Article Cite This: J. Org. Chem. 2019, 84, 289−299
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Phenalenyl Based Aluminum Compound for Catalytic C−H Arylation of Arene and Heteroarenes at Room Temperature Pavan K. Vardhanapu,† Jasimuddin Ahmed,† Anex Jose,† Bikash Kumar Shaw,† Tamal K. Sen,† Amita A. Mathews,‡ and Swadhin K. Mandal*,† †
Department of Chemical Sciences, Indian Institute of Science Education and Research-Kolkata, Mohanpur-741246, India Department of Chemical Sciences, Loyola College, University of Madras, Nungambakkam, Chennai-600034, India
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‡
S Supporting Information *
ABSTRACT: Main group metal based catalysis has been considered to be a cost-effective alternative way to the transition metal based catalysis, due to the high abundance of main group metals in the Earth’s crust. Among the main group metals, aluminum is the most abundant (7−8%) in the Earth’s crust, making the development of aluminum based catalysts very attractive. So far, aluminum based compounds have been popularly used as Lewis acids in a variety of organic reactions, but chemical transformation demanding a redox based process has never utilized an Al(III) complex as a catalyst. Herein, we tuned the redox noninnocence behavior of a phenalenyl ligand by coupling with Al(III) ion, which subsequently can store the electron upon reduction with K to carry out direct C−H arylation of heteroarenes/mesitylene at ambient temperature. A mechanistic investigation revealed that a three-electron reduced phenalenyl based triradical aluminum(III) complex plays the key role in such catalysis. The electronic structure of the catalytically active triradical species has been probed using EPR spectroscopy, magnetic susceptibility measurements, and electronic structure calculations using a DFT method.
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INTRODUCTION The characteristics of most of the transition metal mediated catalysis are facile substrate activation and their rearrangement triggered by switching of different accessible redox states.1 On the other hand, substrate activation by Lewis acidic nontransition metals such as alkali, alkaline earth metals, or by other main group metals is very well-known which works through an entirely different mechanism than that of a typical transition metal mediated process involving redox switching.2 It has long been discussed and considered as an open challenge to mimic the transition metal like behavior by a main group element in catalysis.3 The major impetus of mimicking transition metals by a main group element stems from its low toxicity, less expenses, and high abundance. However, designing a redox chemistry to devise a catalytic organic transformation with the compounds of main group elements is extremely challenging. One school of thought introduced by Power, Roesky, Bertrand, and others considered that the use of low-valent main group complexes may introduce feasibility on the oxidative addition process; however, the reverse reductive © 2018 American Chemical Society
elimination was not favorable which is essential to complete the catalytic cycle as it regenerates the active catalytic species.3−5 As a result, the chemistry of low-valent main group elements can mimic the transition metals only halfway in which the small molecules activation by oxidative addition could be accomplished.6 However, the reductive elimination reaction is the hallmark of a transition metal mediated catalytic process; often, the last step of a typical catalytic cycle remained elusive in such design using low-valent main group chemistry. It thus warrants devising conceptually a new strategy to achieve such a goal. Keeping this aim in mind, herein, we have taken advantage of a well-known redox noninnocent ligand phenalenyl (PLY) and synthesized an Al(III) based complex (Scheme 1) for designing direct C−H arylation catalysis at ambient temperature. Early HMO calculations predicted that phenalenyl can exist in three redox active states such as cation, neutral radical, and Received: October 19, 2018 Published: December 3, 2018 289
DOI: 10.1021/acs.joc.8b02699 J. Org. Chem. 2019, 84, 289−299
Article
The Journal of Organic Chemistry Scheme 1. Redox Catalysis of Al(III) Complex, 1
Figure 1. (a) Redox process for 1. (b) Cyclic voltammetry of complex 1 in chloroform at a scan rate of 0.1 V s−1 acquired using Pt working electrode, Pt wire as the counter electrode and reference electrode, internally referenced to Ag/AgCl.
There has been seminal effort by König and co-workers to avoid transition metals as catalyst to couple aryl diazonium salt with heteroarene partner under light stimulation; however, this method failed to activate an arene partner.20 It may be noted that the coupling between aryl diazonium salt and arene partner was successfully accomplished in 2017 by Lee and coworkers using a dual gold and ruthenium based photoredox catalyst, which induces an electron transfer process to activate the aryl diazonium coupling partner while the Au based catalyst acts as a Lewis acid for electrophilic activation of the arene partner.21 More recently, this coupling reaction between aryl diazonium partner and arene/heteroarene partner was further modified by Ackerman and co-workers using an efficient Mn(I) based catalyst under blue LED irradiation.22 Recently, we have reported the catalytic protocol for this direct
anion using its unique nonbonding molecular orbital (NBMO).7−12 In recent years, we have explored the cationic state of phenalenyl by coordination with appropriate metal ions, and it is capable of accepting electron(s) to form a phenalenyl based radical.13 The in situ generated phenalenyl radical can show a spectacular range of properties spanning from spin based device, fuel cell application, and transition metal mimicking catalysis.14−19 In the present work, the phenalenyl based Al(III) complex was used to store electrons by reducing it with a chemical reductant. Subsequently, the stored electron was transferred through a single electron transfer (SET) process to generate the aryl radical from an aryl diazonium coupling partner which can couple with the arene/heteroarene partner, delivering a facile route to C−H functionalization under mild conditions. 290
DOI: 10.1021/acs.joc.8b02699 J. Org. Chem. 2019, 84, 289−299
Article
The Journal of Organic Chemistry
Figure 2. (a) Preparation of three-electron reduced species IV from complex 1. (b) Spin density plot of IV; Isovalue = 0.02, density = 0.0004 e−/ au3.
Figure 3. (a) Variation of μeff values as a function of temperature for reduced triradical species IV (Blue). Variation of χMT values with temperature (Black). Solid line (Red) represents the theoretically fitted curve, and the points are the experimental data (Black). (b) EPR spectrum of complex 1 and gradual change in the EPR spectrum of complex 1 on addition of 1/2/3 equiv of K. The signal intensities in the EPR spectra are not normalized.
crystallized from CHCl3/hexane and yielded 77% of crystallized product. Complex 1 was further characterized by 1H and 13 C NMR spectroscopy and matched with the earlier report.24 To understand the redox behavior of complex 1, a cyclic voltammetric (CV) experiment was carried out in chloroform which shows three, quasi-reversible, single-electron reductions at −1.28 V (E11/2), −1.52 V (E21/2), and −1.78 V (E31/2), (Figure 1b).25 The CV result clearly indicates that there are three consecutive one-electron reductions which may be attributed to gradual acceptance of three electrons into three phenalenyl units. This observation is in consonance with previous study carried out on a similar PLY based complexes.26−28 It clearly reveals that three consecutive oneelectron reduction processes are generating a radical, biradical,
C−H arylation of arenes and heteroarenes by a redox active phenalenyl assisted Fe(III) complex under ambient conditions.23 However, all of these recent reports failed to provide a completely transition metal free direct C−H arylation catalysis at ambient conditions without using external light stimulation activating both arene and heteroarene coupling partners. The present work addresses this concern.
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RESULTS AND DISCUSSION First, we prepared the aluminum(III) complex (1) with 9hydroxyphenalenone (PLY) ligand using anhydrous AlCl3 precursor following an earlier method after modification.24 Complex 1 was obtained as a yellow precipitate after 16 h reflux in dichloroethane at 100 °C. The product was 291
DOI: 10.1021/acs.joc.8b02699 J. Org. Chem. 2019, 84, 289−299
Article
The Journal of Organic Chemistry and triradical species, respectively (vide infra, Figure 1a). It may be noted that the differences between the consecutive reduction potentials (ΔE2‑1 = −0.24 V and ΔE3‑2 = −0.26 V, respectively) matches very well with earlier electrochemical results.25 Its worth to note that the ΔE is considered as the disproportionation potential and is directly correlated to the on-site Coulomb correlation energy (U).27 The observed ΔE values are in close agreement to bis-phenalenyl-boron complexes (ΔE2‑1 values are −0.29 V and −0.28 V),27 trisphenalenyl-silicon (ΔE2‑1 = −0.24 V and ΔE3‑2 = −0.32 V, respectively), and tris-phenalenyl-germanium complexes (ΔE2‑1 = −0.22 V and ΔE3‑2 = −0.28 V, respectively)28 bearing the same O,O-phenalenyl ligand. These identical ΔE values observed for 1 strongly suggest that successive electron reduction processes occur into phenalenyl moieties. After having this information on redox behavior of complex 1, three electrons were chemically injected to 1 by reduction with 3 equiv of K, yielding a triradical paramagnetic compound IV (Figure 2a). Further, the DFT calculation at the B3LYP/631+g* level reveals that three electrons after reduction populates on PLY moieties as shown in the spin density plot (Figure 2b). The natural bond orbital (NBO) analysis of threeelectron reduced species IV revealed empty 3s and 3p orbitals on aluminum, which clearly supports the presence of an Al(III) center and is consistent with other experimental results (see below). To further explore the electronic spin structure of the catalytically active species IV, we have performed variable temperature dc magnetic susceptibility measurements in an applied field of 0.01 T in the temperature range 2−300 K. The μRT value obtained from the variation of effective magnetic moment (μeff) as a function of temperature (Figure 3a) is 3.92 μB. This is close to the theoretical value (3.87 μB) of triradical species IV generated after successive reduction of three PLY moieties present in complex 1. The existence of a significant antiferromagnetic coupling between these radical spins is revealed from the continuous decrease in the μeff values upon lowering down the temperature to 2 K. To understand the strength of this superexchange coupling, the experimental data were fitted using the following spin Hamiltonian (1) considering three PLY units as equilateral triangular sites.29 The molar susceptibility is plotted in the form of χMT vs T. The sharp fall in the χMT values indicates a significant contribution of temperature independent paramagnetism (TIP) in the species IV. The best fit is obtained considering the superexchange coupling J between the three S = 1/2 PLY•− centers and the Lande g factor (from EPR studies, Figure 3b) as ∼1.99; the extracted parameters are found as J = −25.38 (2) cm−1, TIP = 6 × 10−2 cm3 mol−1. Good agreement factor (R2) in the order of 10−5 is obtained in the least-squares fitting procedure (Figure 3a) H = −(JSiSj + JSjSk + JSiSk)
direct C−H arylation of arenes and heteroarenes with aryl diazonium salt coupling partners under ambient conditions using 1 in the presence of a chemical reductant. In order to design such a reaction, furan (1a) was taken as the arene partner and 4-chloroaryl diazonium salt 2a was taken as the aryl diazonium-coupling partner to obtain the arylated product 3a (Table 1) under different conditions using phenalenyl supported aluminum complex 1. Table 1. Optimization Study for C−H Arylation of Furana
entry
catalyst (mol %)
reductant (mol %)
solvent
time (h)
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
1 (10) 1 (10) 1 (10) 1 (5) 1 (2) 1 (5) 1 (5) 1 (5) 1 (5) 1 (5) 1 (5) none 1 (5) Al(acac)3
K (40) none TDAE (40) K (20) K (8) K (5) K (10) K (15) K (20) K (20) K (20) K (20) K (20) K (20)
DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMF THF DMA DMSO DMSO DMSO
24 24 24 24 24 24 24 24 24 24 24 24 12 14
74 NR
