Measuring H-bonding in supramolecular complexes by gas phase ion-molecule reactions

H/D and guest-exchange ion-molecule reactions have been used as a new tool to elucidate the operation of multiple hydrogen bonding in gas-phase complexes formed between phosphonate cavitands and ethyl-substituted ammonium ions.     

Unusual platinum complexes in the gas phase

Previously unknown cationic platinum complexes Pt(C₅H₄N)(C₅H₅N)⁺ and Pt(C₅H₄N)⁺, where platinum atom forms an unusual three-membered metallacycle with a deprotonated pyridine molecule, were detected in the gas phase by mass spectrometry and structurally characterized by DFT quantum-chemical calculations.     

Palladium acetate complexes in the gas phase

New palladium acetate complexes, Pd₂(OOCMe) ₄ ⁺ , Pd₂(OOCMe) ₃ ⁺ , Pd₂(OOCMe) ₂ ⁺ , and Pd₂(OOCMe)⁺ were detected in the thermal decomposition products of trans-Pd(Py)₂(OOCMe)₂ by mass spectrometry with direct ion source. The geometric and electronic structures of the Pd₂(μ-OOCMe) ₄ ⁺ cation and the Pd₂(OOCMe)₄ molecule were established by quantum chemical calculations (DFT with the PBE1PBE hybrid exchange correlation potential in the 6-31G*/SDD basis set) and natural orbital analysis.     

Elimination reactions of ions in the gas phase

The loss of water from the molecular ion of 2-adamantanol was investigated using specifically labelled deuterium derivatives, and, in particular that stereospecifically labelled in position 4. Water is lost predominantly in a stereospecific 1, 3 fashion by two clearly distinguishable mechanisms. Determination of metastable ion characteristics proved to be essential for drawing this distinction.     

Aggregates of cucurbituril complexes in the gas phase

The self-organization of cucurbit[n]uril (n = 7 and 8, CBs) complexes was probed by electrospray mass spectrometry. The self-association of CB complexes is a general phenomenon but shows some dependence on the absence, presence, and type of included guest molecules.     

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.     

Stereoselectivity in the diels-alder reactions of tropylium ion

New Diels-Alder reactions of tropylium ion are studied, and the observed stereo-selectivities suggest that solvent effects determine the steric course of these reactions.     

Stereoselectivity in 6-halopenicillanate grignard reactions

Structural modifications of 6-halopenicillanate Grignards result in increased “aldol” stereoselectivity. The intermediacy of tetrahedral penicillin carbanions in THF and β-lactam enolates in CH₂Cl₂/toluene is proposed.     

Stereoselectivity in micellar and vesicular reactions

The diastereoselectivity of esterolytic cleavage of certain dipeptide and tripeptide activated esters is lower in thiol-functionalized vesicles than in comparable micelles. This is thought to be a mechanism-specific effect related to the greater molecular ordering in vesicular systems.     

Nature of minor-groove binders-DNA complexes in the gas phase

The structure of noncovalent complexes of DNA duplex with minor groove binders (mG-binders) has been analyzed by state of the art molecular dynamics (MD) simulations. More than 3.3 micros of MD trajectories (including 4 x 0.5 micros trajectories) were collected for the Dickerson's dodecamer bound to DAPI, Hoechst 33258, and Netropsin. Comparison of these trajectories with control simulations in water allowed us to determine that the extreme dehydration and partial neutralization occurring during electrospray experiments does not produce the disruption of the DNA:mG-binder complexes or the dissociation of the two strands of the duplex. Irrespective of the drug and the simulation conditions the mG-binders remains bound to the DNA near the preferential binding position in aqueous conditions. Large distortions appear in the two DNA strands, which maintain however a memory of the original DNA duplex structure in water, and a general helical-like conformation.     

Structural motifs of DNA complexes in the gas phase

DNA duplexes are known to be quite stable in the condensed phase but recent mass spectrometry results have shown that DNA complexes are also stable (at least for a limited time) in the gas phase. However, very little is known about the overall shape of the complexes in a solvent-free environment and what factors influence that shape. In this article, we present recent ion mobility and molecular modeling results that address some issues concerning the gas-phase conformations of DNA duplexes. Examples include the effect of metal ions on Watson–Crick base pairing, investigating the onset of helicity in duplexes as a function of strand length, comparison of the stability of C·G and A·T base pairs, and examining the formation of quadruplex structures.     

Stereoselectivity in reactions of metal complexes IX Stereoselective formation of cobalt(III) complexes with new linear pentadentate ligands

The synthesis of cobalt(III) complexes with the new linear pentadentate ligands meso- and racemic 2,6-bis(3S)-3-carboxy-4-methyl-2-azapentylpyridine ( 2 ) are described. Only one of the different possible isomers is obtained from each ligand. The structure of the complexes has been assigned on the basis of their ¹H-NMR and CD spectra. The structure of the aqua-cobalt(III)- 1a and the aqua-cobalt(III)- 1b has been confirmed by X-ray analysis. Partial resolution of optical antipodes of the aquacobalt(III)- 1b was achieved by column chromatography and a tentative assignment of their absolute configuration is made.     

Stereoselectivity of superacid-catalyzed Pictet-Spengler cyclization reactions

[reaction: see text] High stereoselectivities were found in a wide range of superacid-catalyzed Pictet-Spengler cyclization reactions. Particularly in the cases of 2-alkyl-N-benzylidene-2-phenethylamines, an enhanced stereoselectivity was observed under the superacid conditions as compared with the corresponding weak acid (TFA)-catalyzed (monocationic) cyclization reaction of the N-benzylidene-2-(3',4'-dimethoxy)phenethylamines that bear electron-donating groups on the cyclizing aromatic ring. The computational study also supported the energetic favorability of the cyclization of the N,N-diprotonated imine and revealed a significantly early transition-state structure.     

A new approach to the gas phase chemistry of ternary transition metal complexes: Ion-beam induced reactions

High Energy Cs⁺-ion bombardment of glycerol containing a suitable transition metal chloride, an amino acid (A.A.) and 1,10-phenanthroline (phen) induces the formation of singly charged ternary complexes of the type [Cat(A.A. - H) (phen)]⁺, where Cat = Cr, Mn, Fe, Co or Ni. The investigation of the complexes is performed by MS/MS experiments using a four-sector instrument. Metastable decompositions of iron containing ternary complexes [Fe(A.A. - H) (phen)]⁺ provide abundant fragment ions indicating the coordination of functionalized amino acid side chains. Fragmentation mechanisms and ion structures are discussed.     

Stereoselectivity in reactions of atropisomeric lactams and imides

A range of reactions of cyclic lactam systems is described in which an atropisomeric C-N axis controls the stereochemical outcome of ring substitution or addition. In the case of enantiopure menthol adducts, substitution via N-acyliminium intermediates occurred with essentially complete control. However, the range of nucleophiles that participate in the reaction is very limited and at present the removal of the N-aryl substituent is problematic. A six-membered enamide is of moderate configurational stability and the axis exerts synthetically useful levels of control over enolate alkylations of the system. A novel Lewis acid mediated enamide arylation process was identified.     

Exchange reactions of ternary ion-association complexes directly in the organic phase

The analytical implications of exchanging a highly coloured or fluorescent group R for a non-coloured or non-fluorescent one directly in the organic phase are discussed for ternary ion association complexes of the type [ML(x)(-)]R(+). Several examples are given to illustrate the approach.     

Novel five-membered ring intermediates in gas phase reactions

This review considers the role of closed-ring intermediates in chemistry, in general, and focuses on recent experimental and theoretical works that provide compelling evidence for a fivemembered ring intermediate in the gas phase. The reactions of oxygen atoms with ethyl iodide and larger alkyl iodides produce HOI and a corresponding ethylenic compound via attack of the oxygen atom at the iodine and closure of the five-membered ring between the oxygen and a hydrogen on the beta-carbon. Detailed results are gathered from infrared chemiluminescence and molecular beam scattering, as well as new kinetics and product yield experiments. The theoretical calculations fully support and characterize the novel reaction mechanism. National Research Council Research Associate, National Institute of Standards and Technology     

Stereoselectivity in reactions of metal complexes VII. Asymmetric synthesis of amino acids by metal ion-promoted transamination

Enantioselective synthesis of phenylalanine was performed by reacting phenylpyruvic acid with pyridoxamine followed by ketimine-aldimine isomerization of the Schiff base formed catalyzed by an optically active copper (II)-complex. By UV and CD measurements it was shown that the enantiomeric excess strongly depends on the reaction conditions and on the reaction time. In favorable cases it reached values up to 80%. The selectivity of the reaction is discussed on the basis of possible structures of the intermediate mixed ligand complex.     

ESI-FTICR-MS studies on gas phase fragmentation reactions of ArPd(PPh3)2I complexes

Gas-phase fragmentation reactions of [ArPd(PPh3)2]+ were studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). The results of sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) experiments provide detailed insights into mechanisms for the gas-phase fragmentation reactions of these complex ions. The PC bond cleavage mediated by palladium is investigated in the gas phase. There are two competitive fragmentation pathways for the complex ions [ArPd(PPh3)2]+ (Ar = p-OCH3-C6H4, p-CH3-C6H4, p-tBu-C6H4, p-NH2-C6H4, p-COCH3-C6H4, and p-F-C6H4) of electron-donating and electron-withdrawing aromatic iodides. Path A proceeds through reductive elimination of [ArPd(PPh3)2]+ to produce the product ion [PPh3Ar]+. Path B mostly proceeds via phenyl migration from the triphenylphosphine ligand to the palladium center by cleavage of the phosphorus-phenyl bond to give a palladium-phenyl intermediate, and subsequent reductive elimination of the intermediate to yield a product ion [PPh4]+. The result of deuterium-labeling experiments provides evidence for the phenyl shift between the palladium center and the coordinated ligand through cleavage of the PC bond. The complex ions [(o-CH3-C6H4)Pd(PPh3)2]+, [(o-2,6-Me2-C6H3)Pd(PPh3)2]+, and [(C10H7)Pd(PPh3)2]+ display more fragmentation pathways, two of which are similar to those of the ions [ArPd(PPh3)2]+ (Ar = p-OCH3-C6H4, p-CH3-C6H4, p-tBu-C6H4, p-NH2-C6H4, p-COCH3-C6H4, p-F-C6H4), and the third pathway involves loss of one molecule of benzene and one PPh3 ligand. The electronic effect and steric effect of the aryl groups also exhibit different influences on the fragmentation pathways.