Nucleophilic fluoromethylation of aldehydes with fluorobis(phenylsulfonyl) methane: The importance of strong Li-O coordination and fluorine substitution for C-C bond formation

Article abstract of DOI:10.1002/anie.201006931

The tighter, the better! Fluorobis(phenylsulfonyl)methane reacts readily with aldehydes to give addition products in excellent yields (see scheme; LiHMDS=lithium hexamethyldisilazide), which is in sharp contrast to previous findings. Both strong Li-O coordination at low temperature and fluorine substitution play important roles in successful C-C bond formation. Copyright

Full text of DOI:10.1002/anie.201006931

Communications
DOI: 10.1002/anie.201006931
Fluoromethylation
Nucleophilic Fluoromethylation of Aldehydes with
Fluorobis(phenylsulfonyl)methane: The Importance of Strong Li–O
ꢀ
Coordination and Fluorine Substitution for C C Bond Formation**
Xiao Shen, Laijun Zhang, Yanchuan Zhao, Lingui Zhu, Guangyu Li, and Jinbo Hu*
Recently, the selective introduction of fluorinated moieties
into organic molecules with a-fluorinated carbanions has
attracted substantial interest in organic synthesis, owing to the
increasingly important applications of selectively fluorinated
organics in life and materials sciences.[1–5] In particular,
a-fluoro(phenylsulfonyl)methane (FSM) derivatives, such as
difluoromethyl phenyl sulfone (PhSO2CF2H), fluoromethyl
phenyl sulfone (PhSO2CH2F), and fluorobis(phenylsulfonyl)-
methane (FBSM), have been extensively studied as excellent
precursors of a-fluorinated carbanions for nucleophilic di-
and monofluoromethylations.[2] In this context, FBSM, a
reagent independently reported by Shibata et al. and our
group in 2006,[3a,4] has been widely recognized as a robust
nucleophilic monofluoromethylating agent for many applica-
tions, including catalytic enantioselective monofluoromethy-
lation reactions.[3a,b,4–6] However, although FBSM reacts with
a great variety of electrophiles,[3–5] its nucleophilic addition
reaction with aldehydes still remains a challenge.
nucleophilic addition of FBSM to aldehydes. Our study shows
that both the strong Li–O coordination at low temperatures
and fluorine substitution play very important roles in the
successful nucleophilic addition of FBSM to aldehydes, which
provides new intriguing insights into the nucleophilic fluo-
roalkylation reactions with a-fluorinated carbanions.[8]
The nucleophilic addition reaction of FBSM to aldehydes
was tested by using benzaldehyde (1a) as a model substrate
(Table 1). When lithium hexamethyldisilazide (LiHMDS)
Table 1: Optimization of reaction conditions.[a]
Entry
Base
Solvent
Protonation
conditions
Yield
[%][b]
Very recently, Shibata et al. reported that “FBSM failed to
undergo nucleophilic addition to aldehydes regardless of the
reaction conditions, leading instead to starting materials by a
retro-type reaction”, and they also reasoned that “this
behavior presumably results from the instability of the
1
2
3
4
5
6
7
8
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS[d]
NaHMDS[d]
KHMDS[d]
LiHMDS[e]
THF
THF
HCl (aq), ꢀ308C
HCl (aq), ꢀ788C
HCl (aq), ꢀ788C
CF3COOH, ꢀ788C
CF3COOH, ꢀ788C
CF3COOH, ꢀ788C
CF3COOH, ꢀ788C
CF3COOH, ꢀ948C
CF3COOH, ꢀ948C
CF3COOH, ꢀ948C
CF3COOH, ꢀ948C
CF3COOH, ꢀ948C
0
27
0
THF/HMPA[c]
THF
45
52
85
86
92
92
71
27
83
Et2O
CH2Cl2
PhCH3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
resulting
b-hydroxy-a-fluorobis(phenylsulfonyl)methanes
caused by the steric hindrance of the two phenylsulfonyl
groups” (Scheme 1).[7] Prompted by their report, herein we
wish to disclose our remarkable success in the efficient
9
10
11
12
[a] Reactions were carried out using FBSM (1 equiv), 1a (2.0 equiv), and
base (1.2 equiv) in solvent at ꢀ788C unless otherwise noted. [b] Deter-
mined by 19F NMR spectroscopy. [c] THF/HMPA=10:1 (v/v). [d] 1a
(1.5 equiv) was used. [e] 1a (1.2 equiv) was used.
Scheme 1. Unsuccessful monofluoromethylation of aldehydes with
FBSM.[7]
was added to a CH2Cl2 solution of 1a and FBSM at ꢀ788C
(and the solution was stirred for 30 min), followed by
quenching with hydrochloric acid (10m) at ꢀ308C, no
addition product 2a was observed (Table 1, entry 1). Inter-
estingly, when the reaction was quenched by hydrochloric
acid (10m) at lower temperature (ꢀ788C), product 2a was
formed in 27% yield (Table 1, entry 2). Increasing the solvent
polarity by adding hexamethylphosphoramide (HMPA)
resulted in the failure of the addition reaction (Table 1,
entry 3). When the addition reaction was quenched by
trifluoroacetic acid, the yield of 2a increased to 45%
(Table 1, entry 4). After further scanning of the reaction
solvents, temperatures, reactant ratios, and bases (Table 1,
[*] X. Shen, L. Zhang, Y. Zhao, L. Zhu, G. Li, Prof. Dr. J. Hu
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences
345 Ling-Ling Road, Shanghai 200032 (China)
Fax: (+86)21-6416-6128
E-mail: jinbohu@sioc.ac.cn
[**] Support of our work by the National Natural Science Foundation of
China (20772144, 20825209, 20832008) is gratefully acknowledged.
We thank Liping Shi for her help in VT NMR experiments and Dr.
Huaping Mo and Bo Chen for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006931.
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Table 2: Monofluoromethylation of aldehydes with FBSM.[a]
entries 5–12), we found that an optimal yield of 2a (92%)
could be obtained when the reaction was carried out in
CH2Cl2 at ꢀ788C (with a reactant ratio FBSM/1a/LiHMDS =
1:1.5:1.2) and quenched by trifluoroacetic acid at ꢀ948C
(Table 1, entry 9).
It should be mentioned that, among the lithium, sodium,
and potassium hexamethyldisilazides, LiHMDS serves as the
best base for this addition reaction, which indicates that the
strong Li–O coordination in the carbinolate intermediate 3
plays a very important role in the success of the reaction
(Scheme 2). This assumption is supported by the experimen-
Entry
Aldehyde 1
Product 2
Yield
[%][b]
1
2
3
4
5
6
7
8
9
10
1a
1b
1c
1d
1e
1 f
1g
1h
1i
R=Ph
2a
2b
2c
2d
2e
2 f
2g
2h
2i
92
88
87
89
92
91
95
90
85
93
R=p-Me-C6H4
R=p-MeO-C6H4
R=2,5-MeO-C6H4
R=o-Cl-C6H4
R=p-Cl-C6H4
R=2,4-Cl-C6H4
R=p-Br-C6H4
R=2-furyl
=
1j
R=(E)-PhCH CH
2j
11
1k
2k
87
12[c]
13[c]
14[c]
1l
1m
1n
R=PhCH2CH2
R=(CH3)2CH
R=cyclohexyl
2l
2m
2n
93
94
82
Scheme 2. Formation of carbinol 2a through lithium carbinolate 3.
tal observations that the use of weak-coordinating solvent
(such as CH2Cl2) and low reaction/quenching temperatures
facilitates the addition of FBSM to benzaldehyde 1a
(Table 1). Furthermore, the existence of strong coordination
between Li and three neighboring oxygen atoms in lithium
carbinolate 3 was confirmed by our computationally opti-
mized structure (the Li–O distances are 1.761, 1.985, and
1.986 ꢀ, respectively; see Figure 3A).[9]
[a] Reactions were carried out using FBSM (1.0 equiv), aldehyde
(1.5 equiv), and LiHMDS (1.2 equiv) in CH2Cl2 at ꢀ788C for 30 min
unless otherwise noted. [b] Yield of isolated product. [c] FBSM was
stirred with LiHMDS first in CH2Cl2 at ꢀ788C for 30 min, and then the
aldehyde was added and the mixture was stirred for an additional 30 min.
With the optimized conditions in hand, we then examined
the substrate scope of this nucleophilic monofluoromethyla-
tion reaction between FBSM and aldehydes 1. As shown in
Table 2, the reaction turned out to be general and a variety of
structurally diverse aldehydes were successfully monofluoro-
methylated by FBSM to give the corresponding carbinols 2 in
high yields (82–95%). The reaction tolerates many substitu-
ents such as methoxy, bromo, and chloro groups (Table 2,
entries 3–8). Heteroaryl aldehyde 1i can also react with
FBSM, affording carbinol product 2i in 85% yield (Table 2,
entry 9). a,b-Unsaturated aldehydes 1j and 1k are also
compatible with the reaction, and only 1,2-addition products
2j and 2k were formed in 93 and 87% yield, respectively
(Table 2, entries 10 and 11). It is remarkable that the current
nucleophilic monofluoromethylation reaction is also amena-
ble to aliphatic aldehydes 1l–1n, giving the products 2l–2n in
82–94% yield (Table 2, entries 12–14). All the products 2a–
2n were purified and isolated by silica gel chromatography,
and the structure of 2h was confirmed by
X-ray crystal structure analysis (Figure 1).[10]
To gain more insight into the efficient nucleophilic
addition of FBSM to aldehydes, we carefully monitored the
progress of the reaction of FBSM, benzaldehyde (1a), and
LiHMDS in CD2Cl2 by variable-temperature (VT) 19F NMR
spectroscopy (Figure 2).[11] First of all, under a N2 atmosphere,
a mixture of 1a (0.15 mmol) and FBSM (0.10 mmol) in
CD2Cl2 (1 mL) was placed in an NMR tube, and a peak (d =
ꢀ170 ppm) of FBSM was observed at ꢀ768C (Figure 2A).
Thereafter, LiHMDS (0.12 mmol) was added to the NMR
tube and the temperature was kept at ꢀ798C. The peak of
Figure 1. X-ray crystal structure of product 2h.[10]
FBSM diminished quickly, and new peaks ranging from d =
ꢀ121 to ꢀ128 ppm appeared with a major peak at d =
ꢀ126 ppm, which represents the lithium alcoholate 3 (Fig-
ure 2B). When the temperature of the NMR tube was slowly
raised from ꢀ79 to 258C, the major peak at d = ꢀ126 ppm
diminished slowly (Figure 2B–F), and finally several broad
peaks ranging from d = ꢀ121 to ꢀ128 ppm were left (Fig-
ure 2F).[11] This indicates that at higher temperatures (such as
above ꢀ198C) a quick equilibrium may exist between lithium
alcoholate 3 and the retro-addition products [that is, aldehyde
1a and (PhSO2)2CFꢀLi+ (8), as shown in Eq. (1)].
ðPhSO2Þ2CFꢀLiþ ð8Þ þ 1 a Ð 3
ð1Þ
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Scheme 3. Unsuccessful nucleophilic additions to PhCHO (1a) with
(PhSO2)2CH2 (BSM) and (PhSO2)2CHCl (CBSM).
CBSM could be attributed to the instability of lithium
carbinolates 4 and 5 when compared to the fluorine-
substituted analogue 3 (Scheme 2).
However, the fact that fluorine-substituted lithium
carbinolate 3 possesses higher stability than nonsubstituted
and chlorine-substituted analogues 4 and 5 is anti-intuitive,
especially when one considers that: a) the steric strain of 3
is not likely to be significantly smaller than that of 4 and 5,
b) the Li–O coordination is similarly strong in 3–5
(Figure 3), and c) the electronegative fluorine atom may
Figure 2. Monitoring of the reaction of FBSM, PhCHO (1a), and
LiHMDS by VT 19F NMR spectroscopy. A) An NMR sample containing
1a (0.15 mmol), FBSM (0.10 mmol), and CD2Cl2 (1 mL) at ꢀ768C.
B) LiHMDS (0.12 mmol) was added to the above sample at ꢀ798C.
C–F) The temperature of the sample was raised to ꢀ49, ꢀ19, 0, and
258C, respectively. G) The sample was cooled to ꢀ798C again.
H) CF3COOH (0.5 mL) was added to the sample at ꢀ798C.
Furthermore, when we cooled the NMR tube to low
temperature (ꢀ798C) again, a peak corresponding to 3 (d =
ꢀ126 ppm) reappeared (Figure 2G), which suggested that
low temperature is beneficial to push the equilibrium
[Eq. (1)] to the right-hand side, thus facilitating the formation
of 3. Finally, after the NMR sample was quenched with
CF3COOH at ꢀ798C, the peaks of carbinol product 2a (d =
ꢀ134 ppm) and of FBSM (d = ꢀ169 ppm) appeared (Fig-
ure 2H).[12] These results clearly demonstrate that not only
does the strong Li–O coordination play an important role for
the generation of 3 (as shown in Table 1), but also the control
of the reaction at low temperature is crucial to stabilize the
carbinolate 3. This probably also explains why when Shibata
and co-workers performed the same addition reaction of
FBSM to benzaldehyde 1a by using other bases (such as
tBuOK, Et3N, 1,8-diazabicyclo[5.4.0]undec-7-ene, and 1,4-
diazabicyclo[2.2.2]octane) at room temperature, no addition
product 2a could be obtained.[7]
Figure 3. A) Optimized structure of fluorine-substituted lithium carbi-
ꢀ
nolate (3). The length of the FC COLi bond is 1.570 ꢀ, and the lengths
ꢀ
of the three O Li coordination bonds are 1.761, 1.985, and 1.986 ꢀ,
respectively. B) Optimized structure of lithium carbinolate (4). The
ꢀ
length of the HC COLi bond is 1.605 ꢀ, and the lengths of the three
ꢀ
O Li coordination bonds are 1.769, 1.942, and 2.01 ꢀ, respectively.
C) Optimized structure of chlorine-substituted lithium carbinolate (5).
ꢀ
The length of the ClC COLi bond is 1.622 ꢀ, and the lengths of the
ꢀ
three O Li coordination bonds are 1.761, 1.933, and 1.967 ꢀ, respec-
tively. All three optimized structures were calculated at the B3LYP/6-
31+G(d,p) level, and all the hydrogen atoms in these structures have
been omitted for clarity.
Encouraged by these results, we further tested the
nucleophilic additions of bis(phenylsulfonyl)methane
(BSM) and chlorobis(phenylsulfonyl)methane (CBSM) to
benzaldehyde 1a. The reaction conditions were exactly the
same as those for the reaction with FBSM (as described in
Table 1, entry 9). Much to our surprise, neither of these two
reactions produced the desired addition product 6 or 7
(Scheme 3), and the BSM or CBSM starting material was
recovered almost quantitatively. It should be mentioned that
compound 6, which could be prepared by a different
method,[13] was found to be stable even at room temperature.
Therefore, the failure of the addition reactions with BSM and
make the fluorobis(phenylsulfonyl)methyl group a better
leaving group than the (phenylsulfonyl)methyl group itself.
To understand the “unusual” stability of 3, we first inves-
tigated the optimized gas-phase structures of 3–5 by DFT
calculation based on the B3LYP/6-31 + G(d,p) level of theory
(Figure 3).[9] It was found that the length of the newly formed
ꢀ
C C bond (from the addition reaction between FBSM and
1a) in 3 is 1.570 ꢀ, which is shorter than those in 4 and 5
ꢀ
(1.605 and 1.622 ꢀ, respectively). The different C C bond
lengths of carbinolates 3–5 suggest that species 3 possesses the
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least tendency to undergo decomposition through retro-
addition reaction.
aldehydes, but also it opens up new synthetic possibilities such
as the design of enantioselective fluoromethylations of
carbonyl compounds. Further investigations in this direction
are currently under way in our laboratory.
Secondly, based on DFT calculations, Prakash et al.
reported that the gas-phase proton affinities of the anions
are: (PhSO2)2CFꢀ, 335.1 kcalmolꢀ1; (PhSO2)2CHꢀ, 332.7 kcal
molꢀ1; and (PhSO2)2CClꢀ, 330.1 kcalmolꢀ1.[6] These proton
affinity data indicate that among these three anions,
(PhSO2)2CFꢀ possesses the highest basicity and the lowest
leaving-group ability, which is in good agreement with our
experimental result that 3 is the most stable lithium carbino-
late species among 3–5.[14]
Thirdly, we investigated the gas-phase Gibbs free energies
of three addition reactions between carbanions 8–10 (9 =
(PhSO2)2CHꢀLi+; 10 = (PhSO2)2CClꢀLi+) and benzaldehyde
1a by DFT calculation based on the B3LYP/6-311 + G(2d,p)//
B3LYP/6-31 + G(d,p) level of theory.[9] It was found that the
Gibbs free energies (DG) of these addition reactions at
ꢀ788C (1 atm) are ꢀ2.3, + 6.5, and + 5.5 kcalmolꢀ1 for the
formation of 3–5, respectively. These DG values suggest that
among these three reactions, only the formation of 3 can
spontaneously proceed at ꢀ788C, but adducts 4 and 5 prefer
decomposing into 9 (or 10) and 1a at the same temperature,
which is also in good agreement with our experimental
observations (see Schemes 2 and 3).
Received: November 4, 2010
Revised: December 10, 2010
Published online: February 15, 2011
Keywords: aldehydes · fluorine · fluoromethylation ·
.
nucleophilic addition · sulfones
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Scheme 4. Synthetic application of product 2a. Bz=benzoyl, AIBN=a-
zobisisobutyronitrile.
compound 11 was converted into monofluorinated alkene 12
with excellent stereoselectivity (Z/E = 99:1). The phenyl-
sulfonyl group in 12 was efficiently transformed into a
tributylstannyl group in 13 by using nBu3SnH/AIBN, possibly
through a radical addition–elimination process.[15] Finally, a
stereospecific Stille coupling reaction efficiently converted 13
into fluorinated E-stilbene derivatives 14.
In conclusion, the nucleophilic addition reaction between
FBSM and an aldehyde, a reaction that was previously
believed to be unattainable,[7] has been successfully accom-
plished. Our experimental results showed that both the strong
Li–O coordination at low temperature and fluorine substitu-
tion play very important roles in the successful nucleophilic
addition of FBSM to aldehydes, which was further supported
by our VT NMR study and DFT calculations. Not only does
our study provide new mechanistic insights into understand-
ing the nucleophilic addition of substituted methides to
[7] T. Furukawa, Y. Goto, J. Kawazoe, E. Tokunaga, S. Nakamura,
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1647.
[8] For a review on fluorinated carbanions, see: W. R. Farnham,
Chem. Rev. 1996, 96, 1633 – 1640.
[9] For details of the DFT calculations, see the Supporting
Information.
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[10] CCDC 798880 (2h) contains the supplementary crystallographic
[13] Compound 6 was prepared by nucleophilic addition of bis(phe-
nylthio)methane to benzaldehyde, followed by oxidation with
meta-perbenzoic acid (see the Supporting Information). It
should be noted that when we added LiHMDS to the CH2Cl2
solution of 6 at ꢀ788C, then quenched with CF3COOH at
ꢀ948C, compound 6 decomposed to give PhCHO and BSM.
[14] For discussions on basicity and leaving-group ability, see: E.
Buncel, J. M. Dust, Carbanion Chemistry: Structures and Mech-
anisms, Oxford University Press, New York, 2003.
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[11] For more detailed VT 19F NMR spectra, see the Supporting
Information.
[12] Since it is difficult to stir the solution in the NMR tube during the
quenching process, a local exothermic reaction may lead to the
generation of (PhSO2)2CFꢀLi+ (8) from 3. This partially explains
why the yield of 2a in the NMR experiments was lower than
those in flask reactions (Table 1).
[15] J. R. McCarthy, D. P. Matthews, D. M. Stemerick, E. W. Huber,
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