Regio- and enantioselective monoborylation of alkenylsilanes catalyzed by an electron-donating chiral phosphine-Copper(I) complex

Article abstract of DOI:10.1002/adsc.201300765

An asymmetric monoborylation of alkenylsilanes catalyzed by a copper(I) complex with the chiral bisphosphine ligand BenzP* is reported. The reaction proceeded with excellent regioselectivity and high enantioselectivity to afford the corresponding opticall

Full text of DOI:10.1002/adsc.201300765

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DOI: 10.1002/adsc.201300765
Regio- and Enantioselective Monoborylation of Alkenylsilanes
Catalyzed by an Electron-Donating Chiral Phosphine–Copper(I)
Complex
Koji Kubota,a Eiji Yamamoto,a and Hajime Itoa,*
a
Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Fax : (+81)-(0)11-706-6561; e-mail: hajito@eng.hokudai.ac.jp
Received: August 23, 2013; Published online: December 6, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300765.
Abstract: An asymmetric monoborylation of alken-
ACHTUNGTRENNUNGylsilanes catalyzed by a copper(I) complex with the
bonds in the substrate to form b-borylated alkylcop-
per(I) intermediate A, which is stabilized by the elec-
tronic effect of the silicon group, followed by stereo-
specific intramolecular substitution to give the corre-
sponding annular vicinal borosilane (Scheme 1a).[9]
We anticipated that efficient protonation of the alkyl-
copper(I) intermediate A’ could also afford optically
active linear vicinal borosilanes (Scheme 1b).[10]
At the time that we initiated this investigation,
there were no reports of such an asymmetric boryla-
tion; however, Hoveyda and co-workers have since
reported that chiral N-heterocyclic carbine–copper
complexes catalyze the enantioselective b-borylation
of (E)-alkenylsilanes.[11] Although the reaction pro-
ceeded with high regioselectivity, the degree of enan-
tioselection was moderate in some cases and depend-
ed on the structure of the substrate (68–93% ee).
They only reported the reaction with (E)-isomers.
chiral bisphosphine ligand BenzP* is reported. The
reaction proceeded with excellent regioselectivity
and high enantioselectivity to afford the corre-
sponding optically active organoboronate esters
À
with a stereogenic C B bond containing a vicinal
silyl group. The synthetic utility of the product is
demonstrated through stepwise transformations to
multifunctional optically active compounds in a ste-
reospecific manner.
Keywords: asymmetric catalysis; borylation;
copper; silanes; synthetic methods
Enantioenriched chiral organoboron compounds have Herein we report an efficient catalysis using a cop-
been recognized as important chiral building blocks in per(I) complex of the chiral electron-donating ligand
organic synthesis because they undergo stereospecific BenzP* to produce optically active linear vicinal
À
transformations of the stereogenic C B bonds to boroACTHNUGRTNEUNGsilanes with a high degree of enantioselectivity
[1]
À
À
À
form C O, C N, or C C bonds. The copper(I)-cata- (88–97% ee) from (Z)-alkenylsilane substrates. We
À
lyzed asymmetric borylation reactions of C C double
bonds have emerged as a powerful tool to prepare
various optically active organoboronates, which have
been researched extensively recently.[2–7] The key step
À
for the efficient construction of a stereogenic C B
bond is the regio- and enantioselective addition of the
borylcopper(I) species to prochiral alkene sub-
strates.[4,6,7]
An asymmetric b-borylation of alkenylsilanes can
provide enantiomerically enriched vicinal borosilanes,
which can be derivatized through stepwise, stereospe-
cific transformations of the boron and silicon functio-
nalities.[3c,8] We previously reported the highly enan-
tioselective copper(I)-catalyzed intramolecular 1,2-
carboboration of (Z)-alkenylsilane derivatives.[3c,h]
This reaction involved the addition of a chiral boryl-
copper(I)/bisphosphine complex to the C=C double tions of (Z)-alkenylsilane substrates.
Scheme 1. Copper(I)-catalyzed asymmetric borylation reac-
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Table 1. Copper(I)-catalyzed enantioselective monoborylation of alkenylsilanes 1 with bis(pinacolato)diboron 2 under vari-
ous conditions.[a]
Entry
R3Si
Chiral Ligand
Yield [%][b]
ee [%]
3
4
1
2
3
4
5
6
7
8
Me3Si (1a)
Me3Si
Me3Si
Me3Si
Me3Si
Me3Si
Me3Si
PhMe2Si (1b)
BnMe2Si (1c)
Ph2MeSi (1d)
Me3Si
U
68
<1
64
<1
–
<1
<1
–
–
5
5
6
92
–
74
85[c]
–
29
<1
<1
91
94
92
–
95
94
94
91
97
9
10
74
81
9
<1
11[d]
[a]
Conditions: 1a (0.5 mmol), CuCl (0.025 mmol), ligand (0.025 mmol), KACTHNUTRGNE(NUG O-t-Bu)/THF (0.6M, 1.0 mL), 2 (0.75 mmol),
MeOH (2.0 mmol).
[b]
[c]
[d]
1
Yield and ratio of 3/4 were determined by H NMR spectroscopy of the crude reaction mixture.
R isomer.
The reaction was carried out at 08C.
found that an (E)-alkenylsilane also gave the borylat-
ed product with opposite stereochemistry in a good BenzP*,[12] which has a similar structure to QuinoxP*.
enantiomeric purity (89% ee). However, BenzP* is more electron-donating than
Following on from our previous research, the reac- QuinoxP*, which can facilitate the interaction be-
tion of (Z)-1a was first investigated with CuCl/(R,R)- tween the HOMO of the borylcopper(I) complex and
QuinoxP* chiral catalyst (5 mol%) in the presence of LUMO of the alkenylsilane substrate, enhancing reac-
bis(pinacolato)diboron 2 (1.5 equiv.), K
(O-t-Bu) base tivity while keeping high enantioselectivity.[13,14] As
Then, we investigated the reaction with (R,R)-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(1.2 equiv.) and MeOH (4.0 equiv.) as a proton source a result, we successfully improved the yield (91%)
in THF at 308C (Table 1). Pleasingly, the reaction and enantioselectivity (95% ee) of the reaction using
proceeded with good enantioselectivity (92% ee), but the chiral ligand (R,R)-BenzP* (entry 7). A small
gave only a moderate yield (68%) (entry 1). To our amount of a b-elimination product 4a (5%) was also
disappointment, using the chiral ligand (R)-Segphos, detected. Enantiomerically enriched organoboronates
which has previously been found to catalyze the inter- with a dimethylphenylsilyl or benzyldimethylsilyl
molecular 1,2-carboboration of (Z)-alkenylsilane de- group, which can be converted into other functional
rivatives,[3c] instead of (R,R)-QuinoxP*, resulted in groups more easily than a trimethylsilyl group, were
almost no reaction (entry 2). A series of other chiral also synthesized with excellent enantioselectivity
ligands was also screened, including (R,R)-Me- (94% ee) through reaction with the copper(I)/ACHTUNGTRENNUNG(R,R)-
Duphos, (S,S)-BDPP, (R)-BINAP and the ferrocenyl BenzP* catalytic system (entries 8 and 9, respective-
chiral ligand (R,S)-Josiphos; they too showed poor re- ly). The reaction of the substrate with a more sterical-
sults (entries 3–6).
ly hindered diphenylmethylsilyl group also gave
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Regio- and Enantioselective Monoborylation of Alkenylsilanes
Table 2. Copper(I)-catalyzed enantioselective monoborylation of various alkenylsilanes.[a]
[a]
Conditions: 1a (0.5 mmol), CuCl (0.025 mmol), ligand (0.025 mmol), K
(0.6M, 1.0 mL), 2 (0.75 mmol), MeOH (2.0 mmol). Isolated yields.
ACHTUNGTREN(NUNG O-t-Bu)/THF
a good result (entry 10). Higher enantioselectivity thermore, the reaction also proceeded in the presence
(97% ee) was obtained when the reaction was con- of prenyloxy ether (3l), cyano group (3m) and N-pro-
ducted at 08C without formation of the b-elimination tected indole (3n) substrates, resulting in good yields
side-product 4a (entry 11).
with excellent enantioselectivities (88–96% ee).
Various alkenylsilanes were then subjected to the
The synthetic utility of the monoborylation prod-
asymmetric monoborylation in the presence of the ucts was demonstrated through selective derivatiza-
copper(I)/(R,R)-BenzP* catalyst (Table 2). Optically tion. The product 3j was subjected to sequential
ACHTUNGTRENNUNG
active organoboronates that possess alkyl substituents NaBO3 and Tamao oxidation to afford the desired
[R=CH2CH2Ph (3a), Me (3e), and n-Bu (3f)] were enantiomerically enriched 1,2-diol 5 (64%, 3 steps;
À
obtained with high enantioselectivity (94–97% ee) 90% ee) in a stereospecific manner for the C B bond
from the corresponding alkenylsilanes.
(Scheme 2).[8a,16] Taking advantage of the reactivity
Although the reaction of a substrate with a b- difference between the silicon and boron groups, we
branched alkyl substituent (R=CH2Cy, 3g) resulted also conducted the stereospecific amination of 3f with
in good enantioselectivity (89% ee), the reaction rate benzyl azide to give the corresponding amine, fol-
À
was relatively slower (58%) because of the considera- lowed by oxidation of the Si C bond to obtain the
ble steric congestion around the C=C double bond. optically active 1,2-amino alcohol 6 in good yield with
Alkenylsilanes with functional groups such as silyl excellent enantiomeric purity (62%, 3 steps; 93%
ether (3h), benzyl ether (3i), and benzoate (3j) were ee).[17]
compatible with the reaction. A substrate containing
The E or Z configuration of the substrate influ-
a chloro group (3k), which is reactive for copper(I)- enced the enantioselectivity of the borylation reaction
catalyzed boryl substitution, was also applicable (Scheme 3). Reaction of (E)-1a under our standard
(82%, 95% ee) with no side-product detected.[15] Fur- conditions gave (R)-3a in lower yield and enantiose-
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Scheme 2. Stepwise transformations of monoborylation
products: synthesis of 1,2-diol and 1,2-amino alcohol deriva-
tives.
Figure 1. Transition state models explaining the higher enan-
tioselectivity for (Z)-configuration.
Experimental Section
General Remarks
Scheme 3. Copper(I)-catalyzed enantioselective monobory-
lation of (E)-1a.
Materials were obtained from commercial suppliers and pu-
rified by standard procedures unless otherwise noted. Sol-
vents were also purchased from commercial suppliers, de-
gassed via three freeze-pump-thaw cycles, and further dried
over molecular sieves (MS 4ꢁ). NMR spectra were record-
ed on a JEOL JNM-ECX400P spectrometer (1H: 400 MHz
and 13C: 100 MHz). Tetramethylsilane (1H) and CDCl3 (13C)
were employed as external standards, respectively. CuCl
lectivity (61%, 89% ee) than those obtained for the
same reaction with (Z)-1 (81%, 97% ee).[11]
The observed stereochemical outcome of the cop-
per(I)-catalyzed monoborylation of (Z)- or (E)-alken-
(ReagentPlusꢂ grade, 224332-25G, ꢀ99%) and K
ACHTUNGTRENN(UNG O-t-Bu)/
ACHTUNGTRENNUNGylsilanes can be explained by the transition states of
THF (1.0M, 328650-50ML) were purchased from Sigma–Al-
drich Co. and used as received. Mesitylene was used as an
internal standard to determine NMR yields. GLC analyses
were conducted with a Shimadzu GC-2014 or GC-2025
equipped with an ULBON HR-1 glass capillary column
(Shinwa Chemical Industries) and an FID detector. HPLC
analyses with chiral stationary phase were carried out using
a Hitachi LaChrome Elite HPLC system with an L-2400
UV detector. Elemental analyses and high-resolution mass
spectra were recorded at the Center for Instrumental Analy-
sis, Hokkaido University.
À
Cu B addition across the C=C double bond, as
shown in Figure 1. In the case of the reaction with
(Z)-1, the favored transition state TS1 is free from
steric congestion between the substituents of (Z)-
1 and the t-Bu group of the BenzP* chiral ligand, thus
producing (S)-3 as the major enantiomer. In contrast,
the less favored transition state TS2 is destabilized be-
cause one of the t-Bu groups of the ligand is close to
both the silyl groups and substituents of (Z)-1. In the
case of (E)-1, one of the t-Bu groups of the ligand is
involved in steric interactions with either the substitu-
ents (TS3) or silyl groups of (E)-1 (TS4). Thus, the
energy difference between TS3 and TS4 is smaller
than that between TS1 and TS2, resulting in higher
Representative Procedure for Copper(I)-Catalyzed
Asymmetric Monoborylation of (Z)-1a
enantioselectivity for the substrates with (Z)-configu- Copper chloride (2.5 mg, 0.025 mmol) and bis(pinacolato)di-
boron (190.5 mg, 0.75 mmol), and (R,R)-BenzP* (7.1 mg,
0.025 mmol) were placed in an oven-dried reaction vial.
After the vial was sealed with a screw cap containing
a teflon-coated rubber septum, the vial was connected to
a vacuum/nitrogen manifold through a needle. It was evacu-
ated and then backfilled with nitrogen. This cycle was re-
ration.
In summary, we have developed the copper(I)/
BenzP* complex-catalyzed monoborylation of (Z)-
alkenylsilanes to afford synthetically useful optically
active linear vicinal borosilanes. The reaction pro-
ceeded with excellent regioselectivities and high enan-
tioselectivities. The synthetic utility of the protocol
was demonstrated by the stepwise and stereoselective
transformation of the products into enantioenriched
1,2-diol and 1,2-amino alcohol derivatives.
ACHTUNGTRENNUNG
peated three times. THF (0.4 mL) and KACTHUNRTGNEUNG(O-t-Bu)/THF
(1.0M, 0.6 mL, 0.6 mmol) were added in the vial through
the rubber septum. After (Z)-alkenylsilane 1a (102.2 mg,
0.5 mmol) had been added to the mixture at 08C, MeOH
(80.9 mL, 2.0 mmol) was added dropwise. After the reaction
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Regio- and Enantioselective Monoborylation of Alkenylsilanes
was complete, the reaction mixture was passed through
a short silica gel column eluting with Et2O/hexane (20:80).
The crude mixture was further purified by flash column
chromatography (SiO2, Et2O/hexane, typically 0:100–4:96)
to give the corresponding monoborylation product 3a as
a colorless oil. The flash column chromatography should be
done within 5 min after the crude mixture was applied on
the silica gel surface; otherwise the products are obtained in
low yield. The ee value was determined by HPLC analysis
of the corresponding alcohol after NaBO3 oxidation of the
borylated product in comparison of the racemic sample.
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[8] a) M. Oestreich, E. Hartmann, M. Mewald, Chem. Rev.
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Acknowledgements
This work was financially supported by the Funding Program
for Next Generation World-Leading Researchers (NEXT
Program, No. G002) from the Japan Society for the Promo-
tion of Science (JSPS). We would like to thank Nippon
Chemical Industrial Co., Ltd. for the gift of BenzP*.
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