Copper(i)-catalyzed regioselective monoborylation of 1,3-enynes with an internal triple bond: Selective synthesis of 1,3-dienylboronates and 3-alkynylboronates

Article abstract of DOI:10.1002/anie.201007182

Hooray for hydroboration! The products afforded by the title reaction depend on the substitution pattern on the double bond moiety of 1,3-enyne substrates (see scheme). These types of products, either 1,3-dienylboronates or 3-alkynylboronates, are difficult to obtain by other methods. Interestingly, ligand-controlled borylation was observed with high selectivity in some cases. pin=pinacolato, THF=tetrahydrofuran. Copyright

Full text of DOI:10.1002/anie.201007182

Communications
DOI: 10.1002/anie.201007182
Catalytic Hydroboration
Copper(I)-Catalyzed Regioselective Monoborylation of 1,3-Enynes
with an Internal Triple Bond: Selective Synthesis of
1,3-Dienylboronates and 3-Alkynylboronates**
Yusuke Sasaki, Yuko Horita, Chongmin Zhong, Masaya Sawamura, and Hajime Ito*
Organoboron compounds are useful reagents and the hydro-
boration of simple alkenes or alkynes is one of the most
efficient and straightforward methods to access a variety of
organoboron compounds.[1,2] However, for the preparation of
polyconjugated hydrocarbon compounds, there are limited
types of regio- and stereoselective hydroboration reac-
tions.[3–10] This type of transformation is still challenging in
both transition-metal-catalyzed and noncatalyzed hydrobora-
tion.
Hydroboration of 1,3-enyne compounds, for example,
gives limited types of the organoboron products.[4–9] This
transformation can theoretically produce six possible product
isomers (Scheme 1, types I–VI). However, there has been no
clear-cut report on the selective 1,2-hydroboration of 1,3-
enynes (types I and II).[4,5] Allenylboron compounds can be
obtained through palladium-catalyzed 1,4-hydroboration of
1,3-enynes (type III),[6] whereas the type IV product has not
been reported. The 3,4-hydroboration of 1,3-enynes is the
most common reaction pattern; the type V product, which is
the 1,3-dienylboron compound, is a useful synthetic precur-
sor. However, in this reaction the type VI product is detected
as a minor product. Currently, the successful reaction patterns
are limited to the production of type III and V products. In
addition, most examples for type III and V products require
the substrate structure to have a terminal alkyne moiety.[7–9]
Although hydroboration is a general and widely used
synthetic procedure, the application of hydroboration to 1,3-
enynes, especially those with an internal alkyne moiety,
remains undeveloped.
Very recently, our research group reported the copper(I)-
catalyzed, regio- and enantioselective monoborylation of 1,3-
diene compounds.[10] This process can be extended for the
development of novel regioselective borylation reactions of
other conjugated systems; namely, where conventional hydro-
boration can not be used effectively.[11,12] Herein, we report a
copper(I)-catalyzed, highly regioselective monoborylation of
1,3-enyne compounds. In this catalysis reaction, either 3-
alkynylboronates or 1,3-dienylboronates were obtained with
high regioselectivity (Scheme 2). Substrates with a terminal
double bond exclusively afforded unprecedented type I
products (Scheme 2a), whereas highly substituted substrates
gave type V product with high regioselectivity—even when
the substrates have an internal alkyne moiety (Scheme 2c).
Interestingly, in the reaction of 1,3-enynes that have moderate
Scheme 1. Possible product isomers in the hydroboration of 1,3-enyne
compounds.
[*] Y. Sasaki, Y. Horita, Dr. C. Zhong, Prof. Dr. M. Sawamura
Department of Chemistry, Faculty of Science
Hokkaido University (Japan)
Prof. Dr. H. Ito
Graduate School of Engineering, Hokkaido University
Sapporo 060-8628 (Japan)
and
PRESTO (JST)
Honcho, Kawaguchi, Saitama 332-0012 (Japan)
Fax: (+81)11-706-6561
E-mail: hajito@eng.hokudai.ac.jp
[**] This study was supported by a Grant-in-Aid for Scientific Research
(B) from MEXT, and by PRESTO from JST. Y.S. thanks the JSPS for a
fellowship.
Scheme 2. Regioselective copper(I)-catalyzed monoborylation of 1,3-
enyne compounds. THF=tetrahydrofuran, xantphos=4,5-bis(diphenyl-
phosphanyl)-9,9-dimethylxanthene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007182.
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Table 2: Monoborylation of 1,3-enyne compounds with 1- or 2-substitu-
steric demand around the double bond (Scheme 2b), ligand-
controlled regioselective borylation was observed. The syn-
thetic utility of the reaction products was further demon-
strated through the Suzuki–Miyaura cross-coupling and the
Diels–Alder reaction. In addition, a preliminary result for the
asymmetric 1,2-monoborylation of 1,3-enyne (84% ee) is also
reported.
The regioselectivity in the copper(I)-catalyzed monobor-
ylation of 1,3-enyne compounds containing several substitu-
tion patterns was investigated. We initially studied the 1,3-
enyne with a terminal double bond and an internal triple bond
such as 1-octen-3-yne (1a). The reaction was initiated by the
addition of 2.0 equivalents of methanol to the mixture of 1a,
1.5 equivalents of bis(pinacolato)diboron 2, and 5 mol% of
Cu(OtBu)/xantphos in THF at room temperature (Table 1,
tion.[a]
Entry Substrate
Ligand
Product
Yield 3/5[c]
[%][b]
1[d]
2[e]
xantphos
PPh3
65
64
>99:1
7:93
3
4
xantphos
PPh3
58
66
>99:1
1:>99
Table 1: Monoborylation of 1,3-enyne compounds bearing a terminal
double bond.[a]
5[f]
6[f]
xantphos 3d
61
80
>99:1
1:>99
PPh3
5e
7[e]
8[e]
xanphos
PPh3
52
65
92:8
Entry Substrate
Ligand
Product
Yield 3/4[c]
[%][b]
1:>99
1
xantphos
dppe
dppbz
PPh3
87
61
60
80
0
>95:5
2[d]
3[d]
4[d]
5
>95:5
>95:5
>95:5
–
[a] Reaction conditions: 1 (0.25 mmol), 2 (0.375 mmol), Cu(OtBu)
(5 mol%, 0.0125 mmol), ligand (5 mol%, 0.0125 mmol), THF
(0.25 mL), and methanol (0.5 mmol). [b] Yield of isolated product.
[c] Determined by 1H NMR or GC analysis of the crude reaction mixture.
[d] Reaction time was 2.8 h. [e] 1.1 equivalents of diboron 2 was used.
[f] 5 mol% of CuCl and 50 mol% of K(OtBu) were used instead of
Cu(OtBu).
none
6
7
xantphos
PPh3
88
89
>95:5
>95:5
8
9
xantphos
PPh3
83
79
94:6
>95:5
or 2-monosubstitution around the double bond afforded
either 3-alkynylboronate 3 or 1,3-dienylboronate 5 selectively
(Table 2).[14] With the xantphos ligand, the reaction of 1-
substituted 1,3-enyne 1d afforded the corresponding 3-
alkynylboronate 3d with excellent regioselectivity (3/5 >
99:1; Table 2, entry 1). In contrast, the reaction with PPh3
gave 1,3-dienylboronate 5d with high regioselectivity (3/5 =
7:93; Table 2, entry 2). The reaction with an (E)-alkene
substrate also gave either 3-alkynylboronate 3d and 1,3-
dienylboronate 5e, respectively, thus demonstrating that the
alkene geometry (E or Z) did not affect the selectivity
outcome (Table 2, entries 3 and 4). This reaction was also
performed with the easily available CuCl/K(OtBu) precata-
lyst instead of Cu(OtBu) (Table 2, entries 5 and 6). This same
type of product profile was also observed in the reaction with
2-substituted 1,3-enyne 1 f (Table 2, entries 7 and 8).
We further tested the reaction of 1,3-enyne compounds
with di- or trisubstitution around the double bond (Table 3).
1-Propynylcyclohexene 1g was converted into the corre-
sponding 1,3-dienylboronate 5g in quantitative yield and with
high regioselectivity (5/6 > 95:5; Table 3, entry 1). Other
possible regioisomers, such as 3-alkynylboronate or 1,2-
dienylboronate were not detected. Using other bidentate
ligands afforded 5g in high regioselectivity; however, the
yields were lower when compared with the reaction using
[a] Reaction conditions: 1 (0.25 mmol), 2 (0.275–0.375 mmol), Cu-
(OtBu) (5 mol%, 0.0125 mmol), ligand (5 mol%, 0.0125 mmol), THF
(0.25 mL), and methanol (0.5 mmol). [b] Yield of isolated product.
[c] Determined by 1H NMR or GC analysis of the crude reaction mixture.
[d] Yield based on 1H NMR analysis of the crude reaction mixture.
dppbz=1,2-bis(diphenylphosphonio)benzene, dppe=l,2-bis(diphenyl-
phosphino)ethane, pin=pinacolato.
entry 1). The reaction was complete within 2 hours and gave
3-alkynylboronate 3a in 87% yield with high regioselectivity
(3/4 > 95:5). This reaction is the first example of the type I
hydroboration of 1,3-enynes. Reactions using other diphos-
phine ligands such as dppe and dppbz resulted in lower yields
(60–61%; Table 1, entries 2 and 3). The reaction with PPh3
also afforded 3a in high yield (80%; Table 1, entry 4). In the
absence of the ligand, the reaction did not proceed (Table 1,
entry 5).[13] The reaction of 1,3-enynes with cHex or Ph groups
at the 4-position proceeded to furnish the corresponding 3-
alkynylboronates 3 selectively (79–89%, 3/4 = 94:6 to > 95:5;
Table 1, entries 6–9).
We next investigated the reaction of 1,3-enyne com-
pounds with other substituent patterns. Interestingly, by
changing the ligand the reaction with 1,3-enynes bearing 1-
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Table 3: Monoborylation of 1,3-enyne compounds bearing 1,1-di-, 1,2-di-,
and 1,1,2-trisubstitution.[a]
ceeded with high regioselectivity but the yield was lower
even in the presence of 10 mol% of the catalyst (16%;
Table 3, entry 12). This outcome was probably a result of
the large steric hindrance around the double bond.
Interestingly, the reaction of a 1,3-enyne with a phenyl
group at the 4-position (1m) predominantly gave the
type VI regioisomer (6m) with good selectivity (5/6 =
15:85; Table 3, entry 13).
Entry Substrate
Ligand
Product
Yield 5/6[c]
[%][b]
The usefulness of 1,3-dieneylboronate 5g was also
demonstrated (Scheme 3). Palladium-catalyzed cross-cou-
1[d,g]
2[d,g]
3[d,g]
4[d,g]
5[d]
xantphos
dppe
dppbz
PPh3
none
xantphos
quant. >95:5
77
85
97
0
>95:5
>95:5
>95:5
–
6[e]
89
>95:5
7
8
9
PPh3
PPh3
PPh3
76
58
79
92:8
89:11
Scheme 3. Derivatization of 1,3-dienylboronate (5g). Reaction con-
ditions: path a) (E)- or (Z)-1-bromopropene, [Pd(PPh3)4]/SPhos
(cat.), aq NaOH (2 m), 60 8C, 2.5–3 h; path b) 1-iodohexyne, [Pd-
(PPh3)4] (5 mol%), aq NaOH (2 m), 80 8C, 4 h; path c) N-phenyl-
mareimide, 120 8C, 3 days. SPhos=2-dicyclohexylphosphino-2’,6’-
dimethoxybiphenyl.
>95:5
10
11
xantphos
PPh3
92
93
>95:5
>95:5
pling of 5g with (E)-, (Z)-1-bromopropene, and 1-iodohex-
yne gave the corresponding trienes and dienyne in high
yields (7a: 92%, 7b: 89%, and 8: 83%; Scheme 3,
path a,b). Furthermore, the Diels–Alder reaction[15] with
N-phenylmaleimide afforded the unprecedented cyclic
allylboronate 9, which has four contiguous stereocenters,
including one quaternary carbon atom, with high diaste-
reoselectivity (69%, endo/exo = 92:8; Scheme 2, path c).
The reaction for the asymmetric synthesis of enantioen-
riched 3-alkynylboronate with a chiral copper(I) catalyst
(5 mol% of Cu(OtBu)/(R,R)-quinoxP*) resulted in a good
ee value with excellent regioselectivity (84% ee, 3/5 = 95:5;
Scheme 4); however, the yield was moderate (34%)
12[f,g]
xantphos
xantphos
16
89
95:5
13
15:85
[a] Reaction conditions: 1 (0.25 mmol), 2 (0.275–0.5 mmol), Cu(OtBu)
(5 mol%, 0.0125 mmol), ligand (5 mol%, 0.0125 mmol), THF (0.25 mL),
and methanol (0.5 mmol). [b] Yield of isolated product. [c] Determined by
1H NMR or GC analysis of the crude reaction mixture. [d] Reaction was
carried out on a 0.5 mmol scale. [e] 5 mol% of CuCl and 50 mol% of
K(OtBu) were used instead of 5 mol% of Cu(OtBu). [f] Catalyst loading was
10 mol%. [g] Yield based on 1H NMR analysis of the crude reaction mixture.
xantphos (77–85%; Table 3, entries 2 and 3). The reaction
with PPh3 gave excellent yield and regioselectivity (97%,
5/6 > 95:5; Table 3, entry 4), but no reaction was observed in
the absence of the ligand (Table 3, entry 5). The CuCl/
K(OtBu) precatalyst was also operative without significant
loss of regioselectivity and yield (89%, 5/6 > 95:5; Table 3,
entry 6). The reaction of a 1,3-enyne bearing 1,1-disubstitu-
tion (1h) afforded the corresponding 1,3-dienylboronate 5h
with high regioselectivity (5/6 = 92:8; Table 3, entry 7). This
reaction was applicable to the 1,3-enynes with an ether or a
benzyloxy functionality (Table 3, entries 8 and 9). Reactions
with 1,3-enynes bearing a terminal alkyne moiety also
afforded the corresponding 1,3-dienylboronate 5k with high
selectivities (5/6 > 95:5; Table 3, entries 10 and 11). The
reaction of a 1,3-enyne bearing 1,1,2-trisubstitution pro-
Scheme 4. Asymmetric catalytic monoborylation of 1,3-enyne 1d.
because of the formation of multiborylation by-products.
This reaction is the first example of an asymmetric synthesis
of 3-alkynylboronates.
A tentative explanation of the regioselective outcome of
the hydroboration reaction is presented in Scheme 5. Accord-
ing to the mechanistic investigation reported by Marder, Lin,
and co-workers,[16] the interaction between the HOMO of the
borylcopper intermediate and the electrophile LUMO is
decisive in the regioselectivity of the borylcopper addition to
unsaturated bonds. Orbital population analysis showed that
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[2] For reviews of catalytic hydroboration of alkenes, see: a) C. M.
Vogels, S. A. Westcott, Curr. Org. Chem. 2005, 9, 687; b) K.
Burgess, M. J. Ohlmeyer, Chem. Rev. 1991, 91, 1179; for catalytic
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Rubin, V. Gevorgyan, J. Am. Chem. Soc. 2003, 125, 7198; d) C.
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2008, 130, 3734.
[3] For non-asymmetric catalytic hydroboration of 1,3-dienes, see:
a) M. Satoh, Y. Nomoto, N. Miyaura, A. Suzuki, Tetrahedron
Lett. 1989, 30, 3789; b) M. Zaidlewicz, J. Meller, Tetrahedron
Lett. 1997, 38, 7279; c) J. Y. Wu, B. Moreau, T. Ritter, J. Am.
Chem. Soc. 2009, 131, 12915; d) D. Nakagawa, M. Miyashita, K.
Tanino, Tetrahedron Lett. 2010, 51, 2771.
[4] a) H. Brown, A. Moerikofer, J. Am. Chem. Soc. 1963, 85, 2063;
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3395; c) G. Zweifel, N. Polston, J. Am. Chem. Soc. 1970, 92, 4068.
[5] Selective hydroboration of double bonds in the presence of triple
bonds with 9-BBN in nonconjugated systems has been reported,
see: C. A. Brown, R. A. Coleman, J. Org. Chem. 1979, 44, 2328.
[6] a) M. Satoh, Y. Nomoto, N. Miyaura, A. Suzuki, Tetrahedron
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Organometallics 1992, 11, 2732.
[7] Hydroboration of 1,3-enynes with a terminal alkyne moiety
usually affords 1,3-dienylboronate as the major product. For
selected papers, see: a) L. Garnier, B. Plunian, J. Mortier, M.
Vaultier, Tetrahedron Lett. 1996, 37, 6699; b) C. E. Tucker, J.
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Tetrahedron Lett. 1988, 29, 3541; f) M. Tortosa, N. A. Yakelis,
W. R. Roush, J. Am. Chem. Soc. 2008, 130, 2722; for Rh catalysis,
see: g) S. Lꢀpez, J. Montenegro, C. Saꢁ, J. Org. Chem. 2007, 72,
9572; for Zr catalysis, see: h) N. PraveenGanesh, S. dꢂHondt,
P. Y. Chavant, J. Org. Chem. 2007, 72, 4510; i) K. C. Nicolaou,
A. L. Nold, R. R. Milburn, C. S. Schindler, K. P. Cole, J.
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[8] For regioselective hydroboration of 1,3-enynes bearing an
internal alkyne with catecholborane have been previously
reported. However, these reactions are only applicable for 1,3-
enynes bearing 1,2-disubstitution at the olefin double bond, see:
a) S. J. Eade, M. W. Walter, C. Byrne, B. Odell, R. Rodriguez,
J. E. Baldwin, R. M. Adlington, J. E. Moses, J. Org. Chem. 2008,
73, 4830; b) I. Paterson, M. V. Perkins, J. Am. Chem. Soc. 1993,
115, 1608; for regioselective and stereoselective hydroboration
of 1-haloenynes with thexylalkylborane, see: c) G. Zweifel, T.
Shoup, Synthesis 1988, 130.
[9] To the best of our knowledge, there has only been one report for
transition-metal-catalyzed hydroboration (Ni catalyst) of 1,3-
enynes with an internal alkyne, see: a) M. Zaidlewicz, J. Meller,
J. Collect. Czech. Chem. C 1999, 64, 1049. Our attempted
reaction of noncatalyzed and rhodium-catalyzed hydroboration
of (E)-non-2-en-4-yne (1e) gave a mixture of 1,3- and 1,2-
dienylboronate with low selectivity.
Scheme 5. a) DFT population analysis and isosurface of the LUMO of
1-buten-3-yne (B3LYP/6-31G(d,p), Gaussian 09W). b,c) Proposed
explanation for product selectivities.
the 2p orbitals of the alkene carbon atoms (Scheme 5a; C1:
0.43; C2: 0.25 for 1-buten-3-yne) have a significantly larger
contribution than those of the alkyne carbon atoms (C3: 0.07;
C4: 0.23).
Thus, it is reasonable that the borylcupration takes place
at the olefin double bond when steric perturbation of the
substrate is not significant (type I; Scheme 5b). In the case of
highly substituted 1,3-enynes, steric hindrance around the
double bond would render borylcopper addition to the
electronically less favorable alkyne moiety to produce 1,3-
dienylboronates (type V; Scheme 5c). However, the selectiv-
ity outcome observed for moderately substituted 1,3-enynes
were counterintuitive along this line. Further investigation is
required to resolve this point.
In summary, we have developed copper(I)-catalyzed
regioselective monoborylation of 1,3-enyne compounds.
This catalysis includes the first examples for type I and VI
hydroborations, and efficient type V hydroboration of 1,3-
enynes with an internal triple bond. The regioisomeric
preference (1,3-dienylboronates or 3-alkynylboronates) was
primarily determined by the substrate structure, whereas the
regioselectivity for moderately substituted 1,3-enynes was
controlled by the ligand of the catalyst. It is noted that these
selectivity features are different from those for 1,3-dienes in
our previous study.[10] This copper(I)-catalyzed selective
monoborylation is a complementary method to conventional
hydroboration reactions for 1,3-enynes.
Received: November 16, 2010
Published online: February 24, 2011
Keywords: asymmetric catalysis · boron · copper · enynes ·
.
hydroboration
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