Infrared Multiphoton Dissociation Spectroscopic Analysis of Noncovalent Interactions in Organocatalysis

Holland, Mareike C.; Berden, Giel; Meijer, Anthony J. H. M.; Schäfer, Matthias; Gilmour, Ryan

doi:10.1002/ejoc.201402845

Cited by 10 publications

(4 citation statements)

References 47 publications

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“…Their synthetic value as readily accessible and very powerful enantioselective organocatalysts was shown in a series of publications particularly from the MacMillan group . These organocatalysts perform well in iminium‐based as well as enamine‐based catalytic reactions. We are particularly fascinated by the role they can play in synthetic procedures based on open‐shell species.…”

Section: Introductionmentioning

confidence: 90%

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

2017

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Electron ionisation mass spectra of N-methylimidazolidin-4-one organocatalysts were studied by experimental and theoretical means. The molecular ions mostly undergo alpha cleavages of exocyclic substituents that leave the five-membered ring intact. The type of substituent strongly dominates the appearance of the spectra. Fragmentation cascades are corroborated by metastable ion mass spectra. Quantum Chemistry Electron Ionisation Mass Spectra calculations correlate reasonably well with the experimental electron ionisation spectra and reveal mechanistic details of fragmentation pathways. The drawbacks and benefits of such calculations are discussed. Copyright © 2017 John Wiley & Sons, Ltd.

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Section: Introductionmentioning

confidence: 90%

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

2017

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“…Upon unification with an α,β-unsaturated aldehyde, the intermediate iminium salt has noteworthy on account of the three sp 2 -centers embedded in the 5-membered ring, together with a gem -dimethyl unit. This generates a planar core that allows the flanking benzyl group to engage in stabilizing CH−π interactions with the syn -methyl group [N + CH 2 CH 2 –H δ+ ···Ph], or to form π–π-interactions with the pendant iminium chain. − Indeed, a “ windshield wiper ” model has been invoked to describe this dynamic conformational behavior by Seebach, Grimme, and co-workers . Indeed, structural analyses by this laboratory have established a correlation between the quadrupole moment of the aryl shielding group and the enantioselectivity in model transformations indicating that the conformation of the iminium salts is a manifestation of a cation-π interaction. , …”

Section: Fluorine Stereoelectronic Control In Iminium Ion Organocatal...mentioning

confidence: 99%

Informing Molecular Design by Stereoelectronic Theory: The Fluorine Gauche Effect in Catalysis

2018

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The axioms of stereoelectronic theory constitute an atlas to navigate the contours of molecular space. All too rarely lauded, the advent and development of stereoelectronic theory has been one of organic chemistry's greatest triumphs. Inevitably, however, in the absence of a comprehensive treatise, many of the field's pioneers do not receive the veneration that they merit. Rather their legacies are the stereoelectronic pillars that persist in teaching and research. This ubiquity continues to afford practitioners of organic chemistry with an abundance of opportunities for creative endeavor in reaction design, in conceiving novel activation modes, in preorganizing intermediates, or in stabilizing productive transition states and products. Antipodal to steric governance, which mitigates destabilizing nonbonding interactions, stereoelectronic control allows well-defined, often complementary, conformations to be populated. Indeed, the prevalence of stabilizing hyperconjugative interactions in biosynthetic processes renders this approach to molecular preorganization decidedly biomimetic and, by extension, expansive. In this Account, the evolution and application of a simple donor-acceptor model based on the fluorine gauche effect is delineated. Founded on reinforcing hyperconjugative interactions involving C(sp)-H bonding orbitals and C(sp)-X antibonding orbitals [σ → σ*], this general stratagem has been used in conjunction with an array of secondary noncovalent interactions to achieve acyclic conformational control (ACC) in structures of interest. These secondary effects range from 1,3-allylic strain (A) through to electrostatic charge-dipole and cation-π interactions. Synergy between these interactions ensures that rotation about strategic C(sp)-C(sp) bonds is subject to the stereoelectronic requirement for antiperiplanarity (180°). Logically, in a generic [X-CH-CH-Y] system (X, Y = electron withdrawing groups) conformations in which the two C(sp)-X bonds are synclinal (i.e., gauche) are significantly populated. As such, simple donor-acceptor models are didactically and predictively powerful in achieving topological preorganization. In the case of the gauche effect, the low steric demand of fluorine ensures that the remaining substituents at the C(sp) hybridized center are placed in a predictable area of molecular space: An exit vector analogy is thus appropriate. Furthermore, the intrinsic chemical stability of the C-F bond is advantageous, thus it may be considered as an inert conformational steering group: This juxtaposition of size and electronegativity renders fluorinated organic molecules unique among the organo-halogen series. Cognizant that the replacement of one fluorine atom in the difluoroethylene motif by another electron withdrawing group preserves the gauche conformation, it was reasoned that β-fluoroamines would be intriguing candidates for investigation. The burgeoning field of Lewis base catalysis, particularly via iminium ion activation, provided a timely platform from which to explore a postulated ...

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“…This computational procedure is an improvement on the procedure from our earlier work, which already gave a good correlation with experiments. [36][37][38] All the calculations performed on these systems were done in vacuo. Intrinsic reaction coordinate (IRC) calculations were performed with the calculation of frequencies at every step of the profile.…”

Section: Computationsmentioning

confidence: 99%

Hydrogen Tunneling above Room Temperature Evidenced by Infrared Ion Spectroscopy

et al. 2017

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While hydrogen tunneling at elevated temperatures has, for instance, often been postulated in biochemical processes, spectroscopic proof is thus far limited to cryogenic conditions, under which thermal reactivity is negligible. We report spectroscopic evidence for H-tunneling in the gas phase at temperatures around 320-350 K observed in the isomerization reaction of a hydroxycarbene into an aldehyde. The charge-tagged carbene was generated in situ in a tandem mass spectrometer by decarboxylation of oxo[4-(trimethylammonio)phenyl]acetic acid upon collision induced dissociation. All ion structures involved are characterized by infrared ion spectroscopy and quantum chemical calculations. The charge-tagged phenylhydroxycarbene undergoes a 1,2-H-shift to the corresponding aldehyde with an half-life of about 10 s, evidenced by isomer-selective two-color (IR-IR) spectroscopy. In contrast, the deuterated (OD) carbene analogue showed much reduced 1,2-D-shift reactivity with an estimated half-life of at least 200 s under the experimental conditions, and provides clear evidence for hydrogen atom tunneling in the H-isotopologue. This is the first spectroscopic confirmation of hydrogen atom tunneling governing 1,2-H-shift reactions at noncryogenic temperatures, which is of broad significance for a range of (bio)chemical processes, including enzymatic transformations and organocatalysis.

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Infrared Multiphoton Dissociation Spectroscopic Analysis of Noncovalent Interactions in Organocatalysis

Cited by 10 publications

References 47 publications

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

Informing Molecular Design by Stereoelectronic Theory: The Fluorine Gauche Effect in Catalysis

Hydrogen Tunneling above Room Temperature Evidenced by Infrared Ion Spectroscopy

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