Polarity reversal catalysis in radical reductions of halides by N-heterocyclic carbene boranes

Article abstract of DOI:10.1021/ja300416f

Otherwise sluggish or completely ineffective radical reductions of alkyl and aryl halides by N-heterocyclic carbene boranes (NHC-boranes) are catalyzed by thiols. Reductions and reductive cyclizations with readily available 1,3-dimethylimidazol-2-ylidene borane and a water-soluble triazole relative are catalyzed by thiophenol and tert-dodecanethiol [C₉H ₁₉C(CH₃)₂SH]. Rate constants for reaction of the phenylthiyl (PhS?) radical with two NHC-boranes have been measured to be ～10⁸ M^-1 s^-1 by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis.

Full text of DOI:10.1021/ja300416f

Article
pubs.acs.org/JACS
Polarity Reversal Catalysis in Radical Reductions of Halides by N-
Heterocyclic Carbene Boranes
Xiangcheng Pan,† Emmanuel Lacote,‡ Jacques Lalevee,*,§ and Dennis P. Curran*,†
̂
́
†Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
‡ICSN CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
§Institut de Science des Mater
́
iaux de Mulhouse, IS2M − LRC CNRS 7228 − ENSCMu-UHA, 15 rue Jean Starcky, 68057 Mulhouse
Cedex, France
S
* Supporting Information
ABSTRACT: Otherwise sluggish or completely ineffective radical
reductions of alkyl and aryl halides by N-heterocyclic carbene
boranes (NHC-boranes) are catalyzed by thiols. Reductions and
reductive cyclizations with readily available 1,3-dimethylimidazol-
2-ylidene borane and a water-soluble triazole relative are catalyzed
by thiophenol and tert-dodecanethiol [C9H19C(CH3)2SH]. Rate
constants for reaction of the phenylthiyl (PhS•) radical with two NHC-boranes have been measured to be ∼108 M−1 s−1 by laser
flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis.
has good solubility in many solvents and does not dissociate on
heating. It contains only hydrogen and second-row elements,
has a low molecular weight (110 g/mol), and forms byproducts
that are easy to separate from the target products. The triazole
analogue, diMe-Tri-BH3 (2), is generally similar to 1 but is also
soluble in water.8e,9
Boranes 1 and 2 smoothly reduce xanthates [X =
OC(S)SCH3] and related functional groups.8e However, radical
reductions of the all-important halides (X = Br, I) have been
limited to alkyl precursors R that have electron-withdrawing
groups near the halide.8f Simple alkyl halides such as adamantyl
iodide and bromide have not been reduced efficiently, and aryl
halides have been untouched by NHC-boranes.
The problem in alkyl halide reductions seems to be the
hydrogen atom transfer (step 2 in Figure 1). Secondary alkyl
radicals react with NHC-boranes with rate constants of only
∼104 M−1 s−1.8c,e In contrast, the rate constants for reactions of
alkyl radicals with tin hydrides are 100−500 times larger.10 The
only successful halide reductions match electron-poor radicals
with the “nucleophilic” hydrogen donor properties of NHC-
boranes.8f This need for polarity matching is a significant
limitation.
INTRODUCTION
■
Reductive radical reactions and reaction cascades are commonly
used in organic synthesis,1 so the development of practical,
efficient, and economical radical hydrogen transfer reagents is
an important goal.2 The use of organoboron reagents for radical
hydrogen atom transfer reactions was pioneered by Roberts
over 20 years ago.3 This work was seminal because so little was
then known about boryl radicals. However, the amine borane
reagents (R3N−BH3) that emerged have seen sporadic use in
radical chemistry because they are rather poor hydrogen atom
donors4 and because they thermally dissociate to give
aggressive free boranes.5 While boron-based hydrogen transfer
chemistry has been fallow, other uses of boron reagents in
radical chemistry have flourished.6
Recently, N-heterocyclic carbene boranes (NHC-boranes)7
have revived the promise of boron-based radical hydrogen
transfer reagents.8 The second-generation reagent 1,3-dimethy-
limidazol-2-ylidene borane (diMe-Imd-BH3, 1; Figure 1)8e is a
readily available and stable white solid that is easy to handle. It
Problems with slow hydrogen transfer have been solved in
silane reductions and other types of radical reactions by
addition of a “polarity reversal catalyst”, often a thiol or
selenide.11 Here we report that NHC-borane reductions are
subject to polarity reversal catalysis with thiols. This catalysis
significantly extends the scope and practicality of radical
reductions by NHC-boranes.
Figure 1. (top) Structures of NHC-borane radical hydrogen transfer
reagents 1−3. (bottom) Proposed propagation steps for chain
reductions of halides.
Received: January 13, 2012
Published: March 6, 2012
© 2012 American Chemical Society
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Table 1. Scouting Experiments with Adamantyl Iodide (4a), diMe-Imd-BH3 (1), and Various Initiators, with and without Thiol
Sources
a
entry
init
equiv
temp
5% PhSH
time
conv 4a
yield of 5
1
2
3
4
5
6
7
8
AIBN
AIBN
Et3B
1
80 °C
80 °C
rt
no
yes
no
yes
no
yes
no
5 h
5 h
5 h
2 h
5 h
2 h
1 h
1 h
65%
82%
62%
76%
86%
82%
39%
99%
41%
49%
16%
76%
46%
82%
38%
99%
1
1
Et3B
0.5
1
rt
TBHN
TBHN
DTBP
DTBP
80 °C
80 °C
0.1
1
b
hν
b
0.2
hν
yes
a
b
NMR yields determined using 1,3,5-trimethoxybenzene as an internal standard. The NMR tube was warmed by the lamp to an estimated
temperature of 50−60 °C.
Table 2. Preparative Reductions of Alkyl and Aryl Iodides and Bromides Catalyzed by Thiols
a
entry
substrate
conditions
thiol
product
yield
b
1
2
4b
4b
6
TBHN, 4 h
DTBP, 1 h
Et3B, 1 h
PhSH
PhSH
PhSH
PhSH
PhSH
5
97%
b
5
92%
3
7
82%
79%
79%
96%
4
6
TBHN, 2 h
DTBP, 1 h
TBHN, 2 h
TBHN, 2 h
TBHN, 2 h
TBHN, 2 h
TBHN, 2 h
TBHN, 3 h
TBHN, 3 h
TBHN, 3 h
TBHN, 3 h
DTBP, 7 h
TBHN, 3 h
DTBP, 7 h
7
5
6
7
c
6
8a
8a
8b
TDT
9
d
7
PhSH
9
81%
c
8
TDT
9
95%
95%
90%
−
e
c
9
8b
TDT
9
f
c
10
11
12
13
14
15
16
17
8b
TDT
9
c
10a
10b
12
TDT
11
11
13
15
15
15
c
TDT
91%
c
bg
,
TDT
96%
86%
86%
81%
82%
b
b
b
b
14a
14a
14b
14b
PhSH
PhSH
PhSH
PhSH
15
a
b
Conditions: Et3B (0.5 equiv), air, rt; TBHN (0.2 equiv), 80 °C; DTBP (0.2 equiv), hν (50−60 °C). NMR yield determined using 1,3,5-
c
d
trimethoxybenzene as an internal standard. TDT = tert-dodecanethiol. diMe-Tri-BH3 (2) was used in place of 1; isolated yield after aqueous
extraction. In C6H5CF3. In C6H5CH3. 1.5 equiv of 1 was used.
e
f
g
ideal for quickly ascertaining whether thiols and NHC-boranes
interact antagonistically or synergistically.
RESULTS AND DISCUSSION
■
Discovery and Development of Thiol Catalysis. Thiols
are powerful hydrogen atom donors that can suppress or
interfere with radical chains as well as promote them. We
selected adamantyl iodide (4a) for preliminary experiments
because it resists ionic reduction12 and is inefficiently reduced
by NHC-boranes alone under radical conditions. Thus, it is
Table 1 summarizes selected scouting experiments in the
reduction of 4a by 1. Little or no conversion occurred upon
heating or irradiation of these reactants without an added
initiator. Entries 1−8 are alternating pairs of experiments with
standard initiators,13 with and without 5 mol % thiophenol.
Room-temperature experiments were conducted with triethyl-
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borane,14 while azobis(isobutyronitrile) (AIBN) and di-tert-
butyl hyponitrite (TBHN)15 experiments were performed at
80 °C. Photolysis at 50−60 °C was used with di-tert-butyl
peroxide. The conversions of iodide 4a and yields of
adamantane 5 were measured by H NMR integration against
an internal standard.
With AIBN (entries 1 and 2), the yields were moderate both
without and with thiophenol (41 and 49%). However, under
the other three sets of conditions, the addition of thiophenol
significantly increased the yields (compare entry 3 with 4, 5
with 6, and 7 with 8). Furthermore, the percent conversions
and yields were essentially the same when thiophenol was
added (entries 4, 6, and 8), indicating a smooth radical chain. In
contrast, the control experiments (especially entries 3 and 5),
where the percent conversion exceeded the yield, indicated that
other pathways were competing. One equivalent of initiator was
used to maximize the conversion in the control experiments,
but in the thiophenol experiments with TBHN and DTBP
(entries 6 and 8), the amount of initiator was decreased to 10−
20% while maintaining high yield and conversion.
bromide (12) under TBHN conditions give mesitylene (13) in
95% yield after 3 h (entry 13). Without the thiol, no mesitylene
was formed. Aryl iodide 14a and bromide 14b were also cleanly
reduced to 15 with TBHN and DTBP (81−86% yield; entries
14−17).
All of these reactions were conducted in benzene for
consistency, but we expect that other solvents can also be used.
Preferred alternatives to benzene in preparative chemistry
include benzotrifluoride (C6H5CF3)18 and toluene. Reductions
of 8b in these two solvents gave about the same yield as the
reduction in benzene (compare entries 8−10: C6H6, 95%;
C6H5CF3, 95%, C6H5CF3, 90%).
1
Reductive cyclizations of both alkyl and aryl radicals derived
from halides can also be catalyzed by thiols, as shown by the
examples in Table 3. All of the yields reported in this table were
Table 3. Thiol-Catalyzed Radical 5-Exo-Trig Cyclizations of
a
Halides with diMe-Imd-BH3 1
11B NMR experiments showed that the expected boron
product (diMe-Imd-BH2I) was formed cleanly with high
conversions and yields in several of the reactions of 4a.
Additional experiments (see the Supporting Information)
showed the importance of the combination of the NHC-
borane and the thiol. Replacement of 1 with tetrabutylammo-
nium cyanoborohydride (Bu4NBH3CN)16 resulted in moderate
conversions and yields in the reduction of 4a, while
triphenylphosphine borane3d and trimethylamine borane4
gave very low conversion. The thiol could not be replaced by
a phenol or catechol,2c and benzeneselenol11d gave inferior
results to thiophenol. On the other hand, while pentadecane-
thiol (C15H31SH) did not match thiophenol, tert-dodecanethiol
[C9H19C(CH3)2SH, TDT] performed at least as well and
possibly better. TDT also has a weak odor, and it was used in
several of the subsequent preparative experiments.
Table 2 summarizes the results of a series of preparative
experiments under several of the conditions identified in Table
1. Yields for low-molecular-weight products such as adaman-
1
tane and mesitylene were determined by H NMR integration
against an internal standard, while the larger products were
isolated by flash chromatography.
Adamantyl bromide (4b) was reduced in yields comparable
to those for the iodide 4a with TBHN (97%) and DTBP (92%)
(entries 1 and 2). In contrast, little conversion was observed
with adamantyl chloride (see the Supporting Information).
Glucose diacetonide 6 is a halide that can be reduced directly
with 1,8f but addition of thiophenol gave faster conversions and
higher yields (67−81%; entries 3−5). Cholesterol iodide (8a)
and bromide (8b) were not reduced by 1 alone, but both were
cleanly reduced in the presence of thiophenol (96 and 95%
yield; entries 6 and 8). The iodide was also reduced with the
triazole borane 2, after which the crude product was partitioned
between ether and water and the ether phase was evaporated.
This provided a good-quality sample of 9 in 81% yield, free of
both thiophenyl- and boron-containing products (entry 7). A
limitation was encountered in reductions of the derived
epoxides 10a and 10b. Bromide 10b was reduced cleanly
with TBHN initiation (91%; entry 12), but the iodide gave a
complex mixture (entry 11). This may be due to the Lewis
acidity of the boron-derived product, diMe-Imd-BH2I.17
In a major expansion of scope, aryl iodides and bromides are
also reduced by 1 and PhSH. For example, reduction of mesityl
a
All used 0.2 M substrate under TBHN-initiated conditions: TBHN
b
(20 mol %), 1 (1.1 equiv), TDT (5 mol %), 80 °C, 2 h. 17:18 ratio.
c
9/1 mixture of diastereomers; the major product is shown.
obtained after flash chromatography. Cyclization of bromide 16
(0.2 M starting concentration) provided a mixture of cyclized
product 17 and directly reduced product 18 in an 83/17 ratio
in 90% yield (entry 1). The formation of significant amounts of
18 shows the effect of the thiol as a hydrogen donor (as 1 itself
is not capable of intercepting the radical before cyclization).
Compound 19, the precursor of a more rapidly cyclizing alkyl
radical, provided only the cyclized product 20 in 83% yield
(entry 2). Stork−Ueno cyclization19 of bromoacetal 21 gave
cyclic acetal 22 in 76% yield (entry 3). In addition, aryl radical
reactions could be conducted, as shown by the cyclizations of
23a, 23b, and 25 (80−83% yield; entries 4−6).
Mechanism of Thiol Catalysis. Figure 2 shows suggested
propagation steps for these halide reductions. We focus the
mechanistic analysis on alkyl radicals because currently no rate
constants for reactions involving aryl radicals and NHC-
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radicals are conveniently produced by photolysis of diphenyl
disulfide. In addition, NHC-boryl radicals produced by other
means can add to the spin trap N-tert-butyl-α-phenylnitrone
(PBN) to give a stable nitroxide spin adduct.20 Thus, photolysis
of PhSSPh, 1, and PBN should product a nitroxide spin adduct,
as shown in Scheme 1.
Scheme 1. Spin Trapping Provides Evidence for Hydrogen
Transfer
Figure 2. Suggested propagation steps with rate constants for alkyl
radicals R•.
boranes are known. In the reaction without thiophenol, the
carbene boryl radical NHC-BH2• abstracts a halogen atom
from RX to give an alkyl radical R• (step 1), which in turn
abstracts a hydrogen atom from the precursor NHC-BH3 (step
2). The measured rate constants k1 for abstraction of bromine
and iodine by NHC-boryl radicals20 are 1−3 orders of
magnitude larger than k2 for hydrogen abstraction by alkyl
radicals (<1 × 105 M−1 s−1).8b,d Thus, the slow hydrogen
transfer step (step 2) presumably limits the chain propagation.
When thiophenol is added, this slow hydrogen transfer
reaction is superseded by two fast reactions. First, thiophenol
donates a hydrogen to the alkyl radical (step 3). Rate constants
k3 for this reaction are in the range (0.7−1) × 108 M−1 s−1.21
We propose that the phenylthiyl radical PhS• next abstracts a
hydrogen atom from NHC-borane 1 to give the starting radical
R• and regenerate the thiophenol catalyst (step 4).
The spin adduct was detected by ESR experiments, as shown
in Figure 3. In a control experiment, 1 and PBN were irradiated
Figure 3. ESR spin-trapping spectra for PhSSPh/1 in tert-
butylbenzene: (1) experimental and (2) simulated spectra for the
nitroxide spin adduct.
The thermodynamics of step 4 can be assessed on the basis
of bond dissociation energies (BDEs). The experimental BDE
of the S−H bond of thiophenol is ∼83 kcal/mol.22 An estimate
(from an Evans−Polyani relationship) places the BDE of an
NHC-borane at ∼87 kcal/mol,8d which would mean that step 4
is endothermic. However, several calculations of the B−H
BDEs of 1 and related NHC-boranes at the UB3LYP/6-31+G*
level provide BDEs of 80−82 kcal/mol.8c,20 At the same level,
PhS−H is calculated to have a BDE of 79 kcal/mol, which
would mean that step 4 is almost thermoneutral.
Thermochemistry aside, it is the rate of step 4 that dictates its
importance.23 Here, the polarity matching between the
electrophilic thiyl radical and the NHC-borane is expected to
lower the barrier of the forward and reverse reactions. Polarity
effects are reflected by the difference between the electro-
negativities of the radicals involved in the hydrogen atom
transfer reactions (PhS• and NHC-BH2•). The Mulliken
electronegativity (χM) of a radical is defined by eq 1:
in tert-butylbenzene in an ESR cavity, but no signal was
observed. Irradiation of a mixture of PhSSPh, 1, and PBN gave
a strong signal of a spin adduct that was simulated with
hyperfine coupling constants aN = 15.4 G, aH = 2.1 G, and aB =
4.3 G. These values agree with previous data on NHC-boryl
radical spin adducts with PBN.9a,20 Accordingly, a photolytically
generated PhS• radical abstracts hydrogen from 1, and the
resulting NHC-boryl radical adds to PBN.
We next used laser flash photolysis (LFP) experiments to
measure the rate constant for the reaction between phenylthiyl
radical and diMe-Imd-BH3 1. The phenylthiyl radical was again
generated by irradiation of diphenyl disulfide and allowed to
react with increasing concentrations of 1. The results of these
experiments are summarized in Figure 4. The rate constant for
the reaction of PhS• with 1 was determined according to the
classical Stern−Volmer equation (eq 2),
IP + EA
χ
=
(1)
M
2
1
τ
1
τ
0
=
+ k [1]
H
(2)
The ionization potential (IP) and electron affinity (EA) for the
PhS• and NHC-BH2• radicals were calculated at the UB3LYP/
6-31+G* level by optimization of the relevant radicals and ions
(see the Supporting Information). Remarkably, PhS• and
diMe-Imd-BH2• were found to have χM = 5.33 and 2.72 eV,
respectively. This large electronegativity difference anticipates
polar effects in the hydrogen transfer reactions, in which PhS•
is electrophilic and diMe-Imd-BH2• is nucleophilic.
where kH is the rate constant for hydrogen transfer and τ0 is the
PhS• lifetime in the absence of quencher. A plot of the
reciprocal of the lifetime of the radical against the concentration
of 1 provided the rate constant k4 = 1.2 × 108 M−1 s−1. On the
basis of the high values of k3 and k4, we conclude that
thiophenol can behave as a catalyst for such reductions through
the chain in Figure 2 that involves step 1 but bypasses the slow
step 2 in favor of the fast steps 3 and 4. It is not currently clear
The viability of step 4 was first supported by electron spin
resonance (ESR) spin-trapping experiments. Phenylthiyl
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Supporting Information). From this, we estimate that kH for the
reaction of PhS• with Ph3P−BH3 is at least 2 orders of
magnitude smaller (<106 M−1 s−1) than kH for 1. This is
consistent with the earlier observation that Ph3P−BH3 and
thiophenol do not reduce adamantyl iodide effectively.
CONCLUSIONS
■
In summary, we have developed several practical recipes for
thiol-catalyzed reductive dehalogenations with convenient and
readily available NHC-boranes. The use of thiols considerably
expands the scope of reductions of alkyl halides and for the first
time adds aryl halides as substrates for NHC-borane reductions.
In the case of alkyl radicals, an analysis of available and new rate
constants suggests that the thiol accelerates the radical
hydrogen atom transfer reaction through polarity reversal
catalysis. Rate constants for the reaction of the phenylthiyl
radical with two NHC-boranes were measured to be ∼108 M−1
s−1 by laser flash photolysis experiments. These values are large
for reactions that that are probably nearly thermoneutral, no
doubt thanks to the favorable polarity effect. Thus, the scope of
halide reductions by carbene boranes has been significantly
expanded, and the basis for thiol catalysis has been placed on a
firm mechanistic foundation.
Figure 4. Decay of the PhS• signal at 480 nm in ethylbenzene/
acetonitrile with increasing concentrations of 1. The arrow shows that
the thiyl lifetime decreases as the concentration of 1 increases. ΔOD is
the change in optical density associated with the light excitation. Inset:
the associated Stern−Volmer treatment.
whether step 4 is reversible. The reverse reaction certainly has a
relatively high rate constant, but the concentration of PhSH is
relatively low. The reaction in step 1 competes for NHC-BH2•,
and this reaction could well be faster than the reverse of step
4.23
We could not directly detect the formation of diMe-Imd-
BH2• in these experiments because its UV absorption overlaps
with that of PhS•. Accordingly, we carried out complementary
experiments with the B-phenyl-substituted analogue diMe-Imd-
BH2Ph (3) (Figure 1) because the UV−visible spectrum of its
derived boryl radical is characterized by a strong red shift (λmax
= 550 and 350 nm for diMe-Imd-BHPh• and diMe-Imd-BH2•,
respectively).18a Indeed, on photolysis of PhSSPh and 3, both
PhS• and diMe-Imd-BHPh• were observed (Figure 5). This
further supports the conclusion that PhS• and NHC-boranes
react by hydrogen atom transfer.
ASSOCIATED CONTENT
* Supporting Information
Full details for all experiments and calculations and crystallo-
graphic data (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
jacques.lalevee@uha.fr; curran@pitt.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. National Science Foundation, the French
Agence Nationale de Recherche (Grant ANR-11-BS07-008,
NHCX), UHA, ENSCMu, and CNRS for funding this work.
E.L. thanks Actelion and ENSCMu for the Actelion Award.
́
Prof. Louis Fensterbank (Universite Pierre et Marie Curie,
Paris) is acknowledged for helpful discussions. We thank Dr.
Steven J. Geib for solving the crystal structure of 2.
REFERENCES
■
(1) (a) Renaud, P.; Sibi, M. P. Radicals in Organic Synthesis, 1st ed.;
Wiley-VCH: Weinheim, Germany, 2001. (b) Radicals in Synthesis I:
Figure 5. Transient spectra for PhS−SPh and diMe-Imd-BH2Ph (3) in
acetonitrile from 0.5 to 36.5 μs in 4.5 μs steps (represented as different
colors).
Methods and Mechanisms; Gansauer, A., Ed.; Topics in Current
Chemistry, Vol. 263; Springer: Berlin, 2006. (c) Radicals in Synthesis II:
̈
Complex Molecules; Gansauer, A., Ed.; Topics in Current Chemistry,
̈
A series of LFP experiments were conducted with different
concentrations of 3, and these data are shown in the Supporting
Information. The results were analyzed by the usual Stern−
Volmer treatment, which provided kH = 7 × 107 M−1 s−1 for the
hydrogen abstraction reaction of PhS• from 3. The small
decrease in the rate constant relative to 1 caused by the phenyl
substituent in 3 is in line with prior rate measurements with
alkyl radicals8e and alkoxy radicals.18a
We also conducted a similar LFP experiment with
triphenylphosphine borane (Ph3P−BH3) in place of the
NHC-borane. However, this reagent did not quench the
PhS• radical under the conditions of the experiment (see the
Vol. 264; Springer: Berlin, 2006. (d) Rowlands, G. J. Tetrahedron
2009, 65, 8603−8655. (e) Rowlands, G. J. Tetrahedron 2010, 66,
1593−1636.
(2) (a) Baguley, P. A.; Walton, J. C. Angew. Chem., Int. Ed. 1998, 37,
3072−3082. (b) Studer, A.; Amrein, S. Synthesis 2002, 835−849.
(c) Luthy, M.; Darmency, V.; Renaud, P. Eur. J. Org. Chem. 2011,
̈
547−552.
(3) (a) Paul, V.; Roberts, B. P. J. Chem. Soc., Perkin Trans. 2 1988,
1183−1193. (b) Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trans
2 1988, 1195−1199. (c) Kirwan, J. N.; Roberts, B. P. J. Chem. Soc.,
Perkin Trans. 2 1989, 539−550. (d) Dang, H. S.; Diart, V.; Roberts, B.
P.; Tocher, D. A. J. Chem. Soc., Perkin Trans. 2 1994, 1895.
(e) Lucarini, M.; Pedulli, G. F.; Valgimigli, L. J. Org. Chem. 1996,
5673
dx.doi.org/10.1021/ja300416f | J. Am. Chem. Soc. 2012, 134, 5669−5674

Journal of the American Chemical Society
Article
61, 1161−1164. (f) Barton, D. H. R.; Jacob, M. Tetrahedron Lett. 1998,
39, 1331−1334. (g) Renaud, P.; Beauseigneur, A.; Brecht-Forster, A.;
Becattini, B.; Darmency, V.; Kandhasamy, S.; Montermini, F.; Ollivier,
C.; Panchaud, P.; Pozzi, D.; Scanlan, E. M.; Schaffner, A. P.; Weber, V.
Pure Appl. Chem. 2007, 79, 223−233.
2261−2267. (c) Lalevee
́
, J.; Morlet-Savary, F.; Roz, M. E.; Allonas, X.;
Fouassier, J. P. Macromol. Chem. Phys. 2009, 210, 311−319.
(21) (a) Newcomb, M.; Choi, S. Y.; Horner, J. H. J. Org. Chem. 1999,
64, 1225−1231. (b) Franz, J. A.; Bushaw, B. A.; Alnajjar, M. S. J. Am.
Chem. Soc. 1989, 111, 268−275.
(22) Borges dos Santos, R. M.; Muralha, V. n. S. F.; Correia, C. F.;
Guedes, R. C.; Costa Cabral, B. J.; Martinho Simoes, J. A. J. Phys.
Chem. A 2002, 106, 9883−9889.
(23) Cai, Y. D.; Roberts, B. P. J. Chem. Soc., Perkin Trans. 2 2002,
1858−1868.
(4) Sheeller, B.; Ingold, K. U. J. Chem. Soc., Perkin Trans. 2 2001,
480−486.
̃
(5) (a) Carboni, B.; Carreaux, F. Sci. Synth. 2004, 6, 455−484.
(b) Staubitz, A.; Robertson, A. P. M.; Sloan, M. E.; Manners, I. Chem.
Rev. 2010, 110, 4023−4078.
(6) Darmency, V.; Renaud, P. Top. Curr. Chem. 2006, 263, 71−106.
(7) Curran, D. P.; Solovyev, A.; Makhlouf Brahmi, M.; Fensterbank,
L.; Malacria, M.; Laco
10317.
(8) (a) Ueng, S.-H.; Makhlouf Brahmi, M.; Derat, E.; Fensterbank,
L.; Lacote, E.; Malacria, M.; Curran, D. P. J. Am. Chem. Soc. 2008, 130,
10082−10083. (b) Ueng, S.-H.; Solovyev, A.; Yuan, X.; Geib, S. J.;
Fensterbank, L.; Lacote, E.; Malacria, M.; Newcomb, M.; Walton, J. C.;
Curran, D. P. J. Am. Chem. Soc. 2009, 131, 11256−11262. (c) Walton,
J. C.; Makhlouf Brahmi, M.; Fensterbank, L.; Lacote, E.; Malacria, M.;
̂
te, E. Angew. Chem., Int. Ed. 2011, 50, 10294−
́
̂
̂
̂
Chu, Q.; Ueng, S.-H.; Solovyev, A.; Curran, D. P. J. Am. Chem. Soc.
2010, 132, 2350−2358. (d) Solovyev, A.; Ueng, S.-H.; Monot, J.;
Fensterbank, L.; Malacria, M.; Laco
̂
te, E.; Curran, D. P. Org. Lett.
te, E.;
Malacria, M.; Curran, D. P. Org. Lett. 2010, 12, 3002−3005. (f) Ueng,
S.-H.; Fensterbank, L.; Lacote, E.; Malacria, M.; Curran, D. P. Org.
2010, 12, 2998−3001. (e) Ueng, S.-H.; Fensterbank, L.; Laco
̂
̂
Biomol. Chem. 2011, 9, 3415−3420. (g) Related cyanoborohydride
anions also show promise. See ref 16 and: Kawamoto, T.; Fukuyama,
T.; Ryu, I. J. Am. Chem. Soc. 2012, 134, 875−879.
(9) (a) Tehfe, M.-A.; Monot, J.; Malacria, M.; Fensterbank, L.;
̂ ́
Fouassier, J.-P.; Curran, D. P.; Lacote, E.; Lalevee, J. ACS Macro Lett.
2012, 1, 92−95. (b) The crystal structure of 2 has now been solved
(see the Supporting Information).
(10) Chatgilialoglu, C.; Newcomb, M. Adv. Organomet. Chem. 1999,
44, 67−112.
(11) (a) Barrett, K. E. J.; Waters, W. A. Discuss. Faraday Soc. 1953,
14, 211−277. (b) Chatgilialoglu, C.; Ferreri, C.; Gimisis, T. In
Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y.,
Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, pp 1539−1579;
(c) Roberts, B. P. Chem. Soc. Rev. 1999, 28, 25−35. (d) Crich, D.;
Grant, D.; Krishnamurthy, V.; Patel, M. Acc. Chem. Res. 2007, 40,
453−463.
(12) (a) Chu, Q.; Makhlouf Brahmi, M.; Solovyev, A.; Ueng, S.-H.;
Curran, D.; Malacria, M.; Fensterbank, L.; Laco
̂
te, E. Chem.Eur. J.
2009, 15, 12937−12940. (b) Horn, M.; Mayr, H.; Laco
̂
te, E.; Merling,
E.; Deaner, J.; Wells, S.; McFadden, T.; Curran, D. P. Org. Lett. 2012,
14, 82−85.
(13) Kita, Y.; Matsugi, M. In Radicals in Organic Synthesis, 1st ed.;
Renard, P., Sibi, M., Eds.; Wiley-VCH: Weinheim, Germany, 2001;
Vol 1, pp 1−10.
(14) (a) Chatgilialoglu, C.; Bertrand, M. P.; Ferreri, C. In Sulfur-
Centered Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, U.K., 1999; pp
311−354. (b) Yorimitsu, H.; Oshima, K. In Radicals in Organic
Synthesis, 1st ed.; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim,
Germany, 2001; Vol. 1, pp 11−27.
(15) Mendenhall, D. G. Tetrahedron Lett. 1983, 24, 451−452.
(16) Ryu, I.; Uehara, S.; Hirao, H.; Fukuyama, T. Org. Lett. 2008, 10,
1005−1008.
(17) Solovyev, A.; Chu, Q.; Geib, S. J.; Fensterbank, L.; Malacria, M.;
̂
Lacote, E.; Curran, D. P. J. Am. Chem. Soc. 2010, 132, 15072−15080.
(18) (a) Ogawa, A.; Curran, D. P. J. Org. Chem. 1997, 62, 450−451.
(b) Maul, J. J.; Ostrowski, P. J.; Ublacker, G. A.; Linclau, B.; Curran, D.
P. Top. Curr. Chem. 1999, 206, 80−104.
(19) Villar, F.; Equey, O.; Renaud, P. Org. Lett. 2000, 2, 1061−1064.
(20) (a) Tehfe, M.-A.; Monot, J.; Makhlouf Brahmi, M.; Bonin-
Dubarle, H.; Curran, D. P.; Malacria, M.; Fensterbank, L.; Laco
Lalevee, J.; Fouassier, J.-P. Polym. Chem. 2011, 2, 625−631. (b) Tehfe,
M.-A.; Makhlouf Brahmi, M.; Fouassier, J.-P.; Curran, D. P.; Malacria,
M.; Fensterbank, L.; Lacote, E.; Lalevee, J. Macromolecules 2010, 43,
̂
te, E.;
́
̂
́
5674
dx.doi.org/10.1021/ja300416f | J. Am. Chem. Soc. 2012, 134, 5669−5674