The aza-Morita-Baylis-Hillman reaction of electronically and sterically deactivated substrates.

The aza-Morita–Baylis–Hillman (azaMBH) reaction has been studied for electronically and sterically deactivated Michael acceptors. It is found that electronically deactivated systems can be converted with electron-rich phosphanes and pyridines as catalysts equally well. For sterically deactivated systems clearly better catalytic turnover can be achieved with pyridine catalysts. This is in accordance with the calculated affinities of the catalysts towards different Michael-acceptors

Theoretical studies of 31P NMR spectral properties of phosphanes and related compounds in solution

Selected theoretical methods, basis sets and solvation models have been tested in their ability to predict 31P NMR chemical shifts of large phosphorous-containing molecular systems in solution. The most efficient strategy was found to involve NMR shift calculations at the GIAO-MPW1K/6-311++G(2d,2p)//MPW1K/6-31G(d) level in combination with a dual solvation model including the explicit consideration of single solvent molecules and a continuum (PCM) solvation model. For larger systems it has also been established that reliable 31P shift predictions require Boltzmann averaging over all accessible conformations in solution

Factorizing Yields in Buchwald-Hartwig Amination

The data collected in (1) are revisited and rigorously analyzed. We show that the matrix of descriptors used in the analysis of (1) is up to a linear transformation equivalent to a dummy coded matrix that reflects the design of the underlined experiment. Furthermore, it is argued that the reaction yield is better be modeled as a continuous Bernoulli random variable. Finally, a four-way ANOVA model with single replicates following a continuous Bernoulli distribution is fitted to the data under the assumption of the sparsity of the parameters, and estimated parameters are interpreted. Thereby, a novel regularisation technique based on the partial least squares algorithm is applied

The performance of computational techniques in locating the charge separated intermediates in organocatalytic transformations

The formation of zwitterionic adducts between neutral nucleophiles such as NMe3 and PMe3 with neutral electrophiles such as methyl vinyl ketone (MVK) has been studied with a wide variety of theoretical methods. It has been found that hybrid density functional methods such as B3LYP are not capable of describing these zwitterionic structures as minima on the potential energy surface. This is also true for combinations of MP2 theory with basis sets lacking diffuse basis functions. The mPW1K hybrid functional, in contrast, correctly describes zwitterionic adducts as true intermediates on the PES. On the basis of this insight, a new version of the G3 compound energy scheme has been developed for the accurate description of zwitterionic structures. It has also been verified that modifications of the B2-PLYP double-hybrid functional are equally capable of the proper description of zwitterionic adducts. The applicability of this latter class of methods to a larger dataset involving combinations of different nucleophiles and electrophiles has been documented

Calculated Nuclear Magnetic Resonance Spectra of Polytwistane and Related Hydrocarbon Nanorods

Polytwistane is an intriguing hydrocarbon nanorod that has not been experimentally realized to date. To facilitate its identification in complex reaction mixtures, the ¹H and ¹³C nuclear magnetic resonance (NMR) spectra of idealized polytwistane were calculated using two distinct quantum chemical approaches. In addition, the NMR spectra of related hydrocarbon nanorods were determined. On the basis of these data, we speculate whether polytwistane and its congeners correspond to a crystalline one-dimensional sp³ carbon nanomaterial formed by high-pressure solid-state polymerization of benzene

Importance of dipole moments and ambient polarity for the conformation of Xaa-Pro moieties - a combined experimental and theoretical study

NMR spectroscopic studies with a series of proline derivatives revealed that the polarity of the environment has a significant effect on the trans : cis isomer ratio of Xaa–Pro bonds. Computational studies showed that this effect is due to differences in the overall dipole moments of trans and cis conformers. Comparisons between the conformational properties of amide and ester derivatives revealed an intricate balance between polarity effects and n → π* interactions of adjacent carbonyl groups. The findings have important implications for protein folding and signaling as well as the performance of proline-based stereoselective catalysts.ISSN:2041-6520ISSN:2041-653

Intermolecular ¹¹⁹Sn,³¹P Through-Space Spin–Spin Coupling in a Solid Bivalent Tin Phosphido Complex

A bivalent tin complex [Sn­(NP)₂] (NP = [(2-Me₂NC₆H₄)­P­(C₆H₅)]⁻) was prepared and characterized by X-ray diffraction and solution and solid-state nuclear magnetic resonance (NMR) spectroscopy. In agreement with the X-ray structures of two polymorphs of the molecule, ³¹P and ¹¹⁹Sn CP/MAS NMR spectra revealed one crystallographic phosphorus and tin site with through-bond ¹<i>J</i>(^117/119Sn,³¹P) and through-space ^TS<i>J</i>(^117/119Sn,³¹P) spin–spin couplings. Density functional theory (DFT) calculations of the NMR parameters confirm the experimental data. The observation of through-space ^TS<i>J</i>(^117/119Sn,³¹P) couplings was unexpected, as the distances of the phosphorus atoms of one molecule and the tin atom of the neighboring molecule (>4.6 Å) are outside the sum of the van der Waals radii of the atoms P and Sn (4.32 Å). The intermolecular Sn···P separations are clearly too large for bonding interactions, as supported by a natural bond orbital (NBO) analysis

Mechanistic Pathways in Amide Activation: Flexible Synthesis of Oxazoles and Imidazoles

The preparation of substituted aminooxazoles and aminoimidazoles from α-arylamides and α-aminoamides through triflic anhydride-mediated amide activation is reported. These reactions proceed via the intermediacy of nitrilium adducts and feature <i>N</i>-oxide-promoted umpolung of the α-position of amides as well as a mechanistically intriguing sequence that results in sulfonyl migration from nitrogen to carbon. Quantum-chemical mechanistic analysis sheds light on the intricacies of the process