Addressing Challenges in Palladium‐Catalyzed Cross‐Coupling Reactions Through Ligand Design

The development of palladium‐catalyzed cross‐coupling reactions has revolutionized the synthesis of organic molecules on both bench‐top and industrial scales. While significant research effort has been directed toward evaluating how modifying various reaction parameters can influence the outcome of a given cross‐coupling reaction, the design and implementation of novel ancillary ligand frameworks has played a particularly important role in advancing the state‐of‐the‐art. This Review seeks to highlight notable examples from the recent chemical literature, in which newly developed ancillary ligands have enabled more challenging substrate transformations to be addressed with greater selectivity and/or under increasingly mild conditions. Throughout, the importance and subtlety of ligand effects in palladium‐catalyzed cross‐coupling reactions are described, in an effort to inspire further development and understanding within the field of ancillary ligand design.     

Palladium‐Catalyzed Cross‐Coupling of Alkenyl Carboxylates

Carboxylate esters have many desirable features as electrophiles for catalytic cross‐coupling: they are easy to access, robust during multistep synthesis, and mass‐efficient in coupling reactions. Alkenyl carboxylates, a class of readily prepared non‐aromatic electrophiles, remain difficult to functionalize through cross‐coupling. We demonstrate that Pd catalysis is effective for coupling electron‐deficient alkenyl carboxylates with arylboronic acids in the absence of base or oxidants. Furthermore, these reactions can proceed by two distinct mechanisms for C?O bond activation. A Pd^0/II catalytic cycle is viable when using a Pd⁰ precatalyst, with turnover‐limiting C?O oxidative addition; however, an alternative pathway that involves alkene carbopalladation and β‐carboxyl elimination is proposed for Pd^II precatalysts. This work provides a clear path toward engaging myriad oxygen‐based electrophiles in Pd‐catalyzed cross‐coupling.     

Six‐ and Eightfold Palladium‐Catalyzed Cross‐Coupling Reactions of Hexa‐ and Octabromoarenes

Palladium‐catalyzed sixfold coupling of hexabromobenzene ( 20 ) with a variety of alkenylboronates and alkenylstannanes provided hexaalkenylbenzenes 1 in up to 73 % and 16 to 41 % yields, respectively. In some cases pentaalkenylbenzenes 21 were isolated as the main products (up to 75 %). Some functionally substituted hexaalkenylbenzene derivatives containing oxygen or sulfur atoms in each of their six arms have also been prepared (16 to 24 % yield). The sixfold coupling of the less sterically encumbered 2,3,6,7,10,11‐hexabromotriphenylene ( 24 ) gave the desired hexakis(3,3‐dimethyl‐1‐butenyl)triphenylene ( 25 ) in 93 % yield. The first successful cross‐coupling reaction of octabromonaphthalene ( 26 ) gave octakis‐(3,3‐dimethyl‐1‐butenyl)naphthalene ( 27 ) in 21 % yield. Crystal structure analyses disclose that, depending on the nature of the substituents, the six arms are positioned either all on the same side of the central benzene ring as in 1 a and 1 i , making them nicely cup‐shaped molecules, or alternatingly above and below the central plane as in 1 h and 23 . In 27 , the four arms at C‐1,4,6,7 are down, while the others are up, or vice versa. Upon catalytic hydrogenation, 1 a yielded 89 % of hexakis(tert‐butylethyl)benzene ( 23 ). Some efficient accesses to alkynes with sterically demanding substituents are also described. Elimination of phosphoric acid from the enol phosphate derived from the corresponding methyl ketones gave 1‐ethynyladamantane ( 3 b , 62 % yield), 1‐ethynyl‐1‐methylcyclohexane ( 3 c , 85 %) and 3,3‐dimethylpentyne ( 3 e , 65 %). 1‐(Trimethylsilyl)ethynylcyclopropane ( 7 ) was used to prepare 1‐ethynyl‐1‐methylcyclopropane ( 3 d ) (two steps, 64 % overall yield). The functionally substituted alkynes 3 f – h were synthesized in multistep sequences starting from the propargyl chloride 11 , which was prepared in high yields from the dimethylpropargyl alcohol 10 (94 %). The alkenylstannanes 19 were prepared by hydrostannation of the corresponding alkynes in moderate to high yields (42–97 %), and the alkenylboronates 2 and 4 by hydroboration with catecholborane (27–96 % yield) or pinacolborane (26–69 % yield).     

Gold‐Catalyzed Homogeneous Oxidative Cross‐Coupling Reactions

Oxidizing gold? A gold(I)/gold(III) catalytic cycle is essential for the first oxidative cross‐coupling reaction in gold catalysis. By using Selectfluor for gold(I) oxidation, this chemistry reveals the synthetic potential of incorporating gold(I)/gold(III) catalytic cycles into contemporary gold chemistry and promises a new area of gold research by merging powerful gold catalysis and oxidative metal‐catalyzed cross‐coupling reactions.

     

Ate Complexes in Iron‐Catalyzed Cross‐Coupling Reactions

Iron‐catalyzed cross‐coupling reactions have an outstanding potential for sustainable organic synthesis, but remain poorly understood mechanistically. Here, we use electrospray‐ionization (ESI) mass spectrometry to identify the ionic species formed in these reactions and characterize their reactivity. Transmetalation of Fe(acac)₃ (acac=acetylacetonato) with PhMgCl in THF (tetrahydrofuran) produces anionic iron ate complexes, whose nuclearity (1 to 4 Fe centers) and oxidation states (ranging from ?I to +III) crucially depend on the presence of additives or ligands. Upon addition of iPrCl, formation of the heteroleptic Fe^III complex [Ph₃Fe(iPr)]^? is observed. Gas‐phase fragmentation of this complex results in reductive elimination and release of the cross‐coupling product with high selectivity.     

Synthesis of Fluorinated Dienes by Palladium‐Catalyzed Coupling Reactions

The synthesis of partly fluorinated 1,3‐ and 1,4‐dienes by palladium‐catalyzed coupling makes these compounds available on the laboratory scale. Several catalyst systems were tested to maximize the yields and minimize the by‐products. The molecular structures of 1,1,2,4,4‐pentafluorobutadiene, chloro(N,N′‐tetramethylethylenediamine)(trifluorovinyl)zinc, PCy₂R, and P(O)Cy₂R (Cy=cyclohexyl, R=2‐(1‐naphthyl)phenyl) were elucidated by X‐ray crystallography.