Copper-Catalyzed Diastereoselective Addition of Diborylmethane to N-tert-Butanesulfinyl Aldimines: Synthesis of β-Aminoboronates

Article abstract of DOI:10.1021/acs.orglett.6b00376

We have developed a highly chemo- and diastereoselective alkylation of N-tert-butanesulfinyl aldimines with diborylmethane. Whereas the addition of diborylmethane under metal-free conditions shows poor diastereoselectivity, the use of a copper catalyst an

Full text of DOI:10.1021/acs.orglett.6b00376

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Diastereoselective Addition of Diborylmethane to
N-tert-Butanesulfinyl Aldimines: Synthesis of β‑Aminoboronates
Jinyoung Park, Yeosan Lee, Junghoon Kim, and Seung Hwan Cho*
Department of Chemistry and Division of Advanced Nuclear Engineering, Pohang University of Science and Technology
(POSTECH), Pohang, 790-784, Republic of Korea
S
* Supporting Information
ABSTRACT: We have developed a highly chemo- and diastereoselective
alkylation of N-tert-butanesulfinyl aldimines with diborylmethane. Whereas the
addition of diborylmethane under metal-free conditions shows poor
diastereoselectivity, the use of a copper catalyst and a bidentate phosphine
ligand significantly enhances the diastereoselectivity, providing chiral β-
aminoboronates in good yields. On the basis of the stereochemical outcome,
we propose that the reaction likely proceeds via a boron-chelating six-
membered chairlike transition state.
transformations of 1,1-diboron reagents are particularly
he nucleophilic addition of organometallic reagents to
Timines bearing a chiral auxiliary is a powerful synthetic appealing.10 Hall11a,b and Yun11c independently reported the
stereospecific cross-coupling of optically pure 1,1-diboron
compounds. Morken and co-workers described the Pd-
catalyzed enantioselective Suzuki−Miyaura coupling of 1,1-
diborylalkanes with aryl10a and vinyl halides.10b The same group
also reported the stereoselective deborylative alkylation of
1,1,2-tris(boronates) with alkyl halides to afford enantiomeri-
cally enriched internal 1,2-diboryl compounds through the
intermediacy of an α-borylcarbanion.10c More recently, Meek
reported the enantio- and diastereoselective addition of 1,1-
diborylalkanes to aldehydes to afford 1,2-hydroxyboronates in
the presence of a copper−chiral monodentate phosphoramidite
complex.10d,e These examples illustrate interesting features of
the chemo-, enantio-, and stereoselective coupling of 1,1-
diboron compounds; yet, the discovery of new reactivity would
be of utmost synthetic importance.
Herein, we describe the first chemo- and diastereoselective
alkylation of N-tert-butanesulfinyl aldimines with diboryl-
methane catalyzed by a copper salt and an achiral bidentate
phosphine ligand (Scheme 1B), likely via an uncommon six-
membered transition state in metal-catalyzed 1,2-addition of
organoboron reagents to N-tert-butanesulfinyl aldimines. The
reaction provides β-aminoboronates, which can be further
functionalized to generate synthetically valuable intermediates.
Inspired by developments utilizing 1,1-diborylalkanes in
stereoselective transformations, we initially reasoned that the α-
borylcarbanion generated from 1,1-diborylalkanes could react
with N-sulfinyl aldimines to provide β-aminoboronates.8e,12 To
probe the viability of this envisioned strategy, at first we
investigated the reaction of (R)-N-(benzylidene)-2-methylpro-
pane-2-sulfinamide (1a) with diborylmethane (2a) under
metal-free conditions. When 1a was treated with 2a in toluene
strategy that provides access to chiral α-branched amines, which
are prevalent in natural products, pharmaceuticals, and
asymmetric catalysts.1 The diastereoselective addition of
organometallics to N-tert-butanesulfinyl imines is one of the
more reliable and efficient methods in this context.2,3 However,
these reagents typically suffer from drawbacks such as limited
functional group tolerance and air and moisture sensitivity.
Recent progress of metal-catalyzed 1,2-addition of boronic
acids or potassium organotrifluoroborates to N-tert-butanesul-
finyl imines has significantly expanded the scope of this
transformation. In particular, Ellman4 and others5 established a
highly efficient transition-metal-catalyzed diastereoselective 1,2-
addition reaction using aryl,4a−e,5a−c alkenyl,4f propargyl,5f and
allyl1b,5d,e boron reagents (Scheme 1A). Despite these
significant advances, the diastereoselective 1,2-addition using
alkylboron reagents remains largely undeveloped.6,7
1,1-Diboron compounds, which contain two boryl groups at
the same sp3 carbon center, have been successfully employed in
the synthesis of alkylboron compounds.8,9 Because of their
great synthetic potential, the enantio- and diastereoselective
Scheme 1. Addition of Organometallics to N-tert-
Butanesulfinylimine
Received: February 8, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00376
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
conversion of 1a (entries 13 and 14).13 Moreover, poor results
were obtained when the solvent was switched from toluene to
THF (entry 15).14 The catalyst loading of the reaction could be
reduced, albeit in lower yield (entry 16).
a
Table 1. Optimization Study
The relative stereochemistry of the major isomer was
assigned as (Rs, R), as determined by single-crystal X-ray
analysis of 4f (Figure 1). On the basis of the observed
conv
(%)
yield
b
c
entry
base
[M]
L
(%)
dr
1
LiOtBu
NaOtBu
KOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
−
−
−
−
−
−
−
<1
100
100
39
<1
96
86
15
66
20
32
54
15
50
61
91
24
<1
45
40
n.d.
2
77:23
54:46
83:17
97:3
95:5
94:6
95:5
96:4
95:5
98:2
99:1
n.d.
3
4
[Rh(COD)Cl]2 L1
5
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
CuCl
L1
L2
L3
L4
L5
L6
L1
L1
−
78
6
46
7
51
8
56
9
33
10
11
12
62
90
CuBr
100
50
Figure 1. X-ray structure of product 4f.
d
13
CuBr
e
14
CuBr
L1
L1
L1
<1
100
84
n.d.
diastereoselectivity, we assumed that the reaction proceeds via a
chairlike six-membered cyclic transition state, in which the
boron species was coordinated to the oxygen atom of the
sulfinyl moiety (Scheme 2).15,16 This is particularly noteworthy
considering that previously developed diastereoselective 1,2-
addition reactions of organoboron reagents to N-tert-
butanesulfinyl imines completely favored an open transition
state (TS-I), because of the steric congestion caused by the
ligand bound to the metal species.4,5 A similar boron-chelated
transition state was reported in the diastereoselective reduction
of N-tert-butanesulfinyl imines employing 9-BBN as a reducing
reagent.17
f
15
LiOtBu
CuBr
85:15
95:5
g
16
LiOtBu
CuBr
a
Reaction conditions: 1a (0.2 mmol), 2a (1.5 equiv), catalyst (10 mol
%) and ligand (10 mol %) in toluene (1.0 mL ) at 50 °C for 12 h, then
b
NaBO·4H2O, THF/H2O, rt for 2 h. Determined by 1H NMR
c
analysis using dibromomethane as an internal standard. Diastereo-
meric ratio was determined by HPLC (see the Supporting
Information). Without ligand. Without LiOtBu. THF was used
instead of toluene as a solvent. 5 mol % of CuBr and L1 was used.
d
e
f
g
n.d. = not detected.
Scheme 2. Rationale for the Diastereoselectivity: Open
Transition State (TS-I) and Chairlike Transition State (TS-
II)
at 50 °C in the presence of a stoichiometric amount of LiOtBu
as a base, no desired product was formed (Table 1, entry 1). In
contrast, when NaOtBu (entry 2) or KOtBu (entry 3) were
employed, the corresponding β-amino alcohol 4a was obtained
after oxidation of 3a in high yields albeit with poor
diastereoselectivity. On the basis of these observations, we
envisioned that generation of the metal α-boryl species and
suppression of the formation of α-borylcarbanion would
promote the diastereoselective 1,2-addition to N-sulfinyl
aldimines.10d
Thus, we set out to investigate the effect of various metal
complexes on the diastereoselective 1,2-addition of 2a to 1a in
the presence of LiOtBu as a base. The reaction using
[Rh(COD)Cl]2 and 1,2-bis(diphenylphosphino)benzene (L1)
ligand forms the corresponding β-amino alcohol (4a) in low
yield with moderate diastereoselectivity (entry 4).4 Switching
the metal precursor from rhodium to copper yielded 4a in 66%
yield with excellent diastereoselectivity (97:3 dr, entry 5).
Further screening of phosphine ligands showed lower
conversion (entries 6−10) with a diastereoselectivity com-
parable to that obtained with L1. The yield of the desired
product was significantly improved by changing from Cu(II) to
Cu(I) salts (entries 11 and 12). In particular, copper bromide
(entry 12) provided excellent yield with excellent diastereo-
meric ratio (99:1 dr). Reactions with CuBr in the absence of a
phosphine ligand and LiOtBu base resulted in very low
With the optimal conditions in hand, we then examined the
scope of N-tert-butanesulfinyl aldimines 1 with diborylmethane
2a (Table 2).18 In all cases, the crude mixtures were directly
oxidized to the corresponding β-amino alcohols to facilitate
chromatographic separation and HPLC determination of the
diastereomeric ratio. The 1,2-addition to the substrate bearing a
methyl group in the para position of the aromatic ring
proceeded in good yield and moderate diastereoselectivity.
However, the reaction of substrate 1b at room temperature
afforded 4b with almost complete diastereoselectivity (99:1 dr),
albeit with slightly lower efficiency after a prolonged reaction
time (24 h). In addition, the electron-poor substrate bearing a
B
DOI: 10.1021/acs.orglett.6b00376
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b c
, ,
oxidation to give aliphatic β-amino alcohol 4m in 60% yield
(over 3 steps) with good diastereoselectivity (97:3 dr). Thus, in
view of the poor conversion of aliphatic N-sulfinyl aldimine 1m
under our reaction conditions, this strategy provides a useful
alternative for the preparation of aliphatic β-amino alcohol.
Because our method provides N-protected β-aminoboro-
nates, a tandem diastereoselective alkylation/deprotection
sequence was examined. As depicted in Scheme 4, this strategy
Table 2. Substrate Scope
b
c
entry
R1
product
yield (%)
dr
Scheme 4. Deprotection of N-Sufinyl Group and Synthesis of
Antitubercular Agent
1
2
C6H5
4a
4b
4b
4c
4d
4e
4f
85
76
57
87
75
82
70
61
69
73
53
67
96
<1
99:1
90:10
99:1
97:3
98:2
97:3
98:2
97:3
97:3
96:4
96:4
98:2
94:6
−
(4-Me)C6H4
(4-Me)C6H4
(4-CF3)C6H4
(4-F)C6H4
(4-Cl)C6H4
(4-Br)C6H4
(3-Me)C6H4
(3-Cl)C6H4
(3-Br)C6H4
(2-Me)C6H4
2-furyl
d
3
4
5
6
7
8
4g
4h
4i
9
10
11
12
13
14
4j
4k
4l
2-thienyl
PhCH2CH2
4m
a
Reaction conditions: 1 (0.2 mmol), 2a (1.5 equiv), CuBr (10 mol %),
L1 (10 mol %) and LiOtBu in toluene (1.0 mL) at 50 °C for 12 h,
b
then NaBO3·4H2O, THF/H2O, rt for 2 h. Isolated yields of the
provided the corresponding β-aminoboronate hydrochloride 5
in 92% yield over two steps. Notably, in the event, the Bpin unit
remained intact. In addition, the Cu-catalyzed diastereoselective
alkylation of 1a with diborylmethane followed by the sequential
oxidation of Bpin and deprotection of the sulfinyl group
afforded the corresponding β-amino alcohol hydrochloride 6 in
84% overall yield.
The β-aminoboronate motif is commonly found in
pharmacologically active agents; thus, the synthetic utility of
product 5 can be highlighted by its application to the synthesis
of β-substituted β-aminoboron compound 8, a highly potent
antitubercular agent.19 The reaction of β-aminoboronate
hydrochloride 5 with Boc-L-alanine in the presence of 2-
chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) and N-methyl-
morpholine (NMM)20 followed by deprotection of the Boc
group under acidic conditions gave 7 in 70% yield, without
epimerization of the stereocenter. The obtained product 7
could be converted into the corresponding antitubercular agent
8 following literature procedures.19a
In summary, we have developed the first example of a
copper-catalyzed chemo- and diastereoselective addition of 1,1-
diborylalkanes to N-tert-butanesulfinyl aldimines. Whereas the
reaction proceeds with low diastereoselectivity under metal-free
conditions, when a copper catalyst and a bidentate phosphine
ligand are employed, the corresponding β-aminoboronates are
obtained in good yields and high diastereoselectivity. On the
basis of the observed diastereoselectivity, we propose that the
reaction likely proceeds through a boron-chelating chairlike
transition state.
c
corresponding 1,2-amino alcohol. Diastereoselectivity was determined
d
by HPLC (see the Supporting Information). Runs at room
temperature for 24 h.
trifluoromethyl group in the para position gave product 4c in
87% yield with excellent diastereoselectivity (97:3 dr). The
reactions of N-tert-butanesulfinyl aldimines with halides in the
para position provided the corresponding β-amino alcohols
with satisfactory yield and diastereoselectivity (70−82%, 97:3−
98:2 dr). N-tert-Butanesulfinyl aldimines with substituents at
the meta or ortho position of the aromatic ring also reacted
smoothly to produce 4g−4j with good diastereomeric ratios
(96:4−97:3 dr). The reactions of heteroaryl-substituted N-
sulfinyl aldimines were also examined. The substrate containing
a furyl group reacted with 2a to deliver 4k in acceptable yield
(67%) and excellent diastereoselectivity (98:2 dr). The N-
sulfinyl aldimine containing a thienyl group also resulted in the
formation of desired product 4l in high yield (96%) with
moderate diastereoselectivity (94:6 dr). Unfortunately, an
aliphatic N-tert-butanesulfinyl aldimine failed to provide the
corresponding product under the standard conditions.
It should be noted that the developed chemo- and
diastereoselective alkylation reaction could be applied also to
α,β-unsaturated N-tert-butanesulfinyl aldimines (Scheme 3).
When 1n was treated with 2a under the standard conditions,
the corresponding α,β-unsaturated β-aminoboronate product
was formed, which was directly subjected to hydrogenation and
Scheme 3. Copper-Catalyzed Diastereoselective Addition of
ASSOCIATED CONTENT
* Supporting Information
2b to α,β-Unsaturated N-tert-Butanesulfinylimine
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b00376.
Experimental details, spectral data for all compounds,
and crystallographic data for 4f (PDF)
C
DOI: 10.1021/acs.orglett.6b00376
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Zhang, Y.; Wang, J. Org. Lett. 2014, 16, 448. (g) Li, H.; Zhang, Z.;
Shangguan, X.; Huang, S.; Chen, J.; Zhang, Y.; Wang, J. Angew. Chem.,
Int. Ed. 2014, 53, 11921. (h) Zhang, Z.-Q.; Yang, C.-T.; Liang, L.-J.;
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2014, 16, 6342. (i) Xu, S.; Shangguan, X.; Li, H.; Zhang, Y.; Wang, J. J.
Org. Chem. 2015, 80, 7779.
(9) We recently reported a copper-catalyzed SN2′-selective allylic
substitution reaction using 1,1-diborylalkanes as facile nucleophiles:
Kim, J.; Park, S.; Park, J.; Cho, S. H. Angew. Chem., Int. Ed. 2016, 55,
1498.
(10) (a) Sun, C.; Potter, B.; Morken, J. P. J. Am. Chem. Soc. 2014,
136, 6534. (b) Potter, B.; Szymaniak, A. A.; Edelstein, E. K.; Morken, J.
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Moyer, B. S.; Meek, S. J. J. Am. Chem. Soc. 2015, 137, 6176.
(e) Joannou, M. V.; Moyer, B. S.; Goldfogel, M. J.; Meek, S. J. Angew.
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(11) (a) Lee, J. C. H.; McDonald, R.; Hall, D. G. Nat. Chem. 2011, 3,
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: seunghwan@postech.ac.kr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT & Future
Planning (NRF-2015R1C1A1A02036326).
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D
DOI: 10.1021/acs.orglett.6b00376
Org. Lett. XXXX, XXX, XXX−XXX