Article pubs.acs.org/Organometallics
Facile Generation and Isolation of π‑Allyl Complexes from Aliphatic Alkenes and an Electron-Deficient Rh(III) Complex: Key Intermediates of Allylic C−H Functionalization Yu Shibata,*,† Eiji Kudo,† Haruki Sugiyama,‡ Hidehiro Uekusa,‡ and Ken Tanaka*,† †
Department of Applied Chemistry, and ‡Department of Chemistry and Material Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan S Supporting Information *
ABSTRACT: It has been established that a strongly electrophilic η5-cyclopentadienylrhodium complex, [CpERhCl2]2, is capable of reacting with aliphatic alkenes in the presence of a silver salt and cesium acetate at room temperature to give the corresponding π-allyl complexes in high yields. The use of an alkenyltosylamide as the alkene also afforded the corresponding π-allyl complex. Treatment of the thus obtained π-allyl complex with a silver(I) salt and copper(II) acetate afforded the allylic amination product, which proves the intermediacy of this π-allyl complex in the rhodium(III)-catalyzed intramolecular oxidative allylic amination.
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[CpERhCl2]2 (1), bearing two ethoxycarbonyl groups on the cyclopentadienyl moiety, and have disclosed its high catalytic activity toward the oxidative sp2 C−H functionalization reactions of electron-rich arenes.17 In these reactions, sp2 C−H bond cleavage through concerted metalation−deprotonation might be accelerated as a result of a strong interaction between the aromatic π bond and the highly electrophilic rhodium(III) center. Similarly, the deprotonation of the allylic sp3 C−H bond would be accelerated by strong interaction of an electron-rich alkene with the electrophilic rhodium(III) center. In this paper, we disclose the generation and isolation of π-allyl complexes from aliphatic alkenes and [CpERhCl2]2 (1) in the presence of a silver salt and cesium acetate at room temperature. An alkenyltosylamide-derived (π-allyl)rhodium(III) complex (R = (CH2)4NHTs, Scheme 2) was also prepared, and its intermediacy in the rhodium(III)-catalyzed intramolecular
INTRODUCTION Allylic substitution through transition-metal π-allyl complexes as represented by the Tsuji−Trost reaction is an important bond-forming transformation in organic synthesis.1 The π-allyl complexes are typically generated by the reactions of transitionmetal complexes with allylic compounds possessing heteroatom-containing leaving groups.1 On the other hand, generation of the π-allyl complexes through deprotonation of aliphatic alkenes with the transition-metal complexes is a direct and atom-economical process and is involved in allylic sp3 C−H functionalization as a key step.2−7 Especially, allylic sp3 C−H functionalization via (π-allyl)palladium(II) complexes has been extensively studied and the proposed (π-allyl)palladium(II) intermediates have been isolated and well characterized.2−4 Recently, (π-allyl)rhodium(III) complexes8−12 have been proposed as key intermediates in rhodium(III)-catalyzed allylic sp3 C−H functionalization.13−16 For example, Cossy and co-workers reported the cationic Cp*RhIII-catalyzed intramolecular oxidative allylic amination of alkenyltosylamides leading to cyclic amines at elevated temperature (83 °C), while the corresponding (π-allyl)rhodium(III) intermediates have not been isolated and characterized (Scheme 1).14 In 2012, we developed the method for the synthesis of the electron-deficient cyclopentadienyl rhodium(III) complex
Scheme 2. Generation and Isolation of (π-Allyl)rhodium Complexes from Aliphatic Alkenes and Rh(III) Complex 1
Scheme 1. Rh(III)-Catalyzed Intramolecular Allylic Amination via (π-Allyl)rhodium Intermediates
Special Issue: Organometallics in Asia Received: February 20, 2016 Published: March 28, 2016 © 2016 American Chemical Society
1547
DOI: 10.1021/acs.organomet.6b00143 Organometallics 2016, 35, 1547−1552
Article
Organometallics oxidative allylic amination was confirmed by formation of the allylic amination product through treatment with a silver(I) salt and copper(II) acetate (Scheme 2).
Table 1. Optimization of Reaction Conditions for Preparation of (π-Allyl)rhodium Complex 6ba
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RESULTS AND DISCUSSION We recently reported the room-temperature oxidative annulation of anilides and benzyl alcohols with alkynes catalyzed by [CpERhCl2]2 (1), AgSbF6, and Cu(OAc)2·H2O.17a−c Subsequently, we reported that the oxidative olefination of acetanilide (1) with the aliphatic alkene 2a proceeds at room temperature by using the same catalyst system to give the corresponding arylalkene 3a in good yield (Scheme 3, top).17d This result
yield (%)b
Scheme 3. Reactions of Anilide 1 and Benzyl Alcohol 4 with Aliphatic Alkenes in the Presence of Catalyst 1
entry
additive
6b
7b
1 2 3 4 5 6 7 8 9 10 11c 12c
4 equiv AgSbF6 4 equiv AgBF4 4 equiv AgPF6 4 equiv AgNTf2 4 equiv AgOTf 2 equiv AgBF4 none 4 equiv AgBF4, 2 equiv Cu(OAc)2·H2O 4 equiv AgBF4, 2 equiv CsOAc 4 equiv AgBF4, 4 equiv CsOAc 4 equiv AgBF4 4 equiv AgBF4, 2 equiv CsOAc
5 22 24 9 9 4 0 62 95
