We have measured the conductance of single-molecule junctions created with three different molecular wires using the scanning tunneling microscope-based break-junction technique. Each wire contains one of three different cyclic five-membered rings: cyclopentadiene, furan, or thiophene. We find that the single-molecule conductance of these three wires correlates negatively with the resonance energy of the five-membered ring; the nonaromatic cyclopentadiene derivative has the highest conductance, while the most aromatic of this series, thiophene, has the lowest. Furthermore, we show for another wire structure that the conductance of furan-based wires is consistently higher than for analogous thiophene systems, indicating that the negative correlation between conductance and aromaticity is robust. The best conductance would be for a quinoid structure that diminishes aromaticity. The energy penalty for partly adopting the quinoid structure is less with compounds having lower initial aromatic stabilization. An additional effect may reflect the lower HOMOs of aromatic compounds.

Helical chiral 2-aminopyridinium ions were designed as a significantly more acidic (active) dual hydrogen-bonding catalyst than commonly used (thio)urea-based systems. The helicene framework was specifically utilized to position an inherently chiral barrier on the hydrogen-bonding side of the catalyst. The catalyst reactivity and enantioselectivity were successfully demonstrated in additions of 4,7-dihydroindoles to nitroalkenes (0.5-2 mol % catalyst loadings, up to 98:2 er).

In the formose reaction, formaldehyde is converted to glycolaldehyde, its dimer, under credible prebiotic conditions. Breslow proposed a mechanism for the process in 1959, but recent studies by Benner showed that it was wrong in detail. Our present studies clarify the mechanism, which involves the original Breslow intermediates but some different connecting steps.

Chiral β-fluoroamines are increasingly prevalent in medicinal compounds, but there are few efficient methods to access them from achiral starting materials. To address this, a multicomponent organocascade reaction was developed in which chiral α-fluoro-β-amino aldehydes were generated in a single flask from achiral α,β-unsaturated aldehydes (2), using catalyst 12a. Conversions up to 85%, dr's up to 98:2, and ee's up to 99% of the corresponding alcohol (9) were achieved in this reaction.The incorporation of fluorine into medicinal compounds is increasingly prevalent with, "20-25% of drugs in the pharmaceutical pipeline contain [ing] at least one fluorine atom."1 MK-0731 (1 , Figure 1) is one example, developed by Merck for the treatment of Taxanerefractory cancer.2 More specifically, β-fluoroamines, as in 1, are increasingly common substructures in medicinal compounds, because fluorine can improve the bioavailability of amine drugs by decreasing the basicity of neighboring amine groups.1 Additionally, α-fluoro-β-amino acids are useful building blocks of therapeutic β-peptides.3 Despite the increasing use of β-fluoroamino moieties in medicinal compounds, efficient synthetic methods for their preparation are sparse. Recently, Pd-catalyzed olefin aminofluorination reactions were reported, in which achiral β-fluoroamines were generated in a single step.4 β-Fluoroamines containing a single stereocenter can be produced using a one-pot organocatalytic α-fluorination/reductive amination protocol.5 Accessing chiral β-fluoroamines with vicinal stereocenters (as in 1) usually requires the use of chiral starting materials. We are aware of only two efficient (i.e., one-pot) methods for preparing these compounds from achiral starting materials. One is an asymmetric olefin aminofluorination reaction, in which chiral β-fluoroamines are generated from achiral α,β-unsaturated esters.6 The asymmetric induction in this method is not catalytic; stoichiometric quantitites of a chiral amine nucleophile are used. In addition, an organocatalytic Mannich reaction using α-fluoro-β-dicarbonyl compounds as nucleophiles produced chiral β-fluoroamines in high yield and selectivity (dr ≥ 92:8).7 However, the dr decreased to 4:1 during subsequent decarboxylation to produce α-fluoro-β-amino acid derivatives.7b We supposed that an organocascade reaction (Scheme 1) would be a highly selective and green chemical method SBrenner@brooklyn.cuny.edu. Supporting Information Available General experimental conditions and full characterization data for compounds 2i, 9a, 9b, 9c, 9d, 9e, 9f, 9g, 9h, 9i and 13. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public Access Author ManuscriptOrg Lett. Author manuscript; available in PMC 2011 August 6. Iminium-catalyzed conjugate additions of amine nucleophiles to α,β-unsaturated aldehydes have been reported,8 as have enamine-catalyzed fluorinations of saturated aldehydes.9 It was anticipated that these two complementary functionalizations could be combined as an org...