Tris(1,2-diphenylethylenediamine)cobalt(III) Complexes: Chiral Hydrogen Bond Donor Catalysts for Enantioselective α-Aminations of 1,3-Dicarbonyl Compounds

Abstract

The enantiopure salt Δ-[Co((S,S)-dpen)3](3+)2Cl(-)B(C6F5)4(-) is an effective hydrogen bond donor catalyst for additions of 1,3-dicarbonyl compounds to di-tert-butyl azodicarboxylate in the presence of N-methylmorpholine (1.0:1.0:0.10) in CH3CN at 0 °C, as illustrated with educts derived from five- or six-membered ring ketones (99-88% yields, >99-91% ee) and cycloheptanone (94%, 72% ee) as well as 2-cyanocyclopentanone (92%, 45% ee) and an acyclic system (98%, >99% ee).

Recent developments in enantioselective cobalt-catalyzed transformations

“…In 2016, Gladysz and co-workers reported the use of Werner complex 129 based on chiral trication [Co((S,S)-dpen) 3 ] 3+ (dpen = 1,2-diphenylethylenediamine) to promote enantioselective a-aminations of 1,3-dicarbonyl compounds and related substrates, such as cyanoketones, in acetonitrile at 0°C [77]. As shown in Scheme 45, the reaction of various cyclic b-ketoesters 130 (X = CO 2 Me, CO 2 Et) with di-tert-butyl azodicarboxylate 131a catalyzed by 5 mol% of cobalt complex 129 in the presence of N-methylmorpholine as base afforded the corresponding chiral tertiary amines 132 in high to quantitative yields (88-98%) and moderate to excellent enantioselectivities (72->99% ee).…”

Enantioselective Additions of Stabilized Carbanions to Imines Generated from α-Amido Sulfones By Using Lipophilic Salts of Chiral Tris(1,2-diphenylethylenediamine) Cobalt(III) Trications as Hydrogen Bond Donor Catalysts

“…7 We have been engaged in developing novel types of chiral metal-containing hydrogen bond donors for use in enantioselective catalysis. [9][10][11][12][13][14][15][16] This represents an overlooked type of catalyst, as many researchers approach this subject from the organocatalysis angle. All too frequently, this community erects a 'Maginot line' when it comes to transition metals, rather than thinking opportunistically about all classes of hydrogen bond donors.…”

“…17,18 Most, but not all of our efforts have involved substituted tris(ethylenediamine) complexes of cobalt(III). [9][10][11][12]14,15 This focus was inspired by the pioneering work of Werner, who first resolved the helically chiral enantiomers of the watersoluble trication [Co(en) 3 ] 3+ -the configurations of which are denoted ∆ and Λ -via easily separated diastereomeric tartrate salts some 105 years ago. 19 Despite the promising beginning, this system has languished without applications in synthesis, as the cobalt center is 'substitution inert' (low spin d 6 with high field ligands) and therefore not accessible to organic substrates for activation via the usual modes of transition-metal-mediated catalysis.…”

“…The dominant configuration of 3aa switched from R to S, establishing the cobalt configuration as the principal determinant of the product configuration, as usually observed with other reactions. 10,11 Additional experiments with this catalyst, salts of each diastereomer with al-ternative counter anions, and analogues with other aryl groups in place of the phenyl moieties of dpen, are presented (entries [11][12][13][14][15][16][17]. None of these offered any improvement, although switching the tetraarylborate anion of the Δ diastereomer from BAr f to BAr f20 gave only a slight drop in ee (87%).…”

Readily Available Chiral Benzimidazoles-Derived Guanidines as Organocatalysts in the Asymmetric α-Amination of 1,3-Dicarbonyl Compounds

“…Among the different strategies developed to accomplish this, the electrophilic amination of 1,3-dicarbonyl compounds [ 4 , 5 , 6 , 7 ] has emerged as an attractive methodology. Since the pioneering work developed by the Jørgensen group, who published a metal- [ 8 ] and an organocatalyzed [ 9 ] amination of β-keto esters and β-keto nitriles with azodicarboxylates, the work done in this field has grown considerably [ 10 , 11 , 12 , 13 , 14 , 15 ]. In the last years, the work developed in the field of organocatalysis for this transformation has been quite prolific [ 16 , 17 , 18 , 19 , 20 ], especially thanks to the irruption of the hydrogen-bond organocatalysis, which has resulted in the publication of several works employing this strategy [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ].…”