Alkene aziridination and epoxidation catalyzed by chiral metal salen complexes

O'Connor, Kenneth J.; Wey, Shiow‐Jyi; Burrows, Cynthia J.

doi:10.1016/s0040-4039(00)91844-6

Cited by 153 publications

(62 citation statements)

References 19 publications

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“…Route 3: synthesis of 3-(tert-butyldimethylsilyl)-2-hydroxybenzaldehyde: This procedure was similar to one reported in reference [64] with some modifications. o-Bromophenol (1.2 mL, 10 mmol) was added to a suspension of 60 % NaH in oil (400 mg, 10 mmol) in THF (10 mL) at 0 8C under N 2 and the resulting mixture was stirred for 1 h at 0 8C.…”

Section: -Hydroxy-5-(trifluoromethyl)-3-(trimethylsilyl)benzaldehydementioning

confidence: 99%

Stereoselective Ring‐Opening Polymerization of a Racemic Lactide by Using Achiral Salen– and Homosalen–Aluminum Complexes

Nomura

Ishii

Yamamoto

et al. 2007

Chemistry A European J

387

246

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Highly isotactic polylactide or poly(lactic acid) is synthesized in a ring-opening polymerization (ROP) of racemic lactide with achiral salen- and homosalen-aluminum complexes (salenH(2)=N,N'-bis(salicylidene)ethylene-1,2-diamine; homosalenH(2)=N,N'-bis(salicylidene)trimethylene-1,3-diamine). A systematic exploration of ligands demonstrates the importance of the steric influence of the Schiff base moiety on the degree of isotacticity and the backbone for high activity. The complexes prepared in situ are pure enough to apply to the polymerizations without purification. The crystal structures of the key complexes are elucidated by X-ray diffraction, which confirms that they are chiral. However, analysis of the (1)H and (13)C NMR spectra unambiguously demonstrates that their conformations are so flexible that the chiral environment of the complexes cannot be maintained in solution at 25 degrees C and that the complexes are achiral under the polymerization conditions. The flexibility of the backbone in the propagation steps is also documented. Hence, the isotacticity of the polymer occurs due to a chain-end control mechanism. The highest reactivity in the present system is obtained with the homosalen ligand with 2,2-dimethyl substituents in the backbone (ArCH==NCH(2)CMe(2)CH(2)N==CHAr), whereas tBuMe(2)Si substituents at the 3-positions of the salicylidene moieties lead to the highest selectivity (P(meso)=0.9(8); T(m)=210 degrees C). The ratio of the rate constants in the ROPs of racemic lactide and L-lactide is found to correlate with the stereoselectivity in the present system. The complex can be utilized in bulk polymerization, which is the most attractive in industry, although with some loss of stereoselectivity at high temperature, and the afforded polymer shows a higher melting temperature (P(meso)=0.9(2), T(m) up to 189 degrees C) than that of homochiral poly(L-lactide) (T(m)=162-180 degrees C). The "livingness" of the bulk polymerization at 130 degrees C is maintained even at a high conversion (97-98 %) and for an extended polymerization time (1-2 h).

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Section: -Hydroxy-5-(trifluoromethyl)-3-(trimethylsilyl)benzaldehydementioning

confidence: 99%

Stereoselective Ring‐Opening Polymerization of a Racemic Lactide by Using Achiral Salen– and Homosalen–Aluminum Complexes

Nomura

Ishii

Yamamoto

et al. 2007

Chemistry A European J

387

246

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“…Much recent attention has focused on developing eco-friendly aerobic catalytic processes based on efficient catalysts with easy recoverability. Schiff base transition metal complexes are considered as promising candidates because of their excellent catalytic performances in a wide range of oxidation reactions [3][4][5][6]. Frequently, these homogeneous Schiff base complexes are supported on inert porous solids such as alumina [7], silica and zeolites, or encapsulated in their pores to increase catalyst stability and allow for catalyst recycling and product separation.…”

Section: Introductionmentioning

confidence: 99%

Tethering of Cu(II), Co(II) and Fe(III) tetrahydro-salen and salen complexes onto amino-functionalized SBA-15: Effects of salen ligand hydrogenation on catalytic performances for aerobic epoxidation of styrene

Yang¹,

Zhang²,

Hao³

et al. 2011

Chemical Engineering Journal

104

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“…The asymmetric epoxidation of styrenes also has received a considerable amount of interest. Various chiral catalysts and reagents have been investigated for the epoxidation of styrenes, including chiral porphyrin complexes (16 -34), chiral salen complexes (35)(36)(37)(38)(39)(40)(41)(42), chiral oxaziridines and oxaziridinium salts (43)(44)(45)(46), and enzymes (47)(48)(49)(50)(51). Metal catalysts such as chiral porphyrin and salen complexes have been studied extensively for the epoxidation of these alkenes, and the enantioselectivities have reached the 80% range in a number of cases (21,23,29,31,37,38,40,41), with 96% enantiomeric excess (ee) being obtained in one case (3,5-dinitrostyrene) (23).…”

mentioning

confidence: 99%

Highly enantioselective epoxidation of styrenes: Implication of an electronic effect on the competition between spiro and planar transition states

Hickey

Goeddel

Crane

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

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Asymmetric epoxidation of various styrenes using carbocyclic oxazolidinone-containing ketone 3 has been investigated. High enantioselectivity (89 -93% enantiomeric excess) has been attained for this challenging class of alkenes. Mechanistic studies show that substituents on the ketone catalyst can have electronic influences on secondary orbital interactions, which affects the competition between spiro and planar transition states and, ultimately, enantioselectivity. The results described herein not only reveal the potential of chiral dioxirane catalyzed asymmetric epoxidation as a viable entry into this important class of olefins but also further enhance the understanding of the mechanistic aspects of chiral ketone-catalyzed asymmetric epoxidation. Epoxides are important intermediates for the synthesis of complex molecules. Asymmetric epoxidation of prochiral alkenes presents a powerful strategy for the synthesis of enantiomerically enriched epoxides. Great progress has been made in the areas of asymmetric epoxidation of allylic alcohols (1-3), chiral metal complex-catalyzed epoxidation of unfunctionalized alkenes (particularly with conjugated cis-and trisubstituted alkenes) (4 -7), and asymmetric epoxidation of electron-deficient alkenes under nucleophilic conditions (8 -10). Styrene oxides are extremely useful and can be prepared by a number of methods such as asymmetric reduction of ␣-halo acetophenones (refs. 11 and 12 and references therein), asymmetric dihydroxylation of styrenes (13,14), and kinetic resolution of racemic epoxides (15). The asymmetric epoxidation of styrenes also has received a considerable amount of interest. Various chiral catalysts and reagents have been investigated for the epoxidation of styrenes, including chiral porphyrin complexes (16 -34), chiral salen complexes (35-42), chiral oxaziridines and oxaziridinium salts (43-46), and enzymes (47-51). Metal catalysts such as chiral porphyrin and salen complexes have been studied extensively for the epoxidation of these alkenes, and the enantioselectivities have reached the 80% range in a number of cases (21,23,29,31,37,38,40,41), with 96% enantiomeric excess (ee) being obtained in one case (3,5-dinitrostyrene) (23).Among other oxidants, dioxiranes either isolated or generated in situ are powerful epoxidation agents (Scheme 1; refs. 52-54). Chiral dioxiranes were shown recently to be effective for asymmetric epoxidation of trans-, trisubstituted (55-91), and certain cis-alkenes (92-95). Asymmetric epoxidation of styrenes with chiral dioxiranes has been studied also (61,66,68,81,88,90,(93)(94)(95); however, the enantioselectivity has not exceeded 85% (93-95). Generally speaking, the highly enantioselective epoxidation of styrenes still remains a formidable challenge.During our studies, we found that varying the catalyst structure could have a dramatic effect on enantioselectivity for the dioxirane-mediated epoxidation of styrene. Fructosederived ketone 1 (Scheme 2), a very effective catalyst for transand trisubstituted alkenes, gave only 24...

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Alkene aziridination and epoxidation catalyzed by chiral metal salen complexes

Cited by 153 publications

References 19 publications

Stereoselective Ring‐Opening Polymerization of a Racemic Lactide by Using Achiral Salen– and Homosalen–Aluminum Complexes

Stereoselective Ring‐Opening Polymerization of a Racemic Lactide by Using Achiral Salen– and Homosalen–Aluminum Complexes

Tethering of Cu(II), Co(II) and Fe(III) tetrahydro-salen and salen complexes onto amino-functionalized SBA-15: Effects of salen ligand hydrogenation on catalytic performances for aerobic epoxidation of styrene

Highly enantioselective epoxidation of styrenes: Implication of an electronic effect on the competition between spiro and planar transition states

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