Boron trihalide mediated substitution of hydroxyl groups with alkenyl, alkynyl, and allyl moieties

Article abstract of DOI:10.1055/s-2007-990816

The coupling of alcohols with alkenyl- and alkynylboron dihalides with high olefin stereoselectivity is described. The reaction provides a facile route to internal acetylenes. Notably, the allylation of propargylic alcohols mediated by boron trichloride p

Full text of DOI:10.1055/s-2007-990816

PRACTICAL SYNTHETIC PROCEDURES
325
Boron Trihalide Mediated Substitution of Hydroxyl Groups with Alkenyl,
Alkynyl, and Allyl Moieties
Boron
H
alides in S
e
yntheses orge Kabalka,* Scott Borella, Min-Liang Yao
Departments of Chemistry and Radiology, The University of Tennessee, Knoxville,
TN 37996-1600, USA
Fax +1(865)9742997; E-mail: kabalka@utk.edu
PSP
Received 1 June 2007; revised 26 June 2007
No110
Abstract: The coupling of alcohols with alkenyl- and alkynylboron dihalides with high olefin stereoselectivity is described. The reaction
provides a facile route to internal acetylenes. Notably, the allylation of propargylic alcohols mediated by boron trichloride proceeds smooth-
ly at room temperature and gives excellent regioselectivity.
Key words: coupling, boron halide, substitution, Lewis acid
Ph
Procedure 1
Cl
BCl2
Cl
OH
Ph
Ph
n-BuLi
Ph
Ph
Ph
CH2Cl2
CH2Cl2, r.t.
2a (83% yield)
Procedure 2
Cl
Cl
Ph
BCl2
CH2Cl2, r.t.
n-BuLi
CH2Cl2
Ph
Ph
OH
Ph
3a (84% yield)
Procedure 3
Cl
Cl
TMS
BCl3, CH2Cl2, r.t.
n-BuLi
CH2Cl2
OH
4a (74% yield)
Scheme 1
Introduction
for direct carbon–carbon bond coupling using these alco-
hols generally require transition-metal catalysts that in-
Allylic, benzylic and propargylic alcohols constitute ideal clude palladium,1 ruthenium,2 rhenium,3 copper,4 gold,5
starting materials in synthesis due to their structural diver- and rhodium.6 These reactions typically require stringent
sity and ready availability. Classic methods for substitu- reaction conditions (high reaction temperatures and
tion of the hydroxyl group in these types of alcohols are sealed tubes) and prolonged periods of time. Recently, our
centered on transformation of the hydroxyl moiety into a research group demonstrated the direct cross-coupling of
more reactive leaving group (Br, I, OTf, etc.). Strategies allylic alcohols with aryl- and vinylboronic acids in ionic
liquids.6 Main group Lewis acids such as bismuth7 and
indium8 halides have also been successfully employed for
nucleophilic allylation in benzylic and propargylic alco-
hols.
SYNTHESIS 2008, No. 2, pp 0325–0329
Advanced online publication: 25.09.2007
DOI: 10.1055/s-2007-990816; Art ID: Z14007SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
0
7

326
G. Kabalka et al.
PRACTICAL SYNTHETIC PROCEDURES
X
R
BX2
Our research group has focused on developing boron tri-
halide (BCl3 and BBr3) mediated carbon–carbon bond-
forming reactions. We have described the coupling of al-
lylic and benzylic alcohols with alkenylboron dihalides9–11
and with alkynylboron dihalides12 (Procedures 1 and 2,
Scheme 1). Based on the observation that the oxyboron
halide moiety can function as a leaving group to generate
an intermediate carbocation (Procedures 1 and 2,
Scheme 1), we turned our attention to an allylation reac-
tion.13 The representative reaction is shown in Procedure
3 (Scheme 1). This method complements classic methods
for generating carbocations from alcohols since it pro-
vides a Brønsted acid free route to carbocations from al-
cohol l.
BX3, CH2Cl2
X = Cl, Br
R
H
n-BuLi
BCl3
R
BCl2
CH2Cl2
Scheme 2 Synthesis of organoboron dihalides
lective vinylboron dihalides and alkynylboron dichlorides
were generated from terminal alkynes under mild reaction
conditions (Scheme 2).
Allylic alcohols can couple readily with stereodefined
alkenylboron dichlorides and alkenylboron dibromides
derived from aryl alkynes; it has been observed that alke-
nylboron dibromides (but not alkenylboron dichlorides)
derived from aliphatic alkynes undergo the coupling reac-
tions (Table 1, entry 4). NMR analysis also revealed that
unsymmetrical allylic alcohols produce regiochemically
pure products; this circumvents a drawback of transition-
metal-catalyzed reactions in which isomeric products are
often formed via p-allyl intermediates.15 We also applied
this methodology to benzylic alcohols (Table 1, entries 6–
10) and propargylic alcohols (Table 1, entries 11–13). In
all cases the reactions produce the Z-stereoisomers as the
major product, thus the configuration of the halovinyl
group is retained. The Z-stereochemistry is supported by
the X-ray crystal structure analysis of 2h.
Scope and Limitations
The exploration of the substitution of hydroxyl groups
with alkenyl and alkynyl moieties using organoboron di-
halides is a consequence of our previous discovery of a
boron trihalide mediated alkyne–aldehyde coupling reac-
tion.14 Preliminary mechanistic studies of this coupling re-
action indicated that it involved a halovinyl group
migration from boron to carbon. To validate the vinyl
group migration, we designed the reaction shown in Pro-
cedure 1 (Scheme 1) and finally extended the reaction to
alkynylboron dichlorides. The highly regio- and stereose-
Table 1 Representative Examples of Alkenylation of Alcohols 1a
X
R1
X
BX2
R1
R2
n-BuLi
R
OH
R2
R
CH2Cl2
1
2
Entry
1
R1
R2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Ph
R
X
Product
2a
Yield (%)b
(E)-PhCH=CH
(E)-PhCH=CH
Ph
Cl
Cl
Cl
Br
Br
Cl
Cl
Cl
Br
Br
Cl
Cl
Br
83
78
66
62
68
79
69
58
64
44
90
60
85
2
3-FC6H4
2b
2c
3
(E,E)-Ph(CH=CH)2
(E)-PhCH=CH
(E)-PhCH=CH
Ph
Ph
4
n-Bu
2d
2e
5
4-MeC6H4
6
Ph
2f
7
4-ClC6H4
Ph
Ph
2g
8
4-MeC6H4
2h
2i
9
Ph
Ph
10
11
12
13
4-NO2C6H4
PhC≡C
4-MeC6H4
2j
4-ClC6H4
Ph
Ph
Ph
Ph
2k
2l
n-BuC≡C
PhC≡C
4-FC6H4
2m
a Stereodefined vinylboron dihalides were prepared in situ at 0 °C and used directly for the coupling reaction (see experimental section for de-
tails).
b Isolated yields based on the precursor alkynes.
Synthesis 2008, No. 2, 325–329 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Boron Halides in Syntheses
327
Table 2 Representative Examples of Alkynylation of Alcohols 1a
R1
R1
n-BuLi
R
BCl2
R
OH
CH2Cl2
R2
R2
1
3
Entry
R1
Ph
R2
R
Product
3a
Yield (%)b
1
2
3
4
5
6
7
8
4-ClC6H4
4-MeOC6H4
Ph
Ph
84
77
68
57
86
71
83
69
4-MeOC6H4
Ph
4-MeC6H4
Ph
3b
3c
4-FC6H4
4-FC6H4
Ph
4-MeOC6H4
2-FC6H4
Ph
3d
(E)-PhCH=CH
3e
(E,E)-Ph(CH=CH)2
PhC≡C
Ph
3f
4-MeC6H4
4-ClC6H4
4-MeC6H4
Ph
3g
PhC≡C
3h
a Alkynylboron dichlorides were prepared in situ at 0 °C and used directly for the coupling reaction (see experimental section for details).
b Isolated yields based on the precursor alkynes.
Although transition-metal-catalyzed and acid-mediated The generation of cations from alkoxides using alkenyl/
methods can be used to replace hydroxyl groups with aryl, alkynylboron dihalides, through the intermediacy of
allyl and alkenyl moieties,16 the replacement of hydroxyl alkoxyboron monohalides, led us to investigate the use of
groups with stereodefined halovinyl moieties had not boron trihalides as reagents in the new coupling reactions.
been reported. The direct substitution of the hydroxyl We discovered that the allylation of alcohols using allyl-
group in alcohol 1 with an alkenyl moiety using alkenyl- silane proceeded smoothly in the presence of boron
boron dihalides can be viewed as a formal transition- trichloride at room temperature (Table 3). No reaction oc-
metal-free Suzuki reaction.17 In addition, the reaction curred between allyltrimethylsilane and boron trichloride,
products provide a potential route to trisubstituted olefins. which precludes the formation of an intermediate allylbo-
ron dichloride. The generation of a cation intermediate
Encouraged by the successful replacement of hydroxyl
from an alkoxide by boron trihalide was further supported
groups with halovinyl moieties using halovinylboron di-
by the coupling of lithium alkoxide with alkynes.13
halides, we postulated that the replacement of hydroxyl
groups with alkynyl moieties using alkynylboron diha-
Table 3 Representative Examples of Allylation of Alcoholsa
lides might also occur (Procedure 2, Scheme 1). This
R
would provide a novel route to internal acetylene deriva-
tives, specifically, secondary alkylacetylenes.
R1
R
R1
R2
TMS
n-BuLi
R2
OH
CH2Cl2
BCl3
Traditional preparations of secondary alkylacetylene de-
rivatives rely on methods such as substitution of second-
ary halides or elimination reactions involving 1,2-
dihalides. Since these reactions often require the use of
strong bases or high temperatures, the desired products
are often contaminated with allene by-products that are
difficult to remove. Although there have been advances in
Sonogashira chemistry,18 the preparation of secondary
acetylenes is still a synthetic challenge due to their ten-
dency to isomerize to allenes.
1
4
Entry R1
R2
R
Product Yield
(%)b
1
2
3
4
5
6
7
n-BuC≡C
4-ClC6H4
H
H
4a
4b
4c
4d
4e
4f
74
53
65
72
96
90
78
3-FC6H4C≡C
PhC≡C
3,4,5-(MeO)3C6H2
4-ClC6H4
Me
Me
H
4-MeC6H4C≡C 4-ClC6H4
We examined the feasibility of coupling alcohols with
alkynylboron dichlorides. The versatility of the reaction is
clearly illustrated by its applicability to allylic, benzylic
and propargylic, alcohols (Table 2). Similar to the cou-
pling reactions with alkenylboron dihalides, the reactions
of alkynylboron dichlorides with primary alcohols do not
work. Most likely, the reaction proceeds via a cationic
mechanism.
Ph
Ph
4-ClC6H4
Ph
H
(E)-PhCH=CH Ph
H
4g
a See experimental section for details.
b Isolated yields based on the starting alcohols.
Synthesis 2008, No. 2, 325–329 © Thieme Stuttgart · New York

328
G. Kabalka et al.
PRACTICAL SYNTHETIC PROCEDURES
Anal. Calcd for C21H17Cl: C, 82.75; H, 5.62. Found: C, 82.57; H,
5.65.
In summary, we have discovered that hydroxyl groups,
via their alkoxides (RO–), can be readily replaced by ste-
reodefined halovinyl, alkynyl and allyl moieties at room
temperature. These new reactions obviate many difficul-
ties associated with transition-metal-catalyzed syntheses,
especially those involving Csp2–Csp3, Csp–Csp3 carbon–
carbon bond formation. We believe that there are two fac-
tors leading to weakening of the C–O bond and the subse-
quent generation of a carbocation: steric hindrance in the
unstable complexes lengthens the C–O bond, and the elec-
tronegativity of chlorine strengthens the B–O bond.
2k
1H NMR (CDCl3): d = 7.29–7.62 (m, 14 H), 6.27 (d, J = 9.3 Hz, 1
H), 5.30 (d, J = 9.3 Hz, 1 H).
13C NMR (CDCl3): d = 137.9, 137.2, 133.6, 133.1, 131.7, 129.0,
128.8, 128.7, 128.6, 128.4, 128.3, 126.7, 126.6, 123.0, 87.6, 84.2,
37.4.
Anal. Calcd for C23H16Cl2: C, 76.04; H, 4.44. Found: C, 75.88; H,
4.22.
Coupling of Alcohols 1 with Alkynylboron Dihalides (Proce-
dure 2)
All chemicals were used as received. The glassware was oven-dried
for a period of 24 h prior to use. CH2Cl2 was distilled from an ap-
propriate drying agent (CaH2) prior to use. BCl3 (1.0 M CH2Cl2 so-
lution) and BBr3 were used as received. Reactions were
magnetically stirred and monitored by TLC using 254 nm UV light
or staining with a 50% solution of phosphomolybdic acid in EtOH.
Products were purified by flash chromatography using silica gel
(230–400 mesh, 60 Å). 1H NMR and 13C NMR spectra were record-
Under N2, 1-phenyl-1-(4-chlorophenyl)methanol (349 mg, 1.6
mmol) in anhyd CH2Cl2 (8 mL) was treated with n-BuLi (1.0 mL of
a 1.6 M solution in hexanes) at 0 °C and then warmed to r.t. After
stirring at r.t. for 30 min, the solution was transferred to the 2-phe-
nylaceteyleneboron dichloride solution and the mixture allowed to
stir overnight. H2O (20 mL) was added to quench the reaction. The
mixture was extracted with EtOAc (3 × 12 mL), the combined or-
ganic layers were dried (MgSO4), and the solvent was removed in
vacuo. The product was purified by silica gel column chromatogra-
phy using hexane as eluent to give 3a (380 mg, 84%). Typical ana-
lytic data are provided for 3a, 3e, and 3g.
1
ed at 250.13 and 62.89 MHz, respectively. Chemical shifts for H
NMR and 13C NMR spectra were referenced to TMS and measured
with respect to the residual protons in the deuterated solvents. Mi-
croanalysis was performed by Atlantic Microlab, Inc. Norcross,
Georgia.
3a
1H NMR (CDCl3): d = 7.21–7.47 (m, 14 H), 5.14 (s, 1 H).
Alkenylboron Dihalides; General Procedure
13C NMR (CDCl3): d = 141.2, 140.3, 132.7, 131.6, 129.2, 128.7,
128.2, 128.1, 127.8, 127.1, 123.2, 89.6, 85.2, 43.1.
Boron trihalide (1.5 mmol, 1.5 mL of a 1.0 M solution in CH2Cl2),
alkyne (1.5 mmol), and anhyd CH2Cl2 (8 mL) were combined in a
50 mL flask at 0 °C and stirred for 1 h under N2.
Anal. Calcd for C21H15Cl: C, 83.30; H, 4.99. Found: C, 83.77; H,
5.04.
Alkynylboron Dichlorides; General Procedure
A solution of alkyne (1.5 mmol) in anhyd hexane (8 mL) was treat-
ed with n-BuLi (1.0 mL of a 1.6 M solution in hexanes) at 0 °C un-
der N2. After stirring at r.t. for 30 min, BCl3 (1.5 mmol, 1.5 mL of a
1.0 M solution in CH2Cl2) was added to the mixture at 0 °C.
3e
1H NMR (CDCl3): d = 7.19–7.47 (m, 13 H), 6.89–6.97 (m, 1 H),
6.71 (d, J = 15.5 Hz, 1 H), 6.30 (dd, J = 15.4, 6.19 Hz, 1 H), 4.71
(d, J = 6.19 Hz, 1 H).
13C NMR (CDCl3): d = 164.3, 160.4, 141.8, 139.9, 136.7, 130.6,
129.8, 129.2, 128.5, 127.6, 126.5, 125.3, 125.2, 118.7, 118.3, 115.5,
115.0, 89.9, 84.2, 41.1.
Coupling of Alcohols 1 with Alkenylboron Dihalides (Proce-
dure 1)
Under N2, 1,3-diphenylprop-2-en-1-ol (336 mg, 1.6 mmol) in anhyd
CH2Cl2 (8 mL) was treated with n-BuLi (1.0 mL of a 1.6 M solution
in hexanes) at 0 °C and stirred at r.t. for 1 h. The solution was then
transferred to the (Z)-2-chloro-2-phenylvinylboron dihalide solu-
tion and allowed to stir at r.t. overnight. H2O (20 mL) was added,
the mixture was extracted with EtOAc (3 × 12 mL), and the com-
bined organic layers were dried (MgSO4). The solvents were re-
moved under reduced pressure and the crude product was purified
by silica gel column chromatography using hexane as eluent to give
2a (410 mg, 83%). Typical analytic data are provided for 2a, 2f, and
2k.
Anal. Calcd for C23H17F: C, 88.43; H, 5.49. Found: C, 88.65; H,
5.81.
3g
1H NMR (CDCl3): d = 7.16–7.56 (m, 14 H), 5.15 (s, 1 H), 2.33 (s, 3
H).
13C NMR (CDCl3): d = 137.1, 135.0, 131.7, 129.4, 128.1, 127.2,
123.0, 86.8, 82.6, 29.7, 21.1.
Anal. Calcd for C24H18: C, 94.08; H, 5.92. Found: C, 94.47; H, 5.84.
Boron Trichloride Mediated Coupling of Alcohols 1 with Allyl-
silanes (Procedure 3)
2a
1H NMR (CDCl3): d = 7.57–7.60 (m, 2 H), 7.15–7.37 (m, 13 H),
6.34–6.58 (m, 3 H), 4.94 (dd, J = 9.0, 8.9 Hz, 1 H).
Under N2, a solution of 1-(4-chlorophenyl)-3-butylpropargylic al-
cohol (333 mg, 1.5 mmol) in anhyd CH2Cl2 (10 mL) was treated
with n-BuLi (1.0 mL of a 1.6 M solution in hexanes) at 0 °C and the
mixture warmed to r.t. After stirring at r.t. for 30 min, allyltrimeth-
ylsilane (1.8 mmol) and BCl3 (1.5 mL of a 1.0 M solution in
CH2Cl2) were added sequentially. The mixture was allowed to stir
for 10 h at r.t. H2O (20 mL) was added to quench the reaction. The
crude product was extracted into EtOAc (3 × 12 mL) and the com-
bined organic layers were dried (MgSO4). The solvent was removed
in vacuo and the product purified by silica gel column chromatog-
raphy using hexane as eluent to give 4a (270 mg, 74%). Typical an-
alytic data are provided for 4a, 4e, and 4g.
13C NMR (CDCl3): d = 142.1, 137.9, 137.1, 133.4, 130.9, 130.3,
128.7, 128.5, 128.3, 127.8, 127.4, 126.9, 126.6, 126.3, 48.5.
Anal. Calcd for C23H19Cl: C, 83.50; H, 5.79. Found: C, 83.57; H,
5.65.
2f
1H NMR (CDCl3): d = 7.56–7.59 (m, 2 H), 7.17–7.30 (m, 13 H),
6.59 (d, J = 9.5 Hz, 1 H), 5.42 (d, J = 9.5 Hz, 1 H).
13C NMR (CDCl3): d = 142.9, 137.8, 133.4, 129.5, 128.6, 128.3,
126.6, 50.8.
Synthesis 2008, No. 2, 325–329 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Boron Halides in Syntheses
329
4a
(3) Luzung, M. R.; Toste, F. D. J. Am. Chem. Soc. 2003, 125,
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(4) Wolter, M.; Nordmann, G.; Job, G. E.; Buchwald, S. M. Org.
Lett. 2001, 4, 973.
1H NMR (CDCl3): d = 7.11–7.34 (m, 4 H), 5.73–5.84 (m, 1 H),
4.91–5.06 (m, 2 H), 3.52–3.64 (m, 1 H), 2.23–2.45 (m, 2 H), 2.15–
2.21 (m, 2 H), 1.37–1.55 (m, 4 H), 0.91 (t, J = 6.53 Hz, 3 H).
(5) Georgy, M.; Boucard, V.; Campagne, J.-M. J. Am. Chem.
Soc. 2005, 127, 14180.
(6) Kabalka, G. W.; Dong, G.; Venkataiah, B. Org. Lett. 2003,
5, 893.
(7) (a) De S, K.; Gibbs, R. A. Tetrahedron Lett. 2005, 46, 8345.
(b) Zhan, Z.-p.; Yang, W.-z.; Yang, R.-f.; Yu, J.-l.; Li, J.-p.;
Liu, H.-j. Chem. Commun. 2006, 3352.
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Ed. 2004, 43, 1414.
(9) Kabalka, G. W.; Wu, Z.; Ju, Y. Org. Lett. 2004, 6, 3929.
(10) Kabalka, G. W.; Yao, M.-L.; Borella, S.; Wu, Z. Chem.
Commun. 2005, 2492.
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2005, 7, 2865.
(12) Kabalka, G. W.; Yao, M.-L.; Borella, S. Org. Lett. 2006, 8,
879.
(13) Kabalka, G. W.; Yao, M.-L.; Borella, S. J. Am. Chem. Soc.
2006, 128, 11320.
(14) Kabalka, G. W.; Wu, Z.; Ju, Y. Org. Lett. 2002, 4, 1491.
(15) (a) Uozumi, Y.; Danjo, H.; Hayashi, T. J. Org. Chem. 1999,
64, 3384. (b) Miyaura, N.; Yamada, K.; Suginome, H.;
Suzuki, A. J. Am. Chem. Soc. 1985, 107, 972. (c) Brown,
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(17) For examples of transition-metal-free Suzuki reactions, see:
(a) Leadbeater, N. E.; Marco, M. Angew. Chem. Int. Ed.
2003, 42, 1407. (b) Li, C. J. Angew. Chem. Int. Ed. 2003, 42,
4856. (c) Leadbeater, N. E.; Marco, M. J. Org. Chem. 2003,
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Williams, V. A.; Granados, P.; Singer, R. D. J. Org. Chem.
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13C NMR (CDCl3): d = 140.6, 135.3, 132.3, 128.9, 128.4, 116.9,
84.1, 80.6, 42.9, 37.4, 31.1, 21.9, 18.4, 13.6.
Anal. Calcd for C16H19Cl: C, 77.87; H, 7.76. Found: C, 77.35; H,
7.64.
4e8
1H NMR (CDCl3): d = 7.03–7.26 (m, 9 H), 5.65–5.76 (m, 1 H),
4.91–5.04 (m, 2 H), 3.92–3.98 (m, 1 H), 2.75–2.81 (m, 2 H), 2.25
(s, 3 H).
13C NMR (CDCl3): d = 144.7, 141.5, 136.7, 135.5, 129.1, 128.3,
127.8, 127.7, 126.0, 116.1, 50.8, 40.0, 20.9.
4g19
1H NMR (CDCl3): d = 7.12–7.34 (m, 10 H), 6.34–6.36 (m, 2 H),
5.70–5.80 (m, 1 H), 4.95–5.07 (m, 2 H), 3.49–3.51 (m, 1 H), 2.54–
2.60 (m, 2 H).
13C NMR (CDCl3): d = 137.4, 136.5, 133.4, 129.7, 128.5, 127.7,
127.1, 126.3, 126.2, 116.3, 48.9, 40.2.
Acknowledgment
We wish to thank the Donor of the American Chemical Society
Petroleum Research Foundation (ACS-PRF #41768-AC1) and the
Robert H. Cole Foundation for support of this research.
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Synthesis 2008, No. 2, 325–329 © Thieme Stuttgart · New York