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Catalytic 1,2-Regioselective Dearomatization of N-Heteroaromatics Via a Hydroboration Heng Liu, Maxim Khononov, and Moris S. Eisen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00074 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018
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ACS Catalysis
Catalytic 1,2-Regioselective Dearomatization of N-Heteroaromatics Via a Hydroboration Heng Liu,a Maxim Khononov,a Moris S. Eisen*a a
Schulich Faculty of Chemistry, Technion – Israel Institute of Technology. Haifa City, 32000, Israel.
ABSTRACT: The thorium-methyl and -hydride complexes (C 5 Me 5 ) 2 ThMe 2 and [(C 5 Me 5 ) 2 Th(H)(μ-H)] 2 catalyzed highly 1,2regioselective dearomatization of pyridines via a hydroboration process is reported herein. Twelve different kinds of meta- and parasubstituted pyridines are applicable to this reaction, giving the corresponding N-boryl-1,2-dihydropyridine products in high yields. Other N-heteroaromatic compounds, such as benzofused N-heterocycles, pyrazines, pyrimidines, 1,3,5-triazine and benzothiazole, were also found to be hydroborated with high chemoselectivity. Kinetics including isotope effect studies revealed a first-order dependence on the concentration of catalyst, pyridine, and pinacolborane, with releasing the dearomatized final product as the rate determining step. A plausible mechanism is proposed based on stoichiometric reaction and kinetic studies. Keywords: Thorium, dearomatization, pyridine, N-heteroaromatic, 1,2-regioselective
INTRODUCTION The 1,2-regioselective dearomatizing of pyridines is a challenging task. It represents an important strategy to prepare valuable 1,2-dihydropyridines intermediates, which are frequently used as an important structural motif in the synthesis of alkaloids and pharmaceutically molecules, such as anticancer drug lepadin B and the antiviral drug oseltamivir (Tamiflu).1-6 Different from other N-heteroaromatics,7-14 pyridine compounds are much more difficult to be dearomatized due to their high resonance stabilization energy,15-18 and up to date, only a handful of methodologies are available. Hydrogenation is the simplest way to reduce pyridine aromatic rings, whereas the harsh conditions and the over-reduction reaction to afford piperidine often make this strategy difficult.19-21 Alternatively, catalytic hydrosilylation has emerged as a recent method to prepare dihydropyridines in recent years, however, it should be noted that most of the hydrosilylation reactions give rise to the 1,4-dihydropyridine,22-25 and this methodology sometimes also have encountered low selectivities,26 over-reactions,27-28 and a narrow substrate scope.25, 29 Therefore, developing a facile, atom-efficient and highly 1,2-regioselective method to dearomatize pyridine compounds is highly demanding. Our research interest is mainly focused on organoactinide precatalysts, due to the unique characteristics of actinide metals, such as their large ionic radii and the presence of the 5f-orbitals, which generally enable them to display complementary catalytic behaviors as compared to early or late transition metals, and lanthanide catalysts.30-43 N''
N''
Dipp
Th N
N
N
Dipp
H
N Si
Me3Si
Th N''
Th N''
CH3 CH3
H
Th
H
Th
H
N'' Th1
Th2
Th3
Th4
(N'' = N(SiMe3)2)
Figure 1. Structures of the thorium precatalysts Th1 - Th4.
Table 1. Hydroboration of pyridine under different conditions. H O + H B
O
N 5a
Cat.
N
70oC, 24h
Bpin
Bpin 7a
6 (HBpin)
N
H
7a' not observed
Entry[a]
Cat.
5a/6
Sol.
Temp. (oC)
Yield[b]
1 2 3 4 5 6 7 8 9 10
none Th1 Th2 Th3 Th4 Th4 Th4 Th4 Th4 Th4
1/1 1/1 1/1 1/1 1/1 1/1.5 1/2 2/1 1/1 1/1
C6D6 C6D6 C6D6 C6D6 C6D6 C6D6 C6D6 C6D6 C6D6 C6D6
70 70 70 70 70 70 70 70 50 25
---58 87 83 80 89[c] 33 4
[a]
Conditions unless specified otherwise: pyridine (0.25 mmol), HBpin (0.25 mmol), thorium precatalysts (5 μmol), 600 μL C 6 D 6 , 24 h. [b]The yield was determined by 1H NMR spectroscopy of the crude based on pyridine. [c] The yield was calculated based on HBpin. Recently, we introduced a series of imidazolin-2-iminato actinide complexes that were found active in promoting the hydroelementation reaction, in which a catalytic addition of unsaturated bonds into a large scope of E-H (E = N, O, P, C, S, Si) moieties were observed.44-50 Encouraged by these results, we, therefore, entered the idea that these actinide complexes might be also active for the hydroboration of the pyridine C=N bond,
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which would be a quite meaningful subject for actinide chemistry. It is of note that previously reported hydroboration of pyridines by Mg,51 Fe,52 Rh, 53 La,54 and recently a Zr-MOF system 55 have been disclosed. In some of these systems, a good 1,2regioselectivities were observed. Herein we disclose the rapid and selective hydroboration of pyridines and other N-heterocyclic systems. We commenced the pyridine hydroboration studies by examining four different types of thorium precatalysts (Figure 1) under different conditions, and pinacolborane was chosen as the hydroboration reagent (Table 1). Initial evaluation revealed that the thorium amido complexes, Th1 and Th2, were not effective for the hydroboration process and no reduced product 7a was detected within 24 hours. We subsequently shifted to study the thorium methyl complex Th3 and the thorium hydride complex Th4, and we were pleased to observe that the latter both systems catalyzed the reaction effectively, giving the N-borylated 1,2dihydropyridine product 7a in 58% and 87% yields, respectively. Increasing the amount of HBpin showed little influence on the yield of 7a. Increasing the amount of pyridine, however, resulted in a slight decrease in the yield. Attempts to carry out the reactions at even milder conditions with the complex Th4, i.e. lower temperatures, proved less efficient, affording only 4% and 33% yields at 25 oC and 50 oC, respectively. It is important to point out, and different from the Mg51, 56 and Rh53 based systems, for all the Th-based reactions, the regioisomeric N-boryl1,4-dihydropyridine 7a’ was not detected all through the hydroboration process. This result indicates the high 1,2-regioselectivities of the thorium precatalysts. Substituted pyridines (5b - 5p) were subjected to the hydroboration reaction in the presence of an equimolar of HBpin to study the substrate scope capabilities (Table 2). It is observed that a wide range of pyridine compounds with different electron-donating and electron-withdrawing groups are capable to undergo the 1,2-regioselective hydroboration to afford the corresponding dearomatized products in moderate to high yields. For meta-substituted pyridines, electron deficient 3-halopyridines generally performed better reactivities with HBpin than electron rich counterparts. Hence, the reactions of HBpin with 3-fluoropyridine (5b), 3-chloropyridine (5c), and 3-iodopyridine (5d) proceeded efficiently to give the corresponding 1,2-dihydroboration products 7b – 7d in 88%, 90% and 93% overall yields, respectively. Whereas for the substrate 3-methoxy pyridine (5f) a remarkably lower yield of 67% was obtained. Due to the asymmetric nature of the meta-substituted pyridines, two different hydroboration isomers, namely 3- and 5- substituted-1,2-dihydropyridines, are possible to be obtained. For the thorium complex, Th4, we obtained excellent regioselectivities to yield preferentially the 3-substituted-1,2-dihydropyridine, as the final product. Interestingly complex Th4, show a much higher regioselectivity as comparing to the literature values. 54 For some of the substrates, such as 3-fluoropyridine, 3-methylpyridine, and 3-methoxy pyridine, 3-substituted-1,2dihydropyridines were detected as the sole dearomatization products. Until now, it is still unclear what factors influence the product regioselectivity, however, in a similar study of La-catalyzed hydroboration of 3-halopyridines, Marks et al. rationalized that bonding interactions between C3-halogen atoms and newly emerged C2-CH 2 might influence the regioselectivity of 3-substituted pyridine dearomatization.54 For para- substituted pyridines, electron-withdrawing groups are also found to be favorable to facilitate the hydroboration rate significantly. Hence,
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in the reaction of 4-trifluoromethyl pyridine (5h) and HBpin, a much higher yield of 7h is obtained, even with shorter reaction times, as compared to substrate 5i. Table 2. Thorium complex Th4 catalyzed 1,2-hydroboration of substituted pyridines. R2
R2 R3
O
R1
H B +
R4
R3 Cat.
O
(5b
- 5p
H
Bpin - 7p (7b )
6 (HBpin)
)
N
R4
70oC, 24h
N
R1
Yield of product (%)[b]
Entry[a]
R1
R2
R3
1
F
H
H
H
5b
2[c]
Cl
H
H
H
5c
3[c]
I
H
H
H
5d
4 5 6 7[d] 8 9 10 11 12 13 14 15
Me OMe H H H H H H Me H H H
H H Ph CF 3 Me Et OMe NMe 2 Me H H H
H H H H H H H H H H H H
H H H H H H H H H Cl Br Me
5e 5f 5g 5h 5i 5j 5k 5l 5m 5n 5o 5p
R4
88 (7b) 90 (7c/7c’) (93/7) 93 (7d/7d’) (92/8) 95 (7e) 67 (7f) 99 (7g) 96 (7h) 72 (7i) 73 (7j) 60 (7k) 0 (7l) 68 (7m) 0 (7n) 0 (7o) 0 (7p)
[a]
Conditions unless specified otherwise: substituted pyridine (0.25 mmol), HBpin (0.25 mmol), thorium precatalyst Th4 (5 μmol), 600 μL C 6 D 6 , 70 oC, 24 h. [b] The yield was determined by 1H NMR spectroscopy of the crude based on pyridines. [c] Two isomer products were detected, and the ratio was shown in the bracket. [d] In 12 h. For reactions of pyridines having electron-donating groups at the para- position with HBpin, gradual decreasing yields in the order of Me~Et>OMe>NMe 2 have been observed, and due to the strong electron donating ability of the dimethylamino group, completely inactive substrate 5l was observed. H
O + N
H Bpin
O
Cat. 70oC, 24h
Bpin
N
(>99%, 7q) Bpin
N
N + 2 H Bpin
Cat. 70oC, 24h
H
H O N
+ 2 H Bpin OMe
O
Cat. 70oC, 24h
Bpin
N N
N H
H
(>99%,7r) Bpin (>99%,7s)
Scheme 1. Hydroboration of pyridines having other unsaturated functional groups. Different from the reported pyridine hydroboration systems,51, 53-54, 57 the thorium hydride complex Th4, displayed no activity
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ACS Catalysis in hydroboration of ortho-substituted pyridine compounds, probably due to the steric encumbrance of these substrates. The substrate scope was also expanded into pyridines bearing other unsaturated functionalities such as aldehyde, cyano- and ester functionalities (Scheme 1). Interestingly the thorium complex Th4 was able to induce the quantitative hydroboration of the aldehyde, cyano, and the ester moieties to give the corresponding products 7q - 7s. Addition of another equivalent of HBpin did not induce the hydroboration of the pyridine even after extended reaction times. Table 3. Thorium complex Th4 catalyzed hydroboration of different kinds of N-heteroaromatics. [a] X' X
Y
O
Z
+
H B O
N
Y'
Z'
Cat.
N
70oC
Bpin
(X, Y, Z = C or N)
H
(X', Y', Z' = C or N-Bpin) (8 - 16)
6 (HBpin)
Bpin N
N H Bpin
(> 99%, 24 h, 8) H
Cl
H N Bpin
(> 99%,12 h, 9)
N H Bpin
(> 99%, 6 h, 10) H
H
N
Bpin N
N H Bpin
Bpin
Br
N
Bpin
H N Bpin
H N Bpin
(> 99%, 12 h, 11)
H
(> 99%, 12 h, 12)
(90%, 12 h, 13)
H Bpin H
N
N
Bpin
H N Bpin
(96%, 6 h, 14)
present reaction, affording the hydroborated products 15 in moderate yields. Other types of N-heteroaromatics, including benzoxazole and benzimidazole, were also attempted for the hydroboration reaction, and no corresponding hydroborated products were detected. Stoichiometric reactions were studied in order to gain insights into the mechanism of this hydroboration reaction. The reaction of complex Th4 with pyridine rapidly gave the Cp* 2 Th(dihydropyridine) intermediate via a Th-H 1,2-addition across the pyridine C=N bonds, which was identified from in situ 1H NMR spectra by the disappearance of a characteristic hydride singlet (Th-H) at δ = 19.08 ppm and simultaneous appearance of the 1,2-dihydropyridine anion signals.22 Subsequent addition of HBpin to the above reaction mixture yields the dearomatized Nboryl-1,2-dihydropyridine product. Based on these studies, a plausible hydroboration mechanism is proposed in Scheme 2. The activation of the reaction cycle is firstly achieved by the insertion of Th-H moiety (A) into the C=N bond of the coordinated pyridine molecule, which gave rise to the thorium-dihydropyridine intermediate C (For Th3 mediated system, the ThMe bond undergoes protonolysis with HBpin to afford the ThH moieties firstly58). In the presence of HBpin, a subsequent Th-N/H-B σ-bond metathesis through the transition state D produces the target N-boryl-1,2-dihydropyridine products, and simultaneously regenerate the active species A to complete the catalytic cycle. Kinetic studies of pyridine/HBpin/Th3 system revealed a first-order dependence on pyridine, HBpin, and Th3, giving rise to the rate equation (1): 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕
= 𝑘𝑘𝑜𝑜𝑏𝑏𝑏𝑏 ∙ [𝐓𝐓𝐓𝐓𝐓𝐓][Pyridine][HBpin]
(1)
Bpin N H S
(55%, 24 h, 15)
[a]
Conditions unless specified otherwise: thorium precatalyst Th4 (5 μmol), 0.25 mmol HBpin, [C=N]/[HBpin] = 1/1 (0.25 mmol of heteroaromatics in case of compound 8, 9, 15, 0.12 mmol of heteroaromatics in case of compound 10, 11, 12, 13, 0.08 mmol of heteroaromatics in case of compound 14), 600 μL C 6 D 6 , 70 oC. The yield was determined by 1H NMR spectroscopy of the crude based on heteroaromatics. Based on the aforementioned hydroboration of pyridines, the present thorium hydride complex Th4 was next applied to additional types of N-heteroaromatics (Table 3). Quinoline and 8chloroquinoline rapidly reacted with equimolar of HBpin to afford the reduced products 8 and 9 almost quantitatively. In addition, we were also pleased to observe that pyrazine, pyrimidine and substituted pyrimidine compounds were able to undergo the double hydroboration to afford the corresponding dearomatized products in high yields. It is notable that the carbon-carbon double bonds in products 10, 11 and 13 successfully survived during the hydroboration reaction, indicating a highly chemoselective fashion of the precatalyst. Interestingly, 1,3,5-triazine underwent a triple hydroboration reaction with three equivalents of HBpin to give a saturated product 14 in excellent yield within 6h. Moreover, five-membered aromatic Nheterocycles, such as benzothiazole, was also subjected to the
Scheme 2. Proposed mechanism for the pyridine hydroboration reaction. In Th3 [Th] = Cp 2 *ThMe; in A-D, [Th] = Cp 2 *ThH. Deuterium labeling studies with DBpin afforded a kinetic isotopic effect (KIE) of K H /K D = 2.75, corroborating that the rate determining step (r.d.s) of the catalytic cycle is Th-N/H-B σbond metathesis. The thermodynamic activation parameters were experimentally calculated from the Erying and Arrhenius plots (Supporting Information), affording an activation barrier (E a ) of 20.3(1) kcal mol-1, and the enthalpy (ΔH≠) and entropy (ΔS≠) of activation of 19.6(5) kcal mol-1 and -23.5(1) e.u. re-
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spectively. The ΔH≠ and ΔS≠ data are consistent with a concurrent bond-cleavage and -formation events and the four-membered ring, respectively. In conclusion, we have established an efficient method for the dearomatization of pyridine compounds using a thorium-catalyzed hydroboration reaction. Highly 1,2-regioselective formation of N-boryl-1,2-dihydropyridine products was achieved when using meta- and para-substituted pyridine substrates. Various N-heteroaromatics are also applicable to this reaction, giving the hydroborated products in high yields. Studies to expand the substrate scope by using other types of actinide precatalysts are currently ongoing in our lab.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details, characterization data.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. Phone: +972-4-8292680 (M.S.E.).
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the Israel Science Foundation administered by the Israel Academy of Science and Humanities under Contract No. 78/14, and by the PAZY Foundation Fund (2015) administered by the Israel Atomic Energy Commission. H.L. thanks the Technion-Guangdong Fellowship Program.
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X
Y
or (C5Me5)2ThMe2 [(C5Me5)2Th(H)(µ-H)]2 Z
X'
1.0 equiv. HBpin, 70o C, 24h
N (X, Y, Z = C or N)
(substituted pyridines, quinoline, pyrimidine, pyrazine, triazine)
Y'
Z'
N
H
Bpin (X', Y', Z' = C or N-Bpin) Diverse substrates; High activities; High 1,2-regioselectivity;
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