Finding Opportunities from Surprises and Failures. Development of Rhodium-Stabilized Donor/Acceptor Carbenes and Their Application to Catalyst-Controlled C-H Functionalization

Advances in the catalyst- and reagent-controlled site-divergent intermolecular functionalization of C(sp 3)–H bonds

Intermolecular C(sp 3)–H bond functionalization reactions promise to revolutionize how we synthesize organic molecules by enabling the introduction of functionality at previously inert sites. However, one of the greatest challenges in this research field is site-selectivity, wherein chosen C(sp 3)–H bonds must be selectively functionalized and other C(sp 3)–H bonds with similar stereoelectronic properties must remain intact. To address this problem, chemists have developed methods that rely on targeting innately more reactive C(sp 3)–H bonds or on using pre-installed functional groups to direct a catalyst or reagent to a particular C(sp 3)–H bond. However, such approaches invariably have limited applicability because only a handful of innately reactive C(sp 3)–H bonds or those nearby certain functional groups can be functionalized with good site-selectivity. To overcome these limitations, chemists also have developed catalysts and reagents that control the site of C(sp 3)–H bond functionalization and have begun to unlock the potential of these reactions to achieve the site-divergent functionalization of C(sp 3)–H bonds, wherein the site of functionalization is changed by modulating the stereoelectronic properties of the catalyst or reagent. This short review will provide a summary of selected examples of catalyst- and reagent-controlled site-divergent intermolecular functionalization of C(sp 3)–H bonds, the factors responsible for modulating the site selectivity of these reactions, and will identify potential areas worthy of future research in this field.

Distal Allylic/Benzylic C-H Functionalization of Silyl Ethers Using Donor/Acceptor Rhodium(II) Carbenes

Regio- and stereoselective distal allylic/benzylic C-H functionalization of allyl and benzyl silyl ethers was achieved using rhodium(II) carbenes derived from N-sulfonyltriazoles and aryldiazoacetates as carbene precursors. The bulky rhodium carbenes led to highly site-selective functionalization of less activated allylic and benzylic C-H bonds even in the presence of electronically preferred C-H bonds located α to oxygen. The dirhodium catalyst Rh2 (S-NTTL)4 is the most effective chiral catalyst for triazole-derived carbene transformations, whereas Rh2 (S-TPPTTL)4 works best for carbenes derived from aryldiazoacetates. The reactions afford a variety of δ-functionalized allyl silyl ethers with high diastereo- and enantioselectivity. The utility of the present method was demonstrated by its application to the synthesis of a 3,4-disubstituted l-proline scaffold.

Tandem Carbenoid C-H Functionalization/Conia-ene Cyclization of N-Propargyl Indoles Generates Pyrroloindoles under Cooperative Rh(II)/Zn(II) Catalysis

The decomposition of diazodicarbonyl compounds in the presence of various metal catalysts has become a reliable method for the functionalization of indoles via carbenoid intermediates. Exploiting the nucleophilic reactivity of the in situ generated malonic ester product formed, we herein report a tandem C-H functionalization/Conia-ene cyclization of N-alkyne tethered indoles. This double functionalization of diazodicarbonyls generates a range of pyrrolo[1,2-a]-, pyrido[1,2-a]-, and azepino[1,2-a]indole products with good synthetic efficiency.

C-H Insertion Reactions of Donor/Donor Carbenes: Inception, Investigation, and Insights

Insertion reactions of donor/donor carbenes have emerged from obscurity to become a versatile method for the synthesis of a variety of cyclic structures with excellent control of diastereo- and enantioselectivity. This Account describes the origin of this project as part of a natural product synthesis and the ensuing decade of reaction development that has resulted in new asymmetric methods as well as intriguing tangential observations.

Iron catalysts with N-ligands for carbene transfer of diazo reagents

This review provides an overview of the catalytic activity of iron complexes of nitrogen ligands in driving carbene transfers towards CC, C–H and X–H bonds. The reactivity of diazo reagents is discussed as well as the proposed reaction mechanisms.