Sensitive and Accurate ¹³C Kinetic Isotope Effect Measurements Enabled by Polarization Transfer

Abstract

Polarization transfer is demonstrated as a sensitive technique for the measurement of isotopic fractionation of protonated carbons at natural abundance. This method allows kinetic isotope effects (KIEs) to be determined with substantially less material or shorter acquisition time compared with traditional experiments. Computations quantitatively reproduce the KIEs in a Diels–Alder reaction and a catalytic glycosylation. The glycosylation is shown to occur by an effectively concerted mechanism.

Blue Light Photocatalytic Glycosylation without Electrophilic Additives

“…Such a concerted process can be interpreted as either a direct attack on the activated donor or an attack on the initially formed radical ion triplet before diffusive equilibration (Scheme 4). There is a steadily increasing body of kinetic evidence 28–37 that a variety of stereoselective glycosylation reactions proceed by associative reaction pathways rather than by dissociatively free glycosyl oxocarbenium ions that have only been observed in super acidic media. 22,23,38,39 In such classical glycosylation reactions, the associative pathways are more prevalent with more reactive acceptor alcohols, 40–43 consistent with the observed pattern in Table 2.…”

Stereospecific β‐l‐Rhamnopyranosylation through an SNi‐Type Mechanism by Using Organoboron Reagents

“…In the case of glycoside hydrolyses and glycosylation reactions,the 13 CKIEs for S N 1and dissociative S N 2r eactions are around 1.00 and 1.03, respectively. [9,15,16] Forthe present glycosylation, the experimental 13 C KIE at the anomeric carbon was 1.0034(51) (Table S1), which indicates that this glycosylation also can be regarded as either an S N i-type reaction, ac oncerted S N ir eaction with an exploded transition state,o ra nS N 1r eaction with an extremely short-lived intermediate. [9,15,16] Forthe present glycosylation, the experimental 13 C KIE at the anomeric carbon was 1.0034(51) (Table S1), which indicates that this glycosylation also can be regarded as either an S N i-type reaction, ac oncerted S N ir eaction with an exploded transition state,o ra nS N 1r eaction with an extremely short-lived intermediate.…”

“…[14] In addition, the 13 CK IEs for stereospecific glycosylations with highly dissociative transition states have been approximately 1.00. [9,15,16] Forthe present glycosylation, the experimental 13 C KIE at the anomeric carbon was 1.0034(51) (Table S1), which indicates that this glycosylation also can be regarded as either an S N i-type reaction, ac oncerted S N ir eaction with an exploded transition state,o ra nS N 1r eaction with an extremely short-lived intermediate.…”

Versatile Glycosyl Sulfonates in β‐Selective C‐Glycosylation

“…To rule out the possibility of a base‐catalyzed epimerization of the α‐C‐glycoside in situ, we turned to primary 13 C kinetic isotope studies . Using Jacobsen and Kwan's modification of the Singleton procedure, we measured a KIE value of 1.029 for the reaction. This value is typical of S N 2‐like glycosylations, and is in line with values that have previously been reported by the Crich, Chan and Bennet, and our own groups …”