Gold(I) Complexes of Conformationally Constricted Chiral Ferrocenyl Phosphines

Hydrogen-Bonded Matched Ion Pair Gold(I) Catalysis

The enantioselective reaction of 1,6-enynes with O-, N-, and C-nucleophiles has been developed by matched ion pair gold(I) catalysis in which the chiral gold(I) cation and anion are H-bonded through a urea group. Very high levels of enantiocontrol are achieved (up to >99:1 er) for a broad scope of substrates. DFT studies demonstrate the importance of the H-bond donor group in anchoring the matched chiral cation- and anion-favoring additional noncovalent interactions.

H-Bonded Counterion-Directed Catalysis: Enantioselective Gold(I)-Catalyzed Addition to 2-Alkynyl Enones as a Case Study

H-bonded counterion-directed catalysis (HCDC) is a strategy wherein a chiral anion that is hydrogen-bonded to the achiral ligand of a metal complex is responsible for enantioinduction. In this article we present the application of H-bonded counterion-directed catalysis to the Au(I)-catalyzed enantioselective tandem cycloisomerization-addition reaction of 2-alkynyl enones. Following the addition of C-, N- or O-centered nucleophiles, bicyclic furans were obtained in moderate to excellent yield and enantioselectivity (28 examples, 59-96 % yield, 62 : 38 to 95 : 5 er). The optimal catalytic system, comprising a phosphinosquaramide Au(I) chloride complex and a BINOL-derived phosphoramidate Ag(I) salt, was selected in a combinatorial fashion from a larger library with the help of high-throughput screening. An enantioselectivity switch of ca. 120 Δee% was observed upon addition of the achiral Au(I) component to the Ag(I) salt.

H-Bonded Counterion-Directed Enantioselective Au(I) Catalysis

A new strategy for enantioselective transition-metal catalysis is presented, wherein a H-bond donor placed on the ligand of a cationic complex allows precise positioning of the chiral counteranion responsible for asymmetric induction. The successful implementation of this paradigm is demonstrated in 5-exo-dig and 6-endo-dig cyclizations of 1,6-enynes, combining an achiral phosphinourea Au(I) chloride complex with a BINOL-derived phosphoramidate Ag(I) salt and thus allowing the first general use of chiral anions in Au(I)-catalyzed reactions of challenging alkyne substrates. Experiments with modified complexes and anions, ¹H NMR titrations, kinetic data, and studies of solvent and nonlinear effects substantiate the key H-bonding interaction at the heart of the catalytic system. This conceptually novel approach, which lies at the intersection of metal catalysis, H-bond organocatalysis, and asymmetric counterion-directed catalysis, provides a blueprint for the development of supramolecularly assembled chiral ligands for metal complexes.

Synthesis and characterization of enantiopure planar-chiral phosphorus-linked diferrocenes

Six new homochiral diferrocenyl derivatives have been synthesized, one of which is purely planar–chiral. Even if the two diferrocene subunits are identical, they are distinguished due to their positions relative to the substituents at the phospho­rous prochiral centre.

In the course of an ongoing synthetic project on cyclic diferrocenylphosphines, we obtained a group of planar–chiral diferrocenyl com­pounds useful as precursors for subsequent cyclization. Here we report the crystal structures of two sym­metric com­pounds [(Fc^A)₂(Ph)P], one of which contains four stereogenic centres (two C chiral and two planar chiral centres), i.e. 1,1′-(phenyl­phos­phane­di­yl)bis­{(2S_p)-2-[(1R)-1-(acet­yloxy)eth­yl]ferrocene}, [Fe₂(C₅H₅)₂(C₂₄H₂₅O₄P)], and the other phosphine sulfide is a purely planar–chiral com­pound (two planar chiral centres), i.e. bis[(2S_p)--2-ethenylferrocen-1-yl]phenylphosphane sulfide, [Fe₂(C₅H₅)₂(C₂₀H₁₇PS)]. Owing to the stereocentres present, reactions performed on [(Fc^A)₂(Ph)P]-type com­pounds strongly favour one ferrocene unit over the other due to diastereoselectivity. Furthermore, we present four related structures where the ferrocene units are not identical [(Fc^A)(Fc^B)(Ph)P]. These are {(2S_p)-2-[(1R)-1-(acet­yloxy)eth­yl]ferrocen-1-yl}[(2S_p)-2-ethenylferrocen-1-yl]phenyl-(S)-phosphine sulfide, [Fe₂(C₅H₅)₂(C₂₂H₂₁O₂PS)], [(2S_p)-2-ethenylferrocen-1-yl]{(2S_p)-2-[(1R)-1-hy­droxy­eth­yl]ferrocen-1-yl}phenyl-(S)-phosphine sulfide, [Fe₂(C₅H₅)₂(C₂₀H₁₉OPS)], {(2S_p)-2-[(1R)-1-(acet­yloxy)eth­yl]ferro­cen-1-yl}{(2S_p)-2-[(1R)-1-hy­droxy­eth­yl]ferrocen-1-yl}phenyl-(R)-phos­phine sulfide, [Fe₂(C₅H₅)₂(C₂₂H₂₃O₃PS)], and {(2S_p)-2-[(1R)-1-benzyl­amino)­eth­yl]ferrocen-1-yl}[(2S_p)-2-ethenylferrocen-1-yl]phenyl-(S)-phosphine sulfide, [Fe₂(C₅H₅)₂(C₂₇H₂₆NPS)]. All of the structures are accessible in one step from known precursors.

Planar-Chiral P and N-bridged Diferrocenes

We describe the synthesis of planar-chiral diferrocene compounds 4, 7 and 12, intended as key precursors for a new family of asymmetric catalyst ligands with less complex structures than their popular equivalents. In contrast to conventional 1,2-disubstituted ferrocenes containing center-chiral and planar-chiral elements, these compounds are purely planar-chiral due to the absence of α-substiuted ethyl groups. The title compounds 4, 7 and 12 were obtained from known precursors in 87%, 59% and 60% yields, respectively.

(2Sp,4R,8Sp)-4-Methyl-1-phenyl-diferroceno-5-Z-ethylene-1-phosphinoxide

We present a synthesis of the title compound, a homochiral diferroceno cycle intended for use in asymmetric catalysis. We present a hydrovinylation protocol, which allows the production of the title compound in two steps from a known diferrocenyl precursor. We obtained one of two possible diastereomers selectively and propose a plausible regio- and stereoselective reaction mechanism.

(2Sp,4R,6R,8Sp)-4,6-Dimethyl-1-phenyl-diferroceno-1-phosphines

Ferrocene-based compounds are powerful asymmetric ligands usable for chemical catalysis. We present the synthesis of six new homochiral diferroceno cycles potentially useful as P,O-, P,N- or P,S-ligands. Due to the stereoconservative nature of the SN1 reaction at carbons adjacent to ferrocene units, we obtained a single diastereomer of the 8-membered diferroceno[c,f]heterophosphocines in all cases.

Formaldehyde-1,1″-(phenylphosphinidenesulfide)bis[(2S)-2-[(1R)-1-(methylamino)-ethyl]]ferrocene-aminal

In the course of an ongoing synthetic project, we observed an unprecedented reactivity of N-methyl groups in bis(N,N-dimethylaminoethylferrocenyl)phenylphosphinesulfide upon treatment with manganese dioxide (MnO2). The intramolecular course of this reaction resulted in the formation of an unexpected homochiral diaza macrocycle. The target structure was accessible in two steps from known N,N-dimethylaminoethylferrocene.

Synthesis and Coordination Behavior of a Flexible Bis(phosphinoferrocene) Ligand

A symmetrical flexible bis(phosphinoferrocene) derivative, viz. bis[1′-(diphenylphosphino)ferrocenyl]methane (1), was prepared and studied as a ligand in Pd(II) and Au(I) complexes. The reactions of 1 with [PdCl₂(cod)] (cod = cycloocta-1,5-diene) and [Pd(μ-Cl)(L^NC)]₂ (L^NC = [2-(dimethylamino-κN)methyl]phenyl-κC¹) produced bis(phosphine) complex trans-[PdCl₂(1-κ²P,P′)] (4), wherein the ligand spans trans positions in the square-planar coordination sphere of Pd(II) and the tetranuclear, P,P-bridged complex [(μ(P,P′)-1){PdCl(L^NC)}₂] (5), respectively. In reactions with the Au(I) precursors [AuCl(tht)] and [Au(tht)₂][SbF₆] (tht = tetrahydrothiophene), ligand 1 gave rise to tetranuclear Au₂Fe₂ complex [(μ(P,P′)-1)(AuCl)₂] (6) and to symmetrical macrocyclic tetramer [Au₄(μ(P,P′)-1)₄][SbF₆]₄ (7). All compounds were characterized by spectroscopic methods. In addition, the structures of compound 1, its synthetic precursor bis[1′-(diphenylphosphino)ferrocenyl]methanone (3), and all aforementioned Pd(II) and Au(I) complexes were determined by single-crystal X-ray diffraction analysis (some in solvated form).

Anatomy of gold catalysts: facts and myths

This review article covers the main types of gold(i) complexes used as precatalysts under homogeneous conditions in organic synthesis and discusses the different ways of catalyst activation as well as ligand, silver, and anion effects.