Cytotoxic Rhodium(III) and Iridium(III) Polypyridyl Complexes: Structure–Activity Relationships,Antileukemic Activity,and Apoptosis Induction

Whereas the cytostatic agents mer‐[RhX₃(DMSO)(pp)] (X=Cl, Br; pp=phen, dpq) are considerably more potent than their facial isomers, this order is reversed for the analogous kinetically more inert Ir^III polypyridyl complexes. The complexes induce specific apoptotic cell death in leukemia and lymphoma cells via the intrinsic mitochondrial pathway and cause negligible necrotic damage.

     

Chelating Bis(1,2,3‐triazol‐5‐ylidene) Rhodium Complexes: Versatile Catalysts for Hydrosilylation Reactions

NHC‐rhodium complexes (NHC=N‐heterocyclic carbenes) have been widely used as efficient catalysts for hydrosilylation reactions. However, the substrates were mostly limited to reactive carbonyl compounds (aldehydes and ketones) or carbon‐carbon multiple bonds. Here, we describe the application of newly‐developed chelating bis(tzNHC)‐rhodium complexes (tz=1,2,3‐triazol‐5‐ylidene) for several reductive transformations. With these catalysts, the formal reductive methylation of amines using carbon dioxide, the hydrosilylation of amides and carboxylic acids, and the reductive alkylation of amines using carboxylic acids have been achieved under mild reaction conditions.

     

Enantioselective Hydrogenation of β‐Acylamino Acrylates Catalyzed by Rhodium(I)‐Monophosphite Complexes

Chiral rhodium(I) complexes bearing monophosphite ligands, prepared from chiral Binol and (L )‐menthol, were found to be efficient catalysts for the asymmetric hydrogenation of β‐acylamino acrylates with ee values up to 94%.     

DiPAMP’s Big Brother “i‐Pr‐SMS‐Phos” Exhibits Exceptional Features Enhancing Rhodium(I)‐Catalyzed Hydrogenation of Olefins

Switching Knowles DiPAMP’s {DiPAMP=1,2‐bis[(o‐anisyl)(phenyl)phosphino]‐ ethane} MeO groups with i‐PrO ones led to the i‐Pr‐SMS‐Phos {i‐Pr‐SMS‐Phos=1,2‐bis[(o‐isoprop‐ oxyphenyl)(phenyl)phosphino]ethane} ligand which displayed a boosted catalyst activity coupled with an enhanced enantioselectivity in the rhodium(I)‐catalyzed hydrogenation of a wide‐range of representative olefinic substrates (dehydro‐α‐amido acids, itaconates, acrylates, enamides, enol acetates, α,α‐diarylethylenes, etc). The rhodium(I)‐(i‐Pr‐SMS‐Phos) catalytic profile was investigated revealing its structural attributes and robustness, and in contrast to the usual trend, ³¹P NMR analysis revealed that its methyl (Z)‐α‐acetamidocinnamate (MAC) adduct consisted of a reversed diastereomeric ratio of 1.4:1 in favour of the most reactive diastereomer.     

Asymmetric Hydrogenation Catalyzed by (S,S)‐R‐BisPast;‐Rh and (R,R)‐R‐MiniPHOS Complexes: Scope,Limitations, and Mechanism

A new class of chiral C₂‐symmetric bis(trialkyl)phosphine ligands has been prepared and used in Rh(I)‐catalyzed asymmetric hydrogenation reactions. The ligands, 1,2‐bis(alkylmethylphosphino)ethanes 1a‐g (abbreviated as BisP*, alkyl = t‐butyl, 1‐adamantyl, 1‐methylcyclohexyl, 1,1‐diethylpropyl, cyclopentyl, cyclohexyl, isopropyl) and 1,2‐bis(alkylmethylphosphino)methanes 2a‐d (abbreviated as MiniPHOS, alkyl = t‐butyl, cyclohexyl, isopropyl, phenyl) are prepared by a simple synthetic approach based on the air‐stable phosphine–boranes. These new ligands give the corresponding Rh(I) complexes, which are effective catalytic precursors for the asymmetric hydrogenation of a representative series of dehydroamino acids and itaconic acid derivatives. Enantioselectivities observed in these hydrogenations are universally high and in many cases exceed 99%. X‐Ray characterization of four precatalysts, study of the pressure effects, deuteration experiments, and characterization of the wide series of intermediates in the catalytic cycle are used for the discussion of the possible correlation between the structure of the catalysts and the outcome of the catalytic asymmetric hydrogenation.     

Chiral NCN Pincer Rhodium(III) Complexes with Bis(imidazolinyl)phenyl Ligands: Synthesis and Enantioselective Catalytic Alkynylation of Trifluoropyruvates with Terminal Alkynes

A series of new chiral C₂‐symmetrical NCN pincer rhodium(III) complexes with bis(imidazolinyl)phenyl ligands have been conveniently synthesized from easily available materials. The complexes were subsequently applied in the enantioselective addition of terminal alkynes to trifluoropyruvates. With catalyst loading of 1.5–3.0 mol%, the alkynylation of ethyl or methyl trifluoropyruvate with a variety of electronically and structurally diverse terminal alkynes gave the optically active trifluoromethyl‐substituted tertiary propargylic alcohols with enantioselectivities of up to >99% ee and high yields. Although good to excellent enantioselectivities (85–98% ee) could be achieved only for some of the aliphatic terminal alkynes under the optimized conditions, the enantioselectivities were consistently excellent (94% to >99% ee) in the case of aromatic as well as heteroaromatic alkynes and enynes.     

Aqueous Biphasic Hydroformylation Catalysed by Protein‐Rhodium Complexes

The water‐soluble complex derived from Rh(CO)₂(acac) and human serum albumin (HSA) proved to be efficient in the hydroformylation of several olefin substrates. The chemoselectivity and regioselectivity were generally higher than those obtained by using the classic catalytic systems like TPPTS‐Rh(I) (TPPTS=triphenylphosphine‐3,3′,3″‐trisulfonic acid trisodium salt). Styrene and 1‐octene, for instance, were converted in almost quantitative yields into the corresponding oxo‐aldehydes at 60 °C and 70 atm (CO/H₂=1) even at very low Rh(CO)₂(acac)/HSA catalyst concentrations. The possibility of easily recovering the Rh(I) compound makes the system environmentally friendly. The circular dichroism technique was useful for demonstrating the Rh(I) binding to the protein and to give information on the stability in solution of the catalytic system. Some other proteins have been used to replace HSA as complexing agent for Rh(I). The results were less impressive than those obtained using HSA and their complexes with Rh(I) were much less stable.     

Impact of Incorporating Substituents onto the P‐o‐Anisyl Groups of DiPAMP Ligand on the Rhodium(I)‐Catalyzed Asymmetric Hydrogenation of Olefins

The introduction of 1,2‐bis[(o‐anisyl)(phenyl)phosphino]ethane (DiPAMP) as a P‐stereogenic ligand for rhodium(I)‐catalyzed hydrogenation by Knowles et al. came after their evaluation of several diphosphines. However, no in‐depth study was carried out on incorporating various substituents on its P‐o‐anisyl groups. In this work, we have prepared a large series of enantiopure and closely related DiPAMP analogues possessing various substituents (MeO, TMS, t‐Bu, Ph, fused benzene ring) on the o‐anisyl rings. The new ligands were evaluated in rhodium‐catalyzed hydrogenation of several model substrates: methyl α‐acetamidoacrylate, methyl (Z)‐α‐acetamidocinnamate, methyl (Z)‐β‐acetamidocrotonate, dimethyl itaconate, and atropic acid. They displayed enhanced activities and increased enantioselectivities, particularly the P‐(2,3,4,5‐tetra‐MeO‐C₆H)‐substituted ligand (4MeBigFUS). Interestingly enough, 88% ee was obtained in the hydrogenation of atropic acid using the Rh‐(4MeBigFUS) catalyst under mild conditions (10 bar H₂, room temperature) versus 7% ee using Rh‐DiPAMP. Conversely, the ligand possessing P‐(2,6‐di‐MeO‐C₆H₃) groups proved to slow down considerably the hydrogenation. X‐Ray structures of their corresponding Rh complexes are presented and discussed.     

Enhanced Hydrogenation Activity and Recycling of Cationic Rhodium Diphosphine Complexes through the Use of Highly Fluorous and Weakly‐Coordinating Tetraphenylborate Anions

The effect of the nature of the anion on the performance of ionic rhodium catalysts has received little attention. Herein it is shown that the use of highly fluorous tetraphenylborate anions can enhance catalyst activity in both conventional and fluorous media. For hydrogenation catalysts of the type [Rh(COD)(dppb)][X] {COD=1,5‐cis,cis‐cyclooctadiene; dppb=1,4‐bis(diphenylphosphino)butane; X=BF₄⁻ ( 1a ), [BPh₄]⁻ ( 1b ), [B{C₆H₄(SiMe₃)‐4}₄]⁻ ( 1c ), [B{C₆H₃(CF₃)₂‐3,5}₄]⁻ ( 1d ), [B{C₆H₄(SiMe₂CH₂CH₂C₆F₁₃)‐4}₄]⁻ ( 1e ), [B{C₆H₄(C₆F₁₃)‐4}₄]⁻ ( 1f ) and [B{C₆H₃(C₆F₁₃)₂‐3,5}₄]⁻ ( 1 g )} the activity towards the hydrogenation of 1‐octene in acetone increased in the order 1c < 1b < 1e < 1a < 1d ~ 1f < 1g with 1g being twice as active as the commonly applied 1a . Despite the fluorophilic character introduced by the substituted tetraarylborate anions, the presence of some perfluoroalkyl‐substituents in the cation was still required for achieving high partition coefficients. Therefore, [Rh(COD)(Ar₂PCH₂CH₂PAr₂)][X] {Ar=C₆H₄(SiMe₂CH₂CH₂C₆F₁₃)‐4, X=[B{C₆H₃(C₆F₁₃)₂‐3,5}₄]⁻ ( 3f ); Ar=C₆H₄(SiMe(CH₂CH₂C₆F₁₃)₂)‐4 and X=[B{C₆H₄(C₆F₁₃)‐4}₄]⁻ ( 2g )} were prepared, which were active in the hydrogenation of 1‐octene, 2g even more so than 3f . Both these highly fluorous catalysts could be recycled with 99% efficiency through fluorous biphasic separation, whereas the corresponding BF₄⁻ complex of 2g ( 2a ) did not show any affinity for the fluorous phase.     

Synthesis of Olefin‐Oxazoline Ligands (OlefOx) by Rhodium(III)‐Catalyzed Oxidative Olefination

The rhodium(III)‐catalyzed oxidative olefination of 2‐aryloxazolines is described and has been employed for the synthesis of olefin‐oxazoline ligands (OlefOx). The highly modular synthesis starting from readily available 2‐aryloxazolines can be performed under an atmosphere of air as the terminal oxidant with catalytic amounts of copper(II)‐acetate.     

Rhodium‐Catalyzed Hydrogenation of Alkenes by Rhodium/Tris(fluoroalkoxy)phosphane Complexes in Fluorous Biphasic System

The reaction of [Ir(μ‐Cl)(COD)]₂ with various fluorous derivatives of triphenylphosphane containing a para‐, meta‐, or ortho‐(1H,1H‐perfluoroalkoxy)‐substituted fluorous phosphane P(C₆H₄‐ORf)₃ (Rf=CH₂C₇F₁₅, CH₂CH₂CH₂C₈F₁₇) and CO (1 atm) gives the corresponding trans‐[Ir(μ‐Cl)(CO){P(C₆H₄ORf)₃}₂]. The IR ν_CO values of these complexes give some information on the donor/acceptor properties of the phosphanes. These fluorous derivatives of triphenylphosphane, as well as a phosphane bearing two (1H,1H‐perfluoroalkyloxy) chains at the 3,5‐positions, were used in association with [Rh(μ‐Cl)(COD)]₂ or [Rh(COD)₂]PF₆ in the reduction of methyl cinnamate, 2‐cyclohexen‐1‐one, cinnamaldehyde, and methyl α‐acetamidocinnamate in a two‐phase system D‐100/ethanol under 1 bar hydrogen at room temperature. Some differences in catalytic activity were observed in the reduction of methyl cinnamate, the most active catalyst being the rhodium complex containing the phosphane with the p‐fluorous ponytail. Recycling of the fluorous catalyst was possible, particularly using the p‐substituted phosphane, where no significant loss of catalyst or activity was observed, and generally with very low leaching of rhodium or phosphane in the organic phase.     

Highly Selective Apoptotic Cell Death Induced by Halo‐Salane Titanium Complexes

Metal‐based antitumor agents : Halogen‐substituted titanium salane complexes showed IC₅₀ values comparable to cisplatin. In contrast to their alkyl‐substituted congeners, they almost exclusively induced apoptotic cell death. This unique combination of very low IC₅₀ values and pronounced preference for apoptosis makes them promising therapeutic agents.

     

Cationic and Neutral N‐Heterocyclic Carbene Gold(I) Complexes: Cytotoxicity,NCI‐60 Screening,Cellular Uptake,Inhibition of Mammalian Thioredoxin Reductase,and Reactive Oxygen Species Formation

A structurally diverse library of 14 gold(I) cationic bis(NHC) and neutral mono(NHC) complexes (NHC: N‐heterocyclic carbene) was synthesized and characterized in this work. Four of them were new cationic gold(I) complexes containing functionalized NHCs, and their X‐ray crystal structures are presented herein. All of the complexes were investigated for their anticancer activities in four cancer cell lines, including a cisplatin‐resistant variant, and a noncancerous cell line. Seven of the cationic gold(I) complexes were found to display high and specific cytotoxic activities toward cancer cells. Two of them were even able to overcome cisplatin resistance. Two highly potent cationic complexes ( 11 and 15 ) were also submitted to the NCI‐60 cancer panel for further cytotoxicity evaluation. Complex 15 showed a surprisingly high potency toward leukemia among the nine examined cancer subtypes, particularly toward the CCRF‐CEM leukemia cell line with a concentration for 50 % inhibition of growth down to 79.4 nm . In addition, cationic complex 13 , which demonstrated a remarkable cytotoxicity against hepatocellular carcinoma, was selected to obtain insight into the mechanistic aspects in HepG2 cells. Cellular uptake measurements were indicative of good bioavailability. By various biochemical assays, this complex was found to effectively inhibit thioredoxin reductase and its cytotoxicity toward HepG2 cells was found to be reactive oxygen species dependent.     

Design and Synthesis of DNA‐Interactive β‐Carboline–Oxindole Hybrids as Cytotoxic and Apoptosis‐Inducing Agents

A new series of (E)‐3‐[(1‐aryl‐9H‐pyrido[3,4‐b]indol‐3‐yl)methylene]indolin‐2‐one hybrids were synthesized and evaluated for their in vitro cytotoxic activity against a panel of selected human cancer cell lines, namely, HCT‐15, HCT‐116, A549, NCI‐H460, and MCF‐7, including HFL. Among the tested compounds, (E)‐1‐benzyl‐5‐bromo‐3‐{[1‐(2,5‐dimethoxyphenyl)‐9H‐pyrido[3,4‐b]indol‐3‐yl]methylene}indolin‐2‐one ( 10 s ) showed potent cytotoxicity against HCT‐15 cancer cells with an IC₅₀ value of 1.43±0.26 μm and a GI₅₀ value of 0.89±0.06 μm . Notably, induction of apoptosis by 10 s on the HCT‐15 cell line was characterized by using different staining techniques, such as acridine orange/ethidium bromide (AO/EB) and DAPI. Further, to understand the mechanism of anticancer effects, various assays such as annexin V‐FITC/PI, DCFDA, and JC‐1were performed. The flow cytometric analysis revealed that compound 10 s arrests the HCT‐15 cancer cells at the G0/G1 phase of the cell cycle. Additionally, western blot analysis indicated that treatment of 10 s on HCT‐15 cancer cells led to decreased expression of anti‐apoptotic Bcl‐2 and increased protein expression of both pro‐apoptotic Bax and caspase‐3, ‐8, and ‐9, and cleaved PARP with reference to actin. Next, a clonogenic assay revealed the inhibition of colony formation in HCT‐15 cancer cells by 10 s in a dose‐dependent manner. Moreover, upon testing on normal human lung cells (HFL), the compounds were observed to be safer with a low toxicity profile. In addition, viscosity and molecular‐docking studies showed that compound 10 s has typical intercalation with DNA.     

Rhodium(I)‐Catalyzed Diastereoselective Cycloisomerization of Enynes with Tethered (S)‐2‐Methyl‐2‐propanesulfinyl Imine

The rhodium(I)‐catalyzed cycloisomerization of enynes with tethered (S)‐2‐methyl‐2‐propanesulfinyl imine affords 5‐ or 6‐membered cyclic compounds containing exocyclic 1,3‐diene moieties in a stereoselective manner. The reaction proceeds through β‐hydride elimination of a 7‐membered azarhodacycle intermediate, which is generated from three unsaturated bonds (i.e., alkene, alkyne, and CN bonds) and an Rh(I) complex. The resultant cyclic compounds could be reacted with various dienophiles to afford spiroamides as single isomers through the Diels–Alder reaction.

     

Rhodium(III)‐Catalyzed Synthesis of Indole Derivatives From Pyrimidyl‐Substituted Anilines and Diazo Compounds

An efficient method for the synthesis of indole derivatives from readily available pyrimidyl‐substituted anilines and diazo compounds via rhodium(III)‐catalyzed C H bond activation has been developed. This cyclization reaction displays excellent functional group compatibility and regioselectivity, which overcomes some drawbacks of the classical indole synthetic methods and provides a facile approach for the construction of multi‐substituted indole derivatives. The redox‐neutral intermolecular annulation procedure comprises tandem C H bond activation, cyclization, and condensation steps, releasing water and nitrogen as by‐products.

     

Rhodium(III)‐Catalyzed Oxidative Olefination of Picolinamides: Convenient Synthesis of 3‐Alkenylpicolinamides

A rhodium(III)‐catalyzed selective olefination of picolinamide derivatives has been developed. The reaction shows high regioselectivity, low catalyst loading (0.5 mol%), high yield and good functional group tolerance, providing a convenient strategy for the synthesis of 3‐alkenylpicolinamides.