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Direct Catalytic Alcoholysis of Unactivated 8-Aminoquinoline Amides Toru Deguchi, Hai-Long Xin, Hiroyuki Morimoto, and Takashi Ohshima ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00442 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017
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Direct Catalytic Alcoholysis of Unactivated 8-Aminoquinoline Amides Toru Deguchi, Hai-Long Xin, Hiroyuki Morimoto* and Takashi Ohshima* Graduate School of Pharmaceutical Sciences, Kyushu University, Maidashi 3-1-1 Higashi-ku, Fukuoka, Japan, 812-8582. ABSTRACT: Direct catalytic alcoholysis of unactivated amides is one of the most difficult challenges in organic chemistry, and an applicable method for cleaving amides used as directing groups in regioselective functionalization reactions has not been reported. Herein we report direct catalytic alcoholysis of 8-aminoquinoline amides, highly effective directing groups in regioselective functionalization reactions. The reactions proceeded with a simple combination of substrates, air-stable catalysts, and alcohols, affording the corresponding esters in good yield with broad functional group tolerance. Highly chemoselective cleavage of the 8aminoquinoline amides in the presence of related carbonyl functionalities and preliminary mechanistic studies are also described.
KEYWORDS: alcoholysis, amides, homogeneous catalysis, chemoselectivity, 8-aminoquinoline Amides are ubiquitous functionalities and important structural motifs in organic molecules, such as proteins, polymers, and biologically active compounds, due to their high stability. Amides have recently also been used as reliable directing groups for regioselective functionalization reactions.1 Enhancing the utility of these functionalization reactions, selective transformation of the directing group amides into more useful carbonyl groups, such as esters, is an important issue. In most cases, however, harsh reaction conditions with stoichiometric amounts of strong acids or bases are used, which limit the functional group compatibility. Another option is to attach an activating functionality, such as a tert-butoxycarbonyl group, before the cleavage, but such indirect methods require additional reaction steps as well as additional reagents. Direct catalytic alcoholysis of unactivated amides is a stepeconomical way to convert directing group amides into more useful esters.2 The development of such potentially useful transformations, however, is hampered by the following factors: (1) low reactivity of unactivated amides due to their high stability produced by delocalization of nitrogen lone pairs, (2) low nucleophilicity of alcohols compared with amines,3 and (3) thermodynamic stability of the starting materials compared with the products. Although the recent seminal contributions from Mashima (N to O acyl transfer and Lewis acid catalysis)4 and Garg and Houk (oxidative addition of Ni(0))5 offer possible solutions (Scheme 1), neither of these strategies has been applied for the catalytic cleavage of directing group amides for regioselective functionalization reactions. In addition, high chemoselectivity is desirable for removing directing group amides in the presence of related carbonyl functionalities. Based on our continued interest in the cleavage of unactivated amides6 as well as chemoselective catalytic transformations,7 we hypothesized that simultaneous catalytic activation of directing group amides and alcohols through a Lewis acidBrønsted base bifunctional activation mode8 would allow us to overcome the limitations of the presently available direct catalytic alcoholysis reactions. Herein we report the direct catalyt-
ic alcoholysis of 8-aminoquinoline amides. Broad functional group tolerance was achieved due to the nature of the catalytic conditions. Chemoselective cleavage of the 8-aminoquinoline amides in the presence of related functionalities and preliminary mechanistic studies are also described. Scheme 1. Direct Catalytic Alcoholysis of Unactivated Amides Zn(OTf)2 (5 mol %) (EtO)2CO (2 equiv)a
HO O CbzHN
N H
R1
OMe
nBuOH, reflux
Ar
Ni(cod)2 (10 mol %) SIPr (10 mol %)b R4OH
Ni(tmhd)2 (1–15 mol %)
O R1
+
N H
R4OH
N 1
a
toluene, 80 °C
80–120 °C this work
2
OnBu R1
O
O R2 + N R3
O CbzHN
O Ar
OR4
+
H
N R3
R2
O R1
OR4 3
+ H2N N recoverable
Reference 4a. bReference 5a.
To test our hypothesis, we selected 8-aminoquinoline amide 1a, a highly effective and frequently used directing group amide introduced by Daugulis,9 as a model substrate, for which a catalytic cleavage method is unknown.10 We screened several catalysts reported to coordinate with 8-aminoquinoline amides9,11 with methanol (2a) as a solvent, and found that nickel(II) triflate11a and cobalt(II) acetylacetonate11b afforded the product 3a, albeit in low yield (Table 1, entries 1–4). These results led us to examine nickel(II) acetylacetonate,11d and we obtained a much better yield (entry 5). Further screening of metal acetylacetonates revealed that nickel(II) was the most effective metal among the tested metal acetylacetonates (entries 3–7). Examination of other nickel(II) diketonate catalysts revealed that nickel(II) bis(2,2,6,6-tetramethyl-3,5heptanedionate) (Ni(tmhd)2) had the best reactivity (entries 8–
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10), and an optimal result was obtained by increasing the reaction time to 24 h to afford ester 3a in 94% isolated yield (entry 11). Notably, Ni(cod)2 did not efficiently promote the reaction (entry 12), suggesting that Ni(0) is not the active species under the present reaction conditions.12 a Standard conditions: 0.25 mmol of 1a, 5 mol % of catalyst in 0.10 M of MeOH (2a) at 80 °C for 12 h. bDetermined by 1H NMR analysis of the crude reaction mixture. cFor 24 h. dIsolated yield. OTf = trifluoromethanesulfonate. acac = acetylacetonate. hfacac = 1,1,1,5,5,5-hexafluoroacetylacetonate. dbm = 1,3-diphenyl-1,3-propanedionate. tmhd = 2,2,6,6tetramethyl-3,5-heptanediaonate. cod = 1,4-cyclooctadiene. n.r. = no reaction. Table 1. Optimization of Reaction Conditionsa catalyst (5 mol %)
O N H
+ MeOH
1a
Ni(tmhd)2 (5 mol %)
O R1
+ MeOH
N H
N 1
2a
N H
yield (%)b
entry
catalyst
yield (%)b
1
Pd(OAc)2
n.r.
7
Pt(acac)2
n.r.
2
Ni(OTf)2
26
8
Ni(hfacac)2
57
3
Co(acac)2· 2H2O
20
9
Ni(dbm)2
52
4
Fe(acac)3
n.r.
10
Ni(tmhd)2
85
5
Ni(acac)2
45
11c
Ni(tmhd)2
95 (94)d
6
Pd(acac)2
n.r.
12
Ni(cod)2
15
N 1b 100 °C, 24 h, 97% (1.4 g) + 8-aminoquinoline 92% (0.79 g) Br O N H
N 1d 80 °C, 12 h, 95% OH O
O
N H
N
Cl
1e 100 °C, 24 h, 76%
SiiPr3
O NC
N
1f 100 °C, 48 h, 76%d
N H
O N
MeO
N H
O
Under these optimized reaction conditions, we first examined the substrate scope of 8-aminoquinoline amides 1 (Scheme 2). It is noted that most of these 8-aminoquinoline amides 1 used in this study (11 out of 14) are regioselective functionalization products reported in the literature.13 Alkyl, α,β-unsaturated and aryl-substituted 8-aminoquinoline amides reacted to give methyl esters 3 in good yields. The reaction with 1a proceeded even in the presence of 1 equiv of water, under air or with 1 mol % of catalyst loading, without a significant reduction of the yield. The reaction conditions were applicable for a gram-scale synthesis of 3b with 92% recovery of 8-aminoquinoline. α-Tetrasubstituted amide 1g also gave the product 3g. Broad functional groups were tolerated such as alkene, alkyne, chloro, bromo, hydroxy, cyano and methoxycarbonyl moieties, most of which are susceptible to either conventional acidic/basic conditions or other amide-cleaving conditions reported in the literature.5,10 Furthermore, structurally complex amide 1i afforded the corresponding ester 3i in good yield. Scheme 2. Scope of 8-Aminoquinoline Amides
3
N H
N H Me N 1c 100 °C, 24 h, 85%
N H
catalyst
OMe
O
O
3a
entry
80–100 °C R1 12–48 h
N
1a 80 °C, 24 h, 92%a, 87%b 100 °C, 24 h, 97%c
OMe
2a
O
O
O
80 °C, 12 h
N
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N
1h 80 °C, 24 h, 87%
1g 100 °C, 24 h, 84%
O Me
N H
MeO Me H
Me
H MeO
H OMe
N 1i 100 °C, 48 h, 81%e
H
a
With 1 equiv of H2O. bUnder air. cWith 1 mol % of catalyst. d With 15 mol % of catalyst in DMSO (1.0 M). eWith 10 mol % of catalyst.
Next, we examined the scope of alcohols. Primary alcohols such as ethanol and 1-butanol gave esters 3 in high yields under similar reaction conditions, whereas secondary and tertiary alcohols gave esters 3 in reduced yields (Scheme 3 (a)). Although these simple esters can be used as intermediates for a variety of further transformations, we also developed one-pot, direct catalytic alcoholysis-catalytic transesterification reactions using nickel(II) and iron(III) catalysts14 to demonstrate the feasibility of alcohols (Scheme 3 (b)). As a result, a variety of esters 3 were obtained from 1.5 equivalents of primary, secondary and tertiary alcohols in good two-step yields without isolation of the intermediate methyl ester 3b. Scheme 3. Scope of Alcohols
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Ni(tmhd)2 (5 mol %)
O N H
(a)
O
+ R4OH
OR4
R1
N
1a
2
+ MeOH
N H
N 1
3j (R4 = Et): 100 °C, 48 h, 94% 3k (R4 = nBu): 120 °C, 24 h, 94% 3l (R4 = iPr): 120 °C, 48 h, 67%a 3m (R4 = tBu): 120 °C, 48 h, 9%a,b
2a
N
O Ni(tmhd)2 (5 mol %)
O
1b [(salen)Fe(III)]2O (5 mol %) R4OH (1.5 equiv) PhCl (0.14 M) reflux, 24–48 h one-pot
N
O
3
N
3n (R4 = Bn): 92% 3o (R4 = (–)-menthyl): 85% 3p (R4 = 1-adamantyl): 61%c 4 OR (all yields are in 2 steps)
N
3s 100 °C, 24 h, 78% (+ 20% 1s)
O N
To test whether our Ni(II) catalysis is highly chemoselective for 8-aminoquinoline amides, we examined competitive reactions using 8-aminoquinoline amides containing related carbonyl group functionalities (Scheme 4 (a)). We found that aromatic and aliphatic amides, an imide and tert-butyl carbamate remained intact during the cleavage of 8-aminoquinoline amides, demonstrating the highly chemoselective nature of the present reaction conditions. It is also noteworthy that no racemization of 3r was observed, implying that the present reaction conditions are less basic than conventional cleavage conditions. The mildness of the current conditions was further demonstrated with amide 1t having a TBS-protected hydroxy group, for which our catalytic reaction conditions provided the desired product 3t in good yield while conventional acidic/basic conditions gave undesired TBS-deprotected byproduct 4t as the major product (Scheme 4 (b)). In addition, treatment of amide 1u under our reaction conditions gave the desired ester 3u in 90% yield whereas acidic conditions previously used for the cleavage of 8-aminoquinoline amides10a,b provided undesired Boc-deprotected byproduct 4u. Scheme 4. Chemoselective Alcoholysis of 8Aminoquinoline Amides
N H
5
O 1r (97% ee) Me
O
1
10 mol % of Ni(tmhd)2 were used. Determined by H NMR analysis of the crude mixture. c10 mol % of the iron(III) catalyst were used.
3r 100 °C, 12 h 82% yield, 97% ee (+ 14% 1r)
O
H N
N H
tBuO 3b (crude)
b
OMe
3q 80 °C, 12 h, 89%
N H
1q
O
OMe
100 °C 24 h 2a
80–100 °C R1 12–24 h
O
+ MeOH N H
O
O
H N
(b)
a
Ni(tmhd)2 (5 mol %)
O
O
1s
N H
N
(b) HCl (1 equiv) MeOH rt, 2 h
N H
3
N H 4t 97%
3
N
O
MeOH TBSO 100 °C, 24 h
3t 82%
NaOMe (1 equiv) 3t MeOH 3% 90 °C, 24 h HCl (2 equiv)
Ni(tmhd)2 (5 mol %)
O 3
O
N H
N
MeOH 80 °C, 24 h
1u BF3•OEt2 (1 equiv)
4t + 1t 59% 33%
+
O ClH3N
N H 4u 72%
3
MeOH 50 °C, 6 h
H N
OMe
3
1t
tBuO
N
Ni(tmhd)2 (5 mol %)
O TBSO
O HO
H N
tBuO
N O 3
OMe
O 3u 90%
3u + 4u + 1u MeOH 26% 26% 31% 50 °C, 24 h
To elucidate the mechanism of the observed chemoselectivity, we performed preliminary mechanistic studies (Scheme 5). First, simple Lewis acidic catalysts Zn(OTf)2 and Sc(OTf)3 previously used for alcoholysis of amides4 were not effective; nor was a catalytic amount of BF3·OEt2, which is an effective reagent for cleaving 8-aminoquinoline amides (eq. 1).10b These results and the results with Ni(II) catalysts shown in Table 1 suggest that the observed reactivity of Ni(tmhd)2 is not solely dependent on the Lewis acidity of the metal catalysts. Second, the coordination mode of 8-aminoquinoline amides was investigated using amide 5a (lacking nitrogen atom on the aminoquinoline ring) and 6a (without the amide N–H moiety); cleavage of 5a did not proceed at all, while reaction with 6a proceeded faster than that of 1a (eq. 2). These results suggested that coordination of quinoline to the Ni(II) catalyst is essen-
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tial, while formation of the metal-amide bond may not always be necessary for promoting the alcoholysis reaction, although different mechanism could be operative between N-H amide 1a and N-Me amide 6a since formation of the metal-amide bond is proposed in most Ni-catalyzed C–H activation reactions for promoting regioselective functionalization.15 Third, we examined the effects of 8-aminoquinoline and acetylacetone, both of which are good ligands for the Ni(II) center. Indeed, these additives reduced the reactivity (eq. 3), suggesting reversible and competitive binding to the Ni(II) center with 8-aminoquinoline amides and 8-aminoquinoline as well as acetylacetone. Scheme 5. Preliminary Mechanistic Studies catalyst (5 mol %)
O N H
O
1a
3a Zn(OTf)2: 6% Sc(OTf)3: 14% BF3•OEt2: 3%
Ni(acac)2 (20 mol %)
O N R3
1a (R3 = H, X = N) 5a (R3 = H, X = CH) 6a (R3 = Me, X = N)
N H
3j 81% from 1a n.r. from 5a 99% from 6a
Notes The authors declare no competing financial interest.
OEt (3)
3j additive = 8-aminoquinoline: 57% additive = acetylacetone: 4%
1a
Based on these results and information from the literature,16 we propose the following possible catalytic cycle (Scheme 6): (diketonato)Ni(II) act as a Brønsted base to produce (alkoxo)Ni(II) species,16 followed by coordination with 8aminoquinoline amide 1 to give intermediate I through coordination of the quinoline.15 After cleavage of the amide bond via intermediate II and the release of ester 3, the resulting (alkoxo)(amido)Ni(II) III reacts with 8-aminoquinoline amide 1 to close the catalytic cycle. The importance of such a Lewis acid-Brønsted base bifunctional nature is distinguishable from the reported direct catalytic alcoholysis of amides. Further investigation of the details, such as the kinetics and nature of the Ni(II) species, is underway. Scheme 6. A Possible Catalytic Cycle
R
R4O R1
R O
O
1 and 2
[NiII] O R
R
O
O
O 1
R
N
II
N
R4O [NiII] I
N
[NiII]
R4OH 2
N
H2N O
N HN O R1
R4O [NiII] N H
III 1 N
N
Corresponding Authors *E-mail:
[email protected] *E-mail:
[email protected] O
EtOH 100 °C, 12 h
N
The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION OEt (2)
Ni(acac)2 (20 mol %) additive (1.0 equiv)
O
ASSOCIATED CONTENT
Experimental details, characterization data and copies of spectra of compounds (PDF)
O
EtOH 100 °C, 12 h
X
In summary, we developed a concise catalytic method to selectively transform 8-aminoquinoline amides into esters. This is the first effective direct catalytic alcoholysis reaction for cleaving directing group amides used in regioselective functionalization reactions with rarely used Ni(II) diketonate catalysts, and this reaction system is useful because only 8aminoquinoline amides, alcohols, and commercially available air-stable Ni(II) diketonate catalysts were used to provide esters with high chemoselectivity and broad functional group tolerance. Further elucidation of the reaction mechanism as well as application of this strategy to related reactions are in progress.
Supporting Information OMe (1)
MeOH 80 °C, 12 h
N
Page 4 of 6
R1
OR4 3
ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research on Innovative Area (JSPS KAKENHI Grant No. JP15H05846 in Middle Molecular Strategy), Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant No. JP24390004 for T.O.) and (C) (JSPS KAKENHI Grant No. JP15K07860 for H.M.) from JSPS, Platform for Drug Discovery, Informatics, and Structural Life Science from AMED, and Naito Foundation. We thank R. Horikawa for [(salen)Fe(III)]2O catalyst and the research group of Prof. Go Hirai at Kyushu University for the use of a polarimeter.
REFERENCES (1) For selected reviews on directing group amides: (a) Rouquet, G.; Chatani, N. Angew. Chem., Int. Ed. 2013, 52, 11726–11743. (b) Daugulis, O.; Roane, J.; Tran, L. D. Acc. Chem. Res. 2015, 48, 1053– 1064. (c) Liu, J.; Chen, G.; Tan, Z. Adv. Synth. Catal. 2016, 358, 1174–1194. (2) For selected recent examples of alcoholysis of amides, see: (a) Bröhmer, M. C.; Mundinger, S.; Bräse, S.; Bannwarth, W. Angew. Chem., Int. Ed. 2011, 50, 6175–6177. (b) Hutchby, M.; Houlden, C. E.; Haddow, M. F.; Tyler, S. N. G.; Lloyd-Jones, G. C.; BookerMilburn, K. I. Angew. Chem. Int. Ed. 2012, 51, 548–551. (c) Siddiki, S. M. A. H.; Touchy, A. S.; Tamura, M.; Shimizu, K.-i. RSC Adv. 2014, 4, 35803–35807. (d) Balachandra, C.; Sharma, N. K. Org. Lett. 2015, 17, 3948–3951. (e) Yamada, K.; Karuo, Y.; Tsukada, Y.; Kunishima, M. Chem.—Eur. J. 2016, 22, 14042–14047. (f) Adachi, S.; Kumagai, N.; Shibasaki, M. Chem. Sci. 2017, 8, 85–90. (3) Based on Mayr’s reactivity parameters, the difference of nucleophilicity between amines and alcohols is about 105 orders of magnitude. For details, see: http://www.cup.lmu.de/oc/mayr/reaktionsdatenbank/ accessed on Dec. 15, 2016. (4) (a) Kita, Y.; Nishii, Y.; Higuchi, T.; Mashima, K. Angew. Chem., Int. Ed. 2012, 51, 5723–5726. (b) Kita, Y.; Nishii, Y.; Onoue, A.; Mashima, K. Adv. Synth. Catal. 2013, 355, 3391–3395. (c) Atkinson, B. N.; Williams, J. M. J. Tetrahedron Lett. 2014, 55, 6935–6938.
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(d) Nishii, Y.; Akiyama, S.; Kita, Y.; Mashima, K. Synlett 2015, 26, 1831–1834. (5) (a) Hie, L.; Nathel, N. F. F.; Shah, T. K.; Baker, E. L.; Hong, X.; Yang, Y.-F.; Liu, P.; Houk, K. N.; Garg, N. K. Nature 2015, 524, 79–83. (b) Hie, L.; Baker, E. L.; Anthony, S. M.; Desrosiers, J.-N.; Senanayake, C.; Garg, N. K. Angew. Chem., Int. Ed. 2016, 55, 15129– 15132. (6) (a) Shimizu, Y.; Morimoto, H.; Zhang, M.; Ohshima, T. Angew. Chem., Int. Ed. 2012, 51, 8564–8567. (b) Shimizu, Y.; Noshita, M.; Mukai, Y.; Morimoto, H.; Ohshima, T. Chem. Commun. 2014, 50, 12623–12625. (c) Noshita, M.; Shimizu, Y.; Morimoto, H.; Ohshima, T. Org. Lett. 2016, 18, 6062–6065. (7) For reviews on chemoselectivity, see: (a) Afagh, N. A.; Yudin, A. K. Angew. Chem., Int. Ed. 2010, 49, 262–310. (b) Mahatthananchai, J.; Dumas, A. M.; Bode, J. W. Angew. Chem., Int. Ed. 2012, 51, 10954–10990. For our recent contributions regarding catalystcontrolled chemoselectivity, see: (c) Uesugi, S.; Li, Z.; Yazaki, R.; Ohshima, T. Angew. Chem., Int. Ed. 2014, 53, 1611–1615. (d) Tokumasu, K.; Yazaki, R.; Ohshima, T. J. Am. Chem. Soc. 2016, 138, 2664–2669. (8) Review: (a) Shibasaki, M.; Kumagai, N. in Cooperative Catalysis: Designing Efficient Catalysts for Synthesis, Peters, R. Ed., WileyVCH, 2015, pp. 1–34. For similar activation mode in Cu(II)-mediated alcoholysis of amides, see: (b) Barrera, I. F.; Mawell, C. I.; Nererov, A. A.; Brown, R. S. J. Org. Chem. 2012, 77, 4156–4160. (9) Zaitsev, V. G.; Shabashov, D.; Daugulis, O. J. Am. Chem. Soc. 2005, 127, 13154–13155. See also ref. 1. (10) For acid-promoted direct alcoholysis of 8-aminoquinoline amides, see: (a) Ano, Y.; Tobisu, M.; Chatani, N. J. Am. Chem. Soc. 2011, 133, 12984–12986. (b) Tran, L. D.; Daugulis, O. Angew. Chem., Int. Ed. 2012, 51, 5188–5191. For an ozonolysis approach, see: (c) Berger, M.; Chauhan, R.; Rodrigues, C. A. B.; Maulide, N. Chem.—Eur. J. 2016, 22, 16805–16808. (11) (a) Aihara, Y.; Chatani, N. J. Am. Chem. Soc. 2013, 136, 898– 901. (b) Grigorjeva, L.; Daugulis, O. Org. Lett. 2014, 16, 4684–4687. (c) Shang, R.; Ilies, L.; Matsumoto, A.; Nakamura, E. J. Am. Chem. Soc. 2013, 135, 6030–6032. (d) Wu, X.; Zhao, Y.; Ge, H. J. Am. Chem. Soc. 2014, 136, 1789–1792. (12) We also examined cleavage of 1a with Ni(cod)2/SIPr and Ni(cod)2/terpy catalysts under the reported conditions,5 but only trace or no 3a was detected. See Supporting Information for details. (13) 1a, 1c, 1s: (a) Shabashov, D.; Daugulis, O. J. Am. Chem. Soc. 2010, 132, 3965–3972. 1b, 1i, 1t: (b) Gou, Q.; Zhang, Z.-F.; Liu, Z.C.; Qin, J. J. Org. Chem. 2015, 80, 3176–3186. 1e: (c) Shang, R.; Ilies, L.; Asako, S.; Nakamura, E. J. Am. Chem. Soc. 2014, 136, 14349–14352. 1f: (d) Singh, B. K.; Jana, R. J. Org. Chem. 2016, 81, 831–841. 1g: (e) Reddy, M. D.; Watkins, E. B. J. Org. Chem. 2015, 80, 11447–11459. 1h: (f) Ano, Y.; Tobisu, M.; Chatani, N. J. Am. Chem. Soc. 2011, 133, 12984–12986. 1u: (g) Gurak, J. A. Jr.; Yang, K. S.; Liu, Z.; Engle, K. M. J. Am. Chem. Soc. 2016, 138, 5805–5808. The functionalized positions of 8-aminoquinoline amides 1 are summarized in the Supporting Information. (14) Horikawa, R.; Fujimoto, C.; Yazaki, R.; Ohshima, T. Chem.— Eur. J. 2016, 22, 12278–12281. (15) For the formation of related Ni(II) intermediates, see: Misal Castro, L. C.; Chatani, N. Chem. Lett. 2015, 44, 410–421 and references cited therein. (16) For generation of (κ2-acetylacetonato)(methoxo)Ni(II) species from Ni(acac)2 in methanol, see: (a) Ginsberg, A. P.; Bertrand, J. A.; Kaplan, R. I.; Kirkwood, C. E.; Martin, R. L.; Sherwood, R. C. Inorg. Chem. 1971, 10, 240–246. (b) Darensbourg, M.; Buonomo, R. M.; Reibenspies, J. H. Z. Kristallogr. 1995, 210, 469–470. Although the structure of the active Ni(II) species is yet to be clarified, we assume that oligomeric Ni(II) species is likely to be involved in the catalytic cycle because of the feasibility of the formation of µ-alkoxo-bridged oligomeric Ni(II) species. It is also noted that our reaction proceeded even with a catalytic amount of nickelocene (50% yield under the conditions in Table 1), which is known to react with alcohols to give (alkoxo)nickel(II) species: (c) Slushkov, A. M.; Petrov, B. I.; Domrachev, G. A. Russ. Chem. Bull. 1987, 36, 385–388.
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tBu
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tBu O
O Ni
O tBu O R1
N H
+ R4OH N
O
tBu Ni(tmhd)2 (1–15 mol %) O 80–120 °C 12–48 h
R1
OR4
+H N 2
N 61–97% recoverable R1: alkyl, alkenyl, aryl 20 examples R4: Me, Et, nBu, iPr other 1°/2°/3°-alkyl esters accecible via one-pot catalytic transesterification compatible functional groups alkene, alkyne, Cl, Br, OH, OTBS, CN, CO2Me, BocNH, amides, imide
ACS Paragon Plus Environment
