Palladium‐Catalyzed Decarbonylative Difluoromethylation of Acid Chlorides at Room Temperature

Carboxylic-Phosphoric Anhydrides as Direct Electrophiles for Decarbonylative Hirao Cross-Coupling of Carboxylic Acids: DFT Investigation of Mechanistic Pathway

In this anniversary issue, we present a DFT study of the mechanism of decarbonylative Hirao cross-coupling of carboxylic-phosphoric anhydrides to afford aryl phosphonates. Traditionally, the direct activation of carboxylic acids to participate in decarbonylative couplings is performed in the presence of carboxylic acid anhydride activators. We discovered that direct dehydrogenative decarbonylative phosphorylation of benzoic acid can be performed in high yield via dehydrogenative and decarbonylative coupling in the presence of phosphite as dual activating and nucleophilic reagent, enabling direct decarbonylative phosphorylation. Control studies demonstrated that carboxylic-phosphoric anhydride (acyl phosphate) is an intermediate in this process. DFT studies were conducted to gain insight into this decarbonylative process and compare the selectivity of C-O and P-O bond activations. Considering the utility of ubiquitous carboxylic acids, this alternative activation pathway may find applications in decarbonylative coupling of carboxylic acids for the synthesis of valuable molecules in organic synthesis.

Palladium-Catalyzed Aryldifluoromethylation of Aryl Halides with Aryldifluoromethyl Trimethylsilanes

Diaryl difluoromethanes are valuable targets for medicinal chemistry because they are bioisosteres of diaryl ethers and can function as replacements for diaryl methane, ketone, and sulfone groups. However, methods to prepare diaryl difluoromethanes are scarce, especially methods starting from abundant aryl halides. We report the Pd-catalyzed aryldifluoromethylation of aryl halides with aryldifluoromethyl trimethylsilanes (TMSCF2 Ar). The reaction occurs when the catalyst contains a simple, but unusual, dialkylaryl phosphine ligand that promotes transmetallation of the silane. Computational studies show that reductive elimination following transmetallation occurs with a low barrier, despite the fluorine atoms on the α-carbon, due to coordination of the difluorobenzyl π-system to palladium. The co-development of a cobalt-catalyzed synthesis of the silanes broadened the scope of the process including several applications to the synthesis biologically relevant diaryl difluoromethanes.

Highly Regio- and Enantioselective Hydrosilylation of gem-Difluoroalkenes via Nickel Catalysis

The synthesis of small organic molecules with a difluoromethylated stereocenter is particularly attractive in drug discovery. Herein, we developed an efficient method for the direct generation of difluoromethylated stereocenters through Ni(0)-catalyzed regio - and enantioselective hydrosilylation of gem -difluoroalkenes. The reaction also represents the enantioselective construction of carbon(sp 3 )-silicon bonds with nickel catalysis, which provides an atom- and step-economical synthesis route of high-value optically active α-difluoromethylsilanes. This protocol features with readily available starting materials and commercial chiral catalysis, broad substrates spanning a range of functional groups with high yield (up to 99% yield) and excellent enantioselectivity (up to 96% ee). The enantioenriched products undergo a variety of stereospecific transformations. Preliminary mechanistic studies were performed.

Palladium-Catalyzed Decarbonylative Iodination of Aryl Carboxylic Acids Enabled by Ligand-Assisted Halide Exchange

We report an efficient and broadly applicable palladium-catalyzed iodination of inexpensive and abundant aryl and vinyl carboxylic acids via in situ activation to the acid chloride and formation of a phosphonium salt. The use of 1-iodobutane as iodide source in combination with a base and a deoxychlorinating reagent gives access to a wide range of aryl and vinyl iodides under Pd/Xantphos catalysis, including complex drug-like scaffolds. Stoichiometric experiments and kinetic analysis suggest a unique mechanism involving C-P reductive elimination to form the Xantphos phosphonium chloride, which subsequently initiates an unusual halogen exchange by outer sphere nucleophilic substitution.

Nickel or Palladium-Catalyzed Decarbonylative Transformations of Carboxylic Acid Derivatives

Carboxylic acid derivatives containing acyl halides, anhydrides, esters, amides and acyl nitriles are highly appealing electrophiles in transition-metal-catalyzed carbon-carbon bond-forming reactions due to their ready availability and low cost, which can provide divergent transformations of carboxylic acids into other value-added products. In this Minireview, we focus on the recent advances of decarbonylative transformations of carboxylic acid derivatives in carbon-carbon bond formations using Ni or Pd catalysts. A series of reaction types, product classifications and reaction pathways are presented herein, which show the advantageous features of carboxylic acid derivatives as alternative to aryl or alkyl halides in terms of reactivity and compatibility. The well-accepted mechanism of nickel- or palladium-catalyzed decarbonylative transformations involves initial oxidative addition of carboxylic acid derivatives, followed by decarbonylation or transmetalation (or insertion), and reductive elimination to generate the products, thereby regenerating the catalysts.

Accessing Difluoromethylated and Trifluoromethylated cis-Cycloalkanes and Saturated Heterocycles: Preferential Hydrogen Addition to the Substitution Sites for Dearomatization

Reported here is a straightforward process in which a cyclic (alkyl)(amino)carbene/Rh catalyst system facilitates the preferential addition of hydrogen to the substitution sites of difluoromethylated and trifluoromethylated arenes and heteroarenes, leading to dearomative reduction. This strategy enables the diastereoselective synthesis of cis-difluoromethylated and cis-trifluoromethylated cycloalkanes and saturated heterocycles, and even allows formation of all-cis multi-trifluoromethylated cyclic products with a defined equatorial orientation of the di- and trifluoromethyl groups. Deuterium-labeling studies indicate that hydrogen preferentially attacks the substitution sites of planar arenes, resulting in dearomatization, possibly with heterogeneous Rh as the reactive species, followed by either reversible or irreversible hydrogen addition to the nonsubstitution sites.

Redox-Neutral Decarbonylative Cross-Couplings Coming of Age

Major progress has recently been made in the challenging redox-neutral decarbonylative cross-coupling of carboxylic acids. For example, the use of acid fluorides as effective cross-coupling partners has been found to enable control of the decarbonylation selectivity and facilitates challenging Pd⁰ -catalyzed nucleophilic trifluoromethylation and exogenous base-free Suzuki cross-coupling reactions. In another recent advance, the use of acid chlorides in room temperature difluoromethylation and direct decarbonylative cross-coupling of carboxylic acids allows these classical substrates to be used as aryl electrophiles in cross-coupling reactions. Further challenges that are yet to be addressed in redox-neutral decarbonylative cross-couplings are also briefly summarized.

Palladium-Catalyzed Decarboxylative Alkynylation of α-Acyloxyketones by C(sp3 )-O Bond Cleavage

Palladium-catalyzed decarboxylative alkynylation of α-acyloxyketones triggered by C(sp³ )-O bond cleavage is disclosed. The decarboxylation strategy featuring a neutral reaction condition enabled an unprecedent catalytic alkynylation of a ketone enolate. The reaction was applied to a variety of substrates, giving desired products in good yields. We successfully obtained X-ray crystallography of a new palladium-enolate intermediate that was synthesized by a reaction of [Pd(cod)(CH₂ TMS)₂ ] with XPhos and α-acyloxyketone at room temperature, indicating facile C(sp³ )-O bond disconnection.