Group 9 and 10 Metal Complexes of an Ylide-Substituted Phosphine: Coordination versus Cyclometalation and Oxidative Addition

Computer-Driven Development of Ylide Functionalized Phosphines for Palladium-Catalyzed Hiyama Couplings

Palladium-catalyzed couplings of silicon enolates with aryl electrophiles are of great synthetic utility, but often limited to expensive bromide substrates. A comparative experimental study confirmed that none of the established ligand systems allows to couple inexpensive aryl chlorides with α-trimethylsilyl alkylnitriles. In contrast, ylide functionalized phosphines (YPhos) led to encouraging results. A statistical model was developed that correlates the reaction yields with ligand features. It was employed to predict catalyst structures with superior performance. With this cheminformatics approach, YPhos ligands were tailored specifically to the demands of Hiyama couplings. The newly synthesized ligands displayed record-setting activities, enabling the elusive coupling of aryl chlorides with α-trimethylsilyl alkyl nitriles. The preparative utility of the catalyst system was demonstrated by the synthesis of pharmaceutically meaningful α-aryl alkylnitriles, α-arylcarbonyls and biaryls.

Formation of exceptional monomeric YPhos-PdCl2 complexes with high activities in coupling reactions

The use of well-defined palladium(ii) complexes as precatalysts for C-X cross-coupling reactions has improved the use of palladium catalysts in organic synthesis including large-scale processes. Whereas sophisticated Pd(ii) precursors have been developed in the past years to facilitate catalyst activation as well as the handling of systems with more advanced monophosphine ligands, we herein report that simple PdCl₂ complexes function as efficient precatalysts for ylide-substituted phosphines (YPhos). These complexes are readily synthesized from PdCl₂ sources and form unprecedented monomeric PdCl₂ complexes without the need for any additional coligand. Instead, these structures are stabilized through a unique bonding motif, in which the YPhos ligands bind to the metal through the adjacent phosphine and ylidic carbon site. DFT calculations showed that these bonds are both dative interactions with the stronger interaction originating from the electron-rich phosphine donor. This bonding mode leads to a remarkable stability even towards air and moisture. Nonetheless, the complexes readily form monoligated LPd(0) complexes and thus the active palladium(0) species in coupling reactions. Accordingly, the YPhos-PdCl₂ complexes serve as highly efficient precatalysts for a series of C-C and C-X coupling reactions. Despite their simplicity they can compete with the efficiency of more complex and less stable precatalysts.

Ylide-stabilized phosphenium cations: Impact of the substitution pattern on the coordination chemistry

Although N‐heterocyclic phosphenium (NHP) cations have received considerable research interest due to their application in organocatalysis, including asymmetric synthesis, phosphenium cations with other substitution patterns have hardly been explored. Herein, the preparation of a series of ylide‐substituted cations of type [YPR]⁺ (with Y=Ph₃PC(Ph), R=Ph, Cy or Y) and their structural and coordination properties are reported. Although the diylide‐substituted cation forms spontaneous from the chlorophosphine precursor, the monoylidylphosphenium ions required the addition of a halide‐abstraction reagent. The molecular structures of the cations reflected the different degrees of electron donation from the ylide to the phosphorus center depending on the second substituent. Molecular orbital analysis confirmed the stronger donor properties of the ylide systems compared to NHPs with the mono‐ylide substituted cations featuring a more pronounced electrophilicity. This was mirrored by the reaction of the cations towards gold chloride, in which only the diylide‐substituted cation [Y₂P]⁺ formed the expected LAuCl]⁺ complex, while the monoylide‐substituted compounds reacted to the chlorophosphine ligands by transfer of the chloride from gold to the phosphorus center. These results demonstrate the tunability of ylide‐functionalized phosphorus cations, which should allow for further applications in coordination chemistry in the future.

Ylide-Substituted Phosphines with a Cyclic Ylide-Backbone: Angle Dependence of the Donor Strength

Ylide-substituted phosphines (YPhos) have been shown to be highly electron-rich and efficient ligands in a variety of palladium catalyzed transformations. Here, the synthesis and characterization of novel YPhos ligands containing a cyclic backbone architecture are reported. The ligands are easily synthesized from a cyclic phosphonium salt and the chlorophosphines Cy2PCl (L1) and Cy(FluMe)PCl (L2, with FluMe = 9-methylfluorenyl) and were characterized in both solution and solid states. The smaller PCy2-substituted ligand, L1, readily formed the biscoordinate L1 2 Pd species when treated with Pd2(dba)3 and showed no activity in palladium-catalyzed amination reactions even when applied as defined palladium(II) η3-allyl, t-Bu-indenyl, or cinnamyl precursors. Bulkier fluorenyl-substituted ligand L2 similarly was inactive, despite its ability to form the stable monophosphine complex L2·Pd(dba). Assessment of the electronic properties by experimental and computational methods revealed that L1 and L2 are considerably less electron-rich than previously synthesized YPhos ligands. This was shown to be the result of the small P-C-S bond angle, which is sterically enforced due to the cyclic nature of the backbone. Density functional theory calculations revealed that the small angle results in an increased s-character of the lone pair at the ylidic carbon atom and leads to a polarization of the C-P bond toward the carbon atom, thus decreasing the electron density at the phosphorus atom. The results demonstrate the tunability of the donor strength of YPhos ligands by modification of the ligand backbone beyond simple changes of the substitution pattern and are thus important for future ligand design, with a careful balance of many factors to be considered to achieve catalytic activity.

Phosphorus-Containing Superbases: Recent Progress in the Chemistry of Electron-Abundant Phosphines and Phosphazenes

The renaissance of Brønsted superbases is primarily based on their pronounced capacity for a large variety of chemical transformations under mild reaction conditions. Four major set screws are available for the selective tuning of the basicity: the nature of the basic center (N, P, …), the degree of electron donation by substituents to the central atom, the possibility of charge delocalization, and the energy gain by hydrogen bonding. Within the past decades, a plethora of neutral electron-rich phosphine and phosphazene bases have appeared in the literature. Their outstanding properties and advantages over inorganic or charged bases have now made them indispensable as auxiliary bases in deprotonation processes. Herein, an update of the chemistry of basic phosphines and phosphazenes is given. In addition, due to widespread interest, their use in catalysis or as ligands in coordination chemistry is highlighted.

Three-Coordinate Pd(0) with Rare-Earth Metalloligands: Synergetic CO Activation and Double P-C Bond Cleavage-Formation Reactions

Metalation of β-diketiminato rare-earth metal complexes L^nacnacLn(PhNCH₂PPh₂)₂ (Ln = Y, Yb, Lu) with (COD)Pd(CH₂SiMe₃)₂ afforded three-coordinate Pd(0) complexes supported by two sterically less bulky phosphines and a Pd → Ln dative interaction. The Pd(0) center is prone to ligation with isonitrile and CO; in the latter case, the insertion of a second CO with the Y-N bond was assisted via a precoordination of CO on the Pd(0) center, which led to the formation of an anionic Pd(0) carbamoyl. The reaction of the Pd-Y complex with iodobenzene showed a remarkable double P-C bond cleavage-formation pathway within the heterobimetallic Pd-Y core to afford (Ph₃P)₂PdI(Ph), imine PhNCH₂, and a β-diketiminato yttrium diiodide. In the related reaction of L^nacnacY(PhNCH₂PPh₂)₂ with (Ph₃P)₂PdI(Ph), the P-C bond cleavage following with a N-C bond formation was observed. Computational studies revealed a synergetic bimetallic mechanism for these reactions.

Towards the rational design of ylide-substituted phosphines for gold(i)-catalysis: from inactive to ppm-level catalysis

The implementation of gold catalysis into large-scale processes suffers from the fact that most reactions still require high catalyst loadings to achieve efficient catalysis thus making upscaling impractical. Here, we report systematic studies on the impact of the substituent in the backbone of ylide-substituted phosphines (YPhos) on the catalytic activity in the hydroamination of alkynes, which allowed us to increase the catalyst performance by orders of magnitude. While electronic changes of the ligand properties by introduction of aryl groups with electron-withdrawing or electron-donating groups had surprisingly little impact on the activity of the gold complexes, the use of bulky aryl groups with ortho-substituents led to a remarkable boost in the catalyst activity. However, this catalyst improvement is not a result of an increased steric demand of the ligand towards the metal center, but due to steric protection of the reactive ylidic carbon centre in the ligand backbone. The gold complex of the thus designed mesityl-substituted YPhos ligand Y_MesPCy₂, which is readily accessible in one step from a simple phosphonium salt, exhibited a high catalyst stability and allowed for turnover numbers up to 20 000 in the hydroamination of a series of different alkynes and amines. Furthermore, the catalyst was also active in more challenging reactions including enyne cyclisation and the formation of 1,2-dihydroquinolines.

A Phosphonium Ylide as a Ligand for [3 + 2] Coupling Reactions of Epoxides with Heterocumulenes under Mild Conditions

The potential of carbonyl-stabilized phosphonium ylides as ligands for novel catalysis was explored. We found that the combination of phosphonium ylides and metal halide salts efficiently catalyzed the reaction of epoxides with carbon dioxide under mild conditions. Five-membered cyclic carbonates, including disubstituted cyclic carbonates, were obtained in good yields with the use of 1 atm of carbon dioxide at 35 °C. Terminal epoxides could be converted to N-aryl oxazolidinones in the reaction with isocyanates under a similar catalytic system.