Showing papers by "Hans J. Reich published in 2005"

The Strength of Parallel-Displaced Arene-Arene Interactions in Chloroform

Abstract: Triptycene-derived compounds have been prepared to serve as conformational equilibrium reporters for direct measurements of arene−arene interactions in the parallel-displaced orientation. A series of such compounds bearing arenes with different substituents were synthesized, and the ratios of the syn and anti conformers were determined by variable-temperature NMR spectroscopy. The syn conformer allows attached arenes to interact with each other while the anti conformer does not. The free energies derived from the syn/anti ratios in chloroform range from slightly positive (0.2 kcal/mol) to considerably negative (−0.98 kcal mol) values. The interactions between the arenes bearing electron-donating groups (EDG) are either negligible or slightly repulsive, while the interactions between arenes bearing electron-withdrawing groups (EWG) are attractive. Intermediate free energy values are obtained for those compounds bearing arenes with one EDG and one EWG.

Off-Center Oxygen−Arene Interactions in Solution: A Quantitative Study

Abstract: Triptycenes with C(1)−MeO/RCOO (R = H, Me, Et, i-Pr, CF3) and C(9)−XC6H4CH2 (X = Me, H, F, CN, CF3) have been prepared to determine lone pair−arene interactions in the off-center configuration. The ratios of the syn and anti conformers were determined by low-temperature NMR spectroscopy. The syn conformer allows the attached arene and the MeO/ester to interact with each other while the anti conformer does not. The free energies of interaction have been derived from the syn/anti ratios. Compound 7 in the ester series with X = H and R = CF3 is the only compound that shows a slightly repulsive interaction (0.08 kcal/mol). Compound 2e in the MeO series with X = CF3 exhibits an attractive interaction (−0.47 ± 0.05 kcal mol). All other compounds show smaller attractive interactions.

Solution structure and chelation properties of 2-thienyllithium reagents.

Abstract: [reaction: see text] The solution and chelation properties of 2-thienyllithium reagents with potential amine and ether chelating groups in the 3-position and related model systems have been investigated using low temperature 6Li, 7Li, 13C, and 31P NMR spectroscopy, 15N-labeling, and the effect of solvent additives. In THF-ether mixtures at low temperature 3-(N,N-dimethylaminomethyl)-2-thienyllithium (4) is ca. 99% dimer (which is chelated) and 1% monomer (unchelated), whereas 3-(methoxymethyl)-2-thienyllithium (5) is <10% dimer. Compound 5 crystallizes as a THF-solvated dimer, but there is no indication that the ether side chain is chelated in solution. Both 4 and 5 form PMDTA-complexed monomers almost stoichiometrically, similar to the model compound 2, in sharp contrast to phenyl analogues, which show very different behavior. The barriers to dimer interconversion are ca. 2 kcal/mol lower and chelation is significantly weaker in the 2-thienyllithium reagents than in their phenyl analogues.

Mechanism of the lithiumiodine exchange in an iodothiophene

Abstract: Solutions of 2-lithio-5-methylthiophene (4) were characterized using DNMR techniques and shown to be a mixture of monomer and dimer in THF–Et2O (3:2). The hypervalent iodine ate complex 5 (Ar2I–Li+), a presumed intermediate in the Li–I exchange with 2-iodo-5-methylthiophene, was observed by 13C and 7Li NMR spectroscopy at low temperatures (–130 °C). At higher temperatures, the ate complex coalesced with 2-lithio-5-methylthiophene. A kinetic scheme was developed, which accounts for the exchange of the monomer 4M, dimer 4D, and 2-iodo-5-methylthiophene (6) with the ate complex 5. The rates of the various exchanges were obtained through a DNMR analysis of the variable temperature 13C and 7Li NMR spectra, and the thermodynamic and activation parameters were calculated. The monomer 4M and the ate complex 5 have similar reactivity as aryl donors in the Li–I exchange reaction, but 4M is at least 1000 times as reactive as the dimer 4D towards the iodide.Key words: halogen–metal exchange, lithium iodinate, iodine ...

Reaction of metalated nitriles with enones.

Abstract: There have been a number of reports of the kinetic conjugate (1,4) addition of metalated arylacetonitriles to enones. Several proposals have been made to explain this behavior based on nucleophile structure or aggregation state or on the HSAB properties of the reactants. A reexamination of these studies showed that in each case the 1,4 adducts resulted from equilibration of the kinetically formed 1,2 adducts to the more stable 1,4 adducts. Thus, no conclusions about the origins of 1,4 selectivity can be drawn from these experiments. The 1,2 addition, retro-1,2 addition, 1,4 addition, and retro-1,4 addition of lithiophenylacetonitrile to benzylideneacetone were examined, and a free energy level diagram was constructed for the reaction.

Introducing JCE ChemInfo: Organic

Abstract: “JCE ChemInfo: Organic” is a collection of Web pages containing information useful to teachers, researchers, and students in organic chemistry, biochemistry, and medicinal chemistry. The pages have...