Regioselective and stereospecific copper-catalyzed aminoboration of styrenes with bis(pinacolato)diboron and O-benzoyl- N,N-dialkylhydroxylamines

Article abstract of DOI:10.1021/ja4007645

A Cu-catalyzed regioselective and stereospecific aminoboration of styrenes with bis(pinacolato)diboron and O-benzoyl-N,N-dialkylhydroxylamines that delivers the corresponding β-aminoalkylboranes in good yields has been developed. The Cu catalysis enables introduction of both amine and boron moieties to C-C double bonds simultaneously in a syn fashion. Moreover, the use of a chiral biphosphine ligand, (S,S)-Me-Duphos, provides a catalytic enantioselective route to optically active β-aminoalkylboranes.

Full text of DOI:10.1021/ja4007645

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Communication
Regioselective and Stereospecific Copper-Catalyzed
Aminoboration of Styrenes with Bis(pinacolato)diboron
and O-Benzoyl-N,N-dialkylhydroxylamines
Naoki Matsuda, Koji Hirano, Tetsuya Satoh, and Masahiro Miura
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4007645 • Publication Date (Web): 15 Mar 2013
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Regioselective and Stereospecific Copper-Catalyzed Aminoboration of Styrenes with
Bis(pinacolato)diboron and O-Benzoyl-N,N-dialkylhydroxylamines
Naoki Matsuda, Koji Hirano,* Tetsuya Satoh, and Masahiro Miura*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
Supporting Information Placeholder
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electrophilic aminations by our group11 and others.12 An initial
ligand exchange of a Cu(I) complex and MO-t-Bu (A)13 followed
by σ-bond metathesis with bis(pinacolato)diboron generates a
borylcopper species B.14 Subsequent insertion of the alkene into
Cu–B bond of B furnishes a borylated alkylcopper intermediate
Abstract:
stereospecific
bis(pinacolato)diboron
dialkylhydroxylamines has been developed to deliver the
corresponding β-aminoalkylboranes in good yields. The
copper catalysis enables introduction of both amine and
boron moieties to C–C double bonds simultaneously in a
syn fashion. Moreover, the use of chiral biphosphine ligand,
(S,S)-Me-Duphos, can provide a catalytic enantioselective
route to optically active β-aminoalkylboranes.
A
copper-catalyzed
aminoboration
and
regioselective
styrenes
and
with
of
O-benzoyl-N,N-
C.15
An umpolung, electrophilc amination with the O-
benzoylhydroxylamine then occurs to form the desired
aminoboration product and a CuOBz complex D.12j Final ligand
exchange with MO-t-Bu regenerates the starting CuO-t-Bu
complex A to complete the catalytic cycle.11c,12j,16 If the reaction
of the borylcopper B with the alkene proceeded selectively even
in the presence of the O-benzoylhydroxylamine, the
chemoselective aminoboration could be realized.17 An additional
conceivable problem is a control of regio- and stereochemistry. In
particular, the stereochemical course of the C–N forming process
somewhat remains elusive,18 while the alkenes are known to insert
into the borylcopper Cu–B in a syn fashion.15
Organoborons constitute an important class of compounds in
organic synthesis owing to their high utilities for carbon–carbon
and carbon–heteroatom bond forming reactions. Now they are
ubiquitously found in the synthesis of complex natural products,
biologically active compounds, and functional materials.1 Among
numerous approaches to organoboron compounds, transition-
metal-catalyzed addition reactions of boron functionalities to C–C
multiple bonds have recently received significant attention. In
particular, catalytic difunctionalization are strongly appealing
because they enable introduction of both boron and other
functional groups to organic molecules in one synthetic operation
and provide a facile access to complex, densely functionalized
organoboron compounds. To date, B–B,2 B–Si,3 B–Ge,4 B–Sn,5
B–S,6 and B–C7 single bonds are added across alkenes, alkynes,
and dienes in the presence of a catalytic amount of metal
complexes. In such a reaction, the special organoboron reagents
are prepared in advance, and their boron-element single bonds are
often activated through the oxidative addition to the low-valent
metal center. A good alternative is the transmetalative approach,
in which boron and carbon functionalities are incorporated from
two different components.8 Despite the above advances in this
field, there is no successful report of catalytic, simultaneous
addition of boron and nitrogen groups into C–C unsaturated
molecules (aminoboration). Given the ubiquity of amino groups
in natural products and pharmaceutical targets,9 the expected
product can be a highly useful building block in synthetic
chemistry, and thus the development of a new catalytic system
directed toward the aminoboration is strongly desired. Herein, we
Scheme 1. Working Hypothesis
R2
R1
Cu Bpin
pinB Bpin
B
MO-t-Bu
ligand
Cu
R2
Cu(I)X
Cu O-t-Bu
R1
C
A
Bpin
R3
MOBz
N
R4
OBz
Cu OBz
MO-t-Bu
D
R4 R3
N
• efficiency?
• regioselectivity?
• stereoselectivity?
R2
R1
O
B
O
pinB =
Bpin
In accordance with the above assumption, we began our
optimization studies with trans-β-methylstyrene ((E)-1a) and O-
benzoyl-N,N-diethylhydroxylamine (2a) as model substrates,
because styrene derivatives tend to react with the borylcopper
species regioselectively and thus regioselective issues can be
obviated.10,15 After the extensive screening of various copper salts,
ligands, bases, and solvents, we were pleased to find that a
combination
of
CuCl/dppbz
(dppbz
=
1,2-
bis(diphenylphosphino)benzene) catalyst and LiO-t-Bu catalyzed
the desired transformation in THF even at room temperature
(Table 1, entry 1).19 To our delight, the product 3aa was obtained
in a good yield as the single regioisomer and diastereomer
(syn/anti = >99:1).20 Under the optimized conditions, we
performed the catalytic aminoboration of (E)-1a with a variety of
report a
bis(pinacolato)diboron
dialkylhydroxylamines. The reaction proceeds very smoothly
even at room temperature with high regioselectivity and
stereospecificity.
enantioselective variant is achieved by using an appropriate chiral
biphosphine ligand.
Our scenario for the catalytic aminoboration of alkenes is
illustrated in Scheme 1. The working hypothesis is prompted by
recent developments of copper-catalyzed hydroboration chemistry
with bis(pinacolato)diboron10 and current studies on umpolung,
copper-catalyzed aminoboration of styrenes with
and O-benzoyl-N,N-
Moreover, the preliminary catalytic
O-benzoyl-N,N-dialkylhydroxylamines 2.
Benzyl-substituted
amines 2b and 2c underwent the reaction very smoothly to afford
the corresponding aminoborated products 3ab and 3ac in 83% and
73% isolated yields, respectively; additional derivatization of
which could be easily operative after the appropriate deprotection
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Page 2 of 5
of benzyl groups21 (entries
2
and 3).
The O-
We next investigated the scope of alkene substrates with 2b as
an electrophilic nitrogen source (Table 2). trans-β-
1
2
3
4
5
6
7
8
benzoylhydroxylamine 2d that bears a 1-pentenyl substituent
furnished the usual product 3ad exclusively (entry 4), excluding
the possibility of an aminyl radical pathway.22 Not only acyclic
but also cyclic amines also participated in the reaction. The six-
membered piperidine and seven-membered azepane were
introduced to (E)-1a efficiently (entries 5 and 6). Moreover,
Methylstyrenes that bear electron-donating methoxy as well as
electron-withdrawing groups underwent the aminoboration
without any difficulties (entries 1 and 2). Terminal styrenes also
reacted with the diboron and 2b regioselectively, in which the
boron was introduced to the terminal position while the nitrogen
atom was attached to the benzylic position. Electronically and
sterically diverse substituents were tolerated under reaction
conditions (entries 4–6). Particularly notable is the compatibility
with the aryl-Br bond (entry 6).23 The fused naphthalene ring did
not interfere with the reaction (entry 7). It is noteworthy that the
cinnamyl alcohol derivative 1i produced the corresponding
densely functionalized alkylboranes 3ia and 3ib in synthetically
useful yields (entries 8 and 9). Moreover, the simple aliphatic
olefin, 1-octene, also could be aminoborated with good
regioselectivity (88:12) with 4,5-bis(diphenylphosphino)-9,9’-
dimethylxanthene (xantphos) as a ligand under otherwise identical
conditions (entry 10). The catalysis accommodated the steric
hinderance at the allylic positions; 1k and 1l also underwent the
aminoboration without any difficulties (entries 11 and 12). On the
other hand, (E)-ethyl crotonate and (E)-crotonitrile provided the
corresponding hydroborated products, in which the boryl group
and hydrogen atom were introduced to the β and α positions,
respectively, but the origin of the hydrogen atom was not clear at
this stage (data not shown).
morpholine,
Boc-protected
piperazine,
and
bicyclic
tetrahydroisoquinoline are also available for use (entries 7–9).
Notably, in latter two cases, NaO-t-Bu gave a better result than
LiO-t-Bu (entries 8 and 9). In addition, the reaction could be
carried out on a 4-fold larger scale, indicating the good reliability
and reproducibility of the process (entry 2). On the other hand,
bis(neopentylglycolato)diboron (neoB–Bneo) instead of pinB–
Bpin gave a lower yield of the aminoborated product (entry 10).
Regardless of steric and electronic nature of O-benzoyl-N,N-
dialkylhydroxylamines, the aminoboration proceeded with
excellent regioselectivity and diastereoselectivity: the amine and
boron groups are selectively installed to the benzylic and
homobenzylic positions, respectively, and the only syn-
stereoisomer was detected in all entries.
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Table 1. Copper-Catalyzed Aminoboration of trans-b-
Methylstyrene ((E)-1a) with Bis(pinacolato)diboron and
Various O-Benzoylhydroxylamines 2a
R2 R1
10 mol % CuCl
10 mol % dppbz
N
R1
+
+
N
R2
OBz
pinB Bpin
Ph
Ph
LiO-t-Bu
Bpin
THF, rt, 4 h
(E)-1a
2
3
syn/anti = >99:1
Table 2. Copper-Catalyzed Aminoboration of Various
Alkenes 1 with Bis(pinacolato)diboron and O-Benzoyl-N,N-
dibenzylhydroxylamines (2b)a
entry
1
2
Et
3, yield (%)b
N
OBz
2a
3aa, 81 (66)
Ph Ph
Et
10 mol % CuCl
10 mol % dppbz
N
Ph
Ph
Ph
3ab, 86 (83, 85c)
3ac, 86 (73)
R2
Bpin
R2
N
OBz
2b
2
3
+
+
OBz pinB Bpin
N
R1
R1
LiO-t-Bu
THF, rt, 4 h
Ph
1
2b
3
Ph
syn/anti = >99:1
N
OBz
2c
Me
n-Bu
entry
1
R1, R2 1
3, yield (%)b
3bb, 81 (69)
3cb, 81 (76)d
3db, 91 (74)
3eb, 78 (76)
3fb, 77 (73)
3gb, (66)
N
OBz
R1 = 4-MeOC6H4, R2 = Me (1b)
R1 = 4-CF3C6H4, R2 = Me (1c)
R1 = Ph, R2 = H (1d)
R1 = 4-MeOC6H4, R2 =H (1e)
R1 = 4-CF3C6H4, R2 = H (1f)
R1 = 2-BrC6H4, R2 = H (1g)
R1 = 2-naphthyl, R2 = H (1h)
R1 = Ph, R2 = CH2OMe (1i)
1i
4
3ad, 91 (86)
2d
2c
3
N
OBz
2e
5
6
3ae, 74
3af, 95
4
5
N
OBz
2f
6
7
8e
3hb, (51)
2g
O
N
OBz
7
3ag, 44 (43)
3ah, (58)
3ia, 64 (48)
3ib, 43 (42)
3jb, 71 (71)g
3kb, (79)
8d
BocN
N
OBz
2h
9
10f
11h
R1 = n-C6H13, R2 = H (1j)
R1 = 3-ClC6H4, R2 = i-Pr (1k)
R1 = c-C6H11, R2 = H (1l)
9d
3ai, (64)
3ab’, 30
N
OBz
2i
12f
3lb, (67)i
10e
a
2a
a,b
c
See the footnote of Table 1. With a 96:4 mixture of E/Z
d
A mixture of CuCl (0.025 mmol), dppbz (0.025 mmol), (E)-
1a (0.25 mmol), 2 (0.38 mmol), pinB–Bpin (0.38 mmol), and
LiO-t-Bu (0.75 mmol) in THF (1.5 mL) was stirred at room
temperature for 4 h under N2. Yield estimated by 1H NMR with
isomer.
Isolated as a 96:4 mixture of syn- and anti-
f
e
stereoisomers. With 2a instead of 2b. With xantphos instead
of dppbz. g Isolated as an 88:12 regioisomeric mixture of 3jb and
3jb’. With a 92:8 mixture of E/Z isomer. Isolated as a 90:10
regioisomeric mixture of 3lb and 3lb’.
b
h
i
1-methylnaphthalene as an internal standard. Yield of isolated
product in parenthesis. The lower isolated yield is due to the
partial decomposition during chromatographic purification. See
In entry 2 of Table 2, we observed a small but significant
amount of anti-stereoisomer probably owing to the contamination
of Z-isomer of the starting styrene 1c.24 The phenomenon
suggests the potential of stereospecificity under the present
catalysis. To check the possibility, we implemented
aminoboration of cis-β-methylstyrene ((Z)-1a) (Scheme 2).
c
the Supporting Information for the detailed procedure. On a 1.0
d
mmol scale. With 2 (0.30 mmol), pinB–Bpin (0.30 mmol), and
e
NaO-t-Bu (0.50 mmol).
(neoB–Bneo) instead of pinB–Bpin.
With bis(neopentylglycolato)diboron
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Pleasingly, the reaction occurred stereospecifically and anti-3ab
was formed exclusively,20 while the efficiency was relatively
low.25 The cyclic Z-alkene, indene (1m) was also transformed
syn-1,2-diamine 5aa at the synthetically useful level. These
sequential manipulations are a good alternative to the precedented
osmium-catalyzed oxyamination29 and diamination30 of styrenes.
1
2
3
4
5
6
7
8
9
stereospecifically into the cis-1,2-aminoborane 3me.
The
Scheme 4. Transformation of Aminoborated Products
stereochemical outcomes again confirm the syn-addition mode of
the present aminoboration.26 Given the syn addition of the
borylcopper Cu–B across alkenes (Scheme 1, B to C), the C–N
bond formation occurs with retention of configuration (Scheme 1,
C to D).
10 mol % CuCl
10 mol % dppbz
R
+
+
pinB Bpin
Ph
N
OBz
LiO-t-Bu
THF, rt, 4 h
R
(E)-1a
2
R
Ph
Et
R
N
N
NaBO3•OH2
THF/H2O
R
R
4aa: R = Et 54%
4ab: R = Bn 80%
Scheme 2. Catalytic Aminoboration of Z-Styrenes
N
OH
Et
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Ph Ph
Ph
1) BuLi, MeONH2
THF
Bpin
10 mol % CuCl
10 mol % dppbz
N
3
5aa 52%
2) Boc2O
Ph
Ph
+
+
pinB Bpin
2b
Ph
NHBoc
LiO-t-Bu
Bpin
THF, rt, 4 h
(Z)-1a
3ab 52%
Finally, we applied the present protocol to the catalytic
enantioselective aminoboration by using an appropriate optically
active ligand. Preliminary investigation into some representative
chiral biphosphines identified a Duphos-type ligand to be a
promising candidate (Scheme 5). The aminoboration of (E)-1a
with 2a in the presence of (S,S)-Me-Duphos afforded the
corresponding aminoborane 3aa in 83% yield with 92:8 er.31
Similar enantiomer ratios were observed for other substrate
combinations. Further efforts on increasing the enantioselectivity
and elucidation of the stereochemical course are now in progress.
syn/anti = <1:99
10 mol % CuCl
10 mol % dppbz
N
+
+
pinB Bpin
2e
NaO-t-Bu
Bpin
THF, rt, 4 h
1m
3me 61% (NMR)
cis/trans = >99:1
In the above studies, some aminoborated products were
unstable for the column chromatographic purification, and the
relatively lower isolated yields were obtained compared to those
checked by 1H NMR in the crude materials.
Moreover,
contamination of some impurities was inevitable in several cases.
Thus, to modify the purification process, we attempted the direct
conversion into trifluoroborate salts. Gratifyingly, upon exposure
of the crude reaction mixture into KHF2 in a THF/H2O mixed
solvent, the corresponding borate salts 3-BF3 could be obtained
generally in higher yields through simple filtration.27 Analogous
to Molander’s original work,28 all 3-BF3 were obtained as not a
potassium salt but an internal ammonium salt (Scheme 3). It
should be noted that the trifluoroborates were isolated with high
Scheme 5. Catalytic Enantioselective Aminoboration
R2
+
+
N
OBz
pinB Bpin
R2
2a: R2 = Et
2b: R2 = benzyl
R1
1a: R1 = H
1b: R1 = OMe
R2 R2
N
10 mol % CuCl
10 mol % (S,S)-Me-Duphos
P
P
LiO-t-Bu, THF, rt, 4 h
Bpin
R1
3
(S,S)-Me-Duphos
1
syn/anti = >99:1
purity, judged by H NMR. The analytically pure salts are quite
Ph Ph
N
stable and can be stored under ambient conditions at least for
three months. Additionally, with the modified procedure, an
acceptable isolated yield of 3aa-BF3 was observed even in the
presence of 5 mol % of CuCl/dppbz.
Ph Ph
Et
Et
N
N
Bpin
Bpin
Bpin
MeO
3aa 83%, 92:8 er
3ab 72%, 93:7 er
3bb 51%, 90:10 er
Scheme 3. Direct Conversion into Internal Borate Salts
10 mol % CuCl
10 mol % dppbz
R1
N
R2
In conclusion, we have developed a copper-catalyzed
R2
+
+
OBz
pinB Bpin
R1
aminoboration of styreneswith bis(pinacolato)diboron and O-
benzoyl-N,N-dialkylhydroxylamines. The key to success is the
introduction of the umpolung, electrophilic amination chemistry.
The catalytic reaction takes place very smoothly even at room
temperature as well as regisoselectively and stereospecifically.
Moreover, the asymmetric catalysis is also achieved by using an
appropriate chiral biphosphine ligand, while further improvements
are essential. Current research seeks to elucidate the detailed
reaction mechanism,32 expand the substrate scope, and develop
additional useful transformations of the aminoborated products.
LiO-t-Bu
THF, rt, 4 h
1
2
H
R2 R1
N
R2 R1
N
1) KHF2, THF/H2O, rt, 2 h
2) filtration
R2
R2
R1
R1
BF
Bpin
3 3-BF3
3
syn/anti = >99:1
H
H
H
H
N
Me
Et
Et
Et
Et
H
N
Ph
N
N
N
Ph
Ph
Ph
Ph
OMe
Ph
BF3
BF3
BF3
BF3
BF3
3aa-BF3 83% 3ac-BF3 65% 3ae-BF3 77% 3af-BF3 73% 3ia-BF3 68%b
ASSOCIATED CONTENT
(75%)a
Supporting Information
Experimental details and characterization data for new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
a With 5 mol % of CuCl/dppbz. b With NaO-t-Bu instead of LiO-t-Bu.
To demonstrate synthetic utilities of the present aminoboration,
transformation of the products was carried out (Scheme 4). The
catalytic aminoboration followed by oxidation with NaBO3•OH2
afforded the corresponding syn-1,2-aminoalcohols 4 in good
overall yields. Moreover, the stereoretentive amination of the
resultant C–B bond12m with MeONHLi also proceeded to form the
AUTHOR INFORMATION
Corresponding Author
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Recent examples: (e) Berman, A. M.; Johnson, J. S. J. Am. Chem. Soc.
2004, 126, 5680. (f) Liu, S.; Liebeskind, L. S. J. Am. Chem. Soc. 2008,
k_hirano@chem.eng.osaka-u.ac.jp;
u.ac.jp
Notes
The authors declare no competing financial interests.
miura@chem.eng.osaka-
1
2
3
4
5
6
7
8
130, 6918. (g) He, C.; Chen, C.; Cheng, J.; Liu, C.; Liu, W.; Li, Q.; Lei,
A. Angew. Chem., Int. Ed. 2008, 47, 6414. (h) Barker, T. J.; Jarvo, E. R. J.
Am. Chem. Soc. 2009, 131, 15598. (i) Hatakeyama, T.; Yoshimoto, Y.;
Ghorai, S. K.; Nakamura, M. Org. Lett. 2010, 12, 1516. (j) Rucker, R. P.;
Whittaker, A. M.; Dang, H.; Lalic, G. J. Am. Chem. Soc. 2012, 134, 6571.
(k) Grohmann, C.; Wang, H.; Glorius, F. Org. Lett. 2012, 14, 656. (l)
Xiao, Q.; Tian, L.; Tan, R.; Xia, Y.; Qiu, D.; Zhang, Y.; Wang, J. Org.
Lett. 2012, 14, 4230. (m) Mlynarski, S. N.; Karns, A. S.; Morken, J. P. J.
Am. Chem. Soc. 2012, 134, 16449. (n) Zhu, C.; Li, G.; Ess, D. H.; Falck, J.
R.; Kürti, L. J. Am. Chem. Soc. 2012, 134, 18253. (o) Miura, T.;
Morimoto, M.; Murakami, M. Org. Lett. 2012, 14, 5214.
ACKNOWLEDGMENT
This work was supported by Grants-in-Aid for Scientific
Research from MEXT and JSPS, Japan. K.H. acknowledges “The
Uehara Memorial Foundation” for financial support.
(13) (a) Tsuda, T.; Hashimoto, T.; Saegusa, T. J. Am. Chem. Soc. 1972, 94, 658.
(b) Lemmen, T. H.; Goeden, G. V.; Huffman, J. C.; Geerts, R. L.; Caulton,
K. G. Inorg. Chem. 1990, 29, 3680.
(14) Laitar, D. S.; Müller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005, 127,
17196.
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(2) Selected examples: Palladium: (a) Pelz, N. F.; Woodward, A. R.; Burks, H.
E.; Sieber, J. D.; Morken, J. P. J. Am. Chem. Soc. 2004, 126, 16328. (b)
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Platinum: (c) Ishiyama, T.; Matsuda, N.; Miyaura, N.; Suzuki, A. J. Am.
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B.; Miller, S. P.; Morken, J. P. J. Am. Chem. Soc. 2003, 125, 8702. Gold:
(k) Baker, R. T.; Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1336. Silver: (l) Ramírez, J.; Corberán, R;
Sanaú, M.; Peris, E.; Fernandez, E. Chem. Commun. 2005, 3056. Copper:
(m) Yoshida, H.; Kawashima, S.; Takemoto, Y.; Okada, K.; Ohshita, J.;
Takaki, K. Angew. Chem., Int., Ed. 2012, 51, 235. Organocatalytic
process: (n) Bonet, A.; Pubill-Ulldemolins, C.; Bo, C.; Gulyás, H.;
Fernández, E. Angew. Chem., Int. Ed. 2011, 50, 7158.
(15) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. Organometallics 2006, 25, 2405.
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5792. (b) Ohishi, T.; Zhang, L.; Nishiura, M.; Hou, Z. Angew. Chem., Int.
Ed. 2011, 50, 8114.
(17) In the course of this study, copper-catalyzed carboboration and
stannylboration of alkynes based on a similar concept were reported; (a)
Zhang, L.; Cheng, J.; Carry, B.; Hou, Z. J. Am. Chem. Soc. 2012, 134,
14314. (b) Alfaro, R.; Parra, A.; Alemán, J.; Ruano, J. L. G.; Tortosa, M.
J. Am. Chem. Soc. 2012, 134, 15165. (c) Takemoto, Y.; Yoshida, H.;
Takaki, K. Chem. Eur. J. 2012, 18, 14841.
(18) For a precedented study, see; Campbell, M. J.; Johnson, J. S. Org. Lett.
2007, 9, 1521.
(19) In the absence of CuCl, dppbz, or CuCl/dppbz, no aminoborated product
was detected. See the Supporting Information for the detailed
optimization studies.
(20) The relative stereochemistry was confirmed after oxidation to the
corresponding aminoalcohol. See the Supporting Information for details.
(21) Wang, J.-Y.; Wang, D.-X.; Zheng, Q.-Y.; Huang, Z.-T.; Wang, M.-X. J.
Org. Chem. 2007, 72, 2040.
(22) (a) Noack, M.; Göttlich, R. Chem. Commun. 2002, 536. Also see: (b)
Tsuritani, T.; Shinokubo, H.; Oshima, K. Org. Lett. 2001, 3, 2709. (c)
Tsuritani, T.; Shinokubo, H.; Oshima, K. J. Org. Chem. 2003, 68, 3246.
(23) The conceivable Br migration reaction was not detected at all. See; Grigg,
R. D.; Hoveln, R. V.; Schomaker, J. M. J. Am. Chem. Soc. 2012, 134,
16131.
(24) Although the alkene 1k also contained the Z-isomer, only the syn-
aminoborated product was observed. This is probably because the Z-
isomer of much lower reactivity could not participated in the reaction.
See ref 25 for the effect of 1,3-allylic strain in the insertion step.
(25) Such a reactivity trend is consistent with reported literature. This is
probably because a 1,3-allylic strain decrease the aryl-alkene conjugation,
which lowers alkene π* and more effective back-bonding. See; (a) Dang,
Li.; Zhao, H.; Lin, Z.; Marder, T. B. Organometallics 2007, 26, 2824. (b)
Deng, L.; Lin, Z.; Marder, T. B. Organometallics 2008, 27, 4443, and ref
10a.
(3) Very selected recent publications: (a) Ohmura, T.; Taniguchi, H.; Kondo,
Y.; Suginome, M. J. Am. Chem. Soc. 2007, 129, 3518. (b) Ohmura, T.;
Matsuda, K.; Suginome, M. J. Am. Chem. Soc. 2008, 130, 1526. (c)
Ohmura, T.; Takasaki, Y.; Furukawa, H.; Suginome, M. Angew. Chem.,
Int. Ed. 2009, 48, 2372, and references therein. A KO-t-Bu-mediated
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8006.
(4) Suginome, M.; Matsuda, T.; Ito, Y. Organometallics 1998, 17, 5233.
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(26) Cyclohexene
gave
the
corresponding
cis-1,2-aminoborane
(6) Ishiyama, T.; Nishijima, K.; Miyaura, N.; Suzuki, A. J. Am. Chem. Soc.
1993, 115, 7219.
stereospecifically albeit in only 17% yield, judged by 1H NMR.
(27) See the Supporting Information for the detailed procedure.
(28) Raushel, J.; Sandrock, D. L.; Josyula, K. V.; Pakyz, D.; Molander, G. A. J.
Org. Chem. 2011, 76, 2762.
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(31) The enantiomer ratio and absolute configuration were after oxidation into
the corresponding aminoalcohol. See the Supporting Information for
details.
(32) LiO-t-Bu also can accelerate some elementary steps through its
coordination to the copper or boron center. For a relevant discussion, see;
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