Iron-Catalyzed Hydroboration of Vinylcyclopropanes

Ligand-Controlled Cobalt-Catalyzed Regiodivergent and Stereoselective Ring-Opening Isomerization of Vinyl Cyclopropanes

A ligand-controlled regiodivergent and stereoselective ring-opening isomerization of vinylcyclopropane was developed with cobalt catalysis. Employing the commercially available Xantphos ligand, the reactions afforded exclusively linear-type 1,3-dienes as the products. Interestingly, when switching the ligand to an amido-diphosphine ligand (PNP), branched-type 1,3-dienes were obtained with high regioselectivity and stereoselectivity. Preliminary mechanistic investigations suggested that a π-allyl metal and a metal-hydride species are involved as key intermediates in the two transformations, respectively.

Chalcone-Based Synthesis of Tetrahydropyridazines via Cloke-Wilson-Type Rearrangement-Involved Tandem Reaction between Cyclopropyl Ketones and Hydrazines

A facile and efficient approach for the synthesis of multisubstituted tetrahydropyridazines starting from cyclopropyl ketones and hydrazines has been developed. The transformation is chalcone-based and takes place via a Cloke-Wilson-type rearrangement-involved tandem reaction catalyzed by TfOH in HFIP.

Copper-catalyzed synthesis of β-boryl cyclopropanes via 1,2-borocyclopropanation of aryl olefins with CO as the C1 source

Cyclopropane represents one of the most critical rings and has been found present in various bioactive compounds, especially in clinical medicines. It can be synthesized by the reaction of olefins with diazo-derived carbenoids which are potentially hazardous. Carbonylation is a powerful tool for synthesizing carbonylated or carbon-extended compounds. In this communication, we describe a straightforward approach for synthesizing β-boryl cyclopropane derivatives catalyzed by an inexpensive copper catalyst with CO as the C1 source. This reaction was mediated by an in situ generated carbene intermediate and afforded a wide range of cyclopropane-containing organoboron compounds in moderate to good yields.

Mechanistic Studies of Alkyne Hydroboration by a Well-Defined Iron Pincer Complex: Direct Comparison of Metal-Hydride and Metal-Boryl Reactivity

Iron-hydride and iron-boryl complexes supported by a pyrrole-based pincer ligand, ^tBuPNP (PNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole), were employed for a detailed mechanistic study on the hydroboration of internal alkynes. Several novel complexes were isolated and fully characterized, including iron-vinyl and iron-boryl species, which represent likely intermediates in the catalytic hydroboration pathway. In addition, the products of alkyne insertion into the Fe-B bond have been isolated and structurally characterized. Mechanistic studies of the hydroboration reaction favor a pathway involving an active iron-hydride species, [FeH(^tBuPNP)], which readily inserts alkyne and undergoes subsequent reaction with hydroborane to generate product. The iron-boryl species, [Fe(BR₂)(^tBuPNP)] (R₂ = pin or cat), was found to be chemically competent, although its use in catalysis entailed an induction period whereby the iron-hydride species was generated. Stoichiometric reactions and kinetic experiments were performed to paint a fuller picture of the mechanism of alkyne hydroboration, including pathways for catalyst deactivation and the influence of substrate bulk on catalytic efficacy.

Metal-ligand cooperative approaches in homogeneous catalysis using transition metal complex catalysts of redox noninnocent ligands

Catalysis offers a straightforward route to prepare various value-added molecules starting from readily available raw materials. The catalytic reactions mostly involve multi-electron transformations. Hence, compared to the inexpensive and readily available 3d-metals, the 4d and 5d-transition metals get an extra advantage for performing multi-electron catalytic reactions as the heavier transition metals prefer two-electron redox events. However, for sustainable development, these expensive and scarce heavy metal-based catalysts need to be replaced by inexpensive, environmentally benign, and economically affordable 3d-metal catalysts. In this regard, a metal-ligand cooperative approach involving transition metal complexes of redox noninnocent ligands offers an attractive alternative. The synergistic participation of redox-active ligands during electron transfer events allows multi-electron transformations using 3d-metal catalysts and allows interesting chemical transformations using 4d and 5d-metals as well. Herein we summarize an up-to-date literature report on the metal-ligand cooperative approaches using transition metal complexes of redox noninnocent ligands as catalysts for a few selected types of catalytic reactions.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Recent advances in asymmetric borylation by transition metal catalysis

Chiral organoboronates have played a critical role in organic chemistry and in the development of materials science and pharmaceuticals. Much effort has been devoted to exploring synthetic methodologies for the preparation of these compounds during the past few decades. Among the known methods, asymmetric catalysis has emerged as a practical and highly efficient strategy for their straightforward preparation, and recent years have witnessed remarkable advances in this respect. Approaches such as asymmetric borylative addition, asymmetric allylic borylation and stereospecific cross-coupling borylation, have been extensively explored and well established employing transition-metal catalysis with a chiral ligand. This review provides a comprehensive overview of transition metal-catalysed asymmetric borylation processes to construct carbon-boron, carbon-carbon, and other carbon-heteroatom bonds. It summarises a range of recent achievements in this area of research, with considerable attention devoted to the reaction modes and the mechanisms involved.

Radical 1,5-Chloropentafluorosulfanylation of Unactivated Vinylcyclopropanes and Transformation into α-SF5 Ketones

The radical 1,5-chloropentafluorosulfanylation of vinyl cyclopropanes (VCPs) initiated by Et₃B/O₂ affords allylic pentafluorosulfanyl/homoallylic chloride products through the ring-strain release of the cyclopropane. The VCP substitution pattern was investigated. The utility of this reaction was illustrated in post-transformation of the C═C bond by ozonolysis, giving access to valuable α-SF₅ carbonyl compounds.

Copper-Catalyzed Highly Selective Protoboration of CF3 -Containing 1,3-Dienes

The copper-catalyzed highly selective protoboration of CF₃ -containing conjugated diene with proton source and B₂ Pin₂ has been developed. This chemistry could suppress the well-known defluorination and provide borated reagents with an intact CF₃ -group. Further studies indicated that the functional group tolerance of this chemistry is very well, and the products could be used as versatile precursors for different types of transformations. Importantly, using chiral diphosphine ligand, we have developed the first example for using such starting material to synthesis allylic boron-reagents which bearing a CF₃ -containing chiral center. Notably, the reaction mechanism was intensively studied by DFT calculations, which could reveal the reason that defluorination was inhibited.

Ni-Catalyzed Reductive Allylation of α-Chloroboronates to Access Homoallylic Boronates

The transition-metal-catalyzed allylation reaction is an efficient strategy for the construction of new carbon-carbon bonds alongside allyl or homoallylic functionalization. Herein we describe a Ni-catalyzed reductive allylation of α-chloroboronates to efficiently render the corresponding homoallylic boronates, which could be readily converted into valuable homoallylic alcohols or amines or 1,4-diboronates. This reaction features a broad substrate scope with good functional group compatibility that is complementary to the existing methods for the preparation of homoallylic boronates.

Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes

Transition metal catalyzed asymmetric hydrofunctionalization of readily available unsaturated hydrocarbons presents one of the most straightforward and atom-economic protocols to access valuable optically active products. For decades, noble transition metal catalysts have laid the cornerstone in this field, on account of their superior reactivity and selectivity. In recent years, from an economical and sustainable standpoint, first-row, earth-abundant transition metals have received considerable attention, due to their high natural reserves, affordable costs, and low toxicity. Meanwhile, the earth-abundant metal catalyzed hydrofunctionalization reactions have also gained much interest and been investigated gradually. However, since chiral ligand libraries for earth-abundant transition-metal catalysis are limited to date, the development of highly enantioselective versions remains a significant challenge.This Account summarizes our recent efforts in developing suitable chiral ligands for iron and cobalt catalysts and their applications in the highly enantioselective hydrofunctionalization reactions (hydroboration and hydrosilylation) of alkenes and alkynes. In ligand design, we envisioned that chiral unsymmetric NNN-tridentate (UNT) ligand scaffolds could promote these enantioselective transformations with earth-abundant metals. Therefore, several types of chiral UNT ligands were designed and prepared in our laboratory, utilizing readily available natural amino acids as chiral sources. In the very beginning, chiral oxazoline iminopyridine (OIP) ligands were proposed and investigated through the rational combination of nitrogen-containing ligand scaffolds. After a systematic survey of the ligand effects, the imine moiety in the rigid OIP ligands was replaced by a conformationally more flexible amine unit, leading to the construction of reactive oxazoline aminoisopropylpyridine (OAP) ligands. Subsequently, imidazoline iminopyridine (IIP) and thiazoline iminopyridine (TIP) ligands were prepared by altering the oxygen atom of oxazoline with nitrogen and sulfur linkers, respectively. To further expand the chiral ligand library, other tridentate ligands containing a twisted pincer, anionic, and nonrigid backbone were also designed and synthesized, including iminophenyl oxazolinyl phenylamine (IPOPA) and imidazoline phenyl picolinamide (ImPPA). The efficacy of these chiral UNT ligands for asymmetric induction in iron and cobalt catalysis has been demonstrated through asymmetric hydrofunctionalization of alkenes and asymmetric sequential hydrofunctionalization of alkynes, which exhibit excellent reactivity as well as high chemo-, regio-, and stereoselectivity with broad functional group tolerance. Notably, highly regio- and enantioselective hydrofunctionalization of challenging substrates, such as 1,1-disubstituted aryl alkenes and terminal aliphatic alkenes, was also achieved. Furthermore, the development of asymmetric sequential isomerization/hydroboration of internal alkenes and sequential hydrofunctionalization of alkynes further demonstrates the synthetic power of these catalytic systems. The chiral enantioenriched products obtained by these methodologies could be potentially utilized in organic synthesis, medicinal chemistry, and materials science. We believe that our continuous efforts in this field would be beneficial to the development of asymmetric earth-abundant metal catalysis.

Nickel-catalyzed alkenylboration of alkenylarenes to access homoallylic boronic esters

A practical nickel-catalyzed alkenylboration of alkenylarenes with excellent chemo- and regio-selectivity has been developed.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Silver-promoted oxidative sulfonylation and ring-expansion of vinylcyclopropanes with sodium sulfinates leading to dihydronaphthalene derivatives

Silver-promoted sulfonylation and ring-expansion of vinylcyclopropanes with sodium sulfinates is established for the construction of 1-sulfonylmethylated 3,4-dihydronaphthalenes. This sulfonylation process involves a radical pathway, including sulfonyl radical formation, radical addition, ring-opening and cyclization. The 1-sulfonylmethylated 3,4-dihydronaphthalenes can be converted into other useful products.

Methodologies and Strategies for Selective Borylation of C-Het and C-C Bonds

Organoborons have emerged as versatile building blocks in organic synthesis to achieve molecular diversity and as carboxylic acid bioisosteres with broad applicability in drug discovery. Traditionally, these compounds are prepared by the substitution of Grignard/lithium reagents with electrophilic boron species and Brown hydroboration. Recent developments have provided new routes for the efficient preparation of organoborons by applying reactions using chemical feedstocks with leaving groups. As compared to the previous methods that used organic halides (I, Br, and Cl), the direct borylation of less reactive C-Het and C-C bonds has become highly important to get efficiency and functional-group compatibility. This Review aims to provide a comprehensive overview of this topic, including (1) C-F bond borylation, (2) C-O bond borylation, (3) C-S bond borylation, (4) C-N bond borylation, and (5) C-C bond borylation. Considerable attention is given to the strategies and mechanisms involved. We expect that this Review will inspire chemists to discover more efficient transformations to expand this field.

Iron-Catalyzed Asymmetric Hydrosilylation of Vinylcyclopropanes via Stereospecific C-C Bond Cleavage

An iron-catalyzed highly anti-Markovnikov selective, enantioselective hydrosilylation of vinylcyclopropanes with PhSiH₃ was reported for the preparation of valuable chiral allylic silanes via stereospecific C-C bond cleavage. Simultaneously, difficultly prepared chiral VCPs could be also obtained with moderate to excellent enantioselectivity via this kinetic resolution pathway. The chiral Z-allylic silanes could be converted to various chiral allylic derivatives. A possible mechanism via an iron-silyl species was proposed based on experimental and computational studies.

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Iron-catalyzed 1,5-hydrosilylation of VCPs via C-C bond cleavage was first established

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Chiral allyl silanes and chiral VCPs were obtained with high enantioselectivity

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Various chiral allylic derivatives were delivered from chiral Z-allylic silanes

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A possible mechanism via an iron-silyl species was proposed

Intra- and intermolecular Fe-catalyzed dicarbofunctionalization of vinyl cyclopropanes

Design and implementation of the first (asymmetric) Fe-catalyzed intra- and intermolecular difunctionalization of vinyl cyclopropanes (VCPs) with alkyl halides and aryl Grignard reagents has been realized via a mechanistically driven approach. Mechanistic studies support the diffusion of alkyl radical intermediates out of the solvent cage to participate in an intra- or intermolecular radical cascade with a range of VCPs followed by re-entering the Fe radical cross-coupling cycle to undergo (stereo)selective C(sp2)-C(sp3) bond formation. This work provides a proof-of-concept of the use of vinyl cyclopropanes as synthetically useful 1,5-synthons in Fe-catalyzed conjunctive cross-couplings with alkyl halides and aryl/vinyl Grignard reagents. Overall, we provide new design principles for Fe-mediated radical processes and underscore the potential of using combined computations and experiments to accelerate the development of challenging transformations.

Ligand-promoted cobalt-catalyzed radical hydroamination of alkenes

Highly regio- and enantioselective intermolecular hydroamination of alkenes is a challenging process potentially leading to valuable chiral amines. Hydroamination of alkenes via metal-catalyzed hydrogen atom transfer (HAT) with good regioselectivity and functional group tolerance has been reported, however, high enantioselectivity has not been achieved due to the lack of suitable ligands. Here we report a ligand-promoted cobalt-catalyzed Markovnikov-type selective radical hydroamination of alkenes with diazo compounds. This operationally simple protocol uses unsymmetric NNN-tridentate (UNT) ligand, readily available alkenes and hydrosilanes to construct hydrazones with good functional group tolerance. The hydrazones can undergo nitrogen–nitrogen bond cleavage smoothly to deliver valuable amine derivatives. Additionally, asymmetric intermolecular hydroamination of unactivated aliphatic terminal alkenes using chiral N-imidazolinylphenyl 8-aminoquinoline (IPAQ) ligands has also been achieved to afford chiral amine derivatives with good enantioselectivities.

Asymmetric intermolecular hydroamination of alkenes is a challenging process, potentially leading to useful chiral amines. Here, the authors report unsymmetric NNN tridentate ligands promoting the cobalt-catalyzed radical hydroamination of alkenes via hydrogen atom transfer, also in an asymmetric fashion.

Organocatalytic 1,5-trifluoromethylthio-sulfonylation of vinylcyclopropane mediated by visible light in the water phase

1,2-Difunctionalization has developed into a robust tool for the preparation of complex organic molecules, and remote difunctionalization has also aroused widespread interest to achieve pluripotency of difunctionalization.

Iron catalyzed stereoselective alkene synthesis: a sustainable pathway

Replacing expensive or toxic transition metals with iron has become an important trend. This article summarises the recent progresses of a wide range of Fe-catalyzed reactions for accessing various stereodefined alkenes.

Synthesis of a Base-Stabilized Silicon(I)-Iron(II) Complex for Hydroboration of Carbonyl Compounds

The reaction of the amidinatosilicon(I) dimer [LSi:]₂ (1; L = PhC(N tBu)₂) with FeBr₂ in tetrahydrofuran (THF) at ambient temperature afforded the silicon(I)-iron(II) dimer [LSi(FeBr₂·THF)]₂ (2) after 40 h. Compound 2 can catalyze hydroboration of aliphatic and aromatic ketone compounds with HBpin in the absence of any strong reducing agent. Mechanistic studies show that complex 2 reacts with ketone compounds to form a zwitterionic intermediate in the first step of catalysis. Subsequent reaction with HBpin affords the corresponding boron esters and then regenerates complex 2.

Iron(ii) coordination polymer catalysed hydroboration of ketones

Catalytic hydroboration of ketones with pinacolborane was achieved with a 2D iron(ii) coordination polymer (CP) of a divergent 4,2';6',4''-terpyridine (tpy) derivative under mild conditions with high efficiency. This solid iron catalyst system is more active towards the hydroboration of ketones than that of aldehydes, displaying a different trend of reactivity from known homogeneous iron hydroboration catalysts.

Nickel-catalyzed enantioselective sequential Nazarov cyclization/decarboxylation

The nickel-catalyzed enantioselective sequential Nazarov cyclization/decarboxylation has been developed.

10 gram-scale synthesis of a chiral oxazoline iminopyridine ligand and its applications

This method reports on the preparation of a chiral oxazoline iminopyridine (OIP) ligand and its applications on a scale of 10 grams.