Characterization of Elusive Reaction Intermediates Using Infrared Ion Spectroscopy: Application to the Experimental Characterization of Glycosyl Cations

Abstract

A detailed understanding of the reaction mechanism(s) leading to stereoselective product formation is crucial to understanding and predicting product formation and driving the development of new synthetic methodology. One way to improve our understanding of reaction mechanisms is to characterize the reaction intermediates involved in product formation. Because these intermediates are reactive, they are often unstable and therefore difficult to characterize using experimental techniques. For example, glycosylation reactions are critical steps in the chemical synthesis of oligosaccharides and need to be stereoselective to provide the desired α- or β-diastereomer. It remains challenging to predict and control the stereochemical outcome of glycosylation reactions, and their reaction mechanisms remain a hotly debated topic. In most cases, glycosylation reactions take place via reaction mechanisms in the continuum between S_N1- and S_N2-like pathways. S_N2-like pathways proceeding via the displacement of a contact ion pair are relatively well understood because the reaction intermediates involved can be characterized by low-temperature NMR spectroscopy. In contrast, the S_N1-like pathways proceeding via the solvent-separated ion pair, also known as the glycosyl cation, are poorly understood. S_N1-like pathways are more challenging to investigate because the glycosyl cation intermediates involved are highly reactive. The highly reactive nature of glycosyl cations complicates their characterization because they have a short lifetime and rapidly equilibrate with the corresponding contact ion pair. To overcome this hurdle and enable the study of glycosyl cation stability and structure, they can be generated in a mass spectrometer in the absence of a solvent and counterion in the gas phase. The ease of formation, stability, and fragmentation of glycosyl cations have been studied using mass spectrometry (MS). However, MS alone provides little information about the structure of glycosyl cations. By combining mass spectrometry (MS) with infrared ion spectroscopy (IRIS), the determination of the gas-phase structures of glycosyl cations has been achieved. IRIS enables the recording of gas-phase infrared spectra of glycosyl cations, which can be assigned by matching to reference spectra predicted from quantum chemically calculated vibrational spectra. Here, we review the experimental setups that enable IRIS of glycosyl cations and discuss the various glycosyl cations that have been characterized to date. The structure of glycosyl cations depends on the relative configuration and structure of the monosaccharide substituents, which can influence the structure through both steric and electronic effects. The scope and relevance of gas-phase glycosyl cation structures in relation to their corresponding condensed-phase structures are also discussed. We expect that the workflow reviewed here to study glycosyl cation structure and reactivity can be extended to many other reaction types involving difficult-to-characterize ionic intermediates.

Scheme 1. Stereoselective Glycosylation via (A) the Neighboring Group Participation of a C-2 Acyl Group and (B) the Neighboring Group Participation of a C-3 Acyl Group and (C) the Glycosylation Mechanisms of Glycosyl Triflate Intermediates

Figure 1

(A) Principle of electrospray ionization. (B) Direct in-source formation of a glycosyl cation. (C) Formation of a glycosyl cation through CID.

Scheme 2. (A) Formation of glycosyl cations from acetylated glycoside precursors using CID (RA = Relative Abundance of the Glycosyl Cation Fragment), (B) Influence of the Protecting Group Nature on Glycosyl Cation Formation, (C) Influence of Protecting Groups and Their Dipole Moment on the Energetics of Sialyl Oxocarbenium Ions, and (D) Evaluation of the Effects of Linkage Stereochemistry, Leaving Group Geometry, and Protecting Group Pattern on the Stability of the Glycosyl Linkage Using Survival Yield Analysis

Figure 2

(A) Principle of IRMPD spectroscopy at ambient temperature. (B) Principle of helium nanodroplet spectroscopy at 0.37 K.

Figure 3

Comparison of computed IR spectra (filled gray) and measured IRMPD spectra (black line). IRMPD spectra of (A) a mannosyl oxocarbenium ion and (B) a mannosyl dioxolanium ion.

Scheme 3. (A) Proposed Intermediates in the Reaction of 4-Benzyl-6,3-uronic Acid Mannolactones, (B) Overview of Characterized Glycosyl Cations Derived from 6,3-Uronic Acid Lactone Donors (6 and 8, LG = SOPh; 7 and 9, LG = SPh), and (C) Proposed Intermediates in the Reaction of 4-Acetyl-6,3-uronic Acid Mannolactones

Scheme 4. Overview of Characterized Glycosyl Cations Derived from Galactosides Using He Nanodroplet Spectroscopy

15–18, LG = TCAI.

Scheme 5. Overview of Glycosyl Cations Characterized from Monoacetyl Glycoside Precursors

22–24, LG = SEt; 25 and 30, LG = SPh; 26–29, LG = SOPh.

Scheme 6. (A) Experimentally Determined Stereoselectivities for Model Glycosylations of C-3 Acyl Glycosides and (B) Detection of the Dioxanium Ion via CEST-NMR

HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol, TFE = 2,2,2-trifluoroethanol, DFE = 2,2-difluoroethanol, and MFE = 2-fluoroethanol.