Palladium-catalyzed addition of alkynes to cyclopropenes

NHC-Ni(II)-catalyzed cyclopropene-isocyanide [5 + 1] benzannulation

Isocyanides are common compounds in fine and bulk chemical syntheses. However, the direct addition of isocyanide to simple unactivated cyclopropene via transition metal catalysis is challenging. Most of the current approaches focus on 1,1-insertion of isocyanide to M-R or nucleophilc insertion. That is often complicated by the competitive homo-oligomerization reactivity occurring at room temperature, such as isocyanide 1,1-insertion by Ni(II). Here we show a (N-heterocyclic carbene)Ni(II) catalyst that enables cyclopropene-isocyanide [5 + 1] benzannulation. As shown in the broad substrate scope and a [trans-(N-heterocyclic carbene)Ni(isocyanide)Br₂] crystal structure, the desired cross-reactivity is cooperatively controlled by the high reactivity of the cyclopropene, the sterically bulky N-heterocyclic carbene, and the strong coordination ability of the isocyanide. This direct addition strategy offers aromatic amine derivatives and complements the Dötz benzannulation and Semmelhack/Wulff 1,4-hydroquinone synthesis. Several sterically bulky, fused, and multi-substituted anilines and unsymmetric functionalized spiro-ring structures are prepared from those easily accessible starting materials expediently.

The direct addition of isocyanides to cyclopropenes is challenging. Here, the authors report a catalytic cyclopropene-isocyanide [5 + 1] benzannulation catalyzed by an (N-heterocyclic carbene)Ni(II) complex; this method enables the preparation of fused and multi-substituted anilines and unsymmetrically functionalized spiro-ring structures.

Pd-Catalyzed Enantioselective Hydroalkynylation of Cyclopropenes

We report herein an easy, mild, and robust Pd-catalyzed enantioselective hydroalkynylation reaction of achiral cyclopropenes. Commercially available Pd(acac)₂ and (R)-DM-BINAP proved to be the best combination to reach high diastereo- and enantioselectivities.

Stereoselective Access to ( E)-1,3-Enynes through Pd/Cu-Catalyzed Alkyne Hydrocarbation of Allenes

Pd^II and Cu^I cooperate in catalyzing the alkynes hydrocarbation of allenes (AHA) giving ( E)-1,3-enynes with high yields, atom economy, and high regio-/stereoselectivities. We devised new efficient conditions and expanded the substrate scope. Experimental and computational studies support a nonorthodox Pd^II/Pd^IV catalytic cycle involving an oxidative addition triggered by a stereodeterminant H⁺ transfer. This reaction is leveraged in a new strategy of stereoselective synthesis of 1,3-dienes.

Directed Cu(I)-Catalyzed Carbomagnesiation of 1-Arylcycloprop-2-ene-1-carboxamides En Route to Densely Substituted Functionalized Cyclopropanes

Copper-catalyzed, directed addition of Grignard reagents across the strained C═C bond of cyclopropene-3-carboxamides was developed. It was demonstrated that the amide functionality serves as an ultimate directing group allowing for highly efficient control of diastereoselectivity of addition including stereoselectivity of electrophilic trapping with prochiral aldehydes. Also, regioselectivity of carbomagnesiation of cyclopropenes with a monosubstituted double bond is investigated. It was shown that in many cases this selectivity is controlled by steric factors and allows for preparation of products with a "reversed" regiochemistry.

Rapid access to cyclopentadiene derivatives through gold-catalyzed cycloisomerization of ynamides with cyclopropenes by preferential activation of alkenes over alkynes

Double bond activation: the current transformation represents a rare example of preferential activation of alkenes over alkynes under gold catalysis.

Iridium-Catalyzed Enantioselective Hydroalkynylation of Enamides for the Synthesis of Homopropargyl Amides

Reported is an iridium-catalyzed asymmetric hydroalkynylation of enamides with terminal alkynes. The reaction occurs regioselectively at the β-position of an enamide to produce homopropargyl amides. Good to high enantioselectivity was observed with an iridium complex ligated by a chiral bis(phosphine) ligand. This method provides a straightforward route to synthesize chiral homopropargyl amides with a stereocenter β to the amide.

Facially selective Cu-catalyzed carbozincation of cyclopropenes using arylzinc reagents formed by sequential I/Mg/Zn exchange

Described is a Cu-catalyzed directed carbozincation of cyclopropenes with organozinc reagents prepared by I/Mg/Zn exchange. This protocol broadens the scope with respect to functional group tolerance and enables use of aryl iodide precursors, rather than purified diorganozinc precursors. Critical to diastereoselectivity of the carbozincation step is the removal of magnesium halide salts after transmetalation with ZnCl(2).

Directed carbozincation reactions of cyclopropene derivatives

A diastereoselective procedure has been developed for the Cu-catalyzed addition of diorganozinc reagents to cyclopropene derivatives. Ester and oxazolidinone functions direct the addition of a variety of organozinc reagents with excellent facial selectivity. The resulting cyclopropylzinc reagents can be captured via stereospecific reactions with electrophiles. Cycloprop-2-ene carboxylic esters, which are directly available from the transition-metal-catalyzed reactions of alkynes with alpha-diazo esters, can be utilized directly in carbozincation protocols. Both diastereoselectivity and regioselectivity are high for the carbozincation reactions of 2-alkylcycloprop-2-ene carboxylate esters. The scope of the method is broadened by the ability to utilize organozinc reagents that have been generated in situ from Grignard reagents. Chiral oxazolidinone auxilaries are effective in controlling the diastereoselectivity of the carbometalation reactions.

Nickel-catalyzed, regio- and stereoselective hydroalkynylation of methylenecyclopropanes with retention of the cyclopropane ring, leading to the synthesis of 1-methyl-1-alkynylcyclopropanes

The C-H bond of triisopropylsilylacetylene adds to the C=C bond of substituted methylenecyclopropanes in a regio- and stereoselective manner at room temperature in the presence of nickel catalysts bearing PMePh(2). The C-C bond formation takes place at the internal sp(2) carbon atom with high regioselectivity from the pi face opposite the substituents located in the three-membered rings.

Nickel-catalyzed regioselective hydroalkynylation of styrenes: improved catalyst system, reaction scope, and mechanism

Addition of the sp-C-H bond of triisopropylsilylacetylene to the carbon-carbon double bonds of styrenes bearing functional groups proceeded efficiently at room temperature in the presence of 3 mol % of Ni(cod)(2) with a PMePh(2) ligand. Use of 2-deuteriotriisopropylsilylacetylene in the hydroalkynylation of styrenes resulted in regioselective incorporation of deuterium into the beta-positions of recovered styrenes, along with its regioselective introduction into the product's methyl group.