Decarbonylative Phosphorylation of Amides by Palladium and Nickel Catalysis: The Hirao Cross‐Coupling of Amide Derivatives

Selective C-P(O) Bond Cleavage of Organophosphine Oxides by Sodium

Sodium exhibits better efficacy and selectivity than Li and K for converting Ph₃P(O) to Ph₂P(OM). The destiny of PhNa co-generated is disclosed. A series of alkyl halides R⁴X and aryl halides ArX all react with Ph₂P(ONa) to produce the corresponding phosphine oxides in good to excellent yields.

Rh-Catalyzed Base-Free Decarbonylative Borylation of Twisted Amides

We report the rhodium-catalyzed base-free decarbonylative borylation of twisted amides. The synthesis of versatile arylboronate esters from aryl twisted amides is achieved via decarbonylative rhodium(I) catalysis and highly selective N-C(O) insertion. The method is notable for a very practical, additive-free Rh(I) catalyst system. The method shows broad functional group tolerance and excellent substrate scope, including site-selective decarbonylative borylation/Heck cross-coupling via divergent N-C/C-Br cleavage and late-stage pharmaceutical borylation.

Phosphorylation of Carboxylic Acids and Their Derivatives with P(O)–H Compounds Forming P(O)–C Bonds

Herein, we highlight advances in the phosphorylation of readily available carboxylic acids and their derivatives forming synthetically important P(O)–sp3C, P(O)–sp2C, and P(O)–spC bonds, with an emphasis on the results demonstrated since 2010. This review examines the challenges associated with the use of this strategy for the synthesis of organophosphorus compounds and details advances in the design of catalytic systems that suppress these problems thus resulting in notable progress. Mechanistic details are discussed where available.

1 Introduction

2 Formation of P(O)–sp3C Bonds

3 Formation of P(O)–sp2C Bonds

4 Formation of P(O)–spC Bonds

5 Outlook and Conclusion

Focusing on the Catal. of the Pd- and Ni-Catalyzed Hirao Reactions

The Hirao reaction involving the phosphinoylation or phosphonation of aryl halides by >P(O)H reagents is a P–C bond forming transformation belonging to the recently very hot topic of cross-couplings. The Pd- or Ni-catalyzed variations take place via the usual cycle including oxidative addition, ligand exchange, and reductive elimination. However, according to the literature, the nature of the transition metal catalysts is not unambiguous. In this feature article, the catalysts described for the Pd(OAc)₂-promoted cases are summarized, and it is concluded that the “(HOY₂P)₂Pd(0)” species (Y = aryl, alkoxy) is the real catalyst. In our model, the excess of the >P(O)H reagent served as the P-ligand. During the less studied Ni(II)-catalyzed instances the “(HOY₂P)(−OY₂P)Ni(II)Cl⁻” form was found to enter the catalytic cycle. The newest conclusions involving the exact structure of the catalysts, and the mechanism for their formation explored by us were supported by our earlier experimental data and theoretical calculations.

Decarbonylative Suzuki-Miyaura Cross-Coupling of Aroyl Chlorides

Herein, we report a catalyst system for Pd-catalyzed decarbonylative Suzuki-Miyaura cross-coupling of aroyl chlorides with boronic acids to furnish biaryls. This strategy is suitable for a broad range of common aroyl chlorides and boronic acids. The synthetic utility is highlighted in the direct late-stage functionalization of pharmaceuticals and natural products capitalizing on the presence of carboxylic acid moiety. Extensive mechanistic and DFT studies provide key insight into the reaction mechanism and high decarbonylative cross-coupling selectivity.

Phosphine Oxides from a Medicinal Chemist's Perspective: Physicochemical and in Vitro Parameters Relevant for Drug Discovery

Phosphine oxides and related phosphorus-containing functional groups such as phosphonates and phosphinates are established structural motifs that are still underrepresented in today's drug discovery projects, and only few examples can be found among approved drugs. In this account, the physicochemical and in vitro properties of phosphine oxides and related phosphorus-containing functional groups are reported and compared to more commonly used structural motifs in drug discovery. Furthermore, the impact on the physicochemical properties of a real drug scaffold is exemplified by a series of phosphorus-containing analogs of imatinib. We demonstrate that phosphine oxides are highly polar functional groups leading to high solubility and metabolic stability but occasionally at the cost of reduced permeability. We conclude that phosphine oxides and related phosphorus-containing functional groups are valuable polar structural elements and that they deserve to be considered as a routine part of every medicinal chemist's toolbox.

A methylation platform of unconventional inert aryl electrophiles: trimethylboroxine as a universal methylating reagent

Methylation is one of the most fundamental conversions in medicinal and material chemistry. Extension of substrate types from aromatic halides to other unconventional aromatic electrophiles is a highly important yet challenging task in catalytic methylation. Disclosed herein is a series of transition metal-catalyzed methylations of unconventional inert aryl electrophiles using trimethylboroxine (TMB) as the methylating reagent. This transformation features a broad substrate type, including nitroarenes, benzoic amides, benzoic esters, aryl cyanides, phenol ethers, aryl pivalates and aryl fluorides. Another important merit of this work is that these widespread “inert” functionalities are capable of serving as directing or activating groups for selective functionalization of aromatic rings before methylation, which greatly expands the connotation of methylation chemistry.

A universal platform for catalytic methylation of a series of unconventional inert aryl electrophiles with trimethylboroxine (TMB) is established, which provides a synthetic chemist's toolbox for the construction of methylated arenes.

Nickel-Catalyzed Deaminative Acylation of Activated Aliphatic Amines with Aromatic Amides via C-N Bond Activation

Deaminative functionalization of aliphatic primary amines has great synthetic utility. Herein, we describe a Ni-catalyzed reductive deaminative cross-electrophile coupling reaction between Katritzky salts and aromatic amides. This work provides examples of the synthesis of various ketones from alkylpyridinium salts, including both primary and secondary alkylamines. Given its mild reaction conditions and high functional group tolerance, this cross-coupling strategy is expected to be useful for late-stage functionalization of complex compounds.

Cobalt Catalyzed C-P Bond Formation by Cross-Coupling of Boronic Acids with P(O)H Compounds in Presence of Zinc

In our current work, we have reported the first cobalt-catalyzed cross-coupling of arylboronic acid with alkyl/aryl phosphites under mild conditions. The reaction was carried out in the presence of zinc powder as an additive and ter-pyridine as a ligand. The use of non-precious cobalt salt makes the protocol advantageous, as it is inexpensive and more abundant than the previously used methods where precious metal salts (Pd and Pt) were used. The reaction has a wide substrate scope and the products were obtained in good yields.

Ni-Catalyzed Suzuki-Miyaura Cross-Coupling of Aliphatic Amides on the Benchtop

Suzuki-Miyaura cross-couplings of amides offer an approach to the synthesis of ketones that avoids the use of basic or pyrophoric nucleophiles. However, these reactions require glovebox manipulations, thus limiting their practicality. We report a benchtop protocol for Suzuki-Miyaura cross-couplings of aliphatic amides that utilizes a paraffin capsule containing a Ni(0) precatalyst and NHC ligand. This methodology is broad in scope, is scalable, and provides a user-friendly approach to convert aliphatic amides to alkyl-aryl ketones.

Palladium-catalyzed denitrative Sonogashira-type cross-coupling of nitrobenzenes with terminal alkynes

Described herein is a palladium-catalyzed cross-coupling reaction between nitroarenes and terminal alkynes, offering a facile method for C(sp²)–C(sp) bond formation.

Selective C–C bond cleavage of amides fused to 8-aminoquinoline controlled by a catalyst and an oxidant

Herein, copper-catalyzed direct C–C bond cleavage of amides fused to 8-aminoquinoline as a directing group to form urea in the presence of amines and dioxygen is reported.

Mechanism of C–P bond formation via Pd-catalyzed decarbonylative phosphorylation of amides: insight into the chemistry of the second coordination sphere

DFT computations establish a detailed reaction mechanism for the first Pd-catalyzed decarbonylative phosphorylation of amides forming C–P bonds, which includes non-covalent interactions as well as proton transfer in the second coordination sphere.

A surprising mechanism lacking the Ni(0) state during the Ni(II)-catalyzed P–C cross-coupling reaction performed in the absence of a reducing agent – An experimental and a theoretical study

The Hirao reaction, i.e. the P–C coupling between a bromoarene and a >P(O)H reagent performed in most cases in the presence of a Pd(0) complex incorporating a P-ligand may also be carried out applying a Ni(II) catalyst precursor with or without Zn or Mg as the reducing agent. The Ni catalysts may include P- or N-ligands. B3LYP/6-31G(d,p)//PCM(MeCN) quantum chemical calculations suggested that the mechanism of the NiX2 catalyzed (X=Cl or Br) P–C couplings performed in the absence of a reducing agent, and in the excess of the >P(O)H reagent serving as the P-ligand (via its tautomeric >POH form) is completely different from that of the Pd(OAc)2 promoted version, as no reduction of the Ni(II) occurs. In the two variations mentioned, the active catalyst is the dehydrobrominated species derived from primary complex [(HO)Y2P]2Ni(II)Br2, and the [(HO)Y2P]2Pd(0) complex itself, respectively. Both species undergo temporary oxidation (to “Ni(IV)” and “Pd(II)”, respectively) in the catalytic cycle. During the catalysis with “P2Ni(II)X2”, one of the P-ligands serves the >P(O)H function of the ArP(O)H <  product. The consequence of this difference is that in the Ni(II)-catalyzed case, somewhat less >P(O)H-species is needed than in the Pd(0)-promoted instance. Applying 10 % of the Pd(OAc)2 or NiX2 precursor, the optimum quantity of the P-reagent is 1.3 equivalent and, in the first approach, 1.1 equivalent, respectively. Preparative experiments justified the new mechanism explored. The ligation of Ni(II) was also investigated by theoretical calculations. It was proved that the bis-complexation is the most favorable energetically as compared to the mono-, tri- and tetra-ligation.

Palladium-catalyzed decarbonylative Suzuki-Miyaura cross-coupling of amides by carbon-nitrogen bond activation

Palladium-catalyzed Suzuki-Miyaura cross-coupling or aryl halides is widely employed in the synthesis of many important molecules in synthetic chemistry, including pharmaceuticals, polymers and functional materials. Herein, we disclose the first palladium-catalyzed decarbonylative Suzuki-Miyaura cross-coupling of amides for the synthesis of biaryls through the selective activation of the N-C(O) bond of amides. This new method relies on the precise sequence engineering of the catalytic cycle, wherein decarbonylation occurs prior to the transmetallation step. The reaction is compatible with a wide range of boronic acids and amides, providing valuable biaryls in high yields (>60 examples). DFT studies support a mechanism involving oxidative addition, decarbonylation and transmetallation and provide insight into high N-C(O) bond activation selectivity. Most crucially, the reaction establishes the use of palladium catalysis in the biaryl Suzuki-Miyaura cross-coupling of the amide bond and should enable the design of a wide variety of cross-coupling methods in which palladium rivals the traditional biaryl synthesis from aryl halides and pseudohalides.

Resonance promoted ring-opening metathesis polymerization of twisted amides

The living ring-opening metathesis polymerization (ROMP) of an unsaturated twisted amide using the third-generation Grubbs initiator is described. Unlike prior examples of ROMP monomers that rely on angular or steric strain for propagation, this system is driven by resonance destabilization of the amide that arises from geometric constraints of the bicyclic framework. Upon ring-opening, the amide can rotate and rehybridize to give a stabilized and planar conjugated system that promotes living propagation. The absence of other strain elements in the twisted amide is supported by the inability of a carbon analogue of the monomer to polymerize and computational studies that find resonance destabilization accounts for 11.3 kcal mol-1 of the overall 12.0 kcal mol-1 ring strain. The twisted amide polymerization is capable of preparing high molecular weight polymers rapidly at room temperature, and post-polymerization modification combined with 2D NMR spectroscopy confirms a regioirregular polymer microstructure.

Na2CO3-promoted thioesterification via N–C bond cleavage of amides to construct thioester derivatives

A mild, efficient, and transition-metal-free catalytic strategy is developed to construct thioesters via selective N–C bond cleavage of Boc2-activated primary amides. This strategy is successfully carried out with stoichiometric Na2CO3 as the base and provides the corresponding products in moderate to excellent yields.

Cesium Carbonate Catalyzed Esterification of N-Benzyl-N-Boc-amides under Ambient Conditions

We report a general activated amide to ester transformation catalyzed by Cs₂CO₃. Using this approach, esterification proceeds under relatively mild conditions and without the need for a transition metal catalyst. This method exhibits broad substrate scope and represents a practical alternative to existing esterification strategies. The synthetic utility of this protocol is demonstrated via the facile synthesis of crown ether derivatives and the late-stage modification of a representative natural product and several sugars in reasonable yields.

Carbon-Carbon Bond Formation of Trifluoroacetyl Amides with Grignard Reagents via C(O)-CF3 Bond Cleavage

The reaction of trifluoroacetyl amides with Grignard reagent for the substitution of CF₃ group with various alkyl or aryl groups is described. A variety of aryl, quinolin-8-yl, and (hetero)alkyl functional groups as well as F, Cl, and Br atoms are well tolerated. These moisture-stable and easily available trifluoroacetyl amides can be conveniently obtained and used as new versatile precursors for isocyanates. The control experiments show that the reaction proceeds via an isocyanate intermediate and/or alkoxide/amide dual anionic intermediate.

Decarbonylative Borylation of Amides by Palladium Catalysis

The development of transition-metal-catalyzed borylation reactions is of significant importance for the fields of organic synthesis and medicinal chemistry because of the versatility of organoboron functional groups. Herein, we report the direct decarbonylative borylation of amides by highly selective carbon-nitrogen bond cleavage by palladium catalysis. The approach capitalizes on the ground-state destabilization of the amide bond in N-acyl glutarimides to achieve Pd-catalyzed insertion into the amide N-C bond and decarbonylation (deamidation). Mechanistic studies and the utility of this methodology in orthogonal sequential cross-couplings of robust, bench-stable amides are reported.

Triflamides: Highly Reactive, Electronically Activated N-Sulfonyl Amides in Catalytic N-C(O) Amide Cross-Coupling

The direct, highly chemoselective Suzuki-Miyaura cross-coupling of trifluoromethanesulfonamides (triflamides) by selective N-C(O) amide bond cleavage is reported. This operationally simple, mild, and user-friendly method accomplishes the direct synthesis of ketones from amides by a catalytic manifold as a powerful alternative to Weinreb amides. Mechanistic studies support rotational inversion and electronic activation, favoring selective insertion under mild conditions. Our data strongly suggest that triflamides should be routinely considered as precursors in amide bond cross-coupling.