Modular Diamino- and Dioxophosphine Oxides and Chlorides as Ligands for Transition-Metal-Catalyzed C?C and C?N Couplings with Aryl Chlorides

Nickel-Catalyzed Atroposelective C-H Alkylation Enabled by Bimetallic Catalysis with Air-Stable Heteroatom-Substituted Secondary Phosphine Oxide Preligands

The catalytic asymmetric construction of axially chiral C-N atropisomers remains a formidable challenge due to their low rotational barriers and is largely reliant on toxic, cost-intensive, and precious metal catalysts. In sharp contrast, we herein describe the first nickel-catalyzed atroposelective C-H alkylation for the construction of C-N axially chiral compounds with the aid of a chiral heteroatom-substituted secondary phosphine oxide (HASPO)-ligated Ni-Al bimetallic catalyst. A wide range of alkenes, including terminal and internal alkenes, were well compatible with the reaction, providing a variety of benzimidazole derivatives in high yields and enantioselectivities (up to 97:3 e.r.). The key to success was the identification of novel HASPOs as highly effective chiral preligands. Mechanistic studies revealed the catalyst mode of action, and in-depth data science analysis elucidated the key features of the responsible chiral preligands in controlling the enantioselectivity.

Ni-catalyzed C-F activation to construct C-P bond with P-P(O) and P(O)OR mediation

Here we developed an efficient Ni-catalyzed C-F bond phosphorylation of aryl fluorides via the crucial intermediates of P-P(O) and P(O)OR. P-P(O) mediated organophosphorus generation is observed for active aryl fluorides, whereas inactive aryl fluorides can also be activated and phosphorylated via a P(O)OR-mediated pathway, which is barely reported yet. Facile scale-up to the gram level and the upgrading of the bioactive molecule make this protocol to have promising applications in synthetic chemistry.

Organic-acid catalysed Minisci-type arylation of heterocycles with aryl acyl peroxides

A metal-free method for the Minisci-type arylation of heterocycles with aryl acyl peroxides has been reported. This strategy enables the rapid and simple synthesis of a series of Minisci-type adducts...

Site-Selective Cross-Coupling of Polyhalogenated Arenes and Heteroarenes with Identical Halogen Groups

Methods to functionalize arenes and heteroarenes in a site-selective manner are highly sought after for rapidly constructing value-added molecules of medicinal, agrochemical, and materials interest. One effective approach is the site-selective cross-coupling of polyhalogenated arenes bearing multiple, but identical, halogen groups. Such cross-coupling reactions have proven to be incredibly effective for site-selective functionalization. However, they also present formidable challenges due to the inherent similarities in the reactivities of the halogen substituents. In this Review, we discuss strategies for site-selective cross-couplings of polyhalogenated arenes and heteroarenes bearing identical halogens, beginning first with an overview of the reaction types that are more traditional in nature, such as electronically, sterically, and directing-group-controlled processes. Following these examples is a description of emerging strategies, which includes ligand- and additive/solvent-controlled reactions as well as photochemically initiated processes.

Visible-Light-Driven Organophotocatalyzed Mono-, Di-, and Tri-C(sp3)-H Alkylation of Phosphoramides

A photocatalytic metal-free, visible-light-driven, highly atom-economic, direct multiple α-C(sp³)-H alkylation of phosphoramides and thiophosphoramides is demonstrated under environmentally benign conditions. Economically viable and commercially available Eosin-Y is used as an HAT photocatalyst for mono-α-C(sp³)-H alkylation of phosphoramide derivatives. Remarkably, di- and tri-C(sp³)-H alkylation of phosphoramides and thiophosphoramides using an acridinium photocatalyst is reported with good yield and selectivity. Mechanistic studies reveal that monoalkylation of phosphoramides by Eosin-Y follows the HAT mechanism, whereas di- and tri-C(sp³)-H alkylation by the acridinium photocatalyst follows the SET mechanism.

Synthesis of Sterically Demanding Secondary Phosphides and Diphosphanes and Their Utilization in Small-Molecule Activation

Sterically demanding secondary potassium phosphides (4) were synthesized and investigated. Reaction with halophosphanes (5) yields diphosphanes (6), whereas reaction with CS₂ yields phosphanyl dithioformates (10). These can be further converted to the corresponding phosphanyl esters of dithioformic acid R₂P-C(S)S-PR₂ (8). One of these thioesters (8) was found to undergo a migration reaction, resulting in the formation of a phosphanylthioketone with an additional phosphanylthiolate group (9), which was used as a chiral ligand in gold coordination chemistry. The phosphanyl migration reaction was investigated by spectroscopic and theoretical methods, revealing a first-order reaction via a cyclic transition state. All species mentioned were fully characterized.

Palladium Complexes Based on Ylide-Functionalized Phosphines (YPhos): Broadly Applicable High-Performance Precatalysts for the Amination of Aryl Halides at Room Temperature

Palladium allyl, cinnamyl, and indenyl complexes with the ylide-substituted phosphines Cy3 P+ -C- (R)PCy2 (with R=Me (L1) or Ph (L2)) and Cy3 P+ -C- (Me)PtBu2 (L3) were prepared and applied as defined precatalysts in C-N coupling reactions. The complexes are highly active in the amination of 4-chlorotoluene with a series of different amines. Higher yields were observed with the precatalysts in comparison to the in situ generated catalysts. Changes in the ligand structures allowed for improved selectivities by shutting down β-hydride elimination or diarylation reactions. Particularly, the complexes based on L2 (joYPhos) revealed to be universal precatalysts for various amines and aryl halides. Full conversions to the desired products are reached mostly within 1 h reaction time at room temperature, thus making L2 to one of the most efficient ligands in C-N coupling reactions. The applicability of the catalysts was demonstrated for aryl chlorides, bromides and iodides together with primary and secondary aryl and alkyl amines, including gram-scale applications also with low catalyst loadings of down to 0.05 mol %. Kinetic studies further demonstrated the outstanding activity of the precatalysts with TOF over 10.000 h-1 .

Group 9 and 10 Metal Complexes of an Ylide-Substituted Phosphine: Coordination versus Cyclometalation and Oxidative Addition

Ylide-substituted phosphines (YPhos) have been shown to be excellent ligands for several transition metal catalyzed reactions. Investigations of the coordination behavior of the YPhos ligand YSPPh2 (1) [with YS = (Ph3P)(SO2Tol)C] toward group 9 and 10 metals revealed a surprisingly diverse coordination chemistry of the ligand. With Ni(CO)4, the formation of a di- as well as tricarbonyl complex is observed depending on the reaction conditions. In [( κP,η 2 -benzene-1)Ni(CO)2] the phosphine ligand also coordinates via a phosphonium bound phenyl group to the metal leading to a unique nickel η 2 -arene interaction, which can be viewed as an intermediate state toward P-C bond activation. Full cleavage of the P-C bond takes place with [Rh(COD)Cl]2 leading to a complex salt with [( κP,κO-1)Rh(COD)]+ as cation and a dirhodium trichloride complex anion. Here, YSPPh2 underwent P-C bond cleavage to thus act as an anionic diphosphine ligand. In contrast, in [( κP,κO-1)Rh(COD)]+ as well as [( κP,κO-1)Rh(CO)Cl], formed from the reaction of 1 with [Rh(CO)2Cl]2, the YPhos ligand acts as bidentate ligand complexing the metal via the phosphine and sulfonyl moiety with an intact PPh3 unit. A further type of coordination is observed with [Ir(COD)Cl]2. Here, phosphine coordination is accompanied by C-H activation at one of the phosphonium bound phenyl groups leading to a cyclometalated complex.

Facile Buchwald-Hartwig coupling of sterically encumbered substrates effected by PNP ligands

The diphosphinoamine ligands [(Ph2P)2N(Ar); 1 (Ar = C6H5), 2 (Ar = 2,6-iPr2C6H3)] were effectively utilized in Buchwald-Hartwig coupling of a range of sterically demanding substrates. The reaction was carried out using conventional and microwave routes and the latter reduces the reaction time from 3 d to 15-30 min. A broad substrate scope was achieved in this protocol and most of the coupling products are isolated on a mutligram scale. DFT calculations were carried out to elucidate the reaction mechanism.

A Highly Active Ylide-Functionalized Phosphine for Palladium-Catalyzed Aminations of Aryl Chlorides

Ylide-functionalized phosphine ligands (YPhos) were rationally designed to fit the requirements of Buchwald-Hartwig aminations at room temperature. This ligand class combines a strong electron-donating ability comparable to NHC ligands with high steric demand similar to biaryl phosphines. The active Pd species are stabilized by agostic C-H⋅⋅⋅Pd rather than by Pd-arene interactions. The practical advantage of YPhos ligands arises from their easy and scalable synthesis from widely available, inexpensive starting materials. Benchmark studies showed that YPhos-Pd complexes are superior to the best-known phosphine ligands in room-temperature aminations of aryl chlorides. The utility of the catalysts was demonstrated by the synthesis of various arylamines in high yields within short reaction times.

Coordination chemistry and catalysis with secondary phosphine oxides

Review on synthesis, coordination chemistry and catalysis with secondary phosphine oxides.

Accessing 4-oxy-substituted isoquinolinones via C-H activation and regioselective migratory insertion with electronically biased ynol ethers

The rhodium-catalyzed C-H activation and annulation with ynol ethers to directly provide 4-oxy substituted isoquinolinones is reported. The polarized nature of ynol ethers provides an electronic bias for controlling the regioselectivity of the migratory insertion process. While the highly reactive nature of ynol ethers presents a challenge, mild conditions were found to provide product in moderate to good yield. Utility was demonstrated by application in the synthesis of a prolyl-4-hydroxylase inhibitor framework.

Site-Selective C-S Bond Formation at C-Br over C-OTf and C-Cl Enabled by an Air-Stable, Easily Recoverable, and Recyclable Palladium(I) Catalyst

This report widens the repertoire of emerging PdI catalysis to carbon-heteroatom, that is, C-S bond formation. While Pd0 -catalyzed protocols may suffer from the formation of poisonous sulfide-bound off-cycle intermediates and lack of selectivity, the mechanistically diverse PdI catalysis concept circumvents these challenges and allows for C-S bond formation (S-aryl and S-alkyl) of a wide range of aryl halides. Site-selective thiolations of C-Br sites in the presence of C-Cl and C-OTf were achieved in a general and a priori predictable fashion. Computational, spectroscopic, X-ray, and reactivity data support dinuclear PdI catalysis to be operative. Contrary to air-sensitive Pd0 , the active PdI species was easily recovered in the open atmosphere and subjected to multiple rounds of recycling.

Ni-Al Bimetallic Catalyzed Enantioselective Cycloaddition of Cyclopropyl Carboxamide with Alkyne

A Ni-Al bimetallic catalyzed enantioselective cycloaddition reaction of cyclopropyl carboxamides with alkynes has been developed. A series of cyclopentenyl carboxamides were obtained in up to 99% yield and 94% ee. The bifunctional-ligand-enabled bimetallic catalysis proved to be an efficient strategy for the C-C bond cleavage of unreactive cyclopropanes.

Room-Temperature CuI-Catalyzed Amination of Aryl Iodides and Aryl Bromides

A general and effective CuI/N',N'-diaryl-1H-pyrrole-2-carbohydrazide catalyst system was developed for the amination of aryl iodides and bromides at room temperature with good chemoselectivity between -OH and -NH₂ groups. Only 5 mol % of CuI and ligands was needed in this protocol to effect the amination of various aryl bromides and aryl iodides with a wide range of aliphatic and aryl amines (1.3 equiv).

P,N,N-pincer nickel-catalyzed cross-coupling of aryl fluorides and chlorides

P,N,N-Pincer nickel complexes [Ni(Cl){N(2-R2PC6H4)(2'-Me2NC6H4)}] (R = Ph, 3a; R = Pr(i), 3b; R = Cy, 3c) were synthesized and their catalysis toward the Kumada or Negishi cross-coupling reaction of aryl fluorides and chlorides was evaluated. Complex 3a effectively catalyzes the cross-coupling of (hetero)aryl fluorides with aryl Grignard reagents at room temperature. Complex 3a also catalyzes the cross-coupling of (hetero)aryl chlorides and arylzinc reagents at 80 °C with low catalyst loadings and good functional group compatibility.

A diphenyl ether derived bidentate secondary phosphine oxide as a preligand for nickel-catalyzed C–S cross-coupling reactions

A bidentate secondary phosphine oxide is a more effective preligand than Ph₂P(O)H in nickel-catalyzed C–S cross-coupling reactions.

Palladium(II)-catalyzed Heck reaction of aryl halides and arylboronic acids with olefins under mild conditions

A series of general and selective Pd(II)-catalyzed Heck reactions were investigated under mild reaction conditions. The first protocol has been developed employing an imidazole-based secondary phosphine oxide (SPO) ligated palladium complex (6) as a precatalyst. The catalytic coupling of aryl halides and olefins led to the formation of the corresponding coupled products in excellent yields. A variety of substrates, both electron-rich and electron-poor olefins, were converted smoothly to the targeted products in high yields. Compared with the existing approaches employing SPO-Pd complexes in a Heck reaction, the current strategy features mild reaction conditions and broad substrate scope. Furthermore, we described the coupling of arylboronic acids with olefins, which were catalyzed by Pd(OAc)2 and employed N-bromosuccinimide as an additive under ambient conditions. The resulted biaryls have been obtained in moderate to good yields.

Generation of [(IPr)Pd(PR2Cl)] complexes via P-Cl reductive elimination

Treatment of [(IPr)Pd(Cl)(2)(PR(2)H)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; R = Cy, tBu, or 1-Ad) with NaN(SiMe(3))(2) generated isolable [(IPr)Pd(PR(2)Cl)] complexes (68-75%) that have been crystallographically characterized. The formation of these mixed-ligand Pd(0) species in this manner corresponds to an unusual net dehydrohalogenation/P-Cl reductive elimination sequence.

P-H activation using alkynylgold substrates: steric and electronic effects

The susceptibility of a prototypical hydrogen phosphonate to undergo P-H activation upon treatment with alkynylgold complexes has been studied. Dynamic solution behavior was observed during reactions involving triphenylphosphine ligated substrates and was attributed to rapid phosphine exchange between the alkynylgold starting material and the gold phosphonate product. The use of bulky biaryldialkylphosphine ligands eliminated the fluxional behavior, but did not significantly slow the rate of P-H activation. Similarly, changing the supporting ligand to an N-heterocyclic carbene did not significantly slow the rate of the reaction. Despite a number of reports outlining the functionalization of propargyl alcohols using gold catalysts, incorporating these groups into the architecture of the alkynylgold substrates did not alter the product distributions. Although the chemistry tolerated a range of supporting ligands, incorporating electron donating groups into the alkyne increased the rate of the reaction while electron-withdrawing groups slowed the reaction. A possible mechanism for the process includes a transition state containing significant pi-contribution from the alkyne. Due to the high yields of gold phosphonates obtained in this chemistry as well as the mild conditions of the reactions, the interception of intermediates/catalysts by substrates or ligands containing labile P-H donors is an issue that must be circumvented when designing or developing a gold catalyzed reaction that proceeds through alkynylgold intermediates.

Palladium-catalyzed Hiyama cross-coupling of aryltrifluorosilanes with aryl and heteroaryl chlorides

An efficient, palladium-catalyzed Hiyama cross-coupling reaction of aryltrifluorosilanes with aryl chlorides has been developed. A wide variety of functionalized biaryl derivatives were isolated in good to excellent yields. The scope of this reaction has also been extended to heteroaryl chlorides, affording the corresponding heterobiaryl compounds in high yields.