Highly selective copper-catalyzed hydroboration of allenes and 1,3-dienes

Article abstract of DOI:10.1002/chem.201300443

The highly selective copper-catalyzed hydroboration of allenes has been developed. Allylboranes and alkenylboranes were selectively prepared by the judicious choice of catalytic species (copper hydride and boryl copper). Furthermore, two types of alkenylboranes could be selectively synthesized by the choice of an appropriate ligand. Mechanistic studies confirmed that the protonation of a (Z)-σ-allyl copper species, which was isolated and structurally characterized by single-crystal X-ray diffraction, was a key step in these reactions. Besides allenes, this method is also applicable to the selective hydroboration of 1,3-diene derivatives to afford allylboranes and homoallylboranes. Copyright

Full text of DOI:10.1002/chem.201300443

FULL PAPER
DOI: 10.1002/chem.201300443
Highly Selective Copper-Catalyzed Hydroboration of Allenes and 1,3-Dienes
Kazuhiko Semba, Masataka Shinomiya, Tetsuaki Fujihara, Jun Terao, and
Yasushi Tsuji*[a]
Abstract: The highly selective copper-
catalyzed hydroboration of allenes has
been developed. Allylboranes and al-
kenylboranes were selectively prepared
by the judicious choice of catalytic spe-
cies (copper hydride and boryl copper).
Furthermore, two types of alkenylbor-
anes could be selectively synthesized
by the choice of an appropriate ligand.
Mechanistic studies confirmed that the
protonation of a (Z)-s-allyl copper spe-
cies, which was isolated and structural-
ly characterized by single-crystal X-ray
diffraction, was a key step in these re-
actions. Besides allenes, this method is
also applicable to the selective hydro-
boration of 1,3-diene derivatives to
afford allylboranes and homoallylbor-
anes.
Keywords: allenes · copper · di-
enes · hydroboration · ligand effects
Introduction
isomers (Scheme 2).[7] In the uncatalyzed hydroboration of
allenes with reactive di(alkyl)boranes, such as 9-BBN
(9-borabicyclo
[3.3.1]nonane),[8a] HBCy2 (Cy=cyclohexyl),[8a]
HB
(sia)2 (sia=1,2-dimethylpropyl),[8b] and 10-TMS-9-BBD-
H (10-TMS-9-borabicyclo
[3.3.2]decane),[8c] allylboranes (2)
were afforded as the major products. However, the draw-
back of the methods involving these reactive di-
ACHTUNGTREN(NUNG alkyl)boranes is substantial instability of both the borane
AHCTUNGTRENNUNG
Pinacolborane (HB
(B2A
(pin)2)[2] are stable and easy-to-handle representative
borylation reagents. Various transition-metal catalysts show
good catalytic activity in borylation reactions with HB(pin)
(pin)2.[1,2] In particular, in the presence of a copper cat-
(pin))[1] and bis(pinacolato)diboron
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
R
or B2ACHTUNGTRENNUNG
alyst, HB
[3]
À
(pin) generates copper hydride (Cu H) and B2-
[4]
À
(pin)2 affords boryl copper (Cu B) catalytic species. Re-
reagents and the resulting hydroboration products. Further-
more, these reactions often suffered from low regioselectivi-
ty and/or the formation of di-hydroborated byproducts.[8a,b]
cently, we reported the regioselective copper-catalyzed hy-
droboration of unsymmetrical internal alkynes through a
À
clean generation of one of the two catalytic species (Cu H
In contrast, diACTHNGUTERNNU(G alkoxy)boranes are much more stable and
[5]
or Cu B, Scheme 1).
easy-to-handle reagents and the resulting allylborane[9] or al-
kenylborane products[10] can be isolated and stored for fur-
ther reactions. However, the uncatalyzed hydroboration re-
À
As for the hydroboration of allenes,[6] even mono-substi-
tuted substrates may provide up to six regio- and stereo-
action with these less-reactive diACTHUNTGRNEUNG(alkoxy)boranes required
harsh reaction conditions (1308C), as indicated by the reac-
tion of 4,4,6-trimethyl-1,3,2-dioxaborinane (HBR2, R2 =OC-
Me2CH2CHMeO) with allenes,[8d] and the selectivity be-
tween allylboranes and alkenylboranes was insufficient.
To utilize these stable diACTHNUGRTENUNG(alkoxy)boranes, transition-metal-
catalyzed reaction are beneficial because of the mild reac-
tion conditions. However, surprisingly, there are only two
precedents for the transition-metal-catalyzed hydroboration
of allenes.[11a,b] Firstly, Miyaura and co-workers reported the
transition-metal-catalyzed hydroboration of terminal allenes
Scheme 1. Copper-catalyzed regioselective hydroboration of unsymmetri-
cal internal alkynes.
by employing HB
ACHTUNGTRENN(UNG pin) in the presence of a platinum cata-
lyst.[11a] The selectivities of the products (2, 4, and 5) varied
considerably depending on the allene substituents and the
nature of the added phosphane ligands. Secondly, during the
preparation of this manuscript,[12] Yuan and Ma reported the
copper-catalyzed hydroboration of mono-substituted allenes
[a] K. Semba, M. Shinomiya, Prof. Dr. T. Fujihara, Prof. Dr. J. Terao,
Prof. Dr. Y. Tsuji
Department of Energy and Hydrocarbon Chemistry
Graduate School of Engineering, Kyoto University
Kyoto 615-8510 (Japan)
Fax : (+81)75-383-2514
E-mail: ytsuji@scl.kyoto-u.ac.jp
by using B2ACHTNUGRTENUNG(pin)2, thereby providing compounds (Z)-4 and
5.[11b]
Herein, we report the copper-catalyzed hydroboration of
allenes by employing HB(pin) or B2A(pin)2 as a borylating re-
A
CHTUNGTRENNUNG
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300443.
agent (Scheme 3). The selectivity for allylboranes ((E)-2)
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7125

considerably (Table 1, entries 7
and 8). In all of these cases,
other isomers, such as allylbor-
ane 3a and alkenylboranes 4a
and 5a, were not produced.
The hydroboration of various
allenes to afford allylboranes
(2) was achieved by employing
HB
catalytic amount of CuCl with
CF3Ar-Xan as ligand
ACHTUNGERTN(NUNG pin) in the presence of a
Scheme 2. Plausible products from the hydroboration of 1-substituted allenes.
a
(Table 2). Allenes that con-
tained a primary alkyl group (1b and 1c) afforded their cor-
responding allylboranes (2b and 2c) in good yields, albeit
with slightly low E/Z ratios (Table 2, entries 1 and 2). In
contrast, allenes that contained secondary or tertiary alkyl
substituents (1d and 1e) afforded their corresponding allyl-
boranes (2d and 2e) in good yields with high E/Z ratios
(Table 2, entries 3 and 4). In the case of allenes that were
conjugated with an aromatic ring, the products (2 f–2h)
were obtained with perfect stereoselectivity (Table 2, en-
tries 5–7). The reaction of a 1,1-disubstituted allene (1i) also
provided its corresponding allylborane (2i; Table 2, entry 8).
Scheme 3. Copper-catalyzed hydroboration of allenes.
Table 1. Hydroboration of cyclohexylallene (1a) with HB
ACHTUNGTRENNUNG
various copper catalysts.[a]
and alkenylboranes ((Z)-4 and 5) was successfully controlled
À
À
by a judicious choice of the catalytic species (Cu H or Cu
B). Notably, two types of alkenylboranes ((Z)-4 and 5) were
selectively obtained by choosing suitable ligands. Thus, this
method can selectively afford three different hydroboration
products from a single substrate. Furthermore, this catalytic
procedure can also be applicable to the selective hydrobora-
tion of 1,3-dienes.
Entry
Cu catalyst
Yield [%][b]
E/Z[c]
1
2
3
4
5
6
7
8
CuCl/PPh3
CuCl/PCy3
CuCl/dppbz
CuCl/Xan
CuCl/MeAr-Xan
CuCl/CF3Ar-Xan
<1
<1
78
85
99
–
–
88:12
>99:1
93:7
95:5
65:35
49:51
99 (78)[d]
68
12
Results and Discussion
AHCTUNGTRENNUNG
G
Hydroboration of allenes with HBACTHUNTGRNEUNG(pin) to afford allylbor-
[a] Reaction conditions: Cyclohexylallene (0.50 mmol), HBACHTUNGTRENNUNG(pin)
anes (2): First, the copper-catalyzed hydroboration of cyclo-
hexylallene (R1 =Cy: 1a) was performed by employing HB-
(0.60 mmol), CuCl (0.010 mmol, 2.0 mol%)/ligand (0.010 mmol,
2.0 mol%) or [(NHC)CuCl] (0.010 mmol, 2.0 mol%), NaOtBu
(0.060 mmol, 12 mol%), 1,4-dioxane (4.0 mL), 288C, 2 h. [b] Yield of
compound 2a, as determined by GC with an internal standard. [c] Ratio
of the E/Z isomers, as determined by GC. [d] Yield of isolated product,
after column chromatography on silica gel.
ACHTUNGTRENNUNG(pin) as a hydroborating agent in the presence of various li-
gands (Table 1). Monodentate phosphane ligands, such as
PPh3 and PCy3, were not effective and almost no allylborane
(2a) was obtained (Table 1, entries 1 and 2). With bidentate
phosphane ligands, such as 1,2-diphenylphosphinobenzene
(dppbz), Xantphos (Xan, 4,5-bis(diphenylphosphanyl)-9,9-
dimethylxanthene), Me-ArXan, and CF3-ArXan (for the
structures of these ligands, see Figure 1), the yields were
dramatically improved and compound 2a was obtained in
good-to-high yields with high E selectivities (Table 1, en-
tries 3–6). Among these ligands, CF3Ar-Xan[5] was the most
effective, thus giving compound 2a in 99% yield with high
selectivity (E/Z, 95:5; Table 1, entry 6). Notably, compound
(E)-2a could be isolated in its pure form by column chroma-
tography on silica gel in air in 78% yield. When N-heterocy-
clic carbene (NHC) ligands, such as IPr and IMes (Figure 1),
were used, both the yields and regioselectivities decreased
Figure 1. Ligands used in this study.
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Copper-Catalyzed Hydroboration of Allenes and 1,3-Dienes
FULL PAPER
Table 2. Copper-catalyzed hydroboration of various allenes to afford al-
lylboranes.[a]
Table 3. Hydroboration of compound 1a with B2ACTHNGUTERN(NUG pin)2 by using various
copper catalysts.[a]
Entry
Cu catalyst
Yield [%][b]
(Z)-4a/5a[c]
Entry
1
Substrate
Product
Yield [%][b] E/Z[c]
1
2
3
4
CuCl/PPh3
CuCl/PCy3
T
40
56
84
92
84:16
47:53
77:23
86:14
99:1
28:72
2:98
35:65
9:91
74
71
91:9
N
5[d]
6
E
99 (90)[e]
2
3
89:11
CuCl/Xan
CuCl/CF3Ar-Xan
66
73
92
99
7
8
9
70
98:2
A
E
10
[(ClIPrCPh )CuCl]
99
2:98
1:99
4
5
78
60
100:0
100:0
11[f]
[(ClIPrCPh )CuCl]
99 (94)[e]
[a] Reaction conditions: Compound 1a (0.50 mmol),
B2ACHTUNGTRENNUNG(pin)2
(0.53 mmol), MeOH (1.0 mmol), CuCl (0.010 mmol, 2.0 mol%)/ligand
(0.010 mmol, 2.0 mol%) or [(NHC)CuCl] (0.010 mmol, 2.0 mol%),
NaOtBu (0.060 mmol, 12 mol%), toluene (1.0 mL), 288C, 2 h. [b] Yield
of the product, as determined by GC with an internal standard. [c] Ratio
of compounds (Z)-4a/5a, as determined by GC. [d] MeOH (4.0 mmol),
À208C. [e] Yield of the isolated product, after column chromatography
6
68
100:0
100:0
on silica gel. [f] B2ACHTUNTGRENUNG(pin)2 (0.60 mmol), THF (2.0 mL).
7
8
9
58
66
52
stituents on the N-heterocyclic ring, was the most efficient,
giving compound (Z)-4a in high yield and selectivity (92%
yield; (Z)-4a/5a, 86:14; Table 3, entry 4). Gratifyingly, by
lowering the reaction temperature to À208C, compound
(Z)-4a was obtained almost exclusively and in quantitative
yield (99% yield; (Z)-4a/5a, 99:1; Table 3, entry 5). From
the reaction mixture, compound (Z)-4a was isolated in 90%
yield in its pure form. Thus, the catalytic system shown in
Table 3, entry 5 (with MeIMes as the ligand) was best for the
preparation of compounds (Z)-4. Compound 5a was afford-
ed as the major product with the use of Xan as a ligand
((Z)-4a/5a, 28:72), although the yield and selectivity were
unsatisfactory (Table 3, entry 6). By employing CF3Ar-Xan
as a ligand, compound 5a was selectively obtained ((Z)-4a/
5a, 2:98) in 73% yield (Table 3, entry 7). When IPr was
used as the ligand, the yield of the products increased to
92% yield, but with low selectivity ((Z)-4a/5a, 35:65;
Table 3, entry 8). In the case of ClIPr, which contained Cl
substituents on the N-heterocyclic ring, both the yield and
selectivity dramatically improved (99% yield; (Z)-4a/5a,
100:0
[a] Reaction conditions: Allene (0.50 mmol), HBACTHNUGRTNEUNG(pin) (0.60 mmol), CuCl
(0.010 mmol, 2.0 mol%), CF3Ar-Xan (0.010 mmol, 2.0 mol%), NaOtBu
(0.060 mmol, 12 mol%), 1,4-dioxane (4.0 mL), 288C, 2 h. [b] Yield of the
isolated product. [c] Ratio of the E/Z isomers, as determined by GC.
It should be noted that an unsymmetrical 1,3-disubstituted
allene (1j) successfully smoothly underwent the hydrobora-
tion reaction to afford the product as a single isomer (2j;
Table 2, entry 9). This is the first example of the transition-
metal-catalyzed hydroboration of 1,3-disubstituted allenes.
Hydroboration of allenes with B
anes (4 or 5): The hydroboration of compound 1a was car-
ried out by employing B2A(pin)2 in place of HB(pin) in the
presence of a catalytic amount of CuCl with various ligands
(Table 3). Remarkably, compared with the hydroboration re-
2ACHTUNGRTEN(NUNG pin)2 to afford alkenylbor-
G
ACHTUNGTRENNUNG
9:91; Table 3, entry 9). Finally, ClIPrCPh , which contained Cl
3
substituents on the N-heterocycle ring and CPh3 substituents
on the phenyl rings,[13] was the most effective ligand, giving
compound 5a in 99% yield with high selectivity ((Z)-4a/5a,
2:98; Table 3, entry 10). The selectivity was further im-
proved by using THF as a solvent (Table 3, entry 11), from
which compound 5a was isolated in its pure form by column
chromatography on silica gel in 94% yield. Thus, the cata-
actions with HBACHTUNGTRENNUNG(pin), the regioselectivity was completely
switched to afford alkenylboranes ((Z)-4a and 5a) and no
other isomers, such as compounds 2a, 3a, or (E)-4a, were
produced (Table 3). The use of monodentate phosphanes,
such as PPh3 and PCy3, led to a mixture of compounds (Z)-
4a and 5a with low selectivities (Table 3, entries 1 and 2).
The NHC ligand, IMes, tended to preferentially produce
compound (Z)-4a (84% yield; (Z)-4a/5a, 77:23; Table 3,
entry 3). The MeIMes ligand, which contained methyl sub-
lyst system shown in Table 3, entry 11 (with ClIPrCPh as the
ligand) was determined to be the best for the preparation of
compounds 5.
3
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Y. Tsuji et al.
Table 4. Copper-catalyzed hydroboration of various allenes to afford al-
Thus, we have established the optimal catalyst system for
the two alkenylborane derivatives, that is, compounds (Z)-4
(Table 3, entry 5) and 5 (Table 3, entry 11). The structures of
kenylboranes ((Z)-4).[a]
[(MeIMes)CuCl] and [(ClIPrCPh )CuCl] were determined in
3
their monomeric forms by X-ray crystallography, as shown
in Figure 2, which were similar to those of other
[(NHC)CuCl] complexes.[14]
Entry
1
Substrate
Product
Yield [%][b] (Z)-4/5[c]
90
89
90
83
93:7
91:9
97:3
95:5
2
3
4[d]
5[e]
56
81
97:3
91:9
6[f]
[a] Reaction conditions: Allene (0.50 mmol),
B2ACHTGUNTERNNU(G pin)2 (0.53 mmol),
[(MeIMes)CuCl] (0.010 mmol, 2.0 mol%), NaOtBu (0.060 mmol,
12 mol%), toluene (1.0 mL), À208C, 2 h. [b] Yield of the isolated prod-
uct. [c] Ratio of compounds (Z)-4/5, as determined by GC. [d] B2ACHTUNGTRENNUNG(pin)2
(0.75 mmol), [(MeIMes)CuCl] (0.038 mmol, 7.5 mol%), CF3CH(OH)CF3
(2.0 mmol). [e] B2A
(pin)2 (0.75 mmol), [(MeIMes)CuCl] (0.025 mmol,
CTHUNGTRENNUNG
5.0 mol%), CF3CH(OH)CF3 (2.0 mmol). [f] B2ACHTNUGTRNEUNG(pin)2 (0.60 mmol),
[(MeIMes)CuCl] (0.025 mmol, 5.0 mol%), CF3CH(OH)CF3 (2.0 mmol).
Figure 2. Crystal structures of a) [(MeIMes)CuCl] and b) [(ClIPrCPh )-
3
ACHTUNGTRENNUNGCuCl]·CHCl3.
arylallenes 1 f–1h also proceeded in high yields and selectiv-
ities (Table 5, entries 5–7). The presence of electron-donat-
ing and -withdrawing groups on the aryl ring did not affect
the yields or selectivities. Not only 1-substituted allenes but
also 1,1-disubstituted (1i) and 1,3-disubstituted allenes (1j)
gave their corresponding products (5i and 5j) with high se-
lectivities (Table 5, entries 8 and 9).
The hydroboration of various allenes to afford compounds
(Z)-4 was achieved with
B2ACHTGUNETRN(NUG pin)2 in the presence of
[(MeIMes)CuCl] as a catalyst (Table 4). Very recently, Yuan
and Ma reported that a CuCl/P(p-MeOC6H4)3 catalytic
system afforded compound (Z)-4 by the hydroboration of
[11b]
monosubstituted allenes with B
G
(see above). How-
ever, with this catalyst system, only aryl-substituted allenes
were applicable; alkyl-substituted allenes could not be used
owing to the poor selectivities for the products. In sharp
contrast, by using our catalyst system, monosubstituted al-
lenes that contained primary alkyl (Table 4, entries 1 and 2)
or secondary alkyl substituents (Table 4, entry 3), as well as
aromatic ones (Table 4, entries 4–6), selectively gave com-
pounds (Z)-4b–4h in good-to-high yields. With arylallenes
1 f–1h, the reactions proceeded smoothly with a more-acidic
alcohol, CF3CH(OH)CF3 (4.0 equiv), in place of MeOH
(Table 4, entries 4–6) as the proton source.
Mechanism of the hydroboration reaction: The mechanism
of the hydroboration reaction to provide allylboranes (2)
must be similar to that of alkynes[5] and a possible catalytic
cycle is shown in Scheme 4. In this cycle, copper hydride
would be generated by the reaction between a copper alkox-
ide and HB
[(ClIPr)CuH] was obtained quantitatively by a stoichiometric
reaction of [(ClIPr)Cu (pin).[5] The copper
(OtBu)] with HB
ACHTUNGTRENN(UNG pin). We had previously confirmed that
A
ACHTUNGTRENNUNG
hydride inserts into an allene from the sterically less-encum-
bered face of the allene to give the (Z)-s-allyl copper spe-
cies as a kinetic product (Scheme 4, step a), which isomeriz-
es into the corresponding thermodynamically stable (E)-s-
allyl copper species. s-Bond metathesis between an (E)-s-
On the other hand, alkenylboranes 5 were selectively pro-
duced with [(ClIPrCPh )CuCl] as the catalyst (Table 5). Al-
3
lenes that contained primary (Table 5, entries 1 and 2), sec-
ondary (Table 5, entry 3), and tertiary alkyl groups (1b–1e;
Table 5, entry 4) provided the corresponding products in
good-to-high yields with high selectivities. The reactions of
allyl copper species and HB
ACHTUNGTNER(NUNG pin) gives compounds (E)-2
and LCuH[15] (Scheme 4, step b).
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Copper-Catalyzed Hydroboration of Allenes and 1,3-Dienes
FULL PAPER
Table 5. Copper-catalyzed hydroboration of various allenes to afford al-
With regards to the mechanism of the hydroboration reac-
tion to afford alkenylboranes (4 or 5), some stoichiometric
reactions were performed by employing MeIMes and ClIPr as
the ligands. In the catalytic reactions, MeIMes was an effec-
tive ligand for preparing compounds (Z)-4 (Table 3, entry 5)
and ClIPr was a suitable ligand for preparing compounds 5
(Table 3, entry 9). First, two boryl copper complexes,
kenylboranes (5).[a]
Entry
1
Substrate
Product
Yield [%][b] 5/(Z)-4[c]
[(MeIMes)CuB
prepared in good yields by the reaction of [(NHC)Cu-
(OtBu)][16] with B
2A(pin)2 (Scheme 5a and b), according to a
literature procedure for [(IPr)CuB
(pin)].[17] Isolated com-
(pin)] (6a) and [(ClIPr)CuB
ACHUTGTNRENNUG ACHUTNTGREN(NUGN pin)] (6b), were
88
90
97:3
98:2
A
CHTUNGTRENNUNG
AHCTUNGTRENNUNG
2
3
plexes 6a and 6b were stable in the solid state under a N2
atmosphere. In toluene solution, compound 6a decomposed
at room temperature, whilst compound 6b was stable at
room temperature for a few hours. The crystal structure of
compound 6b was determined by X-ray crystallography
(Figure 3). The copper atom has an almost-linear geometry
88
98:2
4
5
94
82
100:0
98:2
6
7
80
90
99:1
97:3
8
9
56
85
100:0[d]
100:0[e]
Figure 3. Crystal structure of compound 6b.
[a] Reaction conditions: Allene (0.50 mmol),
B2ACHTGUNTERNNU(G pin)2 (0.60 mmol),
[(ClIPrCPh )CuCl] (0.010 mmol, 2.0 mol%), NaOtBu (0.060 mmol,
12 mol%), THF (2.0 mL), 288C, 2 h. [b] Yield of the isolated product.
[c] Ratio of compounds 5/(Z)-4, as determined by GC. [d] 5i/2i, 93:7.
[e] (Z)-5j/(E)-5j, 83:17.
3
with a carbon atom on the NHC ligand and a boron atom.
The C-Cu-B bond angle is 165.51(15)8 which is comparable
to that of [(IPr)CuBACTHNUTRGNE(UNG pin)] (168.07(10)8).
Boryl copper complexes 6a and 6b smoothly reacted with
compound 1a[18] and the formation of (Z)-s-allyl copper
1
species 7a and 7b was confirmed by H NMR and NOESY
measurements (Scheme 6a and b). All attempts to grow
single crystals of compounds 7a and 7b that were suitable
for X-ray crystallographic analysis failed. On the other
hand, a similar s-allyl copper complex (7c) was prepared
from compounds 6b and 1 f in 61% yield (Scheme 6c) and
they afforded good-quality single crystals. Notably, the (Z)-
s-allyl structure of compound 7c was confirmed by X-ray
crystallographic analysis (Figure 4). In the unit cell, there
are two independent complexes, one of which is shown in
À
Figure 4. The average length of the C1 C2 bond
À
(1.361(4) ꢁ) is considerably shorter than that of the C2 C3
bond (1.492(4) ꢁ), which clearly indicates the s-allyl struc-
ture of compound 7c.
When compound 7a was protonated with MeOH at
À208C, the ratio of compounds (Z)-4a/5a was 47:53 (Sche-
me 6a), which was not as selective as the reaction with
Scheme 4. Plausible reaction mechanism that employs HBACTHUNTGRNEUNG(pin).
Chem. Eur. J. 2013, 19, 7125 – 7132
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Y. Tsuji et al.
[(MeIMes)CuCl] as the catalyst
1
(Table 3, entry 5). The H NMR
spectra of compound 7a at
208C, À108C, À308C, and
À508C almost did not change
(for details, see the Supporting
Information). However, proto-
nation at À808C provided com-
pounds (Z)-4a/5a in
a 99:1
ratio (Scheme 6a), which was
comparable to that in the cata-
lytic reaction. On the other
hand, similar protonation of
compound 7b with MeOH se-
lectively provided compound
Scheme 5. Synthesis of boryl copper complexes 6a and 6b.
5a ((Z)-4a/5a, 6:94) in a SE2’
fashion at both room tempera-
ture and at À208C (Sche-
me 6b). This selectivity is reminiscent of the selectivity in
the hydroboration reaction catalyzed by [(ClIPr)CuCl]
(Table 3, entry 9).
From these results in the stoichiometric reactions, a possi-
ble catalytic cycle for the hydroboration reaction with
B2ACHTNUGTRNEUNG(pin)2 is shown in Scheme 7. First, a boryl copper species
Scheme 6. Stoichiometric reactions between compound 1a and
[(MeIMes)CuB(pin)] (6a) or [(ClIPr)CuB
ACHTUNGTRENNUNG ACHTUGNTREN(NUGN pin)] (6b).
Scheme 7. Probable reaction mechanism that employs B2ACTHNUTRGNEN(UG pin)2.
is generated from the reaction between a copper alkoxide
and B2A(pin)2. The boryl copper species (6) inserts into an
CTHUNGTRENNUNG
allene (1) from the sterically less-encumbered face of the
allene, thus exclusively affording a (Z)-s-allyl copper inter-
mediate (Scheme 7). In the case of bulky NHC ligands, such
as ClIPr and ClIPrCPh , the (Z)-s-allyl copper species is prefer-
3
entially protonated in a SE2’ fashion to afford compounds 5
(Scheme 7, step b). On the other hand, with the less-bulky
MeIMes as the ligand, the (Z)-s-allyl copper is preferentially
protonated in a SE2 fashion, thus giving compounds (Z)-4
(Scheme 7, step b’). To the best of our knowledge, as clearly
indicated in Scheme 6, we first show that the site of protona-
Figure 4. Crystal structure of compound 7c.
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Chem. Eur. J. 2013, 19, 7125 – 7132

Copper-Catalyzed Hydroboration of Allenes and 1,3-Dienes
FULL PAPER
tion of the s-allyl copper species could be controlled by the
Conclusion
choice of ligands.[19]
We have developed a highly selective copper-catalyzed hy-
droboration of allenes to give an allylborane (2) and two al-
kenylboranes (4 and 5). The key to the success of this selec-
tive reaction is controlling the generation of the catalytic
Hydroboration of 1,3-diene derivatives: The hydroboration
of 1,3-dienes is a useful reaction for synthesizing allylbor-
anes and homoallylboranes, which are important intermedi-
ates in organic synthesis.[20,21] We anticipated that analogous
catalytic systems for allenes could be applicable to the selec-
tive hydroboration of 1,3-diene derivatives (Scheme 8). By
À
À
species (Cu H or Cu B), as well as the selective protona-
tion of the allyl copper species. An analogous catalytic
system was also applicable to the selective hydroboration of
1,3-dienes, which gave allylboranes (9) and homoallylbor-
anes (10), respectively. This method should be widely appli-
cable to other catalytic reactions.
Experimental Section
General procedure for the reactions shown in Table 2: CuCl (0.99 mg,
0.010 mmol, 2.0 mol%), CF3Ar-Xan (11.2 mg, 0.010 mmol, 2.0 mol%),
and NaOtBu (5.77 mg, 0.060 mmol, 12 mol%) were placed in an oven-
dried Schlenk flask (20 mL). The flask was evacuated and backfilled with
argon three times. 1,4-Dioxane (1.0 mL) was added and the mixture was
stirred for 15 min at RT under an argon atmosphere. HBACTHNUTRGENUG(N pin) (87 mL,
0.60 mmol) was added to the resulting solution at 08C and the mixture
was stirred at 08C for 5 min. An allene (0.50 mmol) was added to the
mixture at 08C and the mixture was stirred at 288C for 2 h. After the re-
action had reached completion, the selectivity of the product was deter-
mined by GC analysis. The mixture was filtered through a pad of silica
gel and the volatile compounds were removed in vacuo. The products
were purified by column chromatography on silica gel (n-hexane/Et2O).
General procedure for the reactions shown in Table 4: [(MeIMes)CuCl]
(4.31 mg, 0.010 mmol, 2.0 mol%) and NaOtBu (5.77 mg, 0.060 mmol,
12 mol%) were placed in an oven-dried Schlenk flask (20 mL). The flask
was evacuated and backfilled with argon three times. Toluene (1.0 mL)
was added and the mixture was stirred for 15 min at RT under an argon
atmosphere. B2ACHTNUGTRENUNG(pin)2 (133 mg, 0.53 mmol) was added to the resulting sol-
ution and the mixture was stirred at RT for 5 min. Then, MeOH (168 mL,
4.0 mmol) and an allene (0.50 mmol) were added successively at À208C
and the mixture was stirred at À208C for 2 h (in the cases of Table 4, en-
tries 4–6, CF3CH(OH)CF3 (208 mL, 2.0 mmol) was used instead of
MeOH). After the reaction, the selectivity of the product was determined
by GC analysis. The mixture was filtered through a pad of Celite and
silica gel. All of the volatile compounds were removed in vacuo. The
products were obtained by column chromatography on silica gel (n-
hexane/Et2O).
Scheme 8. Copper-catalyzed hydroboration of 1,3-dienes.
employing 1,3-cyclohexadiene (8a) as a substrate and HB-
ACHTUNGTRENNUNG(pin) as a hydroboration reagent, an allylborane (9a) was
selectively isolated in 81% yield with DTBMAr-Xan (for
the structure, see Figure 1) as the ligand (Scheme 8a). On
switching the boron source from HBACHTNUGTRENNUG(pin) to B2ACHTUNGTREN(NUGN pin)2, a
homoallylborane (10a) was selectively isolated in 77% yield
with ClAr-Xan as the ligand in THF at 608C (Scheme 8b).
Notably, when 1-phenyl-1,3-butadiene (8b) was used as the
substrate, an allylborane (9b) and a homoallylborane (10b)
were selectively isolated in high yields by employing HB-
General procedure for the reactions shown in Table 5: [(ClIPrCPh )CuCl]
3
(10.4 mg, 0.010 mmol, 2.0 mol%) and NaOtBu (5.77 mg, 0.060 mmol,
12 mol%) were placed in an oven-dried Schlenk flask (20 mL). The flask
was evacuated and backfilled with argon three times. THF (2.0 mL) was
added and the mixture was stirred for 15 min at RT under argon atmos-
phere. B2ACTHNUGTRNE(UNG pin)2 (152 mg, 0.60 mmol) was added to the resulting solution
and the mixture was stirred at RT for 5 min. Then, MeOH (42 mL,
1.0 mmol) and an allene (0.50 mmol) were added successively at 288C
and the mixture was stirred at 288C for 2 h. After the reaction, the selec-
tivity of the product was determined by GC analysis. The mixture was fil-
tered through a pad of Celite and silica gel. All of the volatile com-
pounds were removed in vacuo. The products were obtained by column
chromatography on silica gel (n-hexane/Et2O).
A
(pin)2 as the boron source, respectively (Sche-
2A(pin)2 as the
CTHUNGTRENNUNG
boron source and [(IMes)CuCl] as the catalyst, a 1,4-hydro-
borated allylborane (9b’) was isolated with high selectively
(Scheme 8e).
Crystallographic data: CCDC-926626 ([(MeIMes)CuCl]), CCDC-926627
([(ClIPrCPh )CuCl]), CCDC-926628 (6b), and CCDC-926629 (7c). contain
3
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Chem. Eur. J. 2013, 19, 7125 – 7132
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7131

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Acknowledgements
This work was supported by Grants-in-Aid for Scientific Research on In-
novative Areas (“Organic Synthesis Based on Reaction Integration” and
“Molecular Activation Directed toward Straightforward Synthesis”) from
MEXT, Japan, as well as by the Mitsubishi Foundation. K.S. is grateful
for the award of a Research Fellowship for Young Scientists from the
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Received: February 4, 2013
Published online: April 9, 2013
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Chem. Eur. J. 2013, 19, 7125 – 7132