Expanding the Protecting Group Scope for the Carbonyl Olefin Metathesis Approach to 2,5-Dihydropyrroles

InCl3-catalyzed intramolecular carbonyl-olefin metathesis

An efficient and novel synthetic strategy for the generation of different carbocyclic moieties by ring closing carbonyl-olefin metathesis is reported. Herein, we describe a sustainably attractive protocol for one of the most powerful carbon-carbon bond-forming reactions, based on solvent-reduction, use of InCl3 catalyst, and microwave irradiation, affording target compounds with yields up to 96%.

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

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.

Supramolecular Capsule-Catalyzed Highly β-Selective Furanosylation Independent of the SN1/SN2 Reaction Pathway

The resorcin[4]arene capsule was found to catalyze β-selective furanosylation reactions for a variety of different furanosyl donors: α-d- and α-l-arabinosyl-, α-l-fucosyl-, α-d-ribosyl-, α-d-xylosyl-, and even α-d-lyxosyl fluorides. The scope is only limited by the inherently finite volume inside the closed capsular catalyst. The catalyst is readily available on a multi-100 g scale and can be recycled for at least seven rounds without significant loss in activity, yield, and selectivity. The mechanistic investigations indicated that the furanosylation mechanism is shifted toward an SN1 reaction on the mechanistic continuum between the prototypical SN1 and SN2 substitution types, as compared to the pyranosylation reaction inside the same catalyst. This is especially true for the lyxosyl donor, as indicated by the nucleophile reaction order of 0.26, and supported by metadynamics calculations. The mechanistic shift toward SN1 is of high interest as it indicates that this catalyst not only enables β-selective furanosylations and pyranoslyations independently of the substrate configuration but in addition also independently of the operating mechanism. To our knowledge, there is no alternative catalyst available that displays such properties.

Toward Homogeneous Brønsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Metathesis Reactions

The carbonyl-olefin metathesis (COM) reaction is an attractive approach for the formation of a new carbon-carbon double bond from a carbonyl precursor. In principle, this reaction can be promoted by the activation of the carbonyl group with a Brønsted acid catalyst; however, it is often complicated as a result of unwanted side reactions under acidic conditions. Thus, there have been only a very few examples of Brønsted-acid-catalyzed COM reactions, all of which required specially designed setups. Herein, we report a new practical homogeneous Brønsted-acid-catalyzed protocol using nitromethane, a readily available solvent, to promote intramolecular ring-closing COM reactions.

A resorcin[4]arene hexameric capsule as a supramolecular catalyst in elimination and isomerization reactions

The hexameric resorcin[4]arene capsule as a self-assembled organocatalyst promotes a series of reactions like the carbonyl–ene cyclization of (S)-citronellal preferentially to isopulegol, the water elimination from 1,1-diphenylethanol, the isomerization of α-pinene and β-pinene preferentially to limonene and minor amounts of camphene. The role of the supramolecular catalyst consists in promoting the protonation of the substrates leading to the formation of cationic intermediates that are stabilized within the cavity with consequent peculiar features in terms of acceleration and product selectivity. In all cases the catalytic activity displayed by the hexameric capsule is remarkable if compared to many other strong Brønsted or Lewis acids.