Biomedical subjects

Alain Goeppert

Publications and source records attributed to Alain Goeppert.

4 recordsLinked to original sources

PubMed2006

Chiral alpha-branched benzylic carbocations: diastereoselective intermolecular reactions with arene nucleophiles and NMR spectroscopic studies.

The chiral benzylic alcohols 1-6 were prepared and subjected to S(N)1-type displacement reactions with various arene nucleophiles in acidic medium. Under optimized conditions (HBF(4).OEt(2), CH(2)Cl(2), -78 degrees C --> r.t.) the corresponding 1,1-diarylalkanes 11-18 and 20 were obtained in good chemical yields (48-99%). The facial diastereoselectivity of the reaction is high (d.r. = 91/9-97/3) when the substrate bears a stereogenic carbon center -CHtBuMe in the alpha-position to the electrophilic carbon atom. If the starting material was enantiomerically pure, no significant racemization was observed (94% ee --> 92% ee). The reactions proceed stereoconvergently as demonstrated by the conversion of the separated diastereoisomers syn-1a and anti-1a in separate reactions to the same product syn-11 (d.r. = 97/3). Further evidence for long-lived chiral benzylic carbocations as reaction intermediates was obtained from NMR studies in superacidic medium. The chiral cation 24 was generated in SO(2)ClF as the solvent at -70 degrees C employing SbF(5) as the Lewis acid and characterized by its (1)H and (13)C NMR spectra. NOE measurements suggest a preferred conformation in which the diastereotopic faces of the cation are differentiated by the two carbon substituents R and Me at the stereogenic carbon center in the alpha-position. The hypothesis is further supported by the observation that the diastereoselectivity of the substitution reaction decreases if the bulky tert-butyl (R = tBu) substituent in the substrate 1a is replaced by a smaller ethyl group (2a, R = Et).

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PubMed2005

Ionic liquid and solid HF equivalent amine-poly(hydrogen fluoride) complexes effecting efficient environmentally friendly isobutane-isobutylene alkylation.

Isoparaffin-olefin alkylation was investigated using liquid as well as solid onium poly(hydrogen fluoride) catalysts. These new immobilized anhydrous HF catalysts contain varied amines and nitrogen-containing polymers as complexing agents. The liquid poly(hydrogen fluoride) complexes of amines are typical ionic liquids, which are convenient media and serve as HF equivalent catalysts with decreased volatility for isoparaffin-olefin alkylation. Polymeric solid amine:poly(hydrogen fluoride) complexes are excellent solid HF equivalents for similar alkylation acid catalysis. Isobutane-isobutylene or 2-butene alkylation gave excellent yields of high octane alkylates (up to RON = 94). Apart from their excellent catalytic performance, the new catalyst systems significantly reduce environmental hazards due to the low volatility of complexed HF. They represent a new, "green" class of catalyst systems for alkylation reactions, maintaining activity of HF while minimizing its environmental hazards.

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PubMed2004

Carbocationic rearrangement of pivaloyl cation and protonated pivalaldehyde in superacid medium: a novel solution equivalent of the McLafferty rearrangement.

Both pivaloyl cation in the presence of hydride donors and protonated pivalaldehyde in superacid media (both aprotic and protic) rearrange to protonated methyl isopropyl ketone involving gitionic dicationic intermediates. In our earlier studies we have found that the rearrangement of pivaladehyde to methyl isopropyl ketone occurs quantitatively in the presence of various superacidic media such as anhydrous HF, triflic acid, boron trifluoride-2,2,2-trifluoroethanol complex (BF(3).2CF(3)CH(2)OH) etc. Our present study with environmentally more benign and stable amine:HF complexes, namely pyridinium poly(hydrogen fluoride) (PPHF) (5), poly(4-vinylpyridinium) poly(hydrogen fluoride) (6), and poly(ethyleniminium) poly(hydrogen fluoride) (PEIHF) (7) shows that these modified HF equivalents can carry sufficient amount of immobilized HF and provide ample acidity for complete isomerization of pivalaldehyde to methyl isopropyl ketone. Calculations on protioformyl, acetyl and pivaloyl dications at the B3LYP/6-311 ++ G(d,p) and CCSD(T)/6-311 ++ G(d,p)//B3LYP/6-311 ++ G(d,p) levels have been performed to compare the nature of protosolvation of formyl, acetyl, pivaloyl cations and protonated pivaladehyde in superacid media. These studies further suggest protosolvation of protonated pivalaldehyde leading to gitionic dications at high acidities resulting in the carbocatioinic rearrangement. The reported carbocationic rearrangement under superacidic activation represents a novel solution chemistry equivalent of the well known gas-phase McLafferty rearrangement.

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PubMed2002

Methane activation and oxidation in sulfuric acid.

The H/D exchange observed when methane is contacted with D(2)SO(4) at 270-330 degrees C shows that the alkane behaves as a sigma base and undergoes rapid and reversible protonation at this temperature. DFT studies of the hydrogen exchange between a monomer and a dimer of sulfuric acid and methane show that the transition states involved in the exchange are bifunctional, that is one hydrogen atom is transferred from a hydroxy group in sulfuric acid to methane, while one hydrogen atom is abstracted from methane by a non-hydroxy oxygen atom in sulfuric acid. All the transition states include a CH(5) moiety, which shows similarities to the methanium ion CH(5) (+). The calculated potential activation energy of the hydrogen exchange for the monomer is 174 kJ mol(-1), which is close to the experimental value (176 kJ mol(-1)). Solvation of the monomer and the transition state of the monomer with an extra sulfuric acid molecule, decrease the potential activation energy by 6 kJ mol(-1). The acid-base process is in competition, however, with an oxidative process involving methane and sulfuric acid which leads to CO(2), SO(2), and water, and thus to a decrease of acidity and loss of reactivity of the medium.

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