New Asymmetric Catalysis by (Salen)cobalt(III) Complexes (Salen=[Bis(salicylidene)ethylenediaminato]={{2,2′-[ethane-1,2-diyl]bis[(nitrilo-N)methylidyne]bis[phenolato-O]}(2−)}) of cis--Structure: Enantioselective Baeyer-Villiger Oxidation of Prochiral Cyclobutanones

Abstract

New Asymmetric Catalysis by (Salen)cobalt(III) Complexes (Salen [Bis(salicylidene)ethylenediaminato] {{2,2'-[ethane-1,2-diyl]bis[(nitrilo-kN)methylidyne]bis[phenolato-kO]}(2 À)}) of cis-b-Structure: This paper is dedicated with admiration and respect to Professor Dieter Seebach on the occasion of his sixty-fifth birthday A series of chiral (salen)cobalt(III) complexes (salen [bis(salicylidene)ethylenediaminato] {{2,2'-of cis-b structure were prepared and used for the enantioselective Baeyer-Villiger oxidation of prochiral cyclobutanones with hydrogen peroxide as terminal oxidant. Both cationic (salen)cobalt(III) and neutral iodo(salen)cobalt(III) complexes 3 ± 5 and 7 ± 12, respectively, all having a chiral binaphthalenediamine unit, were found to be effective catalysts for the enantioselective Baeyer-Villiger oxidation (Tables 1, 3, and 4). In particular, complex 8 bearing electronwithdrawing F-atoms showed a good level of enantioselectivity (75 ± 79% ee) in the reactions of 3arylcyclobutanones (Scheme 4). On the other hand, complex 12 bearing t Bu groups at C(3) and C(3') and electron-withdrawing NO 2 groups at C(5) and C(5') (trivial numbering) exhibited a high enantioselectivity of 98% ee in the reaction of a tricyclic cyclobutanone ( Table 4).

Asymmetric Baeyer–Villiger Oxidation of 2,3‐ and 2,3,4‐Substituted Cyclobutanones Catalyzed by Chiral Phosphoric Acids with Aqueous H2O2 as the Oxidant

“…The reaction was quenched by addition of an aqueous solution of Na 2 SO 3 (0.1 mL), and the product 5 was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, 6:1) in 99 % yield. [ α ] D 20 = –66.5 ( c 0.71, CHCl 3 ), 95 % ee [ref 10c. [ α ] D 20 = +62 ( c 1.0, CHCl 3 ) for (1 R ,4 S ,7 S ,10 R )‐isomer with >98 % ee ].…”

Catalytic Asymmetric Oxygenations with the Environmentally Benign Oxidants H2O2and O2

“…Uchida and Katsuki showed that cobalt salen complexes of the type 166 (5 mol %) mediated the oxidation of 3-phenyl-substituted cyclobutanone with H 2 O 2 and UHP, demonstrating good yields (72–92%) and moderate to good enantioselectivities (50–77% ee). , Ethanol was identified as the preferred reaction solvent, and UHP was shown to afford chiral lactones with higher optical yields than H 2 O 2 . Strukul and co-workers conducted the oxidation of cyclobutanones with H 2 O 2 in water in the presence of cobalt salen complex 167 (1 mol %); for the solubilization of the hydrophobic substrate and catalyst, micelles were used .…”

Rearrangements of organic peroxides and related processes

“…In recent years, the inexpensive, commercially available, and environmentally friendly H 2 O 2 was utilized in the Baeyer–Villiger reaction with increasing frequency. Various catalysts that activate hydrogen peroxide, such as heterogeneous catalysts based on solid acids [ 201 ], zeolites [ 202 – 203 ], Se [ 204 ], As [ 205 ], Co [ 206 ], sulfonated organic ion exchange resins [ 203 , 207 ], and homogeneous catalysts based on Pt [ 208 ], Zr [ 209 ], Re [ 210 – 211 ], Se [ 212 – 213 ], As [ 205 ], Mo [ 214 ], Co [ 215 ], Brønsted [ 216 ], and Lewis acids [ 217 ] are described in the literature. The general mechanism of a Lewis acid-catalyzed Baeyer–Villiger rearrangement is presented in Scheme 3 [ 200 , 218 ].…”