From dipivaloylketene to tetraoxaadamantanes
Dipivaloylketene (2) is obtained by flash vacuum pyrolysis of furan-2,3-dione 6 and dimerizes to 1,3-dioxin-4-one 3, which is a stable but reactive ketene. The transannular addition and rearrangement of enols formed by the addition of nucleophiles to the ketene function in 3 generates axially chiral 2,6,9-trioxabicyclo[3.3.1]nonadienes (bisdioxines) 4. When arylamines are used as the nucleophiles under neutral conditions, decarboxylation occurs during the formation of bisdioxines 8. However, when water or alcohols are added to 3 under acidic conditions, bisdioxine-carboxylic acids and esters 10 and 11 are obtained. Acid hydrolysis of the bisdioxines proceeds through the addition of water to a C=C double bond and results in a second transannular oxa-Michael-type reaction and generation of tetraoxaadamantanes 5. This reaction is decarboxylative when free carboxylic acid functions are present in the bisdioxines, thus forming 21 and 22, but carboxylic acid derivatives are preserved to yield compounds 20, 23, 25, 28, and 29. A hydrogenolysis of the dibenzyl ester 23 yields the free dicarboxylic acid 24. The tetraoxaadamantanes are formed in high yields (65–95%) in most cases, but the addition of water to the concave inside of the bisdioxines becomes severely hindered in cyclic derivatives, so that the 38-membered ring compound 32 requires microwave heating at 170 °C to form tetraoxaadamantane 33, and the catenated compound 36 and calix[6]arene derivative 37 did not form tetraoxaadamantanes. The reaction mechanisms of bisdioxine and tetraoxaadamantane formation are discussed.
How Acid-Catalyzed Decarboxylation of 2,4-Dimethoxybenzoic Acid Avoids Formation of Protonated CO2.
The decarboxylation of 2,4-dimethoxybenzoic acid (1) is accelerated in acidic solutions. The rate of reaction depends upon solution acidity in a manner that is consistent with the formation of the conjugate acid of 1 (RCO2H2(+)), with its higher energy ring-protonated tautomer allowing the requisite C-C bond cleavage. However, this would produce the conjugate acid of CO2, a species that would be too energetic to form. Considerations of mechanisms that fit the observed rate law were supplemented with DFT calculations. Those results indicate that the lowest energy pathway from the ring-protonated reactive intermediate involves early proton transfer from the carboxyl group to water along with C-C bond cleavage, producing 1,3-dimethoxybenzene and CO2 directly.
Study on formation and stability of azacage metal complexes using electrospray mass spectrometry
Abstract In this study electrospray mass spectrometry was used to investigate spherical azacages as host molecules for metal ions systems. Two different pyridino cryptands, [bis-tren(tris-pyridine)amine L1 and bis-tren(tris-pyridine)phenylamide L2] were examined with regard to their affinity for coordinating metal ions such as Ag+, Hg2+, Ni2+, Zn2+, and Co2+. The complex formation was performed in a water/methylene chloride system as well as in pure water, and subsequent electrospray mass spectrometry described the behavior and partition of the complex species. Generally, L1 shows higher ability in the coordination of metal ions and the strongest binding for Ag+. L2 gives significant interactions only with Ag+ and Co2+. Collision activation experiments were used to study the stability of complex species and to distinguish between species formed in solution and during electrospray.