Ligand controlled highly selective copper-catalyzed borylcuprations of allenes with bis(pinacolato)diboron

Article abstract of DOI:10.1002/adsc.201100929

Copper-catalyzed highly selective borylcuprations of allenes with bis(pinacolato)diboron produce two different types of alkenylboranes by applying a ligand effect. In the presence of tris(para-methoxyphenyl)phosphine [P(C ₆H₄OMe-p)₃], the reaction of aryl-1,2-dienes affords 2-alken-2-yl boronates as the only product with exclusive Z-geometry; the regioselectivity is switched to afford the 1-alken-2-yl boronates as the major products when the bidentate phosphine 2,2'-bis(diphenylphosphino)biphenyl is used as the ligand. Copyright

Full text of DOI:10.1002/adsc.201100929

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DOI: 10.1002/adsc.201100929
Ligand Controlled Highly Selective Copper-Catalyzed
Borylcuprations of Allenes with Bis(pinacolato)diboron
Weiming Yuana and Shengming Maa,b,*
a
Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal
University, 3663 North Zhongshan Lu, Shanghai 200062, Peopleꢀs Republic of China
Fax : (+86)-21-6260-9305; e-mail: masm@sioc.ac.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, 345 Lingling Lu, Shanghai 200032, Peopleꢀs Republic of China
Received: November 29, 2011; Revised: February 16, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201100929.
Abstract: Copper-catalyzed highly selective boryl-
cuprations of allenes with bis(pinacolato)diboron
produce two different types of alkenylboranes by
applying a ligand effect. In the presence of tris-
ACHTUNGTRENNUNG(para-methoxyphenyl)phosphine [P(C6H4OMe-p)3],
the reaction of aryl-1,2-dienes affords 2-alken-2-yl
boronates as the only product with exclusive Z-ge-
ometry; the regioselectivity is switched to afford
the 1-alken-2-yl boronates as the major products
when the bidentate phosphine 2,2’-bis(diphenyl-
phosphino)biphenyl is used as the ligand.
Scheme 1. Monohydroboration of allenes.
Keywords: allenes; bis(pinacolato)diboron; copper;
regioselectivity; stereoselectivity
hydroboration product in 48% yield and 50% conver-
sion; hydroboration of 3-methyl-1,2-butadiene and
2,3-pentadiene with 4,4,6-trimethyl-1,3,2-dioxabori-
nane afforded a mixture of 2 and 4; using 9-BBN as
Organoboranes are important intermediates widely hydroboration agent, the hydroboration of 3-methyl-
used in various carbon-carbon bond forming reac- 1,2-butadiene and 2,3-pentadiene afforded 4 as the
tions,[1] such as Pd-catalyzed Suzuki cross-coupling re- only product. Roushꢀs group recently reported the hy-
action,[2] transition metal-catalyzed 1,2-addition of droboration of allenes with 10-TMS-9-borabicyclo-
carbonyl compounds[3] and 1,4-addition of electron-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
anes. However, a significant regio- and stereoselectiv- lenes with pinacolborane affording one of three possi-
ity issue arises: in principle, it is possible to produce ble monohydroboration products 2–4 (Scheme 1) by
six regio- and stereoisomers even if the further hydro- choosing an appropriate phosphine ligand:[7] for ex-
boration reaction to the remaining C=C bond is not ample, hydroboration of alkoxyallenes with PtACHTUNGTRENNUNG(dba)2/
considered (Scheme 1). In traditional uncatalyzed hy- PCy3 afforded allylborane exclusively in moderate to
droboration of allenes with boranes, the regio- and good yields; the addition of pinacolborane with ali-
stereoselectivity were usually controlled by the sub- phatic and aromatic allenes with PtACHTUNGRTENNUG(dba)2/2PACHTUNGTRENNUNG(t-Bu3)
strates and hydroboration agents and the reaction re- occurred at the internal double bond to selectively
quires high temperature with low conversions and provide 1-alken-2-yl boronates at 508C in toluene. In
yields:[5] hydroboration of propadiene with 4,4,6-tri- this report, we wish to report a highly selective
methyl-1,3,2-dioxaborinane at 1308C afforded a mix- copper-catalyzed hydroboration of allenes with bis(pi-
ture of 2-allylborane and 2-isopropenylborane in nacolato)diboron forming 2-alken-2-yl Z-2 or 1-alken-
~86:14 regioselectivity in 25% yield, while hydrobora- 2-yl boronates 3, respectively, by applying a ligand
tion of propadiene with 9-BBN afforded only the di- effect (Scheme 1).
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Table 1. Optimization of reaction conditions for copper-catalyzed regio- and stereoselective hydroboration of allenes using
monodentate phosphine ligands to produce stereodefined 2-alken-2-yl boronate Z-2a.[a]
Entry
Ligand
Solvent
T [oC]/t [h]
Z-2a/3a[b]
Yield of Z-2a [%][c]
1
PPh3
PPh3
TFP
THF
THF
THF
THF
THF
THF
THF
THF
40/1.2
40/9
40/5
40/1.3
40/1.3
40/1.3
40/1
40/1
rt/1.3
0/1
r.t./1.5
r.t./1
r.t./1
r.t./1
r.t./1
r.t./1
89/11
–
72
2[d]
3
5 (82)[e]
54
73/27
80/20
88/12
80/20
90/10
92/8
4
5
6
7
8
9
10
11
12
13
14
15
16
P
P
P
N
73
64
67
78
79
80
66
34
76
71
76
69
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3[g]
PCy3
L
THF
THF
93/7
89/11
85/15
89/11
92/8
97/3
88/12
100/0
CH3CN
PhCH3
CH2Cl2
Et2O
Et2O
Et2O
72
[a]
Reaction conditions: 0.2 mmol of allene, 0.24 mmol of bis(pinacolato)diboron, 5 mol% CuCl, 6 mol% monodentate
ligand, 20 mol% NaO-t-Bu, and 0.4 mmol of MeOH were used.
[b]
[c]
[d]
[e]
1
The selectivity was determined by analysis of H NMR spectra of the crude reaction products.
The yield was determined by 1H NMR spectra analysis using 1,3,5-trimethylbenzene as the internal standard.
NaO-t-Bu was not added.
1
The number in the parenthesis is the recovery of the starting material as determined by H NMR spectra analysis using
1,3,5-trimethylbenzene as the internal standard.
L=P(C6H4OMe-p)3, TFP=triACHTNUGTREN(NNUG 2-furyl)phosphine, PACHTUNRTGEG(NNUN o-tol)3 =triACTHNUGTREN(NUGN o-tolyl)phosphine.
10 mol% PCy3 was used.
[f]
[g]
Initially, we used phenylallene 1a as the model sub- vent affording Z-2a with the highest regioselectivity
strate to explore such a feasibility: When we treated (entry 14). Running the reaction at 08C gave no
1a with bis(pinacolato)diboron in the presence of better result (entry 10). When we used the
[8–11]
5 mol% Pd
(PPh3)4
and 1 equiv. AcOH in THF P(C6H4OMe-p)3 instead of PCy3 as the ligand by con-
under reflux,[12–15] disappointedly, the result was com- sidering the electron-rich nature of these two ligands,
plicated. Then we used CuCl (5 mol%) combined fortunately, the product Z-2a was afforded with an ex-
with 6 mol% PPh3 instead of Pd
ACHTUNGTRENNUNG
recently developed for hydroboration of alkynes by
A
Yun et al.[16] In the presence of 20 mol% NaO-t-Bu P(C6H4OMe-p)3, 20 mol% NaO-t-Bu, 2 equiv. MeOH,
and 2 equiv. of MeOH, as expected, the reaction in Et2O, and room temperature) were applied for the
THF at 408C afforded a mixture of 2-alken-2-yl boro- highly regio- and stereoselective hydroboration of
nate Z-2a and 1-alken-2-yl boronate 3a in a regioselec- aryl-1,2-allenes affording Z-2. We investigated the
tivity of ~8:1 with Z-2a being the major product in scope of the reaction using different aryl-substituted
72% yield (entry 1, Table 1). Although the regioselec- allenes with B2ACTHNUTRGNEU(GN pin)2 (Table 2): the arylallenes with
tivity is poor, it is interesting to note that in both electron-donating substituents at the para or meta po-
products the boryl group is connected to the central sition can afford Z-2b–Z-2f as the only isomer in
carbon atom of allenes, indicating allyl copper com- good yields (entries 3–7); heteroaryl substituents, such
pounds. Then, a series of monodentate phosphine li- as 2-thienyl and 3-thienyl, were also tolerated (en-
gands was screened to improve the regioselectivity tries 8 and 9). In addition, the reaction of 1a proceed-
and PCy3 was shown to be the best (entry 8). The re- ed smoothly to afford Z-2a on a one-gram scale in
action could not proceed without NaO-t-Bu (entry 2). 69% yield (entry 2). It should be noted that the selec-
The effects of the solvents and temperature were then tivity for the reaction of alkylallenes with B
examined (entries 9–14). A comprehensive study on under conditions A is poor.
the solvent effect indicated that Et2O is the best sol-
2ACHTUNGTRENNUNG(pin)2
2
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Ligand Controlled Highly Selective Copper-Catalyzed Borylcuprations of Allenes
Table 2. Copper-catalyzed addition of bis(pinacolato)diboron with different arylallenes under conditions A affording stereo-
defined 2-alken-2-yl boronates Z-2.[a]
Entry
Ar
t [min]
Isolated yield of Z-2 [%]
1
Ph (1a)
Ph (1a)
40
20
30
50
50
40
50
30
40
72 (Z-2a)
69 (Z-2a)
71 (Z-2b)
70 (Z-2c)
66 (Z-2d)
73 (Z-2e)
63 (Z-2f)
76 (Z-2g)
64 ( Z-2h)
2[b]
3
p-Me-C6H4 (1b)
m-Me-C6H4 (1c)
p-MeO-C6H4 (1d)
p-Bu-C6H4 (1e)
p-i-Pr-C6H4 (1f)
3-thienyl (1g)
2-thienyl (1h)
4
5
6
7
8
9
[a]
The reaction was carried out with allene (0.5 mmol), CuCl (5 mol%), P(C6H4OMe-p)3 (6 mol%), NaO-t-Bu (20 mol%),
2A(pin)2 (0.6 mmol), MeOH (1.0 mmol), and Et2O (2 mL) at room temperature. The reaction was monitored by TLC on
B CHTUNGTRENNUNG
silica gel.
[b]
The reaction is carried out in a one-gram scale.
Furthermore, it was observed that the regioselectiv- phine ligands was screened and BIPHEP gave the
ity may be switched when bidentate phosphine li- best regioselectivity for producing 3a (entry 6). We
gands were applied (Table 3). Firstly, the reaction also investigated the effects of the solvents and tem-
with dppe afforded a mixture of 2-alken-2-yl boronate perature, no better results was observed when other
Z-2a and 1-alken-2-yl boronate 3a in a reversed regio- solvents used (entries 9–12); running the reaction at
selectivity of ~3:7 with 3a being the major product in 08C gave no better result (entry 8).
47% yield (entry 1)! Then, a series of bidentate phos-
Table 3. Optimization of reaction conditions for copper-catalyzed regio- and stereoselective hydroboration of allenes using
bidentate phosphine ligands yielding 1-alken-2-yl boronate 3a.[a]
Entry
Ligand
Solvent
T [oC]/t [h]
3a/Z-2a[b]
Yield of 3a [%][c]
1
2
3
4
5
6
7
8
dppe
THF
THF
THF
THF
THF
THF
THF
THF
CH3CN
Et2O
PhCH3
CH2Cl2
40/1.1
40/1
40/1.2
40/1.1
40/1.1
40/1
r.t./1
0/1
r.t./1
r.t./1
r.t./1
r.t./1
71/29
74/26
84/16
93/7
93/7
95/5
95/5
95/5
–
47
23
70
56
57
62
63
57
NR
57
67
69
Xantphos
DPEphos
dppf
BINAP
BIPHEP
BIPHEP
BIPHEP
BIPHEP
BIPHEP
BIPHEP
BIPHEP
9
10
11
12
89/11
93/7
93/7
[a]
Reaction conditions: 0.2 mmol of allene, 0.24 mmol of bis(pinacolato)diboron, 5 mol% CuCl, 5 mol% bidentate ligand,
20 mol% NaO-t-Bu, and 0.4 mmol of MeOH were used.
[b]
[c]
The selectivity was determined by 1H NMR spectra analysis of the crude reaction products prior to purification.
1
The yield was determined by H NMR spectra analysis using 1,3,5-trimethylbenzene as the internal standard. dppe=1,2-
bis(diphenylphosphino)ethane, Xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, DPEphos=(oxydi-2,1-phe-
nylene)bis(diphenylphosphine), dppf=1,1’-bis(diphenylphosphino)ferrocene, BINAP=(Æ)-2,2’-bis(diphenylphosphino)-
1,1’-binaphthalene, BIPHEP=2,2’-bis(diphenylphosphino)biphenyl.
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Table 4. Copper-catalyzed addition of bis(pinacolato)diboron with different allenes under conditions B to afford 1-alken-2-yl
boronates 3.[a]
Entry
R
t [min]
Isolated yield of (3+Z-2) [%]
Ratio (3/Z-2)[b]
1
Ph (1a)
Ph (1a)
40
20
40
50
40
45
60
45
45
90
70
50
50
60
67 (3a)
70 (3a)
70 (3b)
64 (3c)
70 (3d)
73 (3e)
60 (3f)
62 (3g)
73 (3h)
54 (3i)
76 (3j)
64 (3k)
71 (3l)
63 (3m)
ꢀ95/5
ꢀ95/5
ꢀ94/6
ꢀ95/5
ꢀ94/6
ꢀ96/4[d]
ꢀ96/4
>99/1
ꢀ96/4[d]
>99/1
ꢀ96/4[d]
99/1
2[c]
3
p-Me-C6H4 (1b)
m-Me-C6H4 (1c)
p-Bu-C6H4 (1e)
p-Cl-C6H4 (1i)
m-Cl-C6H4 (1j)
o-Cl-C6H4 (1k)
p-Br-C6H4 (1l)
o-Br-C6H4 (1m)
p-EtO2C-C6H4 (1n)
Bn (1o)
4
5
6
7
8
9
10
11
12[e]
13[e]
14[e]
C8H17 (1p)
C6H13 (1q)
ꢀ96/4
ꢀ97/3[f]
[a]
The reaction was carried out with allene (0.5 mmol), CuCl (5 mol%), BIPHEP (5 mol%), NaO-t-Bu (20 mol%), B
(0.6 mmol), MeOH (1.0 mmol), and THF (2 mL) at room temperature.
The selectivity was determined by 1H NMR spectra analysis of the products obtained before and after separation.
The reaction is carried out on a one-gram scale.
The selectivity before separation is 97/3.
The ligand is dppf.
2ACHTUNGTRENNUNG(pin)2
[b]
[c]
[d]
[e]
[f]
The selectivity before separation is 98/2.
With the optimized conditions in hand, we applied ceeded smoothly to afford 3a on a one-gram scale in
conditions (5 mol% CuCl, 5 mol% BIPHEP, 70% overall yield with the same regioselectivity
B
20 mol% NaO-t-Bu, 2 equiv, MeOH, THF, and room (entry 2). The structure of compound 3h was con-
temperature) to investigate the scope of the hydrobo- firmed by an X-ray single crystal diffraction study
ration of allenes to afford 3 (Table 4). The substituent (Figure 1).[17] It should be pointed out that the recent-
on the aryl group shows very limited effect on the ly developed hydroboration of alkyl- or benzyl-substi-
yields: arylallenes with either an electron-withdrawing tuted terminal alkynes by Hoveyda et al. provides
or electron-donating substituent at the ortho, meta, or a mixture of two regioisomers with the traditional hy-
para position of the aryl group all reacted smoothly droboration products being the major[16c] and, thus,
to afford 3b–3j in good yields and excellent regiose- failed to provide such products 3 from the corre-
lectivity (entries 3–11); alkyl- or benzyl-substituted al- sponding alkynes highly selectively.[18]
lenes also afford the corresponding 1-alken-2-yl boro-
To explore the mechanism, a deuterium-labeling
nates in good yields and excellent regioselectivitity experiment was tried: it is interesting to observe that
(entries 12–14). In addition, the reaction of 1a pro- the reaction can afford 72% of Z-2a-d with 97% D in-
corporation and 61% of 3a-d in ꢀ94:6 regioselectivity
with 91% D incorporation at the allylic position
(Scheme 2).
Based on these observations, a rationale for the ob-
served regioselectivity was proposed (Scheme 3).[19]
Firstly, the copper-boryl complex LCu-BACTHNURGTNE(NUG pin) adds to
the terminal or internal double bond of allene with
boron always connected to the middle carbon atom:
For P(C6H4OMe-p)3, the reaction proceeds with the
more-substituted C=C bond forming Int 1 while for
sterically bulky BIPHEP the terminal C=C was react-
ed producing Int 2. Then g-protonolysis of the allylic
À
Figure 1. ORTEP representation of 3h.
C Cu bond in Int 1 and Int 2 by MeOH produces the
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Ligand Controlled Highly Selective Copper-Catalyzed Borylcuprations of Allenes
Experimental Section
Typical Procedure for the Preparation of Z-2
To an oven-dried Schlenk tube equipped with a magnetic
stirring bar were added sequentially bis(pinacolato)diboron
(2.6200 g,
10.33 mmol),
P(C6H4OMe-p)3
(182.0 mg,
0.52 mmol), CuCl (42.6 mg, 0.43 mmol), NaO-t-Bu
(165.4 mg, 1.72 mmol), 1a (1.0102 g, 8.61 mmol), MeOH
(0.7 mL, d=0.7915 gcmÀ3, 551.7 mg, 17.22 mmol), and Et2O
(20 mL) under argon. The mixture was stirred at room tem-
perature for 20 min as monitored by TLC. Upon comple-
tion, the resulting mixture was filtered through a short
column of silica gel eluting with Et2O and concentrated. The
residue was purified by chromatography on silica gel
(eluent: petroleum ether/ethyl ether=50/1) to afford Z-2a
as a liquid; yield: 1.4544 g (69%). 1H NMR (300 MHz,
CDCl3): d=7.41–7.30 (m, 4H), 7.28–7.20 (m, 2H), 1.99 (s,
Scheme 2. Deuterium labeling experiments.
3H, CH3), 1.32 [s, 12H, BACHTNUTGRNEUNG
(pin)]; 13C NMR (75.4 MHz,
Z-2 and 3, respectively. Obviously, the steric hin-
drance of the ligand determines the regioselectivity.
This also nicely explains the stabilization effect of the
aryl group for selectivity shown in Table 2. Of course,
an a-protonation mechanism may not be completely
excluded.
CDCl3): d=142.3, 137.8, 129.3, 127.9, 127.0, 83.3, 24.7, 15.8;
IR (neat): n=2978, 1618, 1491, 1448, 1405, 1367, 1346, 1311,
1272, 1209, 1145, 1104 cmÀ1; MS (70 eV, EI): m/z (%)=244
(M+, 71.67), 43 (100).
In summary, we have developed an efficient non-
classic hydroboration of allenes with bis(pinacolato)-
diboron based on copper catalysis, which affords two
different types of vinylboronates with high regio- and
stereoselectivity by using different ligands. These
borylated products are not readily available from the
classic hydroboration of alkynes or the recently devel-
oped reaction of alkynes with bis(pinacolato)diboron
and obviously will be very useful intermediates for
subsequent elaboration to more complex structures
Typical Procedure for the Preparation of 3
To an oven-dried Schlenk tube equipped with a magnetic
stirring bar were added sequentially bis(pinacolato)diboron
(2.6200 g, 10.33 mmol), BIPHEP (224.7 mg, 0.43 mmol),
CuCl
(42.6 mg,
0.43 mmol),
NaO-t-Bu
(165.4 mg,
1.72 mmol), 1a (1.0010 g, 8.61 mmol), MeOH (0.7 mL, d=
0.7915 gcmÀ3, 551.7 mg, 17.22 mmol), and THF (20 mL)
under argon. The mixture was stirred at room temperature
for 20 min as monitored by TLC. Upon completion, the re-
sulting mixture was filtered through a short column of silica
although the effect of ligands on the regio- and ste- gel eluting with Et2O and concentrated. The residue was pu-
rified by chromatography on silica gel (eluent: petroleum
ether/ethyl ether=50/1) to afford a mixture of 3a and Z-2a,
molar ratio by NMR: ꢀ95:5) as a liquid; yield: 1.4630 g
(70%). 3a: 1H NMR (300 MHz, CDCl3): d=7.28–7.08 (m,
5H, Ar-H), 5.83 (s, 1H, one proton of =CH2), 5.58–5.48 (br
s, 1H, one proton of =CH2), 3.47 (s, 2H, CH2), 1.19 [s, 12H,
reoselectivity requires further attention. Studies in-
cluding synthetic applications and the factors control-
ling the regio- and stereoselectivity are under way in
this laboratory.
BACTHNUTRGNEUNG
(pin)]; 13C NMR (75.4 MHz, CDCl3): d=140.6, 129.7,
129.0, 128.0, 125.6, 83.4, 41.3, 24.8, 24.6; IR (neat): n=2979,
1618, 1494, 1429, 1364, 1310, 1272, 1210, 1136, 1111, 1073,
Scheme 3. Ligand-controlled borylcuprations followed by g-protonolysis.
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Weiming Yuan and Shengming Ma
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1030 cmÀ1; MS (70 eV, EI): m/z (%)=244 (M+, 1.80), 41
(100); HR-MS: m/z=244.1637, calcd. for C15H21BO2 (M+):
244.1635.
J. P. Morken, Org. Lett. 2006, 8, 4557; d) H. E. Burks, S.
Liu, J. P. Morken, J. Am. Chem. Soc. 2007, 129, 8766.
[11] For Pd-catalyzed silaboration of allenes, see: a) M. Su-
ginome, Y. Ohmori, Y. Ito, Synlett 1999, 1567; b) M. Su-
ginome, Y. Ohmori, Y. Ito, J. Organomet. Chem. 2000,
611, 403; c) S. Onozawa, Y. Hatanaka, M. Tanaka,
Chem. Commun. 1999, 1863; d) M. Suginome, T.
Ohmura, Y. Miyake, S. Mitani, Y. Ito, M. Murakami, J.
Am. Chem. Soc. 2003, 125, 11174; e) T. Ohmura, H. Ta-
niguchi, M. Suginome, J. Am. Chem. Soc. 2006, 128,
13682.
Acknowledgements
Financial support from the National Basic Research Program
of China (No. 2011CB808700) and National Natural Science
Foundation of China (No. 20732005) is greatly appreciated.
We thank Mr Qiankun Li in this group for reproducing the
results of Table 2 (entry 5), Table 4 (entries 8 and 14) present-
ed in this study.
[12] a) S. Ma, N. Jiao, L. Ye, Chem. Eur. J. 2003, 9, 6049;
b) S. Ma, H. Guo, F. Yu, J. Org. Chem. 2006, 71, 6634;
c) H. Guo, S. Ma, Synthesis 2007, 2731.
[13] C. H. Oh, T. W. Ahn, R. V. Reddy, Chem. Commun.
2003, 2622.
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Y. Guo, S. Ma, Angew. Chem. 2005, 117, 4849; Angew.
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R1=0.0353,
wR2=0.0815,
a=19.691(3) ꢂ,
b=
6.1598(10) ꢂ, c=24.966(4) ꢂ, a=908, b=908, g=908,
V=3028.2(9) ꢂ3, T=173(2) K, Z=8, reflections col-
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servations [> 2s(I)] 2286, parameters: 180. CCDC
845986 contains the supplementary crystallographic
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