Alkylboronic esters from copper-catalyzed borylation of primary and secondary alkyl halides and pseudohalides

Article abstract of DOI:10.1002/anie.201106299

Easy access: An unprecedented copper-catalyzed cross-coupling reaction of the title compounds with diboron reagents is described (see scheme; Ts=4-toluenesulfonyl). This reaction can be used to prepare both primary and secondary alkylboronic esters having diverse structures and functional groups. The resulting products would be difficult to access by other means. Copyright

Full text of DOI:10.1002/anie.201106299

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Angewandte
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DOI: 10.1002/anie.201106299
Cross-Coupling
Alkylboronic Esters from Copper-Catalyzed Borylation of Primary and
Secondary Alkyl Halides and Pseudohalides**
Chu-Ting Yang, Zhen-Qi Zhang, Hazmi Tajuddin, Chen-Cheng Wu, Jun Liang, Jing-Hui Liu,
Yao Fu, Maria Czyzewska, Patrick G. Steel,* Todd B. Marder,* and Lei Liu*
Dedicated to Christian Bruneau on the occasion of his 60th birthday
Alkylboronic acid derivatives are interesting compounds in
medicinal chemistry (e.g., bortezomib).[1] They are more
often used as synthetic intermediates for transition-metal-
catalyzed cross-coupling of C(sp3) organometallics with
electrophiles.[2] Compared to other C(sp3) organometallics
such as alkylmagnesium, alkylzinc, and alkylindium reagents,
alkylboronic acid derivatives can be readily purified prior to
utilization and have superior shelf stability.[3] Their cross-
coupling reactions also show excellent compatibility with a
wide range of functional groups. Classical methods for the
synthesis of alkylboronic acid derivatives involve the reaction
of alkyllithium or alkylmagnesium reagents with suitable
boron compounds. However, these methods suffer from poor
functional-group tolerance.[4] Thus, recent attention has been
given to the development of transition-metal-catalyzed meth-
ods for the preparation of alkylboronic acid derivatives. Some
important examples include Rh- and Ir-catalyzed hydrobora-
tion of the corresponding alkyl halides and pseudohalides,
thus providing a practical means for the preparation of
alkylboronic esters with diverse skeletons and functional
groups. Moreover, this method expands the concept and
scope of copper-catalyzed cross-coupling reactions[9] in a
fundamental sense.
In 2009, Marder et al. reported that CuI in the presence of
phosphines catalyzes the borylation of aryl halides with di-
boron reagents to generate arylboronic esters (Scheme 1a).[10]
More recently, Liu et al. found that under similar reaction
tion of alkenes,[5] Ir-, Rh-, Ru-, and Re-catalyzed C H
À
activation/borylation of alkanes,[6] and Pt-, Pd-, Ni-, Rh-,
and Cu-catalyzed b-boration of unsaturated carbonyl com-
pounds.[7] Herein, we describe a new and more general
method for the synthesis of unactivated primary and secon-
dary alkylboronic esters[8] through copper-catalyzed boryla-
[*] C.-T. Yang, Z.-Q. Zhang, C.-C. Wu, J. Liang, J.-H. Liu, Y. Fu,
Prof. Dr. L. Liu
Joint Laboratory of Green Synthetic Chemistry, Department of
Chemistry, University of Science and Technology of China
Hefei 230026 (China)
and
Department of Chemistry, Tsinghua University
Beijing 100084 (China)
E-mail: lliu@mail.tsinghua.edu.cn
Scheme 1. a) Marder’s aryl borylation reaction. b) Liu’s aryl–alkyl cou-
pling reaction. c) Initial experiment attempting to combine the two
methods into a one-pot borylation/cross-coupling reaction.
DMF=N,N’-dimethylformamide, Ms=methanesulfonyl, Ts=4-tolue-
nesulfonyl.
H. Tajuddin, M. Czyzewska, Dr. P. G. Steel, Prof. Dr. T. B. Marder
Department of Chemistry, Durham University
South Road, Durham DH1 3LE (U.K.)
E-mail: p.g.steel@durham.ac.uk
conditions, CuI can catalyze the cross-coupling of unactivated
alkyl electrophiles with arylboronic esters (Scheme 1b).[11]
On the basis of these findings, we explored the reaction of
an aryl halide (1), a diboron reagent (2a), and an alkyl
electrophile (3a) in the presence of the CuI/PnBu3 catalyst
system. We proposed that this would lead initially to the
formation of the arylboronic ester 5 which would then react
in situ with 3a to afford the aryl–alkyl cross-coupling product
4a. Although 1, 2a, and 3a were all consumed rapidly in the
reaction and 5 was generated as anticipated, we were not able
to obtain 4a in good yield. In an independent experiment we
todd.marder@durham.ac.uk
[**] We thank the NSFC (Nos. 20932006 and 20972148), EPSRC, and
GlaxoSmithKline (GSK) for financial support of this work (Grant EP/
F068158/1) and Drs A. Maxwell and L. Shukla (GSK) for helpful
discussions. T.B.M. thanks the EPSRC for an Overseas Research
Travel Grant, the RCUK China Office for support of the first UK-
China Research Workshop on “Metals in Organic Synthesis: Toward
Cleaner, Greener Chemical Processes”, the Royal Society for a
Wolfson Research Merit Award, and AllyChem Co. Ltd. for a
generous donation of diboron reagents.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106299.
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confirmed that 5 reacts with 3a to produce 4a under the same
reaction conditions. All of these observations indicated that
3a must be consumed by an alternative pathway that is faster
than its reaction with 5. A thorough analysis of the reaction
mixture then revealed that the alkylboronic ester 6a was
could be isolated in 36% yield. Given the low mass balance of
borylated products, a second experiment using 3,5-dimethy-
liodobenzene in the presence of the less volatile electrophile,
3-phenylpropyltosylate, was undertaken. This produced the
corresponding alkylboronate in 85% yield (GC/MS) accom-
panied by smaller amounts of the aryl boronate (63%), thus
suggesting that the lower yields in the initial experiment
resulted from losses during isolation.[12]
We hypothesized that 6a was produced by an unprece-
dented copper-catalyzed cross-coupling reaction between the
alkyl halide and diboron reagent. To test this assumption, we
treated 3a with 2a in the presence of the copper catalyst
(Table 1). Gratifyingly, the desired alkylboronic ester 6a was
obtained in 84% yield at 258C in 18 hours (entry 1). To
improve the yield, different bases were tested (entries 1–6)
with LiOMe proving to be optimal giving a yield of 91%
(entry 6). Next, we optimized the ligand (entries 7–9), copper
salt (entries 10–12), and solvent (entries 13 and 14). Although
6a was successfully obtained under all reaction conditions, the
highest yield was obtained with CuI/PPh3 in DMF. In addition
to B2pin2 (2a), other diboron reagents such as bis(neopentyl
glycolato)diboron (B2neop2) function equally effectively
(entry 15). The necessity for copper in these reactions was
confirmed by the observation that without adding the catalyst
the reaction does not occur (entry 21). Moreover, the possible
involvement of palladium or nickel contamination in the
catalyst was eliminated by the observation that palladium and
nickel salts provide only a trace amount of 6a under the
optimized reaction conditions (entries 19 and 20). Finally, the
reaction is not significantly sensitive to moisture, because the
addition of 4 equivalents of water only reduces the yield to
77% (entry 22).
n-Hexyl iodide, chloride, and tosylate are also viable
substrates with optimal yields of 90%, 86%, and 76%,
respectively (Table 1, entries 16–18). However, higher tem-
peratures (608C) and the addition of (Bu4N)I are required for
reaction of the chloride and tosylate. Presumably, these
proceed via the iodide and, interestingly, for this substrate the
PPh3 ligand is not needed; however, the optimal base changes
from LiOMe to LiOtBu. Overall, the reactivity decreases in
the order: iodide > bromide > chloride ꢀ tosylate (entries 16–
18). This observation is consistent with previous copper-
catalyzed couplings of Grignard[9c] or organoboron
reagents[11] with alkyl electrophiles. This reactivity difference
can be exploited to allow the selective substitution of the
bromine atom of 6-chlorohexyl bromide at room temperature
(Scheme 2). However, on increasing the reaction temperature
to 608C, and in the presence of Bu4NI, both bromide and
chloride react efficiently.
Table 1: Borylation of n-hexyl bromide under various conditions.
Entry Catalyst
Ligand
Base
Sol.
T
Yield
(10 mol%) (13 mol%)
[8C] [%][a]
1
2
3
4
5
6
CuI
CuI
CuI
CuI
CuI
CuI
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
LiOtBu DMF
KOtBu DMF
NaOtBu DMF
25 84
25 28
25 24
25 13
25 trace
25 91
(89)[i]
LiHMDS DMF
Li2CO3
LiOMe
DMF
DMF
7
8
9
CuI
CuI
CuI
PnBu3
PtBu3
LiOMe
LiOMe
DMF
DMF
DMF
25 78
25 70
25 65
1,10-phenanthro- LiOMe
line
10
11
12
13
14
CuBr
CuCl
Cu(OTf)2
CuI
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
DMF
DMF
DMF
DMSO 25 57
THF
25 72
25 56
25 60
CuI
25 35
25 87
Scheme 2. Site-selective borylation.
15[b] CuI
DMF
(83)[i]
16[c] CuI
17[d] CuI
18[e] CuI
–
–
–
LiOtBu THF
LiOtBu THF
LiOtBu MeCN 60 76
LiOMe
LiOMe
LiOMe
LiOMe
25 90
60 86
With optimized reaction conditions identified, we exam-
ined the scope of the new borylation reaction (Scheme 3).
Many synthetically important functional groups including
ester (6b), cyano (6c), ketone (6d), ether (6 f, 6j), olefin (6g),
amide (6i, 6v), ketal (6k), and silyl ether (6t) groups are well
tolerated with yields of the desired, isolated alkylboronates
ranging from about 50% to 80%. Furthermore, arene- and
heterocycle-containing compounds (6m, 6s, 6w) are good
substrates for the borylation process. Significantly, even the
presence of a free alcohol group (6h, 6l) does not interfere
with the reaction. This feature compares favorably with early
alkylboronate syntheses starting from alkyllithium or alkyl-
magnesium reagents, in which nearly all of the alkyl groups
are only hydrocarbons.[4] More reactive electrophiles such as
19[f] Pd(OAc)2 PPh3
DMF
DMF
DMF
DMF
25 trace
25 trace
25 trace
25 77
20[g] NiI2
21
22[h] CuI
PPh3
PPh3
PPh3
–
[a] Yields as determined by GC analysis after 18 h (average of two runs).
[b] Bis(neopentyl glycolato)diboron was used in the coupling. [c] n-Hexyl
iodide was used. [d] n-Hexyl chloride was used and 1 equiv of N(Bu)4I
was added. [e] n-Hexyl tosylate was used and 1 equiv of N(Bu)4I was
added. [f] 2 mol% of Pd catalyst was added. [g] 2 mol% of anhydrous
NiI2 used. Similar negative results were obtained with NiCl2·6H2O and
NiBr2·3H2O. [h] 18 mL (1 mmol) of water was added. [i] Yield of isolated
product. DMSO=dimethylsulfoxide, HMDS=hexamethyldisilazide,
Tf =trifluoromethanesulfonyl, THF=tetrahydrofuran.
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Scheme 4. The borylation reaction of secondary alkyl halides. Reac-
tions were carried out at 378C for 24 h using 10 mol% CuI, 0.38 mmol
B2pin2, 0.5 mmol base, and 0.25 mmol alkyl bromide. Yields quoted
are those for purified, isolated products. [a] X=I, solvent=THF,
base=LiOtBu, T=258C. PPh3 was not added. [b] 2 equiv of B2pin2 was
used. [c] Polymer-supported PPh3 was used. Boc=tert-butoxycarbonyl.
primary halides a similar reactivity profile is observed;
cyclohexyl iodide is readily converted into 6aa (yield =
76%), whilst cyclohexyl chloride only affords moderate
yields (30%) under the current reaction conditions. It is
important to note that in most previous copper-catalyzed
cross-couplings of organometallic reagents with aliphatic
electrophiles, secondary alkyl halides have seldom been
used successfully.[11,13] This new borylation reaction therefore
provides an interesting option for copper-catalyzed cross-
coupling reactions of secondary alkyl electrophiles.
Commercial polystyrene-bound PPh3 can be employed in
the reaction with no loss in efficiency. For example, borylation
of 2-bromoheptane led to a crude reaction mixture which
required two chromatographic runs to separate the heptyl-2-
Bpin product from PPh3. With polymer-supported PPh3, clean
formation of the product and no separation problems were
encountered (6ag; Scheme 4).
Scheme 3. Substrate scope of the borylation reaction. Reactions were
carried out at 258C for 18 h using 10 mol% CuI, 0.38 mmol B2pin2,
0.5 mmol base, and 0.25 mmol alkyl bromide unless otherwise stated.
Yields quoted are those for purified, isolated products. [a] X=I.
[b] X=Cl. [c] X=OTs. For detailed reaction conditions, see the Sup-
porting Information.
The mechanism of this transformation is not immediately
obvious. In analogy to previous studies on the copper-
catalyzed cross-coupling of aliphatic electrophiles,[9c,11,13] the
mechanism of the present borylation reaction might involve
an SN2-type substitution with a CuI/boryl complex[7b,14]
generated through transmetalation[10] between CuI and
B2pin2. Alternatively, the alkyl halide might interact with
the CuI/boryl complex via an oxidatively added transition
state (OATS) similar to that proposed for the copper-
catalyzed borylation of aryl halides.[10] Although additional
experimental and theoretical studies are underway to obtain a
full understanding of the mechanism, a few preliminary
experiments provided interesting results. For example, bor-
ylation of exo-2-bromo norbornane (7) proceeded with
overall retention of configuration, as shown by single-crystal
X-ray diffraction studies of the ethanolamine stabilized exo-2-
boron-substituted product 8 (see the Supporting Informa-
tion). In addition, it was surprising to find that the borylation
of 6-bromohex-1-ene does not afford 9 as anticipated
(Scheme 5). Instead, the cyclopentylmethyl boronate 10 is
produced. This product, along with the formation of bibenzyl
by-products from benzyl halides, suggested the possiblity of a
benzyl (6n–6q)[8h,10] and allyl bromides (6r) can be readily
borylated by the present method. With the former, small
amounts of the corresponding bibenzyl can be observed,
although its formation can be minimized by using two
equivalents of the diboron reagent. Finally, we were able to
confirm that aryl halides are in fact less reactive than alkyl
halides (e.g., 6q). This allows haloalkyls bearing bromo- and
chloro-substituted arene rings to be used successfully, thus
providing the potential for subsequent modifications through
additional cross-coupling reactions at the halogenated posi-
tions.
In addition to primary alkyl electrophiles, secondary alkyl
halides can also be borylated (Scheme 4). For cyclohexyl
bromide, the yield reaches about 60% at 258C in 24 hours.
Increasing the reaction temperature to 378C enables the
desired secondary alkylboronate 6aa to be produced in a
79% yield (isolated). Other cyclic and acyclic secondary
bromides can be smoothly borylated (6ab–6ag). As with
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Scheme 5. Borylation of 6-bromohex-1-ene.
radical mechanism similar to that proposed for nickel-
catalyzed Suzuki–Miyaura reactions of alkyl halides.[15] How-
ever, radical scavenger experiments show that the borylation
yields are not sensitive to the presence of cyclohexa-1,4-diene,
militating against the possibility of a radical mechanism
(Scheme 6).
Scheme 9. Suzuki–Miyaura coupling of alkylboronic esters. Reactions
were carried out at 808C for 24 h on a 0.25 mmol scale. Yields quoted
are those for purified, isolated products. [a] X=Cl, 1108C for 24 h.
dba=dibenzylideneacetone, Ruphos=2-dicyclohexylphosphino-2’,6’-
diisopropoxybiphenyl.
Scheme 6. Radical scavenger experiments.
through the use of polymer-supported ligands. We also report
a mild and practical protocol for the Suzuki–Miyaura
coupling of alkylboronic esters with both aryl bromides and
chlorides. Further optimization, applications, and mechanistic
studies of these new methods are in progress and will be
reported in due course.
Having established an efficient and highly versatile entry
to alkyl boronic esters it was of interest to explore their utility.
Treatment with aqueous hydrochloric acid affords the corre-
sponding alkylboronic acids[16] (Scheme 7), valuable alterna-
tives to the corresponding esters when used either as
Received: September 6, 2011
Published online: December 1, 2011
Keywords: alkanes · boronates · copper · cross-coupling ·
.
Scheme 7. Conversion from boronic ester into boronic acid.
synthetic methods
pharmaceutical agents or synthetic intermediates. Treatment
with KHF2 converts the alkyl boronic esters to the corre-
sponding organotrifluoroborates (Scheme 8).[17] Applications
of alkylboronic esters in Suzuki–Miyaura coupling reactions
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(Scheme 9).
In conclusion, we have developed an unprecedented
copper-catalyzed cross-coupling reaction of unactivated
alkyl halides and pseudohalides with diboron reagents. This
reaction can be used to prepare primary and secondary
alkylboronic esters with diverse structures and functional
groups, many of which would be difficult to access by other
means. The reaction is efficient, practically simple, and gives
easy isolation of the products which can be further enhanced
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