Manganese-catalyzed oxidative azidation of cyclobutanols: Regiospecific synthesis of alkyl azides by C-C bond cleavage

Article abstract of DOI:10.1002/anie.201506578

A novel, manganese-catalyzed oxidative azidation of cyclobutanols is described. A wide range of primary, secondary, and tertiary alkyl azides were generated in synthetically useful yields and exclusive regioselectivity. Aside from linear alkyl azides, oth

Full text of DOI:10.1002/anie.201506578

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International Edition: DOI: 10.1002/anie.201506578
German Edition: DOI: 10.1002/ange.201506578
À
C C Activation
Manganese-Catalyzed Oxidative Azidation of Cyclobutanols:
À
Regiospecific Synthesis of Alkyl Azides by C C Bond Cleavage
Rongguo Ren, Huijun Zhao, Leitao Huan, and Chen Zhu*
Abstract: A novel, manganese-catalyzed oxidative azidation
of cyclobutanols is described. A wide range of primary,
secondary, and tertiary alkyl azides were generated in syntheti-
cally useful yields and exclusive regioselectivity. Aside from
linear alkyl azides, otherwise elusive medium-sized cyclic
azides were also readily prepared. Preliminary mechanistic
studies reveal that the reaction likely proceeds by a radical-
À
À
mediated C C bond cleavage/C N3 bond formation pathway.
O
rganoazides, in particular alkyl azides, are widely used as
versatile synthetic intermediates to construct nitrogen-con-
taining molecules owing to their unique reactivity.[1] The
synthetic utility of alkyl azides has been explicitly demon-
strated by their employment in many powerful organic and
bioorthogonal transformations.[2–5] Moreover, the incorpora-
tion of an azide moiety into a molecule usually leads to
a remarkable improvement in the biological activities.[6]
Therefore, the development of mild and efficient azidation
methods is of great significance for multiple fields of
chemistry, medicine, biology, and materials sciences.
Scheme 1. Transition-metal-catalyzed synthesis of alkyl azides by cleav-
ing inert chemical bonds. TMS=trimethylsilyl.
The most common route to alkyl azides relies on the
nucleophilic substitution of alkyl halides by inorganic
azides.[7] However, the halide precursors are sometimes
difficult to obtain. During the past decade, widespread
interest in the azidation of olefins has provided another
À
butanols to efficiently generate alkyl azides by C C bond
cleavage. A wide range of g-carbonyl-containing primary,
secondary, and tertiary alkyl azides were readily furnished in
synthetically useful yields (Scheme 1B).
solution for the formation of alkyl azides.[8] Alkyl C H and
À
À
C C bonds are inert but abundant in organic compounds;
À
À
therefore, the direct elaboration of C H and C C bonds into
target functional groups represents the ideal and most
straightforward way for the introduction of functional
groups. Very recently, the groups of Hartwig[9a] and Groves[9b]
Cyclopropanols and cyclobutanols can be regarded as
readily available precursors for the synthesis of b- and
g-substituted ketones by radical-clock strategies.[11] Reactions
of cyclopropanols that proceed by single-electron oxidation
can be easily realized owing to the high reactivity that is
induced by the notable ring strain of the three-membered
ring, and have attracted much interest.[12] In contrast, radical-
mediated transformations of cyclobutanols have rarely been
reported,[13,14] even though cyclobutane has a strain energy
(26.3 kcalmolÀ1) similar to that of cyclopropane (29.0 kcal
molÀ1).[15] This surprising finding could be rationalized by the
fact that the ring strain of gem-disubstituted cyclobutanes is
significantly lower owing to the Thorpe–Ingold effect, which
thus stabilizes cyclobutanols.[16] On the other hand, in situ
formed g-benzoylpropyl radicals, as the open-chain tautomers
of the cyclobutoxy radicals, rapidly undergo intramolecular
cyclization to generate 1-tetralones rather than intermolecu-
lar trapping processes with extrinsic radical scavengers.[17]
Therefore, the efficient capture of open-chain alkyl radicals
remains to be a significant challenge.
À
independently developed elegant C H bond azidation pro-
cesses by means of iron and manganese catalysis to produce
alkyl azides in modest chemical yields (Scheme 1A).[9,10]
Although these azidation processes are robust, they primarily
occurred at tertiary and benzylic carbon atoms. We herein
disclose a novel, manganese-catalyzed azidation of cyclo-
[*] R. Ren, H. Zhao, L. Huan, Prof. Dr. C. Zhu
Key Laboratory of Organic Synthesis of Jiangsu Province, College of
Chemistry, Chemical Engineering and Materials Science
Soochow University
199 Ren-Ai Road, Suzhou, Jiangsu 215123 (China)
E-mail: chzhu@suda.edu.cn
Prof. Dr. C. Zhu
Key Laboratory of Synthesis Chemistry of Natural Substances
Shanghai Institute of Organic Chemistry
Chinese Academy of Science
In light of our recent success with the silver-catalyzed
synthesis of g-fluorinated ketones,[18] we wondered whether
a similar ring-opening strategy could be applied to the
345 Lingling Road, Shanghai 200032 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506578.
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Table 1: Variation of oxidant and ligand.[a]
Entry
Oxidant
Ligand
Yield[b] [%]
1
2
3
H2O2
TBHP
K2S2O8
O2
–
–
–
–
0
<5
<5
16
4
5
6
7
PIDA
PhIO
IBX
–
–
–
32
30
66
8
9
10
11
BI-OH
BI-OH
BI-OH
IBX
–
61
82
85
37
phen
bipy
bipy
[a] 1a (0.20 mmol), Mn(OAc)3·2H2O (0.04 mmol, 20 mol%), ligand
(0.044 mmol, 22 mol% when added), TMSN3 (0.40 mmol, 2.0 equiv),
and oxidant (0.40 mmol, 2.0 equiv) in CH3CN (1.5 mL) at room
temperature for 5 min, then 708C for 8 h. [b] Yields of isolated products.
TBHP=tert-butyl hydroperoxide.
construction of alkyl azides. However, to our disappointment,
all attempts with silver catalysts gave rise to the undesired
1-tetralone.[17] These negative results prompted us to re-
investigate various transition-metal catalysts. After consider-
able efforts, we found that the use of Mn(OAc)3 enabled the
conversion of cyclobutanol 1a into the desired azide 2a. The
nature of the oxidant was crucial to the reaction outcome
(Table 1). Whereas commonly used oxidants, such as H2O2,
TBHP, K2S2O8, and O2, gave poor chemical yields (entries 1–
4), hypervalent iodine reagents were promising candidates to
promote the redox cycle (entries 5–8). The lower yields
obtained with PIDA and PhIO could be attributed to the
consumption of TMSN3 through the generation of highly
unstable acyclic azido hypoiodite, which rapidly decomposed
at room temperature.[19] The use of IBX and BI-OH gave
comparable, modest yields (entries 7 and 8). Remarkably, the
addition of N,N-bidentate ligands, such as phen and bipy,
significantly improved the yield; the latter ligand performed
slightly better (entries 9 and 10). The incompatibility of IBX
and bipy was probably caused by oxidation processes
(entry 11).
With the optimized reaction conditions in hand, we set out
to evaluate the generality of the method. The reaction
displayed a superb functional-group tolerance; a wide array
of substituted tertiary cyclobutanols were readily converted
into the corresponding alkyl azides (Scheme 2). Electron-rich
substrates consistently afforded the corresponding products
in good yields regardless of their steric properties (2a–2g).
Various halides were compatible with the reaction conditions
(2h–2j), providing a platform for later manipulations. Sub-
Scheme 2. Ring-opening strategy for the synthesis of alkyl azides.
Reaction conditions: 1 (0.20 mmol), Mn(OAc)3·2H2O (0.04 mmol,
20 mol%), bipy (0.044 mmol, 22 mol%), TMSN3 (0.40 mmol,
2.0 equiv), and BI-OH (0.40 mmol, 2.0 equiv) in CH3CN (1.5 mL),
708C. Yields of isolated products are given. [a] TMSN3 (3.0 equiv) and
BI-OH (3.0 equiv). [b] At room temperature.
strates with strongly electron-withdrawing groups, such as CF3
and CF3O, were also readily converted into the desired alkyl
azides (2k, 2l). Remarkably, heteroaryl and alkyl cyclo-
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butanols were suitable substrates to deliver 2p–2r in syntheti-
cally useful yields. Challenging substrates such as 2s and 2t,
which may experience appreciable Thorpe–Ingold effects
owing to the multisubstituted cyclobutyl ring, furnished the
products in moderate yields. The reaction of 2u was interest-
ing as the azidation regioselectively occurred at the non-
chlorinated carbon atom. Apart from primary azides, secon-
dary and tertiary alkyl azides were also efficiently generated
under these reaction conditions. With cyclobutanols bearing
either alkyl or aryl substituents adjacent to the hydroxy group
(1v–1x), the azide was regioselectively introduced to the
methine position, affording secondary azides as the sole
products. Likewise, the azidation only took place on the ring
to afford cyclic azides when bicyclic substrates were employed
(1y–1aa). Starting from the highly substituted and sterically
crowded substrate 1ab, tertiary azide 2ab was produced in
useful yield. Secondary cyclobutanols were not compatible
with the reaction conditions as they were oxidized into the
corresponding cyclobutanones.
of diastereomers, whereas the formation of single diastereo-
mers would be expected if the reaction proceeded through an
ionic pathway. Then, the electrophilic azidation agent BI-N3
was used instead of BI-OH and TMSN3 under otherwise
identical conditions (Scheme 4). The azidation of 1a did not
take place, explicitly illustrating a) that BI-N3 was not
generated as the actual azidation reagent in the reaction
mixture, and b) that the ionic pathway could be ruled out.
Scheme 4. Control experiment: employing BI-N3 instead of BI-OH and
TMSN3.
Based on these experimental observations, a reaction
Medium-sized all-carbon rings are important and ubiq-
uitous in organic compounds, but usually difficult to obtain. A
series of benzocyclic azides (4a–4h) with seven- to ten-
membered rings were furnished in good yields by making use
of the manganese-catalyzed ring-expansion strategy
(Scheme 3). Remarkably, the transformations proceeded in
the absence of bipy ligand even at room temperature. The
complexity of the products could be easily increased by
installing substituents on either the arene or the cycloalkanol
moiety (4c–4 f).
To gain further insights into the azidation reaction, several
mechanistic experiments were performed. First, under the
standard reaction conditions, TEMPO or BHT was added as
a radical probe. The resulting suppression of the formation of
2a (< 10% yield) might suggest a radical-mediated pathway.
This possibility was corroborated by the reactions of 1v, 1x–
1z, and 1aa, which yielded the azidation products as mixtures
mechanism was postulated (Figure 1). Initially, Mn(OAc)3,
V
À
TMSN3, and BI-OH generate the high-valent Mn N3 species
A.[20] The reaction of A with cyclobutanol 1 leads to the
formation of complex B, which undergoes simultaneous SET
to generate MnIV N3 species C and cyclobutyloxy radical D.
À
The open-chain tautomer of D, alkyl radical E, is then
intercepted by the azidation reagent C, eventually leading to
alkyl azide 2. It is likely that once formed, the intermediates
C, D, and E are confined within a solvent cage, which
promotes the azidation. Otherwise, the free alkyl radical E
would instantly cyclize to generate 1-tetralone.[17]
In summary, we have described a novel and efficient
method for the manganese-catalyzed synthesis of alkyl azides
À
by the C C bond cleavage of cyclobutanols that tolerated
a broad range of functional groups and proceeded with
unique regioselectivity. A wide range of primary, secondary,
and tertiary alkyl azides were generated in synthetically
useful yields. Moreover, a series of medium-sized cyclic azides
were also obtained in good yields. Preliminary mechanistic
studies indicate the involvement of a radical pathway. Further
mechanistic studies and investigations towards the applica-
tion of the manganese-catalyzed cyclobutanol ring-opening
Scheme 3. Ring-expansion strategy for the synthesis of medium-sized
cyclic azides. Reaction conditions: 3 (0.20 mmol), Mn(OAc)3·2H2O
(0.04 mmol, 20 mol%), TMSN3 (0.60 mmol, 3.0 equiv), and BI-OH
(0.60 mmol, 3.0 equiv) in CH3CN (1.5 mL), room temperature. Yields
of isolated products are given.
Figure 1. Proposed reaction mechanism.
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in our laboratory.
Experimental Section
Cyclobutanol 1 (0.2 mmol, 1.0 equiv), Mn(OAc)3·2H2O (0.04 mmol,
0.2 equiv), bipy (0.044 mmol, 0.22 equiv), and BI-OH (0.4 mmol,
2.0 equiv) were added to a flask, which was evacuated and flushed
with nitrogen three times. CH3CN (1.5 mL) was added to the mixture
with a syringe, and the mixture was then stirred at 258C for 5 min.
Subsequently, TMSN3 (0.4 mmol, 2.0 equiv) was added to the reaction
mixture, which was left to stir at 708C until the starting materials had
been completely consumed (determined by TLC analysis). The
reaction mixture was extracted with EtOAc (3 ” 10 mL). The
combined organic extracts were washed by brine, dried over
Na2SO4, filtered, concentrated, and purified by flash column chro-
matography on silica gel (ethyl acetate/petroleum ether) to give
product 2.
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2
À
À
Keywords: azidation · azides · C C cleavage · cyclobutanols ·
manganese
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Received: July 16, 2015
Published online: September 1, 2015
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