Calculated Hydrogen Shift Rate Constants in Substituted Alkyl Peroxy Radicals

Peroxy radical hydrogen shift (H-shift) reactions are key to the formation of highly oxidized organic molecules and particle growth in the atmosphere. In an H-shift reaction, a hydrogen atom is transferred to the peroxy radical from within the same molecule to form a hydroperoxy alkyl radical, which can undergo O2 uptake and further H-shift reactions. Here we use an experimentally verified theoretical approach based on multi-conformer transition state theory to calculate rate constants for a systematic set of H-shifts. Our results show that substitution at the carbon, from which the hydrogen is abstracted, with OH, OOH, and OCH3 substituents lead to increases in the rate constant by factors of 50 or more. Reactions with CO and CC substituents lead to resonance stabilized carbon radicals and have rate constants that increase by more than a factor of 400. In addition, our results show that reactions leading to secondary carbon radicals (alkyl substituent) are 100 times faster than those leading to primary carbon radicals, and those leading to tertiary carbon radicals a factor of 30 faster than those leading to secondary carbon radicals. When the carbon from which the H is abstracted is secondary and has an OH, OOH, OCH3, CO, or CC substituent, H-shift rate constants are larger than 0.01 s–1 and need to be considered in most atmospheric conditions. H-shift reaction rate constants are largest and can reach 1 s–1 when the ring size in the transition state is 6, 7, or 8 atoms (1,5, 1,6, or 1,7 H-shift). Thus, H-shift reactions are likely much more prevalent in the atmosphere than previously considered

Side-by-Side Comparison of Hydroperoxide and Corresponding Alcohol as Hydrogen-Bond Donors

Hydroperoxides are formed in significant amounts in the atmosphere by oxidation of volatile organic compounds and are key in aerosol formation. In a room-temperature experiment, we detected the formation of bimolecular complexes of <i>tert</i>-butyl hydroperoxide (<i>t</i>-BuOOH) and the corresponding alcohol <i>tert</i>-butanol (<i>t</i>-BuOH), with dimethyl ether (DME) as the hydrogen-bond acceptor. Using a combination of Fourier-transform infrared spectroscopy and quantum chemical calculations, we compare the strength of the OH–O hydrogen bond and the total strength of complexation. We find that, both in terms of observed red shifts and determined equilibrium constants, <i>t</i>-BuOOH is a significantly better hydrogen-bond donor than <i>t</i>-BuOH, a result that is backed by a number of calculated parameters and can be explained by a weaker OH bond in the hydroperoxide. On the basis of combined experimental and theoretical results, we find that the hydroperoxide complex is stabilized by ∼4 kJ/mol (Gibbs free energy) more than the alcohol complex. Measured red shifts show the same trend in hydrogen-bond strength with trimethylamine (N acceptor atom) and dimethyl sulfide (S acceptor atom) as the hydrogen-bond acceptors

Mechanisms of carbonyl activation by BINOL N-triflylphosphoramides: enantioselective nazarov cyclizations

BINOL N-triflylphosphoramides are versatile organocatalysts for reactions of carbonyl compounds. Upon activation by BINOL N-triflylphosphoramides, divinyl ketones undergo rapid and highly enantioselective (torquoselective) Nazarov cyclizations, making BINOL N-triflylphosphoramides one of the most important classes of catalysts for the Nazarov cyclization. However, the activation mechanism and the factors that determine enantioselectivity have not been established until now. Theoretical calculations with ONIOM and M06-2X are reported which examine how BINOL N-triflylphosphoramides activate divinyl ketones and control the torquoselectivity of the cyclization. Unexpectedly, the computations reveal that the traditionally accepted mechanisms for these catalysts (i.e., NH⋯O=C hydrogen bonding or proton transfer) are not the dominant activation mechanisms. Instead, the active catalyst is a less-stable tautomer of the phosphoramide containing a P(=NTf)OH group. Proton transfer from the catalyst to the substrate occurs concomitantly with ring closure. The enantioselectivities of Nazarov cyclizations of three different classes of divinyl ketones are shown to depend on a combination of factors, including catalyst distortion, the degree of proton transfer, intramolecular substrate stabilization, and intermolecular noncovalent interactions between the substrate and catalyst in the transition state, all of which relate to how well the cyclizing divinyl ketone fits into the chiral binding pocket of the catalyst

Design of Pure Heterodinuclear Lanthanoid Cryptate Complexes

Heterolanthanide complexes are difficult to synthesize owing to the similar chemistry of the lanthanide ions. Conse-quently, very few purely heterolanthanide complexes have been synthesized. This is despite the fact that such complexes hold inter-esting optical and magnetic properties. To fine-tune these properties, it is important that one can choose complexes with any given combination of lanthanides. Herein we report a synthetic procedure which yields pure heterodinuclear lanthanide cryptates LnLn*LX3 (X = NO3- or OTf-) based on the cryptand H3L = N[(CH2)2N=CH-R-CH=N-(CH2)2]3N (R = m-C6H2OH-2-Me-5). In the synthesis the choice of counter ion and solvent prove crucial in controlling the Ln-Ln*composition. Choosing the optimal solvent and counter ion affords pure heterodinuclear complexes with any given combination of Gd(III)-Lu(III) including Y(III). To demon-strate the versatility of the synthesis all dinuclear combinations of Y(III), Gd(III), Yb(III) and Lu(III) were synthesized resulting in 10 novel complexes of the form LnLn*L(OTf)3 with LnLn* = YbGd 1, YbY 2, YbLu 3, YbYb 4, LuGd 5, LuY 6, LuLu 7, YGd 8, YY 9 and GdGd 10. Through the use of 1H, 13C NMR and mass spectrometry the heterodinuclear nature of YbGd, YbY, YbLu, LuGd, LuY and YGd was confirmed. Crystal structures of LnLn*L(NO3)3 reveal short Ln-Ln distances of ~3.5 Å. Using SQUID magnetometry the exchange coupling between the lanthanide ions was found to be anti-ferromagnetic for GdGd and YbYb while ferromagnetic for YbGd