Enantiomerically Pure ansa- η5-Complexes of Transition Metals as an Effective Tool for Chirality Transfer

Abstract

Chiral ansa-η⁵-complexes of transition metals have shown remarkable efficacy in organometallic synthesis and catalysis. Additionally, enantiomerically pure ansa-complexes hold promise for the development of novel chiral materials and pharmaceuticals. The discovery and synthesis of a diverse range of group IVB and IIIB metal complexes represents a significant milestone in the advancement of stereoselective catalytic methods for constructing metal-C, C-C, C-H, and C-heteroatom bonds. The synthesis of enantiomerically pure metallocenes can be accomplished through several strategies: utilizing optically active precursors of η⁵-ligands, separation of diastereomers of complexes with enantiomerically pure agents, and synthesis via the stereocontrolled reactions of enantiomerically pure σ-complexes with prochiral anions of η⁵-ligands. This review focuses on the analysis of various nuances of the synthesis of enantiomerically pure ansa-η⁵-complexes of titanium and lanthanum families. Their applicability as effective catalysts in asymmetric carbomagnesiation, carbo- and cycloalumination, oligo- and polymerization, Diels-Alder cycloaddition, reactions of zirconaaziridines, cyclization, hydrosilylation, hydrogenation, hydroamination, and other processes are highlighted as well.

Keywords: ansa-metallocenes; asymmetric synthesis; enantiomerically pure complexes; reaction mechanisms; stereoselectivity.

Scheme 1

Pathways for the synthesis of enantiomerically pure ansa-complexes ([aux]*—chiral auxiliary).

Scheme 2

Derivatization of Ti complexes rac-2a,b using S-binaphthol.

Scheme 3

Derivatization of Zr complexes 5 and 6.

Scheme 4

Derivatization of Ti bis-tetrahydroindenyl complex with S-binaphthol.

Scheme 5

Resolution of bis-tetrahydroindenyl Ti and Zr complexes with R-acetylmandelic acid.

Scheme 6

Derivatization of Ti complex rac-11 using S-binaphthol.

Scheme 7

Synthesis of enantiomerically pure Zr complexes p-S,p-S-17 and p-S,p-S-18.

Scheme 8

Resolution of racemic Ti (20a) and Zr (21a) complexes with R-binaphtholate and acetylmandelic acid.

Scheme 9

Derivatization of complexes rac-11 (Ti) and rac-13 (Zr) with a mixture of R-binaphthol and p-aminobenzoic acid.

Scheme 10

Separation of metallocenes rac-13 (Zr) and rac-28 (Hf) using chiral HPLC.

Scheme 11

Synthesis of enantiomerically pure triflates of ansa-metallocenes Zr (29) and Ti (30).

Scheme 12

Synthesis of Ti (p-S,p-S-34) and Zr (p-S,p-S-35) complexes starting from enantiomerically pure ligands.

Scheme 13

Synthesis of optically active titanocene 38.

Scheme 14

Synthesis of enantiomerically enriched Ti and Zr complexes from optically active Br-substituted binaphthyl precursors.

Scheme 15

Synthesis and purification of metallocenes 47–49.

Scheme 16

Synthesis of enantiomerically pure titanocene 52.

Scheme 17

Stereoselective synthesis of titanocene complexes p-S,p-S-58 and p-S,p-S-59.

Scheme 18

Synthesis of optically active meso-isomers of complexes 62a–c and 63a–c.

Scheme 19

Synthesis of optically active Zr complex meso-66.

Scheme 20

Stereoselective synthesis of Zr complex p-R,p-R-68.

Scheme 21

Polycyclic compounds as precursors in the synthesis of chiral titanocenes 73–75.

Scheme 22

Stabilization of ligand conformation as a significant factor in the stereoselective synthesis of chiral complexes.

Scheme 23

Bis-amide R,R-82 in the synthesis of Ti complexes 47A–C.

Scheme 24

Effect of menthyl and neomenthyl substituent in Cp-ring on stereoselectivity of metal complex synthesis.

Scheme 25

Synthesis of mixed menthyl-substituted cyclopentadienyl-octahydrofluorenyl ansa-complexes 93, 94.

Scheme 26

Synthesis of diastereomeric complexes 98.

Scheme 27

Synthesis of indenyl-fluorenyl ansa-zirconocene with a naphthyl bridge p-S-103.

Scheme 28

Synthesis of complexes 109–112.

Scheme 29

Synthesis of enantiomerically pure C₁-symmetric complexes S-116a–c.

Scheme 30

Effect of σ-ligand on the diastereoselectivity in the synthesis of complexes 122 and 123.

Scheme 31

Preparation of enantiomerically pure ansa-complexes of group IIIB metals 127–139.

Scheme 32

Synthesis of chiral C₁-symmetric organolanthanide complexes S-140a–c and S-141a–c.

Scheme 33

Application of enantiomerically pure binaphthol-bridged ligands in the synthesis of ansa-yttrocenes.

Scheme 34

R,R-enantiomer of Zr amide as a precursor in the synthesis of enantiomerically pure ansa-complex p-S,p-S-151.

Scheme 35

Enantioselective synthesis of Zr complex 13 via the association of ZrCl₄ with enantiomerically pure S- or R-binaphthol ethers.

Scheme 36

Complex p-R,p-R-13 as a stereoselective catalyst for alkene carbomagnesiation.

Scheme 37

Carbomagnesation of heterocyclic alkenes catalyzed by enantiomerically pure complexes p-R,p-R-13 and R,p-R,p-R-16.

Scheme 38

Carbomagnesation of rac-5,6-dihydropyrans, catalyzed with p-R,p-R-13.

Scheme 39

Kinetic effect of p-R,p-R-13 in the ethylmagnesiation of pyrans.

Scheme 40

Kinetic resolution of dihydrofurans in the presence of p-R,p-R-13.

Scheme 41

Ethylmagnesiation of seven-membered cycloolefins catalyzed by p-R,p-R-16 or R,p-R,p-R-27.

Scheme 42

Carbomagnesiation of cyclic alkoxy-substituted olefins 166a–v catalyzed by R,p-R,p-R-16 or S,p-S,p-S-16.

Scheme 43

Carbomagnesiation of inactivated alkenes by MgEt₂ in the presence of R,p-R,p-R-16.

Scheme 44

Synthesis of homoallylic alcohols by the reaction of aldehydes with crotyl titanocenes.

Scheme 45

Reaction of allyl- (171) and crotyltitanocenes (172) with aldehydes.

Scheme 46

Carbometalation of aldehydes catalyzed by complex p-S,p-S-4b.

Scheme 47

Dia- and enantioselectivity of titanocenes 52, 58, and 59 in the reactions of 2-alkyl-1,3-butadienes with CO₂ or R’CHO.

Scheme 48

Barbier-type carbonyl compound propargylation or allylation catalyzed with p-S,p-S-11.

Scheme 49

Asymmetric Zr and Ti-catalyzed enantioselective carboalumination and cycloalumination of alkenes.

Scheme 50

Mechanism of alkene methylalumination in the presence of enantiomerically pure ansa-zirconocene complexes.

Scheme 51

Reaction of 2,5-dihydrofurans with AlEt₃ catalyzed by complexes p-R,p-R-13 or R,p-R,p-R-16.

Scheme 52

Propene polymerization in the presence of a catalytic system based on p-R,p-R-17 and MAO.

Scheme 53

Polymerization of 1-pentene catalyzed by R,p-R,p-R-16 or p-R,p-R-17 and MAO, followed by treatment with D₂.

Scheme 54

Oligomerization of propene and 1-butene catalyzed by R,R,p-S,p-S-15 and MAO.

Scheme 55

Cyclopolymerization of 1,5-hexadiene catalyzed by achiral precatalysts Cp₂MX₂ (M = Ti, Zr; X = Cl, Me) and MAO.

Scheme 56

Synthesis of enantiomerically enriched styrene hydrooligomers catalyzed by complex p-R,p-R-16.

Scheme 57

Kinetic resolution of racemic α-olefins bearing bulky substituents during polymerization catalyzed by enantiomerically pure S-116a–d.

Scheme 58

Stereoselectivity control model of propene polymerization catalyzed by enantiomerically pure S-116a–d.

Scheme 59

Stereocontrolled oligomerization of propylene via reaction with ZnEt₂ in the presence of p-S,p-S-13 and MAO.

Scheme 60

Oligomerization of 1-hexene catalyzed by enantiomerically pure Zr ansa-complexes.

Scheme 61

Cycloaddition of enones with cyclopentadiene catalyzed by chiral Ti and Zr complexes.

Scheme 62

Synthesis of zirconaaziridines from dimethyl complex p-S,p-S-17.

Scheme 63

Asymmetric induction in the reaction of amines with alkynes, alkenes, or aldehydes catalyzed by zirconaaziridines 210.

Scheme 64

Reactions of complexes 214 with various alkenes yielding metallocycles 215a–d and synthesis of substituted pyridine 216 catalyzed by p-S,p-S-17.

Scheme 65

Reactions of alkyl- or aryl-substituted zirconaziridine with ethylene carbonate or isocyanates yielding amino acid esters.

Scheme 66

Proposed mechanism of stereoinduction in the reaction of substituted zirconaziridine with ethylene carbonate.

Scheme 67

Reaction of the enantiomerically enriched imido-complex p-S,p-S-222 with allenes.

Scheme 68

Pauson–Khand cyclo-condensation of enynes with carbon monoxide catalyzed with p-S,p-S-226.

Scheme 69

Asymmetric cyclocarbonylation of nitrogen-containing enynes catalyzed with p-S,p-S-226.

Scheme 70

Intramolecular asymmetric reductive coupling of ketones with nitriles catalyzed by p-R,p-R-11.

Scheme 71

Asymmetric hydrosilylation of ketones by polymeric methylhydrosiloxane Me₃SiO[MeSi(H)O]_nSiMe₃ in the presence of R,p-R,p-R-12.

Scheme 72

Hydrosilylation of ketones by silanes catalyzed with alkylated S,p-S,p-S-12.

Scheme 73

Titanocene p-R,p-R-235 as an effective catalyst for the hydrosilylation of ketones by PhSiH₃.

Scheme 74

Mechanism of titanocene-catalyzed hydrosilylation of ketones by PhSiH₃.

Scheme 75

Reduction of aryl-substituted ketones by silanes catalyzed by Ti complexes S-88a, R-88a, p-S,p-S-38, and 1R,2R,4R,5R-53.

Scheme 76

Asymmetric hydrogenation of imines catalyzed by R,p-R,p-R-12 and R,R,R-22.

Scheme 77

Hydrogenation of 1,1-disubstituted enamines catalyzed with R,p-R,p-R-12.

Scheme 78

Asymmetric hydrosilylation of N-aryl-substituted imines using PhSiH₃ or PMHS in the presence of Ti complex p-S,p-S-235.

Scheme 79

Hydrosilylation of rac-2,5-disubstituted pyrroles in the presence of the binaphtholate complex S,p-S,p-S-12.

Scheme 80

Kinetic resolution of N-alkyl imines derived from 3-substituted indanones and 4-substituted tetralones.

Scheme 81

Hydrosilylation of phenylbutene by PhSiH₃ in the presence of samarium complexes R- or S-137a.

Scheme 82

Hydrogenation of 2-phenylbut-1-ene and styrene deuteration catalyzed with Zr, Ti, or Sm complexes.

Scheme 83

Hydrogenation of trisubstituted olefins in the presence of Ti complex S,p-S,p-S-12.

Scheme 84

Hydrogenation of tetrasubstituted olefins in the presence of p-R,p-R- or p-S,p-S-17 and [PhMe₂NH]⁺[B(C₆F₅)₄]⁻.

Scheme 85

Synthesis of enantiomerically enriched N-heterocycles.

Scheme 86

Asymmetric hydroamination/cyclization of aminoalkenes catalyzed with lanthanide η⁵-complexes.

Scheme 87

Asymmetric reduction/cyclization of amino- and phosphinoalkenes in the presence of complexes with octahydrofluorenyl ligand S-141a–c.

Scheme 88

Cyclization of aminodienes and aminoalkenes in the presence of complexes S-141a, S-133b or S-134b.

Scheme 89

Asymmetric catalytic epoxidation of alkyl- and aryl-substituted alkenes.

Scheme 90

Ring-opening of epoxides in the presence of pure ansa-titanocene p-S,p-S-11.

Scheme 91

Homocoupling of benzaldehyde catalyzed with titanocene complexes 52 and 53.

Scheme 92

Ti-catalyzed synthesis of γ-lactol (+)-285.

Scheme 93

Enantioselective isomerization of trans- or cis-4-tert-butyl-1-vinylcyclohexanes in the presence of chiral titanium ansa-complexes 52 or 73.