Copper-Catalyzed Borylcupration of Allenylsilanes

Article abstract of DOI:10.1002/anie.201510354

A highly regio- and stereoselective copper-catalyzed borylcupration of 1,2-allenylsilanes affords an unexpected regioreversed allylic boronate bearing an extra C-Si bond at the 3-position, with a thermodynamically disfavored Z geometry. Such stereodefined allylic boronates containing an extra alkenyl silane moiety are very useful organodimetallic reagents for organic synthesis.

Full text of DOI:10.1002/anie.201510354

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201510354
Allylic Compounds
German Edition:
DOI: 10.1002/ange.201510354
Copper-Catalyzed Borylcupration of Allenylsilanes
Weiming Yuan, Liu Song, and Shengming Ma*
Abstract: A highly regio- and stereoselective copper-catalyzed
borylcupration of 1,2-allenylsilanes affords an unexpected
regioreversed allylic boronate bearing an extra CÀSi bond at
the 3-position, with a thermodynamically disfavored Z geom-
etry. Such stereodefined allylic boronates containing an extra
alkenyl silane moiety are very useful organodimetallic reagents
for organic synthesis.
R
ecently, the highly regio- and stereoselective copper-
catalyzed borylcupration of allenes with bis(pinacolato)di-
boron [B (pin) ], thus providing 1-alkenyl boronates, was
2
2
established to proceed via favored allylic copper intermedi-
[1–4]
ates (Scheme 1a).
On the other hand, allylic metallic
compounds with an extra carbon–metallic bond, such as
allylic boronates with a CÀSi bond (2; Scheme 1b), are
important synthetic reagents because of the stability, low
[
5,6]
toxicity, versatile reactivity as a result of the allylic nature,
and broad functionality compatibility, thus leading to poly-
functional compounds efficiently by the reactions of both
[7]
CÀM bonds. The preparation of compounds of type 2, which
are thermodynamically stable E isomers, has been estab-
[
8]
Scheme 1. Background information and new observation for allene
borylcuprations.
lished. However, the thermodynamically disfavored stereo-
[
9]
isomers (Z)-2 are still difficult to access. Thus, the develop-
ment of an efficient approach towards the stereodefined 3-
silylalk-2(Z)-enyl boronates is of high interest. Herein, we
report an unexpected observation that the borylcupration of
the allenylsilanes 1 exclusively afforded the 3-silylalk-2(Z)-
enyl boronates (Z)-2, via the 1-alkenyl copper intermediate
as the only product with a very low yield; most of the starting
material was recovered (entries 1–6). No allylboronate prod-
uct was detected. Subsequently, bidentate ligands were
investigated and when DPEphos was used we observed the
formation of the 3-silylallylboronate 2m, having exclusively
a Z geometry, albeit together with the regioisomer 4m
(entry 7). Xantphos demonstrated the best result with the
exclusive formation of (Z)-2m in 88% yield within 1 hour
(entry 8). Screening of other solvents led to the observation
(
E)-2M, with the opposite regioselectivity and thermodynam-
ically disfavored stereoselectivity as compared to the reaction
shown in Scheme1a. In addition, the usually expected
products, the 1-alkenyl boronates (Z)-3, (E)-3, and 4, via
the most favored allylic copper intermediates, were not
formed.
[11]
that the reaction in Et O yielded (Z)-2m in a higher yield of
2
We initially studied the borylcupration of the allenylsilane
with B (pin) (Table 1). Firstly, a series of monodentate
2 2
phosphine ligands was screened: the expected 1-alkenyl
95% with the same excellent selectivity (entry 9). Thus, we
successfully established the standard reaction conditions:
CuCl (5 mol%), Xantphos (5 mol%), NaOtBu (20 mol%),
B (pin) (1.2 equiv), iPrOH (2.0 equiv), Et O at RT, for the
[
10]
1
m
boronate 4m, via an allylic copper intermediate, was formed
2
2
2
exclusive formation of the (Z)-2. The formation of the
thermodynamically more stable (E)-2m, the related 3-type
of product (see Scheme 1), and 4m was not observed.
With the optimized reaction conditions in hand, we next
explored the scope of the reaction (Table 2). Firstly, when
[
*] W. Yuan
Shanghai Key Laboratory of Green Chemistry and Chemical Process,
Department of Chemistry, East China Normal University
663 North Zhongshan Lu, Shanghai 200062 (P. R. China)
3
L. Song, Prof. Dr. S. Ma
Department of Chemistry, Fudan University
[
Si] = TMS, substrates with various primary alkyl groups led
to good results (entries 1–3). With secondary alkyl substitu-
ents, such as iPr or Cy, the reaction afforded a mixture of (Z)-
2
20 Handan Lu, Shanghai 200433 (P. R. China)
E-mail: masm@sioc.ac.cn
2
d and 4d and (Z)-2e and 4e, respectively, with a slightly
Prof. Dr. S. Ma
lower regioselectivity. These regioisomers could be easily
separated by chromatography on silica gel (entries 4 and 5).
Moreover, when R = iBu, Bn, or CH CH Ph, the reactions
State Key Laboratory of Organometallic Chemistry, Shanghai,
Institute of Organic Chemistry, Chinese Academy of Sciences
3
45 Lingling Lu, Shanghai 200032 (P. R. China)
2
2
proceeded with excellent regio- and stereoselectivities
(entries 6–8). To our delight, unsaturated functional groups
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201510354.
3
140
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Table 1: Optimization of the reaction conditions for copper-catalyzed
selective borylcupration of the allenylsilane 1m to produce the 3-silyl-
having a TBS-protected hydroxy group also worked well
entries 11 and 12).
Furthermore, with R = CH , the reactions proceeded
(
[
a]
2
(Z)-allylic boronate (Z)-2m.
3
smoothly to afford the corresponding allylic boronates (Z)-2
products in decent yields with excellent selectivities irrespec-
tive of whether the silyl group is SiMe Ph, SiMePh , SiPh , or
2
2
3
[
b]
[d]
Entry
Ligand
t [h]
Yield [%]
(Z)-2m/4m
SitBuMe (Table 3, entries 1–5). In addition, R may also be Et
2
[
c]
4
m
(Z)-2m
or nPr (entries 6 and 7). For R = aryl, the amount of base and
[
[
[
[
[
[
[
[
[
e]
e]
e]
e]
e]
e]
f]
B (pin) had to be increased and the reaction time prolonged
2
2
1
2
3
4
5
6
7
8
9
PPh3
TFP
PCy3
P(tBu)3
P(p-MeOC
LBphos
DPEphos
Xantphos
Xantphos
4
4
4
4
4
4
1
1
1
10 (67)
7 (71)
13 (65)
12 (66)
12 (74)
14 (74)
49
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
39
–
–
–
–
–
to 15–17 hours to ensure full conversion (entries 8–11).
Gratifyingly, the reaction could be run on a one-gram scale
to give excellent yield and the same selectivity (entry 2).
Moreover, the stereochemistry of the products was estab-
lished by single-crystal X-ray diffraction of (Z)-2o
6
H
4
)
3
–
44:56
>99:1
>99:1
[12]
f]
(Figure 1).
n.d.
n.d.
88
95
f,g]
To demonstrate the factors controlling the stereoselectiv-
ity, the silyl group was replaced with tBu, and the reaction still
afforded (Z)-2w with high stereoselectivity (Scheme 2).
However, when the silyl group was replaced with nBu, the
reaction afforded 2x in a 73:27 E/Z mixture, thus indicating
the importance of the steric bulkiness of the silyl group on the
[
a] Reaction conditions: 0.2 mmol of 1m, 5 mol% CuCl, 20 mol%
NaOtBu, 0.24 mmol of B (pin) , 0.4 mmol of iPrOH in 2 mL of THFat RT.
b] Determined by NMR spectroscopy using 1,3,5-trimethylbenzene as
an internal standard. [c] The value within the parentheses is that of the
recovered of starting material. [d] The selectivity was determined by
2
2
[
1
HNMR analysis of the crude reaction mixture. [e] 6 mol% ligand was
used. [f] 5 mol% ligand was used. [g] The solvent was Et
tri(2-furyl)phosphine.
2
O. TFP=
Table 3: Highly selective ligand-controlled borylcupration of allenyl-
silanes using various silyl substituents.
[a]
[
b]
Entry
1
R/[Si]
t
Yield [%]
Me/SiMe Ph (1m)
Me/SiMe Ph (1m)
Me/SiMePh (1n)
2
Me/SiPh (1o)
Me/SitBuMe (1p)
Et/SiMe Ph (1q)
40 min
30 min
35 min
40 min
60 min
40 min
60 min
15 h
90 [(Z)-2m]
90 [(Z)-2m]
88 [(Z)-2n]
86 [(Z)-2o]
85 [(Z)-2p]
80 [(Z)-2q]
88 [(Z)-2r]
82 [(Z)-2s]
73 [(Z)-2t]
85 [(Z)-2u]
65 [(Z)-2v]
2
[c]
2
3
4
5
6
7
8
9
1
1
2
Table 2: Highly selective ligand-controlled borylcupration of allenyl-
[
a]
silanes yielding 3-silyl-2(Z)-allylboronates [(Z)-2].
3
2
2
nC H /SiMe Ph (1r)
3
7
2
[
d]
Ph/SiMe Ph (1s)
4-MeC H /SiMe Ph (1t)
3-MeOC H /SiMe Ph (1u)
2-Naphthyl/SiMe Ph (1v)
2
[b]
Entry
R
t [min]
Yield [%]
[d]
17 h
17 h
17 h
6
4
2
[
d]
0
1
6
4
2
1
2
3
4
5
6
7
8
9
nC H (1a)
40
40
60
60
60
60
60
60
40
40
60
90
84 [(Z)-2a]
86 [(Z)-2b]
85 [(Z)-2c]
81 [(Z)-2d]
81 [(Z)-2e]
84 [(Z)-2 f]
80 [(Z)-2g]
85 [(Z)-2h]
81 [(Z)-2i]
80 [(Z)-2j]
75 [(Z)-2k]
74 [(Z)-2l]
4
9
[
d]
2
nC H (1b)
5
11
nC H (1c)
8
17
[a] Reaction conditions: 1.0 mmol of 1, 5 mol% CuCl, 5 mol% Xantphos,
0 mol% NaOtBu, 1.2 mmol of B (pin) , 2.0 mmol of i-PrOH in 3 mL of
[
c]
iPr (1d)
Cy (1e)
iBu (1 f)
Bn (1g)
2
2
2
[d]
Et O at RT. [b] Yield of isolated product. [c] The reaction was conducted
2
on one gram scale. [d] 40 mol% NaOtBu and 1.4 mmol of B (pin) were
2
2
used.
CH CH Ph (1h)
2
2
(CH ) CH=CH (1i)
2
2
2
1
1
1
0
1
2
(CH ) C(Me)=CH (1j)
2 2 2
(CH ) OTBS (1k)
2
2
(CH ) OTBS (1l)
2
3
[
2
a] Reaction conditions: 1.0 mmol of 1, 5 mol% CuCl, 5 mol% Xantphos,
0 mol% NaOtBu, 1.2 mmol of B (pin) , 2.0 mmol of iPrOH in 3 mL of
2
2
Et O at RT. [b] Yield of isolated product. [c] The ratio of (Z)-2d/4d=96:4.
2
[
d] The ratio of (Z)-2e/4e=92:8.
such as homoallylic substituents were also tolerated under the
standard reaction conditions to afford the corresponding
products with exciting results (entries 9 and 10), thus indicat-
ing that the reactivity of the C=C bond in the allene unit is
Figure 1. ORTEP representation of (Z)-2o.
much higher than that of an alkene. Furthermore, a substrate
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In conclusion, we have developed highly selective copper-
catalyzed borylcuprations of the simple 1,2-allenylsilanes,
thereby providing an efficient method for the synthesis of the
thermodynamically disfavored 3-silylalk-2(Z)-enylboronates
in high yields and excellent selectivities starting from
allenylsilanes. Notably, it is the first observation of a boryl
group, B(pin), connected to the terminal carbon atom of an
allene to produce the regioreversed allylboronates in copper-
catalyzed borylcupration of allenes with B (pin) . Control
2
2
experiments show that both the steric effects of the ligand and
substrates determine the regio- and stereoselectivities. Fur-
ther studies, including a look at the mechanism and synthetic
applications, are underway in this laboratory.
Scheme 2. Factors controlling the regio- and stereoselectivities. Con-
ditions A: CuCl (5 mol%), Xantphos (5 mol%), NaOtBu (20 mol%),
B (pin) (1.2 equiv), iPrOH (2.0 equiv), Et O, RT. [a] Used 1.5 equiv
2
2
2
B (pin) .
2
2
Experimental Section
CuCl (5.0 mg, 0.05 mmol), Xantphos (29.1 mg, 0.05 mmol), NaOtBu
(
19.3 mg, 0.2 mmol), bis(pinacolato)diboron (304.6 mg, 1.2 mmol)/
stereoselectivity. In addition, the reaction of 1y afforded the
vinylboronate 4y, thus indicating the effect of the R group, as
shown in the equations for Tables 2 and 3, on the regiose-
lectivity.
Et O (2 mL), 1a (169.6 mg, 1.0 mmol)/Et O (1 mL), iPrOH (153 mL,
2
2
À1
d = 0.784 gmL , 120.2 mg, 2.0 mmol) were added sequentially to an
oven-dried Schlenk tube under argon. The mixture was stirred at RT
for 40 min and monitored by TLC. The resulting mixture was filtered
To capture the organocopper intermediate, the reaction of
through a short column of silica gel, eluting with Et O (3x20 mL) and
2
1
m was conducted with iPrOD, with 80% D-labelling, instead
of iPrOH, and it afforded an 87% yield of [D]-(Z)-2m with
7% deuterium incorporation at the vinylic position
Scheme 3). This result indicates that the vinylic copper
concentrated. The residue was purified by chromatography on silica
gel (eluent: petroleum ether/ethyl ether= 50:1) to afford (Z)-2a
1
(
249.3 mg, 84%) as a liquid: H NMR (300 MHz, CDCl ): d = 6.06 (t,
3
5
(
J = 8.6 Hz, 1H, = CH), 2.04 (t, J = 8.6 Hz, 2H, CH ), 1.78 (d, J =
2
8
.1 Hz, 2H, = CCH ), 1.32–1.19 (m, 16H, 2 ” CH and B(pin)), 0.87
2
2
13
(
(
t, J = 6.8 Hz, 3H, CH ), 0.13 ppm (s, 9H, SiMe ); C NMR
75.4 MHz, CDCl ): d = 138.6, 137.1, 83.0, 38.2, 33.2, 24.7, 22.3, 14.0,
3
3
3
0
1
.2 ppm; IR (neat): ~n = 2957, 2926, 2857, 1607, 1465, 1323, 1248,
À1
+
144 cm ; MS (ESI) m/z (%) 319 [M+Na] ; HRMS Calcd. for
1
33
1
+
C H BO Si [M] : 296.2343, Found: 296.2345.
1
6
2
Acknowledgements
Financial support from the National Natural Science Foun-
dation of China (No. 21232006) is greatly appreciated. We
thank Pengbin Li in this group for reproducing the results for
Scheme 3. Deuterium-labelling experiment and mechanism for the
regio- and stereoselectivity.
(
Z)-2i and (Z)-2q as presented in this study.
Keywords: allylic compounds · boron · copper · regioselectivity ·
stereoselectivity
intermediate A was formed in the reaction. Based on these
factors, a mechanism for this reaction is proposed, as shown in
Scheme 3. The preferable formation of A over B is due to the
favorable trans orientation of the CÀCuL bond and CÀSi
How to cite: Angew. Chem. Int. Ed. 2016, 55, 3140–3143
Angew. Chem. 2016, 128, 3192–3195
bond in A, and is caused by steric effects. The regioselectivity
was most probably determined by the crowded nature of
delocalized allylic copper intermediate C since the reaction of
[
[
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