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Enantioselective Synthesis of Chiral-at-Cage oCarboranes via Pd-Catalyzed Asymmetric B-H Substitution Ruofei Cheng, Bowen Li, Jie Wu, Jie Zhang, Zaozao Qiu, Wenjun Tang, Shu-Li You, Yong Tang, and Zuowei Xie J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018
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Journal of the American Chemical Society
Enantioselective Synthesis of Chiral-at-Cage o -Carboranes via Pd-Catalyzed Asymmetric B−H Substitution Ruofei Cheng,† Bowen Li,‡ Jie Wu,† Jie Zhang,§ Zaozao Qiu,*,†,ǁ Wenjun Tang,‡ Shu-Li You,⊥ Yong Tang,⊥ and Zuowei Xie*,†,§ †
Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Rd, Shanghai 200032, China ‡ State Key Laboratory of Bio-Organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Rd, Shanghai 200032, China § Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China ǁ Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Rd, Shanghai 200032, China ⊥
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Rd, Shanghai 200032, China Supporting Information Placeholder
ABSTRACT: Carborane cage chirality is an outstanding issue of great interest as the icosahedral carboranes have wide applications in medicinal and materials chemistry. The synthesis of optically active carborane derivatives, whose chirality is associated with the substitution patterns on the polyhedron, will open new avenues to carborane chemistry. We report herein an efficient method to achieve chiral-at-cage arylation of o-carboranes with high regioand enantio-selectivities by a strategy of palladium-catalyzed asymmetric intramolecular B−H arylation and cyclization. This represents the first example of the enantioselective reaction on carboranes, providing an efficient way for the construction of chiral-at-cage compounds with new skeletons.
als science and medicinal chemistry, where chirality plays an important role in molecular design. It has been documented that addition of substituents to ocarborane cage can lower the symmetry of the resultant molecules.9 For example, the presence of a substituent at the position 4/5 of 1-substituted o-carborane results in the chirality of the molecule (Chart 1, The observer looks onto the pentagonal plane of C(2)-B(3)-B(4)-B(5)-B(6) in o-carborane and then examines the positions of substituents according to the Cahn-Ingold-Prelog rule for the determination of the cage chirality (R or S)10).11 Inspired by the recent reports on fullerene cage chirality12 and ferrocene planar chirality,13 we have established a research program to develop enantioselective methods for the synthesis of optically pure chiral-at-cage o-carborane derivatives, which should add a new page in carborane chemistry.
The importance of chiral compounds and their enantioselective synthesis has been fully acknowledged to date by scientists in both pharmaceutical industry and academia. The stereoselective creation can be achieved by chiral pool, chiral auxiliary, chiral reagents and enantioselective catalysis of transition-metal complexes, as well as organo- and bio-catalysts. Among these asymmetric syntheses, transition-metal-catalyzed enantioselective reaction is one of the most important methods to obtain enantiomerically enriched compounds.1 In the past decades, many excellent asymmetric reactions have been developed for the synthesis of optically pure organic compounds.2 However, none of these reactions has been extended to the preparation of chiral-at-cage carboranes or boron clusters.
Chart 1. The Inherent Cage B(4/5) Chirality of oCarboranes
Icosahedral carboranes are carbon-boron molecular clusters, often viewed as three-dimensional analogues to benzene, which are finding many applications in medicine as boron neutron capture therapy agents or pharmacophores,3 in coordination/organometallic chemistry as versatile ligands,4 in supramolecular design/nanomaterials as functional building blocks,5 in optoelectronics as unique electron sinks and more.6 Although the inherent chirality of carborane cage has been reported, they are only restricted to chiral resolution7 or those tethered with asymmetric substituents.8 The inherent cage chirality is an important yet unresolved issue in the fields of asymmetric synthesis, materi-
We have recently reported a Pd-catalyzed intramolecular B(4)H arylation of o-carboranes in the presence of phosphine ligands.14 We speculated that the use of chiral phosphine ligands may lead to the development of asymmetric version of such arylation reaction. Herein, we report the first example of enantioselective synthesis of chiral-at-cage o-carborane derivatives via Pdcatalyzed asymmetric catalysis. Theoretically, palladium-catalyzed intramolecular arylation of 1-C(O)(o-C6H4Br)-1,2-C2B10H11 (1a) can afford four isomers (Scheme 1). In our previous work, the B(4)/(5)-arylated products (R)-2a and (S)-2a were obtained as racemate via direct cage B−H
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arylation.14 They are chiral by virtue of the substitution patterns on the icosahedral cluster. When PPh3 was used as a ligand, both enantiomers of (R)-2a and (S)-2a were formed in 1:1 ratio, which were able to be separated by HPLC on the Chiralpak IA-3 column (see Fig. S1-S3 in the SI). Their circular dichroism (CD) spectra exhibited unambiguously mirror images to each other, indicating a pair of enantiomers (Fig. 1). In addition, the absolute configuration of (R)-2a and (S)-2a was determined via single-crystal X-ray analyses of their corresponding B(12) iodinated derivatives (R)-4a and (S)-4a, respectively (Fig. 1). The optical rotation [α]D28 is 176.2 (c = 1.00, CH2Cl2) for (R)-2a and +186.8 (c = 1.00, CH2Cl2) for (S)-2a.
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Table 1. Optimization of Pd Sources and Chiral Ligandsa
Scheme 1. Palladium-Catalyzed Intramolecular B−H Arylation of o-Carborane
Entry
[Pd]
L
1
[Pd(allyl)Cl]2
2 3 4 5
Yieldb (%)
ee of
2a
3a
(S)-2a (%)
L1
-
-
-
Pd(OTf)2
L1
97
3
87
Pd(CN)4(BF4)2
L1
97
3
82
Pd(OAc)2
L1
98
2
90
Pd(OAc)2
L2
78
3
-75
6
Pd(OAc)2
L3
84
3
-53
7
Pd(OAc)2
L4
94
5
39
8
Pd(OAc)2
L5
23
2
13
9
Pd(OAc)2
L6
32
3
-33
10
Pd(OAc)2
L7
18
1
-34
11
Pd(OAc)2
L8
68
4
83
Fig. 1. CD Spectra of (R)-2a (blue)/(S)-2a (red) in MeCN (c = 1.33 mM) and Molecular Structures of (R)-4a and (S)-4a
a
Having aforementioned data on (R)-2a and (S)-2a, we then investigated Pd-catalyzed enantioselective intramolecular B−H arylation of o-carborane using chiral phosphine ligand (R)-BIDIME (L1).15 The screening results with L1 ligand were summarized in Table S1 in the SI. A complete consumption of 1a in the presence of 10 mol% of Pd(OAc)2, 10 mol% of L1 and 2 equiv of Cs2CO3 was observed in toluene, giving 90% ee of 2a at 0 oC for 14 h, followed by room temperature (25 oC) for 26 h. The absolute structure of the product 2a from this reaction was assigned as S-configuration by comparing the HPLC and optical rotation of the standard samples (R)- and (S)-2a shown in Scheme 1. We further screened the Pd sources and phosphine chiral ligands using 1a as a model substrate. Compared with Pd(OAc)2, Pd(OTf)2 and Pd(CN)4(BF4)2 gave slightly lower ee of (S)-2a, while [Pd(allyl)Cl]2 was inactive (Table 1, entries 1-4). Different chiral ligands were also examined. As shown in Table 1, L1 proved to be the most efficient chiral ligand in terms of enantioselectivity and reactivity, giving (S)-2a in 98% GC yield with 90% ee (Table 1, entry 4). L2 with methyl group at the 2-position of the oxophosphole ring and 9’-anthracenyl substituted L3 provided diminished reactivities and opposite enantioselectivities (Table 1, entries 5 and 6). L4 offered high activity but moderate ee (Table 1, entry 7). Bisphosphine ligands L5 and L6 were much less active (Table 1, entries 8 and 9). L7 afforded a very low activity (Table 1, entry 10). L8 gave (S)-2a in 68% GC yield and 83% ee (Table1, entry 11). Under the optimized reaction conditions (Table 1, entry 4), the
substrate scope was then examined and the results were summarized in Table 2. The reaction worked well for 9,12-dimethyl substituted o-carborane, affording (S)-2b in 65% yield and 96% ee. Various substituted arylbromides bearing either electron-donating or electron-withdrawing group were well-tolerated to afford the corresponding chiral cyclization products in good to excellent yields (63−90%) with moderate to excellent enantioselectivity (67−92% ee) (2c−2m), in which 4-methyl and 3-fluoro substituted arylbromides 1g and 1j gave high ee values of 92% and 90%, respectively. The aryl chloride was also tolerated in (S)-2m, which provided opportunities for further functionalization. The reaction with naphthylbromide 1n proceeded smoothly to afford the desired product (S)-2n in 93% yield and 83% ee. Notably, the conversion of different thienylbromide substrates was equally well (86-91% yields), albeit with different enantioselectivity. 2Acylthienyl-3-bromide (1p) gave the B−H arylated o-carborane (S)-2p in 71% ee, whereas 1-acylthienyl-2-bromide (1o) and 2acylthienyl-1-bromide (1q) resulted in very low ee’s of 9% and 26%, respectively, which may be related to the chelation of sulfur atom to Pd center.
Reactions were conducted at 0.02 mmol scale in 0.5 mL of toluene at 0 oC for 14 h, then rt for 26 h. bGC yields.
A catalytic cycle for the enantioselective formation of (S)-2a, involving oxidative addition of the C(sp2)−Br bond onto palladium(0), intramolecular B−H palladation and reductive elimination was proposed in Schemes S2-S3 (see the SI). In the crucial B−H bond activation step, the Pd with a chiral ligand can discriminate one among four ortho-B−H bonds. The regioselectivity between B(4/5)- and B(3/6)-positions on o-carborane cage may result from
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Journal of the American Chemical Society Table 2. Reaction Scopea,b,c
nyl group in TS-R, leading to a bigger Pd-P-C(tBu) angle of 123.9o than that of 116.1o in TS-S. The calculated energy of TS-R is 2.0 kcal/mol higher than that of TS-S, which is in accord with the enantioselectivity observed experimentally. In summary, the first enantioselective synthesis of chiral-atcage o-carboranes has been developed via Pd-catalyzed asymmetric intramolecular B–H arylation under mild reaction conditions. The absolute configuration of the products has been unambiguously assigned. Generally good to excellent yields with up to 96% ee can be achieved. The use of chiral monophosphine ligand is essential for such enantioselective B–H arylation. This work opens avenues to a new class of chiral-at-cage o-carboranes, and also sets a very good example for the development of enantioselective B–H functionalization in other carborane/borane clusters.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, characterization of the products (PDF), and crystal structures (CIF)
AUTHOR INFORMATION Corresponding Author
[email protected] [email protected] Notes The authors declare no competing financial interests. a
Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10 mol%), L1 (10 mol%) and Cs2CO3 (0.4 mmol) in toluene (2 mL) in a closed flask at 0 oC for 14 h, then rt for 26 h. bIsolated yield. cDetermined by chiral HPLC analysis and optical rotation data.
ACKNOWLEDGMENT This work was supported by grants from National Natural Science Foundation of China (no. 21772223 to Z.Q.), CAS-Croucher Funding Scheme, and NSFC/RGC Joint Research Scheme (N_CUHK442/14 to Z.X. and 21461162002 to Y.T.). We thank X.-L. Wan in Shanghai Institute of Organic Chemistry for his assistance with HPLC analysis.
REFERENCES
Fig 2. DFT calculated transition states TS-S and TS-R. The bond distances are given in Å and the bond angles are given in deg. both electronic and steric effects.16 For B(4)- and B(5)enantioselectivity, the P-chiral monophosphine ligand L1 plays a crucial role to achieve the cage chirality. To shed some light on the enantioselectivity in the current asymmetric B−H functionalization reactions, the transition states TS-R and TS-S leading to the final intramolecular arylation products in R and S configuration, respectively, were located by DFT calculations on the basis of concerted metalation-deprotonation (CMD) mechanism, in which the bicarbonate anion acts as the counteranion (Fig 2). In both calculated transition states, the Pd(II) has chiral ligand and one oxygen atom of the bicarbonate anion. The [(B···H)−C−P−O] dihedral angle of 20.3o in TS-R is bigger than that of 13.1o in TSS, suggesting a highly distorted coordination plane of the Pd center in TS-R. This difference arises from a greater steric influence of the t-butyl group of (R)-BI-DIME (L1) over the substrate phe-
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