ALBERT

Description: BH 4 - , a well-known and widely used reducing agent for carbonyl compounds, has been reported to have the ability to participate in dihydrogen bonding, an interaction with applications in catalysis, stereoselectivity and crystal engineering. Specifically, α-hydroxycarbonyls are activated for reduction by dihydrogen bonding that occurs between BH 4 - and hydroxyl group. We explored the effect of the interaction on the mechanism of these reactions by examining their activation parameters. We found that dihydrogen bonding activates α-hydroxycyclopentanone for reduction with NBu 4 BH 4 by lowering the activation enthalpy by 6.6 kcal/mol. While the activation entropy is a significant component of the barrier, the changes resulting from the occurrence of dihydrogen bonding are manifested predominantly in the enthalpy term. Computational studies suggest that, while internal hydrogen bonding is allowed by the flexibility of the carbon backbone, that interaction is outweighed by dihydrogen bonding once BH 4 - is present in the system. Experimentally, a red shift of the hydroxyl frequency is observed upon addition of BH 4 - to the reaction mixture, suggesting a dihydrogen bonding interaction. The flexibility of the substrate's skeleton or the selectivity of the hydride sites in BH 4 - does not account for the lack of directing effect of the dihydrogen bonding. When a substrate with a rigid naphthalene backbone moiety, 2-hydroxyacenaphthylen-1(2H)-one, is reduced, the stereochemical outcome is very similar to the one corresponding to the α-hydroxycyclopentanone. Copyright © 2012 John Wiley & Sons, Ltd. Dihydrogen bonding between BH 4 - and hydroxyl group of α-hydroxycyclopentanone, evidenced by a red shift of the hydroxyl frequency, activates α-hydroxycyclopentanone for reduction with NBu 4 BH 4 by lowering the activation enthalpy by 6.6 kcal/mol. While the activation entropy is a significant component of the barrier, the changes due to dihydrogen bonding are manifested predominantly in the enthalpy term. The rigidity of the substrate or the selectivity of the hydride sites in BH 4 - does not have an effect on the stereochemical outcome.

Description: Herschel Island in the southern Beaufort Sea is a push moraine at the northwestern-most limit of the Laurentide Ice Sheet. Stable water isotope (δ 18 O, δD) and hydrochemical studies were applied to two tabular massive ground ice bodies to unravel their genetic origin. Buried glacier ice or basal regelation ice was encountered beneath an ice-rich diamicton with strong glaciotectonic deformation structures. The massive ice isotopic composition was highly depleted in heavy isotopes (mean δ 18 O: −33‰; mean δD: −258‰), suggesting full-glacial conditions during ice formation. Other massive ice of unknown origin with a very large δ 18 O range (from −39 to −21‰) was found adjacent to large, striated boulders. A clear freezing slope was present with progressive depletion in heavy isotopes towards the centre of the ice body. Fractionation must have taken place during closed-system freezing, possibly of a glacial meltwater pond. Both massive ground ice bodies exhibited a mixed ion composition suggestive of terrestrial waters with a marine influence. Hydrochemical signatures resemble the Herschel Island sediments that are derived from near-shore marine deposits upthrust by the Laurentide ice. A prolonged contact between water feeding the ice bodies and the surrounding sediment is therefore inferred. Copyright © 2011 John Wiley & Sons, Ltd.