Evgenii P. Talsi

1H-, 13C-NMR and ethylene polymerization studies of zirconocene/MAO catalysts: effect of the ligand structure on the formation of active intermediates and polymerization kinetics

Abstract Using 1 H- and 13 C-NMR spectroscopies, cationic intermediates formed by activation of L 2 ZrCl 2 with methylaluminoxane (MAO) in toluene were monitored at Al/Zr ratios from 50 to 1000 (L 2 are various cyclopentadienyl (Cp), indenyl (Ind) and fluorenyl (Flu) ligands). The following catalysts were studied: (Cp-R) 2 ZrCl 2 (R=Me, 1,2-Me 2 , 1,2,3-Me 3 , 1,2,4-Me 3 , Me 4 , Me 5 , n -Bu, t -Bu), rac-ethanediyl(Ind) 2 ZrCl 2 , rac-Me 2 Si(Ind) 2 ZrCl 2 , rac-Me 2 Si(1-Ind-2-Me) 2 ZrCl 2 , rac-ethanediyl(1-Ind-4,5,6,7-H 4 ) 2 ZrCl 2 , (Ind-2-Me) 2 ZrCl 2 , Me 2 C(Cp)(Flu)ZrCl 2 , Me 2 C(Cp-3-Me)(Flu)ZrCl 2 and Me 2 Si(Flu) 2 ZrCl 2 . Correlations between spectroscopic and ethene polymerization data for catalysts (Cp-R) 2 ZrCl 2 /MAO (R=H, Me, 1,2-Me 2 , 1,2,3-Me 3 , 1,2,4-Me 3 , Me 4 , Me 5 , n -Bu, t -Bu) and rac-Me 2 Si(Ind) 2 ZrCl 2 were established. The catalysts (Cp-R) 2 ZrCl 2 /AlMe 3 /CPh 3 + B(C 6 F 5 ) 4 − (R=Me, 1,2-Me 2 , 1,2,3-Me 3 , 1,2,4-Me 3 , Me 4 , n -Bu, t -Bu) were also studied for comparison of spectroscopic and polymerization data with MAO-based systems. Complexes of type (Cp-R) 2 ZrMe + ←Me − -Al≡MAO ( IV ) with different [Me-MAO] − counteranions have been identified in the (Cp-R) 2 ZrCl 2 /MAO (R= n -Bu, t -Bu) systems at low Al/Zr ratios (50–200). At Al/Zr ratios of 500–1000, the complex [L 2 Zr(μ-Me) 2 AlMe 2 ] + [Me-MAO] − ( III ) dominates in all MAO-based reaction systems studied. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr=200–1000) for the systems (Cp-R) 2 ZrCl 2 /MAO (R=H, Me, n -Bu, t -Bu), while it is virtually constant in the same range of Al/Zr ratios for the catalytic systems (Cp-R) 2 ZrCl 2 /MAO (R=1,2-Me 2 , 1,2,3-Me 3 , 1,2,4-Me 3 , Me 4 ) and rac-Me 2 Si(Ind) 2 ZrCl 2 /MAO. The data obtained are interpreted on assumption that complex III is the main precursor of the active centers of polymerization in MAO-based systems.

13C-NMR study of Ti(IV) species formed by Cp*TiMe3 and Cp*TiCl3 activation with methylaluminoxane (MAO)

Abstract Using 13 C- and 1 H-NMR spectroscopy, titanium(IV) species formed in the catalytic systems Cp*TiMe 3 /MAO and Cp*TiCl 3 /MAO (Cp*=C 5 (CH 3 ) 5 ) in toluene and chlorobenzene were studied within the temperature range 253–293 K and at Al/Ti ratios 30–300. It was shown that upon activation of Cp*TiMe 3 with methylaluminoxane (MAO) mainly the ‘cation-like’ intermediate Cp*Me 2 Ti + ←Me − ue5f8Alue606(MAO) ( 2 ) is formed. Three types of titanium(IV) complexes were identified in Cp*TiCl 3 /MAO catalytic system. They are methylated complexes Cp*TiMeCl 2 and Cp*TiMe 2 Cl, and the ‘cation-like’ intermediate 2 . Complex 2 dominates in Cp*TiCl 3 /MAO system in conditions approaching to those of practical polymerization (Al/Ti ratios more than 200). According to the EPR measurements, the portion of EPR active Ti(III) species in the Cp*TiCl 3 /MAO system is smaller than 1% at Al/Ti=35, and is about 10% at Al/Ti=700.

Titanium salan catalysts for the asymmetric epoxidation of alkenes: steric and electronic factors governing the activity and enantioselectivity.

A new insight into the highly enantioselective (up to >99.5u2009% ee) epoxidation of olefins in the presence of chiral titanium(IV) salan complexes is reported. A series of 14 chiral ligands with varying steric and electronic properties have been designed, and it was found that electronic effects modulate the catalytic activity (without affecting the enantioselectivity), whereas the steric properties account for the enantioselectivity of the epoxidation. Competitive oxidations of p-substituted styrenes reveal the electrophilic nature of the oxygen-transferring active species, with a Hammett ρ value of -0.51; the enantioselectivity is unaffected by the electron-donating (or withdrawing) ability of the p-substituents. Mechanistic studies provide evidence in favor of a stepwise reaction mechanism: in the first (rate-determining) stage, olefin most probably coordinates to the active species, followed by intramolecular enantioselective oxygen transfer. The enantioselectivity increases with decreasing temperature. The modified Eyring plots for the epoxidation of styrene and (Z)-β-methylstyrene are linear, indicating a single, enthalpy-controlled mechanism of stereoselectivity, with ΔΔH(≠) =-6.6u2005kJu2009mol(-1) and -5.4u2005kJu2009mol(-1) , respectively.

Asymmetric Autoamplification in the Oxidative Kinetic Resolution of Secondary Benzylic Alcohols Catalyzed by Manganese Complexes

Herein, chiral Mn–aminopyridine complexes have been shown to catalyze the oxidation of alkylarenes to enantiomerically enriched 1‐arylalkanols with hydrogen peroxide. The observed enantiomeric excess values result from the direct enantioselective benzylic C−H hydroxylation, accompanied by stereoconvergent oxidative kinetic resolution of the resulting alcohol. Testing several (S,S)‐bipyrrolidine derived Mn complexes has revealed a novel catalyst (6) that exhibits the best kinetic resolution in the series (krel up to 8.8), along with sufficient reactivity and efficiency (>1000 catalytic turnovers). The mechanistic study of the Mn‐mediated alcohol oxidation witnesses electrophilic active species (ρ=−1.2), with rate‐limiting H abstraction (kH/kD=2.2), followed by oxygen rebound and dehydration of the resulting gem‐diol to form the ketone. Intriguingly, while for the resolution of the relatively bulky 1,2‐diphenylethanol, krel is virtually constant throughout the reaction, for less bulky alcohols, krel increases with increasing conversion, in line with the rising optical purity of the 1‐arylalkanol. The latter participates in the oxidation as an auxiliary ligand, assisting the chiral recognition. This effect is related to the previously described asymmetric autocatalysis and asymmetric autoinduction, but is not identical with either of those, with the differences being discussed. To unambiguously identify this effect, the term asymmetric autoamplification (chiral autoamplification) is proposed.

Titanium Salan/Salalen Complexes: The Twofaced Janus of Asymmetric Oxidation Catalysis

Optically pure chiral epoxides and sulfoxides are ubiquitous building blocks in fine organic synthesis, employed in the pharmaceutical, agrochemical, and cosmetic industries. On the road to chiral epoxides and sulfoxides, efficient and stereoselective transition metal-based catalysts are the most promising guides. Among transition metals, we favor titanium for its cheapness and availability, nontoxicity, and well-known ability to catalyze a variety of stereoselective transformations, including oxidations with environmentally benign H2O2. In this personal account, we summarize the state-of-the-art of rational design of chiral titanium(IV) salan and salalen catalysts, and investigations of their catalytic reactivities and stereoselectivities in the epoxidations of olefins and oxidations of thioethers, unraveling the peculiarities and mechanisms of their catalytic action.

Highly Enantioselective C-H Oxidation of Arylalkanes with H2O2 in the Presence of Chiral Mn Aminopyridine Complexes

Bioinspired chiral Mn‐aminopyridine complexes [(S,S)‐LMnII(OTf)2] and [(R,R)‐LMnII(OTf)2] (where (S,S)‐L=(2S,2′S)‐1,1′‐bis((3‐methyl‐4‐(2,2,2‐trifluoroethoxy)pyridin‐2‐yl)methyl)‐2,2′‐bipyrrolidine, and (R,R)‐L=(2R,2′R)‐1,1′‐bis((3‐methyl‐4‐(2,2,2‐trifluoroethoxy)pyridin‐2‐yl)methyl)‐2,2′‐bipyrrolidine) have been shown to efficiently catalyze the benzylic C−H oxidation of arylalkanes with hydrogen peroxide in the presence of carboxylic acid additives, affording enantiomerically enriched 1‐arylalkanols and the corresponding ketones. Optically pure additive N‐Boc‐(L)‐proline, in combination with [(R,R)‐LMnII(OTf)2] complex, affords 1‐arylalkanols in up to 86u2009% ee, which is the highest reported enantioselectivity for direct benzylic hydroxylations with H2O2 in the presence of transition‐metal catalysts. Oxidative kinetic resolution only slightly contributes to the increase of the observed enantiomeric excess over the reaction course. The observed kH/kD values (3.5–3.6 for the oxidation of ethylbenzene/d10‐ethylbenzene) and competitive oxidation data are consistent with either a hydrogen‐atom transfer/oxygen rebound or hydride transfer/oxygen rebound asymmetric hydroxylation mechanism.

Chiral Manganese Aminopyridine Complexes: the Versatile Catalysts of Chemo- and Stereoselective Oxidations with H2O2

In the last decade, manganese(II) complexes with N-donor tetradentate aminopyridine ligands emerged as efficient catalysts of enantioselective epoxidation of olefins and direct selective oxidation of C-H groups in complex organic molecules, with environmentally benign oxidant hydrogen peroxide. In this personal account, we summarize the progress of these catalysts with regard to ligands design, structure-reactivity correlations, evaluation of the substrate scope, as well as mechanistic studies, shedding light on the nature of active sites and the mechanisms of selective oxygenations. Several practically promising catalytic syntheses with the aid of Mn aminopyridine catalysts are exemplified.

Highly Efficient Aromatic C−H Oxidation with H2O2 in the Presence of Iron Complexes of the PDP Family

The catalytic activity of a series of iron complexes of the PDP family (PDP=N,N′‐bis(2‐pyridylmethyl)‐2,2′‐bipyrrolidine) in the oxidation of aromatic substrates with H2O2 has been studied. In the presence of acetic acid, these complexes efficiently catalyze the oxidation of benzene and alkylbenzenes with high selectivity for oxygen incorporation into the aromatic ring (up to 93u2009%), performing up to 84 catalytic turnovers. The parent complex, [(PDP)(OTf)2], has demonstrated the highest catalytic efficiency and aromatic oxidation selectivity. The yield of products of oxidation of different substrates increases in line with increasing number of electron‐donating alkyl groups of the substrates: halogenbenzenes