Recent Advances in Acyl Suzuki Cross-Coupling

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.

Macromolecular Brushes Based on Poly(L-Lactide) and Poly(ε-Caprolactone) Single and Double Macromonomers via ROMP. Synthesis, Characterization and Thermal Properties

Single and double poly(L-lactide) (PLLA) and poly(ε-caprolactone) (PCL) macromonomers having a norbornenyl polymerizable group were prepared by conventional Ring Opening Polymerization (ROP). These macromonomers were further subjected to ring opening metathesis polymerization (ROMP) reactions in order to produce double polymer brushes consisting of PLLA or PCL side chains on a polynorbornene (PNBE) backbone. Statistical or block ring opening metathesis copolymerization of the PLLA and PCL macromonomers afforded the corresponding random and block double brushes. Sequential ROMP of the single PLLA, PCL and PLLA macromonomers resulted in the synthesis of the corresponding triblock copolymer brush. The molecular characteristics of the macromolecular brushes were obtained by 1H-NMR spectroscopy and Size Exclusion Chromatography. The thermal properties of the samples were studied by thermogravimetric analysis, TGA, Differential Thermogravimetry, DTG and Differential Scanning Calorimetry, DSC.

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.

Palladium-Catalyzed Suzuki-Miyaura Reactions of Aspartic Acid Derived Phenyl Esters

Transition-metal-catalyzed transformations of amino acids and peptides could provide a powerful method for their site-selective modification. Here, we report non-decarbonylative Pd-catalyzed Suzuki-Miyaura reactions of phenyl ester derivatives of aspartic acid to form aryl-amino ketones. These products are potentially important in the synthesis of pharmaceuticals, and our methodology represents a new route to access molecules of this type.

Catalytic C-H/C-F Coupling of Azoles and Acyl Fluorides

A method for the palladium/copper-catalyzed direct acylation of azoles with acyl fluorides is described. This study reports the first examples of acyl fluorides being used as acylation reagents in transition-metal-catalyzed aromatic C-H bond functionalization reactions. Depending on the reaction temperature, decarbonylative coupling may also occur. Mechanistic studies suggest that the cleavage of the aromatic C-H bond, promoted by a copper-phosphine species, is not the rate-limiting step of this acylation.

[Pd(NHC)(acac)Cl]: Well-Defined, Air-Stable, and Readily Available Precatalysts for Suzuki and Buchwald-Hartwig Cross-coupling (Transamidation) of Amides and Esters by N-C/O-C Activation

A general class of well-defined, air-stable, and readily available Pd(II)-NHC precatalysts (NHC = N-heterocyclic carbene) for Suzuki and Buchwald-Hartwig cross-coupling of amides (transamidation) and esters by selective N-C/O-C cleavage is reported. Since these precatalysts are highly active and the easiest to synthesize, the study clearly suggests that [Pd(NHC)(acac)Cl] should be routinely included during the development of new cross-coupling methods. An assay for in situ screening of NHC salts in this cross-coupling manifold is presented.

Acid Fluorides in Transition-Metal Catalysis: A Good Balance between Stability and Reactivity

Several recent reports outlined the singular reactivity of acid fluorides as excellent electrophiles in transition-metal catalysis. These species undergo oxidative addition of the metal into the C-F bond; then, retention or release of the CO moiety can occur and be controlled by tuning the catalytic system and the reaction parameters. Acid fluorides, which can be derived from carboxylic acids, show good stability and high reactivity in a wide range of possible functionalizations with nucleophiles. Their use provides an interesting alternative to that of the parent carboxylic acid derivatives (acid chlorides, esters, amides, acids, or aldehydes).

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.