Cobalt-catalyzed oxidative cyclization of gem-disubstituted conjugated alkenols

Tetrahedron Letters xxx (2016) xxx–xxx

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Cobalt-catalyzed oxidative cyclization of gem-disubstituted conjugated alkenols Tânia M. F. Alves, Mateus O. Costa, Beatriz A. D. Bispo, Fabiana L. Pedrosa, Marco A. B. Ferreira ⇑ Department of Chemistry, Laboratório de Química Bio-Orgânica, Federal University of São Carlos—UFSCar, Rod. Washington Luis, km 235, C.P. 676 São Carlos, SP, Brazil

a r t i c l e

i n f o

Article history: Received 10 May 2016 Revised 11 June 2016 Accepted 14 June 2016 Available online xxxx Keywords: Mukaiyama oxidative cyclization 2,5,5-Trisubstituted tetrahydrofuran Metal catalysis

a b s t r a c t Aryl gem-disubstituted conjugated alkenols underwent oxidative cyclization affording 2,5,5-trisubstituted tetrahydrofurans in reasonable yields and good diastereoselectivities using the reductive termination variation of the Mukaiyama aerobic oxidative reaction. Under oxidative termination, the same alkenols produced diols and ketonic by-products via the double hydration and beta-scission competing pathways. Furthermore, the differences in alkenol reactivity under the reductive and oxidative termination conditions were investigated. Ó 2016 Elsevier Ltd. All rights reserved.

Introduction Substituted tetrahydrofurans (THFs) are common structural elements of natural products and other biologically active molecules.1 Therefore, significant efforts have been dedicated to the development of new stereoselective methods for the construction of such moieties.2 In particular, synthetic approaches for the formation of 2,5-disubstituted-THFs have received considerable attention.2 However there is a lack of stereoselective methodologies for 2,5,5-trisubstituted-THFs possessing aromatic motifs in tertiary carbinol centers, presented in some biologically active compounds (Fig. 1).3 A powerful strategy to achieve 2,5-trans-THFs in a stereoselective manner involves the Mukaiyama aerobic oxidative cyclization reaction of bis-homoallylic alcohols under a catalytic amount of Co(II) complexes in the presence of O2 (Scheme 1).4 The low cost and non-toxic metal, open reaction conditions, lack of moisture sensitivity, and use of green solvents make this approach appealing. Therefore, the reaction mechanism has been investigated by Hartung, showing evidences of the formation of carbon free radical intermediates.5 Additionally, with slight variations on reaction conditions, these radical intermediates can be transformed into synthetically useful functional groups such as alcohols,4–6,9 bromides,6b alkyls,6b,7 and alkylsulfanyls.8 They are also able to perform a nucleophilic radical addition to electron-withdrawing conjugated double/triple bonds.7 This methodology has proven to be an

extremely powerful tool in asymmetric synthesis of natural products and biologically active compounds.9 A critical issue for the application of this methodology has been the degree of substitution of the double bond. Previous work has explored tertiary pentenol derivatives forming 2,5,5-trisubstituted-THFs in moderate yields and selectivities (Scheme 2).3c Moreover, excellent selectivities and yields for terminal unsubstituted and gem-alkyl olefins have been achieved.4–9 Alkyl or aryl substituents attached at the terminal position provide excellent 2,5-trans diastereoselectivity independent of the double bond geometry.5a,6b However, the newly formed stereocenter in the side chain is obtained in a ratio of 1:1 from the radical symmetric intermediate under the traditional oxidative termination.5a,6b Harsh reaction conditions were also necessary to cyclize trialkyl-trisubstituted terminal olefins, affording the products with low yield and diastereoselectivity.5 We envisioned whether this transformation could be performed employing aryl gem-disubstituted conjugated double bonds (Scheme 2). We hypothesized that this reaction would stereoselectively form a 2,5,5-trisubstituted THF, thus creating a tertiary carbinol center with an aromatic group. Herein we report our contribution toward extending the scope of this methodology and in understanding the reactivity of these systems. Results and discussion Reductive termination

⇑ Corresponding author. E-mail address: [email protected] (M.A.B. Ferreira).

Initially, we focused on the feasibility of the Mukaiyama cyclization of simple model substrates 1a, under reductive

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T. M. F. Alves et al. / Tetrahedron Letters xxx (2016) xxx–xxx

HO N

O

(7S,10S)-boivinianin B

N F N O

O O

F

N

N

N

SCH 45009

N N

Figure 1. Examples of biologically active compound containing 2,2,5-trisubstituted THF structural motifs.

termination version for construction of the 2a core (Table 1). In this initial screening, we used 1,4-cyclohexadiene (1,4-CHD) as a reducing agent (Table 1). The combination of CoL2 (3, 5, or 6) (Fig. 2), 1,4-CHD (20 equiv) in toluene and open flask conditions (atmospheric air), catalyzed the reaction to afford the expected THFs (2a–c) and complete conversion of alkenol 1a after 4 h (entries 1–3). The mass balance was a critical issue leading us to suspect that under these conditions we had decomposition of the starting material and/or product. A more sterically bulky alkenol could also be associated with the lower selectivity due to repulsive interaction in catalyst coordination.3c Aiming to improve the reaction conditions, we systematically varied the concentration. At lower and higher concentrations (entries 4 and 5, respectively), we experienced difficulties in achieving acceptable yields. It is noteworthy that when only 1,4-CHD is used as solvent, as described by Hartung,5,7 decomposition of the starting material was observed (entry 6). The best yield was achieved with 0.06 M employing catalyst 5 at 15 mol % (entry 7). We were not able to reduce the catalyst loading, as 7.5 mol % of 5 lead to a reduced reaction yield, producing 2a in only 13% (entry 8). No substrate consumption occurred in the absence of O2 and 1,4-CHD (entry 9). Finally, the alkenol 1b (entry 10) lead to similar results to 1a, revealing a minor influence of an electron withdrawing group on aromatic ring on this reaction. However, it was noted a higher yield for the alkenol 1c (entry 11) possessing an electron donating group. The alkenol 1a0 possessing a trisubstituted double bond also generated the cyclic product 2a0 in 31% yield (Scheme 3). However, attempts to lengthen the alkyl chain proved to be unfruitful as no products were observed with the alkenol 1a00 even after 24 h, with partial recovery of the starting material. With the best reaction condition in hand, we next investigated the diastereoselectivity of the Mukaiyama cyclization with alkenols 7a–h (Scheme 4). The reaction gave a separable mixture

radical functionalization

by silica gel chromatography of 2,5-trans (8a–h) and 2,5-cis (9a–h) THFs in moderate to poor yields and good diastereoselectivities in favor of 8a–h (2,5-trans). The relative stereochemistry was confirmed by NOE experiments of both 8b/9b diastereoisomers.10 Better diastereoselectivities were obtained at 60 °C, but a reduction in the yield also occurred. When comparing the reaction involving the compounds 7a and 7h, erosion of diastereoselectivity was observed for electron-donating aromatic substituents. Despite the quantitative consumptions of starting materials the isolated yields after flash chromatography were lower than expected. We were not able to assess the yields from NMR using an internal standard of the crude material due to paramagnetic character of the cobalt catalyst, which produced broad signals in most cases. To examine these results further, we collected kinetic information by monitoring the cyclization of alkenols 7b and 7h at 60 °C via quantitative GC–MS analysis. The consumption of starting material and formation of cyclized products were monitored and the changes in concentration are plotted as a function of time in Figure 3. After approximately 2 and 3 h, respectively, the starting material was completely consumed for the reactions of 7b and 7h. The amount of products remains unchanged after this point, with yields of 49% for 7b and 54% for 7h. The achieved diastereoselectivity ratios were 5.7:1 for 8b:9b and 1.2:1 for 8h:9h. Additionally, the diastereomeric ratios of the products are nearly constant with respect to conversion. While the diastereoselectivities obtained from the kinetic progress were very similar to those obtained from isolated products after silica gel chromatography, the yields decrease by approx. 30–40%. Pagenkopf obtained similar results for 2,5,5-trisubstituted THF.3c We hypothesize in our case a product decomposition by a ring opening due to exposure to silica gel through the formation of a stable tertiary carbocation. Finally, a control experiment was performed in order to verify if the presence of the catalyst was critical to induction of selectivity. The alkenol 7g was cyclized in the presence of p-TSA (1 equiv) and CH2Cl2 (0.06 M), at room temperature affording a separable mixture by silica gel chromatography of 8g:9g in 58% yield with no diastereoselectivity (dr = 1:1), proving the importance of the catalyst in this reaction. Oxidative termination In a second stage of this study, we explored the oxidative termination version of the Mukaiyama cyclization. In this screening, we used catalysts 3–6 (Fig. 2), t-BuOOH as additive, and i-PrOH as solvent under O2 (1 atm) atmosphere. Surprisingly, as shown in Table 2, we were unable to obtain the cyclized product 10 under oxidative termination for alkenols 1a–c. Depending on the cobalt

OH

OH

R

ref 3c oxidative termination reductive termination

R

O

CoL 2 O2, H2X

SR RS-SR 1,4-CHD

i

R

O

OH

1,4-CHD

H EWG

OH R'

ref 4-9

R

CoL 2, O2 reductor 60-80 ˚C

R

OH

O

dr 2:1 to 5:1

O

R

R'

X

dr > 95:5

Br R

O

H

O

R

R' = alkyl, H

BrCCl3 1,4-CHD

EWG 1,4-CHD

CoL 2, O 2

reductor 60-80 ˚C R = alkyl, aryl, alkynyl

O

R

R

O2 PrOH

R

O

R this work

OH R'

R' = Aryl Scheme 1. Synthetic variations of Mukaiyama oxidative cyclization.

CoL 2, O2

R

O

R' X

reductor 60-80 ˚C

Scheme 2. Scope of the Mukaiyama oxidative cyclization.

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T. M. F. Alves et al. / Tetrahedron Letters xxx (2016) xxx–xxx Table 1 Mukaiyama oxidative cyclization reactions of alkenols 1a–c under reductive termination conditions

HO

n-Pr

Ar

8 (15 mol%), air 1,4-CHD (20 equiv)

Ar Me

(±)-8 2,5-trans

F

a b c

CoL2

Alkenol

Concentration

Time

Yielda (2)

1 2 3 4 5 6b 7 8 9c 10 11

3 6 5 5 5 5 5 5 5 5 5

1a 1a 1a 1a 1a 1a 1a 1a 1a 1b 1c

0.04 M 0.04 M 0.04 M 0.02 M 0.1 M 0.6 M 0.06 M 0.06 M 0.06 M 0.06 M 0.06 M

4h 4h 4h 16 h 3h 18 h 3h 18 h 18 h 3h 3h

30% 25% 31% <5% 6% Complex mixture 34% 13% <5% 32% 44%

(15 mol %) (15 mol %) (15 mol %) (15 mol %) (15 mol %) (15 mol %) (15 mol %) (7.5 mol %) (15 mol %) (15 mol %) (15 mol %)

Yield of isolated product 2a–c. The reaction was conducted without solvent. The reaction was conducted without 1,4-CHD and under argon atmosphere.

n-Pr Me +

60 ˚C, PhMe, 3-4 h 7

Entry

Ar

O

O

Br

O

n-Pr

O

n-Pr

Me

8c, 46% dr=4.7:1

Me

Me

O

n-Pr

O

Me

n-Pr

8f, 17% dr=3.4:1

MeO

O

O

n-Pr

Me

*reaction at 80 ˚C

O Me

8e, 39%, dr=3.5:1

PhO

Me

n-Pr

Me

8d, 30% dr=5.3:1

n-Pr

Me

8b, 30% dr=5.4:1 (8b, 53%,dr=5.9:1)*

8a, 43% dr=4.7:1 (8a, 56% dr=2.5:1)*

n-Pr

(±)-9 2,5-cis

Cl

Me

O

8g, 26% dr=4.2:1

n-Pr

Me 8h, 20% dr=2.5:1

Scheme 4. Substrate scope for Mukaiyama cyclization under reductive termination. Yields and diastereoselectivities obtained from isolated products.

MeN

O

O

O

N

O

O C oL 2

t-Bu O

3

O

O

N

t-Bu

O

4 O

CF3

O

F 3C

5

CF3

6

Figure 2. Cobalt catalysts used in this work.

OH ( )n 1a'. n=1 1a''. n=2

8 (15mol%) air (1 atm) 1,4-CHD (20 equiv) PhMe, 80 ˚C, 3 h 31%

n=1

O

2a' n=2

Scheme 3. Mukaiyama oxidative cyclization reactions of alkenols 1a0 and 1a00 under reductive termination conditions.

catalyst and additive (t-BuOOH) used, the formation of by-products 11a and 13a were observed (Table 2, entries 1, 4, 7–9). Hartung and colleagues have detected similar diols using trialkyl-trisubstituted double bonds.5a In some cases (entries 2, 3, 5, and 6), we were not able to detect the formation of these compounds, obtaining only complex mixtures. The use of molecular sieves and dry conditions (entry 8) were also considered in order to avoid the supposed hydration by-product 11a. However a similar result was obtained, revealing a possible radical path. We employed an alternative method involving the pre-activation of catalyst prior the introduction of the alkenol, as described by Pagenkopf and coworkers,11 but this was unsuccessful (entry 9). We investigated the reaction profile using electron-withdrawing (Cl–) and electron-donating (MeO–) groups on aromatic rings by using alkenols 1b and 1c, respectively (entries 10 and 11), however the desired products were not obtained.

Again, the mass balance was a critical issue in all entries leading us to speculate that under these reaction conditions we had decomposition of the starting material. Mechanistic interpretation Based on the experimental results from this study and the literature, the conjugation of aryl gem-disubstituted double bonds appears to prevent oxidative cyclization reactions via traditional oxidative termination, but proceed under reductive termination. Furthermore, gem-disubstituted non-conjugated double bonds, or conjugated E/Z-disubstituted olefins normally react under any activation mode.5a,6b Based on the detailed mechanism study of Hartung and col.,5a it is expected that after the oxygen activation of cobalt-II (I), a superoxo-binding mode provides a stronger oxidant, converting the olefin into radical cation II (Scheme 5a). The lowest-energy chair-like conformer was proposed based on the experimentally observed diastereoselectivities. The decrease in bond order and, consequently, the rotational barrier, can lead to the interconversion of II to II0 by a C–C rotation. This step is the key to understand the reactivity differences between oxidative and reductive termination. The inability of the catalyst to promote the desired reaction via oxidative termination is because the redox potential in the aromatic gem-disubstituted double bond increases substantially compared to vicinal disubstituted or nonsubstituted derivatives.12 In addition, a reduction of the oxidative potential of cobalt complex is expected in polar media.13 A more sterically bulky alkenol could also be associated with the lower reactivity due to repulsive interaction in catalyst coordination.3a The low cyclization reactivity eventually allows the occurrence of secondary radical reactions (Scheme 5b).5a The double hydration of olefins in the presence of i-PrOH, O2, and cobalt-II (via cobalt hydride species) is well-known,14 leading to the formation of radical intermediate V in our system. Hydrogen abstraction from isopropanol by V leads to formation of 11. A beta-scission mechanism can lead to the products 12 (path a) or 13 (path b). Finally, we have noted that only the less polar solvent media, corresponding to the reductive termination variation of oxidative cyclization, converted the olefins into the desired products. Under

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T. M. F. Alves et al. / Tetrahedron Letters xxx (2016) xxx–xxx

(a)

100

100

9h 13h 8h 12h 7h 11h

80

Compound in reaction (%)

80

Compound in reaction (%)

(b)

9b 13b 8b 12b 7b 11b

90

70 60 50 40 30 20

60

40

20

10

0

0 0

50

100

150

200

250

300

0

50

100

150

Time (min)

200

250

300

350

Time (min)

Figure 3. Reaction progress profile for the reaction in Scheme 4 at 60 °C obtained using quantitative GC–MS. Yield versus time for alkenols 7b (a) and 7h (b).

Table 2 Screening of conjugated alkenols 1a–c for the Mukaiyama aerobic oxidative cyclization under oxidative termination

Oxidative cyclization methanism

not observed OH R

CoL 2 (10 mol%) O2 (1 atm)

OH

additive i-PrOH, 60 ˚C 24h

R

2,5-trans

O

+ R

+ 11a-c

OH

+

R

2 H 2O + Ar O

(a)

R

3[ H ] / O 2

a

c d e

H O Ar

L 2CoIII -- O2H

Entry

CoL2

Alkenol

Conva

1 2c 3 4c 5 6c 7 8d 9c,e 10 11

3 3 4 4 5 5 6 3 3 3 3

1a 1a 1a 1a 1a 1a 1a 1a 1a 1b 1c

87% Complex Complex 78% Complex Complex Quant. 89% 79% 79% 85%

O

L 2CoII

Ar

Yieldb 22% (11a) mixture mixture 13% (11a) mixture mixture 19% (11a) 21% (11a) 15% (11a) 21% (11b) 15% (11c)

23% (13a)

—

—

— 25% 19% 12% 12%

R I

C-C rotation

R

2,5-cis

— — — — 25% (12c)

-- O 2

H O Ar R II

III

—

Ar

L 2CoIII -- O 2

13a-c

12c

R

L 2CoII -- O 2

R

b

HO

HO O

O

OH

10a-c

1a. R = Ph 1b. R = p-Cl-C6H 4 1c. R = p-CH3O-C6H 4

R

O

L 2CoII -- O 2 Ar

Ar

H O II' R

(13a) (13a) (13b) (13c)

Conversion based on recovery of starting material. Yield of isolated by-product. t-BuOOH (10 mol %) was used as additive. Use of MS 4 Å, and dry i-PrOH. Catalyst was pre-activated.

(b)

beta-scission mechanism

OH i-PrOH

HO

Ar

11 Ar

O 2CoIIIL

CoL 2, O 2 i-PrOH

path a

Ar

Ar

O

12 IV

OH

O

OH

V

path b

OH

+ HO Ar O

+ CH3 Ar

HO

13

Scheme 5. Proposed mechanism.

this condition, a higher oxidative potential, combined with a complete inhibition of hydration of olefins due to the absence of i-PrOH, explains the reactivity difference between reaction conditions. In regard to diastereoselectivity, formation of a stable benzylic tertiary carbocation may explain the lower diastereoselectivities of 2,5-trans diastereoisomer where C–C rotation and shift equilibrium lead to II0 . However, the preference for the pseudoaxial or equatorial orientation of aryl substituent of intermediate II is not clear. Steric repulsion between the alkenol and the cobalt ligand may play a major role in this equilibrium. Concluding, in this study, aryl gem-disubstituted conjugated alkenols underwent the reductive termination variation of the Mukaiyama aerobic oxidative cyclization, providing 2,5,5-trisubstituted THF with moderate yields and diastereoselectivities. Although the diastereoselectivity of products can be improved, the difference in reactivity under these termination conditions provides further insights into the mechanism of the Mukaiyama oxidative cyclization and competing reactions. Studies regarding

the structural nature of the cobalt complex and ligand coordination in solution are currently underway. Acknowledgments The authors acknowledge FAPESP (13/02311-3), CNPq (477944/2013-2) and CAPES for financial support and fellowships. We also thank Prof. Márcio W. Paixão and Prof. Timothy J. Brocksom for the careful reading of the manuscript and helpful suggestions. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.06. 064.

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