Allene C(sp2)-H Activation and Alkenylation Catalyzed by Palladium

N,N- and N,O-Bidentate-Chelation-Assisted Alkenyl C-H Functionalization

Chelation-assisted olefinic C-H functionalization has been demonstrated to be a powerful method of constructing multi-substituted alkenes from simpler ones. This strategy produces complex alkenes in a regio- and stereoselective manner, followed by C-H endo- and exo-cyclometallation. Among the various directing groups developed, N,N- and N,O-bidentate directing groups are the most widely used to selectively promote C-H functionalization due to their fine, tunable, and reversible coordination with the metal center. In this review, we discuss various N,N- and N,O-bidentate directing group-assisted olefinic C-H bond functionalization reactions, including alkenylation, alkylation, arylation, thiolation, silylation, halogenation, and cyclization.

Catalytic Amino Group Transfer Reactions Mediated by Photoinduced Nitrene Formation from Rhodium-Hydroxamates

Herein, we report a photocatalytic platform to access transient nitrenoids by designing photo-responsive neutral rhodium-hydroxamate complexes. Combined experimental and computational mechanistic studies, including electron paramagnetic resonance (EPR) and mass spectrometric analysis, suggest that electrophilic Fischer-type Rh-acylnitrenoid intermediates could be generated via photoactivation of corresponding Rh-hydroxamates via N-O bond homolysis and redox event. Moreover, catalytic acylnitrenoid transfer was explored toward the amidation of various hydrocarbons, amines, and alcohols to furnish new N-C, N-N, and N-O bonds.

Palladium-Catalyzed, Regio-/Stereo- and Enantiospecific Anti-Carboxylation of Unactivated Internal Allenes

We report herein a directing group-controlled, palladium-catalyzed, regio-, stereo-, and enantiospecific anti-carboxylation of unactivated, internal allenes enabled via the synergistic interplay of a rationally designed bidentate directing group, palladium catalyst, and a multifunctional acetate ligand. The corresponding trans allyl ester was obtained in excellent yields with exclusive δ-regioselectivity and anti-carboxypalladation stereocontrol. The acetate ligand of the palladium catalyst controls the regio-, stereo- and enantioselectivity in the desired transformation. The potential of this concept has been demonstrated by the development of the chiral version of this transformation by using axial-to-central chirality transfer with good yields and enantioselectivities. Detailed investigations, including kinetic studies, order studies, and DFT studies, were performed to validate the ligand-assisted nucleopalladation process and the rationale behind the observed racemization of chiral allenes. The studies also indicated that the anti-carboxypalladation step was the rate-limiting as well as the stereo- and enantiodetermining step.

Site-Selective Direct γ-Difunctionalization of Diazoenals: Application to the Synthesis of Enal-Functionalized Allenes and Furans

A new approach for the synthetically important γ-functionalized enals has been developed. The strategy involves rhodium-catalyzed direct C-C & C-S bond forming site-selective γ-difunctionalization of diazoenals with aryl propargyl sulfides via sulfur ylide [2,3] sigmatropic rearrangement, resulting in the highly functionalized γ-allenyl(sulfanyl)enals in excellent yield at ambient temperature. This highly versatile approach constitutes a viable alternative to the remote carbonyl-directed γ-functionalization of unmodified enals which suffer from competitive side reactions. The synthetic utility of the γ-allenyl(sulfanyl)enals was demonstrated by the InCl3-catalyzed cycloisomerization to the trisubstituted furanyl-enals via an unusual 1,4-sulfanyl migration. In addition, an operationally simple and efficient one-pot [3+2] annulation of diazoenals and propargyl sulfides, involving combined γ-difunctionalization and cycloisomerization was successfully developed for the diverse enal-functionalized furans.

Copper-Catalyzed sp2 C-H Arylation and Alkynylation of Allenes via Hydrogen Atom Abstraction

Development of methods for the sp2 C-H transformations of allenes has received much attention, and it presents a powerful tool for the synthesis of complicated allene-containing bioactive molecules. With a copper-catalyzed radical relay, sp2 allenic C-H arylation and alkynylation were established herein, using various aryl boronic acids and trimethoxysilyl-substituted alkynes as carbon nucleophiles and using electrophilic N-F reagents as nitrogen-centered radical precursors. These methods featured excellent site selectivity to deliver fully substituted allenes efficiently. Moreover, with silyl-substituted allenes as substrates, a subsequent dual sp2 C-H functionalization process was established as well, which allowed for the divergent synthesis of multifunctionalized allenes, significantly expanding their chemical spaces.

Rh(III)-catalyzed regioselective C(sp2)-H alkenylation of isoquinolones with methoxyallene: A facile access to aldehyde-bearing isoquinolones

A simple and rapid access to isoquinolone aldehyde scaffolds has been established by a rhodium-catalyzed reaction between isoquinolone and methoxyallene that forges alkenylation in an explicit regioselective manner. Herein, methoxyallene serving as an acrolein equivalent results in execution of this unique functionalization. Furthermore, the compatibility with complex molecules underscores the significance of this developed protocol. The mechanistic proposal for this regioselective transformation was consistent with kinetic studies and several control reactions.

Pd-Catalyzed C(sp2)-H/C(sp2)-H Coupling of Limonene

Limonene undergoes a regioselective Pd(II)-catalyzed C(sp2)-H/C(sp2)-H coupling with acrylic acid esters and amides, α,β-unsaturated ketones, styrenes, and allyl acetate, affording novel 1,3-dienes. DFT computations gave results in accord with the experimental results and allowed for the formulation of a plausible mechanism. The postfunctionalization of one of the coupled products was achieved via a large-scale Sonogashira reaction conducted under micellar catalysis.

Copper-catalyzed remote double functionalization of allenynes

Addition reactions of molecules with conjugated or non-conjugated multiple unsaturated C-C bonds are very attractive yet challenging due to the versatile issues of chemo-, regio-, and stereo-selectivities. Especially for the readily available conjugated allenyne compounds, the reactivities have not been explored. The first example of copper-catalyzed 2,5-hydrofunctionalization and 2,5-difunctionalization of allenynes, which provides a facile access to versatile conjugated vinylic allenes with a C-B or C-Si bond, has been developed. This mild protocol has a broad substrate scope tolerating many synthetically useful functional groups. Due to the highly functionalized nature of the products, they have been demonstrated as platform molecules for the efficient syntheses of monocyclic products including poly-substituted benzenes, bicyclic compounds, and highly functionalized allene molecules.

A Hydrogen Evolution Catalyst [Co2O2] Metallacycle Enables Regioselective Allene C(sp2)-H Functionalization

Selective functionalization of allenic C(sp2)-H is an ideal approach to upgrading simple allenes to synthetically useful allenes, albeit suffering from challenges associated with inert reactivity and inferior selectivity. Inspired by energy chemistry, a catalytic hydrogen evolution reaction (HER) strategy was leveraged to selectively activate weakly acidic allene C(sp2)-H bonds in a reductive mode. An array of [Co2O2] metallacycle complexes were readily devised starting from amino acids, and they were demonstrated as robust HER catalysts, which would selectively break allenic C(sp2)-H bonds to release hydrogen. With the newly developed HER catalyst, regioselective electrochemical functionalization of allenic C(sp2)-H with alcoholic α C(sp3)-H was unprecedentedly achieved. This strategy features excellent regioselectivity, unconventional chemoselectivity, good functional-group tolerance (62 examples), and mild conditions. Mechanism experiments revealed a reactive hydroxy-coordinated cobalt(II) species in the reaction. Density functional theory (DFT) calculations were also conducted to rationalize the regioselectivity observed in the reaction.

Cross-coupling of organic fluorides with allenes: a silyl-radical-relay pathway for the construction of α-alkynyl-substituted all-carbon quaternary centres

Controlling the transformation of versatile and reactive allenes is a considerable challenge. Herein, we report an efficient silylboronate-mediated cross-coupling reaction of organic fluorides with allenes to construct a series of sterically demanding α-ethynyl-containing all-carbon quaternary centers (ACQCs), using catalyst-free silyl-radical-relay reactions to selectively functionalize highly inert C-F bonds in organic fluorides. The key to the success of this transformation lies in the radical rearrangement of an in situ-generated allenyl radical to form a bulky tertiary propargyl radical; however, the transformation does not show efficiency when using the propargyl isomer directly. This unique reaction enables the cross-coupling of a tertiary carbon radical center with a C(sp2)-F bond or a benzylic C(sp3)-F bond. α-Ethynyl-containing ACQCs with (hetero)aromatic substituents and benzyl were efficiently synthesized in a single step using electronically and sterically diverse organic fluorides and allenes. The practical utility of this protocol is showcased by the late-stage functionalization of bioactive molecules and the modification of a liquid crystalline material.

Palladium-Catalyzed Dehydrogenative Carbonylative Esterification of Allenoic Acids for the Synthesis of γ-Butyrolactone Derivatives

A highly efficient dehydrogenative carbonylative esterification of allenoic acids using Pd-catalysis was developed, providing a novel approach to synthesizing esterified γ-butyrolactone derivatives with consistently good to excellent results demonstrated across over 50 examples. Additionally, we used a heterogeneous catalyst known as Pd-AmP-MCF and harnessed biomimetic-aerobic-oxidation conditions to facilitate the practical execution of this reaction. Furthermore, our detailed study of γ-butyrolactone products highlighted their potential in synthesizing bioactive compounds.

Stereoselective Synthesis of Complex Polyenes through Sequential α-/β-C-H Functionalization of trans-Styrenes

Sequential C-H functionalization of molecules containing multiple C-H bonds can efficiently lead to structural diversity. Herein we present the first chelation-assisted sequential α-/β-C-H functionalization of E-styrenes with simple alkenes and alkynes in excellent regio- and stereo-selectivity. The process involves α C-H functionalization by six-membered exo-cyclopalladation to result in tri- and tetrasubstituted 1,3-dienes and β C-H functionalization through seven-membered endo-cyclopalladation to produce tetra- and pentasubstituted 1,3,5-trienes in up to 97 % yield with up to >99/1 E/Z selectivity, both enabled by the chelation assistance of pyrazinamide. The protocol is demonstrated to be widely applicable, tolerant to a wide range of functional groups and bioactive fragments, and suitable for gram-scale synthesis as well as one-pot and two step preparation of trienes. Mechanistic experiments and density functional theory (DFT) calculations were performed to elucidate the selectivity and reactivity.

Copper-Catalyzed Radical Allene C(sp2 )-H Cyanation

While the hydrogen atom abstraction (HAA) from C(sp3 )-H bond has been well explored, the radical-mediated chemo- and regio-selective functionalization of allenic C(sp2 )-H bond via direct HAA from C(sp2 )-H bond of allene remains an unsolved challenge in synthetic chemistry. This is primarily due to inherent challenges with addition of radical intermediates to allenes, regioselectivity of HAA process, instability of allenyl radical toward propargyl radical et al. Herein, we report a copper catalyzed allenic C(sp2 )-H cyanation of an array of tri- and di-substituted allenes with exceptional site-selectivity, while mono-substituted allene was successfully cyanated, albeit with a low yield. In the developed strategy, steric N-fluoro-N-alkylsulfonamide, serving as precursor of hydrogen atom abstractor, plays a crucial role in achieving the desired regioselectivity and avoiding addition of N-centered radical to allene.

Site-Selective sp2 C-H Cyanation of Allenes via Copper-Catalyzed Radical Relay

Compared with the extensively reported hydrogen atom transfer (HAT) at sp3 C-H, abstraction of hydrogen atoms at the sp2 carbon is extremely rare. Here, we communicate the site-selective cyanation of the sp2 C-H bond of allenes using the strategy of copper-catalyzed radical relay. The reactions afford various allenyl nitriles directly from simple allenes with a broad substrate scope and a remarkable functional group compatibility under mild conditions. These reactions exhibit excellent site-selectivity toward sp2 C-H, which can be attributed to the unique pocket created by the Cu-bound nitrogen-centered radical. The favorable HAT on sp2 C-H is due to crucial hydrogen bonding between the fluoride bonded to the Cu(II) center and the hydrogen atom at the allylic position. These features enable the late-stage functionalization of druglike bioactive molecules containing an allene motif.

Supramolecular Block Copolymers from Tricarboxamides. Biasing Co-assembly by the Incorporation of Pyridine Rings

The synthesis of a series of triangular-shaped tricarboxamides endowed with three picoline or nicotine units (compounds 2 and 3, respectively) or just one nicotine unit (compound 4) is reported, and their self-assembling features investigated. The pyridine rings make compounds 2-4 electronically complementary with our previously reported oligo(phenylene ethynylene)tricarboxamides (OPE-TA) 1 to form supramolecular copolymers. C3 -symmetric tricarboxamide 2 forms highly stable intramolecular five-membered pseudocycles that impede its supramolecular polymerization into poly-2 and the co-assembly with 1 to yield copolymer poly-1-co-2. On the other hand, C3 -symmetric tricarboxamide 3 readily forms poly-3 with great stability but unable to form helical supramolecular polymers despite the presence of the peripheral chiral side chains. The copolymer poly-1-co-3 can only be obtained by a previous complete disassembly of the constitutive homopolymers in CHCl3 . Helical poly-1-co-3 arises in a process involving the transfer of the helicity from racemic poly-1 to poly-3, and the amplification of asymmetry from chiral poly-3 to poly-1. Importantly, C2v -symmetric 4, endowed with only one nicotinamide moiety and three chiral side chains, self-assembles into a P-type helical supramolecular polymer (poly-4) in a thermodynamically controlled cooperative process. The combination of poly-1 and poly-4 generates chiral supramolecular copolymer poly-1-co-4, whose blocky microstructure has been investigated by applying the previously reported supramolecular copolymerization model.

Pd(II)-Catalyzed [5 + 2] Cyclization of N-Triflyl Aryl Indoles and α,γ-Substituted Allenoates: A Route to Indole-Fused Benzodiazepines

A Pd(II)-catalyzed [5 + 2] annulation between N-triflyl aryl indoles and α,γ-substituted allenoates for the synthesis of indole-fused benzodiazepines is reported. This protocol is highly efficient when N-acetylated valine amino acid and DMSO have been used as ligand and cosolvent, respectively. The substrate scope can be further extended to disubstituted allenoates. A reaction mechanism has been proposed based on the mechanistic studies. Mechanistically, the N-acetylated valine amino acid ligand accelerates C-H activation of the C(sp2)-H bond. Consequent cyclopalladation leads to the formation of a six-membered palladacycle. Subsequent coordination and migratory insertion of an allenoate forms the possible eight-membered intermediate. Reductive elimination followed by a [1,3]-H shift results in the indole-fused benzodiazepines.

A Pd-catalyzed highly selective three-component protocol for trisubstituted allenes

Herein we report the first example of a Pd-catalyzed highly selective three-component reaction of alkynyl-1,4-diol dicarbonates, organoboronic acids, and malonate anions for the efficient synthesis of trisubstituted 2,3-allenyl malonates not readily available by the known protocols. The reaction demonstrates an excellent regio- and chemo-selectivity for both the oxidative addition referring to the two C-O bonds and the subsequent coupling with the nucleophile with a remarkable functional group compatibility. A series of control experiments confirm a unique mechanism involving β-O elimination forming alka-1,2,3-triene and the subsequent insertion of its terminal C[double bond, length as m-dash]C bond into the Ar-Pd bond.

Electrochemically Driven Selective Removal of the S═N Bond-Directing Group Using Cyclohexanone Oxime as the Mediator

An inexpensive electrochemical induction system was used for the efficient reductive defunctionalization of sulfoximines through a radical pathway. This practical and robust strategy could be used for the removal of the S═N bond-directing group from various sulfoximines. The practicability of this method was demonstrated by its mild conditions, simple operation, one-pot procedure, gram-scale synthesis, and the undivided cell. Furthermore, preliminary mechanistic studies suggested that the reaction might proceed via a homocoupling reaction and a denitrification procedure.

Synthesis of 1,1-Disubstituted Allenylic Silyl Ethers Through Iron-Catalyzed Regioselective C(sp2)─H Functionalization of Allenes

We report a synthesis of allenylic silyl ethers through iron-catalyzed functionalization of the C(sp2)─H bonds of monosubstituted alkylallenes. In the presence of a cyclopentadienyliron dicarbonyl based catalyst and triisopropylsilyl triflate as a silylation agent, a variety of aryl aldehydes were suitable coupling partners in this transformation, furnishing a collection of 1,1-disubstituted allenylic triisopropylsilyl ethers as products in moderate to excellent yields as a single regioisomer. Lithium bistriflimide was identified as a critical additive in this transformation. The optimized protocol was scalable, and the products were amenable to further transformation to give a number of unsaturated, polyfunctional derivatives.

Picolinamides and Iodoalkynes Enable Palladium-Catalyzed syn-Aminoalkynylation of Di- and Trisubstituted Alkenes to Give Pyrrolidines

Palladium-catalyzed aminoalkynylation of electronically unbiased olefins with iodoalkynes is reported. The picolinamide auxiliary enables for the first time the syn-selective aminoalkynylation of mono-, di- and trisubstituted alkenes to afford the corresponding pyrrolidines in up to 97 % yield and as single diastereomers. Furthermore, through a C-H activation approach, the picolinamide allows the rapid synthesis of functionalized olefins, which are suitable cyclization precursors. Facile and orthogonal deprotection of the amides and Siⁱ Pr₃ -acetylenes in the products, and a subsequent Pictet-Spengler reaction is demonstrated.