Carboxylate switch between hydro- and carbopalladation pathways in regiodivergent dimerization of alkynes

trans-Hydroalkynylation of Internal 1,3-Enynes Enabled by Cooperative Catalysis

A cooperative catalyst system involving a Pd(0)/Senphos complex, tris(pentafluorophenyl)borane, copper bromide, and an amine base, is demonstrated to catalyze trans-hydroalkynylation of internal 1,3-enynes. For the first time, a Lewis acid catalyst is shown to promote the reaction involving the emerging outer-sphere oxidative reaction step. The resulting cross-conjugated dieneynes are versatile synthons for organic synthesis, and their characterization reveals distinct photophysical properties depending on the positioning of the donor/acceptor substituents along the conjugation path.

Palladium Hydride-Enabled Hydroalkenylation of Strained Molecules

We report the first palladium hydride enabled hydroalkenylation of strained molecules. This new mild protocol proceeds via a regio- and chemoselective hydropalladation step, followed by a photoinduced radical alkyl Heck reaction. This methodology represents a new reactivity mode for strained molecules and opens new avenues for photoinduced palladium catalysis. The reaction is compatible with a wide range of functional groups and can be applied to complex structures, delivering a diverse array of highly valuable and modifiable alkenylated cyclobutanes and cyclopropanes. A hydroalkenylation/diastereoselective rearrangement cascade toward a cyclopentene scaffold has also been demonstrated.

Silyl-Osmium(IV)-Trihydride Complexes Stabilized by a Pincer Ether-Diphosphine: Formation and Reactions with Alkynes

Complex OsH₄{κ³-P,O,P-[xant(PⁱPr₂)₂]} (1) activates the Si-H bond of triethylsilane, triphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane to give the silyl-osmium(IV)-trihydride derivatives OsH₃(SiR₃){κ³-P,O,P-[xant(PⁱPr₂)₂]} [SiR₃ = SiEt₃ (2), SiPh₃ (3), SiMe(OSiMe₃)₂ (4)] and H₂. The activation takes place via an unsaturated tetrahydride intermediate, resulting from the dissociation of the oxygen atom of the pincer ligand 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(PⁱPr₂)₂). This intermediate, which has been trapped to form OsH₄{κ²-P,P-[xant(PⁱPr₂)₂]}(PⁱPr₃) (5), coordinates the Si-H bond of the silanes to subsequently undergo a homolytic cleavage. Kinetics of the reaction along with the observed primary isotope effect demonstrates that the Si-H rupture is the rate-determining step of the activation. Complex 2 reacts with 1,1-diphenyl-2-propyn-1-ol and 1-phenyl-1-propyne. The reaction with the former affords Os{C≡CC(OH)Ph₂}₂{=C=CHC(OH)Ph₂}{κ³-P,O,P-[xant(PⁱPr₂)₂]} (6), which catalyzes the conversion of the propargylic alcohol into (E)-2-(5,5-diphenylfuran-2(5H)-ylidene)-1,1-diphenylethan-1-ol, via (Z)-enynediol. In methanol, the hydroxyvinylidene ligand of 6 dehydrates to allenylidene, generating Os{C≡CC(OH)Ph₂}₂{=C=C=CPh₂}{κ³-P,O,P-[xant(PⁱPr₂)₂]} (7). The reaction of 2 with 1-phenyl-1-propyne leads to OsH{κ¹-C,η²-[C₆H₄CH₂CH=CH₂]}{κ³-P,O,P-[xant(PⁱPr₂)₂]} (8) and PhCH₂CH=CH(SiEt₃).

Palladium-Catalyzed Tail-to-Tail Reductive Dimerization of Terminal Alkynes to 2,3-Dibranched Butadienes

The palladium-catalyzed tail-to-tail reductive dimerization of terminal alkynes is described for the first time. Aromatic terminal alkynes bearing diverse and sensitive functional groups as well as aliphatic terminal alkynes are efficiently transformed to 2,3-dibranched butadienes. The key to achieve a selective tail-to-tail reductive dimerization reaction is to control appropriately the acidity of the reaction solution, which is accomplished by a combined use of pivalic acid and para-toluenesulfonic acid. The tail-to-tail reductive dimerization reaction is proposed to proceed via a cationic alkenyl palladium intermediate under acidic conditions.

Synthesis of 2-acetal-1,3-enynes by Sonogashira reaction of bromovinyl acetals with alkynes: application to the formal synthesis of a glucagon antagonist

The preparation of functionalized 1,3-enynes bearing an acetal moiety at the 2-position has been studied through Sonogashira reaction of bromovinyl acetals with various alkyl- and aryl-substituted terminal alkynes.

Preparation of (E)-1,3-Enyne Derivatives through Palladium Catalyzed Hydroalkynylation of Allenes

A general and efficient palladium catalyzed hydroalkynylation of allenes was developed to produce synthetically versatile (E)-1,3-enyne derivatives with high regio- and stereoselectivity. This catalytic system proceeded under mild conditions and was compatible with a broad range of substrates, especially for allenes without electron-bias groups. This work further broadens the synthetic potential of these scaffolds in organic synthesis and medicinal chemistry.

Atom-Economical Cross-Coupling of Internal and Terminal Alkynes to Access 1,3-Enynes

Selective carbon-carbon (C-C) bond formation in chemical synthesis generally requires prefunctionalized building blocks. However, the requisite prefunctionalization steps undermine the overall efficiency of synthetic sequences that rely on such reactions, which is particularly problematic in large-scale applications, such as in the commercial production of pharmaceuticals. Herein, we describe a selective and catalytic method for synthesizing 1,3-enynes without prefunctionalized building blocks. In this transformation several classes of unactivated internal acceptor alkynes can be coupled with terminal donor alkynes to deliver 1,3-enynes in a highly regio- and stereoselective manner. The scope of compatible acceptor alkynes includes propargyl alcohols, (homo)propargyl amine derivatives, and (homo)propargyl carboxamides. This method is facilitated by a tailored P,N-ligand that enables regioselective addition and suppresses secondary E/Z-isomerization of the product. The reaction is scalable and can operate effectively with as low as 0.5 mol % catalyst loading. The products are versatile intermediates that can participate in various downstream transformations. We also present preliminary mechanistic experiments that are consistent with a redox-neutral Pd(II) catalytic cycle.

Recent advances in NHC–palladium catalysis for alkyne chemistry: versatile synthesis and applications

This review summarizes the recent developments in NHC–palladium catalysis for alkyne chemistry: versatile synthesis and applications.

Ligand-Controlled Diastereoselective Cobalt-Catalysed Hydroalkynylation of Terminal Alkynes to E- or Z-1,3-Enynes

A diastereoselective hydroalkynylation of terminal alkynes to form the head‐to‐head dimerization products by two different cobalt‐phosphine catalyst system is reported. The use of the bidentate ligand dppp and additional triphenylphosphine led to the selective formation of the (E)‐1,3‐enynes (E:Z>99:1) in good to excellent yields, while the tridentate ligand TriPhos led to the corresponding (Z)‐1,3‐enynes in moderate to good yields with excellent stereoselectivities (up to E:Z=1:99). Both pre‐catalysts are easy to handle, because of their stability under atmospheric conditions. The optimized reaction conditions were identified by the Design of Experiments (DoE) approach, which has not been used before in cobalt‐catalysed reaction optimisation. DoE decreased the number of required reactions to a minimum.

Rhodium(I)-NHC Complexes Bearing Bidentate Bis-Heteroatomic Acidato Ligands as gem-Selective Catalysts for Alkyne Dimerization

A series of Rh(κ² -BHetA)(η² -coe)(IPr) complexes bearing 1,3-bis-hetereoatomic acidato ligands (BHetA) including carboxylato (O,O), thioacetato (O,S), amidato (O,N), thioamidato (N,S), and amidinato (N,N), have been prepared by reaction of the dinuclear precursor [Rh(μ-Cl)(IPr)(η² -coe)]₂ with the corresponding anionic BHetA species. The Rh^I -NHC-BHetA compounds catalyze the dimerization of aryl alkynes, showing excellent selectivity for the head-to-tail enynes. Among them, the acetanilidato-based catalyst has shown an outstanding catalytic performance reaching unprecedented TOF levels of 2500 h^-1 with complete selectivity for the gem-isomer. Investigation of the reaction mechanism supports a non-oxidative pathway in which the BHetA ligand behaves as proton shuttle through intermediate κ¹ -HBHetA species. However, in the presence of pyridine as additive, the identification of the common Rh^III H(C≡CPh)₂ (IPr)(py)₂ intermediate gives support for an alternative oxidative route.

Palladium-catalyzed decarboxylative gem-selective addition of alkynoic acids to terminal alkynes

Alkynoic acids added to terminal alkynes to give gem-1,3-enynes with high selectivity and good yields. In addition, the reaction of alkynoic acids with propiolic acid provided the corresponding gem-1,3-enynes via double decarboxylation.

Metal-catalyzed regiodivergent organic reactions

This review discusses metal-catalysed regiodivergent additions, allylic substitutions, CH-activation, cross-couplings and intra- or intermolecular cyclisations.

Pseudo-tetrahedral Rhodium and Iridium Complexes: Catalytic Synthesis of E-Enynes

The reactions of the rhodium(I) and iridium(I) complexes [M(PhBP₃ )(C₂ H₄ )(NCMe)] (PhBP₃ =PhB(CH₂ PPh₂ )₃ ^- ) with alkynes have resulted in the synthesis of a new family of pseudo-tetrahedral complexes, [M(PhBP₃ )(RC≡CR')] (M=Rh, Ir), which contain the alkyne as a four-electron donor. The reactions of these unusual compounds with two-electron donors (L=PMe₃ , CNtBu) produced a change in the "donicity" of the alkyne from a 4e^- to a 2e^- donor to give five-coordinate complexes. These were the final products with the iridium complexes, whereas further reactions took place with the rhodium complexes. In particular, C(sp)-H bond activation of the alkyne occurred leading to hydrido alkynyl complexes. This process is essential for the further reactivity of the alkynes, and if the alkyne itself was used as reagent, E-enyne complexes were obtained. As a consequence of this chemistry, we show that the complex [Rh(PhBP₃ )(C₂ H₄ )(NCMe)] is a very efficient pre-catalyst for the regioselective di- and trimerization of terminal alkynes to E-enynes and benzene derivatives, respectively. Interestingly, acetonitrile significantly enhanced the catalytic activity by facilitating the C(sp)-H bond activation step. A hydrometalation mechanism to account for these experimental observations is proposed.

Copper-Catalyzed Regioselective and Stereoselective Coupling of Grignard Reagents with Pent-1-en-4-yn-3-yl Benzoates: A Shortcut to ( Z) -1,5-Disubstituted Pent-3-en-1-ynes from Accessible Starting Materials

Copper-catalyzed coupling of Grignard reagents with pent-1-en-4-yn-3-yl benzoates occurs regioselectively at the terminal alkenyl carbon rather than the alkynyl site, leading to the stereoselective formation of unexpected ( Z) -1,5-disubstituted pent-3-en-1-ynes without generation of the initially expected alkenyl allene products. By using easily accessible starting materials, this reaction can provide direct access to thermodynamically unfavorable Z-configured enynes, which widely exist in many bioactive natural products, such as the anti-inflammatory components in henna.

Revealing the unusual role of bases in activation/deactivation of catalytic systems: O-NHC coupling in M/NHC catalysis

Numerous reactions are catalyzed by complexes of metals (M) with N-heterocyclic carbene (NHC) ligands, typically in the presence of oxygen bases, which significantly shape the performance. It is generally accepted that bases are required for either substrate activation (exemplified by transmetallation in the Suzuki cross-coupling), or HX capture (e.g. in a variety of C-C and C-heteroatom couplings, the Heck reaction, C-H functionalization, heterocyclizations, etc.). This study gives insights into the behavior of M(ii)/NHC (M = Pd, Pt, Ni) complexes in solution under the action of bases conventionally engaged in catalysis (KOH, NaOH, t-BuOK, Cs2CO3, K2CO3, etc.). A previously unaddressed transformation of M(ii)/NHC complexes under conditions of typical base-mediated M/NHC catalyzed reactions is disclosed. Pd(ii) and Pt(ii) complexes widely used in catalysis react with the bases to give M(0) species and 2(5)-oxo-substituted azoles via an O-NHC coupling mechanism. Ni(NHC)2X2 complexes hydrolyze in the presence of aqueous potassium hydroxide, and undergo the same O-NHC coupling to give azolones and metallic nickel under the action of t-BuOK under anhydrous conditions. The study reveals a new role of NHC ligands as intramolecular reducing agents for the transformation of M(ii) into "ligandless" M(0) species. This demonstrates that the disclosed base-mediated O-NHC coupling reaction is integrated into the catalytic M/NHC systems and can define the mechanism of catalysis (molecular M/NHC vs. "NHC-free" cocktail-type catalysis). A proposed mechanism of the revealed transformation includes NHC-OR reductive elimination, as implied by a series of mechanistic studies including 18O labeling experiments.

Phosphinito palladium(ii) complexes as catalysts for the synthesis of 1,3-enynes, aromatic alkynes and ynones

An air-stable phosphinito palladium(ii) complex has been found to be an efficient catalyst in the formation of C–C bonds.

Synthesis of 2-Alkynoates by Palladium(II)-Catalyzed Oxidative Carbonylation of Terminal Alkynes and Alcohols

A homogeneous Pd(II) catalyst, utilizing a simple and inexpensive amine ligand (TMEDA), allows 2-alkynoates to be prepared in high yields by an oxidative carbonylation of terminal alkynes and alcohols. The catalyst system overcomes many of the limitations of previous palladium carbonylation catalysts. It has an increased substrate scope, avoids large excesses of alcohol substrate and uses a desirable solvent. The catalyst employs oxygen as the terminal oxidant and can be operated under safer gas mixtures.

A rhodium(i)–oxygen adduct as a selective catalyst for one-pot sequential alkyne dimerization-hydrothiolation tandem reactions

An air-stable rhodium(i)–oxygen adduct featuring a CNC-pincer ligand, based on 1,2,3-triazol-5-ylidenes, catalyzes the homo-dimerization and hydrothiolation of alkynes, affording the gem-enyne and α-vinyl sulfide isomers, respectively, with excellent selectivity.

N-Substitution dependent stereoselectivity switch in palladium catalyzed hydroalkynylation of ynamides: a regio and stereoselective synthesis of ynenamides

A palladium catalysed regioselective hydroalkynylation of ynamides for ynenamides is achieved with an N-substitution dependent stereoselectivity switch.

Computational insight into the mechanism of the Pd(0)–Brønsted acid cooperatively catalysed head-to-tail dimerization of terminal alkynes

The Pd(0)–Brønsted acid cooperatively catalysed head-to-tail dimerization of terminal alkynes was computationally studied to gain a mechanistic insight.

CuSO4-H-phosphonate catalyzed highly stereo- and regioselective dimerization of terminal alkynes

The readily available CuSO₄-H-phosphonate catalytic system can catalyze the head-to-head dimerization of terminal alkynes to give the corresponding (E) conjugated enynes selectively in high yield.