the metal ions and of the crown-ether oxygens are greatly reduced upon complexation and the complexes undergo hydrophobic hydration in water; the character of each alkali metal ion in solvation is more effectively masked by DB24C8 than by DB18C6, because of the larger and more flexible ring structure of DB24C8. Solvent effects on the complex stabilities are discussed on the basis of the sH₂O values.