Stereochemical Models for Discussing Additions to α,β‐Unsaturated Aldehydes Organocatalyzed by Diarylprolinol or Imidazolidinone Derivatives – Is There an ‘(E)/(Z)‐Dilemma’?

The structures of iminium salts formed from diarylprolinol or imidazolidinone derivatives and α,β‐unsaturated aldehydes have been studied by X‐ray powder diffraction (Fig. 1), single‐crystal X‐ray analyses (Table 1), NMR spectroscopy (Tables 2 and 3, Figs. 2–7), and DFT calculations (Helv. Chim. Acta 2009 , 92, 1, 1225, 2010 , 93, 1; Angew. Chem., Int. Ed. 2009 , 48, 3065). Almost all iminium salts of this type exist in solution as diastereoisomeric mixtures with (E)‐ and (Z)‐configured ⁺NC bond geometries. In this study, (E)/(Z) ratios ranging from 88 : 12 up to 98 : 2 (Tables 2 and 3) and (E)/(Z) interconversions (Figs. 2–7) were observed. Furthermore, the relative rates, at which the (E)‐ and (Z)‐isomers are formed from ammonium salts and α,β‐unsaturated aldehydes, were found to differ from the (E)/(Z) equilibrium ratio in at least two cases (Figs. 4 and 5, a, and Fig. 6, a); more (Z)‐isomer is formed kinetically than corresponding to its equilibrium fraction. Given that the enantiomeric product ratios observed in reactions mediated by organocatalysts of this type are often ≥99 : 1, the (E)‐iminium‐ion intermediates are proposed to react with nucleophiles faster than the (Z)‐isomers (Scheme 5 and Fig. 8). Possible reasons for the higher reactivity of (E)‐iminium ions (Figs. 8 and 9) and for the kinetic preference of (Z)‐iminium‐ion formation are discussed (Scheme 4). The results of related density functional theory (DFT) calculations are also reported (Figs. 10–13 and Table 4).     

锕系元素在熔盐体系中的电化学及萃取特性研究进展

熔融盐是一类非水溶剂,分为高温熔盐、室温熔盐和低温熔盐。 作为反应介质和电解介质,因其优良性能,可以溶解很多难溶于水的活泼金属。 近年来,该领域的研究热点是将熔盐作为干法后处理的电解质分离和回收锕系元素。 本文综述了锕系元素在高温熔盐中的电化学行为、热力学等物理特性,介绍了近几年室温离子液体(RTILs)的研究进展及锕系元素在RTILs中萃取特性的最新研究成果,展望了熔盐体系未来的研究方向。     

Picomolar Detection of Melamine Using Molecularly Imprinted Polymer‐Based Electrochemical Sensors Prepared by UV‐Graft Photopolymerization

Molecularly imprinted polymer (MIP) films of melamine were prepared by photopolymerization of vinylic monomers on diazonium‐modified gold electrodes. The gold‐grafted MIPs are specific and selective for melamine in either organic or aqueous media. The interferent molecules cyromazine and cyanuric acid were not recognized by the MIPs. The limit of detection was as low as 1.75×10^?12 mol L^?1 at S/N=3. Efficiency of melamine rebinding is related to the solubility parameter of the organic solvent or pH and ionic strength of the aqueous medium. It is concluded that diazonium salts permit to design robust electrochemical MIP sensors.     

Chiral Lithiated Allylic α‐Sulfonyl Carbanions: Experimental and Computational Study of Their Structure,Configurational Stability,and Enantioselective Synthesis

X‐ray crystal structure analysis of the lithiated allylic α‐sulfonyl carbanions [CH₂?CHC(Me)SO₂Ph]Li ? diglyme, [cC₆H₈SO₂tBu]Li ? PMDETA and [cC₇H₁₀SO₂tBu]Li ? PMDETA showed dimeric and monomeric CIPs, having nearly planar anionic C atoms, only O?Li bonds, almost planar allylic units with strong C?C bond length alternation and the s‐trans conformation around C1?C2. They adopt a C1?S conformation, which is similar to the one generally found for alkyl and aryl substituted α‐sulfonyl carbanions. Cryoscopy of [EtCH?CHC(Et)SO₂tBu]Li in THF at 164 K revealed an equilibrium between monomers and dimers in a ratio of 83:17, which is similar to the one found by low temperature NMR spectroscopy. According to NMR spectroscopy the lone‐pair orbital at C1 strongly interacts with the C?C double bond. Low temperature ⁶Li,¹H NOE experiments of [EtCH?CHC(Et)SO₂tBu]Li in THF point to an equilibrium between monomeric CIPs having only O?Li bonds and CIPs having both O?Li and C1?Li bonds. Ab initio calculation of [MeCH?CHC(Me)SO₂Me]Li ? (Me₂O)₂ gave three isomeric CIPs having the s‐trans conformation and three isomeric CIPs having the s‐cis conformation around the C1?C2 bond. All s‐trans isomers are more stable than the s‐cis isomers. At all levels of theory the s‐trans isomer having O?Li and C1?Li bonds is the most stable one followed by the isomer which has two O?Li bonds. The allylic unit of the C,O,Li isomer shows strong bond length alternation and the C1 atom is in contrast to the O,Li isomer significantly pyramidalized. According to NBO analysis of the s‐trans and s‐cis isomers, the interaction of the lone pair at C1 with the π* orbital of the CC double bond is energetically much more favorable than that with the “empty” orbitals at the Li atom. The C1?S and C1?C2 conformations are determined by the stereoelectronic effects n_C–σ_SR* interaction and allylic conjugation. ¹H DNMR spectroscopy of racemic [EtCH?CHC(Et)SO₂tBu]Li, [iPrCH?CHC(iPr)SO₂tBu]Li and [EtCH?C(Me)C(Et)SO₂tBu]Li in [D₈]THF gave estimated barriers of enantiomerization of ΔG^≠=13.2 kcal mol^?1 (270 K), 14.2 kcal mol^?1 (291 K) and 14.2 kcal mol^?1 (295 K), respectively. Deprotonation of sulfone (R)‐EtCH?CHCH(Et)SO₂tBu (94 % ee) with nBuLi in THF at ?105 °C occurred with a calculated enantioselectivity of 93 % ee and gave carbanion (M)‐[EtCH?CHC(Et)SO₂tBu]Li, the deuteration and alkylation of which with CF₃CO₂D and MeOCH₂I, respectively, proceeded with high enantioselectivities. Time‐dependent deuteration of the enantioenriched carbanion (M)‐[EtCH?CHC(Et)SO₂tBu]Li in THF gave a racemization barrier of ΔG^≠=12.5 kcal mol^?1 (168 K), which translates to a calculated half‐time of racemization of t_1/2=12 min at ?105 °C.     

Chiral Silver Phosphate Catalyzed Transformation of ortho‐Alkynylaryl Ketones into 1H‐Isochromene Derivatives through an Intramolecular‐Cyclization/Enantioselective‐Reduction Sequence

The transformation of ortho‐alkynylaryl ketones through a cyclization/enantioselective‐reduction sequence in the presence of a chiral silver phosphate catalyst afforded 1H‐isochromene derivatives in high yield with fairly good to high enantioselectivity. An asymmetric synthesis of the 9‐oxabicyclo[3.3.1]nona‐2,6‐diene framework, which has been found in some biologically active molecules, is presented as a demonstration of the synthetic utility of this method.     

The Triple‐Bond Metathesis of Aryldiazonium Salts: A Prospect for Dinitrogen Cleavage

The {N₂} unit of aryldiazonium salts undergoes unusually facile triple‐bond metathesis on treatment with molybdenum or tungsten alkylidyne ate complexes endowed with triphenylsilanolate ligands. The reaction transforms the alkylidyne unit into a nitrile and the aryldiazonium entity into an imido ligand on the metal center, as unambiguously confirmed by X‐ray structure analysis of two representative examples. A tungsten nitride ate complex is shown to react analogously. Since the bonding situation of an aryldiazonium salt is similar to that of metal complexes with end‐on‐bound dinitrogen, in which {N₂}→M σ donation is dominant and electron back donation minimal, the metathesis described herein is thought to be a conceptually novel strategy toward dinitrogen cleavage devoid of any redox steps and, therefore, orthogonal to the established methods.     

Preparation and Structures of 2‐Substituted 5‐Benzyl‐3‐methylimidazolidin‐4‐one‐Derived Iminium Salts,Reactive Intermediates in Organocatalytic Transformations Involving α,β‐Unsaturated Aldehydes

Preparations of the title compounds, 5 – 7 (Scheme 1 and Table 1), of their ammonium salts, 9 – 11 (Scheme 2 and Table 2), and of the corresponding cinnamaldehyde‐derived iminium salts 12 – 14 (Scheme 3 and Table 3) are reported. The X‐ray crystal structures of 15 cinnamyliminium PF₆ salts have been determined (Table 4). Selected ¹H‐NMR data (Table 5) of the ammonium and iminium salts are discussed, and structures in solution are compared with those in the solid state.     

Hydrogen‐deuterium exchange of α‐carbon protons and fragmentation pathways in N‐methylated glycine and alanine‐containing peptides derivatized by quaternary ammonium salts

Recently, we developed a selective and efficient method of hydrogen‐deuterium exchange (HDX) at the α‐carbon (α‐C) of sarcosine residue (N‐methylglycine) in model peptides [B?chor et al. J. Mass Spectrom. 2014, 49, 43]. Here, we report the influence of quaternary ammonium (QA) group on HDX at the α‐C of sarcosine and N‐methylalanine in peptides. The obtained results suggest a significant acceleration of the HDX in sarcosine residue caused by the presence of QA. The effect depends on the distance between the sarcosine residue and QA moiety. The deuterons, introduced at α‐C, are resistant to the back‐exchange in acidic aqueous solution. The collision induced dissociation of the deuterium‐labeled analogs of QA‐tagged oligosarcosine peptides without mobile hydrogen revealed the mobilization of the hydrogens localized at α‐C of sarcosine residue. Copyright © 2014 John Wiley & Sons, Ltd.     

An unexpected reaction of pyridine with acetyl chloride to give dihydropyridine and piperidine derivatives

An unexpected reaction of pyridine with acetyl chloride to give N-acetyl-1,2 and 1,4-dihydropyridyl acetic acid (1, 2) in high yield and regioselectivity has been reported. The key step is the formation of a zwitterionic pyridinium ketene enolate. The effect of different activating agents on the reaction yield and selectivity has been studied. Platinum(IV) mediated hydrogenation of the corresponding methyl esters (7, 8) gave piperidine derivatives (9, 10).