Alkene and Olefin Functionalization by Organoaluminum Compounds, Catalyzed with Zirconocenes: Mechanisms and Prospects

Alkene and olefin functionalization via addition of electro‐ or nucleophilic reagents is one of the convenient synthetic methods for the insertion of heteroatoms into organic molecules. The use of organometallic reagents in these reactions in combination with the specific catalysts provides high substrate conversion and process selectivity. The introduction of this approach into the chemistry of organoaluminum compounds leads to the development of chemo‐, regio‐ and stereoselective catalytic methods of alkene and olefin functionalization. The chapter focuses on the modern concepts of the alkene hydro‐, carbo‐ and cycloalumination mechanisms, that is, the experimental and theoretical data on the intermediate structures involved in the product formation, the effects of the catalyst and organoaluminum compound structure, reaction conditions on the activity and selectivity of the bimetallic systems. The prospects of the development of enantioselective methods using these catalytic systems for the alkene and olefin transformations are considered

Synthesis of Dibenzylbutane and 9,8′-Neo-Lignans via Cyclometalation of Allylbenzene by EtAlCl2 and Mg in the Presence of Zr ansa-Complexes

The aim of the research is the development of a one-pot method for the synthesis of lignans, natural compounds that show a wide spectrum of biological activities. For this purpose, the ansa-zirconocenes of various structures were tested as catalysts of allylbenzene cyclometalation with ethylaluminum dichloride (EtAlCl2) and Mg. The effects of the organophosphorus compounds, hexamethylphosphoramide (HMPA) and triphenylphosphine (PPh3), on the chemo- and regioselectivity of the reaction were studied. The use of η5-indenyl or fluorenyl ansa-complexes with ethanediyl or dimethylsilylene bridges, as well as a biscyclopentadienyl complex with Si-bound ligands as catalysts in the presence of HMPA, yields the formation of cyclometalation products in a total yield of 70%. Cyclometalation product composition is represented by two regioisomers, 3,4-dibenzyl- and 2,4-dibenzyl-substituted alumolanes, with a ratio of (1-2):1, in which hydrolysis provides corresponding dibenzylbutane lignan and 9,8′-neo-lignan

Modification of 1-Hexene Vinylidene Dimer into Primary and Tertiary Alkanethiols

Aliphatic thiols are in high demand in materials chemistry. Herein, a synthesis of thio-derivatives of 1-hexene vinylidene dimer is described. The approach, based on a hydroalumination reaction with further replacement of the organoaluminum function with sulfur using thiourea or dimethyl disulfide, provides anti-Markovnikov products, 2-butyloctane-1-thiol or 5-(methylsulfanylmethyl)undecane, in moderate yields. The reaction of a vinylidene dimer with phosphorus pentasulfide in the presence of catalytic amounts of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) selectively gives the Markovnikov product, 5-methylundecane-5-thiol, with a yield of up to 77%

Catalytic Systems Based on Cp2ZrX2 (X = Cl, H), Organoaluminum Compounds and Perfluorophenylboranes: Role of Zr,Zr- and Zr,Al-Hydride Intermediates in Alkene Dimerization and Oligomerization

The activity and chemoselectivity of the Cp2ZrCl2-XAlBui2 (X = H, Bui) and [Cp2ZrH2]2-ClAlEt2 catalytic systems activated by (Ph3C)[B(C6F5)4] or B(C6F5)3 were studied in reactions with 1-hexene. The activation of the systems by B(C6F5)3 resulted in the selective formation of head-to-tail alkene dimers in up to 93% yields. NMR studies of the reactions of Zr complexes with organoaluminum compounds (OACs) and boron activators showed the formation of Zr,Zr- and Zr,Al-hydride intermediates, for which diffusion coefficients, hydrodynamic radii, and volumes were estimated using the diffusion ordered spectroscopy DOSY. Bis-zirconium hydride clusters of type x[Cp2ZrH2∙Cp2ZrHCl∙ClAlR2]∙yRnAl(C6F5)3−n were found to be the key intermediates of alkene dimerization, whereas cationic Zr,Al-hydrides led to the formation of oligomers

Ti Group Metallocene-Catalyzed Synthesis of 1-Hexene Dimers and Tetramers

1-Hexene transformations in the catalytic systems L2MCl2–XAlBui2 (L = Cp, M = Ti, Zr, Hf; L = Ind, rac-H4C2[THInd]2, M = Zr; X = H, Bu i) and [Cp2ZrH2]2-ClAlR2 activated by MMAO-12, B(C6F5)3, or (Ph3C)[B(C6F5)4] in chlorinated solvents (CH2Cl2, CHCl3, o-Cl2C6H4, ClCH2CH2Cl) were studied. The systems [Cp2ZrH2]2-MMAO-12, [Cp2ZrH2]2-ClAlBui2-MMAO-12, or Cp2ZrCl2-HAlBui2-MMAO-12 (B(C6F5)3) in CH2Cl2 showed the highest activity and selectivity towards the formation of vinylidene head-to-tail alkene dimers. The use of chloroform as a solvent provides further in situ dimer dimerization to give a tetramer yield of up to 89%. A study of the reaction of [Cp2ZrH2]2 or Cp2ZrCl2 with organoaluminum compounds and MMAO-12 by NMR spectroscopy confirmed the formation of Zr,Zr-hydride clusters as key intermediates of the alkene dimerization. The probable structure of the Zr,Zr-hydride clusters and ways of their generation in the catalytic systems were analyzed using a quantum chemical approach (DFT)

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

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 cyclo-alumination of alkenes by organoaluminium compounds (OAC) (AlMe3, AlEt3, HAlBui2) has been studied. It was found that OAC type exhibits the most effect on the reactions chemo- and enantioselectivity. The reaction chemo- and enantio-selectivity depend on the catalyst structure and reaction conditions (solvent type, catalyst concentration, temperature) as well. It is shown that lack of asymmetric induction in the reaction of ?-methylstyrene hydroalumination by HAlBui2, catalyzed with complexes 1 or 3, is the result of the formation of Zr hydride complexes of different structure as reaction intermediates. MTPA was used as derivatization reagent for enantiomeric excess estimation and absolute configuration assignment of ?-chiral alcohols obtained after the oxidation and hydrolysis of reaction products. The applicability of MTPA for the assignment of chiral center absolute configuration in ?–ethyl substituted primary alcohols and ?–alkyl-1,4-butanediols was shown