Progress in Double Dearomatization Reactions

Dearomatization reactions are among the most straightforward and efficient methods for creating sp3-rich cyclic systems from simple, readily available arenes. These reactions have been widely applied in the total synthesis of natural products, medicinal chemistry, and material sciences. The fruitful development of dearomatization strategies and methodologies targeting single aromatic substrate over the past decades has paved the way for more sophisticated multiple dearomatization processes, which offer greater advantages in constructing molecular complexity. Double dearomatization reactions have made significant pioneering strides in recent years. This review will provide an overview of the strategies and detailed examples of multiple dearomatization reactions involving various aromatic compounds, along with a discussion of the related mechanisms and the major challenges that remain in this intriguing yet formidable field.

Nickel-Catalyzed Enantioselective Reductive N-Cyclization-Thiolation Reaction of Alkene-Tethered Oxime Esters and Disulfides

Asymmetric aza-Heck cyclization and coupling reactions offer efficient access to enantioenriched N-heterocycles, yet the current studies focus primarily on sequential C-N and C-C bond formation. Herein, we report an enantioselective reductive aza-Heck cyclization followed by a C-S coupling sequence, ultimately yielding sulfide-containing enantioenriched pyrrolines. The reaction is conducted under mild conditions and tolerates broad functionalities including alkynes, phenols, anilines, amides, nitriles, and bromides.

Through the Looking Glass: Using the Lens of [SNS]-Pincer Ligands to Examine First-Row Metal Bifunctional Catalysts

ConspectusHomogeneous catalysis is at the forefront of global efforts to innovate the synthesis of fine chemicals and achieve carbon-neutrality in energy applications. For decades, the push toward sustainable catalysis has focused on the development of first-row transition metal catalysts to supplant widespread use of precious metals. Metal-ligand cooperativity is an effective strategy to yield high-performing first-row metal molecular catalysts. Despite remarkable progress, state of the art catalysts often employ phosphorus-based ligands which are air-sensitive, potentially toxic, and on occasion offset the cost-savings of the metal. Thus, the development of simple and economical ligands composed of biomimetic donors should be a key focus that cannot be overlooked in the pursuit of sustainable catalyst candidates. This is an Account of our group's efforts to develop first-row transition metal complexes which use [SNS]-pincer ligands for bifunctional catalysis. We have synthesized two potentially tridentate ligands, one bearing an amido and two thioether donors [(S^MeNS^Me), L1] and one which includes thiolate, imine, and thioether donors [(SNS^Me), L2], and used them as platforms upon which to explore the reaction pathways of first-row metals. The [SNS] ligand, L1, leads to formation of high-spin paramagnetic metal complexes of the type M(L1)₂ in which the 6-membered ring thioether donor is hemilabile (M = Mn, Fe, Co). This allows Mn(L1)₂ to function as a carbonyl hydroboration catalyst that operates by a novel hydride-free, inner-sphere reaction pathway. Exploring the reactivity of L2 with Fe and Ni revealed unique coordination chemistry and a variety of mono-, di-, tri-, and tetranuclear complexes enabled by bridging thiolates. Further studies showed L2 undergoes selective C_aryl-S bond cleavage upon coordination to a metal with electron-rich phosphine donors, yielding a new (CNS)^2- pincer ligand. The analogous reaction with L1 afforded a new (CNS^Me)^- pincer ligand via both C_aryl-S and benzylic C-H bond cleavage. In an attempt to prepare Fe(L2)₂, we obtained instead an Fe(N₂S₃) complex in which imine C-C bond formation affords a potentially hexadentate redox-active ligand. The Fe(N₂S₃) complex is a selective catalyst for hydroboration of aldehydes and appears to operate through a complicated mechanism. In contrast, a mechanistic study of Mn(L2)(CO)₃-photocatalyzed dihydroboration of nitriles indicated that both the flexibility of the κ³-SNS^Me ligand (fac- vs mer-coordination) and ability of Mn to undergo a spin-state change are required to access low energy barriers for this transformation. To effectively compare the reactivity of the thiolate vs amido donor, we prepared two Cu complexes, Cu(L1)(IPr) and Cu(L2)(IPr) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], showing that, while both served as carbonyl hydroboration catalysts, only the amido complex was an effective catalyst for carbonyl hydrosilylation. In addition, complexes of the type Zn(L1)₂, Zn(L2)₂, and Zn(L1)(L2), were also effective for catalytic carbonyl hydroboration. While Zn(L1)(L2) was most active, catalyst speciation studies showed that each undergoes bifunctional catalyst activation to form a Zn bis(alkoxide) catalyst. Overall, our observations using [SNS] ligands with first-row transition metals show how the absence of traditional phosphine donors leads to different fundamental reactivity. Furthermore, this Account demonstrates the gap of knowledge which exists in understanding the reactivity of sulfur-based ligands to promote more widespread adoption of sustainable ligands.

Spin-state crossover in photo-catalyzed nitrile dihydroboration via Mn-thiolate cooperation

The role of S-donors in ligand-assisted catalysis using first-row metals has not been broadly investigated. Herein is described a combined experimental and computational mechanistic study of the dihydroboration of nitriles with pinacolborane (HBpin) catalyzed by the Mn(i) complex, Mn(κ3-SMeNS)(CO)3, that features thioether, imine, and thiolate donors. Mechanistic studies revealed that catalysis requires the presence of UV light to enter and remain in the catalytic cycle and evidence is presented for loss of two CO ligands. Stoichiometric reactions showed that HBpin reduces the imine N[double bond, length as m-dash]C of the ligand backbone in the absence of nitrile, forming an inactive off-cycle by-product. DFT calculations showed that the bifunctional thiolate donor, coordinative flexibility of the SMeNS ligand, and access to an open-shell intermediate are all crucuial to accessing low-energy intermediates during catalysis.

Single and double deprotonation/dearomatization of the N,S-donor pyridinophane ligand in ruthenium complexes

We report a series of ruthenium complexes with a tetradentate N,S-donor ligand, 2,11-dithia[3.3](2,6)pyridinophane (N₂S₂), that undergo single and double deprotonation in the presence of a base leading to the deprotonation of one or both pyridine rings. Both singly and doubly deprotonated complexes were structurally characterized by single-crystal X-ray diffraction. The NMR spectra are indicative of the dearomatization of one or both pyridine rings upon the deprotonation of the CH₂-S arm, similar to the dearomatization of phosphine-containing pincer ligands. The deprotonated (N₂S₂)Ru complexes did not show appreciable catalytic or stoichiometric reactivity in transfer hydrogenation, hydrogenation and dehydrogenation of alcohols, and attempted activation of H₂, CO₂, and other substrates. Such a lack of reactivity is likely due to the low stability of the deprotonated species as evident from the structural characterization of one of the decomposition products in which shrinkage of the macrocyclic ring occurs via picolyl arm migration.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.