Broadly Applicable Directed Catalytic Reductive Difunctionalization of Alkenyl Carbonyl Compounds

Nickel-Catalyzed Regioselective Alkenylarylation of γ,δ-Alkenyl Ketones via Carbonyl Coordination

We disclose a nickel-catalyzed reaction, which enabled us to difunctionalize unactivated γ,δ-alkenes in ketones with alkenyl triflates and arylboronic esters. The reaction was made feasible by the use of 5-chloro-8-hydroxyquinoline as a ligand along with NiBr2 ⋅DME as a catalyst and LiOtBu as base. The reaction proceeded with a wide range of cyclic, acyclic, endocyclic and exocyclic alkenyl ketones, and electron-rich and electron-deficient arylboronate esters. The reaction also worked with both cyclic and acyclic alkenyl triflates. Control experiments indicate that carbonyl coordination is required for the reaction to proceed.

Modular synthesis of non-conjugated N-(quinolin-8-yl) alkenyl amides via cross-metathesis

We report a direct and modular method to access non-conjugated alkenyl amides containing the 8-aminoquinoline (AQ) directing auxiliary and related groups via cross-metathesis. In this way, readily available, AQ-containing, terminal β,γ-unsaturated amides can be coupled with various terminal alkenes to furnish internal alkene products that are otherwise difficult to prepare. The value of this family of products stems from their ability to participate in a number of directed alkene functionalization reactions.

Photoinduced 1,2-dicarbofunctionalization of alkenes with organotrifluoroborate nucleophiles via radical/polar crossover

Alkene 1,2-dicarbofunctionalizations are highly sought-after transformations as they enable a rapid increase of molecular complexity in one synthetic step. Traditionally, these conjunctive couplings proceed through the intermediacy of alkylmetal species susceptible to deleterious pathways including β-hydride elimination and protodemetalation. Herein, an intermolecular 1,2-dicarbofunctionalization using alkyl N-(acyloxy)phthalimide redox-active esters as radical progenitors and organotrifluoroborates as carbon-centered nucleophiles is reported. This redox-neutral, multicomponent reaction is postulated to proceed through photochemical radical/polar crossover to afford a key carbocation species that undergoes subsequent trapping with organoboron nucleophiles to accomplish the carboallylation, carboalkenylation, carboalkynylation, and carboarylation of alkenes with regio- and chemoselective control. The mechanistic intricacies of this difunctionalization were elucidated through Stern-Volmer quenching studies, photochemical quantum yield measurements, and trapping experiments of radical and ionic intermediates.

Ir-Catalyzed Regio- and Enantioselective Hydroalkynylation of Trisubstituted Alkene to Access All-Carbon Quaternary Stereocenters

The stereoselective construction of all-carbon quaternary stereocenters, especially acyclic ones, represents an important challenge in organic synthesis. In particular, homopropargyl amides with a quaternary stereocenter β to a nitrogen atom are valuable synthetic intermediates, which could be transformed to diverse chiral structures through alkyne transformations. However, highly enantioselective synthetic methods for homopropargyl amides with a β quaternary stereocenter are extremely rare. We report here unprecedented substrate-directed, iridium-catalyzed enantioselective hydroalkynylations of trisubstituted alkenes to form an acyclic all-carbon quaternary stereocenter β to a nitrogen atom. The hydroalkynylation of enamide occurred with unconventional selectivity, favoring the more hindered reaction site. Homopropargyl amides with β-stereocenters were prepared in high regio- and enantioselectivities. Combined experimental and computational studies revealed the origin of the regio- and enantioselectivities.

Catalytic Regioselective Olefin Hydroarylation(alkenylation) by Sequential Carbonickelation-Hydride Transfer

Alkene hydrocarbofunctionalization represents one of the most important classes of chemical transformations, but related branched-selective examples with unactivated olefins are scarce. Here, we report that catalytic amounts of a dimeric Ni(I) complex and an exogenous alkoxide base promote Markovnikov-selective hydroarylation(alkenylation) of unactivated and activated olefins using organo bromides or triflates derived from widely available phenols and ketones. Products bearing aryl- and alkenyl-substituted tertiary and quaternary centers could be isolated in up to 95% yield and >99:1 regioisomeric ratios. Contrary to previous dual-catalytic methods that rely on metal-hydride atom transfer (MHAT) to the olefin prior to carbofunctionalization with a cocatalyst, our mechanistic evidence points toward a nonradical reaction pathway that begins with site-selective carbonickelation across the C═C bond followed by hydride transfer using alkoxide as the hydride source. Utility of the single-catalyst protocol is highlighted through the synthesis of medicinally relevant scaffolds.

Visible-Light-Induced Sulfur-Alkenylation of Alkenes

A visible-light-induced intermolecular sulfur-alkenylation of alkenes, including both activated and unactivated alkenes, is described. This sulfur-alkenylation reaction proceed in a highly regio- and stereospecific manner involving the visible-light-induced conversion of a ketene dithioacetal to the thiavinyl 1,3-dipole intermediate, followed by a formal [3 + 2] cycloaddition and C-S bond cleavage. Furthermore, it is also an efficient approach for the late-stage functionalization of natural products and complex molecules, even being induced by sunlight under ambient conditions.

Three-Component Alkene Difunctionalization by Direct and Selective Activation of Aliphatic C-H Bonds

Catalytic alkene difunctionalization is a powerful strategy for the rapid assembly of complex molecules and has wide range of applications in synthetic chemistry. Despite significant progress, a compelling challenge that still needs to be solved is the installation of highly functionalized C(sp³ )-hybridized centers without requiring pre-activated substrates. We herein report that inexpensive and easy-to-synthesize decatungstate photo-HAT, in combination with nickel catalysis, provides a versatile platform for three-component alkene difunctionalization through direct and selective activation of aliphatic C-H bonds. Compared with previous studies, the significant advantages of this strategy are that the most abundant hydrocarbons are used as feedstocks, and various highly functionalized tertiary, secondary, and primary C(sp³ )-hybrid centers can be easily installed. The practicability of this strategy is demonstrated in the selective late-stage functionalization of natural products and the concise synthesis of pharmaceutically relevant molecules including Piragliatin.

Enantioselective Alkylamination of Unactivated Alkenes under Copper Catalysis

An enantioselective addition reaction of various alkyl groups to unactivated internal alkenes under Cu catalysis has been developed. The reaction uses amide-linked aminoquinoline as the directing group, 4-alkyl Hantzsch esters as the donor of alkyl radicals, and rarely used biaryl diphosphine oxide as a chiral ligand. β-lactams featuring two contiguous stereocenters at Cβ and the β substituent can be obtained in good yield with excellent enantioselectivity. Mechanistic studies indicate that a nucleophilic addition of the alkyl radical to Cu^II-coordinated alkene is the enantio-determining step.

Ni-Catalyzed Regioselective 1,2-Dialkylation of Alkenes Enabled by the Formation of Two C(sp3)-C(sp3) Bonds

We disclose a Ni-catalyzed vicinal difunctionalization of alkenes with benzyl halides and alkylzinc reagents, which produces products with two new alkyl-alkyl bonds. This alkene dialkylation is effective in combining secondary benzyl halides and secondary alkylzinc reagents with internal alkenes, which furnishes products with three contiguous all-carbon secondary stereocenters. The products can be readily elaborated to access complex tetralene, benzosuberene, and bicyclodecene cores. The reaction also features as the most efficient alkene difunctionalization process to date with catalyst loadings down to 500 ppm and the catalytic turnover number (TON) and turnover frequency (TOF) registering up to 2 × 103 and 165 h-1 at rt, respectively.

Alkyl halides as both hydride and alkyl sources in catalytic regioselective reductive olefin hydroalkylation

Among the plethora of catalytic methods developed for hydrocarbofunctionalization of olefins to date, reactions that regioselectively install a functionalized alkyl unit at the 2-position of a terminal unactivated C=C bond to afford branched products are scarce. Here, we show that a Ni-based catalyst in conjunction with a stoichiometric reducing agent promote Markovnikov-selective hydroalkylation of unactivated alkenes tethered to a recyclable 8-aminoquinaldine directing auxiliary. These mild reductive processes employ readily available primary and secondary haloalkanes as both the hydride and alkyl donor. Reactions of alkenyl amides with ≥ five-carbon chain length regioselectively afforded β-alkylated products through remote hydroalkylation, underscoring the fidelity of the catalytic process and the directing group’s capability in stabilizing five-membered nickelacycle intermediates. The operationally simple protocol exhibits exceptional functional group tolerance and is amenable to the synthesis of bioactive molecules as well as regioconvergent transformations.

Methods that regioselectively install a functionalized alkyl unit at the 2-position of a terminal unactivated C=C bond are scarce. Here, the authors report a Markovnikov-selective hydroalkylation of unactivated amide-tethered alkenes catalyzed by nickel in conjunction with a stoichiometric reductant.

Directed Palladium(II)-Catalyzed Intermolecular Anti-Markovnikov Hydroarylation of Unactivated Alkenes with (Hetero)arylsilanes

We describe herein a regioselective palladium(II)-catalyzed intermolecular hydroarylation of unactivated aliphatic alkenes with electronically and sterically diverse (hetero)arylsilanes under redox-neutral conditions. A removable bidentate 8-aminoquinoline auxiliary was readily employed to dictate the regioselectivity, prevent β-hydride elimination, and facilitate protodepalladation. This silicon-based protocol features a broad substrate scope with excellent functional group compatibility and enables an expeditious route to a variety of γ-aryl butyric acid derivatives in good yields with exclusive anti-Markovnikov selectivity.

Compatibility Score for Rational Electrophile Selection in Pd/NBE Cooperative Catalysis

A mechanistically guided approach is developed to predict electrophile compatibility in the palladium/norbornene (Pd/NBE) cooperative catalysis for the ipso/ortho difunctionalization of aryl halides. A key challenge in these reactions is to identify orthogonal electrophile and aryl hali de starting materials that react selectively with different transition metal complexes in separate oxidative addition events in the catalytic cycle. We performed detailed experimental and computational mechanistic studies to identify the catalytically active Pd(II) intermediate and the substrate-dependent mechanisms in reactions with various types of carbon and nitrogen electrophiles. We introduced the concept of electrophile compatibility score (ECS) to rationally select electrophiles based on the orthogonal reactivity of electrophile and aryl halide towards the Pd(0) and Pd(II) complexes. This approach was applied to predict electrophile compatibility in the Pd/NBE cooperative catalysis with a variety of electrophilic coupling partners used in alkylation, arylation, amination, and acylation reactions.

Walking metals: catalytic difunctionalization of alkenes at nonclassical sites

Migration of metals along a carbon chain is triggered by two of the most common organometallic elementary steps - β-hydride (β-H) elimination and alkene hydrometallation. This process heralds a new future for creating bonds at carbon sites that fall outside the tenets of the conventional wisdom for reactivity and bond formation, and provides an opportunity to leverage β-H elimination to advance the very reaction of alkene difunctionalization it is intrinsically predestined to disrupt. Almost four decades since its genesis, the early adventure for alkene difunctionalization by metal migration was sporadic, and its later development went on a hiatus primarily due to original impetus on arresting β-H elimination for vicinal alkene difunctionalization. With the recent surge on alkene difunctionalization, efforts have been gradually shifting to harnessing the process of β-H elimination to difunctionalize alkenes at sites other than the classical vicinal carbons, termed henceforth nonclassical reaction sites for pedagogical simplicity. In this review article, we extricate and examine the origin and the development of such reactions over the years. This review covers a wide range of reactions for the difunctionalization of alkenes at geminal (1,1), allylic (1,3) and remote (1,n) carbon sites with a variety of coupling partners. These reactions have enabled engineering of complex molecular frameworks with the generation of new carbon-carbon (C-C)/C-C, C-C/C-heteroatom (halogens, O, N, B) and C-B/C-B bonds. The development of these unique transformations is also presented with mechanistic hypotheses and experimental evidences put forward by researchers. Judged by the number of reports emerging recently, it is now strikingly evident that the field of alkene difunctionalization by metal migration has begun to gain momentum, which holds a great future prospect to develop into a synthetic method of enormous potential.

Nickel-Catalyzed Dicarbofunctionalization of Alkenes

1,2-Dicarbofunctionalization of alkenes has emerged as an efficient synthetic strategy for preparing substituted molecules by coupling readily available alkenes with electrophiles and/or nucleophiles. Nickel complexes serve as effective catalysts owing to their tendency to undergo facile oxidative addition and slow β-hydride elimination, and their capability to access both two-electron and radical pathways. Two-component alkene functionalization reactions have achieved high chemo-, regio-, and stereoselectivities by tethering one of the coupling partners to the alkene substrate. Three-component reactions, however, often incorporate directing groups to control the selectivity. Only a few examples of directing-group-free difunctionalizations of unactivated alkenes have been reported. Therefore, great opportunities exist for the development of three-component difunctionalization reactions with broad substrate scopes and tunable chemo-, regio-, and stereoselectivities.

Fe-catalyzed three-component dicarbofunctionalization of unactivated alkenes with alkyl halides and Grignard reagents

A highly chemoselective iron-catalyzed three-component dicarbofunctionalization of unactivated olefins with alkyl halides (iodides and bromides) and sp2-hybridized Grignard reagents is reported. The reaction operates under fast turnover frequency and tolerates a diverse range of sp2-hybridized nucleophiles (electron-rich and electron-deficient (hetero)aryl and alkenyl Grignard reagents), alkyl halides (tertiary alkyl iodides/bromides and perfluorinated bromides), and unactivated olefins bearing diverse functional groups including tethered alkenes, ethers, protected alcohols, aldehydes, and amines to yield the desired 1,2-alkylarylated products with high regiocontrol. Further, we demonstrate that this protocol is amenable for the synthesis of new (hetero)carbocycles including tetrahydrofurans and pyrrolidines via a three-component radical cascade cyclization/arylation that forges three new C-C bonds.