A Traceless Chiral Shift Reagent Based on Nonbonding Interactions with Single-Handed Helical Poly(quinoxaline-2,3-diyl)

A single-handed poly(quinoxaline-2,3-diyl) (PQX) has been found to serve as a new type of chiral shift reagent (CSR) for determining the enantiomeric ratio by NMR spectroscopy. Even though there is no specific binding site in the PQX, its nonbonding interaction with chiral analytes leads to a significant shift of the NMR chemical shift, allowing quantification of the enantiomeric ratio. The new type of CSR has the advantages of a wide scope of analytes including ethers, haloalkanes, and alkanes, easy tunability of the degree of chemical shifts by measurement temperature, and erasability of proton signals of CSR because of the short spin-spin (T₂) relaxation of the macromolecular scaffold.     

Palladium-catalysed cis- and trans-silaboration of terminal alkynes: complementary access to stereo-defined trisubstituted alkenes

Palladium-catalysed cis- and trans-silaboration of terminal alkynes has been developed via the addition of (chlorodimethylsilyl)pinacolborane, followed by a one-pot conversion of the chloro group on the silicon atom to an isopropoxy group.     

Dearomatizing conversion of pyrazines to 1,4-dihydropyrazine derivatives via transition-metal-free diboration, silaboration, and hydroboration

Pyrazines and substituted pyrazines undergo addition of boron reagents having B-B, Si-B, and H-B bonds under transition-metal-free conditions, leading to high-yield synthesis of N-borylated 1,4-dihydropyrazines and 1,2,3,4-tetrahydropyrazine.     

Regioselective synthesis of 1,2-dihydropyridines by rhodium-catalyzed hydroboration of pyridines

Pyridine undergoes addition of pinacolborane at 50 °C in the presence of a rhodium catalyst, giving N-boryl-1,2-dihydropyridine in a high yield. The selective 1,2-hydroboration also takes place in the reactions of substituted pyridines. In the reaction of 3-substituted pyridines, 3-substituted N-boryl-1,2-dihydropyridines are formed regioselectively.     

Enhanced catalyst activity and enantioselectivity with chirality-switchable polymer ligand PQXphos in Pd-catalyzed asymmetric silaborative cleavage of meso-methylenecyclopropanes

The poly(quinoxaline-2,3-diyl)-based helically chiral phosphine ligands PQXphos exhibited high enantioselectivities up to 97% ee in palladium-catalyzed desymmetrization of meso-1,2-dialkylsubstituted-3-methylenecyclopropanes through silaborative cleavage of the C-C bond. The observed enantioselectivities were higher than those obtained with 2-diarylphosphino-1,1'-binaphthyl in our original report. Remarkable rate enhancement was also observed with a series of PQXphos in comparison with the corresponding low-molecular weight ligands.     

Ligand-controlled, complementary stereoselectivity in the platinum-catalyzed intramolecular silaboration of alkenes

An intramolecular silaboration of borylsilanyl homoallyl ethers was achieved using a platinum catalyst, giving 1-oxa-2-silacyclopentanes in high yields. The stereoselectivity of the reactions of sec-homoallyl ethers strongly depended on the phosphorus ligand of the platinum catalysts used. The platinum complex bearing the PCyPh2 ligand was found to be the most trans-selective catalyst (trans/cis = 81:19-92:8), whereas a highly cis-selective cyclization was achieved using a platinum catalyst having tris(2,4-di-tert-butylphenyl)phosphite ligand (trans/cis = 8:92-6:94). The synthetic utility of the intramolecular silaboration was demonstrated by the complementary synthesis of a pair of diastereomers of 6-methylheptane-1,3,5-triol.     

Theoretical study of keto–enol tautomerism by quantum mechanical calculations (the QM/MC/FEP method)

In this study, the tautomeric equilibrium between the keto and enol forms has been studied for five typical ketones and aldehydes: i‐butanal, acetaldehyde, acetone, acetylacetone, and dimedone. The level of theory used in the gas‐phase calculation was Becke, three‐parameter, Lee–Yang–Parr/6‐311G(d,p)//Becke, three‐parameter, Lee–Yang–Parr/6‐31G(d). The free energies of solvation were included in the calculation by using the free‐energy perturbation method based on Monte Carlo simulation, that is, the quantum mechanical/Monte Carlo/free‐energy perturbation method. Three different models, incorporating no‐water, one‐water, and two‐waters, were adopted. The results showed that in the gas phase the addition of water molecules to the reaction mechanism caused the activation barriers (ΔG^?_gas) to decrease by half relative to the water‐free mechanism, but there was no effect on the relative difference in free energy, ΔG_gas. The solvation effects (ΔG_sol), based on quantum mechanical/Monte Carlo/free‐energy perturbation calculations, were added to those of the gas‐phase results of the one‐water and two‐waters models. The two‐waters model produced values that were very consistent with the experimental data for all of the tautomers. The differences in the relative Gibbs free energy (ΔG_rxn) were less than 1.0 kcal mol^–1. In summary, the inclusion of solvent molecules in gas‐phase calculations plays a very important role in producing results consistent with experimental data. Copyright © 2012 John Wiley & Sons, Ltd.     

Facile dissociation of B?S coordination bonds in [2,6-bis(ethylthiomethyl)phenyl]diethylborane by an SN2-type mechanism

The title organoboron complex undergoes dissociation of the intramolecular B–S coordination bond much faster than the corresponding mono ethylthiomethyl complex as revealed by the dynamic NMR study. The facile dissociation is attributable to an S_N2-type mechanism, where the uncoordinated ligand assists the dissociation of the coordinated ligand in the transition state. This mechanism is supported by the initio calculations of a model reaction system. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:241–245, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20005