Rui H. Huang

Substrate-Induced Covalent Assembly of a Chemzyme and Crystallographic Characterization of a Chemzyme−Substrate Complex

A substrate induced covalent assembly of a highly organized chemzyme known to be effective in both catalytic asymmetric aziridination and aza Diels-Alder reactions is described and the information gained from which led to an efficient one-pot aziridination protocol. The crystal structures of two chemzyme-iminium complexes were elucidated by X-ray diffraction analysis that provides critical insights into the binding of the substrates with the chemzyme.

Evidence for a Boroxinate Based Brønsted Acid Derivative of VAPOL as the Active Catalyst in the Catalytic Asymmetric Aziridination Reaction

Studies are described that were designed to determine the structure of the active catalyst in the asymmetric catalytic aziridination of imines with ethyl diazoacetate (AZ reaction). Evidence suggests that the active catalyst contains a boroxine ring in which one of the three boron atoms is spiro-fused with the two phenol groups of the VAPOL ligand. (11)B and (1)H NMR evidence supports the boroxinate structure B in which the counterion to the boroxinate is the protonated form of the imine. The boroxinate structure is also supported by two solid state structures of a VAPOL boroxinate in which the gegen cation is tetramethyl ammonium and 4-dimethylaminopyridinium.

Optically active calixarenes conduced by methylene substitution

The first method for the synthesis of optically active calix[4]arenes that are chiral as a result of substitution on the methylene bridges is described. The key step in the synthesis involves the reaction of a biscarbene complex with a diyne, which generates two of the benzene rings and the macrocyclic ring of the calix in a single transformation. The utility of this triple annulation process is demonstrated in the synthesis of di- and tetramethoxycalix[4]arenes. The flexibility of this synthetic approach is demonstrated by the synthesis of two diastereomers of the tetramethoxycalix[4]arenes in which each is synthesized in a stereoselective fashion by proper control of the absolute configurations of the methoxy groups in the biscarbene complex and in the diyne.

Double Stereodifferentiation in the Catalytic Asymmetric Aziridination of Imines Prepared from α‐Chiral Amines

The catalytic asymmetric aziridination of imines and diazo compounds (AZ reaction) mediated by boroxinate catalysts derived from the VANOL and VAPOL ligands was investigated with chiral imines derived from five different chiral, disubstituted, methyl amines. The strongest matched and mismatched reactions with the two enantiomers of the catalyst were noted with disubstituted methyl amines that had one aromatic and one aliphatic substituent. The synthetic scope for the AZ reaction was examined in detail for α-methylbenzyl amine for cis-aziridines from α-diazo esters and for trans-aziridines from α-diazo acetamides. Optically pure aziridines could be routinely obtained in good yields and with high diastereoselectivity and the minor diastereomer (if any) could be easily separated. The matched case for cis-aziridines involved the (R)-amine with the (S)-ligand, but curiously, for trans-aziridines the matched case involved the (R)-amine with the (R)-ligand for imines derived from benzaldehyde and n-butanal, and the (R)-amine with the (S)-ligand for imines derived from the bulkier aliphatic aldehydes pivaldehyde and cyclohexane carboxaldehyde.

SYNTHESES AND STRUCTURES OF SIX COMPOUNDS THAT CONTAIN THE SODIUM ANION

Abstract Crystal structures have been obtained for the sodide (IUPAC, natride) salts, K+(12–crown–4)2–Na− (I), Na+(cryptand[2.2.1])Na−(II), Rb+(18–crown–6)Na−·CH3NH2 (III), K+(18–crown–6) Na−·2(CH3NH2)·3(18–crown–6)(IV), Cs+(cryptand[3.2.2])Na−(V), and (Li+)2(TMPAND)2–(Na−)2·(CH3NH2)(VI), in which TMPAND is 5,12,17–trimethyl–1,5,9,12,17–pentaazabicyclo [7.5.5] nonadecane). The synthesis, handling and crystal growth techniques are described and the properties and structures are compared with those of other alkalides.

The synthesis of optically active calix[4]arenes with one or three substituents on the methylene bridges

A facile synthesis of optically active mono and trisubstituted calix[4]arenes is described wherein the chirality at the methylene bridges arises from centers of chirality present in the diyne and the bis-carbene complex from which they are constructed.

Building blocks for molecule-based magnets: Radical anions and dianions of substituted 3,6-dimethylenecyclohexane-1,2,4,5-tetrones as paramagnetic bridging ligands

We have prepared four tetraaryl derivatives of 3,6-dimethylene-1,2,4,5-tetraoxocyclohexane (aryl = Ph; 4-MeOPh; 4-Me(2)NPh; and 3,5-(t-Bu)(2)-4-MeOPh) with guidance from an earlier reported ab initio analysis (Misiolek, A. W.; Jackson, J. E. J. Am. Chem. Soc. 2001, 123, 4774-4780). These electron acceptors may be chemically or electrochemically reduced to the mono- and dianions desired as building blocks for the assembly of molecule-based magnets. Cyclic voltammetry shows that the potential of the first reduction wave depends on the electron donor ability of the aryl ring substituents, ranging from -0.28 V for the tetraphenyl derivative to -0.78 V for the p-dimethylamino substituted analogue (vs ferrocene/ferrocinium(+) at 0.46 V). Spin density distributions in the semiquinone moieties were elucidated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) observations of hyperfine couplings to internal (1)H sites and bound alkali metal cations. X-ray diffraction studies of the sodium and potassium salts of the octa-t-butyltetramethoxy derivative reveal the structure of the monoanion and its tendency to self-assemble with metal cations into one-dimensional chains in the solid state. Within the chains the anions display the expected bridging and chelating mode of coordination; SQUID magnetometry revealed weak intermolecular spin-spin couplings of 2J = -0.2 and approximately 0 K for the sodium and potassium salts, respectively. NIR transitions in the electronic spectra of the monoanions in solution are consistent with the expected low energy gap between frontier orbitals and its tunability by substituent variations. EPR studies of the free dianions and monoradical analogues indicate diradical localization into separate triphenylmethyl-like monoradicals via twisting of the diarylmethylene termini.

Low temperature (−80°C) thermionic electron emission from alkalides and electrides

Abstract Electrons are emitted thermionically into vacuum at temperatures as low as −80°C from polycrystalline and thin film samples of alkalides and electrides. Saturation currents are reached at cathode-to-collector potentials of only 5–10 V. The emission current versus temperature follows the Richardson equation and yields apparent work functions between 0.2 and 0.5 eV. The emitted electrons apparently originate from trapped electrons in electrides or defect electrons in alkalides.

Role of Cation Complexants in the Synthesis of Alkalides and Electrides

Publisher Summary This chapter reviews the requirements, design, and methodology involved in the synthesis of alkalides and electrides, culminating with the recent designed synthesis of a thermally stable electride. Solid alkalides and electrides are formed by precipitation, crystallization, or solvent evaporation from solutions that contain complexed cations and either alkali metal anions or solvated electrons. No quantitative equilibrium data are available for complexant-free alkali metal solutions in the solvents used to synthesize alkalides and electrides (MeNH 2 , EtNH 2 , Me 2 O). It is the irreversible decomposition of oxa–complexants rather than decomplexation that limits the temperature stability of alkalides and electrides that contain crown ethers and cryptands. Research with alkalides and electrides would certainly have been simpler if crown ethers and cryptands were able to withstand the strong reducing power of alkali metal anions and trapped electrons. In the study of the decomposition products of some thirty solid alkalides and electrides that contained CH 2 -O-CH 2 -CH 2 -O-CH 2 groups, it was found that the initial products were ethylene and a glycolate.

Correlation of the Structure and Properties of Alkalides and Electrides That Contain Cryptands or Crown Ethers

Abstract The structures of several salts that contain alkali metal anions (alkalides) and two that contain trapped electrons (electrides) are correlated with their optical and NMR spectra, and magnetic susceptibilities. The nature of the channels and trapping sites in the two electrides are described.