J. Du Bois

Metal-Catalyzed Nitrogen-Atom Transfer Methods for the Oxidation of Aliphatic C–H Bonds

For more than a century, chemists have endeavored to discover and develop reaction processes that enable the selective oxidation of hydrocarbons. In the 1970s, Abramovitch and Yamada described the synthesis and electrophilic reactivity of sulfonyliminoiodinanes (RSO(2)N═IPh), demonstrating the utility of this new class of reagents to function as nitrene equivalents. Subsequent investigations by Breslow, Mansuy, and Müller would show such oxidants to be competent for alkene and saturated hydrocarbon functionalization when combined with transition metal salts or metal complexes, namely those of Mn, Fe, and Rh. Here, we trace our own studies to develop N-atom transfer technologies for C-H and π-bond oxidation. This Account discusses advances in both intra- and intermolecular amination processes mediated by dirhodium and diruthenium complexes, as well as the mechanistic foundations of catalyst reactivity and arrest. Explicit reference is given to questions that remain unanswered and to problem areas that are rich for discovery. A fundamental advance in amination technology has been the recognition that iminoiodinane oxidants can be generated in situ in the presence of a metal catalyst that elicits subsequent N-atom transfer. Under these conditions, both dirhodium and diruthenium lantern complexes function as competent catalysts for C-H bond oxidation with a range of nitrogen sources (e.g., carbamates, sulfamates, sulfamides, etc.), many of which will not form isolable iminoiodinane equivalents. Practical synthetic methods and applications thereof have evolved in parallel with inquiries into the operative reaction mechanism(s). For the intramolecular dirhodium-catalyzed process, the body of experimental and computational data is consistent with a concerted asynchronous C-H insertion pathway, analogous to the consensus mechanism for Rh-carbene transfer. Other studies reveal that the bridging tetracarboxylate ligand groups, which shroud the dirhodium core, are labile to exchange under standard reaction conditions. This information has led to the generation of chelating dicarboxylate dinuclear rhodium complexes, exemplified by Rh(2)(esp)(2). The performance of this catalyst system is unmatched by other dirhodium complexes in both intra- and intermolecular C-H amination reactions. Tetra-bridged, mixed-valent diruthenium complexes function as effective promoters of sulfamate ester oxidative cyclization. These catalysts can be crafted with ligand sets other than carboxylates and are more resistant to oxidation than their dirhodium counterparts. A range of experimental and computational mechanistic data amassed with the tetra-2-oxypyridinate diruthenium chloride complex, [Ru(2)(hp)(4)Cl], has established the insertion event as a stepwise pathway involving a discrete radical intermediate. These data contrast dirhodium-catalyzed C-H amination and offer a cogent model for understanding the divergent chemoselectivity trends observed between the two catalyst types. This work constitutes an important step toward the ultimate goal of achieving predictable, reagent-level control over product selectivity.

A Rh‐Catalyzed C−H Insertion Reaction for the Oxidative Conversion of Carbamates to Oxazolidinones

Selective intramolecular alkane oxidations: an RhII carboxylate catalyzed C-H amination reaction facilitates the preparation of 1,2-amino alcohols from primary carbamates. The reaction is stereospecific, providing access to chiral α-branched amines from optically pure starting materials with no loss in enantiomeric excess.

Metal-Catalyzed Oxidations of C–H to C–N Bonds

This chapter offers a general review of selective methods for the oxidative conversion of C-H to C-N bonds. Special focus has been given to the many disparate catalyst types that are capable of promoting this unique transformation.

A diruthenium catalyst for selective, intramolecular allylic C-H amination: reaction development and mechanistic insight gained through experiment and theory.

The mixed-valent paddlewheel complex tetrakis(2-oxypyridinato)diruthenium(II,III) chloride, [Ru(2)(hp)(4)Cl], catalyzes intramolecular allylic C-H amination with bis(homoallylic) sulfamate esters. These results stand in marked contrast to reactions performed with dirhodium catalysts, which favor aziridine products. The following discussion constitutes the first report of C-H amination using complexes such as [Ru(2)(hp)(4)Cl] and related diruthenium adducts. Computational and experimental studies implicate a mechanism for [Ru(2)(hp)(4)Cl]-promoted C-H amination involving hydrogen-atom abstraction/radical recombination and the intermediacy of a discrete, albeit short-lived, diradical species. The collective data offer a coherent model for understanding the preference of this catalyst to oxidize allylic (and benzylic) C-H bonds.

A Chiral Rhodium Carboxamidate Catalyst for Enantioselective C−H Amination

Rh2(S-nap)4, a chiral dirhodium tetracarboxamidate complex, has been developed and shown to be an effective catalyst for the asymmetric, intramolecular C-H amination of sulfamate esters. Enantiomeric excesses range from 60-99% for a collection of disparately substituted 3-arylpropylsulfamates. In addition, Rh2(S-nap)4 is found to promote chemoselective allylic C-H oxidation of unsaturated sulfamates, a property not observed with other dirhodium complexes tested to date.

A synthesis of (+)-saxitoxin.

An asymmetric synthesis of the bis-guanidinium poison, (+)-saxitoxin (STX), is described. Commencing from an N,O-acetal starting material made readily available through sulfamate ester C-H amination, the completed route to STX showcases the utility of oxathiazinane dioxide heterocycles for the assembly of polyfunctionalized amine derivatives. In the final preparative stages, an unusual nine-membered ring guanidine intermediate is oxidized selectively and made to undergo dehydrative cyclization to afford the tricyclic core of the natural product. Access to STX and related structures will provide unique pharmacological tools for the study of voltage-regulated Na+ ion channel proteins.

Understanding the Differential Performance of Rh2(esp)2 as a Catalyst for C–H Amination

Catalytic amination of saturated C-H bonds is performed efficiently with the use of Rh(2)(esp)(2). Efforts to identify pathways for catalyst degradation and/or arrest have revealed a single-electron oxidation event that gives rise to a red-colored, mixed-valence dimer, [Rh(2)(esp)(2)](+). This species is fortuitously reduced by carboxylic acid, a byproduct generated in the reaction cycle with each turnover of the diacyloxyiodine oxidant. These findings have led to the conclusion that the high performance of Rh(2)(esp)(2) is due in part to the superior kinetic stability of its one-electron oxidized form relative to other dimeric Rh complexes.

CH Hydroxylation Using a Heterocyclic Catalyst and Aqueous H2O2

Substituted benzoxathiazines function as catalysts for the selective hydroxylation of tertiary C-H bonds. Mechanistic studies have revealed an unanticipated disparity between oxaziridine reactivity and catalyst performance and have given way to a new catalyst and an aqueous H(2)O(2) reaction protocol that greatly facilitate such transformations (see scheme).

Ruthenium-Catalyzed Hydroxylation of Unactivated Tertiary C−H Bonds

The combination of catalytic RuCl(3) and pyridine with KBrO(3) as the stoichiometric oxidant is shown to efficiently promote the hydroxylation of unactivated tertiary C-H bonds. Substrates possessing different polar functional groups--ester, epoxide, sulfone, oxazolidinone, carbamate, and sulfamate--are found to engage in this reaction to give alcohol products in yields generally exceeding 50%. As judged based on efficiency, ease of operation, substrate scope, and selectivity toward tertiary C-H centers, the method appears competitive with other C-H hydroxylation processes.

Synthesis of 1,3-Diamines Through Rhodium-Catalyzed C—H Insertion

A grand opening: N-Boc-N-alkylsulfamides are effective substrates for the title transformation. Oxidative cyclization is highly chemoselective as well as being both stereospecific and diastereoselective. With the advent of new protocols that facilitate ring opening of the six-membered-ring heterocyclic products, access to differentially protected 1,3-diamines has been made possible (see scheme).