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Catalytic Intermolecular Cross-Couplings of Azides and LUMO Activated Unsaturated Acyl Azoliums Wenjun Li, Manjaly J. Ajitha, ming lang, Kuo-Wei Huang, and Jian Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03674 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017
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Catalytic Intermolecular Cross-Couplings of Azides and LUMO Activated Unsaturated Acyl Azoliums Wenjun Li,† Manjaly J. Ajitha,§ Ming Lang,† Kuo-Wei Huang,*,§ and Jian Wang*,† †School
of Pharmaceutical Sciences, Tsinghua University, Beijing, 100084, China
§Division
of Physical Sciences & Engineering and KAUST catalysis Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia Supporting Information Placeholder
ABSTRACT: An example for the catalytic synthesis of densely functionalized 1,2,3-triazoles through a LUMO activation mode has been developed. The protocol is enabled by intermolecular cross-coupling reactions of azides with in situ generated α,β-unsaturated acyl azoliums. High yields and broad scope as well as the investigation of reaction mechanism are reported. KEWORDS: 1,2,3-triazoles, azides, 1,3-dipolar cycloaddition, organocatalysis, unsaturated acyl azolium The 1,2,3-triazole nucleus is one of the promising heterocycles found in pharmaceutical chemistry and material science.1 It has earned the “privileged structure” title in drug discovery due to its plentiful pharmaceutical properties. On the other hand, ever-increasing demands especially in small molecule drug screening, drive the adventure of new synthetic tools that enable the construction of drug-like molecules with versatile substitutions.2 Within the context of triazole assembly, regioselectivity control has attracted much attention from organic chemistry community. Classic thermal 1,3-dipolar cycloadditions of alkynes and azides require high temperatures and result in triazoles with low levels of regioselectivity control.3 Later on, the problem of regioselectivity control was resolved through the copper-catalyzed azide−alkyne cycloaddition (CuAAC),4 a paradigm of the “click reaction“, thus affording 1,4-disubstituted 1,2,3-triazoles in good yields. In addition, the urgent appearance of Ruthenium-catalyzed (RuAAC)5 and Iridium-catalyzed (IrAAC)6 azide−alkyne reactions enables the regioselective synthesis of 1,5-disubstituted7 or 1,4,5trisubstituted 1,2,3-triazoles8 to be practicable and feasible. Although above methods are very important advances, the availability and the high cost of used starting materials (e.g. alkynes) prompted us to explore additional protocols. As one of the most promising alternative strategyies, the organocatalytic Ramachary−Bressy−Wang [3+2] cycloaddition of azides with in situ generated enamines has significantly contributed to furnishing 1,2,3-triazoles via a highly regioselective fashion (Figure 1(a), I).9 Shortly after, the Ramachary group reported another impressive DBU-catalyzed [3+2] cycloaddition proto-
col of aryl azides with in situ generated enolates,10 thus giving substituted 1,2,3-triazoles in good yields (Figure 1(a), II). In 2014, Wang et al. introduced a new direction to make 1,2,3triazoles through an efficient [3+2] cycloaddition process of
Figure 1. An Overview for Organocatalytic 1,2,3-Triazole Synthesis
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azides with in situ generated zwitterion catalyzed by DBU (Figure 1(a), III).11 It is important to note that the Dehaen group also independently reported an elegant multi-component reaction to access diversely functionalized 1,2,3-triazoles in situ through a DBU-catalyzed Knoevenagel condensation step.12 Although the breakthrough of metal-free synthesis of 1,2,3-triazoles is reported, the development of more sustainable variants is still highly desired as the 1,2,3-triazole core has already demonstrated numerous important applications in pharmaceutical chemistry and biological science.1 Table 1. Optimization of the Reaction Conditionsa,g
entry 1 2 3 4 5 6 7 8 9 10c 11h 12d 13c,e 14c,f
cat. solvent base K2CO3 CHCl3 C-1 CHCl3 K2CO3 C-2 CHCl3 K2CO3 C-3 K2CO3 CHCl3 C-4 CHCl3 K2CO3 C-5 CHCl3 K2CO3 C-6 CHCl3 K2CO3 C-7 THF K2CO3 C-4 THF Cs2CO3 C-4 C-4 THF Cs2CO3 -THF Cs2CO3 THF Cs2CO3 C-4 THF Cs2CO3 C-4 THF Cs2CO3 C-4 Further investigation on oxidant
yield (%)b 13 22 25 47 17 34 29 73 94 91 0 82 79 81
a
Reaction conditions: mixture of 1a (0.20 mmol), 2a (0.10 mmol), MeOH (0.5 mmol), cat. C (20 mol%), DQ as oxidant (0.10 mmol), and base (20 mol%) in solvent (0.3 mL, without degasing) stirred in a sealed tube (25 mL) at 80 oC for 24 h. b Isolated yield. c cat. C-4 (15 mol%) used. d cat. C-4 (10 mol%) used. e 1a (0.1 mmol) was used. f MeOH (0.3 mmol) was used. g DQ = 3,3',5,5'-tetra-tert-butyldiphenoquinone; If other oxidants (e.g. PCC, DDQ) used, none of 3a was detected by either GC or TLC. h None of 3a was observed without catalyst. Despite recent significant progresses on organcatalytic synthesis of 1,2,3-triazoles, a few issues have not yet been fully resolved: i) a compatible synthetic protocol for both 1,4disubstituted and 1,4,5-trisubstituted 1,2,3-triazoles with con-
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siderable molecular diversity enables structure-activity relationship studies (SAR) to be readily available; ii) a practical procedure to synthesize 1,2,3-triazoles from commercially available and commonly used starting materials, bearing versatile functionality. In view of these challenging questions, to develop more general, viable, but complementary routes would be of great relevance to synthetic chemists. Nowadays, N-heterocyclic carbenes (NHCs) have emerged as powerful Lewis base catalysts to construct a vast array of carbocyclic, heterocyclic, and polycyclic compounds.13 Inspired by these elegant works, we here report a novel NHC-catalyzed intermolecular cross-couplings of organic azides with in situ generated unsaturated acyl azoliums14 (Figure 1(b), IV). To our knowledge, this is the first example of 1,2,3-triazole synthesis facilitated by carbene catalysis. Pleasingly, this catalytic reaction can not only provide a high regioselectivity, but also assemble 1,4-disubsituted or 1,4,5trisubstituted 1,2,3-triazole compounds. In addition, the ester moiety on the triazole core skeleton could readily convert to other useful functional groups for further applications. We initiated our reaction condition optimization through screening NHC catalysts building on the model reaction of acrolein (1a), phenyl azide (2a), and MeOH (Table 1). Interestingly, the reaction of 2a with 2.0 equiv of 1a, 5.0 equiv of MeOH, 0.2 equiv of K2CO3, and 0.1 equiv of oxidant DQ (3,3',5,5'-tetra-tert-butyldiphenoquinone) in CHCl3 at 80 oC catalyzed by 20 mol% catalyst C-4 in a sealed tube, furnished 3a as a single regioisomer in moderate yield (Table 1, entry 4, 47%). Same reaction catalyzed by 20 mol% other NHC catalysts furnished 1,2,3-triazole 3a with low chemical yields (Table 1, entries 1−7). Switching the solvent to THF was successful in promoting the efficiency of the reaction (73%, entry 10). The choice of Cs2CO3 as the base resulted in the formation of 3a with excellent yield (Table 1, entry 9, 94%). It is worth to note that no product was observed in the absence of NHC catalysts (Table 1, entry 11). The effect of oxidant was also tested. Surprisingly, other commonly used oxidants (e.g. PCC or DDQ) gave no desire product. The feasibility of using 15 mol% C-4 was surveyed and resulted in a 91% yield (entry 10), however, the use of 10 mol% cat. C-4 caused a slight decrease in chemical yield (entry 12, 82%). In addition, lowering the amount of 1a led to a certain degree of loss in chemical yield (entries 13 and 14). To further clearly elucidate the oxidation process, an additional investigation on oxidation system was conducted and the relevant results were shown in Table 1. We found that the combination of DQ (1.0 equiv.) with air or oxygen is an effective oxidation system in achieving a high conversion (Table 1, air: 91%, 24 h; O2: 94%, 12 h). Surprisingly, when the reaction conducted in a glovebox or under N2, almost no desire product 3a was obtained even with 2.0 equivent DQ as oxidant, but methyl acrylates were mainly formed in above conditions. We envisioned that air or oxygen likely played an important role in triazole formation step. If DQ removed from system but kept oxygen in sealed tube, no 3a was detected. In summary, a combination of DQ and air or oxygen is critical for high conversion of this reaction. With the optimized reaction conditions in hand, we first investigated the scope and limitation of this transformation for the synthesis of 1,4-disubstituted 1,2,3-triazoles (Table 2). Various azides 2 were surveyed (Table 2). Aryl azides with electron–withdrawing or –donating groups in the para-, meta-
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Table 2. Scopea
N N
CHO
+
R1
R2
1
R
cat. C-4 (15 mol%) R3OH THF, Cs2CO3 , DQ 80 o C, 24 h
N3
2
Ph N
2
N N
R1 3
X= H H
N
Ph N
N
NH
X=H H
OH 4
CO2 R 3
N N
b
a
O
H
O
5
N N
N
N CO2Me
X
R1 = H
H N N
X N N
N N
N
N N
N
N
N N
3h; 85% Bn
3i; 83%
N N
N N N
BocHN
H
H Boc
H
CO 2Me
CO 2Me
3j; 93%
N
CO 2Me
CO2Me
3g; 83%
H 7
Scheme 1. Further Manipulation of 4-Methylester 1,2,3Triazole
H
H CO2Me
3k; 74%
Reaction conditions: a) LiOH aq., THF, RT, overnight, 92% yield; b) i. THF, DIBAL-H, RT, 12 h; ii. DCM, PCC, RT, 2h; iii. Isoniazid, EtOH/H2O, RT, 3 h, overall 82% yield (3 steps); c) LiAlH4, THF, -20 oC, 30 min, 95% yield; d) PhB(OH)2, [Pd(PPh3)4], K2CO3, toluene, 90 oC, 2 h, 88% yield.
N H+
CO2Me
3l; 78%
Ph
N N
N
CO 2Me
air
N
H
N
Ph N
CO 2X
X = n-Bu, 3m; 87% X = i-Pr, 3n; 78%
N N
1a
Ph
N CF3
O
O
N N
N
H
Ph
O
3o; 85%
O
N
VII
MeO2 C
Ph
N N
Ph N
CO2 Me X = 2-NO 2, 3q; 84% X = 3-Cl, 3r; 86% X = 4-OMe, 3s; 89%
N N
X N
X CO2Me X = 2-thiophenyl, 3t; 85% X = n-C4H9 , 3u; 83%
N
VI
3a
CO2 Me N N N
O N
Path B (disfavor)
N N
I
Path A (favor)
N
N
N
N N
N Ph
IV
[O] DQ Ph N3 2a
N N
O
O
N
N MeO V
CO2Me
recycle OH
N
N N
Ph N N N N
MeOH
recycle H
N N
cat. c-4
3p; 63%
R1 = H
X
N N
H
cat. c-4
Base
H
O
3a
[O]
VIII
Ph
CO2Me
d
OH
6
3f; 84%
Bn
N
N N
N
CO2Me
X = Cl, 3d; 91% X = Me, 3e; 85%
H
H
H CO2Me
CO 2Me
N N
N N
X = Br
c
N N N
F
N
H
H X = Br, 3b; 92% X = OMe, 3c; 94%
X= H
Ph N
X
N N
N N
MeOH II
air Ph N 3 2a 1,3-dipolar cycloaddition
[O] O
N
N
N
N N
N Ph
III
Ph CO 2Me
Scheme 2. Plausible Reaction Pathways
X = 3-pyridinyl, 3v; 82% X = n-C5 H11, 3w; 86%
a
Reaction conditions: mixture of 1 (0.20 mmol), 2 (0.10 mmol), cat. C-4 (15 mol%), DQ (0.10 mmol), Cs2CO3 (20 mol%) and R3OH (0.50 mmol) in THF (0.3 mL, without degassing) stirred at 80 oC for 24 h in a sealed tube (25 mL). or ortho-position were tolerated, giving the corresponding products 3b–f in high yields (Table 2, 84-94%). Naphthyl, heterocyclic, and alkyl azides also efficiently yielded their corresponding triazoles 3g–j. It is worth to note that this protocol allowes the incorporation of chiral structure motifs (e.g. 3k and 3l) to the N-1 position of triazole core. Next, we examined the scope of coupling partner alcohol 3. Functionalized alcohols, bearing CF3 or allylic group, were compatible with standard conditions, thus giving products 3o and 3p respectively. If the substrate scope range from acrolein to other α,βunsaturated aldehydes, the corresponding 1,4,5-trisubstituted 1,2,3-triazoles were obtained in good yields (3q–w). The structures of products 3a–w were assigned by NMR or based on X-ray single crystal structure analysis of 3d.15
Figure 2. Activation Barriers of 1,3 Dipolar Cycoladdition via Path A and Path B. Relative Free Energies Are Given in kcal/mol As indicated in Scheme 1, methyl 4-carboxylate 1,2,3triazoles could be transformed into other useful molecules, such as carboxylic acid 4, hydrazone 5, alcohol 6, and 1-
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biphenyl triazole 7. Notably, all of these derivatives were obtained in high yields. The mechanistic proposal is depicted in Scheme 2. Inital addition of carbene catalyst C-4 to enal 1a followed by deprotonation generates the Breslow intermediate I.16 Subsequent oxidation of I in the presence of DQ oxidant forms acyl azolium intermediate II.17 Building upon the active intermediate II, four plausbile reaction pathways were postulated. Path A and B are listed in Scheme 2. Full details of Path C and D are summarized in supporting information. For path A: 1,3-dipolar cycloaddition of 2a to II produces the dihydrotriazole intermediate III, which can proceed to the key intermediate IV through an aerobic oxidative aromatization process. IV then undergoes esterification and releases catalyst C-4 to form the cycloaddition product 3a. The high regioselectivity of this reaction is presumably due to the presence of unsaturated acyl azolium intermediate II, which lowers the energy of the lowest unoccupied molecular orbital (LUMO) resulting in a more electrophilic β carbon atom. For path B: an esterification of II delivers methyl acrylate intermediate V, which rearranges to form a Baylis-Hillman-like intermediate VI.18 Subsequent addition of VI to azide 2a forms intermediate VII. Elimination of the carbene catalyst produces the olefin intermediate VIII, which then leads to product 3a through a 6π electrocyclization after subsequent aerobic oxidation. To gain additional insights into the proposed reaction mechanisms, several control experiments were carried out. As indicated in Figure 1 (entry 11), there is no background reaction of 1a and 2a in the absence of NHC catalyst. Instead of acrolein 1a, the reaction between methyl acrylate and 2a was found to be very sluggish, suggested that methyl acrylate might not be generated in path B. In order to corroborate our hypothesis outlined in Scheme 2, we carried out a Mass Spectrometric (MS) study to monitor the reaction of 1a and 2a. The ESI-MS spectrum shows a major ion, m/z 287 (see SI for details), indicating the preference to probably form the key intermediate IV (Scheme 2). Taken together, these experimental observations demonstrated that path A is more favorable than others. Further mechanistic studies of the density functional theory (DFT) calculations of key steps in Path A and Path B were investigated using M06-2X functional with Gaussian 09 (Scheme 2; see full details in the SI). The acycl azolium intermediate II requires to overcome an energy barrier of only 16.5 kcal/mol for the 1,3 dipolar cycloaddition reaction with 2a (Figure 2), whereas the Baylis-Hillman-like intermediate VI has an activation barrier (∆G‡) of 65.7 kcal/mol for the nucleophilic addition reaction with 2a to form intermediate VII. These observations rule out the possibility of path B under the present reaction conditions and hence, it is more likely for the reaction to proceed via path A.
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Figure 3. Highest Occupied Molecular Orbital (HOMO) and Lowest Occupied Molecular Orbital (LUMO) Energies of 1a, 2a, and II Calculated at the M06-2X/6-31+G** Level Further analysis of frontier molecular orbitals showed that the formation of acycl azolium intermediate II significantly reduces the LUMO energy of the α,β-unsaturated system in 1a and facilitates the 1,3 dipolar cycloaddition reaction with 2a (Figure 3). The ∆G‡ of direct 1, 3-dipolar cycloaddition reaction between 1a and 2a of 31.4 kcal/mol (Path C in SI) is significantly reduced to 16.5 kcal/mol via the formation of intermediate II. In summary, we have developed a new NHC-catalyzed organocatalytic cross-coupling reaction to generate highly substituted 1,2,3-triazoles decorated with useful functional groups, which could be used for further manipulation and diversification. This method features not only metal-free and also high levels of regioselectivity. Further extensions of this chemistry to other new transformations are underway.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected], Jian Wang
[email protected], Kuo-Wei Huang Notes The authors declare no competing financial interest.
ASSOCIATED CONTENT Supporting Information. 1H NMR and 13C NMR spectroscopic and analytic data of the compounds 1 and 3 are included. This material is available free of charge via the Internet at http://pubs.acs.org.)
ACKNOWLEDGMENT Generous financial supports are provided by: the National Natural Science Foundation of China (21672121, 21502043), the “Thousand Plan” Youth program of China, the Bayer Investigator fellow.
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