Highly Diastereo- and Enantioselective Cu-Catalyzed Borylative Coupling of 1,3-Dienes and Aldimines

Article abstract of DOI:10.1002/anie.201607493

A Cu-catalyzed diastereo- and enantioselective borylative coupling reaction of 1,3-dienes with imines was realized. Branched homoallylic amines are readily prepared in a syn-selective manner with high regio-, diastereo- and enantioselectivity. Moreover, t

Full text of DOI:10.1002/anie.201607493

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201607493
German Edition: DOI: 10.1002/ange.201607493
Asymmetric Catalysis
Highly Diastereo- and Enantioselective Cu-Catalyzed Borylative
Coupling of 1,3-Dienes and Aldimines
Liyin Jiang, Peng Cao,* Min Wang, Bin Chen, Bing Wang, and Jian Liao*
Dedicated to Professor Yaozhong Jiang on the occasion of his 80th birthday
[
8]
Abstract: A Cu-catalyzed diastereo- and enantioselective
borylative coupling reaction of 1,3-dienes with imines was
realized. Branched homoallylic amines are readily prepared in
a syn-selective manner with high regio-, diastereo- and
enantioselectivity. Moreover, these three-component coupling
reactions feature good functional-group compatibility and easy
access to the substrates and catalyst.
carbon substrates and imines is less developed. Conjugated
dienes and especially 1,3-dienes, as a common feedstock, are
[
9]
ideal nucleophilic allyl metal precursors and readily partic-
ipate in allylic transformations of carbonyls and imines. We
envisioned merging the borylcupration of a conjugated
diene (Scheme 1, step A) and enantioselective imine allyl
ation (Scheme 1, step B) in the presence of a suitable chiral
copper catalyst. This approach could afford useful homoal-
[10]
[
11]
[
1]
A
symmetric allylation reactions of imines have received
[2]
increasing interests from the synthesis community because
enantioenriched homoallylic amine products are useful
building blocks in organic synthesis and medicinal chemis-
try. Copper catalysis is an efficient approach to promote the
enantioselective addition of terminal allyl transmetalation
[3]
[
4]
reagents (e.g., with B, Si, and Sn) to imines, however, Cu-
catalyzed asymmetric, nucleophilic addition to imines with
functionalized (e.g., g-substituted) allyl metal reagents
remain challenging. This is probably due to inefficiency of
the transmetalation event arising from increased steric
hindrance, the weak electrophilicity/reactivity of imines,
and/or difficulties in predicting the regio- and stereochemical
outcome of additions. New and efficient approaches to
generating and utilizing functionalized allyl metal reagents
in the presence of a copper catalyst hold promise for accessing
complex homoallylic amines, and these methods will expand
the application of catalytic asymmetric allylation chemistry.
Since the seminal work of borylcopper catalysis reported
Scheme 1. Cu-catalyzed borylative coupling of conjugated dienes and
imines.
lylic amines containing an easily derivatizable boron motif. In
this method, possible complications include the undesired
[
5]
[6]
by the Ito and the Miyaura groups, Cu-catalyzed boryla-
tive coupling reactions have been recognized as an important
method for generating boron-containing organocopper spe-
[
12]
borylation of imines,
imines or products, and difficulty in controlling the regio-
poisoning of the metal catalyst by
[
7]
[7k,11a]
cies in situ from an unsaturated hydrocarbon. In this
context, Hoveyda pioneered the use of boron-functional
allylcopper intermediates in enantioselective allylation of
(linear vs. branched products)
and stereoselectivity
(enantio- and/or diastereoselectivity).
To test the feasibility of our proposed transformation, we
chose a complex of CuCl and chiral sulfoxide-phophine (SOP,
[
7f]
[7i]
[7k]
aldehydes/ketones, allyllic esters, and enoates.
How-
[
7j]
ever, Cu-catalyzed borylative coupling of unsaturated hydro-
L1) as a catalyst in preliminary experiments. In the presence
of isoprene (5 equiv), 1.5 equiv of B (pin) , and NaOtBu
2
2
(
40 mol %) as base, allylation of several imines (3–7a but not
[
*] L. Jiang, Prof. Dr. P. Cao, Dr. M. Wang, B. Wang, Prof. Dr. J. Liao
Chengdu Institute of Biology, Chinese Academy of Sciences
Chengdu 610041 (China)
N-Boc imine 2) proceeded smoothly to provide branched
homoallylic amines. Interestingly, we found that the diaste-
reoselectivity of the reaction increases with the bulkiness of
the protecting groups on the imines (Ms < Ts < PMP <
OMP < Dpp, Table 1, entries 2–6). With N-diphenylphosphi-
noyl (Dpp) as the protecting group, the homoallylic amine
and
University of Chinese Academy of Sciences
Beijing 100049 (China)
E-mail: caopeng@cib.ac.cn
jliao@cib.ac.cn
8
aa was produced with 90:10 d.r., while the enantioselectivity
B. Chen, Prof. Dr. J. Liao
was not satisfactory with our SOP ligand L1 (Table 1, entry 6).
We then turned our attention to commercially available chiral
ligands. To our delight, (R)-BINAP (L2) gave 8aa in 89% ee
and 86:14 d.r. (Table 1, entry 7). The observed selectivity
College of Chemical Engineering, Sichuan University
Chengdu 610065 (China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607493.
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[
a]
Table 1: Enantioselective borylative aminoalkylation of isoprene
[
b]
[b]
[c]
Entry
Imine
L
Copper salt+base
d.r.
Yield /ee
%/%]
[
t
1
2
3
4
5
6
7
8
9
2
3
4
5
L1
L1
L1
L1
L1
L1
L2
L3
L4
L4
L4
L4
CuCl+ NaO Bu
–
0/–
71/–
70/–
85/–
t
CuCl+ NaO Bu
52:48
57:43
67:33
73:27
90:10
86:14
89:11
93:7
t
CuCl+ NaO Bu
t
CuCl+ NaO Bu
t
6
CuCl+ NaO Bu
76/–
t
7a
7a
7a
7a
7a
7a
7a
CuCl+ NaO Bu
95/23
72/89
89/93
87/94
92/95
98/97
t
CuCl+ NaO Bu
t
CuCl+ NaO Bu
t
CuCl+ NaO Bu
[
[
[
d]
t
10
11
12
CuOAc+ NaO Bu
CuOAc+ KOMe
CuOAc+ KOMe
96:4
96:4
98:2
d,e]
e,f]
98(40)/98
[
a] The reaction was carried out with 1a (1.0 mmol), imine (0.2 mmol),
1
and B (pin) (0.3 mmol) in THF (2 mL). [b] Determined by H NMR
2
2
spectroscopy. [c] The enantiomeric excess was determined by HPLC
analysis of the oxidized product 9aa (for details, see the Supporting
Information). [d] Reacted for 17 h. [e] Toluene as the solvent. [f] In the
presence of 1.5 equiv of KOMe (0.3 mmol) at 08C for 48 h. The yield of
isolated 8aa is shown in parentheses. PMP=p-methoxylphenyl,
OMP=o-methoxylphenyl.
values were enhanced when the more bulky biphosphine
ligands (R)-Tol-BINAP (L3; 93% ee, 89:11 d.r.) and (R)-DM-
BINAP (L4; 94% ee, 93:7 d.r.) were employed (Table 1,
entries 8 and 9; for other chiral ligands screened, see the
Supporting Information.) The selectivity profiles and reaction
efficiency were further improved by using CuOAc as the
copper source (entry 10), KOMe as the base, and toluene as
the solvent (entry 11). Excellent yields and selectivity were
achieved by lowering the reaction temperature to 08C and
increasing the KOMe loading to 1.5 equiv (entry 12).
Scheme 2. Scope with respect to the aldmines. The reaction was
performed with isoprene or 1,3-butadiene (1.0 mmol), imine
(0.2 mmol), and B (pin) (0.3 mmol), the diastereoselectivity was
determined by H NMR analysis of the crude product. Yields of
isolated product are shown. The ee values were determined by HPLC
analysis. [a] The reaction was performed in the presence of 20 mol%
of MeOK at 258C.
2
2
1
mines with para- or meta-methyl, methoxy, or fluorine
substituents gave good yields (81–88%), and o-methoxyl-
phenyl aldimine (the product 9ag) also showed high reac-
tivity. On the other hand, those with electron-withdrawing
substituents gave comparatively lower yields of products
(71% for p-Cl and 77% for p-Br), and the use of 3,4-
dichlorophenyl imine gave 9ak in only 52% yield. Electron-
rich imines apparently display higher reactivity, probably
owing to facile coordination to the allylic copper intermedi-
ate, which accelerates the allylation process. Indeed, the
tested electron-rich 3,4-dimethoxyl phenyl, indolyl, furyl, and
thienyl substrates all gave high yields (86–98%) and excellent
ee (96–98%). Alkyl aldimines were also examined and were
found to decompose under the standard reaction conditions.
With optimized reaction conditions in hand, the scope
with respect to the imine partners was first investigated. The
isolated yields and ee values of the three-component reaction
products were measured following oxidation to the g-hydroxy
[
13]
branched amines 9. As shown in Scheme 2, an array of
Dpp-protected aryl and heteroaryl aldimines react with
isoprene under the reaction conditions, and homoallylic
amines were obtained with good to excellent ee (95–98%)
and excellent d.r. (> 95:5). The electronic and steric proper-
ties of the aryl substituents on the imine partner have little
effect on the observed selectivity of the reaction but
a pronounced impact on the reactivity. For example, aryli-
2
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Nonetheless, 3-phenylpropionaldimine reacted to give 9aq in
To demonstrate the versatility and potential of this
moderate yield with high diastereo- and enantioselectivity
when we decreased the amount of base (20 mol%) and
increased the reaction temperature (258C).
method, several transformations of enantioenriched B(pin)-
or hydroxy-funtionalized homoallylic amines were performed
to access more complex building blocks (Scheme 4). For
The borylative coupling of the parent 1,3-butadiene (1b)
and aryl aldimines was also investigated (Scheme 2). Homo-
allylic amines (9ba–9bd) were obtained in high yields (89–
9
2%) with excellent ee (96–98%) and diastereoselectivity
(
> 95:5). The absolute configuration of 9ba was confirmed by
[14]
X-ray diffraction analysis
and the borylative coupling
products are accordingly assigned as the (1S,2R)-configura-
tion resulting from a syn-addition event.
This three-component coupling reaction is also effective
for an array of 2-substituted 1,3-dienes (Scheme 3). For
example, myrcene (1c) and its derivatives (1d–1 f) were quite
reactive and afforded 9ca–9 fa with excellent enantiomeric
excess. 2-Aromatic dienes also reacted smoothly (9ga–9ia).
Notably, this reaction is also tolerant of heteroatom-contain-
ing functional groups [CH OTBS and CH B(pin)], albeit with
Scheme 4. Further transformation of the products.
2
2
lower yields in some cases (9ja and 9ka). However, 2,3-
dimethyl butadiene (1l) and non-terminal conjugated dienes
example, the Pd/C-catalyzed hydrogenation of 8ba afforded
g-aminoboronic ester 10 in high yield without any loss of ee or
d.r.. The selective transformation of the CÀB bond in 8aa by
(
1m and 1n) did not afford the desired three-component
[
11]
adducts but rather hydroboration products, probably due to
the low reactivity of the corresponding borylcopper species to
imines.
Suzuki–Miyaura coupling was also implemented to give
branched homoallylic amine 11. Other transformations,
namely N-allylation and ring-closing metathesis (RCM),
[15]
allowed the construction of chiral tetrahydropyridine 12
with a high level of optical purity. Furthermore, the N-Dpp
group of the hydroxy homoallylic amine (9aa) was easily
removed under mild conditions. These derivatization pro-
cesses demonstrate the promise of this method in organic
synthesis.
We performed studies to probe the mechanism and
rationalize the stereochemical outcome of this three-compo-
nent reaction. Our initial attempt to prepare the correspond-
ing (BINAP)CuÀallyl intermediate from 1,3-diene was
unfruitful. Considering that N-heterocyclic carbenes (NHC)
[
16]
are effective ligands for stabilizing the CuÀBpin species
[
17]
[11b]
and the corresponding adducts with styrene
and allene,
we decided to make NHC-ligated allylcopper species
[
(NHC)CuÀallyl] from 1,3-diene. Fortunately, when exposing
t
butadiene to a mixture of (Ipr)Cu(O Bu) and B (pin) , s-
2
2
allylcopper complex 14 was isolated and the olefinic config-
uration was confirmed as Z by analysis of the coupling
[
18]
constant of the two olefinic hydrogens [J ꢀ 10 Hz, Eq. (1)].
3
Allyllation of imine 7a with 14 afforded the syn product (8ba)
[
19]
in moderate d.r. (56:44), and this selectivity is consistent
with the catalytic reaction (see the Supporting Information).
Thus, the formal 1,4-migratory insertion of 1,3-dienes into
borylcopper should release the (Z)-allylcopper species. In
the imine allylation event, a closed 6-centered transition
structure could possibly be formed. In N-Dpp (E)-imine
allylation, the observed excellent syn selectivity might arise
Scheme 3. Scope with respect to the conjugated dienes. The reaction
was performed with isoprene (1.0 mmol), 7a (0.2 mmol), and B (pin)
[
20]
2
2
1
(
0.3 mmol), and the diastereoselectivity was determined by H NMR
analysis of the crude product. Yields of isolated product are shown.
The ee values were determined by HPLC analysis.
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[
21]
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repulsion between bulky N-diphenylphosphinoyl group and
the g-substituent group [e.g., -CH B(pin)] of the allylcopper
complex is avoided, although chair-like transition states that
involve the participation of Z-conformed imine
coordination of phosphinoyl oxygen to Lewis acidic Cu
could not be excluded. Moreover, a boat-like stereochemical
model based on analysis of the X-ray crystal structures of
2
[
2] Selected reviews of asymmetric addition of organometallic
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[22]
Cu À(R)-BINAP complexes is proposed to account for the
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noyl group resides far away from either of the axial P-aryl
groups of (R)-DM-BINAP. (Figure 1) This model can also
explain the positive effects of the steric hindrance from the N-
protecting group of the imines and P-aryl groups of the
BINAPs on the stereochemical outcome (see Table 1,
entries 2–10).
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In summary, a highly diatereo- and enantioselective
borylative coupling reaction of 1,3-dienes and N-Dpp aldi-
mines was realized by using a simple chiral biphosphine/
copper catalyst. The favored syn selectivity, unlike that found
for the borylative coupling of allenes and imines, is unexpect-
edly observed. Transition-state models were proposed to
rationalize the enantio- and diastereoselectivity. This three-
component coupling reaction represents a useful method for
the asymmetric allylation of imines. More detailed mecha-
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2
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Keywords: 1,3-dienes · asymmetric catalysis ·
borylative coupling · copper · imines
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uted to the low energe gap between the formed boat and chair
transition station in the imine allylation step. Very recently, an
anti-selective borylative coupling of allenes and imines was
successfully induced by an NHC ligand, wherea chair transition-
station model was proposed to be responsible for the stereo-
chemical outcome (see Ref. [8a,b]).
[20] In some cases, (Z)-allyl metal reagents are thermodynamically
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Received: August 2, 2016
Revised: August 29, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Asymmetric Catalysis
L. Jiang, P. Cao,* M. Wang, B. Chen,
B. Wang, J. Liao*
&&&& — &&&&
Highly Diastereo- and Enantioselective
Cu-Catalyzed Borylative Coupling of 1,3-
Dienes and Aldimines
A multitude of syns: A Cu-catalyzed
borylative coupling reaction of 1,3-dienes
with imines was realized. Branched
homoallylic amines are readily prepared
in a syn-selective manner with high regio-,
diastereo- and enantioselectivity. More-
over, these three-component coupling
reactions feature good functional-group
compatibility and easy access to the
substrates and catalyst.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!