ChiralN-salicylidene vanadyl carboxylate-catalyzed enantioselective aerobic oxidation of α-hydroxy esters and amides

Abstract

A series of chiral vanadyl carboxylates derived from N-salicylidene-L-␣-amino acids and vanadyl sulfate has been developed. These configurationally well defined complexes were examined for the kinetic resolution of double-and mono-activated 2°alcohols. The best chiral templates involve the combination of L-tert-leucine and 3,5-di-t-butyl-, 3,5-diphenyl-, or 3,4-dibromo-salicylaldehyde. The resulting vanadyl(V)-methoxide complexes after recrystallization from air-saturated methanol serve as highly enantioselective catalysts for asymmetric aerobic oxidation of ␣-hydroxyl-esters and amides with a diverse array of ␣-, O-, and N-substituents at ambient temperature in toluene. The asymmetric inductions of the oxidation process are in the range of 10 to >100 in terms of selectivity factors (k rel) in most instances. The previously undescribed aerobic oxidation protocol is also applicable to the kinetic resolution of C-13 taxol side chain with high selectivity factor (k rel ‫؍‬ 35). X-ray crystallographic analysis of an adduct between a given vanadyl complex and N-benzyl-mandelamide allows for probing the stereochemical origin of the nearly exclusive asymmetric control in the oxidation process.alcohol oxidation ͉ ␣-hydroxy acids ͉ asymmetric catalysis ͉ vanadyl(V) methoxides T he oxidation of alcohols normally requires stoichiometric use of DMSO-based reagents or metal oxides of high oxidation state (1). Advances on their aerobic oxidations with catalytic metal oxides [e.g., RuO 2 -H 2 O (2), V 2 O 5 -K 2 CO 3 (3), and OsO 4 -Cu(O 2 CR) 2 (4)], homogeneous metal complexes [e.g., Co(OAc) 2 -N-hydroxyphthalimide (5), Ru(III)NOCl(salen) (6), RuCl 2 (PPh 3 ) 3 -hydroquinone-K 2 CO 3 (7), CuCl-Phen-DBADH 2 (8), RuCl 2 (PPh 3 ) 2 -TEMPO (9)͞CuCl-TEMPO (10), Pd(OAc) 2 -pyridine (11, 12), and Pd 4 Phen 2 (CO)(OAc) 4 (13)], bimetallic complexes [e.g., RuCl 3 -Co(OAc) 2 -aldehyde (14) and MoO 2 (acac) 2 -CuNO 3 ] (14, 15), or heterogeneous metal complexes [e.g., Ru(III) hydroxyapatite (16), polyaniline-supported MoO 2 (acac) 2 (17), and Pd(OAc) 2 -hydrotalcite-pyridine (18)] have been documented. Notably, additives and͞or bases are often needed to increase the catalyst reactivity and͞or to facilitate the turnover process. In addition, the targeted substrates are somewhat limited to primary, benzylic, allylic, and propargylic alcohols. Recently, the asymmetric variants of the aerobic catalytic process have attracted a lot of attention (19,20). So far, Pd(II)-sparteine (21, 22), photoly activated RuNOCl(salen) (23), and Mn(III)(salen)PF 6 (24) have been developed with fair to high enantioselectivity toward kinetic resolutions of 2°benzylic alcohols. In marked contrast, the asymmetric aerobic catalytic process with ␣-hydroxycarboxylic acid derivatives is relatively unexplored (25-31).Optically pure ␣-hydroxycarboxylic acid and mandelic acid derivatives are important precursors toward enantioselective synthesis scopes. Based on our experiences of using vanadyl and oxometallic species in catalyzing C-C and C-X bond forming (47-51), ae...

Kinetic Resolution of Racemic Mandelic Acid Esters by N,N′‐Dioxide–Scandium‐Complex‐Catalyzed Enantiomer‐Selective Acylation

“…[5] Despite the advances made in these asymmetric reactions, kinetic resolution (KR) [6] of racemic a-hydroxy esters is still one of the most important and reliable ways to acquire one isomer in an enantiomerically pure form. Two routes are usually employed in the KR of a-hydroxy esters: oxidative KR [7] and acylative KR [8] (Scheme 1). Chiral transition-metal-complex-catalyzed asymmetric aerobic oxidations transfer one isomer of a-hydroxy esters into a-ketoesters, therefore another unreacted isomer can be reversed enantioselectively.…”

Aerobic Oxidations in Asymmetric Synthesis: Catalytic Strategies and Recent Developments

“…Thus, Chen et al developed a series of chiral N -salicylidene vanadyl carboxylate complexes to perform catalytic kinetic resolution of α-hydroxycarboxylic esters, thioesters, amides ( Weng et al, 2006 ; Chen et al, 2007 ; Salunke et al, 2011 ) and α-hydroxyketones ( Chen et al, 2011 ) by using molecular oxygen as the terminal oxidant at ambient conditions. It was found that aerobic oxidation of ( S )-alcohols proceeds more readily than their ( R )-counterparts in the presence of vanadium complex 12 and its congeners ( Scheme 4 ).…”

“…It was found that aerobic oxidation of ( S )-alcohols proceeds more readily than their ( R )-counterparts in the presence of vanadium complex 12 and its congeners ( Scheme 4 ). As a notable example illustrating high oxidation selectivity, ( R )-isomer of N -benzylamide 13 was obtained in 50% yield and 98% ee ( Weng et al, 2006 ). Drastically slower rate of the oxidation for ( R )-isomer ( k rel = 458) was rationalized based on the results of single crystal X-ray analysis of the corresponding catalyst-substrate adduct 14 .…”

“…SCHEME 3 | Palladium-catalyzed aerobic oxidative kinetic resolution of secondary alcohols and its application in the total synthesis of alkaloids(Ferreira and Stoltz, 2001;Jensen et al, 2001;Caspi et al, 2004;Krishnan et al, 2008). SCHEME 4 | Kinetic resolution of N-benzyl mandelamide 13 via vanadium-mediated aerobic oxidation(Weng et al, 2006). Frontiers in Chemistry | www.frontiersin.org March 2021 | Volume 9 | Article 614944…”

Preface: Recent Advances and Highlights of the 5th International Vanadium Symposium

“…Coordination complexes and some of their applications continue to be reported by many groups, and include salen-type complexes (e.g. 19,66,[171][172][173][174][175][176], pyrone, pyridinone and pyrimidinone complexes (177)(178)(179), several types of Schiff bases and related complexes (66, [180][181][182], picolinato (e.g. 183), quinoline derivatives (184) dipicolinato complexes (185), compounds involving ligands such as amino acids or derivatives (28,(186)(187)(188)(189)(190)(191)(192)(193)(194)(195)(196)(197), hydroxycarboxylates (198,199), but novel classes of coordination compounds and other new reports on previously known compounds are too many to name.…”