Ligand-Based Carbon–Nitrogen Bond Forming Reactions of Metal Dinitrosyl Complexes with Alkenes and Their Application to C–H Bond Functionalization

Abstract

Over the past few decades, researchers have made substantial progress in the development of transition metal complexes that activate and functionalize C-H bonds. For the most part, chemists have focused on aliphatic and aromatic C-H bonds and have put less effort into complexes that activate and functionalize vinylic C-H bonds. Our groups have recently developed a novel method to functionalize vinylic C-H bonds that takes advantage of the unique ligand-based reactivity of a rare class of metal dinitrosyl complexes. In this Account, we compare and discuss the chemistry of cobalt and ruthenium dinitrosyl complexes, emphasizing alkene binding, C-H functionalization, and catalysis. Initially discovered in the early 1970s by Brunner and studied more extensively in the 1980s by the Bergman group, the cyclopentadienylcobalt dinitrosyl complex CpCo(NO)2 reacts reversibly with alkenes to give, in many cases, stable and isolable cobalt dinitrosoalkane complexes. More recently, we found that treatment with strong bases, such as lithium hexamethyldisilazide, Verkade's base, and phosphazene bases, deprotonates these complexes and renders them nucleophilic at the carbon α to the nitroso group. This conjugate anion of metal dinitrosoalkanes can participate in conjugate addition to Michael acceptors to form new carbon-carbon bonds. These functionalized cobalt complexes can further react through alkene exchange to furnish the overall vinylic C-H functionalized organic product. This stepwise sequence of alkene binding, functionalization, and retrocycloaddition represents an overall vinylic C-H functionalization reaction of simple alkenes and does not require directing groups. We have also developed an asymmetric variant of this reaction sequence and have used this method to synthesize C1- and C2-symmetric diene ligands with high enantioinduction. Building upon these stepwise reactions, we eventually developed a simple one-pot procedure that uses stoichiometric amounts of a cobalt dinitrosoalkane complex for both inter- and intramolecular C-H functionalization. We can achieve catalysis in one-pot intramolecular reactions with a limited range of substrates. Our groups have also reported an analogous ruthenium dinitrosyl complex. In analogy to the cobalt complex, this ruthenium complex reacts with alkenes in the presence of neutral bidentate ligands, such as TMEDA, to give octahedral dinitrosoalkane complexes. Intramolecular functionalization or cyclization of numerous ruthenium dinitrosoalkane complexes proceeds under mild reaction conditions to give the functionalized organic products in excellent yields. However, despite extensive efforts, so far we have not been able to carry out intermolecular reactions of these complexes with a variety of electrophiles or C-H functionalization reactions. Although additional work is necessary to further boost the catalytic capabilities of both cobalt and ruthenium dinitrosyl complexes for vinylic C-H functionalization of simple alkenes, we believe this ligand-based vinylic C-H functionalization reaction has provided chemists with a useful set of tools for organic synthesis.