Selectivity in the Olefin-Intervened Macrocyclic Ring-Closing Me

starting α,m,ω-triene (Figure 1d). A pioneering and extraordinary example is the total synthesis of natural .... resulting in the thermodynamic sele...
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Selectivity in the Olefin-Intervened Macrocyclic Ring-Closing Metathesis Ruzhang Liu, Hua Ge, Kuanwei Chen, and Huaiguo Xue ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01084 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Catalysis

Selectivity in the Olefin-Intervened Macrocyclic Ring-Closing Metathesis Ruzhang Liu,* Hua Ge, Kuanwei Chen, Huaiguo Xue College of Chemistry and Chemical Engineering, Yangzhou University, 180 Siwangting Rd. Yangzhou, 225002, China

ABSTRACT: The ring-closing metathesis (RCM) of α,m,ω-triene was employed for the syntheses of four slightly different EE-, EZ-, ZE- and ZZ-isomers of macrocycles with two far separated type I olefins. Kinetic control of RCM reaction using G1 is achieved, in which secondary metathesis of initially formed RCM product that usually afforded the thermodynamically favored product is inhibited; and a reliable model was built to differentiate the kinetic or thermodynamic control in the RCM reaction. Kinetic studies of olefin metathesis using the kinetically controlled Z-selective catalyst Cat-Z were conducted to approve its theremodynamic capability to convert Z-olefin to E-olefin. Finally, the ring size selectivity of α,ω- vs. α,m/m,ω-products in the RCM reaction of α,m,ω-triene revealed that all types of isomeric α,ω-products are favored for the 18-membered ring and above, and α,m/m,ω-products are predominated for the 14-membered ring and less. For the 15-, 16and 17-membered ring, each of E/Z-selective RCM reactions of starting mE- and mZ-trienes has different performance under the optimized conditions. These studies would provide insights in the applications of RCM on the synthesis of macrocycles with two separate type I olefins. KEYWORDS: ring-closing metathesis, ring size selectivity, Z selectivity, kinectics, dimerization

Selectivity is a long-standing problem faced by all types of olefin metathesis that is one of the most powerful tools for the formation of the Carbon – Carbon double bond.1 In the cross metathesis (CM) of two olefins, control of E/Zselectivity is an issue along with the competing selfmetathesis and olefin isomerization (Figure 1a).2 Until recently, highly selective formation of E- or Z-olefins was achieved using the catalysts developed independently by Grubbs and Hoveyda groups.3,4 In the ring-closing metathesis (RCM) of α,ω-dienes,5 double bond isomerization is negligible for the synthesis of most 5- and 6-membered carbo- and heterocycles,6 but is specifically utilized in the tandem RCM reactions (Figure 1b).7 The competing self-metathesis and dimerization of precursors are pronounced in the formation of medium-sized ring.8 The E/Z-selectivity issue of the newly formed double bond may arise for the RCM products with larger than 9membered ring systems.9 Similarly, highly E- or Z-selective macrocyclic RCM was also achieved recently using the newly developed catalysts.10,11 As olefin metathesis goes by, chemical practitioners have been wondering the impact of additional double bond existing in the substrates on CM or RCM. These reactions of conjugated 1,3-dienes, although cumbersome, have been investigated and applied in the synthesis of several natural products and pharmaceuticals (Figure 1c).12 Besides 1,3dienes, highly Z-selective CM of 1,4-dienes is also reported.13 While the internal olefin in the RCM precursor, i.e., α,m,ω-triene (m denoting the position of internal

Figure 1. Olefin metathesis.

olefin), has the possibility to react with α- or ω-olefin, the desired α,ω-RCM product might be complicated with α,mor m,ω-products. However, the relay RCM reaction takes advantage of the less hindered α-olefin in the triene to initiate the RCM, and the release of α,m-product, usually being the easily formed 5-membered ring, furnishes the sterically or electronically encumbered m,ω-product.14 For the syntheses of products with two highly active olefins (i.e., type I) in the ring,2d the RCM reaction is less utilized, possibly due to the uncertainty of reactive sites in the starting α,m,ω-triene (Figure 1d). A pioneering and extraordinary example is the total synthesis of natural

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product cylindrocyclophane F accomplished by Smith, in which the 12-olefin of 1,12,23-triene was reorganized in the RCM reaction to afford the thermodynamically stable one of seven possible isomers with a 22-membered [7,7]paracyclophane ring.15 Another issue is the geometry survival of m-olefin in the RCM reaction, particularly for the less stable Z-olefin, which would be instructive in the total synthesis of natural products with two or more separate olefins.16 Herein, we report the equilibration of EE-, EZ-, and ZZ-isomers of 18-membered ring generated from possible substrates promoted by the first-generation Grubbs’ catalyst G1 and the Z-selective Grubbs’ catalyst Cat-Z, and the application on the macrocyclic RCM reaction revealing the selectivity turn of α,ω- vs. α,m/m,ωproducts. Azonan-5-ene, with 9-membered ring, not only constitutes the fawcettimine-type Lycopodium alkaloids,17 but also promotes the electrophile-triggered transannular cyclization in the synthesis of functionalized indolizidine alkaloids.18 For its synthesis, the sluggish nucleophilic ring closure strategy renders the RCM reaction powerful.19 Initially, when sulfonamide 1 was exposed to 5 mol% of

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HG2 in the refluxing dichloromethane (entry 1, Table 1), surprisingly one compound was obtained in 47% yield and is different from the expected 2, which was prepared by the routine synthetic route and confirmed by X-ray analysis (Table S2, SI). Fortunately, x-ray quality crystal of EE-3, one of three dimeric isomers, was deposited from the mixture (Table S3, SI), and the 13C NMR spectrum and HPLC analysis, in combination with later results, confirmed the existence of another two possible isomers EZ- and ZZ-3 in the mixture with the ratio of 70:27:3. The replacement of CM catalyst by the less active G1 gave 32% of 2, along with 24% of dimer 3 (entry 2, Table 1). The addition of Ti(OiPr)4, the known additive used to prevent the coordination between the substrate and the active ruthenium intermediate,20 increased the production of 2 (entry 3, Table 1). Upon the utilization of the commercialized Z-selective CM catalyst Cat-Z in this reaction (Figure 2), it afforded 34% yield of 3 with a 2:14:84 ratio of three isomers, which reflects the superior Z-selectivity control of this unique catalyst (entry 4, Table 1). The structure of ZZ-3 was ascertained by X-ray crystallography (Table S4, SI).

Table 1. Monomeric and Dimeric Selectivity in the RCM Reaction of 1.a

Scheme 1. Syntheses of RCM Precursors E-6 and Z-6. Table 2. Selectivity in the Macrocyclic Reactions of E6 and Z-6.a

entry 1 2 3 4

cat. HG2 G1 G1d Cat-Z

conc. (mM)

yield of 2 (%)b

8.1

0

3.3 3.3 16.3

32 60 20

yield of 3 (%)b (EE:EZ:ZZ)c 47

entry

sub.

cat.

(mol%)

1

E-6

G1

10

2

E-6

HG2

17

3

E-6

Cat-Z

12

4

Z-6

G1

11

5

Z-6

HG2

19

6

Z-6

Cat-Z

13

(70:27:3) 24 (55:40:5) 32 (59:35:6) 34 (2:14:84)

a Conditions: 1 (0.3-1 mmol), 5 mol% of catalyst, CH2Cl2 (specified concentration), reflux, 18h. bYield of isolated product. cDetermined by HPLC. dTi(OiPr)4 (30 mol%) was added.

yield of 3 (%)b (EE:EZ:ZZ)c 52 (74:25:1) 59 (71:27:2) 79 (13:84:3) 55 (40:51:9) 55 (80:18:2) 72 (2:23:75)

a

Conditions: E- or Z-6 (1.7 μmol), CH2Cl2 (1.7 mM), reflux, 18h. bYield of isolated product. cDetermined by HPLC.

Figure 2. The olefin cross-metathesis catalysts.

Inspired by the predominant production of EZ-3 bearing the superimposable coincidence, two alternative strategies were suggested, one of which is the Z-selective

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ACS Catalysis

RCM (Z-RCM) of E-6, and the other is the E-selective RCM (E-RCM) of Z-6. Beginning with sulfonamide 4, selfmetathesis is conducted under either catalyst G1 or Cat-Z to give a mixture of E-5 and Z-5 with different ratio (Scheme 1). Due to the outstanding crystalline property of E-5, the purity of E-5 can be improved to above 98% by 3-5 times of crystallization, and the removal of E-5, via the crystallization, facilitated the purity improvement of Z-5 over 90% while higher purity is achieved by preparative HPLC. After the preparation of both E-6 and Z-6 with a 98:2 ratio over the other isomer by the di-alkylation, various conditions were applied for the RCM reaction (Table 2). Not surprisingly, catalysts G1 and HG2 gave the large production of EE-3 from E-6 with similar ratio (entries 1-2, Table 2), and the predominant productions of EZ- and ZZ-3 are only achieved by catalyst Cat-Z (entries 3 and 6, Table 2). However, it is astonishing to observe that a significant amount of the Z-olefin was converted to Eolefin in the E-RCM of Z-6 (entries 4 and 5, Table 2). The ratio difference (40:51:9 vs. 80:18:2) reflected the different longevity of CM catalysts. The more active HG2 has a longer lifetime than G1, thereby resulting in more catalytic cycles in the conversion of Z- to E-olefins (Scheme 2). It would be interesting and well worthy to know who reacts first in this conversion: the ring-closure product (i.e., EZ3) or the olefin Z-6, whose difference lies in the conversion occuring before or after ring closure. What’s more important, the optimal condition is needed to be developed for the control of this conversion, which would have an impact on the application of this kind of RCM reaction in the target-oriented synthesis of macrocycles with two or more separate olefins.

as also evidenced in Figure 4a. The Z-6 isomer is completely consumed in 30 min with the summit composition of EE-3:EZ-3:ZZ-3:2 is 7:65:22:6, which indicated that the newly-formed double bond is predominantly with E-configuration and the original Z-olefin is largely untouched on that time. After that, the clear conversion of EZ-3 and ZZ3 to EE-3 is observed, and the equilibration is reached in 48h with the ratio of EE-3:EZ-3:ZZ-3:2 at 42:37:7:12. Importantly, the initiation rates for both E- and Z-6 isomers are too fast to be traced at this condition. When the catalyst loading was decreased to 5 mol% along with lowering the temperature to 0 oC, the initiation rate is controlled with no observance of apparent conversions between EZ-, ZZ- and EE-3 isomers (Figures 3c and 3d). The consumption of >90% of E- and Z-6 is achieved in 6 h, and the similar selectivity of 3:1 and 7:2 individually from E- and Z6 is brought for the newly-formed double bonds. In the conventional olefin metathesis, the higher energy isomer of the primary metathesis products can reenter the catalytic cycle, and is converted to the thermodynamically favored one before the decomposition of catalyst, thereby resulting in the thermodynamic selectivity provided in most reported examples, and the same is for macrocyclic RCM. However, the selectivity in this reaction is the kinetic one due to the inhibition of converting the primary metathesis products, as evidenced by the contrast in Figures 3b and 3d. On the other hand, the kinetic property of this reaction is confirmed by no fluctuation of the selectivity from the beginning to the end of this reaction. The analogs of Z-6 should be a reliable model to answer the question of thermodynamic or kinetic control in the RCM reaction. These results also stated that the ring-closed products are the actual substrates of the Z- to E- conversion (cf. Table

Scheme 2. Mechanistic Analysis of the Conversion of Z- to E-olefins. To understand the transformation of Z- to E- configurations in this macrocyclic RCM reaction, kinetic experiments were carried out under the selected conditions (Figure 3). The initiation rate is an important factor for the success of macrocyclic RCM reaction, and is usually related to lower substrate concentration (mM), to suppress the self-metathesis, and high catalyst loadings (up to 20 mol%), to provide more active ruthenium species and then increase the probability of contact with substrate. The relative composition of substrates (E- and Z-6) and products (2, EE-, EZ- and ZZ-3) was monitored over time by HPLC in the initial condition of 20 mol% of catalyst G1 at 1.3 mM in refluxing dichloromethane (Figures 3a and 3b).21 For E6, the reaction is completed in 40 min, and after that, the ratio of EE-, EZ- and ZZ-3 (70:27:1) almost stayed same. The production of 2 is less than 3% even after 12 days. The unobservable variation reflects that the conversion rate of EE- to EZ/ZZ-isomers after the ring closure is pretty slow,

Figure 3. Kinetic experiments of E-6 and Z-6 at the specified conditions.

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3). Furthermore, the production of 9-membered ring 2 is not observed for both E- and Z-6 isomers under this condition. Encouraged by understanding the relative conversion rate, kinetic experiments were carried out on the pure individual EE-, EZ- and ZZ-3 isomers, which were purified by preparative HPLC (Figure 4). For EE-3, under the catalysis of 15 mol% of G1, the equilibration is reached in 48h with the 73:17:1:7 ratio of EE-3:EZ-3:ZZ-3:2 (Figure 4a). For EZ-3, a significant amount of conversion was observed with the 52:34:4:8 equilibrium ratio of EE-3:EZ-3:ZZ-3:2 (Figure 4b). For ZZ-3, it is surprising to observe the ascending and descending of EZ-3 with the summit composition of EE-3:EZ3:ZZ-3:2 at 20:40:31:9 (Figures 4c), which ascertains the results in Figure 3b. The equilibration is reached in 72h with the 57:24:3:15 ratio of EE-3:EZ-3:ZZ-3:2. When the catalyst loading was decreased to 5 mol%, the conversion rate is decreased, with the equilibrium ratio of 82:14:1:3 of EE3:EZ-3:ZZ-3:2 for EE-3 in 36 h, and 42:48:4:6 for EZ-3 (Figures 4d-e). For ZZ-3 (Figure 4f), the predominant composition of EZ-3 and significant amount of existing ZZ-3 in the products with 20:44:28:8 ratio are resulted from the

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insufficient catalyst capability of G1. No observation of the ascending and descending of EZ-3, compared to Figure 4c, indicated that the high catalyst loading (17 mol%) can bring the eruptive conversions of ZZ-3 to EZ-3, and after conversion of much of ZZ-3, the still active catalyst continues to work on the task of EZ-3 to EE-3; but this is not appropriate for the 5 mol% loading of catalyst. The gradual generation of EE-3 from the beginning presented that the second conversion of EZ-3 to EE-3 did not wait until the total completion of the first conversion of ZZ-3 to EZ-3. In order to compare the relative rate between different types of olefin, the rate constant was calculated upon the assumption of the pseudo-first-order rate in the initial stage of this reaction (Table 3 and Figure S2, SI). Mechanistically, one E-olefin of EE-3 reacted with the active ruthenium species (ARS) to break the double bond, and the following recombination afforded a mixture of EE- and EZ-3 largely with the same selectivity as the approximate 3:1 in the RCM reaction of E-6. The low proportion of Z-olefins resulted in the observed slow conversion rate whereas the actual rate constant of E-olefin with ARS (kE) is almost 4 times of the observed one, 27.0 × 10-3 h-1. This is also applied for ZZ-3, and kZ is almost 9/7 times of the observed one of ZZ-3, 60.3 × 10-3 h-1, after the consideration of 7:2 selectivity in the RCM reaction of Z-6. The observed rate constant of EZ-3 can basically matched with the calculated kE and kZ after the consideration of different ring-closing rates of Eand Z-6. In comparison to CM,22 the relative rate difference of 1,2-disubstituted Z- to E-olefins in this RCM reaction is similar, while this rate difference of monosubstituted to 1,2-disubstituted olefins looks much bigger than CM. Table 3. Rate Constants (k) for RCM Reactions Using Catalyst G1.a entry 1

sub. E-6

temp. 0

oC

0

oC

kobs. (h-1)

kcal. (h-1)

0.904

-

2

Z-6

1.01

-

3

EE-3

reflux

6.76 × 10-3

27.0 × 10-3

4

EZ-3

reflux

29.3 × 10-3

-

reflux

10-3

5

ZZ-3

46.9 ×

60.3 × 10-3

a

Conditions: substrate, 5 mol% of catalyst G1, CH2Cl2 (1.3 or 1.8 mM).

Figure 4. Kinetic experiments of EE-3, EZ-3, and ZZ-3 at the specified conditions.

Inspired by the interesting discovery surrounding catalyst G1, kinetic experiments were carried out for catalyst Cat-Z (Figure 5). Under the condition of 17 mol% of catalyst Cat-Z at 1.7 mM in refluxing dichloromethane for E-6, the ascending and descending of EZ-3 verified that the nature of catalyst Cat-Z is kinetic control, and thermodynamic equilibration can be reached as time prolongs (Figure 5a). At this condition, the conversions are different, 100% for E-6 and 40% for Z-6 (Figures 5a-b). When the catalyst loading was decreased to 5 mol% along with increasing the concentration to 4.2 mM, the conversion of Z-6 is pushed to 78%, and the conversion is dropped to 85% for E-6 (Figures 5c-d). After considering the selectivity, the appropriate reaction time is 24 h because the prolonged time leads to the ratio decrease.

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ACS Catalysis

Table 4. Ring Size Selectivity in the RCM Reactions of E- and Z-isomer.a m

Ts

N

N

E

n

n

Ts

m N

or

Ts

Ts N

Z

Condition A: Condition B: Cat-Z (5 mol%), G1 (5 mol%), CH2Cl2 (1.7 mM) CH2Cl2 (4.2 mM) reflux, 24 h -5 oC, 12 h E

N

Ts

m

n

Ts E

N

Ts N

m Ts N

Z

N

m

Ts

n

E

E n Ts

N

N

m Z

Z

n

Ts

After the deep understanding of reaction conditions, the E-RCM of Z-6 by G1 was conducted at 0 oC, and unfortunately, around 10% conversion of Z- to E-olefins was still observed. And the consumption of Z-6 was less than 40% at the temperature of -10 oC. The E-RCM reactions of E-6 and Z-6 at -5 oC afforded good isolated yields and selectivities (entries 1 and 3, Table 4). The Z-RCM reactions of E-6 and Z-6 by Cat-Z also afforded even better results (entries 2 and 4, Table 4). After that, the precursors of macrocyclic dienes with diverse ring sizes were prepared by one- or two-step dialkylation of E- and Z-5 with alkyl halides, and were tested for E- or Z-RCM (Table 4). Because different products are formed in the Z-RCM of E-isomer and E-RCM of Z-isomer but E/Z-6, total of four different isomers are generated from E/Z-RCM of E- and Z-isomers. For the comparison, compounds 19, 20, and 21 with medium-sized ring was synthesized by the RCM reactions (see SI).23 Regarding the ring size selectivity, the formation of large-sized ring is preferred over 16 in the E-RCM of E-isomer (Figure 6a).24 It is interesting to notice that Z-olefin is preferred to form in the E-RCM of E-9 or E-10 with 16- or 17-membered ring, possibly resulting from the requirement of higher energy in the formation of EE-isomer than EZ-isomer (entries 13 and 17, Table 4). In the Z-RCM of E-isomer, the mediumsized ring is significantly formed on the ring size of 15, but all the generated macrocycles are with high Z-selectivity in all the substrates (Figure 6b).25 In the tough E-RCM of Zisomer, the high reactivity of Z-olefin with G1 resulted in that the medium-sized ring is preferred below the ring size of 17 (Figure 6c).26 Similarly, ZZ-15 is predominantly formed in the E-RCM of Z-9 due to the energy issue. In the Z-RCM of Z-isomer, the production of large-sized ring is gradually increased above the ring size of 17, and good Zcontrol was also achieved in these reactions (Figure 6d).

en t. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

sub. (E/Z)b

Z

E-6 (97:3) E-6 (97:3) Z-6 (1:99) Z-6 (1:99) E-7 (96:4) E-7 (96:4) Z-7 (3:97) Z-7 (3:97) E-8 (99:1) E-8 (99:1) Z-8 (4:96) Z-8 (4:96) E-9 (98:2) E-9

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con d.

con v. (%)

A

98

B

95

A

93

B

89

A

>99

B

>99

A

>99

B

>99

A

93f

B

85g

A B

92 74

A

80

B

77

N Ts

N Ts

m/n

mediumsized ring 2, m/n=2 19, m/n=3 20, m/n=1 21, m/n=0

large-sized ring 3, m=2, n=2 13, m=3, n=3 14, m=2, n=3 15, m=1, n=2 16, m=1, n=1 17, m=0, n=1 18, m=0, n=1

Figure 5. Kinetic experiments of E-6 and Z-6 at the specified conditions.

6, m=2, n=2 7, m=3, n=3 8, m=2, n=3 9, m=1, n=2 10, m=1, n=1 11, m=0, n=1 12, m=0, n=0

l/mc

yield [%]d

(ring size)

(EE:EZ:ZE:ZZ)b

>99:1

93

(18 vs. 9)

(75:23:-:2)e

>99:1

94

(18 vs. 9)

(18:80:-:2)e

98:2

86

(18 vs. 9)

(2:80:-:18)e

93:7

75

(18 vs. 9)

(1:22:-:77)e

>99:1

72

(20 vs. 10)

(38:53:5:4)

>99:1

59

(20 vs. 10)

(4:91:1:4)

>99:1

44

(20 vs. 10)

(10:7:48:35)

>99:1

46

(20 vs. 10)

(0:5:8:87)

>99:1

80

(19 vs. 9/10)

(68:28:3:1)

>99:1

64

(19 vs. 9/10)

(9:89:0:2)

>99:1

85

(19 vs. 9/10)

(3:2:59:36)

>99:1

55

(19 vs. 9/10)

(0:7:8:88)

81:19

50

(17 vs. 8/9)

(34:64:1:1)h

>99:1

42

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(98:2)

15 16 17 18 19 20 21 22 23 24 25 26

Z-9 (1:99) Z-9 (1:99) E-10 (99:1) E-10 (99:1 Z-10 (2:98) Z-10 (2:98) E-11 (96:4) E-11 (96:4) E-12 (98:2) E-12 (98:2) Z-12 (3:97) Z-12 (3:97)

(17 vs. 8/9)

A

85

B

64

A

95

B

82

A

92

B

88

A

96

B

87

A

98

B

60

A

96

B

96

(5:92:1:2)h

38:62

32

(17 vs. 8/9)

(5:13:34:48)h

75:25

63

(17 vs. 8/9)

(1:9:8:82)h

38:62

34

(16 vs. 8)

(0:87:11:2)

95:5

38

(16 vs. 8)

(0:94:1:5)

9:91 (16 vs. 8)

-i

40:60

43

(16 vs. 8)

(0:14:0:86)

4:96 (15 vs. 7/8)

-i

56:44

34

(15 vs. 7/8)

(-)j

2:98 (14 vs. 7) 8:92 (14 vs. 7)