Catalysts For Cyclopropanation Reactions Research Articles

Synthesis of chiral ruthenium and cobalt (meso-2-amidophenyl)porphyrins and their catalytic activity in cyclopropanation reactions

The preparation of two chiral porphyrin ligands 7 and 9 and of their ruthenium and cobalt complexes is reported. These new complexes have been used as catalysts in cyclopropanation reactions. Both 7-Ru and 9-Ru complexes showed a catalytic activity in the enantiomeric cyclopropanation of α-methylstyrene by ethyldiazoacetate whereas amination of ethylbenzene by arylazide in the presence of 7-Ru or 9-Ru did not stop at the corresponding benzylamine stage but instead afforded the corresponding benzylimine.

Effect of support properties on the performance of silica-supported bis(oxazoline)–copper chiral complexes

Mesoporous silica materials with different textural properties have been prepared by sol–gel synthesis under mild acidic conditions, and used as supports for chiral IndaBox ligand. After complexation with copper, these solids are catalysts for cyclopropanation reactions. Enantioselectivities in the range of 52–57% e.e. for cis cyclopropanes and 42–48% e.e. for trans cyclopropanes are obtained without any effect of the textural properties. However, these properties show an important effect on the activity and mainly on the recyclability of the catalyst. In contrast with the expected effect, solids with very large pores but low surface area lead to a significant deactivation upon recovery, whereas solids with highly developed surface area lead to the best overall results.

Immobilization of Pybox Ligand on Modified Starch

Abstract Starch, one of the most abundant natural polymer was used as support for chiral pyridine–bis(oxazoline) ligand. The corresponding ruthenium complex was used as catalyst in cyclopropanation reaction of styrene with ethyl diazoacetate. Yield, diastereoisomeric excess and enantiomeric excess kept constant after 3 runs.

Bis(oxazoline)–copper complexes supported by electrostatic interactions: scope and limitations

Bis(oxazoline)–copper complexes have been supported by cation exchange in laponite and nafion-like solids. The results obtained with these catalysts in cyclopropanation reactions depend on the nature of both the chiral ligand and the support. On using the complex of 2,2′-isopropylidenebis[(4 S)-4- tert-butyl-2-oxazoline] with Cu(II), direct exchange in methanol leads to low enantioselectivities in nafion-type solids. Some improvement in enantioselectivity can be obtained by using the less coordinating nitroethane as the exchange solvent or by exchanging the support prior to use with a more soluble tetraalkylammonium cation. Another strategy consists of the exchange of copper(II) and subsequent treatment with the chiral bis(oxazoline) ligand. This system leads to slower reactions and similar enantioselectivities to those obtained by direct exchange. The best enantioselectivity obtained in the reaction between styrene and ethyl diazoacetate is around 70% ee. The recovery of the catalysts leads to a partial loss of enantioselectivity. The stability of the bis(oxazoline)–copper complex is the crucial factor for this behavior. The selectivity can be improved with the bulkier (1 R,2 S,5 R)-menthyl diazoacetate, up to 83% ee, although the reactivity in this case is much lower.

ChemInform Abstract: Bis(oxazoline)copper Complexes Covalently Bonded to Insoluble Support as Catalysts in Cyclopropanation Reactions.

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Bis(oxazoline)copper complexes covalently bonded to insoluble support as catalysts in cyclopropanation reactions.

Chiral bis(oxazolines) are readily dialkylated in the methylene bridge, opening the way to immobilization at that position, keeping the C(2) symmetry of the chiral ligand. Bis(oxazolines) functionalized with two allyl or vinylbenzyl groups are easily grafted onto mercaptopropylsilica. Another approach to immobilization is the polymerization of the ligands bearing vinylbenzyl groups to yield insoluble polymers. The Cu(OTf)(2) complexes of the immobilized ligands promote the enantioselective cyclopropanation reaction between styrene and ethyl diazoacetate. The results depend on the nature of the support and the method of immobilization. With regard to the type of solid, the best results, which are similar to or even better than those obtained with the corresponding dibenzylated homogeneous catalysts, are obtained with homopolymers. With regard to the bis(oxazoline), that bearing indan groups leads to good results both onto silica and polymers, whereas with the ligand bearing tert-butyl groups good enantioselectivities are only obtained with homopolymeric catalysts. Some of the heterogeneous catalysts can be easily recovered and reused, as much as five times, with the same yield and stereoselectivities.

Polymer-supported Bis(oxazoline)-copper complexes as catalysts in cyclopropanation reactions

Bis(oxazolines) are easily immobilized by functionalization of the central methylene bridge with polymerizable groups and subsequent polymerization. Polymers are transformed into copper catalysts active in the cyclopropanation of styrene with ethyl diazoacetate. The results are similar or even better than those obtained with the similar homogeneous systems, and the catalysts can be easily recovered and reused. The substitution in the methylene bridge leads to a slight reduction in the enantioselectivity and an unexpected cis-preference.

Synthesis and characterisation of chiral Cu(I) complexes of substituted pyrrolidine ligands. Efficient catalysts for cyclopropanation reactions

Abstract A series of new copper(I) complexes of chiral bidentate N,N′-ligands ((S)-2-t-butylaminocarbonylpyrrolidine (bd1), (S)-1-t-butylaminocarbonyl-2-(1-naphthylaminomethyl)pyrrolidine (2, (S)-1-t-butylaminocarbonyl-2-(1-naptghylaminocarbonyl)pyrrolidine (bd3), (S)-1-t-butylaminocarbonyl-2-(1-napthylaminomethyl)pyrrolidine (4), (2S,4S)-1,2-(bis-t-butylaminocarbonyl)-4-aminopyrrolidine (5)) of general formula [Cu(CH3CN)(L-L)]X (X = PF6, ClO4) have been synthesised and fully characterised by elemental analysis, IR, electronic and NMR spectroscopy. These complexes catalyse the cyclopropanation of styrene, used as a model for monosubstituted olefins, with ethyl diazoacetate to yield a mixture of cis/trans ethyl 2-phenylcyclopropan-1-carboxylates with up to 30% ee.

Chiral Pd organometallic complexes as catalysts in cyclopropanation reactions

Asymmetric cyclopropanation of styrene with diazoacetic esters is performed firstly using chiral amino acidato complexes of Pd(II) containing a C,N-cyclometallated group as an ancillary ligand as catalyst precursors. In general these catalytic systems provide good conversion to the corresponding cyclopropyl derivatives with a moderate trans selectivity, although the stereoselectivities obtained were low. With the use of chiral catalysts and chiral diazoacetic esters the diastereoselectivities were slightly improved.