Gold Difluorocarbenoid Complexes: Spectroscopic and Chemical Profiling

The CF3 Group as a Synthon of the CF+ Unit in Palladium Chemistry

The homoleptic trifluoromethyl-palladium(II) complex [Pd(CF3)4]2- (1) is shown to be highly active towards amines. Thus, when treated with primary amines RNH2, it readily undergoes aminolysis of one of the CF3 ligands affording the isocyanide complexes [(CF3)3Pd(CNR)]- (R=aryl). In this process the original CF3 group undergoes total defluorination. Interestingly, the reaction of 1 with secondary amines R2NH proceeds with loss of just two F-substituents, whereby the Fischer-type fluoroaminocarbene complexes [(CF3)3Pd(CFNR2)]- are formed (R=Et, Ph). The reaction of 1 with diamines affords different [(CF3)3Pd(NHC)]- complexes containing sterically non-demanding NHC ligands. Representative examples of various topologies are reported based on the common imidazolidin-2-ylidene or benzimidazolin-2-ylidene rings as well as the expanded-ring perimidin-2-ylidene. This metal-tailored synthetic route, where a CF3 group acts as a pre-carbenic unit, is unprecedented in the vast NHC-chemistry. It takes place under very mild conditions and is envisaged to be extensible to other non-isolable NHC ligands. The key difluorocarbene intermediate [(CF3)3Pd(CF2)]- is experimentally detected.

Gold(I) α-Trifluoromethyl Carbenes: Synthesis, Characterization and Reactivity Studies

Aryl trifluoromethyl diazomethanes 2-R (R=Ph, OMe, CF3 ) are readily decomposed by the (o-carboranyl)diphosphine gold(I) complex 1. The resulting α-CF3 substituted carbene complexes 3-R have been characterized by multi-nuclear NMR spectroscopy as well as X-ray crystallography (for 3-Ph). The bonding situation was thoroughly assessed by computational means, showing stabilization of the electrophilic carbene center by π-donation from the aryl substituent and backdonation from Au, as enhanced by the chelating P^P ligand. Reactivity studies under stoichiometric and catalytic conditions substantiate typical carbene-type behavior for 3-Ph.

Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem-Hydrogenation

gem-Hydrogenation of propargyl alcohol derivatives with [CpX Ru(MeCN)3 ]PF6 (CpX =substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]-sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem-hydrogenation events.

Synthesis of Gold(I)-Trifluoromethyl Complexes and their Role in Generating Spectroscopic Evidence for a Gold(I)-Difluorocarbene Species

Readily prepared and bench-stable [Au(CF₃ )(NHC)] compounds were synthesized by using new methods, starting from [Au(OH)(NHC)], [Au(Cl)(NHC)] or [Au(L)(NHC)]HF₂ precursors (NHC=N-heterocyclic carbene). The mechanism of formation of these species was investigated. Consequently, a new and straightforward strategy for the mild and selective cleavage of a single carbon/fluorine bond from [Au(CF₃ )(NHC)] complexes was attempted and found to be reversible in the presence of an additional nucleophilic fluoride source. This straightforward technique has led to the unprecedented spectroscopic observation of a gold(I)-NHC difluorocarbene species.

Trifluoromethylation of [AuF3 (SIMes)]: Preparation and Characterization of [Au(CF3 )x F3-x (SIMes)] (x=1-3) Complexes

Trifluoromethylation of [AuF3 (SIMes)] with the Ruppert-Prakash reagent TMSCF3 in the presence of CsF yields the product series [Au(CF3 )x F3-x (SIMes)] (x=1-3). The degree of trifluoromethylation is solvent dependent and the ratio of the species can be controlled by varying the stoichiometry of the reaction, as evidenced from the 19 F NMR spectra of the corresponding reaction mixtures. The molecular structures in the solid state of trans-[Au(CF3 )F2 (SIMes)] and [Au(CF3 )3 (SIMes)] are presented, together with a selective route for the synthesis of the latter complex. Correlation of the calculated SIMes affinity with the carbene carbon chemical shift in the 13 C NMR spectrum reveals that trans-[Au(CF3 )F2 (SIMes)] and [Au(CF3 )3 (SIMes)] nicely follow the trend in Lewis acidities of related organo gold(III) complexes. Furthermore, a new correlation between the Au-Ccarbene bond length of the molecular structure in the solid state and the chemical shift of the carbene carbon in the 13 C NMR spectrum is presented.

A Heteroleptic Dirhodium Catalyst for Asymmetric Cyclopropanation with α-Stannyl α-Diazoacetate. "Stereoretentive" Stille Coupling with Formation of Chiral Quarternary Carbon Centers

The heteroleptic dirhodium paddlewheel catalyst 7 with a chiral carboxylate/acetamidate ligand sphere is uniquely effective in asymmetric [2+1] cycloadditions with α‐diazo‐α‐trimethylstannyl (silyl, germyl) acetate. Originally discovered as a trace impurity in a sample of the homoleptic parent complex [Rh₂((R)‐TPCP)₄] (5), it is shown that the protic acetamidate ligand is quintessential for rendering 7 highly enantioselective. The ‐NH group is thought to lock the ensuing metal carbene in place via interligand hydrogen bonding. The resulting stannylated cyclopropanes undergo “stereoretentive” cross coupling, which shows for the first time that even chiral quarternary carbon centers can be made by the Stille–Migita reaction.

Catalytic Asymmetric Fluorination of Copper Carbene Complexes: Preparative Advances and a Mechanistic Rationale

The Cu-catalyzed reaction of substituted α-diazoesters with fluoride gives α-fluoroesters with ee values of up to 95 %, provided that chiral indane-derived bis(oxazoline) ligands are used that carry bulky benzyl substituents at the bridge and moderately bulky isopropyl groups on their core. The apparently homogeneous solution of CsF in C6 F6 /hexafluoroisopropanol (HFIP) is the best reaction medium, but CsF in the biphasic mixture CH2 Cl2 /HFIP also provides good results. DFT studies suggest that fluoride initially attacks the Cu- rather than the C-atom of the transient donor/acceptor carbene intermediate. This unusual step is followed by 1,2-fluoride shift; for this migratory insertion to occur, the carbene must rotate about the Cu-C bond to ensure orbital overlap. The directionality of this rotatory movement within the C2 -symmetric binding site determines the sense of induction. This model is in excellent accord with the absolute configuration of the resulting product as determined by X-ray diffraction using single crystals of this a priori wax-like material grown by capillary crystallization.