Ruthenium catalyzed β-selective alkylation of vinylpyridines with aldehydes/ketones via N2H4 mediated deoxygenative couplings

CCC pincer Ru complex-catalyzed C-H vinylation/6π-E-cyclization of aldimines for constructing 4H-pyrido[1,2-a]pyrimidines

An unusual cascade C-H activation, vinylation and 6π-electrocyclization of 2-pyridyl aldimines with vinyl bromides/triflates was achieved using catalysis with a unique CCC pincer NHC-Ru(iii) complex (Cat B). This reaction was found to enable a rapid and diverse synthesis of polycyclic 4H-pyrido[1,2-a]pyrimidine derivatives in mostly good to high yields, and with a broad substrate scope. A mechanistic study suggested the formation of a semi-opened Ru(iii) intermediate chelating/activating the aldimine, and the occurrence of single-electron transfer (SET) to generate a vinyl radical, followed by vinylation and then an intramolecular 6π-electrocyclization of 1N,3N-hexatrene to form the product. This protocol provides a convenient approach for preparing and seeking new drug candidates.

Phosphoric Acid-Catalyzed Alkene Difunctionalization of 2-Vinylpyridines via HOMO/LUMO Biactivated Diels-Alder Reaction

The difunctionalization of vinylpyridines based on the cyclization strategy remains rare and underdeveloped, in contrast to the well-developed hydrogen functionalization. Current exploration on [4 + 2] cyclization of vinylpyridines mainly relies on extremely high temperatures and the LUMO activation of vinylpyridines using boron trifluoride as a strong Lewis acid. Herein, we established a phosphoric acid-catalyzed [4 + 2] cyclization reaction of 3-vinyl-1H-indoles and 2-vinylpyridines by means of the LUMO/HOMO bifunctional activation model. This protocol features mild reaction conditions, high functional group tolerance, broad substrate compatibility, and high diastereoselectivity, enabling the efficient construction of various functionalized pyridine-substituted tetrahydrocarbazoles with prominent potential in drug discovery.

Unraveling the Potential of Vinyl Ether as an Ethylene Surrogate in Heteroarene C─H Functionalization via the Spin-Center Shift

Despite the simplicity and abundance of ethylene, its practical application presents significant hurdles due to its nature as a highly flammable gas. Herein, a strategic use of easily handled vinyl ether is reported as a latent ethylene surrogate achieved via a spin-center shift (SCS) pathway, enabling the successful three-component reaction that bridges heteroarenes and various coupling partners, including sulfinates, thiols, and phosphine oxides. Through a photoredox catalytic process, α-oxy radicals are generated by combining various radicals with phenyl vinyl ether, which are subsequently added to N-heteroarenes. Subsequently, the radical-mediated SCS pathway serves as the driving force for C─O bond cleavage, effectively engaging the phenoxy group as a leaving group. In addition, by broadening the utility of the method, a valuable synthon is provided for efficient C─H vinylation of N-heteroarenes following sulfonyl group elimination. This approach not only enriches the toolbox of synthetic methodology but also provides a more streamlined alternative, circumventing the challenges associated with direct ethylene gas usage. The versatility of the method, particularly evident in late-stage functionalizations of medicinally relevant molecules and peptides, underscores its capability to produce invaluable three-component compounds and vinylated N-heteroarene derivatives.

Anti-Markovnikov Hydroalkylation of Styrene Derivatives via Hydrazones Catalyzed by Ru-PNP Complex

A well-defined Ru(II)-PNP complex demonstrated high activity in the anti-Markovnikov hydroalkylation of nonpolarized terminal alkenes via hydrazones. Hydrazone served as a carbanion equivalent to combine with the electrophilic alkene substrate upon activation by the ruthenium catalyst, forming a new C-C bond in a concerted pathway with N2 as the only theoretical byproduct. Experimental and computational studies suggested the existence of a push-pull interaction that activated the alkene for hydrazone addition and then deduced the mechanism.

HOME-Chemistry: hydrazone as organo-metallic equivalent

The modern synthetic chemistry heavily relies on the use of stoichiometric organometallic reagents to react with various electrophiles. The dependence on stoichiometric quantities of metals and often organic halides as precursors, in turn both produces copious amounts of metal halide wastes as well as leads to concerns on future metal sustainability. Inspired by the classical Wolff-Kishner reduction, our lab has recently developed a general strategy of HOME-Chemistry , directly using naturally abundant alcohols/aldehydes and ketones as feedstocks with the releasing of innocuous water and nitrogen gas. These reactions include 1,2-carbonyl/imine addition, conjugate addition, carboxylation, olefination, cross-coupling arylation/allylation, alkylation, hydroalkylation and C-heteroatom formations. This article provides a brief summary on this chemistry.

Selective Reductive Coupling of Vinyl Azaarenes and Alkynes via Photoredox Cobalt Dual Catalysis

A visible light-induced Co-catalyzed highly regio- and stereoselective reductive coupling of vinyl azaarenes and alkynes has been developed. Notably, Hünig's base together with simple ethanol has been successfully applied as the hydrogen sources instead of commonly used Hantzsch esters in this catalytic photoredox reaction. This approach has considerable advantages for the straightforward synthesis of stereodefined multiple substituted alkenes bearing an azaarene motif, such as excellent regioselectivity (>20 : 1 for >30 examples) and stereoselectivity (>20 : 1 E/Z), broad substrate scope and good functional group compatibility under mild reaction conditions, which has been utilized in the concise synthesis of natural product monomorine I. A reasonable catalytic reaction pathway involving protolysis of the cobaltacyclopentene intermediate has been proposed based on the mechanistic studies.

Nickel-Catalyzed Stereoselective Alkenylation of Ketones Mediated by Hydrazine

The direct conversion of naturally abundant carbonyl compounds provides a powerful platform for the efficient synthesis of valuable chemicals. In particular, the conversion of ketones to alkenes is a commonly encountered chemical transformation, often achieved via the multistep Shapiro reaction with tosylhydrazone and over stoichiometric organolithium or Grignard reagent. Herein, we report an earth abundant nickel-catalyzed alkenylation of naturally abundant methylene ketones to afford a wide range of alkene derivatives, mediated by hydrazine. The protocol features a broad substrate scope (including alkyl ketones, aryl ketones, and aldehydes), good functional group compatibility, mild reaction conditions, water tolerance, and only environmentally friendly N₂, H₂, and H₂O as theoretical byproducts. Moreover, gram-scale synthesis with good yield and generation of pharmaceutical intermediates highlighted its practical applicability.

Visible-light-induced cross-coupling of aryl iodides with hydrazones via an EDA-complex

A visible-light-induced, transition-metal and photosensitizer-free cross-coupling of aryl iodides with hydrazones was developed. In this strategy, hydrazones were used as alternatives to organometallic reagents, in the absence of a transition metal or an external photosensitizer, making this cross-coupling mild and green. The protocol was compatible with a variety of functionalities, including methyl, methoxy, trifluoromethyl, halogen, and heteroaromatic rings. Mechanistic investigations showed that the association of the hydrazone anion with aryl halides formed an electron donor–acceptor complex, which when excited with visible light generated an aryl radical via single-electron transfer.

Visible-light-induced catalyst-free cross-coupling of aryl iodides with hydrazones via single-electron-transfer was reported. The mechanistic investigations showed that the association of hydrazone anion with aryl iodides formed an EDA complex.

Carbon–carbon bond formation and green chemistry: one dream and 30 years hence

Carbon–carbon bond formation is the core of organic synthesis, in which organometallic reagents play the key role in the forms of 1,2-nucleophilic additions, conjugate additions, and transition-metal catalyzed cross-couplings. These reactions have enabled the production of a wide range of organic molecules in our society. Despite the enormous power of organometallic reagents in chemical synthesis, they have inherent drawbacks in the eyes of future sustainability. This account summarizes our efforts over the past three decades on the exploration of new scientific means to overcome the drawbacks and limitations of these classical organometallic reactions.

Recent advances in visible-light mediated functionalization of olefins and alkynes using copper catalysts

Over the past decade, visible-light photoredox catalysis has blossomed as a powerful strategy and offers a discrete activation mode complementary to thermal controlled reactions. Visible-light-mediated photoredox catalysis also offers exciting...

Deoxygenative Functionalizations of Aldehydes, Ketones and Carboxylic Acids

Conversion of carbonyl compounds, including aldehydes, ketones and carboxylic acids, into functionalized alkanes via deoxygenation would be highly desirable from a sustainability perspective and very enabling in chemical synthesis. This review covers the recent methodology development in carbonyl and carboxyl deoxygenative functionalizations, highlighting some typical and significant contributions in this field. These advances will be categorized based on types of bond formation, and in each part, selected examples will be discussed from their generalized mechanistic perspectives. Four summarized reactivity modes of aldehydes and ketones during the deoxygenation, namely, bis-electrophile, carbenoid, bis-nucleophile and alkyl radical, are presented, while the carboxylic acids are deoxygenated mainly via activated carbonyl or acetal intermediates.

Carbonyl umpolung as an organometallic reagent surrogate

Construction of new carbon-carbon bonds is the cornerstone of organic chemistry. Organometallic reagents are amongst the most robust and versatile nucleophiles for this purpose. Polarization of the metal-carbon bonds in these reagents facilitates their reactions with a vast array of electrophiles to achieve chemical diversification. The dependence on stoichiometric quantities of metals and often organic halides as feedstock precursors, which in turn produces copious amounts of metal halide waste, is the key limitation of the classical organometallic reactions. Inspired by the classical Wolff-Kishner reduction converting carbonyl groups in aldehydes or ketones into methylene derivatives, our group has recently developed strategies to couple various alcohols, aldehydes, and ketones with a broad range of both hard and soft carbon electrophiles in the presence of catalytic amounts of transition metals, via the hydrazone derivatives: i.e., as organometallic reagent surrogates. This Tutorial Review describes the chronological development of this concept in our research group, detailing its creation in the context of a deoxygenation reaction and evolution to a more general carbon-carbon bond-forming strategy. The latter is demonstrated by the employment of carbonyl-derived alkyl carbanions in various transition-metal catalyzed chemical transformations, including 1,2-carbonyl/imine addition, conjugate addition, carboxylation, olefination, cross-coupling, allylation, alkylation and hydroalkylation.

Catalytic Generation of Carbanions through Carbonyl Umpolung

Carbonyl Umpolung is a powerful strategy in organic chemistry to construct complex molecules. Over the last few years, versatile catalytic approaches for the generation of acyl anion equivalents from carbonyl compounds have been developed, but methods to obtain alkyl carbanions from carbonyl compounds in a catalytic fashion are still at an early stage. This Minireview summarizes recent progress in the generation of alkyl carbanions through catalytic carbonyl Umpolung. Two different catalytic approaches can be utilized to enable the generation of alkyl carbanions from carbonyl compounds: the catalytic Wolff–Kishner reaction and the catalytic single‐electron reduction of carbonyl compounds and imines. We discuss the reaction scope, mechanistic insights, and synthetic applications of the methods as well as potential future developments.