Enantioselectivity of chiral zirconocenes as catalysts in alkene hydro-, carbo- and cycloalumination reactions

The enantioselectivity of chiral Zr catalysts L1L2ZrCl2 [L1 = L2 = 1-neomenthylindenyl (Ind∗), 1; L1 = Cp, L2 = Ind∗ 2; L1 = Cp, L2 = 1-neomenthylindenyl-4,5,6,7-tetrahydroindenyl (Cp∗) 3] in the hydro-, carbo- and cycloalumination of alkenes by organoaluminium compounds (OAC) (AlMe3, AlEt3, HAlBu2i) has been studied. It was found that OAC exhibit the most effect on the reaction chemo- and enantioselectivity. The reaction chemo- and enantioselectivity depend on the catalyst structure and reaction conditions (solvent type, catalyst concentration, temperature) as well. It is shown that the lack of asymmetric induction in the reaction of α-methylstyrene hydroalumination by HAlBu2i, catalyzed with complexes 1 or 3, is the result of the formation of Zr hydride complexes of different structures as reaction intermediates. MTPA was used as a derivatization reagent for the enantiomeric excess estimation and absolute configuration assignment of β-chiral alcohols obtained after the oxidation and hydrolysis of the reaction products. The applicability of MTPA for the assignment of the absolute configuration of the stereogenic centre in β-ethyl substituted primary alcohols and β-alkyl-1,4-butanediols is shown.

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(η5-Cyclopentadienyl) (η5-{1-[(1S,2S,5R)-2-isopropyl-5-methylcyclohexyl]indenyl})zirconium dichlorideC24H30Cl2Zr[α]D25=-75.0 (c 0.6, C6H5CH3)Source of chirality: (+)-3-[(1S,2S,5R)-2-isopropyl-5-methylcyclohexyl]indeneAbsolute configuration: (1S,2S,5R)

(2R)-Methyl-1-hexanolC7H16OEe = 69%[α]D25=+10.3 (c 1.9 CH2Cl2)Source of chirality: asymmetric synthesisAbsolute configuration: (2R)

(2R)-Methyl-1-octanolC9H20OEe = 73%[α]D25=+8.9 (c 0.9 CH2Cl2)Source of chirality: asymmetric synthesisAbsolute configuration: (2R)

(2S)-2-Ethyl-1-hexanolC8H18OEe = 68%[α]D25=+2.6 (c 0.3 CH2Cl2)Source of chirality: asymmetric synthesisAbsolute configuration: (2S)

(2S)-Butyl-1,4-butanediolC8H18O2Ee = 36%[α]D25=+3.3 (c 0.4 CH2Cl2)Source of chirality: asymmetric synthesisAbsolute configuration: (2S)