Copper-Catalyzed Oxyboration of Unactivated Alkenes

Article abstract of DOI:10.1002/chem.201503329

The first regiodivergent oxyboration of unactivated terminal alkenes is reported, using copper alkoxide as a catalyst, bis(pinacolato)diboron [(Bpin)₂] as a boron source, and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as an oxygen source. The reaction is compatible with various functional groups. Two regioisomers are selectively produced by selecting the appropriate ligands on copper. The products may be used as a linchpin precursor for various other functionalizations, and net processes such as carbooxygenation, aminooxygenation, and dioxygenation of alkenes can be achieved after C-B bond transformations. Mechanistic studies indicate that the reaction involves the following steps: 1)Transmetalation between CuOtBu and (Bpin)₂ to generate a borylcopper species; 2)regiodivergent borylcupration of alkenes; 3) oxidation of the thus-generated C-Cu bond to give an alkyl radical; 4)trapping of the resulting alkyl radical by TEMPO.

Full text of DOI:10.1002/chem.201503329

DOI: 10.1002/chem.201503329
Communication
&
Copper Catalysis
Copper-Catalyzed Oxyboration of Unactivated Alkenes
Taisuke Itoh,[a] Takumi Matsueda,[a] Yohei Shimizu,*[a] and Motomu Kanai*[a, b]
per(I) species in the borylcupration of alkenes, and the one-
electron redox-active property of alkylcopper species A in the
outer-sphere generation of alkyl radical B, which was trapped
by (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as an oxygen
atom source (Scheme 1b). The novel biphasic mechanism com-
Abstract: The first regiodivergent oxyboration of unacti-
vated terminal alkenes is reported, using copper alkoxide
as a catalyst, bis(pinacolato)diboron [(Bpin)2] as a boron
source, and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)
as an oxygen source. The reaction is compatible with vari-
ous functional groups. Two regioisomers are selectively
produced by selecting the appropriate ligands on copper.
The products may be used as a linchpin precursor for vari-
ous other functionalizations, and net processes such as
carbooxygenation, aminooxygenation, and dioxygenation
of alkenes can be achieved after CꢀB bond transforma-
tions. Mechanistic studies indicate that the reaction in-
volves the following steps: 1) Transmetalation between
CuOtBu and (Bpin)2 to generate a borylcopper species;
2) regiodivergent borylcupration of alkenes; 3) oxidation
of the thus-generated CꢀCu bond to give an alkyl radical;
4) trapping of the resulting alkyl radical by TEMPO.
Scheme 1. a) Previous copper-catalyzed borylative difunctionalization of al-
kenes; b) this work: successive borylcupration–one-electron oxidation path-
way. FG=functional group; TMP=2,2,6,6-tetramethylpiperidin-1-yl.
plements previous copper-catalyzed borylative difunctionaliza-
tions of unactivated alkenes through two-electron pathways
(Scheme 1a).[5] Appropriate selection of the ligand on the
copper catalyst enables a switch in regioselectivity[6] and
allows this method to be used as a linchpin for the synthesis
of various oxygenated vicinally difunctionalized compounds.
Alkylcopper(I) species A, generated through the borylcupra-
tion of an alkene, can act as a two-electron donor nucleophile
(Scheme 1a),[5] and therefore we began our investigation by
seeking an adequate two-electron oxidant for alkylcopper(I) in-
termediate A in the copper-catalyzed oxyboration of 1a
(Table 1, entries 1–4). This reaction, however, did not afford the
desired products 2a or 3a. 4-Phenylbutylboronic acid pinacol
ester, perhaps generated through borylcupration of 1a fol-
lowed by inadvertent protonation, was instead produced in
66% yield (Table 1, entry 2), indicating that two-electron oxy-
genation of the CꢀCuI bond of A was slower than protonation.
Consequently, we investigated one-electron oxidants to con-
vert the CꢀCuI bond to a CꢀO bond in a step-wise manner
through one-electron oxidation of alkylcopper(I) A to alkylcop-
per(II), homolysis of alkylcopper(II) to alkyl radical B to regener-
ate CuI, and trapping of B by an oxygen atom radical
(Scheme 1b). Although the use of molecular oxygen as an oxi-
dant afforded no desired product (Table 1, entry 5), the reac-
tion with TEMPO proceeded efficiently to give 2a in 66% yield
with high regioselectivity (entry 6).[7] The ligand on the copper
catalyst was responsible for both the reactivity and regioselec-
tivity: diphosphine ligands other than Xantphos derivatives af-
forded both a low yield and low selectivity (Table 1, entries 7
and 8; see also the Supporting information for details). Modifi-
Organoboron compounds are synthetically versatile in organic
chemistry and their efficient synthesis has been a subject of in-
terest for several decades. Among various preparation meth-
ods of organoboron compounds, borylative difunctionalization
of alkenes is a particulary attractive approach, because both
a boron atom and another useful functionality can be simulta-
neously introduced to readily available starting materials. The
transition metal-catalyzed borylative difunctionalization of al-
kenes has been intensively studied, including diboration,[1] sila-
boration,[2] carboboration,[3] and aminoboration.[4] To our
knowledge, however, there is no report on oxyboration of al-
kenes. Considering the prevalence of oxygen functionalities in
a variety of organic molecules, oxyboration of alkenes should
be a valuable synthetic method. Herein, we report the first cat-
alytic intermolecular oxyboration of unactivated terminal al-
kenes using copper catalysis. The key to success was a combi-
nation of two distinct properties of copper-containing species
in a catalytic cycle: the nucleophilic property of the borylcop-
[a] T. Itoh, T. Matsueda, Dr. Y. Shimizu, Prof. Dr. M. Kanai
Graduate School of Pharmaceutical Sciences
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: y-shimizu@mol.f.u-tokyo.ac.jp
kanai@mol.f.u-tokyo.ac.jp
[b] Prof. Dr. M. Kanai
JST, ERATO, Kanai Life Science Catalysis Project
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503329.
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Table 1. Optimization of oxyboration of a terminal alkene.
Table 2. Substrate scope of 4’-MeO-Xantphos-ligated CuCl-catalyzed oxy-
boration of alkenes.
Entry
Ligand
Xantphos
[O]
Yield [%][a]
2a/3a
1
2
3
4
TBHP
TBP
0
0
0
0
0
–
–
–
–
–
Xantphos
Xantphos
Xantphos
oxaziridine
(PhCOO)2
O2
5
Xantphos
6
7
8
9
10
11[b]
Xantphos
dppf
dppbz
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
66
19
20
68
76
82 (71)
>20:1
2.3:1
2.5:1
>20:1
>20:1
>20:1
4’-Me-Xantphos
4’-MeO-Xantphos
4’-MeO-Xantphos
General reaction conditions: 1a (0.10 mmol), (Bpin)2 (0.20 mmol), oxidant
(0.20 mmol), CuCl (10 mol%), ligand (12 mol%), NaOtBu (0.15 mmol),
DMF (1.0 mL), 308C, 4 h. Yields and regioisomeric ratio (2a:3a) were de-
1
termined by H NMR using tBuOMe as an internal standard. [a] Combined
yield of 2a and 3a; yield of isolated 2a is given in parentheses.
[b] 0.25 mmol scale; (Bpin)2 (0.72 mmol), and TEMPO (0.70 mmol) were
used.
cation of the substituents on the phosphorus atom of Xant-
phos derivatives increased the reactivity while maintaining the
high regioselectivity, and 4’-MeO-Xantphos was revealed to be
the best ligand (Table 1, entry 10). The yield was further im-
proved to 82% by increasing the amounts of bis(pinacolato)di-
boron [(Bpin)2] and TEMPO (entry 11).
The optimized conditions were applicable to a variety of ter-
minal alkenes (Table 2). In most cases, excellent regioselectivity
(2/3>20:1) was obtained. Due to the instability of the prod-
ucts under purification by silica-gel column chromatography,
the products were isolated with gel-permeation chromatogra-
phy and the yields of isolated product were diminished by ap-
proximately 10–25% compared with the NMR yields. The de-
veloped oxyboration reaction exhibited high functional group
tolerance. Ether-containing alkenes and a ketal-containing
alkene all reacted to give the corresponding products (2b–f) in
moderate to high yields. Notably, the oxyboration reaction was
completely selective to the terminal double bond over the in-
ternal double bond (2d, 2e). A potentially reactive alkyl chlo-
ride moiety was intact under the reaction conditions (2g). The
reaction even proceeded regioselectively with sterically de-
manding vinyl cyclohexane to give 2h. Various carbonyl
groups and a nitrile functional group were tolerated (2i–l). The
reaction of an alkene bearing a strongly coordinating amino
group afforded 2m in good yield without poisoning the
copper catalyst.
Conditions: 1 (0.25 mmol), (Bpin)2 (0.72 mmol), TEMPO (0.70 mmol), CuCl
(10 mol%), 4’-MeO-Xantphos (12 mol%), NaOtBu (0.38 mmol), DMF
(2.5 mL), 308C, 4 h. Yields and regioisomeric ratio (r.r.=2/3) were deter-
mined by 1H NMR using tBuOMe as an internal standard. Combined
yields of isolated 2 and 3 are given in parentheses. [a] Xantphos was
used instead of 4’-MeO-Xatphos. [b] (Bpin)2 (0.66 mmol) and TEMPO
(0.63 mmol) were added dropwise over the course of 20 min.
lated 3a as a major product over 2a [57% combined yield,
2a/3a=1:2.5; IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene]. This result encouraged us to further investigate the
reaction conditions toward catalyst-controlled regiodivergence.
Among various N-heterocyclic carbene (NHC)-ligated copper
catalysts studied, [CyIEtCuCl] was the best catalyst in terms of
reactivity and selectivity (76% yield, 2a/3a=1:5.3; Table 3).[8]
The functional group compatibility was similarly broad to 4’-
MeO-Xantphos-ligated copper catalyst conditions, although
the regioselectivity was generally moderate (Table 3). Ether,
acetal, and chloroalkyl functionalities were tolerated under the
reaction conditions (3b–g). Carbonyl and nitrile functionalities
were also compatible, affording 3i–l in good NMR yields. Isola-
tion of 3i, 3k, and 3l was particularly difficult, however, due to
When [IMesCuCl] was utilized as the catalyst instead of 4’-
MeO-Xantphos-ligated CuCl for the oxyboration of 1a, the re-
gioselectivity switched, preferentially affording internally bory-
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Table 3. Substrate scope of [CyIEtCuCl]-catalyzed oxyboration of alkenes.
Scheme 2. Conversion of the oxyboration product. Reagents and conditions:
a) NaBO3·H2O, THF/H2O, RT; b) MeONH2, nBuLi, THF, ꢀ788C to RT; Boc2O,
NEt3, THF, RT; c) bromochloromethane, nBuLi, THF, ꢀ788C to 608C; d) bro-
mobenzene, Pd(dba)2 (5 mol%), RuPhos (7.5 mol%), NaOtBu, toluene/H2O,
808C; e) Zn, AcOH, THF, 708C; f) Zn, AcOH, THF, 608C.
functionalities on either side of the C=C double bond. Amina-
tion of the CꢀB bond was also possible with MeONH2 and
nBuLi,[9] giving aminooxygenation products 5 and 10. Matte-
son’s homologation in the presence of bromochloromethane
and nBuLi successfully introduced sp3 carbon functionalities,
producing borylmethyl oxygenation intermediates.[10] The ho-
mologated products were directly subjected to oxidation, ami-
nation, and Pd-catalyzed cross-coupling[11] conditions without
purification, giving a variety of 1,3-difunctionalized com-
pounds, 6, 7, 8, 11, 12, and 13. The OTMP group could be con-
verted into the corresponding alcohol in the presence of Zn
and AcOH (4!14, 8!15). Overall, the copper-catalyzed regio-
divergent oxyboration followed by conversion of the CꢀB
bond enabled rapid access to diverse patterns of oxygenative
difunctionalization products, which are difficult to access by
other means, from terminal alkenes.
Conditions:
1 (0.25 mmol), (Bpin)2 (0.66 mmol), TEMPO (0.63 mmol),
[CyIEtCuCl] (10 mol%), LiOtBu (0.38 mmol), DMF (2.5 mL), 308C, 4 h. (Bpin)2
and TEMPO were added dropwise over the course of 20 min. Yields and
regioisomeric ratio (r.r.=2:3) were determined by 1H NMR using tBuOMe
as an internal standard. Combined yields of isolated 2 and 3 are given in
parentheses. [a] [IPrCuCl] was used instead of [CyIEtCuCl]. (Bpin)2 and
TEMPO were added dropwise over the course of 50 min. [b] Yield of iso-
lated product after oxidation with NaBO3·H2O (1.25 mmol). TBDPS=tert-
butyldiphenylsilyl.
To gain insight into the reaction mechanism, following reac-
tions were conducted (Scheme 3). Due to difficult handling of
alkylcopper species derived from unactivated alkenes, we pre-
pared the isolable b-borylalkylcopper 16 from styrene, follow-
ing a procedure reported by Sadighi and co-workers,[12] and
treated this possible intermediate with 2 equivalents of
TEMPO. As expected, the oxygenation proceeded to give oxy-
boration product 2n in 35% yield. When [CyIEtCuCl]-catalyzed
oxyboration was applied to styrene under the conditions given
in Table 3, 2n was produced in 65% yield as the sole regioiso-
mer. Based on this observation and the following results for
the reaction with b-methylstyrene (17), we postulated that
a radical species derived from 16 might be involved in the C-
oxygenation step. In borylcupration of internal alkenes, syn ad-
dition of the boron and copper atoms to a C=C double bond
proceeds. Subsequent protonation[13] or amination[4a] of the Cꢀ
their instability under gel-permeation chromatography and
other purification conditions, and thus they were isolated after
conversion to the corresponding secondary alcohols by treat-
ment with NaBO3·H2O without purification of the oxyboration
products.
Due to the wide substrate scope, regiodivergence, and ver-
satility of the organoboron compounds, the products are
useful precursors for diverse vicinally functionalized molecules.
Therefore, we examined the conversion of the CꢀB bond into
other functionalities (Scheme 2). Oxidation of 2a and 3a by
NaBO3·H2O efficiently produced alcohol products 4 and 9, re-
spectively. Notably, the two-step oxyboration–oxidation se-
quence realized selective introduction of two distinct oxygen
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which affords outer-sphere alkyl radical B and CuIOTMP F
through homolytic cleavage of the unstable CꢀCuII bond of E.
The radical intermediate B is trapped by a second equivalent
of TEMPO to afford oxyboration product 2. Because a stoichio-
metric amount of NaOtBu is necessary for high product yield,
CuOtBu C needs to be regenerated from F and NaOtBu. The re-
gioselectivity shown in Scheme 4 is for L=Xantphos, alhough
the mechanism should be basically the same for the reaction
with an NHC ligand, except for the regioselectivity of the boryl-
cupration step.
In summary, we have developed the first oxyboration of un-
activated alkenes promoted by a copper catalyst. Key to the
success of this reaction was the use of both nucleophilic and
redox-active properties of copper catalysis. The regioselectivity
of the reaction was controlled by the ligands: b-oxygenated
terminal alkylboronates 2 were obtained with 4’-MeO-Xant-
phos-ligated CuCl as catalyst, whereas b-oxygenated internal
alkylboronates 3 were obtained with [CyIEtCuCl]. Conversions
of the CꢀB bond produced molecules with diverse vicinal sub-
stitution patterns, which are difficult to access by other means.
Preliminary mechanistic studies revealed that the reaction in-
volves the borylcupration of alkenes, one-electron oxidation of
the resulting alkylcopper(I) species to give an alkyl radical spe-
cies, and CꢀO bond formation with TEMPO through radical
coupling. Copper-catalyzed borylative C-radical generation will
be applicable to other bond-forming reactions. Further investi-
gation of these applications is ongoing in our laboratory.
Scheme 3. Mechanistic studies.
Cu bond occurs with stereoretention, giving the syn adducts
exclusively. In contrast, the present oxyboration of 17 pro-
duced a 1.8:1 diastereomeric mixture of products 18. This
result can be explained by considering that C-oxygenation of
an organocopper intermediate is a radical coupling process be-
tween a radical generated from the alkylcopper(II) species and
TEMPO. This proposed mechanism is further supported by a fol-
lowing radical clock experiment. When the vinylcyclopropane
19 was subjected to the oxyboration reaction followed by oxi-
dation of the CꢀB bond, the ring-opened product 20 was ob-
tained in 17% yield. This observation is also in accordance
with the involvement of alkyl radical species in the C-oxygena-
tion process.
Acknowledgements
This work was partially supported by ERATO from JST (M.K.),
a Grant-in-Aid for Scientific Research (B) from the JSPS (M.K.),
and a Grant-in-Aid for Scientific Research (C) from the JSPS
(Y.S.). T.I. thanks the JSPS for a fellowship.
These experimental results, combined with previous obser-
vations by other researchers,[14] led us to propose the following
catalytic cycle (Scheme 4). First, copper(I) alkoxide C generated
from CuCl and NaOtBu forms borylcopper(I) species D by trans-
metalation with (Bpin)2. Insertion of the terminal C=C double
bond into the thus-generated borylcopper(I) produces b-bory-
lalkylcopper(I) A. One-electron oxidation of CuI to CuII with
TEMPO results in the generation of alkylcopper(II) species E,
Keywords: alkenes · copper · difunctionalization · boration ·
radical reactions
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Scheme 4. Proposed catalytic cycle.
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Received: August 21, 2015
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COMMUNICATION
&
Copper Catalysis
T. Itoh, T. Matsueda, Y. Shimizu,*
M. Kanai*
The first oxyboration of alkenes has
been realized using copper alkoxide as
a catalyst, (Bpin)2 as a boron source,
and TEMPO as an oxygen source. The
oxyboration proceeds through a unique
mechanism involving both nucleophilic
and one-electron redox-active proper-
ties of copper catalysis. Regiodivergent
oxyboration of alkenes is enabled by se-
lecting appropriate ligands on copper.
&& – &&
Copper-Catalyzed Oxyboration of
Unactivated Alkenes
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