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Synlett 2009(1): 143-146  
DOI: 10.1055/s-0028-1087392
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© Georg Thieme Verlag Stuttgart ˙ New York

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

David N. Zalatan, J. Du Bois*
Department of Chemistry, Stanford University, Stanford, CA 94305-5080, USA
e-Mail: jdubois@stanford.edu;
Further Information

Publication History

Received 2 September 2008
Publication Date:
12 December 2008 (online)

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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

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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.