Boron-Catalyzed Double Hydrofunctionalization Reactions of

Apr 6, 2018 - Similarly, the use of 10 mol % B(C6F5)3·nH2O and 2.0 equiv of allylsilane 2 also promoted the desired reaction; however, the yield of 3...
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Boron-Catalyzed Double Hydrofunctionalization Reactions of Unactivated Alkynes Masatoshi Shibuya, Masaki Okamoto, Shoji Fujita, Masanori Abe, and Yoshihiko Yamamoto ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00955 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

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Boron-Catalyzed Double Hydrofunctionalization Reactions of Unactivated Alkynes Masatoshi Shibuya,* Masaki Okamoto, Shoji Fujita, Masanori Abe, and Yoshihiko Yamamoto Department of Basic Medicinal Sciences, Graduate School of Pharmaceutical Sciences, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan ABSTRACT: Tandem hydroalkoxylation/hydroallylation and hydroalkoxylation/hydrocyanation reactions of alkyl-substituted unactivated alkynes by catalytic systems based on B(C6F5)3·nH2O and silyl nucleophiles were developed. The characteristic high alkynophilicity of B(C6F5)3 enabled the selective activation of the unactivated alkynes in the presence of the reactive alkene of allylsilane. Moreover, the alkynes were electrophilically activated in the presence of cyanide in this catalytic system. Mechanistic studies suggest that the alkynes are activated by the different catalytic species in the two reactions. KEYWORDS: boron, tandem reaction, hydroalkoxylation, hydroallylation, hydrocyanation

Introduction Tandem double hydrofunctionalization reaction of alkynes enables the creation of structures having a tetrasubstituted carbon center when two C–X bonds (X ≠ H) are regioselectively formed on the same internal carbon.1 Intramolecular hydroalkoxylation-initiated double hydrofunctionalization reactions afford cyclic ethers having a tetra-substituted carbon at the 2 position, which are powerful reactions for the rapid syntheses of highly functionalized cyclic ethers. Such reactions can be established by the following tandem process: the intramolecular hydroalkoxylation of alkynes, and the protonation of the resultant 2-alkylidene cyclic ethers to generate their corresponding oxocarbenium intermediates, followed by the addition of nucleophiles.2,3 To date, a number of transition metal catalysts such as Pd, Rh, Ir, Au, and Cu have been reported to promote the intramolecular hydroalkoxylation of unactivated alkynes to yield 2-alkylidene cyclic ethers or their corresponding isomers.2-4 Several studies have reported that 2alkoxy-2-alkyl-substituted cyclic ethers were formed by a double hydroalkoxylation (Figure 1a).2 The syntheses of 2alkyl-2-aryl-substituted cyclic ethers by the hydroalkoxylation/hydroarylation introducing electron-rich heteroaromatic compounds such as indoles and pyrroles were also reported.3 However, reaction introducing other carbon nucleophiles in an intermolecular fashion has not been reported to the best of our knowledge. We, therefore, expect successful double hydrofunctionalization reactions using more versatile nucleophiles such as allyl or cyano group, which can be a foothold to lead to diverse structures by further transformations (Figure 1b). We recently reported the hydroalkoxylation/reduction using a Brønsted acid-silane catalytic system as a part of our interest in the catalytic electrophilic activation of the unactivated alkynes by Brønsted acid (Figure 1a).5 In this case, the formed oxocarbenium species were reduced by Et3SiH to its corresponding saturated cyclic ethers. We envisioned that if this reaction is performed in the presence of an allylsilane instead of the hydrosilane, the intramolecular hydroalkoxylation/hydroallylation reaction will occur.

Figure 1. (a) Previous studies. (b) This study.

Based on this assumption, hydroxy alkyne 1a was treated with triflic imide (Tf2NH) in the presence of allyltrimethylsilane (2) (Scheme S1). This reaction unfortunately resulted in the formation of the silyl ether, as Tf2NH protonated the allylsilane prior to activating alkyne 1a. This result suggests that discrimination between the alkyne and the reactive alkene of the allylsilane is an issue in the development of our desired reaction. We therefore examined the use of tris(pentafluorophenyl)borane, B(C6F5)3,6 which is a unique main group Lewis acid that is strongly acidic, stable, and sterically bulky.6-8 Hashmi and coworkers recently reported the cyclization of enynes with B(C6F5)3,9 where the B(C6F5)3 preferentially activated the alkynes over alkenes. Although allylsilane 2 is more reactive than such alkenes, we expect that B(C6F5)3 will selectively activate any unactivated alkynes in the presence of 2. Thus, we herein report novel catalytic systems based on the use of B(C6F5)3·nH2O10 and silyl nucleophiles for the double hydrofunctionalization reaction

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Table 1. Optimization of tion/Hydroallylation Reaction.

a

the

Hydroalkoxyla-

(i.e., 44%), likely due to the hydrolysis of the ester. 4-Pentyn1-ol (1h), with no substituent on the linker between the hydroxy group and the alkyne, afforded 3h in moderate yield.17 Secondary alcohol 1i efficiently cyclized to give 5-alkylsubstituted product 3i in 72% yield, although 3-alkylsubstituted product 3j was formed in a low yield (i.e., 9%). Notably, enyne 1k selectively produced allyl-substituted 3k in 68% yield without the production of propargyl-substituted 3k’. This result suggests that the alkyne was selectively activated over alkene under the standard reaction conditions. Internal alkyne 1l produced an inseparable mixture of 5-exo cyclization product 3l and the 6-endo cyclization product (i.e. 3m) in a 19% combined yield, while the reaction of δ-hydroxy alkyne 1m efficiently proceeded to give the desired product 3m in 70% yield. Table 2. Substrate Scope of the Hydroalkoxylation/Hydroallylation Reaction

Recovered starting material.

of unactivated alkynes (Figure 1b).11,12 In addition, although the water-tolerance of B(C6F5)3 and the Brønsted acidity of B(C6F5)3·H2O have been reported,10,12,13 our mechanistic investigation uncovered the inherent catalytic behaviors of B(C6F5)3·nH2O in the presence of silyl nucleophiles. The activation modes of the alkynes in the two reactions examined herein differ significantly.

Results and discussion Initially, we treated hydroxy alkyne 1a with 120 mol % B(C6F5)3·nH2O and 2.4 equiv allylsilane 2 in the presence of H2O as a proton source in CH2Cl2 at 40 °C (Scheme S2). These conditions allowed the hydroalkoxylation/hydroallylation reaction to proceed to give the desired product 3a in a good yield. Similarly, the use of 10 mol % B(C6F5)3·nH2O and 2.0 equiv allylsilane 2 also promoted the desired reaction; however, the yield of 3a was not consistent in repeated experiments. Thus, following extensive optimization of the reaction conditions, a reproducible protocol was established (Table 1). More specifically, a solution of 10 mol % B(C6F5)3·nH2O and 0.5 equiv H2O in 1,2-dichloroethane (DCE) was premixed at 60 °C for 3 h, prior to the slow addition of a solution containing 1a and allylsilane 2 in DCE over 2 h. After complete addition of 1a and 2, the reaction mixture was worked up to give the desired allyl-substituted cyclic ether 3a in 82% yield (entry 1). Interestingly, we found that the premixing of B(C6F5)3·nH2O and H2O was essential to ensure reproducible results (entry 2). To account for this observation, a 1:5 mixture of B(C6F5)3·nH2O and H2O was monitored by 19F NMR spectroscopy. However, a significant change in the signals was not observed.14,15 We therefore assumed that premixing was required to ensure the dissolution of H2O in DCE. Slow addition of the reagents was essential to suppress the undesired dimerization reaction (entry 3). An increase in the quantity of H2O to 2.2 equiv had a detrimental effect (entries 4 and 5), and 2.0 equiv of allylsilane 2 was optimal (entries 6 and 7). With the optimized reaction conditions in hand, we moved on to investigate the substrate scope of the hydroalkoxylation/hydroallylation reaction (Table 2). In addition to the formation of 4,4-disubstituted product 3b from 1b, a variety of 4monosubstituted products 3c–3f were successfully obtained from 1c–1f.16 Benzoate 3g was obtained in a moderate yield

a

20 mol % B(C6F5)3·nH2O. b Yield determined by 1H NMR.

Upon screening a range of silyl nucleophiles, we found that the hydroalkoxylation/hydrocyanation reaction was also promoted by the use of trimethylsilyl cyanide (TMSCN, 5) as a nucleophile. It should be noted that this reaction does not require the slow addition technique. Thus, as outlined in Table 3, 4,4-disubstituted products 6a and 6b, 4-monosubstituted products 6c-6f, as well as 6-membered product 6m were provided in high yields (76–95%). The reaction of 4-pentyn-1-ol (1h) efficiently afforded the desired product in 81% NMR yield. Interestingly, the results in the reactions of 1i, 1j, 1k and internal alkynes 1l and 1n are significantly different from those observed in the hydroalkoxylation/hydroallylation reaction (also see Table 2). More specifically, secondary alcohol 1i afforded 5-alkyl-substituted product 6i in a low yield, while the corresponding product 3i was produced in a high yield. On the other hand, 3-alkyl-substituted product 6j was formed efficiently in 82% yield, while the corresponding product 3j was obtained in a low yield. In the reaction of enyne 1k, 13% of propargyl-substituted 3k’ was produced together with allylsubstituted 6k in 54% yield as an inseparable mixture. Furthermore, internal alkynes 1l and 1n selectively gave 5-exo

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ACS Catalysis cyclization products 6l and 6n in high yields (85% and 83%, respectively). Table 3. Substrate Scope of the Hydroalkoxylation/Hydrocyanation Reactiona

ing the addition of 4 equiv TMSCN (5). This species was determined as H+[NCB(C6F5)3]− based on its similarities with the spectrum of [K+(18-crown-6)][NCB(C6F5)3]–.18 In addition, a characteristic shift of the CN stretching vibrational band from 2191 (TMSCN) to 2252 cm–1 was observed in the analysis of the 1:4 solution of B(C6F5)3·nH2O:TMSCN (5) by IR spectroscopy,19 and high-resolution mass spectrometry confirmed the expected molecular ion mass (Calcd. for C19BF15N [M– H]−: 537.9890; found 537.9891). (a)

ortho

para

meta

B(C6F5)3—nH2O

+1 equiv 2

+3 equiv 2

+5 equiv 2 -120.0

-130.0

-140.0

-150.0

-160.0

-170.0

-160.0

-170.0

a

After a solution of 10 mol % B(C6F5)3·nH2O and 2.0 equiv H2O in DCE was premixed at 60 °C for 3 h, alkyne 1 and 2.0 equiv TMSCN were added, and the reaction mixture was stirred at 60 °C. b Yield determined by 1H NMR. c A small amount of impurity could not be removed.

Thus, upon comparison of the substrate scopes of the hydroalkoxylation/hydroallylation and hydroalkoxylation/hydrocyanation reactions, notable differences were observed as indicated above. In addition, when alkene 7 was subjected to the standard reaction conditions, interesting results were obtained (Scheme 1). Although no reaction was observed under standard conditions of the hydroalkoxylation/hydroallylation reaction, hydroalkoxylation of 7 was successful under standard conditions of the hydroalkoxylation/hydrocyanation reaction to give 8 in 79% yield. These results therefore indicate that the active species of these two reactions must differ from one another.

(b) B(C6F5)3—nH2O

+1 equiv 5

+2 equiv 5

+3 equiv 5 +4 equiv 5 -120.0

-130.0

-140.0

-150.0

Figure 2. 19F NMR spectra of the reaction of B(C6F5)3·nH2O with (a) allylsilane 2 and (b) TMSCN (5).

Scheme 1. Hydroalkoxylation of Alkene 7 To gain insight into the catalytically active species involved in these transformations, we monitored the reaction of B(C6F5)3·nH2O and allylsilane 2 by 19F NMR spectroscopy (Figure 2a). Downfield shifts of all three peaks, particularly the para-F signal, were observed depending on the amount of allylsilane 2 until 5 equiv of 2 was added. Interestingly, the final spectrum was comparable with that of anhydrous B(C6F5).18 We also monitored the reaction of B(C6F5)3·nH2O with TMSCN (5)(Figure 2b). In this case, after multiple species were observed, they converged to a single species follow-

Scheme 2. Plausible Reaction Mechanism

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Science Research from AMED. S.F. express his thanks for the Sasakawa Scientific Research Grant from the Japan Science Society.

REFERENCES

Scheme 3. Hydroalkoxylation/Hydrocyanation After in situ – Preparation of H+[NCB(C6F5)3] On the basis of the above experimental observations, plausible reaction mechanisms are shown in Scheme 2. In the hydroalkoxylation/hydroallylation reaction, although the effect of the dehydration of B(C6F5)3·nH2O by allylsilane 2 was unclear, B(C6F5)3 behaved as a Lewis acid catalyst to activate the alkynes, which promote the hydroalkoxylation reaction followed by the protonation and the allylation. In the hydroalkoxylation/hydrocyanation reaction, H+[NCB(C6F5)3]− generated from B(C6F5)3·nH2O and TMSCN (5) behaved as a Brønsted acid catalyst to activate the alkynes and promote the hydroalkoxylation/hydrocyanation reaction, which is consistent with the fact that Brønsted acids such as TfOH and HI promote the intramolecular hydroalkoxylation of alkenes (Scheme 1).20,21 Finally, in order to verify the possibility of H+[NCB(C6F5)3]− as an actual species, we conducted the following control experiment (Scheme 3). After premixing a solution of B(C6F5)3·nH2O and H2O in DCE at 60 °C for 3 h, TMSCN (5) was added. 19F NMR analysis of the resultant solution showed the generation of H+[NCB(C6F5)3]− (Figure S6). After that, 1a was added to the mixture. We confirmed the desired reaction to proceed with the almost same efficiency. In conclusion, we successfully developed B(C6F5)3·nH2Oallylsilane 2 and B(C6F5)3·nH2O-silyl cyanide 5 catalytic systems for the tandem hydroalkoxylation/hydroallylation and hydroalkoxylation/hydrocyanation reactions of unactivated alkynes. These reactions enabled the rapid synthesis of highly functionalized cyclic ethers bearing allyl- or cyano-substituted tetra-substituted carbon centers at the 2 position.22 The mechanistic investigations suggest that the alkynes were activated by B(C6F5)3 as a Lewis acid catalyst in the presence of allylsilane 2, whereas in the presence of TMSCN (5), they were activated by H+[NCB(C6F5)3]− as a Brønsted acid catalyst. These results demonstrate the potency of the catalytic system using B(C6F5)3·nH2O and silyl nucleophiles. Further efforts to expand the utility of the catalytic system are underway.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Control experiments, experimental details, characterization of new compounds, and NMR spectra (PDF)

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI (No. JP16K08162), and the Platform Project for Supporting Drug Discovery and Life

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(16) (a) Schmitt, A.; Reissig, H. U., On the Stereoselectivity of γLactol Substitutions with Allyl- and Propargylsilanes - Synthesis of Disubstituted Tetrahydrofuran Derivatives. Eur. J. Org. Chem. 2000, 38933901. (b)Schmitt, A.; Reissig, H. U. Lewis acid-Promoted Reactions of γlactols with Silyl Enol Ethers - Stereoselective Formation of Functionalized Tetrahydrofuran Derivatives. Eur. J. Org. Chem. 2001, 1169-1174. (17) Kaneti, J.; Kirby, A. J.; Koedjikov, A. H.; Pojarlieff, I. G. ThorpeIngold Effects in Cyclizations to Five-Membered and Six-Membered Rings Containing Planar Segments. The Rearrangement of N(1)-Alkylsubstituted Dihydroorotic Acids to Hydantoinacetic Acids in Base. Org. Biomol. Chem. 2004, 2, 1098-1103. (18) Vei, I. C.; Pascu, S. I.; Green, M. L. H.; Green, J. C.; Schilling, R. E.; Anderson, G. D. W.; Rees, L. H. Synthesis and Study of New Binuclear Compounds Containing Bridging (µ-CN)B(C6F5)3 and (µ-NC)B(C6F5)3 Systems. Dalton Trans. 2003, 2550-2557. (19) Nagata, T.; Matsubara, H.; Kiyokawa, K.; Minakata, S. Catalytic Activation of 1-Cyano-3,3-dimethyl-3-(1H)-1,2-benziodoxole with B(C6F5)3 Enabling the Electrophilic Cyanation of Silyl Enol Ethers. Org. Lett. 2017, 19, 4672-4675. (20) (a) Coulombel, L.; Duñach, E. Triflic Acid-Catalysed Cyclisation of Unsaturated Alcohols. Green Chem. 2004, 6, 499-501. (b) Fujita, S.; Abe, M.; Shibuya, M.; Yamamoto, Y. Intramolecular Hydroalkoxylation of Unactivated Alkenes Using Silane-Iodine Catalytic System. Org. Lett. 2015, 17, 3822-3825. (21) B(C6F5)3·nH2O did not promote the hydroalkoxylation of alkene 7 (Scheme S6). (22) To illustrate the versatility of the allyl- and cyano-substituted cyclic ethers, several transformation of 3a and 6a are shown in Scheme S7.

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