An N-heterocyclic carbene-catalyzed switchable reaction of 9-(trimethylsilyl)fluorene and aldehydes: Chemoselective synthesis of dibenzofulvenes and fluorenyl alcohols

Article abstract of DOI:10.1039/d1ob00065a

An N-heterocyclic carbene-catalyzed synthesis of dibenzofulvenes and fluorenyl alcohols was developed. In the presence of 10 mol% NHC (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and 4 ? molecular sieves, 9-(trimethylsilyl)fluorene undergoes an olef

Full text of DOI:10.1039/d1ob00065a

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An N-heterocyclic carbene-catalyzed switchable
reaction of 9-(trimethylsilyl)fluorene and
aldehydes: chemoselective synthesis of
dibenzofulvenes and fluorenyl alcohols†
a
Cite this: Org. Biomol. Chem., 2021,
19, 3717
Yu-Chuan Ma,a Jin-Yun Luo,a Shi-Chu Zhang,b Shu-Hui Lu,a Guang-Fen Du
*
a
and Lin He
An N-heterocyclic carbene-catalyzed synthesis of dibenzofulvenes and fluorenyl alcohols was developed.
In the presence of 10 mol% NHC (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and 4 Å molecular
sieves, 9-(trimethylsilyl)fluorene undergoes an olefination reaction with aldehydes to produce dibenzoful-
venes in 43–99% yields. However, on reducing the NHC loading to 1 mol% and with the addition of water,
9-(trimethylsilyl)fluorene selectively undergoes nucleophilic addition with aldehydes to afford fluorenyl
alcohols in 40–95% yields.
Received 13th January 2021,
Accepted 23rd March 2021
DOI: 10.1039/d1ob00065a
rsc.li/obc
In particular, NHCs exhibit high reactivity toward activation of
silylated nucleophiles.12 As a result, the NHC-catalyzed cyana-
Introduction
Dibenzofulvenes and fluorenyl alcohols are important struc- tion reaction,13 trifluoromethylation reaction,14a Mukaiyama
tural motifs existing widely in many natural products, pharma- aldol reaction14b and ring-opening reaction15 have been devel-
ceuticals, biologically active compounds1 and optoelectronic oped by different groups. Our group has also independently
materials.2 Due to the remarkable importance of these fluor- developed NHC-catalyzed vinylogous Mukaiyama type
ene frameworks, considerable effort has been exerted to additions, silyl-Reformatsky reaction, Peterson olefination and
develop efficient methods for the synthesis of these versatile other reactions.16 In line with our continuous interest in NHC
scaffolds. Typically, Wittig olefination,3 Peterson olefination,4 catalysis, we envisaged that NHCs can be used as a nucleophi-
and transition-metal catalyzed reactions5 provide efficient lic catalyst to catalyze the reaction between 9-TMSF and carbo-
approaches for the synthesis of dibenzofulvenes, while strong nyl compounds to produce dibenzofulvenes and fluorenyl
base promoted nucleophilic addition of fluorene anions and alcohols.
carbonyl compounds allows facile access to fluorenyl alco-
hols.6 However, stoichiometric amounts of strong bases, such
as n-BuLi, LDA and Grignard reagent, harsh reaction con-
ditions and expensive transition-metal catalysts are usually
Results and discussion
required for these reactions. Therefore, the development of a
more mild and convenient method for the synthesis of diben-
zofulvenes and fluorenyl alcohols is highly significant. As an
important type of organocatalyst, NHCs have been used in a
wide range of reactions,7 such as umpolung reactions,8
cycloadditions,9 redox reactions10 and other transformations.11
With this idea in mind, we commenced our study using the
commercially available 9-(trimethylsilyl)fluorene (9-TMSF) and
p-chlorobenzaldehyde as the model substrates. In the presence
of 10 mol% stable NHC A (1,3-bis(diisopropylphenyl)imid-
azole-2-ylidene, IPr),17 the reaction proceeded in THF at room
temperature to afford 18% yield of dibenzofulvene 3a and 37%
yield of fluorenyl alcohol 4a (Table 1, entry 1). The following
evaluation of the reaction media showed that the polar aprotic
solvents acetonitrile, DMSO, and chlorinated solvents gave
dibenzofulvene 3a as the major product, whereas DMF,
toluene and ethers afforded fluorenyl alcohol 4a as the major
product (Table 1, entries 2–8). Protic solvents, such as metha-
nol and ethanol, were inefficient (Table 1, entries 9 and 10).
Surprisingly, using DMSO as the solvent, NHCs derived from
both imidazolium precursors and the saturated imidazolinium
aKey Laboratory for Green Processing of Chemical Engineering of Xinjiang,
Bingtuan/School of Chemistry and Chemical Engineering, Shihezi University,
Xinjiang Uygur Autonomous Region, 832000, People’s Republic of China.
E-mail: duguangfen@shzu.edu.cn
bCollege of Sciences, Shihezi University, Xinjiang Uygur Autonomous Region, 832000,
People’s Republic of China
†Electronic supplementary information (ESI) available: Experimental procedures
and characterization data. See DOI: 10.1039/d1ob00065a
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Table 1 Optimization of the reaction conditionsa
entry 21). However, further increasing the amount of H2O led
to a dramatic decrease of the yield (Table 1, entry 22). On con-
ducting the reaction at an elevated temperature, the addition
could be completed in a shorter time but the chemoselectivity
was decreased obviously (Table 1, entry 23). Finally, the control
experiment showed that in the absence of NHC, no desired
products were formed (Table 1, entry 24).
Having evaluated the optimal reaction conditions, we then
examined the substrate scope of the olefination reaction and
the results are presented in Table 2. Aromatic aldehydes with
electron-withdrawing, electron-neutral and electron-donating
groups participated in the reaction well, producing the corres-
ponding dibenzofulvenes in moderate to high yields (Table 2,
3a–3h). It’s noteworthy that aromatic aldehydes with electron-
withdrawing substituents gave a higher yield than those
bearing electron-neutral or electron-donating substituents
(Table 2, 3a–3h). In addition, substitutions at the ortho-, meta-
and para-positions of the aromatic ring could be well tolerated
for the reaction (Table 2, 3i–3o). Both α- and β-naphthaldehydes
were proved to be competent substrates for the olefination,
affording 3p and 3q in 87% and 77% yields, respectively
(Table 2, 3p and 3q). Heteroaromatic aldehydes, such as fur-
fural, 2-thienal and 2-thiazol aldehyde, underwent the reaction
to produce the corresponding products in moderate to excel-
lent yields (Table 2, 3r–3t). Heliotropine reacted with 9-TMSF
to afford dibenzofulvene 3u in 50% yield (Table 2, 3u).
Ferrocenealdehyde was proved to be a successful candidate for
T
Yield 3a b Yield 4a b
Entry Conditions
(°C) (%)
(%)
1
2
3
4
5
6
7
8
THF, A (10 mol%), 12 h
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
18
57
79
36
32
12
8
37
16
5
15
3
33
35
37
0
CH3CN, A (10 mol%), 12 h
DMSO, A (10 mol%), 12 h
DCM, A (10 mol%), 12 h
DCE, A (10 mol%), 12 h
DMF, A (10 mol%), 12 h
Toluene, A (10 mol%), 12 h
1,4-Dioxane, A (10 mol%), 12 h
MeOH, A (10 mol%), 24 h
EtOH, A (10 mol%), 24 h
DMSO, B1 (12 mol%)
6
0
9
10
11
0
7
0
41
t-BuOK (10 mol%), 12 h
DMSO, B2 (12 mol%)
t-BuOK (10 mol%), 12 h
DMSO, B3 (12 mol%)
t-BuOK (10 mol%), 12 h
DMSO, C (12 mol%)
t-BuOK (10 mol%), 12 h
DMSO, D (12 mol%)
12
13
14
15
16
rt
rt
rt
rt
rt
rt
5
40
60
55
0
8
4
0
Table 2 NHC-catalyzed synthesis of dibenzofulvenesa
t-BuOK (10 mol%), 24 h
DMSO, E (12 mol%)
0
0
t-BuOK (10 mol%), 24 h
DMSO, A (10 mol%), 4 Å MS, 12 h
DMSO, A (10 mol%), 4 Å MS, 5 h
DMSO, A (1 mol%), 12 h
DMSO, A (0.1 mol%), 12 h
17
18
19
20
21
22
23
24
88
3
0
60 89
rt
rt
6
0
3
0
62
43
74
0
67
0
DMSO, A (1 mol%), H2O (100 μL), 12 h 10
DMSO, A (1 mol%), H2O (500 μL), 12 h 10
DMSO, A (1 mol%), H2O (100 μL), 4 h 60 16
DMSO, 48 h
rt
0
a
1a (1.0 equiv.), 2a (1.0 equiv.), NHC A (10 mol%) or NHC precursor
(12 mol%), t-BuOK (10 mol%). b Isolated yield.
salt catalyzed the reaction to produce fluorenyl alcohol 4a as
the major product (Table 1, entries 11–14). NHCs derived from
thiazolium and triazolium could not catalyze the reaction
(Table 1, entries 15 and 16). The addition of 4 Å molecular
sieves could improve the reaction yield of 3a to 88% (Table 1,
entry 17). Raising the reaction temperature led to a shorter
reaction time (Table 1, entry 18). Interestingly, reducing the
NHC loading to 1 mol% resulted in a switch of the chemo-
selectivity and fluorenyl alcohol 4a was formed as the major
product (Table 1, entry 19). On further reduction of the catalyst
loading to 0.1 mol%, fluorenyl alcohol 4a could still be
obtained in 43% yield (Table 1, entry 20). Intriguingly, the
addition of H2O and lowering of the reaction temperature to
10 °C could improve the reaction yield of 4a to 74% (Table 1,
a Reaction conditions: 1a (0.10 mmol),
(1 mol%), DMSO (1.0 mL), 2–24 h, room temperature; isolated yield.
2 (0.10 mmol), NHC A
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the reaction, providing 3v in 43% yield (Table 2, 3v). However,
when aliphatic aldehyde was used for this olefination, only a
trace amount of the desired product was detected and the
starting materials were recovered in high yields (Table 2, 3w).
The active 2,2,2-trifluoroacetophenone could also undergo the
olefination efficiently to give 3x in 78% yield (Table 2, 3x). It’s
noteworthy that fluorenyl alcohols were obtained as side pro-
ducts in 3–10% yields for these reactions.
The generality of the synthesis of fluorenyl alcohols was
next investigated. As shown in Table 3, aromatic aldehydes
bearing electron-withdrawing substituents have higher yields
than that bearing an electron-donating group (Table 3, 4a–4h).
Different positions of the substituents could be tolerated for
the addition reaction (Table 3, 4i–4k). Both naphthaldehyde
and biphenyl aldehyde performed very well, producing the
corresponding products in good yields (Table 3, 4l–4n).
Heliotropine underwent the addition to give 4o in 40% yield
(Table 3, 4o). 2-Benzofurancarboxaldehyde coupled efficiently
with 9-TMSF to furnish 4p in an excellent yield (Table 3, 4p).
Electron-rich heteroaromatic aldehydes, such as furfural and
2-thienal, reacted with 9-TMSF to produce the desired products
in moderate yields, while electron-deficient 2-thiazo aldehyde
underwent the addition to give 4s in a high yield (Table 3, 4q–
4s).
Scheme 1 Proposed mechanism.
Scheme 2 Control experiment.
On conducting the reaction in undried DMSO, the product
was delivered in a comparable yield (Table 3, 4a). Similarly, the
corresponding dibenzofulvenes were formed as side products
for these additions in 4–21% yields.
Based on the pioneering studies on NHC-catalyzed
additions of silylated nucleophiles,13–16 a plausible mecha-
nism was proposed for the reaction (Scheme 1). NHC acts as a
Lewis base to attack the silicon atom of 9-TMSF to generate
the reactive hypervalent silicon species I, which might trigger
the subsequent nucleophilic addition with aldehyde to
produce intermediate II. The following migration of TMS from
NHC to II results in the formation of intermediate III. At this
time, NHC can act as a carbon-centered Brønsted base16g,18 to
catalyze the elimination of III to produce dibenzofulvene,
while the direct hydrolysis of III will give fluorenyl alcohol.
The control experiment showed that 10 mol% NHC A can
act as a Brønsted base to catalyze dehydration19 of 4a to afford
3a in 75% yield. However, under the catalysis of 1 mol% NHC
A, 3a cannot react with H2O to produce 4a. We concluded that
3a is thermodynamically more stable than 4a, which should be
responsible for this result (Scheme 2).
Table 3 NHC-catalyzed synthesis of fluorenyl alcoholsa
Conclusions
In summary, using the unique nucleophilicity and Brønsted
characteristics of NHCs, we have developed an efficient
method for the synthesis of dibenzofulvenes. We have also
described an organocatalytic method for the preparation of
fluorenyl alcohols via the activation of 9-TMSF by NHCs. The
mild conditions, simple procedure and switchable chemo-
selectivity provide an efficient protocol for the synthesis of
these important fluorene-derived compounds.
a Reaction conditions:
1 (0.10 mmol), 2a (0.10 mmol), NHC A
Conflicts of interest
(1 mol%), DMSO (1.0 mL), H2O (100 μl), 2–24 h, 10 °C; isolated yield.
b Using undried DMSO as the reaction solvent.
There are no conflicts to declare.
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B. Luo, A. Ganesan, F.-H. Wu, R. Pi, P. Huang and S. Wen,
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21662029), the Young Scientists
Foundation of Shihezi University (2015ZRKXJQ05), the
Excellent Young Teachers Plan of Bingtuan (2017CB001,
CZ027203) and the International Cooperation Project of
Shihezi University (no. GJHZ201801).
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