Linear-Selective Hydroarylation of Unactivated Terminal and Internal Olefins with Trifluoromethyl-Substituted Arenes

Practical carbon-carbon bond formation from olefins through nickel-catalyzed reductive olefin hydrocarbonation

New carbon–carbon bond formation reactions expand our horizon of retrosynthetic analysis for the synthesis of complex organic molecules. Although many methods are now available for the formation of C(sp²)–C(sp³) and C(sp³)–C(sp³) bonds via transition metal-catalyzed cross-coupling of alkyl organometallic reagents, direct use of readily available olefins in a formal fashion of hydrocarbonation to make C(sp²)–C(sp³) and C(sp³)–C(sp³) bonds remains to be developed. Here we report the discovery of a general process for the intermolecular reductive coupling of unactivated olefins with alkyl or aryl electrophiles under the promotion of a simple nickel catalyst system. This new reaction presents a conceptually unique and practical strategy for the construction of C(sp²)–C(sp³) and C(sp³)–C(sp³) bonds without using any organometallic reagent. The reductive olefin hydrocarbonation also exhibits excellent compatibility with varieties of synthetically important functional groups and therefore, provides a straightforward approach for modification of complex organic molecules containing olefin groups.

Olefins are employed in many coupling procedures but direct hydrocarbonations of unactivated olefins remain to be developed. Here, the authors report the nickel-catalyzed reductive coupling of olefins with aryl and alky electrophiles under mild conditions and with a broad substrate scope.

A case study of proton shuttling in palladium catalysis

Thanks to mechanistic studies, the catalytic performance of SCS indenediide Pd pincer complexes has been spectacularly enhanced using catechol additives as proton shuttles.

The mechanism of alkynoic acid cycloisomerization with SCS indenediide Pd pincer complexes has been investigated experimentally and computationally. These studies confirmed the cooperation between the Pd center and the backbone of the pincer ligand, and revealed the involvement of a second molecule of substrate. It acts as a proton shuttle in the activation of the acid, it directs the nucleophilic attack of the carboxylic acid on the π-coordinated alkyne and it relays the protonolysis of the resulting vinyl Pd species. A variety of H-bond donors have been evaluated as external additives, and polyols featuring proximal hydroxyl groups, in particular catechol derivatives, led to significant catalytic enhancement. The impact of 4-nitrocatechol and 1,2,3-benzenetriol is particularly striking on challenging substrates such as internal 4- and 5-alkynoic acids. Endo/exo selectivities up to 7.3/1 and 60-fold increase in reactivity were achieved.

Iron-mediated oxidative C–H coupling of arenes and alkenes directed by sulfur: an expedient route to dihydrobenzofurans

A route to medicinally-relevant dihydrobenzofurans utilises a sulfur-directed, iron-mediated C–H ortho-coupling of arenes and alkenes, and a palladium-catalysed heterocyclisation.

Nickel-Catalyzed Arylation of Heteroaryl-containing Diarylmethanes: Exceptional Reactivity of the Ni(NIXANTPHOS)-based Catalyst

Nickel(0)-catalyzed cross-coupling of heteroaryl-containing diarylmethanes with both aryl bromides and chlorides has been achieved. The success of this reaction relies on the introduction of a unique nickel/NIXANTPHOS-based catalyst system, which provides a direct route to triarylmethanes from heteroaryl-containing diarylmethanes. Reactivity studies indicate the Ni(NIXANTPHOS)-based catalyst exhibits enhanced reactivity over XANTPHOS derivatives and other Ni(phosphine)-based catalysts in the reactions examined.

Synthesis of (diarylmethyl)amines using Ni-catalyzed arylation of C(sp3)-H bonds

The first nickel catalyzed deprotonative cross coupling between C(sp3)-H bonds and aryl chlorides is reported, allowing the challenging arylation of benzylimines in the absence of directing group or stoichiometric metal activation. This methodology represents a convenient access to the (diarylmethyl)amine moiety, which is widespread in pharmaceutically relevant compounds.

Nickel-Catalyzed Insertion of Alkynes and Electron-Deficient Olefins into Unactivated sp(3) C-H Bonds

Insertion of unsaturated systems such as alkynes and olefins into unactivated sp(3) C-H bonds remains an unexplored problem. We herein address this issue by successfully incorporating a wide variety of functionalized alkynes and electron-deficient olefins into the unactivated sp(3) C-H bond of pivalic acid derivatives with excellent syn- and linear- selectivity. A strongly chelating 8-aminoquinoline directing group proved beneficial for these insertion reactions, while an air-stable and inexpensive Ni(II) salt has been employed as the active catalyst.

Nickel-catalyzed borylation of arenes and indoles via C-H bond cleavage

The first nickel-catalyzed method for the borylation of carbon-hydrogen bonds in arenes and indoles is described. The use of an N-heterocyclic carbene ligand is essential for an efficient reaction, with an N-cyclohexyl-substituted derivative being optimal. This method is readily applied to the gram scale synthesis of 2-borylindole.

Lewis acid activation of carbodicarbene catalysts for Rh-catalyzed hydroarylation of dienes

The activation of carbodicarbene (CDC)-Rh(I) pincer complexes by secondary binding of metal salts is reported for the catalytic site-selective hydro-heteroarylation of dienes (up to 98% yield and >98:2 γ:α). Reactions are promoted by 5 mol % of a readily available tridentate (CDC)-Rh complex in the presence of an inexpensive lithium salt. The reaction is compatible with a variety of terminal and internal dienes and tolerant of ester, alkyl halide, and boronate ester functional groups. X-ray data and mechanistic experiments provide support for the role of the metal salts on catalyst activation and shed light on the reaction mechanism. The increased efficiency (120 to 22 °C) made available by catalytic amounts of metal salts to catalysts containing C(0) donors is a significant aspect of the disclosed studies.

Remote functionalization of hydrocarbons with reversibility enhanced stereocontrol

Remote functionalization of hydrocarbons could be achieved through successive zirconocene-mediated allylic C-H bond activations followed by a selective C-C bond cleavage. Determination of the reaction mechanism by density functional theory (DFT) calculations shows that the high stereocontrol observed in this process results from a large number of energetically accessible equilibria feeding a preferred reactive channel that leads to the major product. A distinctive consequence of this pattern is that stereoselectivity is enhanced upon heating.

Nickel complexes for catalytic C-H bond functionalization

The direct catalytic functionalization of traditionally unreactive C-H bonds is an atom-economic transformation that has become increasingly important and commonplace in synthetic applications. In general, 2(nd) and 3(rd) row transition metal complexes are used as catalysts in these reactions, whereas the less costly and more abundant 1(st) row metal complexes have limited utility. This Perspective article summarizes progress from our laboratory towards understanding the fundamental issues that complicate the use of Ni complexes for catalytic C-H bond functionalization, as well as approaches to overcoming these limitations. In practice it is found that Ni complexes can functionalize C-H bonds by processes that, to date, have not been observed with the heavier metals. An example is provided by the catalytic stannylation of C-H bonds with tributylvinyltin, Bu3SnCH=CH2, which produces ethylene as a by-product.

C-H activation/functionalization catalyzed by simple, well-defined low-valent cobalt complexes

A facile C-H activation and functionalization of aromatic imines is presented using low-valent cobalt catalysts. Using Co(PMe3)4 as catalyst we have developed an efficient and simple protocol for the C-H/hydroarylation of alkynes with an anti selectivity. Deuterium-labeling experiments, DFT calculations coupled with the use of a well-defined catalyst have for the first time shed light on the elusive black box of cobalt catalyzed C-H functionalization.

Nickel-catalyzed dehydrogenative cross-coupling: direct transformation of aldehydes into esters and amides

By exploring a new mode of nickel-catalyzed cross-coupling, a method to directly transform both aromatic and aliphatic aldehydes into either esters or amides has been developed. The success of this oxidative coupling depends on the appropriate choice of catalyst and organic oxidant, including the use of either α,α,α-trifluoroacetophenone or excess aldehyde. Mechanistic data that supports a catalytic cycle involving oxidative addition into the aldehyde C-H bond is also presented.

Iron-mediated C–H coupling of arenes and unactivated terminal alkenes directed by sulfur

A sulfur-directed Fe(iii)-mediated ortho C–H coupling of arenes with unactivated terminal alkenes gives products of regioselective alkene chloroarylation.