Directed ortho metalation-boronation and Suzuki-Miyaura cross coupling of pyridine derivatives: a one-pot protocol to substituted azabiaryls

A general method for the synthesis of azabiaryls 19a-t by a one-pot procedure involving a Directed ortho metalation (DoM)-boronation-Suzuki-Miyaura cross coupling sequence is described. Aside from the three isomeric pyridyl carboxamides 15a-c, chloro-, fluoro-, and O-carbamoyl pyridines are adapted to this method providing a range of azabiaryls (Table 2). The method has an advantage in that it avoids the recognized difficult isolation of pyridyl boronic acids and their instability toward deboronation. The efficient synthesis of hydroxypicolinamides 12-14 (Scheme 3) by a one-pot metalation-boronation-oxidation sequence with the LDA-B(OiPr)3 in situ procedure that avoids self-condensation of incipient ortho-metalated species (Scheme 2) is delineated. The conversion of azabiaryls 19b,e,h,l into azafluorenones 20b,e,h,l by a directed remote metalation protocol is demonstrated (Table 3). A comprehensive survey of pyridyl boronates, of considerable interest in contemporary heterocyclic synthetic chemistry, is given (Figure 1).     

Infrared spectral identification of the high-energy conformer of <Emphasis Type="Italic">gauche,Trans,trans,Trans,gauche</Emphasis>-octa-1,3,5,7-tetraene

Scaled quantum mechanical force field (SQM-FF) vibrational analyses of the gauche,Trans,trans,Trans,gauche-conformers of octa-1,3,5,7-tetraene (I) with C _i and C ₂ symmetry at the HF/6-31G//HF/6-31G computational level were used to identify the rotational isomer observed in the IR spectrum of this excited molecule in the solid state. Geometry optimizations at the MP2(FC)/aug-cc-pVDZ level show that the two non-planar structures of gTtTg-I have the terminal -CH=CH₂ moieties rotated by ∼30.6° with respect to the remainder of the nearly planar skeleton. The MP2 and HF energies of the C ₂ conformer are slightly lower than the corresponding values of the C _i conformer. A set of the experimental wavenumbers relating to the C ₂ conformer was determined by the detailed analyses of the experimental IR spectra of compounds obtained by Datta et al. via photolysis of 1,3,5-cycloterraene and bicycle[4.2,0]octa-2.4-diene. The available experimental wavenumbers correlate better with the results of our SQM-FF calculations at the HF/6-31G level for the conformer with C ₂ symmetry than for the conformer with C _i symmetry. In further support of these results, the van der Waals molecular volume of the C ₂ conformer was calculated to be somewhat smaller than the volume of the C _i conformer, suggesting that in the solid state, where the close-packing principle comes into play, the C ₂ conformer would be preferred.     

Thermodynamics of boroxine formation from the aliphatic boronic acid monomers R-B(OH)2 (R = H, H3C, H2N, HO, and F): a computational investigation

Boroxines are the six-membered cyclotrimeric dehydration products of organoboronic acids, 3R–B(OH)2 → R3B3O3 + 3H2O, and in recent years have emerged as a useful class of organoboron molecules with applications in organic synthesis both as reagents and catalysts, as structural components in boronic-acid-derived pharmaceutical agents, and as anion acceptors and electrolyte additives for battery materials [Korich, A. L.; Iovine, P. M. Dalton Trans. 2010, 39, 1423?1431]. Second-order M?ller–Plesset perturbation theory, in conjunction with the Dunning–Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets, was used to investigate the structures and relative energies of the endo–exo, anti, and syn conformers of the aliphatic boronic acids R–B(OH)2 (R = H, H3C, H2N, HO, and F), as well as the thermodynamics of their boroxine formation; single-point calculations at the MP2/aug-cc-pVQZ, MP2/aug-cc-pV5Z, and CCSD(T)/aug-cc-pVTZ levels using the MP2/aug-cc-pVTZ optimized geometries were also performed in selected cases. The endo–exo conformer was generally lowest in energy in vacuo, as well as in PCM and CPCM models of aqueous and carbon tetrachloride media. The values of ΔH(298)(0) for boroxine formation via dehydration from the endo–exo conformers of these aliphatic boronic acids ranged from ?2.9 for (H2N)3B3O3 to +12.2 kcal/mol for H3B3O3 at the MP2/aug-cc-pVTZ level in vacuo; for H3B3O3, the corresponding values in PCM/UFF implicit carbon tetrachloride and aqueous media were +11.2 and +9.8 kcal/mol, respectively. On the basis of our calculations, we recommend that ΔHf(298K) for boroxine listed in the JANAF compilation needs to be revised from ?290.0 to approximately ?277.0 kcal/mol.     

Performance of the low pressure MWPCs for f ission fragments under a high background

Two couples of low pressure multi-wire proportional chambers (MWPC) were located in the target chamber to detect fission fragments in a hypernuclei producing experiment at Thomas Jefferson National Laboratory (Jlab). In the experiment, a continuous wave (CW) electron beam was applied to form hypernuclei by electromagnetic interaction. In the target chamber, the high energy (1.853 GeV) and high intensity (500 nA) primary electron beam caused a high particle background, which influenced the detection of the fission fragments. This report described the design of the MWPCs and studied the fission-fragment detecting performance of them under such a high background. The efficiency of the MWPCs was given with the help of a high resolution kaon spectrometer. At the same time, the background particles were discussed with a Monte Carlo code based on GEANT4.     

The 2D Kawahara Equation on a Half-Strip

We formulate on a half-strip an initial boundary value problem for the two-dimensional Kawahara equation. Existence and uniqueness of a regular solution as well as the exponential decay rate for the elevated norm $$\begin{aligned} \Vert u\Vert ^2_{H^1(D)}(t)+\Vert u_{xx}\Vert ^2_{L^2(D)}(t) \end{aligned}$$ of small solutions as \(t \rightarrow \infty \) are proven.     

Fast embedding methods for clustering tens of thousands of sequences

Most sequence clustering methods require a full distance matrix to be computed between all pairs of sequences. This requires computer memory and time proportional to N(2) for N sequences. For small N or say up to 10000 or so, this can be accomplished in reasonable times for sequences of moderate length. For very large N, however, this becomes increasingly prohibitive. In this paper, we have tested variations on a class of published embedding methods that have been designed for clustering large numbers of complex objects where the individual distance calculations are expensive. These methods involve embedding the sequences in a space where the similarities within a set of sequences can be closely approximated without having to compute all pair-wise distances. We show how this approach greatly reduces computation time and memory requirements for clustering large numbers of sequences and demonstrate the quality of the clusterings by benchmarking them as guide trees for multiple alignments. Source code is available on request from the authors.     

A computational characterization of boron-oxygen multiple bonding in HN=CH-CH=CH-NH-BO

Structures, relative energies, and bonding characteristics for various conformers of 3-imino-N-(oxoboryl)prop-1-en-1-amine, HN=CH-CH=CH-NH-BO, and the corresponding borocycle (-HN=CH-CH=CH-NH-B-)O are discussed using results from second-order M?ller-Plesset (MP2) perturbation theory with the Dunning-Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. These MP2 results are compared to those from computationally efficient density functional theory (DFT) calculations using the LDA, PBE, TPSS, BLYP, B3LYP, BVP86, OLYP, O3LYP, and PBE1PBE functionals in conjunction with the economical Pople-type 6-311++G(d,p) basis set to evaluate the suitability of these DFT/6-311++G(d,p) levels for use with larger boron-containing systems. The effects of an aqueous environment were incorporated into the calculations using COSMO methodology. The calculated boron-oxygen bond lengths, orbital compositions, and bond orders in all the (acyclic) HN=CH-CH=CH-NH-BO conformers were consistent with the presence of a boron-oxygen triple bond, similar to that found in H-BO and H2N-BO. The (-HN=CH-CH=CH-NH-B-)O borocycle is predicted to be planar (C2v symmetry), and it is approximately 30 kcal/mol lower in energy than any of the (acyclic) HN=CH-CH=CH-NH-BO conformers; the boron-oxygen bond in this borocycle has significant double bond character, a bonding scheme for which there has been only one experimental structure reported in the literature (Vidovic, D. ; et al. J. Am. Chem. Soc. 2005, 127, 4566- 4569).     

Decay of the energy for the Benjamin–Bona–Mahony equation posed on bounded intervals and on a half‐line

This paper concerns nonlinear dissipative initial boundary value problems for the Benjamin–Bona–Mahony equation posed on a half‐line and on bounded intervals. We prove the existence and uniqueness of global solutions and decay of the energy as time tends to infinity as well as convergence of solutions on bounded intervals to a solution on a half‐line. Copyright © 2012 John Wiley & Sons, Ltd.     

Hydrogen bridging in the compounds X2H (X=Al,Si,P,S)

X2H hydrides (X=Al, Si, P, and S) have been investigated using coupled cluster theory with single, double, and triple excitations, the latter incorporated as a perturbative correction [CCSD(T)]. These were performed utilizing a series of correlation-consistent basis sets augmented with diffuse functions (aug-cc-pVXZ, X=D, T, and Q). Al2H and Si2H are determined to have H-bridged C2v structures in their ground states: the Al2H ground state is of 2B1 symmetry with an Al-H-Al angle of 87.6 degrees, and the Si2H ground state is of 2A1 symmetry with a Si-H-Si angle of 79.8 degrees. However, P2H and S2H have nonbridged, bent Cs structures: the P2H ground state is of 2A' symmetry with a P-P-H angle of 97.0 degrees, and the S2H ground state is of 2A' symmetry with a S-S-H angle of 93.2 degrees. Ground state geometries, vibrational frequencies, and electron affinities have been computed at all levels of theory. Our CCSD(T)/aug-cc-pVQZ adiabatic electron affinity of 2.34 eV for the Si2H radical is in excellent agreement with the photoelectron spectroscopy experiments of Xu et al. [J. Chem. Phys. 108, 7645 (1998)], where the electron affinity was determined to be 2.31+/-0.01 eV.