Oxidative Cyclization of Sulfamate Esters Using NaOCl - A Metal-Mediated Hoffman-Löffler-Freytag Reaction

Abstract

Intramolecular C-H amination with sulfamate esters occurs under the action of dinuclear Rh catalysts and iodine(III) oxidants, and has recently emerged as a powerful tool for synthesis. Insights gained through mechanistic studies of this process suggest that other terminal oxidants, including common halogenating agents such as NaOCl, could function to promote the cyclization reaction. Results described in this report demonstrate that the combination of NaOCl and 3 mol% Rh₂(oct)₄ is effective in select cases for converting sulfamate substrates to the corresponding [1,2,3]-oxa­thiazinane-2,2-dioxide heterocycles. The mechanism for C-H functionalization, however, is distinct from that induced by hypervalent iodine reagents and likely involves the intermediacy of an N-centered radical.

Key words

C-H amination - catalysis - rhodium - bleach - oxathia­zinane

References and Notes

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8

Tentative structural assignment of 2 is based on ^¹H NMR analysis of the reaction mixture.

9

The rather modest isolated yield of this material is likely due to product overoxidation. Analysis of the ^¹H NMR spectrum of the unpurified reaction mixture is quite clean, however. We assume that product(s) of overoxidation are removed in the aqueous workup.

11

The overall efficiency of the reaction is reduced when NaBr is excluded.

12

Optimized Reaction Protocol
To a biphasic solution of CH₂Cl₂ (2.5 mL) and sat. Na₂HPO₄ (2.5 mL, pH ca. 9) was added sequentially sulfamate (0.25 mmol), Rh₂(oct)₄ (6 mg, 7.5 µmol, 0.03 equiv), and NaBr (77 mg, 0.75 mmol, 3.0 equiv). The reaction flask was wrapped in aluminum foil prior to the dropwise addition of 1.6 M aq NaOCl (Acros, 470 µL, 0.75 mmol, 3.0 equiv). The biphasic mixture was stirred for 2 h, following which time the reaction was quenched by the addition of 1.0 mL of 1.0 M aq NaHSO₃. The contents were transferred to a separatory funnel with CH₂Cl₂ (10 mL) and H₂O (5 mL). The organic layer was collected and the aqueous layer was extracted with CH₂Cl₂ (1 × 10 mL). The combined organic fractions were dried over Na₂SO₄ and concentrated under reduced pressure to a yellow oil. Purification of this material by chromatography on SiO₂ (hexanes-EtOAc) afforded the desired oxathiazinane product.

16

Consistent with the proposed mechanism, when N-methyl-3-phenylpropylsulfamate is subjected to the reaction conditions, an oxidized product is formed. This material has been tentatively assigned as N-methyl-3-bromo-3-phenyl-sulfamate.