Isolation and Structure Elucidation of the Heterocumulene Anions [NCC-L]- (L = CO, CS, N2).

Cumulenes are molecules characterized by a series of consecutive double bonds. They serve as important reagents and intermediates in the synthesis of polymers and a wide variety of functionalized compounds, including various heterocycles. Understanding the properties of cumulenes and developing synthetic routes to these often highly reactive species is essential for unlocking new applications.

Cerium(III) yldiide complexes with divergent CO reactivity.

Synthesis of cerium yldiide complexes and their reactivity with CO is demonstrated. In the case of the sulphur-tethered yldiide, the ketenyl complex is formed with release of PPh, while PhPCCO is formed along with a sulfinato ligand in the case of the tosyl-substituted yldiide. Computational analysis shows that this diverging reactivity is due to the stability of the two isomers in the first step of each mechanism.

The lithium effect in ketenyl anion chemistry.

Ketenyl lithium compounds of type [RC(Li)CO] (with R = PhP(E), E = O, S, Se) were found to exhibit lower thermal stabilities than their potassium analogues due to the stronger coordination of the oxygen of the ketene moiety to the harder metal cation, resulting in a more pronounced ynolate character. Using additional ligands allows manipulation of the O-Li interaction, thereby influencing the stability and reactivity of the ketenyl anions.

P,N-Coordinating Ylide-Functionalized Phosphines (NYPhos): A Ligand Platform for the Selective Monoarylation of Small Nucleophiles.

Palladium-catalyzed coupling reactions of small nucleophiles are of great interest, but challenging due to difficulties in selectivity control. Herein, we report the development of a new platform of P,N-ligands consisting of ylide-functionalized phosphines with aminophosphonium groups (NYPhos) to address this challenge. These phosphine ligands are easily accessible in a wide structural diversity with highly modular electronic and steric properties.

Carbon-Based Weakly Coordinating Anions: Molecular Design, Synthesis and Applications.

Although carbanions, which are usually regarded as reactive species and powerful metalation reagents, can be stabilized through choice of the substitution pattern, they have rarely been considered for the design of weakly coordinating anions (WCA). Here, we report on an evaluation of the potential of a series of differently substituted carbanions to serve as WCA by computational methods. This led us to the synthesis of the water- and air-stable allyl anion 1 with triflyl and 3,5-bis(trifluoromethyl)phenyl (Ar) moieties, which can be isolated in high yields even on a gram-scale.

Ligand Exchange at Carbon: Synthetic Entry to Elusive Species and Versatile Reagents.

How different is carbon compared to other elements in the periodic table? Can carbon compounds be regarded as coordination complexes with carbon as the central element undergoing a facile exchange of its ligands? Although carbon clearly plays a special role among the elements of the periodic table, recent studies have drawn parallels between the bonding situation and the reactivity of carbon compounds to transition metal complexes. This Perspective summarizes recent reports about ylidic and zwitterionic compounds that were shown to exhibit ambiguous bonding situations that can be interpreted as donor-acceptor interactions similar to the bond between a metal and a neutral ligand. Based on this conception, ligand exchange reactions prototypical of transition metal complexes were realized at carbon atoms, enabling new synthetic strategies for the synthesis of reactive species and building blocks.

The Cyanoketenyl Anion [NCO].

Cumulenes and heterocumulenes with three or more cumulative multiple bonds are usually reactive species that serve as valuable building blocks for more complex molecules but tend to isomerize or cyclize and therefore are difficult to isolate. Using a mild ligand exchange reaction at the carbon in α-metalated ylides, we have now succeeded in the synthesis and gram-scale isolation of the elusive cyanoketenyl anion [NCO]. Despite its assumed cumulene-like structure and the delocalization of the negative charge across the whole 5-atom molecule, it features a bent geometry with a nucleophilic central carbon atom.

Phosphinoyl-Substituted Ketenyl Anions: Synthesis and Substituent Effects on the Structural Properties.

Ketenyl anions are versatile intermediates in synthetic chemistry and have recently become accessible as isolable reagents from metalated ylides by exchange of the phosphine with CO. Herein, we report on a systematic study of substituent effects on the structure and bonding situation in ketenyl anions. A series of phosphinoyl-substituted ketenyl anions {[RP(X)CCO] with X = O, NTol, S, Se} were prepared by carbonylation of the corresponding yldiides and isolated as their corresponding potassium salts.

From a mercury(II) bis(yldiide) complex to actinide yldiides.

The bis(yldiide) mercury complex, (L-Hg-L) [L = C(PPh)P(S)Ph], is prepared from the corresponding potassium yldiide and used to access the first substituted yldiide actinide complexes [(CMe)An(L)(Cl)] (An = U, Th) salt metathesis. Compared to previously reported phosphinocarbene complexes, the complexes exhibit long actinide-carbon distances, which can be explained by the strong polarization of the π-electron density toward carbon.

Selectivity Control of the Ligand Exchange at Carbon in α-Metallated Ylides as a Route to Ketenyl Anions.

α-Metallated ylides have recently been reported to undergo phosphine by CO exchange at the ylidic carbon atom to form isolable ketenyl anions. Systematic studies on the tosyl-substituted yldiides, R P=C(M)Ts (M=Li, Na, K), now reveal that carbonylation may lead to a competing metal salt (MTs) elimination. This side-reaction can be controlled by the choice of phosphine, metal cation, solvent and co-ligands, thus enabling the selective isolation of the ketenyl anion [Ts-CCO]M (2-M).

Probing the donor strength of yldiide ligands: synthesis, structure and reactivity of rhodium complexes with a PCN pincer ligand.

Control of the metal ligand interaction by changes in the ligand protonation state is vital to many catalytic transformations based on metal-ligand cooperativity. Herein, we report on the coordination chemistry of a new PC(H)N pincer ligand with a central ylide as donor site, which through deprotonation to the corresponding yldiide changes from a neutral L-type ligand to an anionic LX-type PCN ligand. The isolation of a series of rhodium complexes showed that the strong donor ability of the neutral ylide PC(H)N is further increased upon deprotonation, as evidenced by one of the lowest reported CO stretching frequencies in complex [(PCN)Rh(CO)] (2) compared to other known rhodium carbonyl complexes.

Inverting the Electronic Structure of Diylidylgermylenes by Backbone Modification.

Owing to the strong electron-donating ability of ylide substituents, diylidyltetrylenes are usually highly nucleophilic species with strong donor capacities. Here, we demonstrate that their electronic properties are in fact highly flexible and can be effectively tuned through variation of the substituent in the ylide backbone. Initial density functional theory studies showed that cyano groups are particularly capable in lowering the LUMO energy of diylidyl germylenes thus turning these usually highly nucleophilic species into electrophilic compounds.

Correlation of Experimental and Calculated Reaction Enthalpies with Ligand Donor Strengths.

Ylide-functionalized phosphines (YPhos) have recently proven to be strongly donating ligands that enable high catalyst activities in gold(I)-mediated transformations. We now report on a calorimetric study dealing with the [Au(YPhos)Cl] system and assess YPhos-Au bond dissociation enthalpies (BDE). Comparison with other commonly used phosphines confirmed the high binding strengths of the YPhos ligands.

From a Fluorenyl Substituted Ylide-Functionalized Phosphine to a Neutral Phosphide via P-C Bond Cleavage.

Bulky ylide-substituted phosphines have recently found application as potent ligands in homogeneous catalysis. The attempted synthesis of the ylide-substituted fluorenylphosphine Y P(Cy)Flu [Y =Cy P(Ph)C; Flu=9-methylfluorenyl] now resulted in the unexpected elimination of 9-methylenefluorene during the deprotonation step of the intermediary α-phosphino phosphonium salt to yield the secondary ylide-substituted phosphine Y P(Cy)H. This phosphine underwent formal H elimination under basic conditions to form a cyclic phosphonium ylide with a P-C-P-C four-membered ring via deprotonation of one cyclohexyl group of the PCy moiety.

Thiophosphinoyl-Tethered Ylide-Substituted Heavier Carbenes: Synthesis, Structures and Stabilities.

The synthesis and structure analysis of a series of mono and diylide-substituted tetrylenes of type YEX and Y E (E=Ge, Sn, Pb; X=Cl or Br) using a thiophosphinoyl-tethered metallated ylide (Y=Ph P(S)-C-P(pip)Ph with pip=piperidyl) is reported, amongst the first ylide-substituted plumbylenes. The tetrylenes feature distinct trends in the spectroscopic and structural properties of the ylide ligand with increasing atomic number of the tetrel element. For instance, an increasingly high-field shifted signal for the thiophosphinoyl group is observed in the P{ H} NMR spectrum as a consequence of the increasing polarity of the element-carbon bond, which likewise results in a shortening of the ylidic C-P bond in the solid-state structure.

Transition metal-free ketene formation from carbon monoxide through isolable ketenyl anions.

The capacity of transition metals to bind and transform carbon monoxide (CO) is critical to its use in many chemical processes as a sustainable, inexpensive C1 building block. By contrast, only few s- and p-block element compounds bind and activate CO, and conversion of CO into useful carbonyl-containing organic compounds in such cases remains elusive. We report that metalated phosphorus ylides provide facile access to ketenyl anions ([RC=C=O]) by phosphine displacement with CO.

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.

Formation of exceptional monomeric YPhos-PdCl 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 PdCl complexes function as efficient precatalysts for ylide-substituted phosphines (YPhos). These complexes are readily synthesized from PdCl sources and form unprecedented monomeric PdCl complexes without the need for any additional coligand.

Reaction of lithium hexamethyldisilazide (LiHMDS) with water at ultracold conditions.

Alkali metal amides are highly reactive reagents that are broadly applied as strong bases in organic synthesis. Here, we use a combined helium nanodroplet IR spectroscopic and theoretical (DFT calculation) study to show that the reaction of the model compound lithium hexamethyldisilazide (LiHMDS) with water is close to barrierless even at ultra-cold conditions. Upon complex formation of dimeric (LiHMDS) with water in helium nanodroplets as ultra-cold nano-reactors (0.

Single-Site and Cooperative Bond Activation Reactions with Ylide-Functionalized Tetrylenes: A Computational Study.

Due to their transition metal-like behavior divalent group 14 compounds bear huge potential for their application in bond activation reactions and catalysis. Here we report on detailed computational studies on the use of ylide-substituted tetrylenes in the activation of dihydrogen and phenol. A series of acyclic and cyclic ylidyltetrylenes featuring various α-substituents with different - and -donating capabilities have been investigated which demonstrate that particularly -accepting boryl groups lead to beneficial properties and low barriers for single-site activation reactions, above all in the case of silylenes.

From Stable PH-Ylides to α-Carbanionic Phosphines as Ligands for Zwitterionic Catalysts.

Although ylides are commonly used reagents in organic synthesis, the parent methylphosphine MePH only exists in its phosphine form in the condensed phase. Its ylide tautomer H P -CH is considerably higher in energy. Here, we report on the formation of bis(sulfonyl)methyl-substituted phosphines of the type (RO S) C(H)-PR which form stable PH ylides under ambient conditions, amongst the first examples of an acyclic phosphine which only exists in its PH ylide form.

Ylide-Substituted Phosphines: A Platform of Strong Donor Ligands for Gold Catalysis and Palladium-Catalyzed Coupling Reactions.

The development of homogeneous catalysts is strongly connected to the design of new, sophisticated ligands, which resolve limitations of a given reaction protocol by manipulating the electronic properties of the metal and its spatial environment. Phosphines are a privileged class of ligands that find applications in many catalytic transformations, ranging from hydrogenation reactions to hydroformylation and coupling chemistry. For many years, chemists have been trying to improve the efficiency, selectivity, and application of coupling reactions.

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=Ph PC(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.