Intramolecular oxyacylation of alkenes using a hydroxyl directing group

N-Pyridylimidates as Traceless Acyl Equivalents for Directed C-O Bond Functionalization

Directing groups are a common strategy to target traditionally inert bonds, with an easily removable directing group being ideal. Herein we disclose our method for rhodium-catalyzed C-O bond functionalization of N-pyridylimidates using a recyclable and traceless amine directing group. In addition to the substrate scope, we discuss the behavior of this class of compounds and how that behavior affects their reactivity.

Pd-Promoted Cyclization of (Z)-Pent-2-en-4-yn-1-yl Alkanoates Leading to Furans via an Acyl Group Shift and Further Synthetic Transformation

Palladium-promoted acyl migration was observed in the reaction of (Z)-pent-2-en-4-yn-1-yl alkanoates in the presence of DBU to yield 2-(alkanoylmethyl)furan derivatives (2) in high yields. Compounds 2 then underwent oxidative decarbonylation to afford 2-acylfurans with O₂ as the oxidant under mild and metal-free conditions. The mechanistic pathway of the reaction involving an intramolecular cyclization and acyl group shift is discussed.

Hydroxy-directed fluorination of remote unactivated C(sp3)-H bonds: a new age of diastereoselective radical fluorination

We report a photochemically induced, hydroxy-directed fluorination that addresses the prevailing challenge of high diastereoselectivity in this burgeoning field. Numerous simple and complex motifs showcase a spectrum of regio- and stereochemical outcomes based on the configuration of the hydroxy group. Notable examples include a long-sought switch in the selectivity of the refractory sclareolide core, an override of benzylic fluorination, and a rare case of 3,3'-difluorination. Furthermore, calculations illuminate a low barrier transition state for fluorination, supporting our notion that alcohols are engaged in coordinated reagent direction. A hydrogen bonding interaction between the innate hydroxy directing group and fluorine is also highlighted for several substrates with 19F-1H HOESY experiments, calculations, and more.

Regio- and Stereoselective Functionalization Enabled by Bidentate Directing Groups

Chelation-assisted C-H bond and alkene functionalization using bidentate directing groups offers an elegant and versatile approach to overcome regiocontrol issues by allowing the catalyst to come into close proximity with the targeted sites. In this personal account, we highlight our recent works in developing regio- and stereocontrolled functionalizations through transition-metal catalysis enabled by bidentate directing groups. We classify our results into two categories: (1) regioselective alkene functionalization using bidentate directing groups, and (2) asymmetric C-H functionalization using chiral bidentate directing groups. Furthermore, density functional theory studies to elucidate the origin of the regio- and stereoselectivity exerted by bidentate directing groups are discussed.

Effects of ligands on the migratory insertion of alkenes into rhodium-oxygen bonds

Migratory insertions of olefins into metal-oxygen bonds are elementary steps of important catalytic processes, but well characterised complexes that undergo this reaction are rare, and little information on the effects of ancillary ligands on such reactions has been gained. We report a series of alkoxo alkene complexes of rhodium(i) that contain a range of bidentate ligands and that undergo insertion of the alkene. Our results show that complexes containing less electron-donating ancillary ligands react faster than their counterparts containing more electron-donating ancillary ligands, and that complexes possessing ligands with larger bite angles react faster than those with smaller bite angles. External added ligands had several effects on the reactions, including an inhibition of olefin isomerisation in the product and acceleration of the displacement of the product from complexes of ancillary ligands with small bite angles. Complementary computational studies help elucidate the details of these insertion processes.

Oxyacylation of Iodoalkynes: Gold(I)-Catalyzed Expeditious Access to Benzofurans

(Acetonitrile)[1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene] gold(I) catalyzes the cycloisomerization of 2-(iodoethynyl)aryl esters to give 3-iodo-2-acyl benzofurans. This catalytic transformation is the result of an unprecedented selective synthetic event, which comprises a [1,2]-iodine shift, a C-O ring-closure step, and a C-C bond-formation that installs the ketone functionality into the new ring. Experimental evidence supports the involvement of a β-iodo-substituted gold vinylidene as the intermediate species. The reaction tolerates different substitution patterns at the phenol moiety and a wide diversity of groups at the carboxylic fragment, including not only alkyl but also alkenyl, aryl, and heteroaryl groups.

Cross-Coupling of Heteroatomic Electrophiles

At the advent of cross-coupling chemistry, carbon electrophiles based on halides or pseudohalides were the only suitable electrophilic coupling partners. Almost two decades passed before the first cross-coupling reaction of heteroatom-based electrophiles was reported. Early work by Murai and Tanaka initiated investigations into silicon electrophiles. Narasaka and Johnson pioneered the way in the use of nitrogen electrophiles, while Suginome began the exploration of boron electrophiles. The chemistry reviewed within provides perspective on the use of heteroatomic electrophiles, specifically silicon-, nitrogen-, boron-, oxygen-, and phosphorus-based electrophiles in transition-metal catalyzed cross-coupling. For the purposes of this review, a loose definition of cross-coupling is utilized; all reactions minimally proceed via an oxidative addition event. Although not cross-coupling in a traditional sense, we have also included catalyzed reactions that join a heteroatomic electrophile with an in situ generated nucleophile. However, for brevity, those involving hydroamination or C-H activation as a key step are largely excluded. This work includes primary references published up to and including October 2018.

Metal-free alkene carbooxygenation following tandem intramolecular alkoxylation/Claisen rearrangement: stereocontrolled access to bridged [4.2.1] lactones

Alkene carbooxygenation has attracted considerable attention over the past few decades as this approach provides an efficient access to various oxygen-containing molecules, especially the valuable O-heterocycles. However, examples of catalytic alkene carbooxygenation via a direct C-O cleavage are quite scarce, and the C-O cleavage in these cases is invariably initiated by transition metal-catalyzed oxidative addition. We report here a novel Brønsted acid-catalyzed intramolecular alkoxylation-initiated tandem sequence, which represents the first metal-free intramolecular alkoxylation/Claisen rearrangement. Significantly, an unprecedented Brønsted acid-catalyzed intramolecular alkene insertion into the C-O bond via a carbocation pathway was discovered. This method allows the stereocontrolled synthesis of valuable indole-fused bridged [4.2.1] lactones, providing ready access to biologically relevant scaffolds in a single synthetic step from an acyclic precursor. Moreover, such an asymmetric cascade cyclization has also been realized by employing a traceless chiral directing group. Control experiments favor the feasibility of a carbocation pathway for the process. In addition, biological tests showed that some of these newly synthesized indole-fused lactones exhibited their bioactivity as antitumor agents against different breast cancer cells, melanoma cells, and esophageal cancer cells.

Integrating Metal-Catalyzed C-H and C-O Functionalization To Achieve Sterically Controlled Regioselectivity in Arene Acylation

One major goal of organometallic chemists is the direct functionalization of the bonds most recurrent in organic molecules: C-H, C-C, C-O, and C-N. An even grander challenge is C-C bond formation when both precursors are of this category. Parallel to this is the synthetic goal of achieving reaction selectivity that contrasts with conventional methods. Electrophilic aromatic substitution (EAS) via Friedel-Crafts acylation is the most renowned method for the synthesis of aryl ketones, a common structural motif of many pharmaceuticals, agrochemicals, fragrances, dyes, and other commodity chemicals. However, an EAS synthetic strategy is only effective if the desired site for acylation is in accordance with the electronic-controlled regioselectivity of the reaction. Herein we report steric-controlled regioselective arene acylation with salicylate esters via iridium catalysis to access distinctly substituted benzophenones. Experimental and computational data indicate a unique reaction mechanism that integrates C-O activation and C-H activation with a single iridium catalyst without an exogenous oxidant or base. We disclose an extensive exploration of the synthetic scope of both the arene and the ester components, culminating in the concise synthesis of the potent anticancer agent hydroxyphenstatin.

Development and Mechanistic Study of Quinoline-Directed Acyl C-O Bond Activation and Alkene Oxyacylation Reactions

The intramolecular addition of both an alkoxy and acyl substituent across an alkene, oxyacylation of alkenes, using rhodium catalyzed C-O bond activation of an 8-quinolinyl ester is described. Our unsuccessful attempts at intramolecular carboacylation of ketones via C-C bond activation ultimately informed our choice to pursue and develop the intramolecular oxyacylation of alkenes via quinoline-directed C-O bond activation. We provide a full account of our catalyst discovery, substrate scope, and mechanistic experiments for quinoline-directed alkene oxyacylation.