Controlling Catalyst Behavior in Lewis Acid-Catalyzed Carbonyl-Olefin Metathesis

Abstract

Lewis acid-catalyzed carbonyl-olefin metathesis has introduced a new means for revealing the behavior of Lewis acids. In particular, this reaction has led to the observation of new solution behaviors for FeCl3 that may qualitatively change how we think of Lewis acid activation. For example, catalytic metathesis reactions operate in the presence of superstoichiometric amounts of carbonyl, resulting in the formation of highly ligated (octahedral) iron geometries. These structures display reduced activity, decreasing catalyst turnover. As a result, it is necessary to steer the Fe-center away from inhibiting pathways to improve the reaction efficiency and augment yields for recalcitrant substrates. Herein, we examine the impact of the addition of TMSCl to FeCl3-catalyzed carbonyl-olefin metathesis, specifically for substrates that are prone to byproduct inhibition. Through kinetic, spectroscopic, and colligative experiments, significant deviations from the baseline metathesis reactivity are observed, including mitigation of byproduct inhibition as well as an increase in the reaction rate. Quantum chemical simulations are used to explain how TMSCl induces a change in catalyst structure that leads to these kinetic differences. Collectively, these data are consistent with the formation of a silylium catalyst, which induces the reaction through carbonyl binding. The FeCl3 activation of Si–Cl bonds to give the silylium active species is expected to have significant utility in enacting carbonyl-based transformations.

Cation–anion confined hydrogen-bonding catalysis strategy for ring-closing C–O/O–H metathesis of alkoxy alcohols under metal-free conditions

“…Metathesis reactions are widely applied to synthesize complex molecules, pharmaceuticals, and natural products. 1–7 Particularly, ring-closing metathesis (RCM) reactions of multiple bonds, such as olefin-olefin metathesis 2,8–11 and carbonyl-olefin metathesis 5,12–16 have become powerful techniques for the preparation of various unsaturated rings in organic synthesis (Scheme 1a and b). In contrast, the RCM reactions of the C–X bond especially involving the C–O bond are rarely reported and still challenging owing to the low reactivity of single bonds.…”