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Communication
Electrochemically Enabled Carbohydroxylation of Alkenes with HO and Organotrifluoroborates 2
Peng Xiong, Hao Long, Jinshuai Song, Yaohui Wang, Jian-Feng Li, and Hai-Chao Xu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08592 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 2, 2018
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Journal of the American Chemical Society
Electrochemically Enabled Carbohydroxylation of Alkenes with H2O and Organotrifluoroborates Peng Xiong,†,§ Hao Long,†,§ Jinshuai Song,‡ Yaohui Wang,† Jian-Feng Li,† Hai-Chao Xu*,† †State
Key Laboratory of Physical Chemistry of Solid Surfaces, Key Laboratory of Chemical Biology of Fujian Province, Innovative Collaboration Center of Chemistry for Energy Materials, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China ‡Fujian Institute of Research on Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China Supporting Information Placeholder ABSTRACT: Unprecedented hydroxy-alkynylation and alkenylation reactions of aryl alkenes have been developed through electrochemically enabled addition of an organotrifluoroborate reagent and H2O across the double bond of the alkene. The use of electrochemistry to promote these oxidative alkene 1,2difunctionalization reactions not only obviates the need for transition metal catalysts and oxidizing reagents but also ensures high regio- and chemoselectivity to afford homopropargylic or homoallylic alcohols. The possibility of extending the electrochemical alkene difunctionalization strategy to other alkene carboheterofunctionalization reactions has been demonstrated.
Vicinal difunctionalization of alkenes, which adds one substituent to each carbon of a double bond, provides a straightforward and highly attractive strategy for organic synthesis.1 Despite extensive research in this area, only a few reported literatures have explored the synthesis of functionalized alkynes and alkenes via the vicinal addition of a heteroatom group and an alkynyl or alkenyl group, which generally involve transition metal (TM)-catalyzed intramolecular cyclizations with a tethered heteroatom nucleophile.2 In contrast, the more challenging three-component intermolecular hetero-alkynylation and -alkenylation of alkenes remain rare3 and to the best of our knowledge, intermolecular alkene hydroxy-alkynylation and -alkenylation reactions have not been reported. Organic electrochemistry, which employs electrons as traceless redox reagents, provides an enabling and environmentally sustainable tool for organic synthesis.4 Electrochemistry is capable of achieving reactivity umpolung through single electron transfer (SET), allowing the coupling of two or more functionalities that share the same polarity. The feasibility of this concept has been demonstrated in several published studies that have accomplished the intermolecular anodic difunctionalization of alkenes with two nucleophiles (Scheme 1a).5 However, most of these reactions involved the introduction of two heteroatom-based functionalities. Nonetheless, Lin has very recently reported efficient Mn-catalyzed electrochemical chlorotrifluoromethylation of alkenes with MgCl2 and CF3SO2Na via addition of CF3 radical to the alkene (Scheme 1a, top-right).5c An alternative strategy to achieve alkene difunctionalization with two nucleophiles is to use alkene-derived radical cation as the key intermediate.6 Alkenes can lose one electron on the electrode surface to generate a high local concentration of the corresponding radical cation intermediates,
which facilitates their self-coupling or polymerization especially for terminal alkenes.5f,g As a result, electrochemical alkene difunctionalization reactions that proceed through alkene radical cations generally employ a large excess of external nucleophiles, such as solvent molecules, to efficiently trap the radical cations (Scheme 1a, bottom-right).6f-i,7 Alkynyl- and alkenyltrifluoroborate salts have been widely employed in organic synthesis owing to their easy availability, stability and excellent crystallinity.8 Although no reactions between alkynyltrifluoroborates and radical species have been reported, alkenyltrifluoroborates have been shown to react regioselectively with electrophilic radicals9a,b and enamine-derived radical cations9c at the ipso-position. Encouraged by these studies, herein we report the electrochemically enabled regio- and chemoselective hydroxy-alkynylation and -alkenylation reactions of alkenes via sequential addition of alkynyl- or alkenyltrifluoroborate and H2O across the olefinic double bond (Scheme 1b). Our method provides efficient access to homopropargylic and homoallylic alcohols without the need for transition metal-catalysts or oxidizing reagents.10 In addition, extension to other alkene carboheterofunctionalization reactions has been demonstrated.
Scheme 1. Electrochemical 1,2-Difunctionalization of Alkenes with Two Nucleophiles (a) Previous work Radical pathway X
X
NaN3 or MgCl2
R' R X = N3 or Cl ArS
X
Ref. 5a,b
MgCl2 NaO2SCF3
R'
R' Ref. 5c Radical cation pathway O X X S or MeOH R
R
ArSH ROH or RNH2
R' R X = OR or NHR
CF3
Cl
Ref. 5d,e
R' R X = O, OH, OMe
Ref. 5f-i
(b) This work OH
KF3B R3 + R1
R2
R2
+ H 2O
via
R1 R2
R2
R3 =
R
R
1
R
R
3
OH
R3
R
1
R3
OH
R2
OH R1
R2
R3 R1
Our studies began by optimizing the reaction conditions for the difunctionalization of alkene 1 with H2O and alkynyltrifluoroborate 2. The best results were obtained by conducting the electrolysis at
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50 °C, under a constant current of 5 mA, in an undivided cell (a three-necked round-bottomed flask) equipped with a graphite rod anode and a platinum plate cathode, and with a mixed solvent of MeCN/H2O containing KHCO3 as additive. Under these conditions, the desired homopropargylic alcohol product 3 was obtained as the only regioisomer in 78% yield (entry 1). In addition, no other homo-addition products, such as those derived from dihydroxylation or dialkynylation, were detected. Control experiments showed
Table 1. Optimization of Reaction Conditionsa Ph +
Ph
KF3B
1
entry
C
n-Bu 2
OH
Pt
KHCO3 (1.5 equiv) MeCN/H2O, 50 °C
deviation from standard conditions
Ph
n-Bu
Ph 3
yield/%b
1
none
78c
2
Reaction at rt
61
3
No KHCO3
58
4
No electricity
NR
5
K2CO3 as the base
60
6
NaHCO3 as the base
50
7
MeCN/H2O (20:1) as the solent
69
8
MeCN/H2O (5:1) as the solent
71
9
Acetone/H2O (14:1) as the solvent
60
10
7.5 mA
41
11
3 mA
45
aReaction conditions: Undivided cell, graphite rod anode, Pt cathode, 1 (0.2 mmol), 2 (0.6 mmol), MeCN (5.5 mL), H2O (0.4 mL), argon, 5 mA, 3.3 F mol−1 (based on 1). bYield determined by 1H-NMR analysis using 1,3,5-trimethoxybenzene as the internal standard. cIsolated yield. NR = no reaction.
that the reaction became less efficient at room temperature (entry 2) or in the absence of KHCO3 (entry 3), and was completely abolished when there was no electric current (entry 4). Using another base additive such as K2CO3 (entry 5) or NaHCO3 (entry 6), altering the ratio of MeCN/H2O (entries 7 and 8), replacing
MeCN with acetone (entry 9), and changing the current (7.5 mA, entry 10 or 3 mA, entry 11), were all found to lower the yield of 3. An optimal concentration of H2O was need because it served as a reactant and to increase the solubility of the salts. The substrate scope of the reaction was then explored by first varying the substituents of the alkene (Scheme 2). When the 1,1diphenyl alkene 1 was used, the reaction showed broad compatibility with a wide range of substituents with diverse electronic properties on the benzene ring (4–13). Aryl, alkylsubstituted alkenes (14–24) were also well tolerated. Potentially labile functional groups, including silylether (19), free alcohol (20) and ester (21), were all left intact after the electrochemical difunctionalization. Furthermore, alkenes that carried a heteroaryl substituent, such as a 3-pyridyl (25), 2-thiazolyl (26), 2benzothiazolyl (27) or 2-benzoxazolyl (28) group, were all shown to be suitable substrates. However, product formation was significantly hampered when a monosubstituted styrenyl alkene was employed (29, 30). Disubstituted aryl alkene such as 1-buten1-ylbenzene reacted successfully to give the desired product 31 in 55% yield, albeit with low diastereoselectivity (1.3:1 dr). The reaction of 1,1-diphenylpropene, a trisubstituted alkene, afforded the hydroxy-alkynylation product 32 in 27% yield along with 51% of vicinal diol 33, 10% of 3-methyl-2,2-diphenyloxirane and 9% of benzophenone.11,12 This alkene reacted preferentially with H2O over the trifluoroborate probably because of the increased steric demand. We next tested a variety of potassium organotrifluoroborates (Scheme 2). The terminal butyl moiety in 2 could be replaced with a primary alkyl chain bearing a hydroxyl (34), chloro (35) or alkynyl group (36), a more sterically hindered cyclohexyl (37) or tBu (38), or a phenyl ring para-substituted with H (39), OMe (40), Me (41) or halogen (42–44). An alkynyl trifluoroborate capped with an alkenyl group also exhibited moderate reactivity toward 1 affording the enyne product 45. Pleasingly, alkenyltrifluoroborates were also found to be compatible with the reaction system and afforded homoallylic alcohols in moderate to excellent yields (46– 49).13 Importantly, the stereochemistry of the alkenyltrifluoroborate was well-maintained during the reaction, providing stereoselective access to homoallylic alcohols bearing an internal alkene (47–49).
Scheme 2. Scope of the Electrohemical Hydroxy-Alkynylation and -Alkenylation of Alkenesa
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Scope of Alkene OH
KF3B
R
n-Bu
Ph
OH
n-Bu
Ar
KHCO3 (1.5 equiv) CH3CN/H2O, 50 °C
3
OH
OH Me N 26, 50%
N
Me Ph
11, R1 = Br, R2 = H, 65% 12, R1 = F, R2 = OMe, 48%
13, 53%
OH
n-Bu
R
OH
Me
OH N
OH
n-Bu
OH
25, 41%
n-Bu
OH
Ph
Me
Ph Ph
Et
R 29, R = H, 25% 30, R = Br, 20%
n-Bu
Ph
OH
OH
Me
Ph
Ph
Ph
OH Ph
Ph
Ph
Ph
Ph
Ph 45, 48%
Ph Ph
46, 40%
Br
37, 68%
OH
OH
OH Ph
t-Bu
OH
36, 60%
39, R = H, 52% R 40, R = OMe, 54% 41, R = Me, 40% 42, R = F, 68% 43, R = Cl, 60% 44, R = Br, 53%
OH Ph
Ph
Ph 35, 69%
34, 64%
OH Me
32, 27% + 33, 51%b
31, 55% (1.3:1 dr)
OH
Cl
OH
Ph Ph
+
Scope of Organotrifluoroborate OH
n-Bu
Ph
23, n = 0, 58% 24, n = 1, 68%
22, 55%
n-Bu
14, R = H, 67% 15, R = F, 54% 16, R = Br, 51%
n-Bu
n
Me
27, X = S, 50% 28, X = O, 40%
n-Bu
n-Bu
Ph
X
S
R1 OH
OH
n-Bu
OH
n-Bu
R3
Ar
n-Bu
R2
Ph 19, R = TBDPSO, 69% 20, R = CH2CH2OH, 55% 21, R = CH2CH2OOCt-Bu, 66%
R
17, R = Et, 56% 18, R = i-Pr, 68%
OH or
R1
R1
R
Ph
R3
OH
Pt
C
or
4, R = Me, 63% 5, R = F, 74% 6, R = Cl, 65% 7, R = Br, 60% 8, R = CF3, 75% 9, R = CN, 72% 10, R = CONEt2, 70%
R
OH
R3
KF3B +
R2
OH
OH n-Bu
Ph
38, 52%
Me Ph [from E-alkenyltrifluoroborate]
47, 91% (>20:1 E/Z)
n-Bu
Ph
Ph n-Bu [from Z-alkenyltrifluoroborate]
48, 55% (>20:1 E/Z)
49, 78% (>20:1 Z/E)
aReaction b10%
conditions from Table 1, entry 1, alkene (0.2 mmol), trifluoroborate salt (0.6 mmol), 2.8–3.8 h. Isolated yields were reported. of 3-methyl-2,2-diphenyloxirane and 9% of benzophenone were also formed.
employment of tethered oxygen nucleophiles led to the formation of valuable functionalized heterocycles (51–67 and 69).
Scheme 3. Scope of Heteroatom Nucleophiles 1
R1
MeCN/MeOH (5.5:1)
Scheme 4. Gram Transformationsa Ph
51, R = C6H5, 78% 52, R = 4-MeO-C6H4, 72% 53, R = 4-Me-C6H4, 74% 54, R = 4-Cl-C6H4, 68% 55, R = 4-I-C6H4, 76% Ph 56, R = 4-CF3-C6H4, 59% 57, R = 4-CN-C6H4, 52% 58, R = 4-pyridyl, 51% 59, R = 2-thiophenyl, 75%
O
Ph O
OH
Ph 47%
n-Bu
N
52% (1:1 dr)
O n-Bu
67, 52% (only E) O
same as above
n-Bu
Me
68
69, 50% C +
2 (3.0 equiv)
F
Pt
Et4NF•3HF (5.5 equiv) MeCN, 50 °C, 6 mA
Ph
n-Bu
OH f
75 OH
Ph
66, 63%
Ph
n-Bu
Ph
Ph 70, 62%
Further studies (Scheme 3) revealed the electrochemical alkene difunctionalization reaction was compatible with other heteroatom nucleophiles such as MeOH (50), tethered amide carbonyls (51– 67) or a hydroxyl group (68), and even fluoride (70). The
a
g
40%
Ph
62, R = H
Ph
2 (2.5 equiv)
CN
60, R =
Ph O
N
R 63, R = 4-MeO-C6H4, 50% 64, R = 4-F-C6H4, 50% 65, R = 2-naphthyl, 57%
+
Ar
4-Me-C6H4
Ph
Synthesis
and
Ph
49
b
n-Bu
n-Bu
Ph 3 (62%, 1.44 g)
Ph
d
74
n-Bu O
n-Bu Ph Ph
72
Ph c
OH n-Bu
n-Bu
71
Ph
Ph
e
Ph
Product
1+2
n-Bu
76
R2
61, R =
Scale
Ph
O
N
KHCO3 (1.5 equiv) CH3CN/H2O, 80 °C, 5 mA
(2.5 equiv)
Ph
1
Ph 50, 80%
Pt
C
R2 BF3K
+
R
Me
n-Bu
R1
Ar
N
Ph
O
HN
N
OMe
Pt
C
2 (3 equiv)
+
Ph
73
aReaction conditions: (a) H SO , toluene, rt, 70%, 1.8:1 E/Z; (b) 2 4 (i) H2PtCl6, CH2Cl2, rt, 90%; (ii) HOAc, HCl, 120 °C, 60%; (c) (i) PdCl2(PhCN)2, THF, rt, 95%; (ii) Et3SiH, BF3•Et2O, CH2Cl2, −78 °C, 85%; (d) Pd/C, H2, MeOH, rt, 68%; (e) Ni(OAc)2•4H2O, NaBH4, H2, EtOH, rt, 98%; (f) InBr3, TMSCN, CH2Cl2, rt, 60%; (g) SOCl2, pyridine, CH2Cl2, rt, 78%.
To further demonstrate the synthetic utility of the current electrosynthesis, the difunctionalization reaction of 1 with 2 was performed on a gram scale. Gratifyingly, the reaction generated the expected product 3 in 62% yield (Scheme 4). Compound 3 could be converted in one or two steps to various cyclic and acyclic structures such as benzofulvene 71, naphthalene 72, tetrahydrofuran 73, tertiary alcohol 74, homoallylic alcohol 49, nitrile 75, and enyne 76. SET oxidation of aryl alkenes to generate the corresponding radical cations can also be achieved using photocatalysis or chemical oxidants such as cerium ammonium nitrate (CAN).
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Scheme 5. Alkene Functionalization Reactions under Nonelectrochemical Conditions
1
+
cat. Acr+-Mes ClO4 cat. [Co]
2 (3 equiv)
MeCN/H2O (10:1) blue LED, rt, 24 h
1
+
CAN (3 equiv) KHCO3 (1.5 equiv)
2 (3 equiv)
3, 0%
+
3, 0%
+
Ph O Ph 77, 58% Ph
MeCN/H2O (14:1) 50 °C, 4 h
Ph
O 25%
(42% conversion of 1)
Scheme 6. Mechanistic Proposal
e
e
cathode
e
anode + + + + + + + + + + + + + + + + +
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
R
A R1
2H2O B
R2 F3B R3 C
ASSOCIATED CONTENT Supporting Information Full experimental details and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.
Corresponding Author 2e
*E-mail:
[email protected] Author Contributions BF3
2HO + H2
§P.X.
R1 R2
The distinct outcome for the alkene functionalization under the electrochemical conditions is attributable to the unique features of electrochemical oxidation. Under the electrochemical conditions, the alkene A is oxidized to the radical cation B on the anode surface.18 Meanwhile, relatively high concentration of the anionic organotrifluoroborate is probably attracted to the positively charged anode surface.19 As a result, the radical cation B reacts efficiently and preferentially with the organotrifluoroborate reagent. However, cation E reacts selectively with H2O probably due to the steric hindrance posed by the C–C bond formation between two bulky coupling partners (C and E). The cation E was therefore trapped by the less sterically demanding nucleophile H2O. The sensitivity of the organotrifluoroborate reagents to steric hindrance is also manifested in their low reaction efficiency with trisubstituted alkenes that we tested. In summary, we have achieved regio- and chemoselective hydroxy-alkynylation and -alkenylation reactions of aryl alkenes using two easily available and stable nucleophiles. This method takes advantage of the unique aspects of electrochemistry to enable transformations that are difficult for alternative technologies. We are currently assessing the synthetic utility of organotrifluoroborates in other electrochemical heterocarbofunctionalization reactions of alkenes.
AUTHOR INFORMATION
R1 2
Page 4 of 6
R3 D
R1 R3 R2 E [On surface]
and H.L. contributed equally to this work.
Notes H+
The authors declare no competing financial interests.
OH
+ H 2O R2
R R1
3
ACKNOWLEDGMENT
F
A possible mechanism for the electrochemical difunctionalization reaction is shown in Scheme 6. The reaction begins with the anodic oxidation of the alkene A [Ep/2 (1) = 1.57 V vs SCE] to produce the radical cation B,16 which is trapped by the organotrifluoroborate reagent C [Ep/2 (2) = 2.06 V vs SCE] to generate the C-centered radical D. The reactions of enamine radical cations with alkenyltrifluoroborates have been proposed to proceed via a stepwise mechanism involving formation of the C–C bond, SET oxidation and cleavage of the C–B bond.9c However, the stereospecific reaction of the alkenyltrifluoroborates in our work suggests that such a mechanism is not operative here. The reaction of B and C most likely proceeds through a concerted mechanism with concomitant formation of the C–C bond and cleavage of the C–B bond to release BF3. Density functional theory (DFT)-based calculations on the reaction of alkene 1 derived radical cation with an alkenyltrifluoroborate suggests a low activation barrier for such a concerted mechanism (Figure S4). Anodic oxidation of D results in the formation of the carbocation E, which then reacts with H2O to afford the final alcohol product F. At the cathode, H2O is reduced to furnish H2 and HO−. The latter is consumed by H+ produced during the formation of F and the added bicarbonate, preventing accumulation of HO−.17
Financial support of this research from MOST (2016YFA0204100) NSFC (21672178) and the Fundamental Research Funds for the Central Universities.
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(8) Darses, S.; Genet, J.-P. Chem. Rev. 2008, 108, 288. (b) Molander, G. A. J. Org. Chem. 2015, 80, 7837. (9) (a) Yasu, Y.; Koike, T.; Akita, M. Chem. Commun. 2013, 49, 2037. (b) Fernandez Reina, D.; Ruffoni, A.; Al-Faiyz, Y. S. S.; Douglas, J. J.; Sheikh, N. S.; Leonori, D. ACS Catal. 2017, 4126. (c) Kim, H.; MacMillan, D. W. C. J. Am. Chem. Soc. 2008, 130, 398. (10) (a) Ding, C.-H.; Hou, X.-L. Chem. Rev. 2011, 111, 1914. (b) Yus, M.; González-Gómez, J. C.; Foubelo, F. Chem. Rev. 2011, 111, 7774. (c) Diner, C.; Szabó, K. J. J. Am. Chem. Soc. 2017, 139, 2. (11) Please see Figure S1 for details and discussion. (12) (a) Wu, X.; Davis, A. P.; Fry, A. J. Org. Lett. 2007, 9, 5633. (b) Okajima, M.; Suga, S.; Itami, K.; Yoshida, J.-i. J. Am. Chem. Soc. 2005, 127, 6930. (13) Suzuki, J.; Tanigawa, M.; Inagi, S.; Fuchigami, T. ChemElectroChem 2016, 3, 2078. (14) (a) Zhang, G.; Hu, X.; Chiang, C.-W.; Yi, H.; Pei, P.; Singh, A. K.; Lei, A. J. Am. Chem. Soc. 2016, 138, 12037. (b) Yi, H.; Niu, L.; Song, C.; Li, Y.; Dou, B.; Singh, A. K.; Lei, A. Angew. Chem., Int. Ed. 2017, 56, 1120. (15) Nair, V.; Deepthi, A. Chem. Rev. 2007, 107, 1862. (16) Alkylalkenes are not suitable substrates because of their high oxidation potentials. For the functionalization of trisubstituted alkylalkenes using photochemical methods, see: Nguyen, T. M.; Nicewicz, D. A. J. Am. Chem. Soc. 2013, 135, 9588. (17) The reaction of the side product BF3 with H2O also produces acid (HBF4): Wamser, C. A. J. Am. Chem. Soc. 1951, 73, 409. (18) Sperry, J. B.; Wright, D. L. Chem. Soc. Rev. 2006, 35, 605. (19) The absorption of alkynyltrifluoroborate 2 on a gold electrode was observed by Raman spectroscopy. See Figure S5 for details.
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R1 R2
+ H 2O
[nucleophilic]
+
or
R2
KF3B
R3 [nucleophilic]
R3
OH
R3
KF3B
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R1 OH
R2
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