Chromium-Catalysed Asymmetric Dearomatization Addition Reactions of Bromomethylnaphthalenes

Article abstract of DOI:10.1002/adsc.201600962

The asymmetric dearomatization and addition reaction of bromomethylnaphthalenes with aldehydes proceeded smoothly in the presence of carbazole-based bisoxazoline CrCl₂ complex to give the corresponding enanioenriched hydroxylated dearomatizatio

Full text of DOI:10.1002/adsc.201600962

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DOI: 10.1002/adsc.201600962
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Chromium-Catalysed Asymmetric Dearomatization Addition
Reactions of Bromomethylnaphthalenes
+
a
+a
a,
Weiqiang Chen, Jing Bai, and Guozhu Zhang *
a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
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Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China
Fax: (86)-21-64166128; e-mail: guozhuzhang@sioc.ac.cn
+
These authors contribute equally.
Received: August 30, 2016; Revised: December 21, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600962
[
4]
dium catalysed coupling reactions and Grignard
Abstract: The asymmetric dearomatization and
addition reaction of bromomethylnaphthalenes
with aldehydes proceeded smoothly in the presence
[
5]
reaction with aldehyde. To the best of our knowl-
edge, the asymmetric version remains an unmet
challenge. Undoubtedly, new strategy to address this
issue would be synthetically useful and mechanistically
interesting, not only opening new possibilities for this
stock chemicals but also offering a facile and potent
entry to enantioenriched cyclohexane and derivati-
of carbazole-based bisoxazoline CrCl complex to
2
give the corresponding enanioenriched hydroxy-
lated dearomatization products. The excellent che-
mo-, regio-, diastereo- and enantioselecitivity are
remarkable. Furthermore, hydrogenation of the
product led to highly elaborated compound con-
taining three contiguous stereogenic centers.
[
6a]
ves.
Keywords: symmetric; Dearomatization; Chromi-
um catalysis; Naphthalene; Aldehyde
Scheme 1. Reactions of Halomethyl Arenes.
Development of efficient methods for the synthesis of
optically pure alicyclic compounds constitutes one of
the most active and important topics in synthetic
organic chemistry due to the wide occurrence and
We recently reported the first asymmetric dearo-
broad applicability of chiral aliphatic carbocycle matization addition reactions of halomethyl heteroar-
molecules. Among the methods established, the dear- enes with broad range of aldehydes under the
omatization reactions of arenes has become a straight- chromium-catalyzed conditions leading to highly func-
[
6a]
forward and powerful tool since the aromatic com- tionalized heterocycles. In line with our continuing
[
1]
[6]
pounds are stable and widely available. Over the interest on asymmetric chromium catalysis, herein,
past several years, Catalytic Asymmetric DeAromati- we wish to report our preliminary results on the
zation (CADA) reaction has developed into a reliable successful expansion of this novel strategy to bromo-
synthetic method and quite a few aromatic systems methylnaphthalenes. A series of optically pure highly
underwent dearomatization reactions in a highly functionalized dearomatization products can be read-
[
2,3]
diatereoselective and enantioselective manner.
sev- ily accessed under chromium-catalyzed reaction con-
eral elegant asymmetric dearomatization of naphtha- ditions (Scheme 2).
lenes have been reported to construct a variety of
At the outset of this study, commercially available
benzocyclohexane derivatives which occurs widely as 1-(bromomethyl)naphthalene was selected as model
an important structural core in complex natural substrate to test this idea. We first investigated the
products and pharmaceuticals agents (Scheme 1). reaction of 1-(bromomethyl)naphthalene and dihydro-
[
3d,i,j,n]
Halomethyl naphthalenes are one class of cinnamaldehyde under standard NHK conditions
important building blocks leading to alkylated prod- without ligand (i.e., a substoichiometric amount of
ucts. Dearomatization-type reaction of halomethyl- CrCl , stoichiometric amount of Mn as reductant,
2
[
7]
naphthalenes have been scarcely investigated in palla- TMSCl as dissociating reagent). To our delight, a
Adv. Synth. Catal. 2017, 359, 1–6
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and entry 5).less-hindered L4 with the ethyl substitu-
tion was less effective, giving 2a in 39% yield with
30% ee (Table 1, entry 6; Figure 1).
Scheme 2. CADA Addition of Bromomethyl Naphthalene
with Aldehydes.
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small amount of desired dearomatized coupling com-
pound 2a did form with only one diastereomer was
1
Figure 1. Tested ligands.
detected by H NMR. In addition, no benzylic adduct
was observed (Table 1, entry 1). TMSCl was changed
to well-behaved ZrCp Cl , a much improved yield of
2
2
[
8]
[10]
2a was obtained (Table 1, entry 2). Encouraged by
For more information, ligand L5 by Guiry was
this result, we tested asymmetric catalysis of the tested, providing 2a in 40% yield with 40% ee
[
6,9]
[11]
coupling reaction using modified Nakada ligand L1.
(Table 1, entry 7).
Using L1 as ligand, a quite few
Delightfully, the coupling reaction proceeded even solvents were then examined, DME gave the best
better (40% yield), and the ee of 2a was determined result, affording 2a in 45% yield with 89% ee
to be 60% by chiral HPLC analysis; again, only one (Table 1, entry 9). Lowering the temperature to
diastereomer was observed (Table 1, entry 3). The À108C further improved the reaction, the product 2a
following ligands screening showed that Ligand L2 was obtained in 46% yield with 97% ee (Table 1, entry
and L3 with sterically demanding Ph group and iBu 11). Notably, the reaction scale could be increased to
group were not effective for this reaction; 2a was 0.5 mmol with maintenance of the efficiency, giving 2a
obtained with marginal ee (<10%) (Table 1, entry 4 in 97% ee (Table 1, entry 13). The moderate yield was
due to the decomposition of the unstable dearomized
product during the work-up which is in accord with
[
5]
the observations in a literature precedent.
Table 1. Optimization of the Reaction Conditions.
Relying on a single crystal X-ray analysis in
previous dearomatization of 2-chloromethyl benzofur-
[
6a]
an study and a proposed working transition model
(Figure 2), the configuration of the product 2a (>97%
ee) was tentatively assigned to be (S, R) (Figure 2).
[
a]
[b]
[c]
Entry
Ligand Solvent Temp Yield [%] ee [%]
[
d]
1
2
3
4
5
6
7
8
9
–
–
THF
THF
THF
THF
THF
THF
THF
rt
rt
rt
rt
rt
rt
rt
5%
20
40
30
25
39
40
<10%
45
46
–
–
L1
L2
L3
L4
L5
L1
L1
L1
L1
L1
L1
60
10<
10<
30
40
–
89
93
97
-
Figure 2. Proposed transition state.
CH CN rt
3
DME
DME
DME
DME
DME
rt
08C
À108C 46
À208C 14
À108C 47
10
11
12
With the optimized reaction conditions identified,
the coupling of various aldehydes with 1-(bromometh-
yl)naphthalene was explored (Scheme 3). Linear ali-
phatic aldehyde, heptanal participated in coupling
reaction efficiently, the corresponding dearomative
products 2b was isolated in good yield with excellent
enantiomeric excess 91% ee.
A terminal chloro group was well tolerated under
the standard conditions, as 5-chloropentanal reacted
well, giving the desired 2c in 51% yield with 95% ee.
Aldehyde 1d bearing a terminal double bond was
[
e]
1
3
97
[a]
The reactions were carried out at 0.2 mmol scale, ZrCp Cl
2
2
was employed unless noted otherwise.
isolated yield.
Determined by chiral HPLC analysis.
[
[
[
[
b]
c]
d]
e]
TMSCl instead of ZrCp
Cl
2
was employed.
2
0.5 mmol of bromide was used. TMS=trimethylsilane;
Cp=cyclopentadienyl; PS=proton sponge (N1, N1, N8,
N8 – tetramethylnaphthalene – 1, 8-diamine).
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The effect of naphthalene substituents was exam-
ined next. To our delight, naphthalene ring of 1-
(bromomethyl)naphthalene could be freely functional-
ized (Scheme 5), rendering this methodology with
added synthetic value. A weak electron donating
methyl group could be introduced at the 4 position of
1-(bromomethyl)naphthalene, an equal level of reac-
tion efficiency in terms of product yield and enantio-
selectivity (96% ee) was observed.
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Scheme 3. Substrate Scope Studies.
Scheme 5. Substrate Scope Studies.
suitable substrate for this reaction, product 2d was
isolated in 30% yield with 98% ee. A range of
hydroxyl protecting groups such as TBDPS, TBS and
A strong electron donating ethoxy group at the 4
benzyl were examined, providing dearomatized prod- position didn’t significantly impact the dearomatiza-
ucts 2e–2g in high enantiomeric excess (87%–94% tion addition process on yield and stereoselectivity, in
ee). The reaction of naturally occurring aldehyde this case, a slightly dropped enantioselectivity 90% ee
racemic citronellal proceeded well, affording product but better isolation yield was identified. Interestingly,
2h as a mixture of two inseparable diastereomers in a pharmaceutically useful fluorine functionality could
55:45 ratio and over 96% ee for each diastereomer. A be installed at the same position, product 2l was
sensitive ketal moiety was compatible under current obtained in moderate diastereoselectivity (90/10 dr)
reaction conditions, as 2i was obtained in 28% yield as and good enantioselectivity (93% ee). Furthermore, a
a mixture of two diastereomers at the ratio of 62:38 bromo substitution was allowed which provides a
with both highly enantioenriched (>97% ee each). platform for facile derivatization through traditional
Aryl aldehydes and a, b-unsaturated aldehydes were transition-metal-catalyzed cross coupling reactions.
also examined, after numerous trials, unfortunately,
This dearomatization addition protocol could be
we could not obtain enough amount of products for extended to 2-(bromomethyl)-naphthalene and deriv-
characterization due to the quick decomposition of atives (Scheme 6). The reaction of 2-(bromomethyl)-
the dearomatized products under weak acidic con- naphthalene with dihydrocinnamaldehyde proceeded
ditions. Interestingly, in the absence of the chiral smoothly to give the 2n in useful level of yield with
ligand, coupling of 1-(bromomethyl)naphthalene and slightly decreased diastereoselectivity and good enan-
benzaldehyde did proceed to provide the racemic 3 as tioselectivity. To our delight, a methoxy group could
a single diastereomer (Scheme 4).
be introduced at the 3 position to give the highly
functionalized product 2p in good yield with excellent
dr and ee. In addition, a triflate group could be
allowed at the 6-position of naphthalene which serves
as a surrogate to halide for further functionalization
transformations.
The highly functionalized products could be further
elaborated. As shown in Scheme 7, compound 2a
could undergo a three-step sequence procedure, TMS
protection, global hydrogenation and desilylation to
afford product 4 which contains three contiguous
Scheme 4. Substrate Scope Studies.
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Naphthalene (0.05 mmol). Then aldehyde (1.0 mmol) was
added. The resulting suspension was left stirred at À108C for
4 hrs. After the full consumption of Bromomethyl
2
Naphthalene, the reaction mixture was diluted with solvent
of ethyl acetate (10 mL) and Et N (0.2 mL). Then resulting
3
suspension was filtered over a pad of silica gel using
cosolvent (Hexane: EA= 2:1) as eluent. Volatiles were
evaporated in vacuo. The residue was purified by Et N
3
basified chromatography to afford the final product.
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Acknowledgements
We are grateful to NSFC-21421091, XDB20000000, the
“Thousand Plan” Youth program, State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences.
Scheme 6. Substrate Scope Studies.
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was realized under chromium-catalysed conditions,
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Typical procedure for the dearomatization of bromomethyl
naphthalene.
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0
9
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Soc. 2001, 123, 759–760. b) B. Peng, S. Zhang, X. Yu, X.
Feng, M. Bao, Org. Lett. 2011, 13, 5402 and references
therein. c) S. Zhang, X. Yu, X. Feng, Y. Yamamoto, M.
Bao, Chem. Commun., 2015, 51, 3842
[5] For stoichiometric dearomatization addition reaction of
(naphthalen-1-ylmethyl)magnesium bromide with alde-
hyde, see: C. Bernardon, A. Deberly, J. Org. Chem.
(
oxazolinylmethylidene)isoindoline system: i) Q.-H.
Deng, H. Wadepohl, L. H. Gade, Chem.-Eur. J. 2011,
7, 14922
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1
982, 47,463.
6] a) Q. Tian, J. Bai, B. Chen, G. Zhang, Org. Lett. 2016,
8, 1828; b) W. Chen, Q. Yang, T. Zhou, Q. Tian, G.
[
12] a) R. Opatrilova, J. Jampilet, I, Raich, S. Kacerova, J.
Havlicek, T. Pekarek, J. Dohnal, J. Csollei, Current
Organic Chemistry, 2009, 13, 965. b) E. V. Johnston, K.
Bogar, J.-E. Baeckvall, J. Org. Chem. 2010, 75, 4596;
and references therein.
[
1
Zhang, Org. Lett. 2015, 17, 5236–5239; c) Q. Tian, G.
Zhang, Synthesis 2016, 48, DOI: 10.1055/s-0036-
1
589457.
Adv. Synth. Catal. 2017, 359, 1–6
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Chromium-Catalysed Asymmetric Dearomatization
Addition Reactions of Bromomethylnaphthalenes
Adv. Synth. Catal. 2017, 359, 1–6
W. Chen, J. Bai, G. Zhang*
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