Copper-catalyzed borylative aminomethylation of C-C double and triple bonds withN,O-acetal

Article abstract of DOI:10.1039/d1cc00093d

A copper-catalyzed borylaminomethylation of multiple carbon-carbon bonds withN,O-acetal and bis(pinacolato)diboron has been disclosed that offers efficient and expedient access to γ-amino boronates. The products contain a valuable amine and boronate, whic

Full text of DOI:10.1039/d1cc00093d

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Copper-catalyzed borylative aminomethylation
of C–C double and triple bonds with N,O-acetal†
Cite this: Chem. Commun., 2021,
7, 3279
5
Xianfan Qiu, Liugen Xu, Shuxia Wang, Ying Dai, Yunqiu Feng, Chang Gong and
Received 6th January 2021,
Accepted 23rd February 2021
Chuanzhou Tao
*
DOI: 10.1039/d1cc00093d
rsc.li/chemcomm
A copper-catalyzed borylaminomethylation of multiple carbon–
Recently, copper-catalyzed carboboration of multiple carbon–
carbon bonds with N,O-acetal and bis(pinacolato)diboron has been carbon bonds has emerged as an expedient strategy to furnish
disclosed that offers efficient and expedient access to c-amino functionalized organic boronates which are amenable to further
10
boronates. The products contain a valuable amine and boronate, elaboration. The proposed catalytic cycle started with formation of
which are amenable to further elaboration, and have versatile borylcopper species from copper, B pin (bis(pinacolato)diboron)
synthetic utilities. and extrinsic bases (typically tert-butoxide). Subsequent migratory
insertion into double or triple bond generated an organocopper
The aminomethyl subunit is a frequently found structural motif intermediate which was further trapped by a carbon electrophile.
2
2
1
in a wide variety of natural products and pharmaceuticals.
As reported in previous studies, N,O-acetal split into a methylene
Extensive research has been performed to incorporate amino- iminium cation and methoxide anion. We envisioned that an in situ
methyl groups into organic molecules, where a diverse range of
2
aminomethylation reagents have been successfully applied.
Within these reagents, N,O-acetals are attractive precursors to
install the aminomethyl unit owing to their easy preparation
and in situ generation of an active methylene iminium and
3
alkoxide species. Recent years have witnessed the utilization of
2
2
N,O-acetals to construct b -amino acids, since b -amino acids
(
those with substituents at the a-position) are extremely valuable
4
but far less accessible. Gellman and C o´ rdova reported seminal
proline-catalyzed Mannich reaction of aldehydes to achieve
2
b -amino acid synthesis, where N,O-acetals were utilized as ami-
5
nomethylation reagents (Scheme 1a). Then, Chi turned attention
to cooperative catalysts (N-heterocyclic carbene and Brønsted
acid), and a,b-unsaturated aldehydes were aminomethylated
6
(
Scheme 1a). Very recently, diazoesters were chosen as starting
2
materials to synthesize a-alkoxy-b -amino acid ester, where
transition-metal catalysts were employed in the transformations.
Significant recent contributions have been made by the groups of
7
8
9
Xu, Huang and Sun. These precedents have explicated the
convenience and efficiency of N,O-acetals as aminomethylation
precursors (Scheme 1b).
School of Environmental and Chemical Engineering, Jiangsu Ocean University,
Lianyungang 222005, China. E-mail: taocz@jou.edu.cn
†
Electronic supplementary information (ESI) available: Synthetic details, addi-
tional spectroscopic data, and characterization of the new compounds. CCDC
047479. For ESI and crystallographic data in CIF or other electronic format see
2
2
Scheme 1 Synthesis of b -amino acids through aminomethylation with
DOI: 10.1039/d1cc00093d
N,O-acetals and our strategy on borylaminomethylation with N,O-acetal.
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generated methoxy anion could be utilized as a nucleophile to catalyzed the aminomethylation in a moderate yield of 55% (entry
produce the borylcopper species, and the in situ generated iminium 3), and Cu(OAc) performed slightly lower than Cu(CH CN) PF
6
2
3
4
cation might intercept organocopper intermediates that are derived with 70% yield (entry 4). Then, we screened a collection of mono-
from a catalytic borylcupration of multiple carbon–carbon bonds phosphine and bisphosphine ligands. It was found that copper
(Scheme 1c).
complexes supported by the monophosphine including PPh3,
As part of our ongoing research interest in copper-catalyzed XPhos and SPhos, were less effective and 17–50% yields were
1
1
reactions, herein, we disclosed a copper-catalyzed borylative obtained (entries 5–7). Bisphosphine ligands gave worse results
aminomethylation of C–C double and triple bonds with N, in 0–13% yields (entries 8–10). As for the reaction solvent, moderate
O-acetal and bis(pinacolato)diboron (Scheme 1c). This copper- yields were obtained in ethereal solvents, such as THF (65%, entry
catalysis is versatile for various substrates, providing expedient 11) and dioxane (66%, entry 12). Dichloromethane could also be
access to a range of g-amino boronates. Notably, this novel employed as a solvent and a moderate yield of 60% was provided
protocol constitutes a generic platform to prepare valuable (entry 13).
amino acids and ketones.
With the optimal conditions in hand, a diverse set of
We commenced our investigation with styrene (1a) and N, alkenes, alkynes and allenes were used to investigate the scope
N-dibenzyl-1-methoxymethanamine (2) as the model substrates and limitations of this borylative aminomethylation reaction.
to perform borylative aminomethylation of C–C double bonds. Initially, the scope of functional groups on the benzene ring of
After systematically screening the reaction parameters, we obtained the vinyl arenes was surveyed (Table 2). The reaction was
the optimal reaction conditions utilizing Cu(CH
catalyst, Cy-JohnPhos as a ligand, and toluene as a solvent at donating and electron-withdrawing groups, and moderate to
5–30 1C (Table 1, entry 1). During our efforts to optimize the excellent isolated yields of the borylaminomethylation products
3 4 6
CN) PF as a proved to be effective for vinyl arenes carrying electron-
2
reaction conditions, we made several observations summarized in were obtained (49–75%, 3a–3d). It was observed that electron-
Table 1. The choice of catalyst was crucial for the success of the withdrawing groups gave higher yields (see products 3c, 3d)
reaction, and we did not isolate the desired product g-amino than the electron-donating ones (see products 3b). Moderate
boronates with cuprous salt CuCl as a catalyst (entry 2). Cu(OTf)
2
yields (57–59%) were produced with substituents at a meta
Table
1
Optimization of copper-catalyzed borylaminomethylation
Table 2 Substrate scopes of C–C double and triple bonds in the copper-
catalyzed borylaminomethylation with N,O-acetals
a
ab
conditions of vinyl arenes with N,O-acetal
b
Entry
Change from standard conditions
None
3a, Yield [%]
1
2
3
4
5
6
7
8
9
0
1
2
3
75
0
CuCl instead of Cu(CH
Cu(OTf) instead of Cu(CH
Cu(OAc) instead of Cu(CH
Ph P instead of Cy-JohnPhos
3
CN)
4
PF
CN)
CN)
6
2
3
4
PF
PF
6
55
70
17
18
50
13
0
2
3
4
6
3
XPhos instead of Cy-JohnPhos
SPhos instead of Cy-JohnPhos
BINAP instead of Cy-JohnPhos
XantPhos instead of Cy-JohnPhos
Dppbz instead of Cy-JohnPhos
THF instead of toluene
1
1
1
1
0
65
66
60
Dioxane instead of toluene
CH Cl instead of toluene
2 2
a
2 2
Conditions: 0.2 mmol 1a, 0.3 mmol 2, 0.3 mmol B pin , 10 mol%
Cu(CH
Cu(CH
3
CN)
CN)
4
PF
PF
6
and 20 mol% of a monophosphine ligand, or 10 mol%
a
3
4
6
and 10 mol% of a bisphosphine ligand, 1.0 mL of
Conditions: 0.2 mmol 1, 0.3 mmol 2, 0.3 mmol B pin , 10 mol%
2
2
b
appropriate solvent, 25–30 1C, 36 h. The yields are isolated yields with Cu(CH CN) PF and 20 mol% Cy-JohnPhos, 1.0 mL toluene, 25–30 1C,
column chromatography.
3
4
6
b
36 h. The yields are isolated yields with column chromatography.
3
280
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or ortho position on a benzene ring of vinyl arene, including 3-
chloro, 2-chloro and 2-bromo (3e–3g). However, b-substituted
styrene was found to be a reluctant substrate, which failed to
produce the expected product (3h). Vinyl arene containing Scheme 2 Gram-scale synthesis of g-amino boronate 3a.
heterocycles such as thienyl, afforded a low yield of the desired
product (3i, 27%). Unfortunately, 4-vinylpyridine (1j) was
a reluctant substrate and we could not isolate the desired
g-amino boronate. The borylaminomethylation of alkenes
could also be extended to the bicyclic strained alkenes although
with low yield as exemplified with the product of 3k in 29%
yield. Delightfully, a,b-unsaturated esters were accommodated
in this copper-catalyzed transformation, as exemplified with
the product 3l in 65% yield. It is noteworthy that this transfor-
2
mation would directly afford b -amino acid esters with func-
tional group boronates.
Then, we examined the scope of different alkynes. As shown
in Table 2, moderate yields (61–63%) of adducts (3m, 3n) were
obtained with internal alkyne containing 1,2-aryl groups and
1-aryl-2-alkyl alkynes. However, 1,2-alkyl alkynes (1o) were
found to be a reluctant substrate, which failed to produce the
expected product (3o). Besides, heteroaromatic substituted
internal alkyne was examined as well and 40% yield of the
desired product (3p) was isolated.
Finally, the scope of various substituted allenes was evalu-
ated (Table 2). The results showed that the yield of allene
bearing an aryl substituent (3q) was higher than that of allenes
bearing alkyl substituents (3r–3t). In addition, allenes bearing
alkyl substituents such as cyclohexyl and n-decyl both gave
medium yields with an acceptable regioselectivity (3r–3t). Boryla-
minomethylation product 3a was amenable to crystallization, and
allowed for structure determination by X-ray crystallographic
analysis (Fig. 1).
To showcase the scalability of this transformation,
a
Scheme 3 Synthetic utilities of g-amino boronates 3.
gram-scale reaction was performed with styrene (1a, 8.0 mmol),
N,N-dibenzyl-1-methoxymethanamine (2) and bis(pinacolato)
diboron (Scheme 2). Gratifyingly, a satisfactory isolated yield group gave the corresponding Boc-protected g-amino alcohol
1
1a
(
67%, 2.37 g) of g-amino boronate 3a was obtained without 5 (89% yield).
Nevertheless, direct conversion of a g-amino
modification of the standard conditions.
alcohol into a b-amino acid product was hard to realize owing to
1
2
To further demonstrate the synthetic utility and compatibility the benzylic hydrogen of compound 5. Alternatively, a-amino
of this protocol, a series of transformations of the g-amino ketone 6 was isolated when PCC was used as an oxidant. As for
boronates 3a and 3q were executed (Scheme 3). Firstly, we found Dess–Martin periodinane (DMP) reagents, g-amino alcohol 5 was
that the functional group of boronate was compatible with the transformed into b-amino aldehyde 7 in 88% yield. Further
2
high pressure hydrogenolysis protocol, and the benzyl groups oxidization furnished the designed b -amino acid 8 in 71% yield
were removed through hydrogenolysis and replaced by Boc in an with m-CPBA as an oxidant. A dibenzylamino group on 3a was
efficient one-pot operation, affording the Boc-protected g-amino also compatible with an oxidant of H O /NaOH. Successfully, 3a
2 2
boronate 4 in 84% overall yield. Subsequent oxidation of a boryl was transferred to the g-amino alcohol 9 with an excellent yield
of 91%. g-Amino boronate 3a was accommodated in Suzuki–
Miyaura coupling, affording C-arylation product 10 in 86% yield
with bromobenzene. Furthermore, the protodeboronation of
pinacol alkylboronic esters was realized utilizing a copper-
catalysis to give amine 11, the reaction was usually achieved
1
3
under radical chain reaction conditions. Then, we selected
2
alkenyl boronate 3q as a starting substrate, and b -amino ketone
1
2 was formed after treatment with H O /NaOH. Compound 3q
2 2
was also subjected to Suzuki–Miyaura cross-coupling reaction,
Fig. 1 X-ray crystallographic structure of 3a.
generating quinoline amine 13 in 83% yield.
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In summary, we have disclosed a copper-catalyzed borylative
aminomethylation of C–C double and triple bonds with N,
O-acetal and bis(pinacolato)diboron. A series of g-amino boronates
were provided under mild reaction conditions with wide substrate
scopes. The reaction can be performed at gram-scale and the
products containing a valuable amine and boronate providing
expedient access to varied valuable compounds. Given the fact that
amine and boronate are amenable to further elaboration, we
anticipate that this method will find practical applications in
various syntheses of drug and natural products.
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