Chiral 6-NHC ligand and copper complex: Properties, application, and mechanism

Article abstract of DOI:10.1055/s-0031-1290820

We demonstrated the synthesis of chiral annulated six-membered-ring N-heterocyclic carbenes (6-NHC) and their adducts with water and alcohol. We also have shown the donating properties of chiral annulated 6-NHCs is between that of typical 5-NHCs and 6-NHCs. We have shown the synthetic conditions for 6-NHC-copper complex formation. Using the chiral 6-NHC-copper complex, we synthesized unsaturated chiral bimetallic allylic boronate reagents with high enantioselectivity. Finally, we propose a catalytic cycle that justifies observed reactivity and provides a new explanation for the role methanol has in the reaction cycle; our cycle is supported by a primary kinetic isotope effect.

Full text of DOI:10.1055/s-0031-1290820

SPECIAL TOPIC
▌1485
sCpehciailrtoapilc 6-NHC Ligand and Copper Complex: Properties, Application, and
Mechanism
Chiral 6-NHC Ligand and Copper Complex
Jin Kyoon Park, D. Tyler McQuade*
Department of Chemistry & Biochemistry, Florida State University, Tallahassee, FL 32306-4390, USA
E-mail: mcquade@chem.fsu.edu
Received: 02.03.2012; Accepted after revision: 09.03.2012
bene systems reported by the organocatalytic
Abstract: We demonstrated the synthesis of chiral annulated six-
community3e,f and flat imidazoquinazolines which are
membered-ring N-heterocyclic carbenes (6-NHC) and their adducts
known electron-transporting materials in organic light-
with water and alcohol. We also have shown the donating properties
of chiral annulated 6-NHCs is between that of typical 5-NHCs and
emitting diodes (OLED).8 This family of copper(I) com-
6-NHCs. We have shown the synthetic conditions for 6-NHC–cop- plexes exhibits very high turn-over numbers for β-boryla-
per complex formation. Using the chiral 6-NHC–copper complex,
we synthesized unsaturated chiral bimetallic allylic boronate re-
agents with high enantioselectivity. Finally, we propose a catalytic
amine conditions used to create the carbene, evaluate the
cycle that justifies observed reactivity and provides a new explana-
tion for the role methanol has in the reaction cycle; our cycle is sup-
tion reactions and stereoconvergent, as well as catalyst-
controlled, allylic substitution reactions.7 Herein, we ex-
donor properties of the ligand, apply the ligand to the
preparation of chiral bimetallic allyl boronate reagents via
copper(I) catalysis, and offer a catalytic cycle that defines
the role of protic additives.
ported by a primary kinetic isotope effect.
Key words: bimetallic reagents, copper, boron, carbene complex,
enantioselective substitution
Generation of free carbene and carbene adducts. The
synthesis of imidazolium pre-ligand 1a is described else-
where.7a Here, we wish to elaborate on how 1a reacts un-
der basic conditions. Hoping to obtain free carbene 3a, we
deprotonated 1a using lithium hexamethyldisilazanide or
potassium tert-butoxide.4j In the case of lithium hexa-
methyldisilazanide, we observed diastereomeric mixtures
of HMDS adduct 2a when only one equivalent of lithium
N-Heterocyclic carbenes (NHC) are important structures
from
a
biological,1 reactive intermediate,2 and
organocatalytic3 point of view, and as ligands in transi-
tion-metal complexes and catalysts.4 The majority of
NHC research centers around five-membered-ring spe-
cies, in part because these compounds were the first stable
examples.2 Recent investigation has demonstrated that
ring size impacts the donor capacity of the carbene and the
steric environment around the ring.5–7 The stronger basic-
ity of six-membered-ring N-heterocyclic carbenes (6-
NHCs) provides organometallic complexes with distinct
reactivity relative to those of five-membered-ring N-het-
erocyclic carbenes (5-NHC) (Figure 1).6 Though a num-
ber of six- and seven-membered-ring complexes are
known, this class of carbene has only recently shown sig-
nificant benefits over 5-NHCs.6,7
1
hexamethyldisilazanide was used. The complicated H
NMR spectrum of 2a became simpler on addition of a sec-
ond equivalent of lithium hexamethyldisilazanide sug-
gesting the formation of a carbene (Scheme 1). These
observations are similar to that of Lloyd-Jones et al.2c and
Alder et al.2d Unfortunately, efforts to characterize 3a by
13C NMR failed due to the rate of decomposition. Howev-
er, we obtained a crystalline solid after filtration of the
crude mixture and X-ray crystallographic analysis re-
vealed that the hydrate 4 (Scheme 2), presumably pro-
duced via the carbene intermediate 3a, was formed. In the
case of potassium tert-butoxide, we again observed a base
adduct in ~1:1 diastereomeric ratio based as determined
We recently reported the design, synthesis, and novel re-
activity of a family of 6-NHC–copper(I) complexes,
whose design was in part inspired by the annulated car-
1
by H NMR analysis; no further changes were observed
Mes
N
N
CyB
BCy
N
N
N
N
N
N
N
N
N
N
R
R
R
R
R
R
R
R
••
••
••
••
••
more donating/basic
less donating/basic
Figure 1 Relating carbene structures to electronic properties
SYNTHESIS 2012, 44, 1485–1490
Advanced online publication: 19.04.2012
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0031-1290820; Art ID: SS-2012-C0219-ST
© Georg Thieme Verlag Stuttgart · New York

1486
J. K. Park, D. T. McQuade
SPECIAL TOPIC
LiHMDS
(1 equiv)
LiHMDS
(1 equiv)
N
N
N
Ph
Ph
Ph
N
N
N
N
N
N
benzene-d6
benzene-d6
••
Ph
Ph
Ph
HMDS
BF4
1a
2a
3a
Scheme 1 Carbene formation using lithium hexamethyldisilazanide
Scheme 2 Hydration of carbene 3a and crystal structure of hydrate 4
using excess potassium tert-butoxide. However, exchang- Generation of a copper–6-NHC complex and applica-
ing the solvent from tetrahydrofuran-d8 to deuterochloro- tion to the preparation of bifunctional reagents via
form yielded the thermodynamically stable S-isomer 5a asymmetric allylic substitution. Synthesis of copper–6-
(Scheme 3).
NHC complex 9 was challenging because an initial survey
Determination of the donating ability of 6-NHCs. We
have published the synthesis and X-ray crystal structure of
6-NHC–rhodium complex 7.7a Herein we report the 6-
NHC donor capacity by converting 7 into the correspond-
ing carbonyl complex 8 (Scheme 4). The IR stretching fre-
quency of the average CO ligand is 2036 cm–1 (Figure 2)
indicating that the ligand has a donor capacity that is in be-
tween typical 5- and 6-NHC ligands (Figure 3). We pro-
pose that carbene 3a is a weaker donor than other 6-NHCs
because the imidazoline is an electron-withdrawing
group.
OC
N
CO
Rh
Rh
Ph
Ph
Cl
N
Cl
N
Mes
Mes
CO
N
Ph
Ph
CH2Cl2
N
N
7
8
Scheme 4 Synthesis of rhodium dicarbonyl compound 8
N
Ph
N
N
Ph
solvent
exchange to
CDCl3
Ot-Bu
5a
KOt-Bu
(1.1 equiv)
N
N
Ph
+
Ph
N
N
N
N
THF-d8
Ph
Ph
Ot-Bu
BF4
N
5a
1a
Ph
thermodynamically stable
single isomer
N
N
Ph
Ot-Bu
6a
kinetically trapped mixture
Scheme 3 Carbene adduct with potassium tert-butoxide
Synthesis 2012, 44, 1485–1490
© Georg Thieme Verlag Stuttgart · New York

SPECIAL TOPIC
Chiral 6-NHC Ligand and Copper Complex
1487
of enantiopure bimetallic reagents are known and those
described require many steps for their production.10a,b
Herein we describe a simple asymmetric allylic substitu-
tion route to these useful intermediates.
BPin
R
RCHO
M
M
Lewis acid
OBPin
type III
Scheme 6 Type III reagent of bimetallic double-allylation reagents
Our general approach towards the synthesis of bimetallic
reagents is shown in Scheme 7 where the process is a
metathesis/asymmetric allylic boronation (AAB) substi-
tution sequence. For 4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl and trimethylsilyl starting materials such as 10
and 12, first-generation Grubbs catalyst worked well to
give the products in ~70% yield.
Figure 2 IR data for rhodium dicarbonyl compound 8
of many reaction conditions provided only low yields. To
improve the yield, we predicted that alkoxy adducts 5b/6b
would be good precursors based on the observation that
the epimeric mixture equilibrates to a single isomer in di-
chloromethane suggesting that the alkoxide carbene bond
is labile (Scheme 5).2b,4f To our gratification, using a mul-
tisolvent system (THF to CH2Cl2) produced complex 9 in
quantitative yield from 1b. We recently described the use
of 9 in stereoconvergent and catalyst controlled allylic
substitution reactions. Herein, we described the further
application of this unique complex to the synthesis of bi-
functional reagents.
Scheme 8 illustrates the outcome of the allylic substation
step where we obtained very high enantioselectivities
with good yield. In each case, the starting material was a
mixture of cis- and trans-isomers. These results further
underscore the value of the stereoconvergent property of
complex 9.
We have observed in the reactions we have studied using
complex 9 that more than one equivalent of methanol is
required to obtain high yields.11 We recognize that this
methanol dependence appears to be both catalyst and con-
dition dependent.12 We propose the catalytic cycle shown
in Scheme 9. Here the 6-NHC–CuOMe resting state is
produced by reaction of sodium methoxide (generated
Enantiopure bifunctional reagents containing boron and
silicon have emerged as versatile conjugates in organic
synthesis.9,10 Type III bimetallic reagents, as defined by
Hall,10c are broadly used for double allylation reaction
with aldehydes (Scheme 6).10 However, limited examples
L
Cl
Ru
N
OC
CO
Ph
N
N
Ph
N
N
Me
Me
N
N
N
N
N
N
Mes
Mes
Mes
Mes
20295b
20384g
20414j
20196a
(or 2029)5h
2036
vav(CO)/cm–1
two different frequencies are reported
Figure 3 Donation ability of rhodium dicarbonyl compound 8
KOt-Bu
N
N
N
(1 equiv)
CuCl
Ph
Ph
Ph
N
N
N
N
N
N
CH2Cl2
THF
Ph
Ph
Ph
Ot-Bu
Cu
Cl
BF4
1b
5b/6b
9
>95% yield
Scheme 5 Synthesis of 6-NHC–copper chloride complex 9
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 1485–1490

1488
J. K. Park, D. T. McQuade
SPECIAL TOPIC
BPin
M
+
metathesis
AAB
M
M
O
NO2
O
NO2
M = BPin and TMS
Scheme 7 Reaction scheme for bimetallic reagents
9 (1 mol%)
B2Pin2 (1.2 equiv)
BPin
82%
98% ee
BPin
BPin
O
10
NO2
NaOt-Bu (30 mol%)
MeOH (2 equiv)
11
Et2O, –55 °C, 14 h
9 (1 mol%)
B2Pin2 (1.2 equiv)
BPin
75%
>99% ee
Me3Si
Me3Si
O
NO2
NaOt-Bu (30 mol%)
MeOH (2 equiv)
12
13
Et2O, –55 °C, 14 h
Scheme 8 Synthesis of bimetallic allyl boronates
from NaOt-Bu and MeOH). The resting state then reacts a kinetic isotope effect of 2.1 (±0.1) is observed. We assert
with bis(pinacolato)diboron (B2Pin2) to produce the 6- that this small, but primary kinetic isotope effect is consis-
NHC–copper–boryl complex intermediate that forms a
Cu–alkene π-complex with the aryl allyl ether. Insertion
of the Cu–B bond across the alkene via a four-membered
transition state (not shown) gives a β-borylalkyl copper
intermediate that undergoes elimination, regenerating the
resting state.
PinBOMe
NaOt-Bu + MeOH
6-NHC-Cu-BPin
B2Pin2
NaOMe
BPin
6-NHC-CuCl
6-NHC-CuOMe
Based on our observation that addition of methanol is an
essential features of this system and that methanol accel-
erates the rate, we hypothesized that the methanol reacts
via the six-membered transition state 14 (Scheme 9).13
Furthermore, we proposed that this metholysis step is
rate-determining.13 To test this thesis, we measured the ki-
netic isotope effect for the reaction with 12 using two
equivalents of methanol and methanol-d, respectively.
The consumption rate of the trans-isomer of 12 was mon-
itored by GC using an internal standard (dodecane) ac-
cording to the reaction time. Results are shown in Figure 4,
R
OAr
MeOH
R
BPin
+
ArOH
R
OAr
H(D)
Cu-6-NHC
O
Me
14
Scheme 9 Proposed mechanism for allylic substitution reaction
Figure 4 Determination of the kinetic isotope effect for 12
Synthesis 2012, 44, 1485–1490
© Georg Thieme Verlag Stuttgart · New York

SPECIAL TOPIC
Chiral 6-NHC Ligand and Copper Complex
1489
with hexanes. The insoluble solid was filtered off. The filtrate was
concentrated and filtered through short pad of silica gel (hexanes–
EtOAc, 10:1). Remaining 1-(allyloxy)-3-nitrobenzene was re-
moved by Kugelrohr distillation (120 °C/0.67 mbar) to give the de-
sired product 12 (595 mg, 72%) as a brownish oil; ratio trans/cis
4.1:1.
tent with our proposed catalytic cycle and consistent with
other kinetic isotope effects associated with six-mem-
bered-ring proton transfers previously reported.14
In conclusion, the reaction between imidazolium salt 1
and bases yields base adducts that undergo hydrolysis to
produce the crystalline hydrate 4 or the tert-butoxide ad-
duct 5. The donor capacity of the 6-NHC ligand 3a was
determined by measurement of the CO stretching frequen-
cy using infrared spectroscopy. It was found that 3a is in-
termediate between widely known 5-NHCs and 6-NHCs
in terms of donor capacity. A new synthetic method was
developed for bimetallic allylic boronate reagents with
high enantioselectivities and good yields using complex 9,
and justification was provided for the role of methanol in
allylic substitution reactions that is supported, in part, by
a primary kinetic isotope effect. In due time, applications
of these bifunctional reagents will be reported.
trans-Isomer
1H NMR (400 MHz, CDCl3): δ = 7.75 (ddd, J = 8.1, 2.1, 0.90 Hz, 1
H), 7.68 (t, J = 2.3 Hz, 1 H), 7.35 (t, J = 8.2 Hz, 1 H), 7.17 (ddd, J =
8.3, 2.5, 0.95 Hz, 1 H), 5.87–5.78 (m, 1 H), 5.50–5.42 (m, 1 H), 4.50
(dd, J = 6.3, 0.64 Hz, 2 H), 1.50 (d, J = 8.2 Hz, 2 H), –0.06 (s, 9 H).
13C NMR (100 MHz, CDCl3): δ = 159.2, 149.1, 134.1, 129.8, 122.2,
121.8, 115.6, 109.1, 69.7, 23.2 –2.0.
MS (CI+): m/z = 266.2 [M + H]+.
(S)-2,2′-(But-3-ene-1,2-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane) (11)
Allylic aryl ether 10 (638 mg, 2.0 mmol) and bis(pinacolato)dibo-
ron (559 mg, 2.2 mmol) were dissolved in Et2O (10 mL). NaOt-Bu
(58 mg, 0.60 mmol) was added to the mixture, the mixture was
cooled to –55 °C, and MeOH (180 μL, 4.0 mmol) was added. After
5 min, 6-NHC copper catalyst 9 (12 mg, 0.020 mmol) was added.
After 14 h, the mixture was filtered through silica gel and washed
with Et2O. The filtrate was concentrated using a rotary evaporator.
The resulting residue was purified by column chromatography
(hexane–Et2O, 20:1 to 10:1) to afford the desired product 11 (505
mg, 82%) as a clear oil; 98% ee [(R)-4-vinyl-1,3-dioxolan-2-one af-
ter oxidation and protection, GC (β-DM column, 110 °C, 4
mL/min): tR = 3.70 (major), 3.91 min (minor)].
1H and 13C NMR spectra were recorded in CDCl3 on a 400 MHz
spectrometer operating at 400.172 and 100.623 MHz, respectively,
and 600 MHz spectrometer operating at 600.133 and 150.914 MHz,
respectively; 1H NMR reference TMS, 13C NMR reference internal-
ly to TMS. GC analyses were performed using a GC equipped with
an autosampler, a flame ionization detector (FID), and a column
(Varian CP-Sil 5 CB: 15 m × 0.25 mm × 0.25 μm and Astec
CHIRALDEXTM β-DM: 30 m × 0.25 mm × 0.12 μm). Specific ro-
tations were measured on a Jasco polarimeter.
[α]D25 –9.3 (c 0.85, CHCl3).
1H NMR (400 MHz, CDCl3): δ = 5.96 (ddd, J = 17.3, 10.1, 7.2 Hz,
1 H), 4.97 (dt, J = 17.1, 1.7 Hz, 1 H), 4.90 (dt, J = 10.2, 1.6 Hz, 1
H), 2.01 (q, J = 7.5 Hz, 1 H), 1.231 (s, 12 H), 1.228 (s, 12 H), 1.04
(dd, J = 15.9, 9.1 Hz, 1 H), 0.94 (dd, J = 16.0, 6.0 Hz, 1 H).
13C NMR (100 MHz, CDCl3): δ = 141.3, 111.9, 83.1, 83.0, 24.9,
24.77, 24.73, 24.64.
4,4,5,5-Tetramethyl-2-[4-(3-nitrophenoxy)but-2-enyl]-1,3,2-di-
oxaborolane (10)
A 50-mL round-bottom flask was flame-dried and kept under an N2
atmosphere. The flask was charged with 1-(allyloxy)-3-nitroben-
zene (1.12 g, 6.26 mmol, 2 equiv), allylboronic acid pinacol ester
(526 mg, 3.13 mmol, 1 equiv), and CH2Cl2 (31 mL, 0.1 M). Grubbs
I catalyst (77 mg, 0.094 mmol, 0.03 equiv) was added and the mix-
ture was heated to reflux (45 °C) for 15 h, then cooled to r.t. The
crude material was concentrated using a rotary evaporator and then
mixed with hexanes. The insoluble solid was filtered off. The fil-
trate was concentrated and purified by column chromatography
(hexanes–EtOAc, 10:1 to 7:1) to give the desired product 10 (733
mg, 73%) as a brownish oil; ratio trans/cis 1.7:1.
MS (CI+): m/z = 309.3 [M + H]+.
(R)-Trimethyl[2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl]but-3-enyl]silane (13)
Allylic aryl ether 12 (328 mg, 1.24 mmol) and bis(pinacolato)dibo-
ron (315 mg, 1.24 mmol) were dissolved in Et2O (5 mL). NaOt-Bu
(36 mg, 0.37 mmol) was added to the mixture, the mixture was
cooled to –55 °C, and MeOH (90 μL, 2.0 mmol) was added. After 5
min, 6-NHC copper catalyst 9 (7 mg, 0.012 mmol) was added. After
14 h, the mixture was filtered through silica gel and washed with
Et2O. The filtrate was concentrated using a rotary evaporator. The
resulting residue was purified by column chromatography (hexane–
Et2O, 20:1 to 10:1) to afford the desired product 13 together with cis
starting material (295 mg, 82% based on the ratio of product to start-
ing material, 6.7:1, by 1H NMR). Kugelrohr distillation (90 °C/0.67
mbar) removed unreacted cis-starting material to give pure 13 (75%
yield) as a clear oil; >99% ee [GC (β-DM column, 80 °C, 4
mL/min): tR = 17.3 (minor), 17.9 min (major)]. Note: this enantio-
pure boronate was readily racemized under oxidation conditions
(H2O2, aq NaOH).
trans-Isomer
1H NMR (600 MHz, CDCl3): δ = 7.80 (ddd, J = 5.9, 2.1, 0.75 Hz, 1
H), 7.72 (t, J = 2.3 Hz, 1 H), 7.40 (t, J = 8.3 Hz, 1 H), 7.22 (ddd, J =
8.1, 2.4, 0.75 Hz, 1 H), 5.98–5.91 (m, 1 H), 5.70–5.66 (m, 1 H), 4.55
(dd, J = 6.2, 0.78 Hz, 2 H), 1.77 (d, J = 7.5 Hz, 2 H), 1.25 (s, 12 H).
13C NMR (125 MHz, CDCl3): δ = 159.2, 149.1, 134.1, 129.8, 122.2,
121.8, 115.6, 109.1, 69.7, 23.2, –2.0.
cis-Isomer
1H NMR (600 MHz, CDCl3): δ = 7.81 (ddd, J = 5.6, 2.3, 0.75 Hz, 1
H), 7.74 (t, J = 2.4 Hz, 1 H), 7.41 (t, J = 8.3 Hz, 1 H), 7.23 (ddd, J =
7.7, 2.3, 0.75 Hz, 1 H), 5.89–5.83 (m, 1 H), 5.67–5.64 (m, 1 H), 4.67
(d, J = 6.4 Hz, 2 H), 1.81 (d, J = 7.5 Hz, 2 H), 1.26 (s, 12 H).
[α]D25 +8.3 (c 0.83, CHCl3).
MS (CI+): m/z = 320.2 [M + H]+.
1H NMR (600 MHz, CDCl3): δ = 5.83 (ddd, J = 17.8, 9.2, 7.9 Hz, 1
H), 4.99 (dt, J = 16.9, 1.5 Hz, 1 H), 4.89 (d, J = 10.1 Hz, 1 H), 1.93
(q, J = 7.8 Hz, 1 H), 1.24 (s, 12 H), 0.84 (dd, J = 14.6, 7.4 Hz, 1 H),
0.66 (dd, J = 14.6, 7.5 Hz, 1 H), 0.00 (s, 9 H).
13C NMR (150 MHz, CDCl3): δ = 142.0, 112.3, 83.1, 24.70, 24.68,
16.6, –0.91.
Trimethyl[4-(3-nitrophenoxy)but-2-enyl]silane (12)
A 50-mL round-bottom flask was flame-dried and kept under an N2
atmosphere. The flask was charged with 1-(allyloxy)-3-nitroben-
zene (1.12 g, 6.26 mmol, 2 equiv), allyltrimethylsilane (358 mg,
3.13 mmol, 1 equiv), and CH2Cl2 (31 mL, 0.1 M). Grubbs I catalyst
(77 mg, 0.094 mmol, 0.03 equiv) was added and the mixture was
heated to reflux (45 °C) for 15 h , then cooled to r.t. The crude ma-
terial was concentrated using a rotary evaporator and then mixed
MS (CI+): m/z = 255.2 [M + H]+.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 1485–1490

1490
J. K. Park, D. T. McQuade
SPECIAL TOPIC
Cavell, K. J.; Dervisi, A.; Fallis, I. A. Eur. J. Inorg. Chem.
2009, 1913.
Acknowledgment
The authors thank NSF (CHE-0809261, 1152020), Pfizer, Corning
Glass and FSU support and FSU VP of Research and Dean of A&S
for NMR upgrades.
(6) For Heck reaction using 6-NHC, see: (a) Mayr, M.; Wurst,
K.; Ongania, K. H.; Buchmeiser, M. R. Chem.–Eur. J. 2004,
10, 1256. For hydrogenation reaction using 6- and 7-NHC,
see: (b) Binobaid, A.; Iglesias, M.; Beetstra, D. J.; Kariuki,
B.; Dervisi, A.; Fallis, I. A.; Cavell, K. J. Dalton Trans.
2009, 35, 7099. For silylation reaction using 6-NHC, see:
(c) Bantu, B.; Wang, D.; Wurst, K.; Buchmeiser, M. R.
Tetrahedron 2005, 61, 12145.
(7) (a) Park, J. K.; Lackey, H. H.; Rexford, M. D.; Kovnir, K.;
Shatruk, M.; McQuade, D. T. Org. Lett. 2010, 12, 5008.
(b) Park, J. K.; Lackey, H. H.; Ondrusek, B. A.; McQuade,
D. T. J. Am. Chem. Soc. 2011, 133, 2410. (c) Park, J. K.;
McQuade, D. T. Angew. Chem. Int. Ed. 2012, 51, 2717.
(8) Bae, J.-S.; Lee, D.-H.; Lee, D.-W.; Jang, J.-G.; Jeon, S.-Y.
WO 2007069847 2007.
(9) For examples, see: (a) Roush, W. R.; Grover, P. T.
Tetrahedron Lett. 1990, 31, 7567. (b) Barrett, A. G. M.;
Malecha, J. W. J. Org. Chem. 1991, 56, 5243. (c) Roush, W.
R.; Grover, P. T. Tetrahedron 1992, 48, 1981. (d) Hunt, J.
A.; Roush, W. R. J. Org. Chem. 1997, 62, 1112. (e) Roush,
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Braddock, D. C.; de Koning, P. D.; White, A. J. P.; Williams,
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Synthesis 2012, 44, 1485–1490
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