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Relevant academic research and scientific papers

Stereodinamics of N-Ethyl-N-methyl-2-aminobutane

N-Ethyl-N-methyl-2-aminobutane (EMAB) is the simplest acyclic trialkylamine which possesses a chiral carbon bonded to a chiral nitrogen.The chiral carbon is configurationally stable, while the chiral nitrogen is configurationally labile via nitrogen inversion.The 1H dynamic NMR (DNMR) spectra of three selectively deuteriated derivatives of EMAB show a decoalescence into four subspectra with relative populations of 49percent, 22percent, 17percent, and 12percent at 104 K.The DNMR behavior can be simulated in terms of a model which involves a relatively high barrier for interconversion between diastereomers via nitrogen inversion (ΔG=7.3 kcal/mol) and lower barriers for conformational interconversion via isolated rotation (ΔG=6.4 and 5.6 kcal/mol).By using the NMR data and complementary molecular mechanics calculations, the 49percent and 12percent subspectra are assigned to conformations of the RCRN and SCSN stereoisomers, while the 22percent and 17percent subspectra are assigned to the RCSN and SCRN stereoisomers.Preferred conformations have the N-ethyl methyl group in one of the two positions gauche to the nitrogen lone pair, the Cl methyl of the 2-butyl group anti to the lone pair, and the C4 methyl of the 2-butyl group syn periplanar to the lone pair.There is no experimental evidence for conformations which have the methine proton or the N-ethyl methyl group anti to the lone pair.

Oxygen Exchange into 2-Butanol and Hydration of 1-Butene Do Not Proceed through a Common Carbocation Intermediate

The acid-catalyzed isomerization of the 4-methyl group of 1,1,1,2,3,3-hexadeuteriated 2-butanol between the 1- and 4-positions occurs faster than dehydration to 2-butene, as measured by hydrogen incorporation from water into the 3-position of 2-butanol.The rate constants for these processes are kisom = 1.1E-6 s-1 and khyd = 5.6E-7 s-1, respectively, in 0.55 M HClO4 at 100 deg C.These results require that isomerization occurs by hydride transfer from C-3 to C-2, as well as by the formation and hydration of 2-butene.The 6percent initial yield of isomerized product in the acid-catalyzed hydration of -1-butene, before there is significant accumulation of 2-butene (Manassen, J.; Klein, F.J.Chem.Soc. l960, 4023), shows that hydride transfer is also significant in this reaction.Comparison of the product yields from 1-butene and the rate constants for isomerization and oxygen exchange of -2-butanol in the earlier work shows that the two reactions do not occur through a common carbocation intermediate.The preferential formation of cis-2-butene from 1-butene and trans-2-butene from 2-butanol confirms this conclusion.

Deuterium Nuclear Magnetic Resonance Study of Solid Deoxycholic Acid Adducts: Structure, Dynamics, and Photochemical Reactivity

We report cn an 2H NMR study of polycrystalline powders of several deoxycholic acid (DOCA) inclusion compounds between 150 and 350 K.The information obtained relates primarily to the conformation, flexibility, and mobility of the quest molecules, their intrachannel orientation and site multiplicity, and the effect of a photochemical guest-host reaction on these characteristics.With acetophenone, fluoroacetophenone, acetic acid, p- and o-xylene, and acetone rapid internal methyl rotation persist at all temperatures investigated.The C-CD3 bonds were found to fluctuate about a mean orientation within the channel.The amplitude of these fluctuations was found to be side specific, temperature dependent, and suppressible upon cooling and following occurence of the guest-host reaction.Ring inversion was observed with tetrahydrofuran (THF), cyclohexane, and cyclohexanone.Discrete jumps about symmetry axes of the guest molecule or the host lattice were detected with acetophenone and o-xylene, the flip motion taking place between unequally populated sites.In some cases, such as the cyclohexane and acetic acid adducts, there are sites in the host lattice where, on the average, the guest molecules reorient isotropically.

Stereochemical analysis of deuterated alkyl chains by MS/MS

Vicinally deuterated sec-alkyl phenyl ethers, CH3(CH2)(m)CH(OPh)CHD(CH2)(n)CH3, display significant differences in mass spectra between threo and erythro stereoisomers. MS/MS experiments, in which parent ions of a single mass are selected and their fragmentation patterns subsequently measured, show that alkene expulsion represents virtually the only decomposition pathway. Two types of MS/MS experiment are reported: mass- analyzed ion kinetic energy (MIKE) spectroscopy of metastable ions and collisionally activated decomposition (CAD) of stable ions. The expulsion of a deuterated alkene from a monodeuterated precursor yields ionized phenol, PhOH·+ (m/z 94). The expulsion of an undeuterated alkene yields PhOD·+ (m/z 95). Without exception, the ratios (PhOD·+/PhOH·+) from precursors in the threo series have values greater than their diastereomers in the erythro series. The ratio of ratios, r = PhOD·+/PhOH·+ for the threo divided by PhOD·+/PhOH·+ for the erythro, has a value of 1.2 for the 2- phenoxy-3-deuteriobutanes and larger values for all of the higher homologues up through the monodeuterated phenoxyoctanes (m + n = 4). The highest degree of stereoselectivity, r = 5.8, is measured for 3-phenoxy-4-deuteriohexane. Experiments with multiply deuterated analogues show that alkene elimination is highly regioselective, unlike the corresponding decompositions of ionized sec-alcohols or their acetates. The fact that a large fraction of ionized sec-alkyl phenyl ethers undergo stereospecific syn-elimination means that mass spectrometry has a useful capacity to distinguish one isotopically labeled diastereomer from another.