Hybrid Phosphine/Amino-Acid Ligands Built on Phenyl and Ferrocenyl Platforms: Application in the Suzuki Coupling of o-Dibromobenzene with Fluorophenylboronic Acid

We describe the synthesis and characterization of two classes of hybrid phosphino ligands functionalized with amino ester or amino acid groups. These compounds are built either on a rigid planar phenyl platform or on a functionalized - conformationally controlled - rotational ferrocene backbone. Modifications at the -PR₂ phosphino groups (R=aryl and alkyl, with various steric bulk, Ph, Mes, i-Pr, Cy) and at the amino acid/amino ester functions are reported, showing a valuable high modularity. The coordination chemistry of these compounds regarding palladium and gold was investigated, in particular with respect to the coordination mode of the phosphino groups and the preferred interaction with metals for the amino ester and amino acid functions. For all the hybrid ligands, based either on ferrocenyl or phenyl platforms, the (P,N)-chelating effect dominates in solution for coordination to Pd(II), while linear P-Au(I) complexes without interaction with the amino groups are assumed. The investigation of the catalytic activity of these new ligands in the demanding palladium-catalyzed Suzuki-Miyaura coupling of o-dibromoarenes with fluorophenylboronic acid underlined the importance of the amino ester dicyclohexylphosphinoferrocene for avoiding the deleterious homocoupling and arene oligomerization side-reactions that were otherwise observed with the other phosphine ligands.

Coordination behaviour of a hybrid phosphinoguanidine ligand

A triphenylphosphine derivative equipped with a guanidine substituent in the ortho position readily forms P,N-chelate complexes with Pd(ii) and Pt(ii); however, the coordination of the guanidine moiety can be blocked by protonation.

The protonation state governs the coordination of phosphinoferrocene guanidines

Compared to phosphines with guanidinium tags, studied as polar ligands for aqueous catalysis, their counterparts bearing guanidine substituents received only limited attention. This contribution focuses on the coordination of phosphinoferrocene guanidine Ph₂PfcNC(NHiPr)₂ (1iPr, fc = ferrocene-1,1'-diyl) as a hybrid, P,N-donor ligand to Group 10 metals. In its native state, 1iPr coordinated as a P,N-chelating ligand, affording [M(X)(Y)(1iPr-κ²P,N)] (M/X/Y = Pd/Cl/Cl, Pd/Br/4-C₆H₄CN, Pt/Cl/Cl; the corresponding Ni(II) complex was not isolated). While [PdCl₂(1iPr-κ²P,N)] converted into [PdCl(1iPr-κ³Fe,P,N)]⁺ species with Fe-Pd interaction, upon chloride removal, the analogous Pt(II) complex dimerised into [Pt₂(μ-Cl)₂(1iPr-κ²P,N)₂]²⁺. Deprotonation of [PdCl₂(1iPr-κ²P,N)] produced a unique, doubly chelating phosphinoguanidinate complex [PdCl{(1iPr-H)-κ³P,N,N'}], which was smoothly converted into [Pd(MeCN){(1iPr-H)-κ³P,N,N'}][SbF₆]. The latter, a convenient starting material for substitution reactions, was used to prepare either [Pd(L){(1iPr-H)-κ³P,N,N'}][SbF₆] (L = 4-(dimethylamino)pyridine and 2-phenylpyridine), by simple substitution, or the hydroxide and acetylacetonate (acac) complexes, [Pd₂(μ-OH)₂(1iPr-κ²P,N)₂][SbF₆]₂ and [Pd(acac)(1iPr-κ²P,N)][SbF₆], by substitution with concomitant proton transfer. In contrast, protonation of the guanidine moiety prevented its coordination, as shown in reactions of the salts (1iPrH)Cl and (1iPrH)[SbF₆]. Depending on the metal-to-ligand ratio, adding (1iPrH)[SbF₆] to [PdCl₂(MeCN)₂] produced [Pd₂Cl₂(μ-Cl)₂(1iPrH-κP)₂][SbF₆]₂ or [PdCl₂(1iPrH-κP)₂][SbF₆]₂. Analogous reactions involving (1iPrH)Cl were more complicated due to competing coordination of the chloride anion, leading to (in addition to other compounds) the zwitterionic complex [PdCl₃(1iPrH-κP)], which was alternatively obtained by selective protonation of [PdCl₂(1iPr-κ²P,N)] with HCl. Apparently, the protonation state of the guanidine moiety controls the coordination behaviour of phosphinoferrocene guanidines.

Synthesis of Hydrophilic Phosphorus Ligands and Their Application in Aqueous-Phase Metal-Catalyzed Reactions

Transition metal-catalyzed reactions in aqueous media are experiencing a constant increase in interest. In homogenous catalysis the use of water as a solvent offers advantages in cost, safety, the possibility of two-phase catalysis and simplified separation strategies. In the life sciences, transition metal catalysis in aqueous systems enables the ligation or modification of biopolymers in buffer systems or even in their cellular environment. In biocatalysis, aqueous systems allow the simultaneous use of enzymes and transition metal catalysts in cascade reactions. The use of water-soluble phosphine ligands still represents the most reliable and popular strategy for transferring metal catalysts into the aqueous phase. This review summarizes the recent advancements in this field since 2009 and describes current synthetic strategies for the preparation of hydrophilic phosphines and phosphites. In addition, recent applications of transition metal catalysis in aqueous solvents using these hydrophilic ligands are presented.

The chemistry and biology of guanidine secondary metabolites

Covering: 2017-2019Guanidine natural products isolated from microorganisms, marine invertebrates and terrestrial plants, amphibians and spiders, represented by non-ribosomal peptides, guanidine-bearing polyketides, alkaloids, terpenoids and shikimic acid derived, are the subject of this review. The topics include the discovery of new metabolites, total synthesis of natural guanidine compounds, biological activity and mechanism-of-action, biosynthesis and ecological functions.

Synthesis and study of Fe → Pd interactions in unsymmetric Pd(ii) complexes with phosphinoferrocene guanidine ligands

Readily available phosphinoferrocene guanidines coordinate Pd(ii) as P,N-chelating or κ3P,N,Fe-bound ligands. As the latter, they give rise to the first donor-asymmetric complexes featuring Fe-Pd dative bonds, which were studied using direct (spectroscopic and electrochemical) methods and theoretical (DFT) approaches.

Synthesis and characterisation of Pd(ii) and Au(i) complexes with mesoionic carbene ligands bearing phosphinoferrocene substituents and isomeric carbene moieites

While numerous functional ligands combining phosphine and imidazole-2-ylidene (or imidazolin-2-ylidene) donor moieties have already been reported, the chemistry of the corresponding functional mesoionic carbenes (MIC) derived from 1,2,3-triazoles remains nearly untapped. This contribution describes the synthesis of two isomeric series of triazolium salts bearing 1'-(diphenylphosphino)ferrocenyl substituents by [3 + 2] cycloaddition of a P-protected phosphinoferrocene alkyne with azides, or alternatively of a P-protected phosphinoferrocene azide with terminal alkynes, and by subsequent methylation. These salts were used to synthesize structurally unique Pd(ii) complexes featuring a P,C-chelating triazolylidene carbene ligands and Au(i)-MIC complexes with free phosphine groups. The latter were further utilised to prepare Pd(ii)Au(i) heterometallic complexes containing bridging ferrocene phosphino-carbenes as structurally flexible, donor-unsymmetric metalloligands. In addition, the reactivity of the newly prepared, P-protected phosphinoferrocene alkyne Ph₂PfcC[triple bond, length as m-dash]CH·BH₃ (fc = ferrocene-1,1'-diyl) was investigated, and representatives from all reported compound classes were structurally characterised.

Synthesis and characterisation of palladium(ii) complexes with hybrid phosphinoferrocene ligands bearing additional O-donor substituents

Coordination of phosphinoferrocene ligands with O-donor substituents to Pd(ii) is affected by the nature of the oxygen functions and may results in hemilabile species.