Rotational isomer stabilities of 1,2-dichloropropane in the liquid and vapour phases from a reconsideration of available nuclear magnetic resonance and

Reconsideration of data from an earlier nuclear magnetic resonance study of 1,2-dichloropropane indicates that previously reported energy differences of the two G isomers compared to the T isomer (C-Cl bonds trans) should be reassigned for both the liquid and vapour states, i.e., so that the apparently more sterically hindered isomer with the primary chlorine atom trans to hydrogen is of lower energy than the isomer in which the chlorine atom is trans to the methyl group. Using this new assignment and the experimental average coupling constants, the trans and gauche vicinal coupling constants between the methylene and methyne protons are calculated to be 10.9 and 4.4 Hz respectively. It is further proposed that the P'H (primary chlorine trans to hydrogen) carbon-chlorine stretching vibration occurs at ca. 720 cm^?1, about 60 cm^? higher than hitherto expected.     

13C n.m.r. spectra of cyclopropenes

¹³C n.m.r. chemical shifts and one bond ¹³C, ¹H coupling constants for cyclopropene and its 1- and 3-methyl derivatives as well as for methyl cyclopropane have been measured. The data for cyclopropene are 108·9 ppm and 228·2 Hz, 2·3 ppm and 167·0 Hz in the olefinic and allylic position, respectively. Substituent effects for the methyl group are discussed.     

Self-aggregation of a synthetic zinc chlorophyll derivative possessing a 13-dicyanomethylene group as a light-harvesting antenna model

Zinc methyl 3-hydroxymethyl-13¹-dicyanomethylene-pyropheophorbide-a, one of the bacteriochlorophyll-d analogs, was prepared by modifying chlorophyll-a. The semi-synthetic compound self-aggregated in an aqueous Triton X-100 solution to give large oligomers with red-shifted and broadened electronic absorption bands. The J-aggregates were similar to self-aggregates of bacteriochlorophyll-d molecules in natural light-harvesting antennas of green photosynthetic bacteria. The strongly electron-withdrawing dicyanomethylene group was an alternative functional moiety of the hydrogen-bond accepting 13¹-oxo group which had been necessary for such self-aggregation.     

C13-Methylation of methyl pheophorbide a and stereoselective preparation of methyl (13R)-methylpyropheophorbide a

The (13²R)-methoxycarbonyl group of methyl pheophorbide a, one of the chlorophyll-a derivatives, was converted to a methyl group through methylation at the C13² position followed by removal of the methoxycarbonyl group. The methylation of the C13² carboanion gave a 4:1 mixture of methyl 13²-methyl-pheophorbide a and its 13²-epimer. The successive pyrolysis of the major methylated product afforded methyl (13²R)-methyl-pyropheophorbide a with a small amount of its (13²S)-epimer. The substitution effects at the C13² position including stereochemistry were discussed on the basis of 1D/2D NMR, UV–vis absorption, and circular dichroism spectroscopic analyses as well as molecular modeling simulation.     

Enantioselective addition of methyl group to aldehydes catalyzed by titanium complex of fluorous ligand

Titanium complex of fluorous axially dissymmetric ligand ((Ra^∗)-2,2′-bis[(R^∗)-perfluoro-1H-1-hydroxyoctyl]biphenyl, ((Ra^∗)-(R^∗)₂-1)) having perfluoroheptyl carbinol moieties catalyzed enantioselective addition of a methyl group to aldehydes. Dimethylzinc prepared in situ by mixing ZnCl₂ and methyl magnesium bromide without removal of magnesium salt was more reactive for addition of methyl group than dimethylzinc without magnesium salt, and gave up to 99% ee of the products. The high fluorine content of 1 enables it to be recoverable by the selective extraction with perfluorohexane.     

13C, 1H spin coupling constants of dimethylacetylene

¹³C, ¹H spin coupling constants of dimethylacetylene have been determined by the complete analysis of the proton coupled ¹³C NMR spectrum. For the methyl carbon ¹J(CH) = + 130.6₄ Hz and ⁴J(CH) = + 1.5₈ Hz, and for the acetylenic carbon ²J(CH) = ? 10.34 Hz and ³J(CH) = +4.30 Hz. The ⁵J(HH) long-range coupling constant (+2.79 Hz) between the methyl protons was also determined.     

Synthesis of Platinum(II) N-Heterocyclic Carbenes Based on Adenosine

Organometallic derivatization of nucleosides is a highly promising strategy for the improvement of the therapeutic profile of nucleosides. Herein, a methodology for the synthesis of metalated adenosine with a deprotected ribose moiety is described. Platinum(II) N-heterocyclic carbene complexes based on adenosine were synthesized, namely N-heterocyclic carbenes bearing a protected and unprotected ribose ring. Reaction of the 8-bromo-2′,3′,5′-tri-O-acetyladenosine with Pt(PPh₃)₄ by C8−Br oxidative addition yielded complex 1, with a Pt^II centre bonded to C-8 and an unprotonated N7. Complex 1 reacted at N7 with HBF₄ or methyl iodide, yielding protic carbene 2 or methyl carbene 3, respectively. Deprotection of 1 to yield 4 was achieved with NH₄OH. Deprotected compound 4 reacted at N7 with HCl solutions to yield protic NHC 5 or with methyl iodide yielding methyl carbene 6. Protic N-heterocyclic carbene 5 is not stable in DMSO solutions leading to the formation of compound 7, in which a bromide was replaced by chloride. The cis-influence of complexes 1–7 was examined by ³¹P{¹H} and ¹⁹⁵Pt NMR. Complexes 2, 3, 5, 6 and 7 induce a decrease of ¹J_Pt,P of more than 300 Hz, as result of the higher cis-influence of the N-heterocyclic carbene when compared to the azolato ligand in 1 and 4.     

Ring-size controlled dimerization of synthetic zinc chlorophyll derivatives possessing a 1-azacycloalkyl group through mutual coordination of amino moiety to central zinc atom

Zinc methyl 3-devinyl-pyropheophorbides-a bearing a (1-azacycloalkyl)methyl group at the 3-position were prepared by chemically modifying naturally occurring chlorophyll-a. The azacyclobutylated molecules closely dimerized through double coordination of the 3¹-nitrogen atom to the central zinc atom in 1%(v/v) dichloromethane and hexane to give red-shifted visible absorption and reverse S-shaped circular dichroism (CD) bands. The azacyclopentyl analog formed similarly π–π interacted and A-ring overlapped dimers with relatively weakened and lengthened Zn–N bonds to show less split excitonic bands, and its cofacial and anti-parallel supramolecule giving S-shaped CD bands was inversely twisted by the insertion of a methylene group in the 1-azacycloalkyl moiety. One more methylene insertion as in the azacyclohexyl derivative induced a least coordination ability and no more dimerization, so the compound with the most sterically crowded 3¹-N was monomeric in the less polar organic solvent. Steric factor at around the 3¹-nitrogen atom controlled the dimerization and affected the supramolecular structure with excitonically coupling visible absorption and CD bands.     

Conformational preferences of 2-methoxy-acetophenone

On the basis of the presence or absence of long-range spin–spin coupling constants between side–chain and ring nuclei in 2-methoxyacetophenone, some literature ambiguities about the conformational preferences of the side-chains in this compound can be resolved. The long-range coupling between the methoxy protons and the ring proton ortho to the methoxy group, ⁵J(H, CH₃)o, is (?)0.28 ± 0.02 Hz, as expected for a conformation in which the methoxy group lies in the benzene plane and cis to H-3. The methyl protons of the acetyl group do not couple to H-6, implying that this methyl group does not approach H-6 closely. However, the ¹³C nucleus of this methyl group couples by +0.4 Hz to H-5 and not to H-3. This stereospecific five-bond coupling implies that the acetyl group predominantly prefers an arrangement in which the carbonyl group lies trans to the other substituent, as would be expected electrostatically. Large twists out of the ring plane are not consistent with the observed couplings.     

Spectroscopic studies of the monosubstituted amides. VI. Neutron inelastic scattering spectra of N-methyl acetamide

We report the determination of the barriers to methyl group rotation in N-methyl acetamide. These were, for N-methyl V₃ = 590 cm^?1, and for C-methyl V₃ = 670 cm^?1. Some previous spectral assignments of this compound have been confirmed and one new band at about 160 cm^?1 is shown to involve the motion of both methyl groups.     

Asymmetric synthesis of methyl bacteriopheophorbide-d and analogues by stereoselective reduction of the 3-acetyl to the 3-(1-hydroxyethyl) group

Asymmetric borane-reduction of 3-acetyl-13¹-oxo-tetrapyrroles (=bacteriochlorin, chlorin and porphyrin) in the presence of a chiral source gave selectively chiral 3-(1-hydroxyethyl)-13¹-oxo-tetrapyrroles. Oxazaborolidines were effective as chiral auxiliaries. Reduction with (S)-oxazaborolidines led to the (3¹S)-alcohol as the major product, whose stereoselectivity was the opposite of that in the same asymmetric reduction of usual prochiral ketones ArCOR. The asymmetric reduction of the 3-acetyl group in methyl bacteriopyropheophorbide-a and the 7,8-oxidation afforded 3¹-epimeric methyl (3¹S)-bacteriopheophorbide-d (89% de).     

A 1H NMR and molecular modelling investigation of diastereotopic methylene hydrogen atoms

The ¹H NMR spectra of methyl 3‐bromo‐2‐methylpropionate (1a) and the corresponding chloro compound (2a) show no long‐range coupling between the methyl and methylene protons. In contrast, in the analogous dihalocompounds, methyl 2,3‐dibromo‐2‐methylpropionate (1b) and methyl 2,3‐dichloro‐2‐methylpropionate (2b), one of the methylene protons exhibits a large ⁴J_HH coupling (0.8 Hz) to the methyl group, but the other proton shows no observable splitting. This can be explained quantitatively by calculations of the conformational preferences in these compounds combined with the known orientation dependence of the ⁴J_HHcouplings. One conformer predominates in the dihalo compounds 1b and 2b, and this is responsible for the ⁴J_HH coupling. In 1a and 2a all three conformers are populated and the ⁴J_HH couplings average to zero. The technique is a potentially general method of unambiguously assigning diastereotopic methylene protons. Copyright © 2002 John Wiley & Sons, Ltd.     

A Raman spectroscopic study of the low-frequency vibrations in anisole and anisole-d3

Low-frequency Raman spectra of solid anisole and of solid anisole-d₃ have been recorded at 130 K. The phenyl torsion observed at 148 cm^?1 is shifted to 133 cm^?1 upon deuteration of the methyl group. The twofold torsional barriers calculated from these frequencies are 4033 ± 110 cm^?1 and 4094 ± 123 cm^?1 indicating that coupling to other low-frequency modes in both cases is of the same order of magnitude. The methyl torsional mode was observed at 285 cm^?1 in the spectrum of solid anisole and at 183 cm^?1 in the spectrum of anisole-d₃. The threefold barriers calculated using these frequencies are 1847 ± 20 cm^?1 and 1465 ± 18 cm^?1 respectively. These barrier values indicate that the methyl torsion is coupled to another low-frequency mode. A doublet centered at 230 cm^?1 in anisole is shifted to 245 cm^?1 in anisole-d₃; it is proposed that this is due to a ring mode coupled to the methyl torsion. The splitting is interpreted as an example of Davydov splitting.     

Negative solvation of diamagnetic ions in methanol

Analysis of intramolecular contributions calculated from selective T₁ measurements for hydroxyl and methyl protons of methanol solutions of diamagnetic salts provides evidence of a negative solvation effect for I^?, ClO₄^? and CNS^? ions. At low concentrations the microdynamic behaviour of the hydroxyl group is strongly affected whereas that of the methyl group appears to be unchanged.     

Application of chemical shifts in 1H and 13C NMR spectra to configuration assignment of Schiff bases in the liquid phase

In ¹H and ¹³C NMR spectra of N-substituted dimethylketimines chemical shifts of protons and carbon atoms of the methyl groups in the cis-position with respect to the unshared electron pair of the nitrogen are larger than those of the CH₃ groups in the trans-position by 0.2–0.4 and 8–11 ppm respectively. This effect is accompanied by the reduction of the corresponding direct spin-spin coupling constant ¹³C-¹³C by 10 Hz. The experimental trends in the variation of the spectral parameters are well reproduced by ab initio quantum-chemical calculations. The discovered stereochemical dependence of the chemical shifts of ¹H and ¹³C may underlie a simple and efficient method of the configuration assignment in various compounds with a C=N bond.     

Molecular conformations of methyl formate and methyl vinyl ether from ab initio molecular orbital calculations

Ab initio molecular orbital theory with minimal and extended basis sets and a flexible rotor geometric model has been used to investigate the rotational potential surfaces of methyl formate and methyl vinyl ether. For both molecules, the most stable structures (IA and IIA, respectively) are planar cis; additional potential minima are found which correspond to planar trans structures (IB and IIB). The latter lie respectively about 4—8 and 1—2 kcal mol^?1 above the corresponding cis rotational isomers. Methyl rotational barriers have been determined for cis and trans structures of each molecule. For trans methyl formate, there is a slight but unexpected preference for an eclipsed arrangement of the methyl group.     

A suggested revised conformation of methylthionitrite as seen by its spectra and preliminary ab initio calculations

A proof has recently been given that gaseous methylthionitrite, CH₃SNO, occurs exclusively (or mainly) in its anti conformation [1]. The present paper claims that existing spectral evidence and ab initio calculations now performed suggest that gaseous methylthionitrite is mainly the syn conformer, the extra stability being of the order 1–2 kcal mol^?1. Methyl group rotation in the syn conformer is hindered by a three-fold barrier of height 689 cm^?1 while the methyl group rotation in the anti conformer is hardly hindered. The syn/anti energy difference and the barriers hindering methyl group rotation closely parallel the corresponding measured quantities in methylnitrite, CH₃ONO.     

Synthesis and photophysical properties of phenyl-sulfanylated chlororophyll derivatives

Chlorophyll derivatives that possessed a phenylsulfanyl group at the C3¹- or C3²-position were synthesized and their optical properties were investigated. Methyl 3¹-phenylsulfanyl-mesopyropheophorbide-a was prepared by substitutions of the corresponding C3¹-hydroxy-chlorin, methyl bacteriopheophorbide-d, with thiophenol in the presence of zinc iodide or of the corresponding C3¹-bromo-chlorin with thiophenol. The regioisomeric C3²-phenylsulfanyl-chlorin was obtained by addition of thiophenol to the C3-vinyl group of methyl pyropheophorbide-a in the presence of AIBN. Both the synthetic compounds gave similar electronic absorption and emission spectra in chloroform, but fluorescence quantum yield of the C3¹-sulfanyl-chlorin (0.18) was ca. 30% smaller than those of the C3²-sulfanyl-chlorin (0.25) and the C3-ethyl-chlorin (0.24). These observations were consistent with their fluorescence lifetime data. It is suggested that the heavy atom effect of a sulfur atom at the C3¹-position can tune photophysical properties of the chlorophyll derivatives.     

The first example of linear peptides containing a N-trifluoroethylated backbone amide linkage and the surprising solution dynamics observed by F NMR

The α-amino group of (l)phenylalanine methyl ester was trifluoroethylated using (2,2,2-trifluoroethyl)phenyliodonium N,N-bis(trifluoromethylsulfonyl)imide. A dipeptide Gly(l)Phe containing a trifluoroethylated peptide bond was synthesized by removing the α-amino proton of N^α-trifluoroethyl (l)phenylalanine methyl ester followed by coupling with N^α-phthaloyl glycine acid fluoride. The dipeptide was further coupled with (l)leucine methyl ester under conventional carboxyl activation conditions to provide two diastereomers of the tripeptide Gly(d,l)Phe(l)Leu. The solution dynamic behavior of the tripeptide was investigated as a function of solvents, by NOESY and variable temperature (VT) ¹⁹F NMR experiments.     

Synthesis of both RP and SP enantiomers of unsymmetrical methylphosphonates based on a new approach utilizing a P-ester bond with α-hydroxyacids

Condensation of methyl methylphosphonochloridate with the dilithium salt of 2-methyllactic acid gave P-racemic methylphosphonates which unexpectedly contained two units of α-hydroxyacid linked via carboxylic ester bond. The racemic mixture was chromatographically separated via diastereomeric salts with quinine or cinchonine to give, based on the X-ray analysis, pure (R_P)-(+) and (S_P)-(−) enantiomers. Both enantiomers were immobilized on the ArgoGel^®–OH solid support. Condensation of methyl methylphosphonochloridate with α-hydroxyacid methyl esters [2-methyllactate, (S_C)-(−)-lactate, methyl (S_C)-(+) and (R_C)-(−)-mandelates] gave chromatographically inseparable 1:1 mixtures of diastereomers in 63–69% yields. A basic hydrolysis of the latter resulted in a selective and unexpected cleavage of the P–OMe group in a quantitative yield [(S_C)-(+) and (R_C)-(−)-mandelates, (S_C)-(−)-lactate] or simultaneous cleavages of P–OMe and C(O)OMe groups (2-methyllactate).