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The catalytic strategies developed so far for the synthesis of enantiomerically pure benzoins are reviewed. Particular attention is given to their substrate scope and limitations. Finally, this short review highlights the advantages of more atom-economic methods that have been reported recently.1 Introduction2 Benzoin Condensation2.1 Nucleophilic Carbenes as Catalysts2.2 Homocondensation of Aromatic Aldehydes2.3 Cross-Benzoin Condensation2.4 Acyloin Condensation2.5 Biocatalytic Methods3 Organocatalytic Friedel–Crafts Reaction4 Oxidative Methods4.1 α-Hydroxylation of Ketones4.2 Ketohydroxylation of Alkenes4.3 Enantiospecific Oxidation of meso-Hydrobenzoins4.4 Kinetic Resolution of Racemic Hydrobenzoins4.5 Biocatalytic Dynamic Kinetic Resolution5 Asymmetric Hemireduction of Benzils5.1 Biocatalytic Reductions – A Brief Summary5.2 Piers Hydrosilylation of Benzils5.3 Photoreduction of Benzils to Benzoins5.4 Metal-Catalyzed Hemihydrogenation of Benzils6 Conclusion and Future Challenges

The synthesis of enantiomerically pure benzoins by hydrogenation of readily available benzils has been long thwarted by their base-sensitivity. We show here that an iron(II) hydride complex catalyzes the asymmetric transfer hydrogenation of benzils from 2-propanol. When strictly base-free conditions are granted, excellent enantioselectivity is achieved even with o-substituted substrates, which are particularly challenging to prepare with other methods. Hence, under optimized reaction conditions, chiral benzoins were prepared in good yields (up to 83%) and excellent enantioselectivity (up to 98% ee) in short reaction times (30-75 min). Also, this work confirms that both enantiomers of the benzoin products can be accessed when a metal catalyst is used, which is a clear advantage over enzymatic methods.

We report here the tridentate, P-stereogenic, C2-symmetric PNP pincer ligand (SP,SP)-2,6-bis((cyclohexyl(methyl)phosphanyl)methyl)pyridine (1a) and its iron(II) complexes [FeBr2(CO)(1a)] (2a), [FeHBr(CO)(1a)] (3a), and [FeH2(CO)(1a)] (4a). In the presence of base, bromocarbonylhydride 3a catalyzes the hydrogenation of acetophenone to (S)-1-phenylethanol with 48% ee. The transition states of the enantiodetermining transfer of hydride from 3a to the carbonyl group of acetophenone were studied by density functional theory (DFT) with a full conformational analysis of the PNP ligand for the three different mechanistic models recently proposed for a related achiral catalyst. The DFT calculations show that the outer-sphere monohydride mechanism originally proposed by Milstein reproduces the experimentally observed sense of induction (S) and enantioselectivity, whereas the dihydride and inner-sphere pathways predict the formation of the R enantiomer.

The hydride isonitrile complex [FeH(CNCEt3)(1a)]BF4 (2) containing a chiral P2(NH)2 macrocycle (1a), in the presence of 2-propanol as hydrogen donor, catalyzes the base-free asymmetric transfer hydrogenation (ATH) of prostereogenic ketones to alcohols and the hemihydrogenation of benzils to benzoins, which contain a base-labile stereocenter. Benzoins are formed in up to 83% isolated yield with enantioselectivity reaching 95% ee. Ketones give the same enantioselectivity observed with the parent catalytic system [Fe(CNCEt3)2(1a)] (3a) that operates with added NaOtBu

The present account describes the development of chiral, C2-symmetric N/P macrocyclic ligands that attune to the size and electronic properties of the iron(II) ion to give robust complexes under catalysis conditions. This is not trivial, as the complexes of base metals are substantially less stable than those of precious metals. Also, as these N2P2 macrocycles feature stereogenic P atoms, the control of the stereochemistry at phosphorus is a key synthetic issue. Still, as the macrocyclic effect was insufficient to give robust catalysts under hydrogen transfer conditions, we had to dig deeper into the toolbox of coordination chemistry and use strong-field ancillary ligands other than CO, specifically isonitriles, which additionally offer a further handle to tune the catalyst. The reward was the discovery of the first iron(II) catalyst for the asymmetric transfer hydrogenation of polar double bonds that is highly enantioselective for a broad scope of substrates.

The bis(carbonyl) manganese(I) complex [Mn(CO)2 (1)]Br (2) with a chiral (NH)2 P2 macrocyclic ligand (1) catalyzes the asymmetric transfer hydrogenation of polar double bonds with 2-propanol as the hydrogen source. Ketones (43 substrates) are reduced to alcohols in high yields (up to >99 %) and with excellent enantioselectivities (90-99 % ee). A stereochemical model based on attractive CH-π interactions is proposed.

PNP pincer ligands stabilise diamagnetic base metal catalysts, and much effort has been invested in the development of chiral analogues for asymmetric catalysis. Starting from the conformational issues that affect P-stereogenic diphosphines, we extend the analysis to our recently prepared P-stereogenic PNP pincer ligands, which we used in the iron(II)-catalysed hydrogenation of ketones. Backbone rigidity and size (and shape) of the substituents at phosphorus are pivotal in both bi- and tridentate P-stereogenic ligands, and their interplay is discussed, as well as the contribution (and shortcomings) of DFT calculations to the understanding of the conformational flexibility and enantiodiscrimination., Chemical Communications, 55 (63), ISSN:1359-7345, ISSN:1364-548X

Hydride cis-β-[FeH(CNCEt3)(1)]BF4 (5) (1 is a chiral N2P2 macrocycle) is the catalytically active species in the asymmetric transfer hydrogenation of ketones formed upon reaction of [Fe(CNCEt3)2(1)](BF4)2 (3) with base. Stoichiometric reactions show that hydride 5 is formed by H-elimination from the 2-propoxo complex [Fe(O iPr)(CNCEt3)(1)]BF4 (8a) and inserts the C═O bond of acetophenone to give the diastereoisomeric alcoholato complexes [Fe(OCH(Me)Ph)(CNCEt3)(1)]BF4 (10R and 10S). Complexes 5, 8a, and 10 were characterized by NMR spectroscopy, and their structures were calculated by DFT. The DFT study supports a bifunctional mechanism with the alkoxo complexes 8a and 10 as resting species. The stereochemical model reproduces the high enantioselectivity with acetophenone, which results from the combination of the rigid macrocyclic scaffold with the bulky, yet conformationally flexible isonitrile.

We describe herein a new protocol for the synthesis of 2,2′-((1S,1′S)-ethane-1,2-diylbis(phenylphosphanediyl))dibenzaldehyde ((SP,SP)-5), which is the key intermediate in the synthesis of macrocyclic iron(II)/(NH)2P2 catalysts for the highly enantioselective transfer hydrogenation of polar double bonds. The dialdehyde (SP,SP)-5 was obtained as a single diastereoisomer and enantiomer from an optically pure H-phosphinate in 33% yield over five steps. It was further converted to afford multigram quantities of the macrocyclic iron(II)/(NH)2P2 complexes, which were tested in the asymmetric transfer hydrogenation of aryl alkyl ketones and imines in 2-propanol on a 100 mmol scale. Ten substrates, including challenging ones such as tert-butyl phenyl ketone and industrially relevant molecules such as 3,5-bis(trifluoromethyl)acetophenone, were reduced in high yield (89.0–99.7%), excellent enantioselectivity (95.8–99.4% ee), and with low catalyst loadings (S/C up to 10 000/1).

In this account, we summarize our recent work on the preparation of C 2-symmetric macrocyclic N2P2 and open-chain NPPN iron(II) complexes. The investigation of these new ligand classes is enabled by the development of a synthetic route to the key enantiopure, P-stereogenic dialdehyde. Macrocyclization with a diamine affords N2P2 ligands, the iron(II) complexes of which are highly active and selective precatalysts for the asymmetric transfer hydrogenation of ketones. Condensation with monoamines affords open-chain NPPN ligands, of which the highly azaphilic iron(II) complexes are tested in the asymmetric Strecker reaction. 1 Introduction 1.1 Iron Complexes: The Issue of Stability 1.2 Open-Chain PNNP Ligands 1.3 Goals and Motivation 2 N2P2 Macrocycles 2.1 The Key C 2-Symmetric Synthon 2.2 C 2-Symmetric N2P2 Macrocycles 3 Macrocyclic Iron(II) Complexes 3.1 Bis(acetonitrile) N2P2 Complexes 3.2 Bis(acetonitrile) (NH)2P2 Complexes 3.3 Enlarging the Toolbox: Isonitrile (CNR) Ligands 3.4 Asymmetric Transfer Hydrogenation Fe/N2P2 4 Chiral Open-Chain NPPN Ligands 4.1 Synthesis of Bis(acetonitrile) Complexes 4.2 Complex Derivatization 4.3 Asymmetric Transfer Hydrogenation Fe/NPPN 4.4 Strecker Reaction of Azomethine Imines 5 Conclusion and Outlook

A generalized protocol for the synthesis of chiral (NH)2P2 macrocycles allows changing the linker between the phosphines and gives access to a family of such ligands, as demonstrated for the propane-1,3-diyl analogue. The corresponding complexes based on earth-abundant and nontoxic iron were applied as catalysts in the asymmetric transfer hydrogenation of polar double bonds. Thanks to the ligand modularity and to the use of tunable isonitriles as ancillary ligands, the catalyst system can be individually optimized for each substrate to give high enantioselectivity (up to 99.9% conversion and 99.6% ee, TOF up to >3950 h–1) for a broad scope of 26 substrates.

The open chain NPPN ligand (1S1'S) 11' ((ethane 12 diylbis(phenylphosphanediyl))bis(21 phenylene))bis(N cyclohexylmethanimine) (1) was prepared by condensation of cyclohexylamine with enantiomerically pure (1S1'S) 22' (ethane 12 diylbis(phenylphosphanediyl))dibenzaldehyde ((SS) 6). Ligand 1 coordinates to [Fe(OH2)(6)](BF4)(2) or [Fe(MeCN)(6)](SbF6)(2) in acetonitrile to give the dicationic complex [Fe(MeCN)(2)(1)](X)(2) (2) (X = BF4 or SbF6). The corresponding carbonyl (3) bromocarbonyl (4) and bis(tert butylisonitrile) (5) derivatives were prepared and fully characterized. Complex 2 reacts with Me3SiCN to give the corresponding trimethylsilyl isocyanide derivative 18 featuring a Fe CNSiMe3 linkage. The X ray structures of 2 3 5 and 18 show that ligand 1 assumes the Lambda cis alpha geometry which allows comparing the trans influence of these ligands. Complexes 2 3 5 and 18 were applied in the asymmetric addition of trimethylsilyl cyanide to azomethine imines (Strecker reaction) whose enantioselectivity reached 22 ee. The low enantioselectivity can be explained on the basis of the Me3SiCN/Me3SiNC isomerization and of the reaction product partially displacing the NPPN ligand from iron

The previously reported silica-grafted iridium(III) hydride complex [IrH(O–SBA-15)(POCOP)] (2), prepared by treating [IrH2(POCOP)] (1) (POCOP is 1,3-bis((di-tert-butylphosphino)oxy)benzene) with SBA-15 (mesoporous silica), hydrogenates liquid alkenes (1-decene, trans-5-decene, cyclohexene, styrene, and 4-phenyl-1-butene) at room temperature and under 1 atm H2. Internal alkenes react at a lower rate than the terminal ones. For the sake of comparison, the hydrogenation of the same substrates was studied with the homogeneous catalyst [IrH2(POCOP)] (1). The heterogeneous catalyst 2 hydrogenates 1-decene, cyclohexene, and 4-phenyl-1-butene faster than 1, whereas the opposite is true for styrene and trans-5-decene, which suggests that different active species are involved in the heterogeneous and homogeneous reactions. Catalysis by a truly heterogeneous species is supported by a series of “hot filtration tests”. NMR spectroscopic studies showed that 2 does not undergo degrafting upon longer exposure to alkenes compared to ethene under hydrogenation conditions.

Bis(isonitrile) iron(II) complexes bearing a C2 -symmetric diamino (NH)2 P2 macrocyclic ligand efficiently catalyze the hydrogenation of polar bonds of a broad scope of substrates (ketones, enones, and imines) in high yield (up to 99.5 %), excellent enantioselectivity (up to 99 % ee), and with low catalyst loading (generally 0.1 mol %). The catalyst can be easily tuned by modifying the substituents of the isonitrile ligand.

The hydride isonitrile complex [FeH(CNCEt

The N2P2 macrocyclic ligands (5S8S13E14aS18aS19E) 58 diphenyl 567814a1516171818a decahydrotribenzo[bfl][14811]diazadiphosphacyclotetradecine ((1S4S9S10S) 1a) and (5E7R8R9E15S18S) 781518 tetraphenyl 7815161718 hexahydrodibenzo[fl][14811]diazadiphosphacyclotetradecine ((1S4S9R10R) 1b) were prepared by condensing the new enantiomerically pure synthon 22' ((1S1'S) ethane 12 diylbis(phenylphosphinediyl))dibenzaldehyde ((SS) 8) prepared in six steps from (2R4S5R) 34 dimethyl 25 diphenyl 132 oxazaphospholidine 2 borane (3)) with (1S2S) cyclohexane 12 diamine and (1R2R) 12 diphenylethane 12 diamine under high dilution conditions. The opposite enantiomers of the diamines gave oligomeric products. The stereospecificity of the macrocyclization reaction is explained by conformational analysis based on the X ray structures of (1S4S9S10S) 1a and (1S4S9R10R) 1b. The corresponding diamino macrocycles (1S4S9S10S) 2a and (1S4S9R10R) 2b were prepared by reduction of the imine moiety of (1S4S9S10S) 1a and (1S4S9R10R) 1b respectively. Macrocycles (1S4S9S10S) 1a (1S4S9R10R) 1b and (1S4S9S10S) 2a react with [Fe(OH2)6](BF4)2 in acetonitrile to give the corresponding stable diamagnetic bis(acetonitrile) complexes [Fe(MeCN)2(1)](BF4)2 (9a and 9b) and [Fe(MeCN)2(2a)](BF4)2 (10a). Complex 9a exists as a 3:1 mixture of trans and ? cis ß isomers whereas 9b and 10a adopt the ? cis ß configuration exclusively. The bis(acetonitrile) complexes are versatile precursors and were used to prepare the bromocarbonyl analogues [FeBr(CO)(1)]BPh4 (11a and 11b).

Bis(isonitrile) iron(II) complexes bearing a C2-symmetric N2P2 macrocyclic ligand, which are easily prepared from the corresponding bis(acetonitrile) analogue, catalyze the asymmetric transfer hydrogenation (ATH) of a broad scope of ketones in excellent yields (up to 98%) and with high enantioselectivity (up to 91% ee).

Transition metal (TM) ions can be covalently attached to a silica surface by means of a M–OSi bond (grafting) to give site-isolated complexes. Acting as a ligand in such grafted species, the surface influences the electronic nature and the coordination number of the TM ion, in contrast to anchored complexes, in which a linker separates the metal ion and the surface and hence reduces their interaction. This perspective reviews general grafting strategies on an oxide surface and focuses on the peculiarity of silica-grafted catalysts of late TMs, which have a higher tendency to undergo aggregation into metal nanoparticles under reducing conditions than early TMs. Soft ligands (hydride, phosphines) and/or π-acceptors (CO) promote aggregation. In particular, silica-grafted hydride complexes of late TM ions are not stable and tend to undergo reductive elimination of silanol. The coordination chemistry background of this behaviour is discussed, and possible strategies to avoid aggregation are suggested.

The ruthenium/PNNP complexes [RuCl(Et2O)(PNNP)]Y (Y = PF6 4PF6; BF4 4BF4; or SbF6 4SbF6) (10 mol ) catalyze the enantioselective aziridination of imines with ethyl diazoacetate (EDA) as carbene source (PNNP = (1S2S) NN' bis[o (diphenylphosphino)benzylidene]cyclohexane 12 diamine). The highest enantioselectivity was obtained with 4SbF6 which aziridinated N benzylidene 11 diphenylmethanamine (5a) to cis ethyl 1 benzhydryl 3 phenylaziridine 2 carboxylate (cis 6a) with 93 ee at 0 °C. To the best of our knowledge this is the highest enantioselectivity ever obtained in transition metal catalyzed asymmetric aziridination. Aziridine yields were overall moderate to low (up to 33 isolated yield of the cis isomer) because of the competitive formation of diethyl maleate (7). The scope of the catalyst was studied with p and m substituted imines. NMR spectroscopic studies with 13C and 15N labeled EDA indicate that aziridine 6a is formed by carbene transfer from an EDA complex [RuCl(EDA)(PNNP)]PF6 (8) to the imine. The observation of a dinitrogen complex (9) gives further support to this mechanism. The EDA adduct 8 decomposes to the carbene complex [RuCl(CHCO2Et)(PNNP)]+ (10) whose reaction with EDA gives diethyl maleate. This unprecedented mechanism is rationalized on the basis of the nucleophilic nature of diazoalkanes which is enhanced by coordination to a p back donating metal such as ruthenium(II).

The first phosphorus-free, air- and moisture-tolerant Ruthenium-catalyzed asymmetric hydrogenation of methylketones and cyclic ketones, as well as aldehydes using hydrogen is presented.

We describe herein a new protocol for the synthesis of 2,2′-((1S,1′S)-ethane-1,2-diylbis(phenylphosphanediyl))dibenzaldehyde ((SP,SP)-5), which is the key intermediate in the synthesis of macrocyclic iron(II)/(NH)2P2 catalysts for the highly enantioselective transfer hydrogenation of polar double bonds. The dialdehyde (SP,SP)-5 was obtained as a single diastereoisomer and enantiomer from an optically pure H-phosphinate in 33% yield over five steps. It was further converted to afford multigram quantities of the macrocyclic iron(II)/(NH)2P2 complexes, which were tested in the asymmetric transfer hydrogenation of aryl alkyl ketones and imines in 2-propanol on a 100 mmol scale. Ten substrates, including challenging ones such as tert-butyl phenyl ketone and industrially relevant molecules such as 3,5-bis(trifluoromethyl)acetophenone, were reduced in high yield (89.0–99.7%), excellent enantioselectivity (95.8–99.4% ee), and with low catalyst loadings (S/C up to 10 000/1).

Although alkylidene β-ketoesters have been known since the beginning of the 20th century, they have only recently attracted the attention of the synthetic organic community. In these versatile electrophiles, the enone double bond is strongly polarized because of the presence of the ester moiety as a second electron-withdrawing group. The resulting increase in reactivity has recently opened the way to a number of challenging transformations in the field of enantioselective catalysis, but is also responsible for their high sensitivity toward acids, bases, and high temperature, which hampered their application in organic synthesis for a long time. The saturated β-ketoester moiety generated in these reactions has been used for diverse functionalization reactions, some of which lead to quaternary stereocenters. This perspective highlights the origins of alkylidene β-ketoester chemistry and focuses on recent developments of the quickly evolving field of asymmetric catalytic reactions of these substrates. After ...

Hydride abstraction from the β-position of the enolato ligand of the previously reported complex [Ru(3a-H)(PNNP)]PF(6) (5a; 3a-H is the enolate of 2-tert-butoxycarbonylcyclopentanone) with (Ph(3)C)PF(6) gives the dicationic complex [Ru(6a)(PNNP)](2+) (7a) as a single diastereoisomer, which contains the unsaturated β-ketoester 2-tert-butoxycarbonyl-2-cyclopenten-1-one (6a) as a chelating ligand. The methyl analogue 2-methoxycarbonylcyclopentanone (3b) gives [Ru(3b-H)(PNNP)]PF(6) as a mixture of noninterconverting diastereoisomers (ester group of 3b trans to P, 5b; or to N, 5c), which were separated by column chromatography. Hydride abstraction from 5b (or 5c) yields diastereomerically pure [Ru(6b)(PNNP)](2+) (7b or 7c). Complexes 7b and 7c do not interconvert at room temperature in CD(2)Cl(2) and form opposite enantiomers of the Diels-Alder adduct upon reaction with Dane's diene (1 equiv). X-ray studies of 7a, 5b, and 5c give insight into the origin of enantioselection and the sense of asymmetric induction in the previously reported asymmetric Diels-Alder and Ficini cycloaddition reactions with 2,3-disubstituted butadienes and ynamides, respectively. Stoichiometric reactions (substrate coordination, cycloaddition, and product displacement) between [Ru(OEt(2))(2)(PNNP)](2+) (2), 6b (or 6a), and Dane's diene (15, to give estrone derivatives) or N-benzyl-N-(cyclohexylethynyl)-4-methylbenzenesulfonamide (17, to give cyclobutenamides) suggest that product displacement from the catalyst is turnover limiting.

Addition of water to CD2Cl2 solutions of the five-coordinate complex [RuCl(PNNP)]PF6 (2) (formed by treating [RuCl2(PNNP)] (1) with TlPF6 (1 equiv) in dichloromethane) yields the aqua complexes cis-β-[RuCl(OH2)(PNNP)]PF6 (3) and trans-[RuCl(OH2)(PNNP)]PF6 (4) in a 6:1 ratio (PNNP is the chiral ligand (1S,2S)-N,N′-bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine). An excess of H2O (10 equiv) is needed for quantitative conversion of 2. The cis-β isomer 3 is more labile than the trans analogue 4 and is in slow-regime chemical exchange with five-coordinate 2 in CD2Cl2. The trans aqua complex 4 does not dissociate in CDCl3 over 6 h, but undergoes rapid equilibration with 2 and 3 upon dissolution in CD2Cl2. The X-ray structure of (Λ)-cis-β-3 shows an intramolecular hydrogen bond between the aqua and the chloro ligands. The elusive Et2O adduct [RuCl(OEt2)(PNNP)]PF6 (5), obtained by treating 1 with (Et3O)PF6 (1 equiv) in dichloromethane, is formed as the cis isomer only. At room temperature, 5 exists ...

The synthesis, characterization, and application in asymmetric catalytic cyclopropanation of Rh(III) and Ir(III) complexes containing (Sa,RC,RC)-O,O′-[1,1′-binaphthyl-2,2′-diyl]-N,N′-bis[1-phenyl-ethyl]phosphoramidite (1) are reported. The X-ray structures of the half-sandwich complexes [MCl2(C5Me5)(1,κP)] (M = Rh, 2a; M = Ir, 2b) show that the metal–phosphoramidite bond is significantly shorter in the Ir(III) analog. Chloride abstraction from 2a (with CF3SO3SiMe3 or with CF3SO3Me) and from 2b (with AgSbF6) gives the cationic species [MCl(C5Me5)(1,2-η-1,κP)]+ (M = Rh, 3a; M = Ir, 3b), which display a secondary interaction between the metal and a dangling phenethyl group (NCH(CH3)Ph) of the phosphoramidite ligand, as indicated by NMR spectroscopic studies. Complexes 3a and 3b slowly decompose in solution. In the case of 3b, the binuclear species [Ir2Cl3(C5Me5)2]+ is slowly formed, as indicated by an X-ray study. Preliminary catalytic tests showed that 3a cyclopropanates styrene with moderate yield (35%) and diastereoselectivity (70:30 trans:cis ratio) and with 32% ee (for the trans isomer).

The like and unlike isomers of phosphoramidite (P*) ligands are found to react differently with iridium(I), which is a key to explaining the apparently inconsistent results obtained by us and other research groups in a variety of catalytic reactions. Thus, the unlike diastereoisomer (aR,S,S)-[IrCl(cod)(1 a)] (2 a; cod=1,5-cyclooctadiene, 1 a=(aR,S,S)-(1,1'-binaphthalene)-2,2'-diyl bis(1-phenylethyl)phosphoramidite) forms, upon chloride abstraction, the monosubstituted complex (aR,S,S)-[Ir(cod)(1,2-eta-1 a,kappaP)](+) (3 a), which contains a chelating P* ligand that features an eta(2) interaction between a dangling phenyl group and iridium. Under analogous conditions, the like analogue (aR,R,R)-1 a' gives the disubstituted species (aR,R,R)-[Ir(cod)(1 a',kappaP)(2)](+) (4 a') with monodentate P* ligands. The structure of 3 a was assessed by a combination of X-ray and NMR spectroscopic studies, which indicate that it is the configuration of the binaphthol moiety (and not that of the dangling benzyl N groups) that determines the configuration of the complex. The effect of the relative configuration of the P* ligand on its iridium(I) coordination chemistry is discussed in the context of our preliminary catalytic results and of apparently random results obtained by other groups in the iridium(I)-catalyzed asymmetric allylic alkylation of allylic acetates and in rhodium(I)-catalyzed asymmetric cycloaddition reactions. Further studies with the unlike ligand (aS,R,R)-(1,1'-binaphthalene)-2,2'-diyl bis{[1-(1-naphthalene-1-yl)ethyl]phosphoramidite} (1 b) showed a yet different coordination mode, that is, the eta(4)-arene-metal interaction in (aS,R,R)-[Ir(cod)(1,2,3,4-eta-1 b,kappaP)](+) (3 b).

During attempts directed at epoxide ring opening with different HF sources, we discovered that the Lewis acidic [RuCl(PNNP)] + ( 1 ) or [Ru(OEt 2 ) 2 (PNNP)] 2+ ( 2 ) catalysts promote the [1,2]-phenyl shift (Meinwald rearrangement) in phenyl-substituted epoxides to give the corresponding 2-alkylphenylacetaldehydes, which are fluorinated at the α-position in the presence of silver bifluoride (AgHF 2 ) (PNNP is (1 S, 2 S )- N,N ′-bis[ o -(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine). The optimization of aldehyde fluorination with PhCH(R)CHO (R = Me, Et, i Pr, t Bu) as substrates showed that catalyst [ 1 ]SbF 6 gives a moderate degree of enantioselectivity (up to 27% ee) and 35% yield. The substrate scope is limited to benzylic aldehydes. The reaction is unprecedented for transition metal catalysts. Circumstantial evidence suggests that the mechanism involves chemical oxidation followed by enantioselective fluorination with F − .

The chiral dicationic complexes [Ru(4a)(PNNP)]2+ (2a) and [Ru(4h)(PNNP)]2+ (2h, PNNP is (1S,2S)-N,N′-bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine), containing non-enolized 2-tert-butoxycarbonylcyclopentanone (4a) or α-acetyl-N-benzyl-δ-valerolactam (4h), were prepared from [RuCl2(PNNP)] (1) by double chloride abstraction with (Et3O)PF6, followed by reaction with the 1,3-dicarbonyl compound. The estimated aqueous pKa values for 2a (3.3 ± 0.3) and 2h (4.7 ± 0.1) were determined by deprotonation with Ph2NH and pyridine, respectively. The corresponding monocationic enolato complexes 3a and 3h were isolated and structurally characterized. Complex 2a catalyzes the 1,4-addition of 4a to methyl vinyl ketone with up to 93% ee when a CH2Cl2/Et2O (1:1) solvent mixture is used. Oxygen-containing cosolvents enhance both the rate and enantioselectivity of the Michael addition with a number of substrates. This observation is discussed in the context of our previous observation that the combined addition ...

A study of the secondary interactions involving a pendant phenethyl group (NCH(CH3)Ph) of (Sa,RC,RC)-O,O′-[1,1′-binaphthyl-2,2′-diyl]-N,N′-bis[1-phenylethyl]phosphoramidite (1) in d8 and d6 metal complexes is reported. A 1,2-η-phenyl interaction is found in [Pd(η3-allyl)(1,2-η-Ph-1-κP)]+ (3), as indicated by NMR spectroscopy and by an X-ray study. The Rh(I) complexes [RhCl(NBD)(1-κP)] (4a) and [RhCl(COD)(1-κP)] (4b) were prepared, and their X-ray structures were determined. Chloride abstraction from 4a and 4b gave the corresponding cations [Rh(NBD)(1,2-η-Ph-1-κP)]+ (5a) and [Rh(COD)(1,2-η-Ph-1-κP)]+ (5b), which were fully characterized. Besides NMR spectroscopic data, the 1,2-η-phenyl interaction is supported by an X-ray study of 5a. Upon standing in MeOH solution, 5b undergoes loss of the COD and forms [Rh(1-κP)(η6-Ph-1-κP)]+ (6), whose η6-arene coordination was studied by NMR spectroscopy and by X-ray diffraction. For the sake of comparison with the d8 complex 6, the crystal structure of the previously ...

Upon activation with (Et3O)PF6 (2 equiv), the ruthenium(II) complex [RuCl2(PNNP)] (1; PNNP is (1S,2S)-N,N‘-bis(o-(diphenylphosphino)benzylidene)cyclohexane-1,2-diamine) catalyzes the electrophilic ...

Several iron complexes are presented with the pictured compound FEC being one of the most promising catalysts for the title reaction.

Bis(isonitrile) iron(II) complexes bearing a C2-symmetric N2P2 macrocyclic ligand, which are easily prepared from the corresponding bis(acetonitrile) analogue, catalyze the asymmetric transfer hydrogenation (ATH) of a broad scope of ketones in excellent yields (up to 98%) and with high enantioselectivity (up to 91% ee).

Chloride abstraction from the half-sandwich complexes [RuCl2(η6-p-cymene)(P*-κP)] (2a: P* = (Sa,R,R)-1a = (1Sa)-[1,1′-binaphthalene]-2,2′-diyl bis[(1R)-1-phenylethyl)]phosphoramidite; 2b: P* = (Sa,R,R)-1b = (1Sa)-[1,1′-binaphthalene]-2,2′-diyl bis[(1R)-(1-(1-naphthalen-1-yl)ethyl]phosphoramidite) with (Et3O)[PF6] or Tl[PF6] gives the cationic, 18-electron complexes dichloro(η6-p-cymene){(1Sa)-[1,1′-binaphthalene]-2,2′-diyl {(1R)-1-[(1,2-η)-phenyl]ethyl}[(1R)-1-phenylethyl]phosphoramidite-κP}ruthenium(II) hexafluorophosphate (3a) and [Ru(S)]-dichloro(η6-p-cymene){(1Sa)-[1,1′-binaphthalene]-2,2′-diyl {(1R)-1-[(1,2-η)-naphthalen-1-yl]ethyl}[(1R)-1-(naphthalen-1-yl)ethyl]phosphoramidite-κP)ruthenium(II) hexafluorophosphate (3b), which feature the η2-coordination of one aryl substituent of the phosphoramidite ligand, as indicated by 1H-, 13C-, and 31P-NMR spectroscopy and confirmed by an X-ray study of 3b. Additionally, the dissociation of p-cymene from 2a and 3a gives dichloro{(1Sa)-[1,1′-binaphthalene]-2,2′-diyl [(1R)-(1-(η6-phenyl)ethyl][(1R)-1-phenylethyl]phosphoramidite-κP)ruthenium(II) (4a) and di-μ-chlorobis{(1Sa)-[1,1′-binaphthalene]-2,2′-diyl [(1R)-1-(η6-phenyl)ethyl][(1R)-1-phenylethyl]phosphoramidite-κP}diruthenium(II) bis(hexafluorophosphate) (5a), respectively, in which one phenyl group of the N-substituents is η6-coordinated to the Ru-center. Complexes 3a and 3b catalyze the asymmetric cyclopropanation of α-methylstyrene with ethyl diazoacetate with up to 86 and 87% ee for the cis- and the trans-isomers, respectively.

’. This may surprise more than one, considering thathe has dedicated his whole scientific life to the study of transition metals, with a strongovertone on polymerization. However, the statement sounds perfectly logical if oneconsiders that the goal of his research was the understanding of mechanistic detailsof stereoselective transformations of organic molecules induced by transition-metalcomplexes, and that a polymer is nothing else than the repetition of a small molecule.Indeed, browsing through his more than 150 publications, one central and fundamentaltopic emerges as

The dichloro complexes [RuCl2(PNNP)] (2a−c) undergo double chloride abstraction when treated with AgSbF6 (2 equiv) (PNNP is one of (1S,2S)-N,N‘-bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine, 1a; (1S,2S)-N,N‘-bis[o-(bis(4-trifluoromethylphenyl)phosphino)benzylidene]cyclohexane-1,2-diamine, 1b; (1S,2S)-N,N‘-bis[o-(bis(3,5-bis(trifluoromethyl)phenyl)phosphino)benzylidene]cyclohexane-1,2-diamine, 1c). The resulting elusive species form the corresponding bis(aqua) complexes [Ru(OH2)2(PNNP)]2+ by reaction with water. The bis(aqua) complexes [Ru(OH2)2(PNNP)](SbF6)2 (6a−c) were fully characterized, including an X-ray structure of 6c. The X-ray structure of 2c with the new electron-poor chiral tetradentate PNNP ligand 1c is also reported. Complexes 6a and 6c catalyze the cis-selective asymmetric cyclopropanation of styrene, albeit with low yield. The best cis:trans ratio and enantioselectivity were obtained with 6c (87:13 and 92% ee for the cis isomer, respectively). The products of double chloride ...