Rhodium complexes with chiral counterions: achiral catalysts in chiral matrices

The neutral complexes [Rh(I)(NBD)((1S)-10-camphorsulfonate)] (2) and [Rh(I)((R)-N-acetylphenylalanate)] (4) reacted with bis-(diphenylphosphino)ethane (dppe) to form the cationic Rh(I)(NBD)(dppe) complexes, 5 and 6, respectively, accompanied by their corresponding chiral counteranions. Analogously, 4 reacted with 4,4^′-dimethylbipyridine to yield complex 7. Complexes 5 and 6 disproportionated in aprotic solvents to form the corresponding bis-diphosphine complexes 8 and 9, respectively. 8 was characterized by an X-ray crystal structure analysis. In order to form achiral Rh(I) complexes bearing chiral countercations new sulfonated monophosphines 13-16 with chiral ammonium cations were synthesized. Tris-triphenylphosphinosulfonic acid (H₃TPPS, 11) was used to protonate chiral amines to yield chiral ammonium phosphines 14-16. Thallium-tris-triphenylphosphinosulfonate (Tl₃TPPS, 12) underwent metathesis with a chiral quartenary ammonium iodide to yield the proton free chiral ammonium phosphine 13. Phosphines 15 and 16 reacted with [Rh(NBD)₂]BF₄ to afford the highly charged chiral zwitterionic complexes [Rh(NBD)(TPPS)₂][(R)-N,N-dimethyl-1-(naphtyl)ethylammonium]₅ (17) and [Rh(NBD)(TPPS)₂][BF₄][(R)-N,N-dimethyl-phenethylammonium]₆ (18), respectively. Complexes 5, 6, and 18 were tested as precatalysts for the hydrogenation of de-hydro-N-acetylphenylalanine (19) and methyl-(Z)-(α)-acetoamidocinnamate (MAC, 20) under homogeneous and heterogeneous (silica-supported and self-supported) conditions. None of the reactions was enantioselective.     

Efficient preparation of c(2)-symmetrical chiral ferrocenyl diols by catalytic enantioselective reduction of diacylferrocenes

[reaction: see text] Enantioselective borohydride reduction, catalyzed by the optically active beta-ketoiminato cobalt(II) complex, was successfully applied to the 1,1'-dialkanoyl- and 1,1'-dibenzoylferrocenes to afford the corresponding C(2)-symmetrical chiral ferrocenyl diols with high diastereoselectivity and excellent enantioselectivity.     

Asymmetric cyclopropanation of styrenes catalyzed by metal complexes of D2-symmetrical chiral porphyrin: superiority of cobalt over iron

The cobalt(II) complex of D2-symmetric chiral porphyrin [Co(1)] is an effective catalyst for highly diastereoselective and enantioselective cyclopropanation of a broad range of styrene derivatives under mild conditions. Dimerization of diazo compounds, a common side reaction in metal-mediated carbene transfer processes, is minimized in a cobalt porphyrin-based system, obviating the need to employ excess substrates and slow addition of diazo compounds. The high catalytic activity and selectivity of [Co(1)] evidently resulted from the appropriate combination of the cobalt ion and the chiral porphyrin 1 as the use of iron(III) complex of the same ligand [Fe(1)Cl] afforded the desired cyclopropane products in low yields and poor enantioselectivity.     

New chiral thiols and C2-symmetrical disulfides of Cinchona alkaloids: ligands for the asymmetric Henry reaction catalyzed by CuII complexes

Seven Cinchona alkaloids were reacted with thioacetic acid and Bu₃P/diethyl diazadicarboxylate in THF at 0–25 °C to give the corresponding thiolacetates with complete inversion of configuration at the substitution center. The thus obtained chiral thiolesters were converted into thiols and these compounds were oxidized to the respective disulfides of C₂-symmetry. Both C-9 thioles and disulfides were tested as chiral ligands in the Cu-catalyzed asymmetric Henry reaction. When the thiol derivatives of an 8,9-like configuration were applied in the reaction of benzaldehyde and nitromethane, the obtained nitroaldol was of the same absolute configuration as the catalyst and the observed enantioselectivity was up to 83% ee. These ligands gave higher ees than the corresponding thioethers, disulfides, and thioles of 8,9-unlike configuration. The results obtained are in agreement with the proposed transition-state model involving nucleophilic attack of a deprotonated nitromethane directed preferably at one side of the O-complexed benzaldehyde.     

Synthesis and metal complexes of chiral c(2)-symmetric diamino-bisoxazoline ligands

A synthetic route to tetradentate chiral N(4) ligands has been developed with the aim to study the potential of corresponding iron and manganese complexes as catalysts for enantioselective epoxidation. These ligands, which contain two oxazoline rings and two trialkylamino groups as coordinating units, are readily prepared in enantiomerically pure form by the reaction of chiral 2-chloromethyloxazolines with achiral N,N'-dimethylethane-1,2-diamine or chiral (R,R)-N,N'-dimethylcyclohexane-1,2-diamine. The ligands derived from N,N'-dimethylethane-1,2-diamine reacted with anhydrous metal halides MnCl(2) and FeCl(2) in a stereoselective manner to give octahedral mononuclear complexes that have the general formula Delta-[(L)MCl(2)]. In contrast, the ligands derived from N,N'-dimethylcyclohexane-1,2-diamine formed complexes with different coordination modes depending on the diastereomer employed: in one case the metal ion was found to be pentacoordinate, in the other case a hexacoordinated complex was observed. The structure of a series of Fe and Mn complexes was determined by X-ray analysis. The coordination chemistry of these ligands was further studied by X-ray and NMR analyses of the diamagnetic isostructural complexes [(L)ZnCl(2)]. Analogous ionic complexes, which were prepared by removing chloride with silver trifluoromethanesulfonate or hexafluoroantimonate, were tested as catalysts for the epoxidation of olefins.     

Poly(propylene imine) dendrimer complexes as catalysts for oxidation of alkenes

Complexes of poly(propylene imine) dendrimers D8[DAB-dendr-(NH₂)₈] and D32 [DAB-dendr-(NH₂)₃₂] were prepared by interaction of the dendrimers with transition metal salts such as FeCl₃.6H₂O; CoCl₂.6H₂O; CuCl₂.2H₂O; VOSO₄.5H₂O; Na₂MoO₄.2H₂O and Na₂WO₄.2H₂O at room temperature in aqueous solutions. The content of metal ions in the complexes was found to be from 8.2 to 69.6 mg metal ion/g polymer carrier. The complexes were characterized by using IR, UV-VIS, Moessbauer spectroscopy and EPR. The anticipated co-ordination structure of the compounds was suggested. It was found that the order of the catalytic activity of the complexes of poly(propylene imine) dendrimers D8 and D32 in the reaction of epoxidation of cyclohexene with organic hydroperoxides such as tert-butyl hydroperoxide (t-BHP), ethylbenzene hydroperoxide (EBHP) and cumene hydroperoxide (CHP) was as follows: D32-MoО₂²⁺>D32-VО²⁺>D32-WО₂²⁺ > D32-Co²⁺ > D32-Cu²⁺>D32-Fe³⁺. The order of reactivity of organic hydroperoxides in the reaction studied was: t-BHP > EBHP > CHP.     

Cu-Fesulphos complexes: efficient chiral catalysts for asymmetric 1,3-dipolar cycloaddition of azomethine ylides

The Cu^I-Fesulphos catalyst system (≤3 mol %) shows an excellent performance in the asymmetric 1,3-dipolar cycloaddition of azomethine ylides. High to very high levels of reactivity, endo/exo selectivity, and enantioselectivity (69->99% ee) are generally achieved with a very wide range of azomethine ylides and dipolarophiles. Based on experimental and computational studies data, a model that accounts for this high enantiocontrol is proposed.     

Toward stereoselective lactide polymerization catalysts: cationic zinc complexes supported by a chiral phosphinimine scaffold

The P-stereogenic phosphinimine ligands (dbf)MePhP═NAr (7: Ar = Dipp; 8: Ar = Mes; dbf = dibenzofuran, Dipp = 2,6-diisopropylphenyl, Mes = 2,4,6-trimethylphenyl) were synthesized as racemates via reactions of the parent phosphines (rac)-(dbf)MePhP (6) with organoazides. The ligands 7 and 8 were protonated by Br?nsted acids to afford the aminophosphonium borate salts [(7)-H][BAr(4)] (9: Ar = C(6)F(5); 11: Ar = Ph) and [(8)-H][BAr(4)] (10: Ar = C(6)F(5); 12: Ar = Ph). The protonated ligands 9 and 10 were active toward alkane elimination reactions with diethylzinc and ethyl-[methyl-(S)-lactate]zinc to give the heteroleptic complexes [{(dbf)MePhP═NAr}ZnR][B(C(6)F(5))(4)] (Ar = Dipp, 13: R = Et; 15: R = methyl-(S)-lactate; Ar = Mes, 14: R = Et; 16: R = methyl-(S)-lactate). By contrast, reaction of the tetraphenylborate derivative 11 with diethylzinc yielded a phenyl transfer product, [(dbf)MePhP═NDipp]ZnPh(2) (17). Complex 15 was found to catalyze the ring-opening polymerization of rac-lactide.     

Two novel chiral rhodium complexes as catalysts for the enantioselective allylation of arylaldehydes

Two novel chiral rhodium complexes were successfully synthesized from the reaction of a new class of bidentate nitrogen ligands with RhCl₃·3H₂O in ethanol under reflux. The crystal structure of 4a was unambiguously established by X-ray analysis. Their corresponding cationic metal complexes prepared in situ from the reaction of 4a or 4b with AgBF₄ catalyze the enantioselective allylation of arylaldehydes with allylstannane in 5–50% ee.     

Rhodium/tris-binaphthyl chiral monophosphite complexes: Efficient catalysts for the hydroformylation of disubstituted aryl olefins

A family of threefold symmetry phosphite ligands, P(O–BIN–OR)₃ (BIN = 2,2′-binaphthyl; R = Me, Bn, CHPh₂, 1-adamantyl), derived from enantiomerically pure (R)-BINOL, was developed. Cone angles within the range 240–270° were calculated for the phosphite ligands, using the computational PM6 Hamiltonian. Their rhodium complexes formed in situ showed remarkable catalytic activity in the hydroformylation of hindered phenylpropenes, under relatively mild reaction conditions, with full chemoselectivity for aldehydes, high regioselectivity, however with low enantioselectivity. The ether substituents at the ligand affected considerably the catalytic activity on the hydroformylation of 1,1- and 1,2-disubstituted aryl olefins. The kinetics of the hydroformylation of trans-1-phenyl-1-propene, using tris[(R)-2′-benzyloxy-1,1′-binaphthyl-2-yl]phosphite as model ligand, was investigated. A first order dependence in the hydroformylation initial rate with respect to substrate and catalyst concentrations was found, as well as a positive order with respect to the partial pressure of H₂, and a slightly negative order with respect to phosphite concentration and CO partial pressure.     

Synthesis of C2-symmetrical chiral diamines: diastereoselective addition to bis(1,3-oxazolidinyl)alkanes with Grignard reagents

Asymmetric syntheses of C₂-symmetrical chiral 1,4- and 1,5-diamines with stereogenic centers adjacent to the nitrogen atom have been accomplished. Chiral diamines were prepared by diastereoselective alkylations of bisoxazolidine, which was derived from (R)-phenylglycinol. Methyl and phenyl Grignard reagents were employed as alkylating reagents. In addition, tertiary chiral diamines were readily converted to primary diamines in high yield.     

Anionic chiral cobalt(III) complexes as catalysts of asymmetric synthesis of cyanohydrins

Chiral coordinatively saturated cobalt(III) complexes with Schiff bases of enantio-pure amino acids are formed as Λ and Δ-isomers, which are not transformed into each other under normal conditions. These complexes catalyze the formation of enantiomerically enriched cyanohydrins from aldehydes and Me₃SiCN under homo-and heterogeneous catalysis. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 793–799, May, 2006.     

Zinc complexes of chiral phenols as catalysts for enantioselective addition of organozinc reagents to aldehydes

A class of zinc (II) chelates with chiral tertiary amino phenolic alcohols serve as effective catalysts for the enantioselective addition of diethylzinc to aromatic aldehydes with predictable absolute stereochemistry.     

Reversible microencapsulation of pybox-Ru chiral catalysts: scope and limitations

Chiral Pybox-Ru catalysts can be microencapsulated into linear polystyrene as a method to recover and recycle the valuable catalyst. These catalysts allow 60-68% yields to be achieved with enantioselectivities in the range 75-85% ee in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate. The catalyst is soluble in the reaction solvent and is re-encapsulated at the end of the reaction. The great advantage of this methodology is that the chiral ligand does not need to be modified, but the recycling is highly solvent dependent—in contrast with the catalysts immobilized through covalent bonds.     

Asymmetric cyanohydrin synthesis using heterobimetallic catalysts obtained from titanium and vanadium complexes of chiral and achiral salen ligands

Titanium(IV)(salen) and vanadium(V)(salen) complexes are both known to form catalysts for asymmetric cyanohydrin synthesis. When a mixture of titanium and vanadium complexes derived from the same or different salen ligands is used for the asymmetric addition of trimethylsilyl cyanide to benzaldehyde, the absolute configuration of the product and level of asymmetric induction can only be explained by in situ formation of a catalytically active heterobimetallic complex, and is not consistent with two monometallic species acting cooperatively. Combined use of complexes containing chiral and achiral salen ligands demonstrates that during the asymmetry inducing step of the mechanism, the aldehyde is coordinated to the vanadium rather than the titanium ion. The titanium complexes also catalyse the asymmetric addition of ethyl cyanoformate to aldehydes, a reaction in which vanadium(V)(salen) complexes are not active. For this reaction, use of a mixture of titanium and vanadium(salen) complexes results in a complete loss of catalytic activity, a result which again can only be explained by in situ formation of a heterometallic complex. Both the titanium and vanadium based catalysts also induce the asymmetric addition of potassium cyanide/acetic anhydride to aldehydes. For this reaction, combined use of chiral and achiral complexes indicates that during the asymmetry inducing step of the mechanism, the aldehyde is coordinated to titanium rather than vanadium, a result which contrasts with the observed results when trimethylsilyl cyanide is used as the cyanide source.     

Semicorrin metal complexes as enantioselective catalysts. Part 2. Enantioselective cyclopropane formation from olefins with diazo compounds catalyzed by chiral (semicorrinato)copper complexes

Copper complexes of chiral, C₂-symmetric semicorrin ligands were found to be efficient catalysts for the cyclopropane formation from olefins with diazo compounds. In the presence of 1 mode-% of catalyst, alkyl diazoacetates reacted smoothly with terminal olefins such as styrene, butadiene, and 1-heptene to give the corresponding optically active cyclopropanecarboxylic-acid derivatives (Table 1 Scheme 2). With one of the catalysts, enantioselectivities up to 97% ee were obtained (Table 2). Usually, the reactions were carried out using bis(semicorrinato)copper(II) complexes as precatalysts. In order to produce active catalysts, these complexes had to be activated first by heating in the presence of diazoacetate or by treatment with phenylhydrazine. Experiments with (semicorrinato)copper(I) complexes, prepared in situ from copper(I) tert-butoxide (Scheme 4), suggest that the actual catalyst is a [mono(semicorrinato)]copper(I).