Letter pubs.acs.org/OrgLett
Rh(II)-Catalyzed Chemoselective Oxidative Amination and Cyclization Cascade of 1‑(Arylethynyl)cycloalkyl)methyl Sulfamates Dong Pan, Yin Wei, and Min Shi* State Key Laboratory of Organometallic Chemistry, University of Chinese Academy of Sciences, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China S Supporting Information *
ABSTRACT: A Rh(II)-catalyzed chemoselective oxidative amination and cyclization cascade of 1-(arylethynyl)cycloalkyl)methyl sulfamates has been presented. For a cyclopropyl or cyclobutyl moiety containing alkynyl sulfamates, the reactions underwent a metallonitrene-initiated alkyne oxidation along with cyclopropyl ring expansion or alkoxyl moiety migration to give cyclobutane-fused or methylenecyclobutanecontaining heterocycles. In the case of a cyclopentyl or cyclohexyl moiety containing sulfamate, the reaction underwent a direct C−H bond insertion event to afford the corresponding nitrogen-containing heterocyclic product.
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Scheme 1. Previous Work and This Work
atalytically generated metallonitrenes or metal carbenes are highly versatile species and have shown broad applications in modern organic synthetic chemistry.1 Since nitrogen atom is essential to life science, considerable attention has thus been paid to the development of synthetic methods for its incorporation into organic molecules. The metallonitrene chemistry would offer great potential to build structurally complex nitrogencontaining molecules and has attracted much attention over the past decades.2 Besides the typical C−H amination3 and alkene aziridination,4 the cascade reactions of metallonitrene with alkynyl derivatives show more efficiency in multibond formations through the generation of an α-iminometal carbene intermediate. Metal carbene/alkyne cascades have a long history. Hoye5 and Padwa6 did pioneering works on catalytic carbene/alkyne cascade metathesis reactions with alkynyl-tethered diazo compounds. Since then, various cascade reactions of metal carbene/alkyne have been developed.7 For example, May’s group8 recently reported a carbene cascade reaction terminating in C−H insertion (Scheme 1a). As a comparison, the application of metallonitrene chemistry to cascade reactions with alkynyl derivatives has been demonstrated by the pioneering studies of Blakey and co-workers.9 They showed that α-iminometal carbenes could be generated via Rh-catalyzed metallonitreneinitiated alkyne oxidation, and the α-iminometal carbenes could be terminated in an array of reactions, such as oxygen-ylide formation, [2,3]-Wittig rearrangement,9d aromatic substitution, and cyclopropanation9b as well as intermolecular trapping by a variety of allyl ethers (Scheme 1b).9a With these termination methods of carbene/alkyne cascade reactions in mind, we wondered if it could be terminated by an alkyl migration process. On the basis of our ongoing investigation on the chemical transformations of strained small rings,10 we designed 1-(arylethynyl)cycloalkyl)methyl sulfamates for a metallonitrene-initiated metal carbene/alkyne cascade. As shown in Scheme 1c, the α-iminometal carbene II tethered © 2017 American Chemical Society
with an ortho-cyclopropane would be generated from metallonitrene I via a Rh-catalyzed metallonitrene-initiated alkyne oxidation. Intermediate II could be terminated by a cyclopropane ring-expanding process to give product III along with the regeneration of Rh catalyst according to Tang’s previous work.11 Based on this hypothesis, we utilized the sulfamic ester 1a for the initial examination using [Rh2(esp)2] (2.5 mol %) as the catalyst and PhI(OAc)2 (1.3 equiv) as the oxidant with 2.6 equiv of MgO as a base at room temperature in dichloromethane (DCM) under argon atmosphere. We were pleased to find that the desired ring-expanding product 2a was given in 71% NMR yield within 8 h (Table 1, entry 1). This result inspired us to further explore the better conditions for this reaction, and the results are summarized in Table 1. First, we screened various rhodium catalysts such as [Rh 2 (oct) 4 ], [Rh 2 (OAc) 4 ], Received: May 23, 2017 Published: June 28, 2017 3584
DOI: 10.1021/acs.orglett.7b01558 Org. Lett. 2017, 19, 3584−3587
Letter
Organic Letters Table 1. Optimization of the Reaction Conditionsa
entry
cat. (mol %)
oxidant
1 2 3 4 5 6 7 8 9 10 11 12 13 14c 15 16
Rh2(esp)2 Rh2(oct)4 Rh2(OAc)4 Rh2(Piv)4 Rh2(TFA)4 Rh2(esp)2 Rh2(esp)2 Rh2(esp)2 Rh2(esp)2 Rh2(esp)2 Rh2(esp)2 Rh2(esp)2 Rh2(esp)2 Rh2(esp)2
PhI(OAc)2 PhI(OAc)2 PhI(OAc)2 PhI(OAc)2 PhI(OAc)2 PhIO PhI(O2CtBu)2 PhI(O2CtBu)2 PhI(O2CtBu)2 PhI(O2CtBu)2 PhI(O2CtBu)2 PhI(O2CtBu)2 PhI(O2CtBu)2 PhI(O2CtBu)2 PhI(O2CtBu)2
base MgO MgO MgO MgO MgO MgO MgO CaO Cs2CO3 LiOtBu CaO CaO CaO CaO CaO
Rh2(esp)2
solvent
yieldb (%)
DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM toluene benzene DCM DCM DCM
71
