Absolute Autocatalytic Amplification under Heterogenous Phase Conditions Involving Subsequent Hydride Transfer and a Hemiacetal Intermediate

Abstract

In contrast to the homogenous phase, absolute asymmetric amplification of pyridine alkanol is observed under heterogenous conditions. The reaction of iPr2Zn vapor on a solid powder of pyridine carbaldehyde induced autocatalytic amplification of chirality providing the alkanol product with an enantiopurity up to 90 % ee. In this study, side products were identified and their formation is traced back to redox pathways involving hydride transfer, namely a chiral ester resulting most probably from a disproportionation of the starting aldehyde into a hemiacetal and subsequent hydride loss following a Claisen‐Tishchenko mechanism. These observations provide new perspectives in the elucidation of the mechanism of amplification of chirality in the Soai reaction.

One Soai reaction, two mechanisms?

“…Likewise, even though hemiacetals are less likely in the pyridine system, they may not be fully excluded depending on the reaction conditions: Amedjkouh and co-workers found indirect evidence for a transient hemiacetal with tBu Py/ tBu PyII under heterogeneous conditions. 48 It is also worth looking at the initial reaction conditions in both proposals, which favour each the respective defended model: Denmark added the aldehyde to an alcohol initiator/ZniPr 2 -mixture, in which SMStetramers (but no hemiacetals) can already have formed; in the Trapp procedure ZniPr 2 is added to an aldehyde/alcohol initiator mixture, which may have already generated non-metalated hemiacetals but no alkoxide tetramers. Trapp's mixed dialkylzincexperiments show that the starting conditions indeed have a strong influence on the final product ee.…”

Symmetry Breaking and Autocatalytic Amplification in Soai Reaction Confined within UiO‐MOFs under Heterogenous Conditions.

“…Recently, we have reported two set‐ups to perform vapour‐phase reactions [8b] . These set‐ups have been employed for the experiments described in the next section.…”

“…Recently, we have reported two set‐ups to perform vapour‐phase reactions. [8b] These set‐ups have been employed for the experiments described in the next section. In the first set‐up, the Soai reagent (pristine 1 or included in the MOFs) is placed on the top of a cylindrical glass support, arranged inside a glass vial.…”

“… [13] Dimers were proposed as the resting state of the catalyst, while tetramers were found as the active catalytic species. An alternative mechanism involves hemiacetal complexes, first observed as transient intermediates, [16] then by formation of subsequent derivatives under heterogenous conditions [8b] and further by in situ mass spectroscopic investigations. [17] Nevertheless, as proposed by Brown et al., [11c] once the reaction provides a tiny enantiomeric imbalance in the products, the ee can easily be propagated and amplified by the oligomerization of the reaction species.…”

The Use of UiO-Type MOFs in Vapour Phase Soai Reactions

“…Recently, we have reported two set-ups to perform vapour-phase reactions [23] . These set-ups have been employed for the experiments described in the next section.…”