Organoaluminium compounds. XVI. The mechanism of exchange between trimethylaluminium and triphenylaluminium in the presence of pyridine

1968 ◽  
Vol 21 (6) ◽  
pp. 1497 ◽  
Author(s):  
EA Jeffery ◽  
T Mole
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Phenyl Group ◽  
Large Excess ◽  
Methyl Group ◽  
Kinetics Of ◽  
Bimolecular Mechanism

The kinetics of exchange of a methyl group of trimethylaluminiunl for a phenyl group of triphenylaluminium have been studied by the use of proton magnetic resonance (p.m.r.) spectroscopy. An excess of pyridine was always present. Results obtained using a large excess support a bimolecular mechanism without loss of pyridine from the organoaluminium complexes prior to exchange. When the excess of pyridine is small, the rate of exchange depends on the concentration of excess pyridine. Here reaction may proceed by a mechanism in which loss of pyridine from one of the organoaluminium complexes occurs prior to exchange.

Conformations of Chlorophylls a and a′ and their Magnesium-Free Derivatives as Revealed by Circular Dichroism and Proton Magnetic Resonance

1981 ◽  
Vol 36 (8) ◽  
pp. 1000-1009 ◽  
Author(s):  
Paavo H. Hynninen ◽  
Gunnel Sievers
Keyword(s):  
Circular Dichroism ◽  
Magnetic Resonance ◽  
Electronic Absorption ◽  
Proton Magnetic Resonance ◽  
Nmr Spectra ◽  
Cd Spectra ◽  
Steric Strain ◽  
Methyl Group ◽  
H Nmr ◽  
Circular Dichroïsm

Abstract The electronic absorption (UV/VIS), circular dichroism (CD) and proton magnetic resonance (1H NMR) spectra have been recorded for C-10 epimeric chlorophylls a and a′, pheophytins a and a′ as well as pheophorbides a and a′. Although the epimers in each pair showed virtually identical UV/VIS spectra, their CD spectra were profoundly different and exhibited opposite signs at most wavelengths in the UV region. The differences were interpreted as arising, in part, from different C-10 configurations, and, in part, from conformational alterations induced by the steric strain in the crowded periphery of the macrocycle. The conformational alterations were also clearly indicated by the 1H NMR Δδ-values observed for the α,β and δ methine protons, the C-10 protons and most methyl group protons of the epimers in each pair. This was considered to imply changes in the geometry of the whole macrocycle. The Δδ-values were larger for the Mg-free epimers than for the chlorophyll epimers, which shows that the central Mg-atom makes the macrocycle more rigid. Correlations between the signs of the CD bands and configurations are discussed.

Carbon-13 Magnetic Resonance: the Stereochemistry of 1,2- and 1,3-Dimethyl-4-phenylpiperidine Derivatives

1973 ◽  
Vol 51 (11) ◽  
pp. 1782-1789 ◽  
Author(s):  
Alan J. Jones ◽  
A. F. Casy ◽  
K. M. J. McErlane
Keyword(s):  
Magnetic Resonance ◽  
Conformational Change ◽  
Solvent Effects ◽  
Chemical Shifts ◽  
Phenyl Group ◽  
Steric Effects ◽  
Methyl Group ◽  
Charge Polarization ◽  
Magnetic Resonance Spectra

The carbon-13 magnetic resonance spectra of the diastereoisomers of 1,2-dimethyl- (1) and 1,3-dimethyl-4-phenylpiperidin-4-ol (2), their esters and corresponding hydrochlorides have been determined. The observed chemical shifts can be ascribed to charge polarization or steric effects. The latter is particularly important in establishing the axial configuration of the 4-phenyl group and 3-methyl group in the preferred conformations of α-1 and β-2, respectively, and consequently the stereochemistry of these systems. Substituent additivity effects are similar to those in the piperidones and cyclohexanes. Solvent effects on 13C resonances remove the ambiguities of conformational change suggested in earlier proton studies.

Proton Magnetic Resonance Study of the Conformational Equilibria in Some Substituted N-Acetylpyrrolidines

1971 ◽  
Vol 49 (4) ◽  
pp. 639-643 ◽  
Author(s):  
C. M. Wong ◽  
J. Buccini ◽  
R. Schwenk ◽  
J. Te Raa
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Magnetic Resonance Study ◽  
Methyl Group ◽  
Conformational Equilibria ◽  
Proton Magnetic Resonance Study ◽  
Two Peaks

In the p.m.r. spectra of eight N-acetyl-2-(p-methoxybenzyl)-pyrrolidines, the acetamido methyl group gives rise to two peaks in the range 7.7–8.5 τ Compounds 9 and 10 also exhibit two peaks for the acetoxy methyl group in the same range. When the samples were heated, the pairs of peaks coalesced to give singlets. These results are interpreted as evidence for the existence of conformational equilibria.

Proton magnetic resonance spectra of vinylmetallic compounds. Part I. Tetravinyl and triphenylvinyl derivatives of Group IVB elements

1968 ◽  
Vol 46 (14) ◽  
pp. 2373-2384 ◽  
Author(s):  
Seán Cawley ◽  
Steven S. Danyluk
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Phenyl Group ◽  
Coupling Constants ◽  
Multiple Quantum ◽  
Low Field ◽  
Diamagnetic Anisotropy ◽  
Vinyl Group ◽  
Derivatives Of ◽  
Magnetic Resonance Spectra

A study has been made of the proton magnetic resonance spectra for all of the Group IVB derivatives of the series MVi4 and [Formula: see text]([Formula: see text] = phenyl group and Vi = vinyl group). The spectra were measured at 60 MHz as accurately as possible and the assignment of transitions was checked with a variety of supplemental aids including double irradiation, multiple quantum transitions, and medium effects Final, accurate spectral parameters were derived using both iterative and exact computational methods for solution of the three-spin problem Excellent agreement was obtained between the sets of parameters determined by the two methodsThe chemical shifts for both series of compounds display a number of characteristic trends of which the most notable is a displacement of the vinyl proton signals to low field with increasing atomic number of the M atom In each series the largest shift change is noted in going from the carbon to the silicon derivative These deshieldings have been attributed to the enhanced possibility of dπ–pπ interaction between the central M atom and the vinyl group in higher members of the series Marked changes are also the internal shifts of the vinyl protons down both series of compounds It is concluded that these changes are principally due to the effects of the M—C bond diamagnetic anisotropy The trends in internal shifts can be satisfactorily reproduced by the dipole approximation using Δχ values of 4, 6, 8, 12, and 18 × 10−6 cm3 mole−1 for the C, Si, Ge, Sn, and Pb–carbon bonds respectivelyThe signals for the vinyl protons of the [Formula: see text] series are all located to low field relative to the MVi4 series This deshielding is satisfactorily accounted for by the effects of the phenyl ring diamagnetic anisotropy in the former seriesA linear correlation is observed between the sums of the coupling constants and the electronegativities, Em, of the central M atom for both series of compounds However, the ΣJ values for the [Formula: see text] series are all slightly lower than the corresponding sums for the MVi4 series, indicating that the electronegativity of the phenyl group is somewhat larger than for the vinyl groupA significant solvent and concentration dependence is only noted for compounds belonging to the [Formula: see text] series For example, the trans proton of [Formula: see text] shifts up-field by 4 Hz while the cis proton is displaced down-field by 4 Hz as the concentration of [Formula: see text] is increased to 50 mole % in carbon tetrachloride These changes have been interpreted on the basis of a solute–solute interaction scheme of the type proposed previously for phenyl proton shifts.

Use of 13C in biosynthetic studies. Incorporation of acetate and formate into the fungal tropolone, sepedonin

1969 ◽  
Vol 47 (10) ◽  
pp. 945-949 ◽  
Author(s):  
J. Wright ◽  
D. G. Smith ◽  
A. G. McInnes ◽  
L. C. Vining ◽  
D. W. S. Westlake
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Methyl Group ◽  
Biosynthetic Studies ◽  
Magnetic Resonance Spectra

13C-labeled acetate and formate were administered to cultures of Sepedonium chrysospermum. The 13C-labeling patterns in sepedonin (3,6,9-trihydroxy-3-methyl-1,3,4,7-tetrahydrocyclohepta[c]pyran-7-one) were determined by measuring the relative intensities of the 13C–H satellites in the proton magnetic resonance spectra. C-1 was labeled by acetate-1-13C, and C-4, C-5, and the methyl group by acetate-2-13C. Only C-8 was labeled by formate-13C. Malonate-2-14C and succinate-2,3-14C were less efficient precursors than acetate-1-14C, acetate-2-14C, or formate-14C. The available evidence suggests that sepedonin is formed by methylation of a polyketide intermediate derived from acetate and malonate, followed by a stereospecific rearrangement and cyclization.

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