Camphor‐Based Schiff Base Of 3‐Endo‐Aminoborneol (SBAB): Novel Ligand for Vanadium‐Catalyzed Asymmetric Sulfoxidation and Subsequent Kinetic Resolution

Abstract

Novel in situ formed vanadium Schiff base complexes, namely vanadyl Schiff base of 3-exo-aminoisoborneol (V(O)-SBAIB) and vanadyl Schiff base of 3-endo-aminoborneol (V(O)-SBAB), were demonstrated to catalyze an enantioselective sulfoxidation reaction. Among the twenty substitutionally and electronically different (1R)-camphor based Schiff base ligands, 6-bromo-2hydroxy-1-naphthaldehyde Schiff base of 3-endo-aminoborneol, 48 (SBAB-14), produced (S)-ethyl 2-naphthyl sulfoxide in excellent enantioselectivity (> 99 %) with good yield (72 %). In an optimized reaction condition, 1.5 mol% 48 (SBAB-14) catalyzed asymmetric sulfoxidation with concomitant kinetic resolution of various alkyl aryl sulfides and rendered corresponding sulfoxides with high ee (up to 97 %) with moderate yield (up to 74 %).[a] T.Figure 1. Structure of Schiff base ligands that applied in vanadium catalyzed asymmetric sulfoxidation.

Continuous Bioinspired Oxidation of Sulfides

“…Compound 1d was prepared according to the procedure reported in literature. [32] Analytical thinlayer chromatography (TLC) was performed on silica-gel-precoated (60F-254) aluminum foil sheets (Merck, Darmstadt, Germany), using short-wave UV light (TUV T8 HO 95W G13 UVC, Philips,…”

“…Compound 1d was prepared according to the procedure reported in literature. [32] Analytical thinlayer chromatography (TLC) was performed on silica-gel-precoated (60F-254) aluminum foil sheets (Merck, Darmstadt, Germany), using short-wave UV light (TUV T8 HO 95W G13 UVC, Philips, Milano, Italy) as the visualizing agent. NMR spectroscopic (Bruker, Fällanden, Switzerland) experiments were conducted at 25 °C on Bruker DPX and DRX spectrometers operating at the It was observed that in the case of vinyl sulfide 1e, the proposed methodology afforded only the oxidation of the sulfur atom, and in both conditions for the synthesis of the sulfoxide 2d or the sulfone 3d, the C=C was not oxidized.…”

“…Compound 1d was prepared according to the procedure reported in literature. [32] Analytical thinlayer chromatography (TLC) was performed on silica-gel-precoated (60F-254) aluminum foil sheets (Merck, Darmstadt, Germany), using short-wave UV light (TUV T8 HO 95W G13 UVC, Philips, Milano, Italy) as the visualizing agent. NMR spectroscopic (Bruker, Fällanden, Switzerland) experiments were conducted at 25 °C on Bruker DPX and DRX spectrometers operating at the…”

Enantioselective vanadium-catalyzed transformations. An update

“…Later, this methodology has been submitted to modifications [25]. For example, in 2016 Chen and Boobalan designed a novel camphor-based Schiff base ligand 18 to be employed in combination with VO(acac) 2 as a precatalyst to promote the asymmetric sulfoxidation of alkyl aryl sulfides with H 2 O 2 as an oxidant [26]. As illustrated in Scheme 13, when employed at 1.5 mol% of catalyst loading and combined with 1 mol% of VO(acac) 2 in chloroform at 0°C, it promoted the enantioselective sulfoxidation of a range of alkyl aryl sulfides 19a-l with hydrogen peroxide to give the corresponding (S)-configured sulfoxides 20a-l in good yields (60-74%) and uniformly high enantioselectivities (85-97% ee).…”

“…2 (1.35 equiv) 20a: Ar = 2-Naph, R = Et: 74% yield, 94% ee 20b: Ar = 2-Naph, R = Me: 72% yield, 97% ee 20c: Ar = 1-Naph, R = Et: 68% yield, 88% ee 20d: Ar = Ph, R = Me: 63% yield, 88% ee 20e: Ar = Ph, R = n-Bu: 60% yield, 88% ee 20f: Ar = Ph, R = Bn: 63% yield, 92% ee 20g: Ar = p-Tol, R = Me: 73% yield, 85% ee 20h: Ar = m-Tol, R = Me: 74% yield, 86% ee 20i: Ar = p-BrC 6 H 4, R = Me: 67% yield, 94% ee 20j: Ar = p-MeOC 6 H 4, R = Me: 60% yield, 89% ee 20k: Ar = p-O 2 NC 6 H 4, R = Me: 67% yield, Sulfoxidation of alkyl aryl sulfides with H 2 O 2 in the presence of vanadium camphor-based Schiff base catalyst 18[26].…”