Monomeric Rhodium(II) Catalysts for the Preparation of Aziridines and Enantioselective Formation of Cyclopropanes from Ethyl Diazoacetate at Room Temperature

Silver(I)-Catalyzed Oxidative Cyclopropanation of 1,6-Enynes: Synthesis of 3-Aza-bicyclo[3.1.0]hexane Derivatives

A simple Ag(I)-catalyzed oxidative cyclopropanation of heteroatom-tethered 1,6-enynes for the establishment of valuable functionalized 3-aza-bicyclo[3.1.0]hexane is presented, which allows the formation of multiple chemical bonds in one step under 20 mol % silver(I) catalysts and air conditions. This approach is highly atom economical, easy to perform, and free of external oxidants and features good to excellent yields and gram-scale synthesis. The preliminary study showed that an uncommon silver carbenoid intermediate might be involved in this process.

3-Arylaziridine-2-carboxylic Acid Derivatives and (3-Arylaziridin-2-yl)ketones: The Aziridination Approaches

Aziridination reactions represent a powerful tool in aziridine synthesis. Significant progress has been achieved in this field in the last decades, whereas highly functionalized aziridines including 3-arylated aziridine-2-carbonyl compounds play an important role in both medical and synthetic chemistry. For the reasons listed, in the current review we have focused on the ways to obtain 3-arylated aziridines and on the recent advances (mainly since the year 2000) in the methodology of the synthesis of these compounds via aziridination.

Redox noninnocence of carbene ligands: carbene radicals in (catalytic) C-C bond formation

In this Forum contribution, we highlight the radical-type reactivities of one-electron-reduced Fischer-type carbenes. Carbene complexes of group 6 transition metals (Cr, Mo, and W) can be relatively easily reduced by an external reducing agent, leading to one-electron reduction of the carbene ligand moiety. This leads to the formation of "carbene-radical" ligands, showing typical radical-type reactivities. Fischer-type carbene ligands are thus clearly redox-active and can behave as so-called "redox noninnocent ligands". The "redox noninnocence" of Fischer-type carbene ligands is most clearly illustrated at group 9 transition metals in the oxidation state II+ (Co(II), Rh(II), and Ir(II)). In such carbene complexes, the metal effectively reduces the carbene ligand by one electron in an intramolecular redox process. As a result, the thus formed "carbene radicals" undergo a variety of radical-type C-C and C-H bond formations. The redox noninnocence of Fischer-type carbene ligands is not just a chemical curiosity but, in fact, plays an essential role in catalytic cyclopropanation reactions by cobalt(II) porphyrins. This has led to the successful development of new chiral cobalt(II) porphyrins as highly effective catalysts for asymmetric cyclopropanation with unprecedented reactivity and stereocontrol. The redox noninnocence of the carbene intermediates results in the formation of carbene-radical ligands with nucleophilic character, which explains their effectiveness in the cyclopropanation of electron-deficient olefins and their reduced tendency to mediate carbene dimerization. To the best of our knowledge, this represents the first example in which the redox noninnocence of a reacting ligand plays a key role in a catalytic organometallic reaction. This Forum contribution ends with an outlook on further potential applications of one-electron-activated Fischer-type carbenes in new catalytic reactions.

Pincer oxazolines: emerging tools in coordination chemistry and catalysis--where to next?

A brief overview of the coordination chemistry aspects of pincer ligands and their complexes containing at least one oxazoline (i.e., 4,5-dihydro-2-oxazole) unit is presented. This historical perspective is placed into a context of the possible future direction(s) in this particular arena of pincer chemistry. These ideas are compared and contrasted to the overall direction of pincer chemistry since the first such complexes were reported in the 1970s.

Chemo-, regio-, and diastereoselectivity preferences in the reaction of a sulfur ylide with a dienal and an enone

Mechanistic insights into an interesting class of reaction between sulfur ylides with (i) a dienal, and (ii) an enone, obtained by using density functional theory, is reported. The kinetic and thermodynamic factors responsible for chemo-, regio-, and diastereoselectivities are established by identifying all key transition states and intermediates along the reaction pathway for 1,2-, 1,4-, and 1,6- modes of attack of dimethylsulfonium benzylide to 5-phenylpenta-2,4-dienal. The reaction profiles for 1,2- and 1,4- modes of addition are also evaluated for the reaction between dimethylsulfonium benzylide and pent-3-en-2-one. Our results show that the final outcome of the reaction with both these substrates would be decided by the interplay between kinetic and thermodynamic factors. It is found that the addition of a semi-stabilized ylide to conjugated carbonyl compounds prefers to proceed through a 1,4- (conjugate) pathway under thermodynamic conditions, which is in accordance with the available experimental reports. However, the formation of epoxides via a 1,2- (direct) addition pathway is computed to be equally competitive, which could be the favored pathway under kinetic conditions. Even though the lower barrier for the initial addition step is kinetically advantageous for the direct (or 1,2-) addition pathway, the higher energy of the betaine intermediates--as well as the reversibility of the accompanying elementary step--may disfavor product formation in this route. Thus, high diastereoselectivity in favor of 2,3-trans cyclopropanecarbaldehyde is predicted in the case of the dienal, using the most favored conjugate addition (1,4-addition) pathway. Along similar lines, ylide addition to the enone is identified to exhibit a preference toward conjugate addition over direct (1,2-) addition. The importance of transition state analysis in delineating the controlling factors towards product distribution and diastereoselectivity is established.

Seeking passe-partout in the catalytic asymmetric aziridination of imines: evolving toward substrate generality for a single chemzyme

The asymmetric catalytic aziridination reaction (AZ reaction) of imines derived from dianisylmethyl (DAM) amine and tetra-methyldianisylmethyl (MEDAM) amine were examined with boroxinate catalysts prepared from both the VANOL and VAPOL ligands. This included an evaluation of different protocols for the preparation of the catalyst. The AZ reaction of DAM and MEDAM imines prepared from nine different aryl and aliphatic aldehydes were examined. The MEDAM imines were superior to the DAM imines in the AZ reaction, giving much higher asymmetric inductions and higher overall yields of aziridines. The MEDAM imines were found to also be superior to the previously studied diphenylmethyl (benzhydryl or Bh) and tetra-tert-butyldianisylmethyl (BUDAM) imines especially for imines derived from aliphatic aldehydes. The average asymmetric induction over the nine different MEDAM imines studied was 97% ee with the VAPOL catalyst and 96% ee with the VANOL catalyst. The MEDAM imines can be deprotected to give N-H aziridines in all cases except for some electron-rich aryl aldehydes. The MEDAM imines are much more reactive than benzhydryl imines, and this was most evident when a diazoacetate ester is replaced by a diazoacetamide. The less reactive diazoacetamides give very low yields in their reactions with benzhydryl imines but high yields with MEDAM imines.

Trimethylsilyldiazomethane as a versatile stitching agent for the introduction of aziridines into functionalized organic molecules

A highly enantioselective route for the introduction of aziridines into functionalized organic molecules was developed via a tandem acylation and aziridination of TMSCHN(2).

Evidence for a boroxinate based Brønsted acid derivative of VAPOL as the active catalyst in the catalytic asymmetric aziridination reaction

Studies are described that were designed to determine the structure of the active catalyst in the asymmetric catalytic aziridination of imines with ethyl diazoacetate (AZ reaction). Evidence suggests that the active catalyst contains a boroxine ring in which one of the three boron atoms is spiro-fused with the two phenol groups of the VAPOL ligand. (11)B and (1)H NMR evidence supports the boroxinate structure B in which the counterion to the boroxinate is the protonated form of the imine. The boroxinate structure is also supported by two solid state structures of a VAPOL boroxinate in which the gegen cation is tetramethyl ammonium and 4-dimethylaminopyridinium.

Selective C-C coupling of Ir-ethene and Ir-carbenoid radicals

The reactivity of the paramagnetic iridium(II) complex [Ir(II)(ethene)(Me(3)tpa)](2+) (1) (Me(3)tpa=N,N,N-tris(6-methyl-2-pyridylmethyl) amine) towards the diazo compounds ethyl diazoacetate (EDA) and trimethylsilyldiazomethane (TMSDM) was investigated. The reaction with EDA gave rise to selective C--C bond formation, most likely through radical coupling of the Ir-carbenoid radical species [Ir(III){CH.(COOEt)}(MeCN)(Me(3)tpa)](2+) (7) and (the MeCN adduct of) 1, to give the tetracationic dinuclear complex [(MeCN)(Me(3)tpa)Ir(III){CH(COOEt)CH(2)CH(2)}Ir(III)(MeCN)(Me(3)tpa)](2+) (4). The analogous reaction with TMSDM leads to the mononuclear dicationic species [Ir(III){CH(2)(SiMe(3))}(MeCN)(Me(3)tpa)](2+) (11). This reaction probably involves a hydrogen-atom abstraction from TMSDM by the intermediate Ir-carbenoid radical species [Ir(III){CH.(SiMe(3))}(MeCN)(Me(3)tpa)](2+) (10). DFT calculations support pathways proceeding via these Ir-carbenoid radicals. The carbenoid-radical species are actually carbon-centered ligand radicals, with an electronic structure best described as one-electron-reduced Fischer-type carbenes. To our knowledge, this paper represents the first reactivity study of a mononuclear Ir(II) species towards diazo compounds.

Mononuclear Rh(II) PNP-Type Complexes. Structure and Reactivity

The Rh(II) mononuclear complexes [(PNPtBu)RhCl][BF4] (2), [(PNPtBu)Rh(OC(O)CF3)][OC(O)CF3] (4), and [(PNPtBu)Rh(acetone)][BF4]2 (6) were synthesized by oxidation of the corresponding Rh(I) analogs with silver salts. On the other hand, treatment of (PNPtBu)RhCl with AgOC(O)CF3 led only to chloride abstraction, with no oxidation. 2 and 6 were characterized by X-ray diffraction, EPR, cyclic voltammetry, and dipole moment measurements. 2 and 6 react with NO gas to give the diamagnetic complexes [(PNPtBu)Rh(NO)Cl][BF4] (7) and [(PNPtBu)Rh(NO)(acetone)][BF4]2 (8) respectively. 6 is reduced to Rh(I) in the presence of phosphines, CO, or isonitriles to give the Rh(I) complexes [(PNPtBu)Rh(PR3)][BF4] (11, 12) (R = Et, Ph), [(PNPtBu)Rh(CO)][BF4] (13) and [(PNPtBu)Rh(L)][BF4] (15, 16) (L = tert-butyl isonitrile or 2,6-dimethylphenyl isonitrile), respectively. On the other hand, 2 disproportionates to Rh(I) and Rh(III) complexes in the presence of acetonitrile, isonitriles, or CO. 2 is also reduced by triethylphosphine and water to Rh(I) complexes [(PNPtBu)RhCl] (1) and [(PNPtBu)Rh(PEt3)][BF4] (11). When triphenylphosphine and water are used, the reduced Rh(I) complex reacts with a proton, which is formed in the redox reaction, to give a Rh(III) complex with a coordinated BF4, [(PNPtBu)Rh(Cl)(H)(BF4)] (9).

Synthesis and use of bisoxazolinyl-phenyl pincers

The bisoxazolinyl-phenyl (Phebox) ligand is an example of an N,C,N tridentate (pincer type) ligand, which has a central carbon-metal covalent bond and two oxazolines. In this tutorial review, synthetic methods to prepare bisoxazolinyl-phenyl derivatives and their transition-metal complexes including rhodium, iridium, platinum, palladium, nickel and copper, are summarized. In addition, several applications to homogeneous and asymmetric catalysis with chiral bisoxazolinyl-phenyl metal complexes have been reviewed.

Nitrile Biotransformations for the Efficient Synthesis of Highly Enantiopure 1-Arylaziridine-2-carboxylic Acid Derivatives and Their Stereoselective Ring-Opening Reactions

Catalyzed by the Rhodococcus erythropolis AJ270 whole cell catalyst under very mild conditions, biotransformations of racemic 1-arylaziridine-2-carbonitriles proceeded efficiently and enantioselectively to produce highly enantiopure S-1-arylaziridine-2-carboxamides and R-1-arylaziridine-2-carboxylic acids in excellent yields. Although the nitrile hydratase exhibits no selectivity against all nitrile substrates, the amidase is highly R-enantioselective towards 1-arylaziridine-2-carboxamides. When treated with benzyl bromide, 1-phenylaziridine-2S-carboxamide underwent a highly regioselective and enantiospecific ring-opening reaction to afford an almost quantitative yield of R-beta-[(benzyl)phenylamino]-alpha-bromopropanamide (C-2 attack) and R-alpha-[(benzyl)phenylamino]-beta-bromopropanamide (C-3 attack) in a 10.5:1 ratio. Further treatment of the resulting ring-opening products with an N-nucleophilic reagent such as amine and azide led to, through most probably the aziridinium intermediate, the formation of S-alpha-substituted-beta-[(benzyl)phenylamino]propanamides in good chemical yields with high enantiomeric purity.

Construction of Robust Ruthenium(salen)(CO) Complexes and Asymmetric Aziridination with Nitrene Precursors in the Form of Azide Compounds That Bear Easily RemovableN-Sulfonyl Groups

We synthesized new Ru(salen)(CO) complexes of high durability and achieved aziridination with good to excellent enantioselectivity by using azide compounds that contain an easily removable N-sulfonyl group, such as the 2-(trimethylsilyl)ethanesulfonyl group, as a nitrene precursor. Aziridination of less-reactive alpha,beta-unsaturated esters (and amides) proceeded with excellent enantioselectivities, from which it is inferred that an electrophilic species is the active species of this reaction. The present asymmetric aziridination provides a useful tool for introducing optically active nonprotected amine groups.

[Ir(PPh3)2(H)2(ClCH2CH2Cl)][BArF4]: a well characterised transition metal dichloroethane complex

Reaction of [Ir(PPh(3))(2)(COD)][BAr(F)(4)] with H(2) in dichloroethane solution results in [Ir(PPh(3))(2)(H)(2)(ClCH(2)CH(2)Cl)][BAr(F)(4)], which has been fully characterised by X-ray crystallography, NMR spectroscopy and ESI-MS. Its activity towards alkene hydrogenation has been compared with analogous CH(2)Cl(2) complexes.

Sulfamidation of 2-Arylaldehydes and Ketones with Chloramine-T

[reaction: see text] A series of aliphatic and aromatic carbonyl compounds has been transformed into the corresponding sulfamidated products by means of amine-catalyzed nitrene transfer of chloramine-T. Depending on the residues R, either alpha-sulfamidation in the case of aromatic aldehydes and acetone derivatives or direct sulfamidation at the carbonyl functionality of aliphatic aldehydes has been observed. Applying microwave conditions, good to excellent yields under significantly reduced reaction times could be obtained, thus providing a facile access to alpha,alpha-disubstituted amino acids.

Rhodium-Mediated Stereoselective Polymerization of “Carbenes”

Unprecedented rhodium-catalyzed stereoselective polymerization of "carbenes" from ethyl diazoacetate (EDA) to give high molecular mass poly(ethyl 2-ylidene-acetate) is described. The mononuclear, neutral [(N,O-ligand)M(I)(cod)] (M = Rh, Ir) catalytic precursors for this reaction are characterized by (among others) single-crystal X-ray diffraction. These species mediate formation of a new type of polymers from EDA: carbon-chain polymers functionalized with a polar substituent at each carbon of the polymer backbone. The polymers are obtained as white powders with surprisingly sharp NMR resonances. Solution and solid state NMR data for these new polymers reveal a highly stereoregular polymer, with a high degree of crystallinity. The polymer is likely syndiotactic. Material properties are very different from those of atactic poly(diethyl fumarate) polymer obtained by radical polymerization of diethyl fumarate. Other diazoacetates are also polymerized. Further studies are underway to reveal possible applications of these new materials.