Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
N‑Heterocyclic Carbene-Catalyzed Chemoselective S−O Bond Cleavage of Benzenesulfonic Carbamate Zijun Wu and Jian Wang* School of Pharmaceutical Sciences, Key Laboratory of Bioorganic Phosphorous Chemistry & Chemical Biology (Ministry of Education), Tsinghua University, Beijing, 100084, China
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S Supporting Information *
ABSTRACT: An unprecedented example of NHC-catalyzed chemoselective S−O bond cleavage of dinitrobenzenesulfonic carbamates is described. This protocol features several advantages, including mild reaction conditions, broad substrate scope, and operational simplicity, which allows it to be an attractive synthetic method for hydroxylamine synthesis. Notably, dinitrobenzenesulfonic carbamates not only work as “O” synthons but also serve as an efficient oxidant in this reaction.
B
groups, thus resulting in a number of synthetically useful molecules.1 Synthetically, this type of carbamate reagent can be rapidly prepared in two steps from commercially available Nmethyl hydroxylamines.2 Additionally, benzenesulfonic carbamates, commonly presented as uniformly bench-stable solids, can be stored in the air at ambient temperature without any decomposition.2a As essential synthons, benzenesulfonic carbamates can work as either “N” or “O” resources for further assembly, relying on the cleavage of the N−O bond (Figure 1a).1e,f,2,3 In 2013, the MacMillan group2a reported the example of α-amination by employing dinitrobenzenesulfonic carbamate to generate N-centered radicals as “N” resources (Figure 1b). In 2015, Chi4 et al. reported the NHC-catalyzed oxidative SET process by means of nitrobenzenesulfonic carbamates as the oxidant, thus resulting in an incorporation of a hydroxyl functional group to enals at the β-carbon center. Inspired by these elegant examples, we speculated that an amidation process via using dinitrobenzenesulfonic carbamate as the “N” resource and aldehyde as the substrate could be achieved under suitable reaction conditions. Unambiguously, the N-heterocyclic carbene (NHC) as the essential catalyst candidate is highly recommended due to its diverse and powerful activities in a wide range of organic transformations.5 To our surprise, our initial experimental results highlighted that the amidation product originally conceived was not observed, but replaced by finding a O-acyl hydroxylamine carbonyl ester. Upon examination, we found that, in comparison to the use of dinitrobenzenesulfonic carbamates as the electrophilic aminyl radical precursor2,3,6 (Figure 1a, route a), the alternative S−O bond cleavage process has been
enzenesulfonic carbamates are ubiquitous structures due to their wide and easy conversion with other functional
Received: October 18, 2018
Figure 1. Strategies for bond cleavage of benzenesulfonic carbamate. © XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03338 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Optimization of the Reaction Conditionsa
entry
cat.
base
solvent
yield (%)b
1 2 3 4 5 6 7c 8c 9c 10c 11c 12c 13c 14c 15c 16c
C1 C2 C3 C4 C5 C6 C5 C5 C5 C5 C5 C5 C5 C5 C5 C5
K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 Cs2CO3 Li2CO3 KHCO3 t BuOK KOAc
THF THF THF THF THF THF THF t BuOMe CH3CN toluene EOAc THF THF THF THF THF
