Asymmetric rhodium-directed anti-markovnikov regioselective boracyclopentannulation

Article abstract of DOI:10.1021/ja309018f

A Shimoi-type activation of B-H bond of NHC-boranes by a diphosphane-ligated cationic Rh complex was applied in an unprecedented intramolecular hydroboration reaction of simple olefins. The use of NHC-boranes as hydroborating reagents is still undisclosed due to their nonreactivity toward alkenes which could be explained by the high stability of this complex rendering it unable to provide a "free" borane hydroborating reagent. B-H bond Rh activation of NHC-borane circumvents this limitation, and asymmetric Rh-directed anti-Markovnikov boracyclopentannulation reaction led to a library of enantioenriched cyclic boranes in high yield (up to 94%) with high regio- (up to 100%) and enantioselectivity (er up to 99.2:0.8). This new activation mode of NHC-boranes highlights their use in organometallic chemistry and offers a very good approach to access chiral cyclic NHC-boranes.

Full text of DOI:10.1021/ja309018f

Communication
pubs.acs.org/JACS
Asymmetric Rhodium-Directed anti-Markovnikov Regioselective
Boracyclopentannulation
Momar Toure, Olivier Chuzel,* and Jean-Luc Parrain*
́
Aix Marseille Universite, CNRS, ISM2 UMR 7313, case 532, 13397, Marseille cedex 20, France
S
* Supporting Information
ABSTRACT: A Shimoi-type activation of B−H bond of
NHC-boranes by a diphosphane-ligated cationic Rh
complex was applied in an unprecedented intramolecular
hydroboration reaction of simple olefins. The use of NHC-
boranes as hydroborating reagents is still undisclosed due
to their nonreactivity toward alkenes which could be
explained by the high stability of this complex rendering it
unable to provide a “free” borane hydroborating reagent.
B−H bond Rh activation of NHC-borane circumvents this
limitation, and asymmetric Rh-directed anti-Markovnikov
boracyclopentannulation reaction led to a library of
enantioenriched cyclic boranes in high yield (up to 94%)
with high regio- (up to 100%) and enantioselectivity (er
up to 99.2:0.8). This new activation mode of NHC-
boranes highlights their use in organometallic chemistry
and offers a very good approach to access chiral cyclic
NHC-boranes.
Figure 1. Lewis base−borane hydroboration of alkenes: (a) with amine
or phosphine borane; (b) with no reaction with NHC-borane or
reaction via an activation with a borenium species (see ref 8); (c) in this
work.
process, which could be foreseen in an asymmetric version by
using chiral Rh complexes (Figure 1c).
ince the isolation of stable N-heterocyclic carbenes (NHCs)
Sby Arduengo and their intensive use in the past decade as
ligands in organometallic catalysis1 or catalysts in organic
chemistry,2 NHC-boranes emerged also as new reagents3 in
the NHC constellation. They were readily employed in radical4
or ionic5,6 reactions and as efficient co-initiators for photo-
polymerization processes.7 Surprisingly, the use of NHC-borane
as a hydroborating reagent of simple olefins is still undisclosed,
unless a more specific activation is provided, as very recently
reported with borenium ions in a racemic manner (Figure 1b).8
The high stability to decomplexation of NHC-borane complexes
could explain this nonreactivity toward alkenes, in sharp contrast
with their amine9 or phosphine10 borane analogues, which lead
to either inter- or intramolecular hydroboration reactions. In
these cases, the conventional hydroboration step occurred via an
early stage N−B or P−B bond dissociation mechanism, generally
activated by heating (Figure 1a).11
Herein, we disclose our initial studies toward this goal and
report an unprecedented asymmetric Rh-directed anti-Markov-
nikov regioselective boracyclopentannulation (Figure 1c).15
NHC-boranes with a pendant allyl4c group moiety were chosen
because of their nonreactivity as reducing or hydroborating
agents. Regio- and enantioselectivity would be controlled by the
right choice of Rh and chiral ligand sources which are well-known
in the literature for the conventional hydroboration step of
alkenes with borane derivatives.16
NHC-borane 1a (R1 = Ph, R2, R3 = H, n = 1) (Scheme 1)17 was
prepared according to a general procedure described in the
literature.4c Dichloromethane was chosen as a noncoordinating
solvent instead of tetrahydrofuran for conventional hydro-
boration reaction conditions with BH3•SMe2, and classical Rh
sources were then evaluated in the catalytic process. To our
Scheme 1. Asymmetric Intramolecular Rhodium-Catalyzed
Because NHC-boranes were unreactive in hydroborations
compared to other labile borane−Lewis bases (BH3•Me2S or
BH3•THF), we were interested in their potential to be activated
by an organometallic source. As it was already described, in the
case of amine- or phosphine-boranes,12 Braunschweig et al. have
isolated NHC-borane adducts coordinated to a metal center
(Mn, Cr, Mo, and W) through a B−H−M three-center two-
electron bond.13 From these observations, we have hypothesized
that a Rh catalyst could activate the NHC-borane species in a
Shimoi type complex,14 leading to an alkene hydroboration
Hydroboration of NHC-Boranes (1a−1q)
Received: September 11, 2012
Published: October 18, 2012
© 2012 American Chemical Society
17892
dx.doi.org/10.1021/ja309018f | J. Am. Chem. Soc. 2012, 134, 17892−17895

Journal of the American Chemical Society
Communication
delight, a highly regioselective cyclization occurred leading to
cyclic NHC-borane 2a (Scheme 1) in good yields in the case of
neutral Rh species (Table 1, entries 1 and 2) and in low yield with
4 h, with a high enantioselectivity (er = 97.0:3.0) (regioisomer 3a
1
was not detected in the crude by H and 11B NMR). Other
ligands were screened with the [Rh(nbd)2]BF4 source, and
diphosphane ligands with C2 axial symmetry (Table 1, entries 6,
10, and 11) were found to be the most efficient and selective,
especially (S)-L1 and (S)-L5 (er = 97.0:3.0 and 97.6:2.4
respectively). Among the complexes tested, only electron-rich
diphosphane ligated cationic rhodium C4 ((S)-catASium)
yielded the Markovnikov adduct as a minor regioisomer (2a
and 3a in a 2:1 ratio). It should be noted that each ligand, with a
different symmetry element with an (S)-absolute configuration,
gave the (R)-absolute configuration, which was determined by
vibrational circular dichroism (VCD) with a good degree of
confidence (see Supporting Information (SI)).
To optimize these results, several parameters were modified
based, first, on the catalytic system [Rh(nbd)2]BF4 (2.5 mol %),
(S)-L1 (5.0 mol %). Solvent influence, metal/ligand ratio,
catalytic load, concentration, and temperature were evaluated in
the reaction; however no improvement in conversion,
regioselectivity, and enantioselectivity was observed.20 Never-
theless, it was found that the use of [Rh(nbd)2]BF4, (S)-BINAP
in a 1:1 ratio with a 5.0 mol % catalytic load drastically enhanced
the reaction rate (2 min, 100% conv, 95% yield) and maintained
high enantioselectivity (er = 97.0:3.0). In view of these results,
this catalytic system was retained thereafter.
Table 1. Rhodium Source and Chiral Ligand Optimization for
Racemic and Enantioselective Intramolecular Catalyzed
a
Hydroboration of NHC-Borane 1a
b
c
d
entry
Rh source
ligand conv (%)
yield (%) 2a
er (R:S)
e
1
[Rh(cod)Cl]2
Rh(PPh3)3Cl
[Rh(nbd)2]BF4
[Rh(cod)Cl]2
Rh(PPh3)3Cl
[Rh(nbd)2]BF4
[Rh(nbd)2]BF4
[Rh(nbd)2]BF4
C4
−
−
−
L1
L1
L1
L2
L3
−
50
70
20
90
95
100
30
2
45 (90:10)
−
−
−
−
e
2
68 (98:2)
e
3
18 (90:10)
4
−
−
5
−
6
94
25
−
97.0:3.0
7
39.5:60.5
8
−
f
9
70
100
50
40 (20)
76.0:24.0
97.5:2.5
85.0:15.0
10
11
[Rh(nbd)2]BF4
[Rh(nbd)2]BF4
L5
84
40
L6
a
Reactions were carried out in CH2Cl2 (1 mL) at 20 °C under an
argon atmosphere containing 1a (0.125 mmol), [Rh] (2.5 mol %), and
b
c
ligand (5.0 mol %). Determined by 1H NMR. Yield after purification
d
by chromatography on silica gel. Er determined by HPLC analysis
(Chiralcel OD-3, hexane/isopropanol 95/5, 1 mL·min−1); Absolute
e
configuration R determined by VCD. Ratio (2a:3a) determined for
the crude mixture by H NMR. Isolated yield of 3a.
f
1
The scope of the NHC-borane intramolecular asymmetric Rh-
catalyzed hydroboration reaction was then evaluated with L5.21
First, we investigated the influence of the NHC-borane aryl
group, in the enantioselective hydroboration step (Figure 4, 2a−
2h). The enantioselectivity was slightly improved from the
phenyl (2a, er = 97.6:2.4) to mesityl group (2b, er = 98.5:1.5),
but too much steric hindrance led to a lower enantioselectivity
with the 2,6-diisopropylphenyl group (2c, er = 94.5:5.5). With
the alkyl group, high enantioselectivities were obtained, more
particularly with a methyl substituent (2e, er = 95.8:4.2) instead
of a tert-butyl group (2d, er = 92.0:8.0), probably due to the
higher steric hindrance in the latter case. Attempts with vinyl
(1f), allyl (1g), and homoallyl (1h) substituted boranes were
conducted to evaluate the chemoselectivity of the reaction and
the feasibility of two consecutive intramolecular hydroboration
steps. Unfortunately, 1f did not react in the catalytic Rh
hydroboration procedure giving 2f in only trace amounts, and 1f
was fully recovered at the end of the reaction. However, 1g and
1h gave the corresponding cyclic NHC-boranes 2g (er =
96.9:3.1) and 2h (er = 97.3:2.7) in high yields, chemoselectivities
(only hydroboration of the allyl group moiety), and
enantioselectivities. Nevertheless, tricyclic NHC-boranes,
which could be derived from a second intramolecular hydro-
boration step, were not detected. Crystallographic data of 2a
(Figure 3b) indicated that the two fused cycles were quasi
a cationic Rh source (Table 1, entry 3). The structure was
secured by X-ray diffraction analysis (Figure 4b). While the
noncatalyzed hydroboration of alkenes with BH3•SMe2 gave the
terminal-borylated Markovnikov adduct, we observed, predom-
inantly, the five-membered ring α-borylated adduct 2a.18
Due to the impact of the ligand on the reactivity and
regioselectivity of the hydroboration reaction, we next evaluated
various diphosphane chiral ligands L1−L6 (Figure 2),19 and
Figure 2. Ligands and rhodium complex structures.
some results are summarized in Table 1 (entries 4−11). First,
(S)-BINAP was tested with neutral Rh sources (Table 1, entries 4
and 5). Surprisingly, despite very good conversion rates, the
1
cyclic NHC-borane 2a or 3a was not obtained and H NMR
analysis revealed a complex unidentified mixture. No significant
signals were observed beyond those corresponding to an N-
propyl substituent in an imidazolium environment, suggesting
hydrogenation of the allyl group. Fortunately, the use of the
cationic Rh complex [Rh(nbd)2]BF4 in the presence of (S)-
BINAP (Table 1, entry 6) gave only 2a in an excellent yield after
Figure 3. ORTEP diagrams of (a) 1b and (b) 2a, with thermal ellipsoids
at 50% probability. Selected bond distances (Å) and angles (deg). 1b:
B1−C1 1.593(4); B1−C1−N1 129.3(2), B1−C1−N2 126.2(2), N1−
C1−N2 104.5(2). 2a: B1−C1 1.594(5), B1−C11 1.629(5); B1−C1−
N1 143.0(3), B1−C1−N2 111.6(2), C1−B1−C11 99.4(2).
17893
dx.doi.org/10.1021/ja309018f | J. Am. Chem. Soc. 2012, 134, 17892−17895

Journal of the American Chemical Society
Communication
Figure 4. Scope and limitations of NHC-borane asymmetric intramolecular rhodium-catalyzed hydroboration with L5. aWith a stoichiometric amount
of catalyst [Rh(nbd)2]BF4, (rac)-BINAP.
coplanar, and the B1−C1−N1 (143.0°) and B1−C1−N2
(111.56°) angles were very different and clearly displaced
compared to the case of the initial NHC-borane 1b (B1−C1−
N1, 129.3°, B1−C1−N2, 126.2°).
So, from bis-allyl-NHC-borane 1g, a fused tricyclic compound
would be too strained to be stable; nevertheless, in the case of 2h
(more flexible), it seems reasonable to obtain a tricyclic fused
NHC-borane. Another explanation would be that, under these
conditions, the hydrogen atoms from the cyclic NHC-borane
2g−2h would not be sufficiently activated by the Rh catalytic
system for a second hydroboration step. In fact, the catalytic
reaction conducted on 1i and 1j showed, undoubtedly, that the
Figure 5. Deuterated scrambling study in CD2Cl2 (1.0 mL), 11B NMR
Rh-catalyzed hydroboration step did not occur on vinyl or
(128 MHz). Equimolar addition of 1a and 1b•D3 (0.125 mmol). Red
spectrum, t = 0 min before [Rh(nbd), (S)-BINAP]BF4 (5 mol %)
addition; Blue spectrum, t = 10 min, after completion of the Rh-
catalyzed hydroboration.
homoallyl side chains of the NHC-borane. In order to explain the
nonreactivity of 1f, in contrast to that of 1g−1h, we postulated
that the electron-rich vinyl group (that could be compared to an
enamine group) would coordinate the Rh catalytic species as a
ligand and would poison the catalytic reaction. To observe the
hypothetic enamine coordinated species by 1H NMR at 20 °C,
attempts using a stoichiometric amount of the complex
[Rh(nbd)2]BF4-(rac)-L1 and 1f led finally to 2f in 40% yield in
18 h. A large excess in Rh catalyst seemed to overcome the
poisoning effect and slowly catalyzed the hydroboration reaction.
The scope was extended to allyl-(α,β)-disubstituted NHC-
boranes (1k−1p). In all cases, the asymmetric intramolecular
Rh-catalyzed reaction was totally regioselective and only the 5-
membered cyclic NHC-borane adducts were formed. Cyclic
NHC-boranes (2k−2p) were isolated in fair to good yields,22
albeit with excellent enantioselectivities (2p, er = 99.2:0.8; Figure
4). In these cases, the reaction rate strongly decreased and a
complete reaction was obtained after 18 h, certainly due to the
steric hindrance on the double bond of the allyl group. No effect
was observed on reactivity and enantioselectivity by changing the
electronic properties of the double bond (see 2k, 2m−o).
However, attempts to cyclize trisubstituted allyl-NHC-borane 1q
into 2q failed. From these observations, we thought that
boracyclopentannulation would occur only when both olefin
and B−H bonds are simultaneously activated by the Rh catalyst
species. Thus, geometry around the rhodium, including agostic
B−H−Rh and ligated olefin bonds, must be ideal, while too long
tether or steric hindrance near the olefin subunit is unfavorable
for the intramolecular hydroboration.
2b•D3 was observed by 1H NMR and 11B NMR (s, −23.2 ppm)
spectroscopies, with similar enantioselectivity that in the case of
2b (er = 98.7:1.3) revealing no noticeable isotopic effect. A
scrambling study (Figure 5) was also conducted by mixing 1a and
1b•D3 in an equimolar ratio. After completion of the reaction,
1
only cyclic boranes 2a and 2b•D3 were observed by H NMR
and 11B NMR without the migration of hydride or deuteride via a
hypothetical [Rh]-H/-D exchange.
In summary, we have developed the unprecedented activation
of a B−H bond of NHC-boranes by a diphosphane-ligated
cationic Rh complex and applied this reaction in intramolecular
hydroborations of alkenes. Independent of the unreactive N-
substituent, N-allyl NHC boranes yielded a library of
enantioenriched cyclic boranes in high yield (up to 94%),
regio- (up to 100%) and enantioselectivity (up to 99.2:0.8). This
new activation mode of NHC-boranes highlights their use in
organometallic chemistry5a,23,24 and further investigations of
metal-catalyzed intermolecular hydroboration using NHC
boranes are underway and will be reported in due course.
Emerging recently as new promising reagents, these new cyclic
NHC-boranes could be useful tools, notably as chiral hydride
donors.5b,6
ASSOCIATED CONTENT
* Supporting Information
■
S
Finally, to verify if the Rh-catalyzed hydroboration process was
fully an intramolecular process, some deuterated experiments
were conducted. First, mesityl-borane-D3 adduct (Figure 5,
1b•D3) (11B NMR, s, −37.8 ppm) was prepared and then
engaged in the Rh-catalytic procedure. Only the formation of
Contains complete experimental details and additional copies of
NMR and VCD spectra, HPLC traces, CIF for structures 1b
(CCDC 892960) and 2a (CCDC 892959). This material is
available free of charge via the Internet at http://pubs.acs.org.
17894
dx.doi.org/10.1021/ja309018f | J. Am. Chem. Soc. 2012, 134, 17892−17895

Journal of the American Chemical Society
Communication
2004, 69, 4094. (d) De Vries, T. S.; Prokofjevs, A.; Vedejs, E. Chem. Rev.
2012, 112, 4246. (e) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem.
Rev. 2010, 110, 4023.
AUTHOR INFORMATION
Corresponding Author
jl.parrain@univ-amu.fr; olivier.chuzel@univ-amu.fr
■
(12) (a) Kakizawa, T.; Kawano, Y.; Shimoi, M. Organometallics 2001,
20, 3211. (b) 1 Kawano, Y.; Yamaguchi, K.; Miyake, S. Y.; Kakizawa, T.;
Shimoi, M. Chem.Eur. J. 2007, 13, 6920. (c) Piers, W. E. Angew. Chem.,
Int. Ed. 2000, 39, 1923. (d) Chaplin, A. B.; Weller, A. S. Angew. Chem.,
Int. Ed. 2010, 49, 581. (e) Tang, C. Y.; Thompson, A. L.; Aldridge, S.
Angew. Chem., Int. Ed. 2010, 49, 921. (f) Gloaguen, Y.; Alcaraz, G.; Petit,
A. S.; Clot, E.; Coppel, Y.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc.
2011, 133, 17232. (g) Alcaraz, G.; Sabo-Etienne, S. Angew. Chem., Int.
Ed. 2010, 49, 7170.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the French Research Ministry
(Grant 2010-2013, M.T.), Aix Marseille Universite,
(UMR 7313). We thank Dr. M. Giorgi for the X-ray diffraction
studies, and Dr. J.-V. Naubron for the vibrational circular
dichroism spectra (www.spectropole.fr).
■
́
and CNRS
(13) Bissinger, P.; Braunschweig, H.; Kupfer, T.; Radacki, K.
Organometallics 2010, 29, 3987.
(14) Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.; Katoh, K.;
Uruichi, M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704.
(15) Some cyclic NHC-boranes synthesized by C−H insertion were
REFERENCES
■
(1) Selected reviews: (a) Carbenes special issue,. Chem. Rev. 2009, 109,
3209. (b) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem.,
Int. Ed. 2007, 46, 2768. (c) Marion, N.; Nolan, S. P. Acc. Chem. Res.
̂
also reported: (a) Curran, D. P.; Boussonniere, A.; Geib, S. J.; Lacote, E.
Angew. Chem., Int. Ed. 2012, 51, 1602. (b) Bissinger, P.; Braunschweig,
H.; Damme, A.; Dewhurst, R. D.; Kupfer, T.; Radacki, K.; Wagner, K. J.
Am. Chem. Soc. 2011, 133, 19044. (c) Wang, Y. Z.; Robinson, G. H.
Inorg. Chem. 2011, 50, 12326. By ortholithiation: (d) Rao, Y.-L.; Chen,
L. D.; Mosey, N. J.; Wang, S. J. Am. Chem. Soc. 2012, 134, 11026.
2008, 41, 1440. (d) Wurtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523.
̈
(e) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2010, 110, 1746.
(2) Selected reviews: (a) Phillips, E. M.; Chan, A.; Scheidt, K. A.
Aldrichimica Acta 2009, 42, 55. (b) Nair, V.; Vellalath, S.; Babu, B. P.
Chem. Soc. Rev. 2008, 37, 2691. (c) Nair, V.; Menon, R. S.; Biju, A. T.;
Sinu, C. R.; Paul, R. R.; Jose, A.; Sreekumar, V. Chem. Soc. Rev. 2011, 40,
5336. (d) Grossmann, A.; Enders, D. Angew. Chem., Int. Ed. 2012, 51,
314. (e) Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107,
5606. (f) Moore, J. L.; Rovis, T. Top. Curr. Chem. 2009, 291, 77.
(g) Bugaut, X.; Glorius, F. Chem. Soc. Rev. 2012, 41, 3511.
(e) Nagura, K.; Saito, S.; Frohlich, R.; Glorius, F.; Yamaguchi, S. Angew.
̈
Chem., Int. Ed. 2012, 51, 7762.
(16) (a) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957.
(b) Carroll, A. M.; O’Sullivan, T. P.; Guiry, P. J. Adv. Synth. Catal. 2005,
347, 609. (c) Crudden, C. M.; Edwards, D. Eur. J. Org. Chem. 2003,
4695. (d) Edwards, D. R.; Hleba, Y. B.; Lata, C. J.; Calhoun, L. A.;
Crudden, C. M. Angew. Chem., Int. Ed. 2007, 46, 7799. (e) Wadepohl, H.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2441.
(3) Curran, D. P.; Solovyev, A.; Makhlouf Brahmi, M.; Fensterbank, L.;
Malacria, M.; Laco
(4) (a) Ueng, S. H.; Makhlouf Brahmi, M.; Derat, E.; Fensterbank, L.;
Lacote, E.; Malacria, M.; Curran, D. P. J. Am. Chem. Soc. 2008, 130,
10082. (b) Ueng, S. H.; Solovyev, A.; Yuan, X. T.; Geib, S. J.;
Fensterbank, L.; Lacote, E.; Malacria, M.; Newcomb, M.; Walton, J. C.;
Curran, D. P. J. Am. Chem. Soc. 2009, 131, 11256. (c) Walton, J. C.;
Makhlouf Brahmi, M.; Fensterbank, L.; Lacote, E.; Malacria, M.; Chu, Q.
L.; Ueng, S. H.; Solovyev, A.; Curran, D. P. J. Am. Chem. Soc. 2010, 132,
2350. (d) Pan, X.; Lacote, E.; Lalevee, J.; Curran, D. P. J. Am. Chem. Soc.
2012, 134, 5669. (e) Ueng, S. H.; Fensterbank, L.; Lacote, E.; Malacria,
M.; Curran, D. P. Org. Lett. 2010, 12, 3002. (f) Ueng, S. H.; Fensterbank,
L.; Lacote, E.; Malacria, M.; Curran, D. P. Org. Biomol. Chem. 2011, 9,
3415.
(5) (a) Chu, Q.; Makhlouf Brahmi, M.; Solovyev, A.; Ueng, S. H.;
Curran, D.; Malacria, M.; Fensterbank, L.; Laco
̂
te, E. Angew. Chem., Int. Ed. 2011, 50, 10294.
(17) Structures and yields of 1a−1q are described in the SI (see Part 3,
Table 1).
̂
(18) The ratio 2a:3a was evaluated for the crude mixture by 1H NMR,
because the small amount of Markovnikov regioisomer adduct 3a was
not recovered after purification on silica gel.
̂
(19) The complete ligand library evaluated is described in the SI (see
Part 5, Table 2).
̂
(20) Detailed optimization parameters considered in this reaction were
described in the SI (nature of solvents, metal/ligand ratios, catalytic
load, concentration, and temperature) (see Part 5, Tables 3−5).
(21) Both (S)-L1 and (S)-L5 were evaluated in the scope of the
reaction, and generally, L5 gave a slightly higher er. Enantiomeric ratios
with L1 are given in the SI (see Part 5, Figure 1).
̂
́
̂
̂
(22) Full conversions in the corresponding cyclic boranes were
̂
te, E. Chem.Eur. J.
1
observed in both H and 11B NMR, but moderate yields could be
2009, 15, 12937. (b) Lindsay, D. M.; McArthur, D. Chem. Commun.
2010, 46, 2474.
(6) (a) Horn, M.; Mayr, H.; Lacote, E.; Merling, E.; Deaner, J.; Wells,
̂
S.; McFadden, T.; Curran, D. P. Org. Lett. 2012, 14, 82. (b) Taniguchi,
T.; Curran, D. P. Org. Lett. 2012, 14, 4540.
(7) (a) Tehfe, M.-A.; Monot, J.; Makhlouf Brahmi, M.; Bonin-Dubarle,
̂ ́
H.; Curran, D. P.; Malacria, M.; Fensterbank, L.; Lacote, E.; Lalevee, J.;
Fouassier, J. P. Polym. Chem. 2011, 2, 625. (b) Tehfe, M.-A.; Makhlouf
explained by their instability on silica gel during their purification by
chromatography.
(23) Monot, J.; Makhlouf Brahmi, M.; Ueng, S. H.; Robert, C.; Desage-
El Murr, M.; Curran, D. P.; Malacria, M.; Fensterbank, L.; Laco
Lett. 2009, 11, 4914.
̂
te, E. Org.
(24) In this context, treatment of 1b in the standard Rh-catalyzed
intramolecular hydroboration reaction, followed by the oxidation/
hydrolysis process (30% aqueous H2O2/NaOH in MeOH), led to the
functionalized hydroxyl-NHC salt 4b (79% yield) (see SI Part 6). These
products would have interesting applications as chiral bidentate NHC
ligand precursors. For selected examples inthe literature, see: (a) Arnold,
P. L.; Rodden, M.; Wilson, C. Chem. Commun. 2005, 1743. (b) Arnold,
P. L.; Rodden, M.; Davis, K. M.; Scarisbrick, A. C.; Blake, A. J.; Wilson,
Brahmi, M.; Fouassier, J. P.; Curran, D. P.; Malacria, M.; Fensterbank,
L.; Laco
Telitel, S.; Tehfe, M.-A.; Fouassier, J.-P.; Curran, D. P.; Laco
Chem., Int. Ed. 2012, 51, 5958. (d) Tehfe, M.-A.; Monot, J.; Malacria, M.;
Fensterbank, L.; Fouassier, J. P.; Curran, D. P.; Lacote, E.; Lalevee, J.
ACS Macro Lett. 2012, 1, 92.
̂
(8) Prokofjevs, A.; Boussonniere, A.; Li, L.; Bonin, H.; Lacote, E.;
̂
te, E.; Lalevee
́
, J. Macromolecules 2010, 43, 2261. (c) Lalevee
́
, J.;
te, E. Angew.
̂
̂
́
C. Chem. Commun. 2004, 1612. (c) Tissot, M.; Poggiali, D.; Hen
Muller, D.; Guenee
, L.; Mauduit, M.; Alexakis, A. Chem.Eur. J. 2012,
18, 8731. (d) Henon, H.; Mauduit, M.; Alexakis, A. Angew. Chem., Int.
́
on, H.;
̀
́
́
̈
Curran, D. P.; Vedejs, E. J. Am. Chem. Soc. 2012, 134, 12281.
(9) Kanth, J. V. B. Aldrichimica Acta 2002, 35, 57.
́
Ed. 2008, 47, 9122. (e) Martin, D.; Kehrli, S.; d’Augustin, M.; Clavier,
H.; Mauduit, M.; Alexakis, A. J. Am. Chem. Soc. 2006, 128, 8416.
(f) Tissot, M.; Hernandez, A. P.; Muller, D.; Mauduit, M.; Alexakis, A.
Org. Lett. 2011, 13, 1524.
(10) (a) Schmidbaur, H.; Sigl, M.; Schier, A. J. Organomet. Chem. 1997,
529, 323. (b) Gaumont, A. C.; Bourumeau, K.; Denis, J. M.; Guenot, P. J.
Organomet. Chem. 1994, 484, 9.
(11) Hydroboration of alkenes was improved by using amine or
phosphine borenium analogues; see: (a) Scheideman, M.; Wang, G.;
Vedejs, E. J. Am. Chem. Soc. 2008, 130, 8669. (b) Karatjas, A. G.; Vedejs,
E. J. Org. Chem. 2008, 73, 9508. (c) Shapland, P.; Vedejs, E. J. Org. Chem.
17895
dx.doi.org/10.1021/ja309018f | J. Am. Chem. Soc. 2012, 134, 17892−17895