O-Transfer-facilitated cyclizations of propargylamides with TMSN3: Selective synthesis of tetrazoles and dihydroimidazoles

Article abstract of DOI:10.1039/c5cc05715a

An unprecedented formal [3+2] annulation of propargylamides with TMSN₃ to deliver functionalized tetrazoles is developed. Oxygen-atom transfer (OAT) from the amide group to the CC bond was realized via a NIS-triggered-cyclization/ring-opening cascade pathway. The OAT process enables the amide to serve as a two-atom unit in the reactions. Notably, in situ umpolung of azide occurred when terminal propargylamides were employed in this reaction, providing an array of diiodomethylated dihydroimidazoles.

Full text of DOI:10.1039/c5cc05715a

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O-Transfer-facilitated Cyclizations of Propargylamides with TMSN3:
Selective Synthesis of Tetrazoles and Dihydroimidazoles†
Yancheng Hu, Ruxia Yi, Xinzhang Yu, Xiaoyi Xin, Chunxiang Wang,* and Boshun Wan*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An unprecedented formal [3+2] annulation of propargylamides internal N-Ts propargylamides furnished various tetrazoles 2,
with TMSN3 to deliver functionalized tetrazoles is developed. whereas that of terminal substrates gave dihydroimidazoles 4
Oxygen-atom transfer (OAT) from amide group to C≡C bond was (Scheme 1d). Intramolecular OAT from the amide groups to carbon-
realized via
a NIS-triggered-cyclization/ring-opening cascade carbon triple bonds was observed in both cyclizations. The OAT
pathway. The OAT process enables the amide to serve as a two- process enables the amide to serve as a two-atom unit in the
atom unit in the reactions. Notably, in situ umpolung of azide reactions. Thus, we have realized the O-transfer-facilitated
occurred when terminal propargylamides were employed in this cyclizations of propargylamides with TMSN3 to selectively construct
reaction,
providing
an
array
of
diiodomethylated functionalized
tetrazoles[6,7]
and
diiodomethylated
dihydroimidazoles.
dihydroimidazoles[8] for the first time. Herein, we report the
preliminary results of our investigations.
O
Cyclization of alkynes containing adjacent nucleophilic centers
has emerged as an exceptionally mild and practical approach for the
synthesis of functionalized carbo- and heterocycles.[1] Among them,
cyclizations of ketone-, amine N-oxide-, nitro-, sulfoxide-, and
nitrone-alkynes involving an intramolecular oxygen transfer process
have been investigated.[2,3] For example, alkyne-carbonyl
metathesis of ketone-alkynes to produce oxygen transferred
carbocycles through a [2+2] pathway has been reported (Scheme
1a).[2] Transition-metal-catalyzed oxygen transfer reactions of
alkynes possessing the polar Z+-O- groups have been proposed to
generate reactive α-oxo carbenoids, allowing the generation of
various carbonyl compounds (Scheme 1b).[3] However, despite
these achievements, oxygen-atom transfer (OAT) from amide into
alkynes has never been reported before.
R
R
a)
b)
H+or [Au]
R
O
R'
O
heteroenyne
metathesis
R'
R'
R
R
M
R
[M]
M
O
O
M = Au, Rh...
Z
O
+
Z
Z
R'
R'
O-transfer
facilitated
cyclizations
R'
c)
X
X
O
O
X
O
our design
N
R
N
N
R
LG
LG
R
X = I+ or transition metals; LG = Leaving group
d)
R2
R2
I
OH
N
Recently, propargylamides have been well studied as versatile
precursors for the synthesis of 5- or 6-membered oxa-
heterocycles.[4,5] As our continued efforts to explore new reaction
patterns of N-tosyl propargylamides,[5] we envisaged that whether
oxygen-atom transfer could facilitate the cyclizations of
propargylamides with nucleophilic coupling partners to deliver
some unexpected aza-heterocycles (Scheme 1c). Indeed, by
employing TMSN3 as the coupling partner, the iodocyclization of
O
N
NIS/TMSN3
R2 = H
NIS/TMSN3
2 ≠ H
N
I
I
N
O
N
N
R3
R1
R
R1
N
R3
R1
Ts
R3
Ts
4
2
♦ O-transfer-facilitated cyclizations ♦ Amide as a two-atom unit
♦ Substrate-controlled selectivity
♦ Construction of diiodomethyl group
Scheme 1. An overview of oxygen transfer reactions of functionalized alkynes
Our initial studies were carried out using internal
propargylamide 1a as substrate, NIS as iodine source, and TMSN3 as
coupling partner. When the reaction was performed in DCE at 60 °C,
1,5-disubstituted tetrazole 2a was obtained. The structure of 2a
was unambiguously confirmed by X-ray crystallography.[9] It is
noteworthy that the most known synthesis of tetrazoles involves
the cycloaddition reactions between azides and nitriles.[7]
Nonetheless, our method provides an appealing alternative for the
a.Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan
Road, Dalian 116023, China. E-mail: bswan@dicp.ac.cn; cxwang@dicp.ac.cn; Fax:
+86-411-84379223; Tel: +86-411-84379260
†
Electronic Supplementary Information (ESI) available: General experimental
procedures, characterization data, 1H and 13C NMR spectra and X-ray
crystallographic analysis of compounds 2a (CCDC 1026405) and 4a (CCDC
1026409). For ESI and crystallographic data in CIF or other electronic format, see
DOI: 10.1039/x0xx00000x
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synthesis of functionalized tetrazoles from amides and azides,[10] methyl and cyclopentyl substituents in R3 group (3h–3j) were all
which prompted us to screen the reaction conditions for this well tolerated with the reaction. Unexpectedly, the R1 group
process (see the Supporting Information). Gratifyingly, the reaction significantly influences the reactivity. When phenyl group was
of 1a with NIS (2.4 equiv), TMSN3 (3.6 equiv) and H2O (1.2 equiv) in employed as R1, dihydroimidazole 4k was only obtained in 15%
DCE at 80 °C delivered tetrazole 2a in 81% yield (Scheme 2). The yield. Gratifyingly, the protocol can be further extended to oxygen-
scope of this transformation was further examined (Scheme 2). linked propargylic carboxylates bearing various aromatic moieties
Propargylic amides possessing a broad variety of aromatic motifs in (3l–3n), furnishing the products in moderate yields. It is noted that
R1, R2 and R3 groups underwent this reaction smoothly, providing neither NBS nor NCS could promote the cyclization of 3a.
the corresponding tetrazoles (2b–2k) in good yields. The electronic
effects and the positions of the substituents on the phenyl ring had
less influence on the results. However, when aliphatic moiety was
employed as R3 (e.g. R3 = Me) or R1 (e.g. R1 = iPr) group, no reactions
occurred. Butyl substituent in R2 group was also tolerated, albeit
with lower yield (2l). Moreover, NBS could also trigger this
cyclization(2m), whereas NCS gave no desired product.
Scheme 3. Substrate scope. Reaction conditions: 3 (0.20 mmol), NIS (0.48 mmol),
TMSN3 (0.72 mmol), H2O (0.40 mmol), DCE (3.0 mL), 60 °C, 8–18 h; isolated yields
are given. [a] 1,4-dioxane (3.0 mL) was used as solvent.
Scheme 2. Direct transformation of internal propargylic amides into tetrazoles.
Reaction conditions: 1 (0.20 mmol), NIS (0.48 mmol), TMSN3 (0.72 mmol), H2O
(0.24 mmol), DCE (3.0 mL), at RT for 10 min, then 80 °C for 2–8 h; isolated yields
are given.
Having established the transformations of internal
propargylamides, we wondered whether the chemo- and
regioselectivity of the reactions can be tuned by changing the
substituents on the amides. To our delight, when terminal
propargylamide 3a was subjected to the same conditions,
diiodomethylated dihydroimidazole 4a was obtained.[9] Since
Scheme 4. Synthesis of heterocyclic carbaldehydes. Reaction conditions: 4 (0.20
mmol), Martin’s sulfurane dehydrating reagent (0.40 mmol), CHCl3 (2.0 mL), at
room temperature for 12–18 h; isolated yields are given.
dihalogen-derived compounds have been shown to possess useful
biological activity,[11] this reaction might be applicable to prepare
analogous diiodomethylated heterocycles.[12] After careful
To illustrate the synthetic potential of the products, additional
screening of the reaction conditions (see the Supporting
investigations were performed on their further transformations.
Information), we observed that the desired product 4a could be
Pleasingly, we found that the resulting dihydroimidazo-4-ols and
obtained in 80% yield at 60 °C (Scheme 3). Variations on the acyl
dihydrooxazo-4-ols could be efficiently converted into the
moiety (R3) were first examined. Both electron-withdrawing and
corresponding carbaldehydes in the presence of Martin’s sulfurane
dehydrating agent in CHCl3 (Scheme 4).[13] The elimination of H2O
electron-donating substituents (R) on the phenyl ring were
compatible with this transformation, giving the corresponding
leads to diiodomethylimidazoles or diiodomethyloxazoles, followed
dihydroimidazo-4-ols 4b–4g in moderate to good yields. It is
by nucleophilic substitution and elimination of HI to furnish the final
noteworthy that when p-OCH3 substituted propargylamide 3g was
products in 55–89% yields. This general protocol can be readily
submitted to the standard conditions, no desired product was
applied for the construction of diversified heterocyclic
formed. Pleasingly, switching the solvent to 1,4-dioxane could
carbaldehydes under mild conditions.
provide dihydroimidazole 4g in 38% yield. It turns out that 1,4-
To probe the reaction mechanism, isotopic labelling
experiments were first conducted. When 18O was incorporated into
dioxane is much more effective than DCE for the cyclization of
heteroaryl- and alkyl-derived substrates. For example, 2-furyl,
the carbonyl group of propargylamide 1a [Eq. (1)] or 3a [Eq. (2)]
2 | J. Name., 2012, 00, 1-3
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respectively, 18O-labeled products were observed for both intramolecular cyclization affords tetrazole 2. In contrast, a 5-exo-
cyclizations. Besides, when H218O was employed in the reactions, dig cyclization of terminal substrate 3 occurs, resulting in the
only normal products 2a and 4a were obtained (see the Supporting formation of intermediate H. The activation of exo-cyclic double
Information). These results indicate that intramolecular OAT from bond by I+ and intramolecular OAT lead to the ring opening product
amide group to carbon-carbon triple bond occurs in the reaction J. The in situ formed azidocarbenium ion[14] is likely to be
process. The deuterated substrate 3a-d underwent this reaction surrounded by azide anion through a contact ion pair. Subsequently,
efficiently to afford the desired product 4a-d without any loss of an excess of azide anion acts as another molecule of nucleophile to
deuterium (see the Supporting Information), which implies that the promote the cyclization and furnish intermediate K. Ultimately, the
terminal C–H bond of alkyne is not cleaved in the reaction and the succinimide anion traps the azide group of K (when X = N3),
proton of hydroxyl group originates exclusively from H2O [Eq. (3)].
releasing N-azide succinimide by an umpolung step, followed by
protonation to generate the desired product 4. However, attempts
to isolate or detect the byproduct N-azide succinimide did not
succeed, which was presumably due to its lability in the reactions.
Fortunately, the molecular weight of N-phenylsulfonyl succinimide
(239.0) was detected by LC-MS when PhSO2Na was employed in the
reaction [Eq. (4), entry D], thus providing an indirect evidence to
support the proposed mechanism.
Notably, only one equivalent of azide or nitrogen atom is
ultimately incorporated into the product, while much excess
amount of TMSN3 (3.6 equiv) is required to promote the reactions.
This phenomenon indicates that the role of TMSN3 is not only
limited to be a nucleophile (or coupling partner), but might be a
counteranion.[14] Meanwhile, in situ umpolung of azide[15] may also
occur in the reaction. To confirm this hypothesis, control
experiments were then carried out [Eq. (4), entries A–F]. Indeed,
the reaction of 3a with 1 equivalent of TMSN3 only gave trace
amount of 4a [Eq. (4), entry A]. In contrast, with the assistance of
-
other anions (e.g. I-, Br- and PhSO2 ), non-negligible amount of
product could be produced [Eq. (4), entries B–D]. It is noteworthy
that the using of NaN3 as additive could also promote this reaction
[Eq. (4), entry E], albeit with much lower yield than its TMSN3
analogue [Eq. (4), entry F].
On the basis of these preliminary mechanistic studies,
a
plausible mechanism is proposed (Scheme 5). In the case of internal
substrate 1, an attack of the carbonyl oxygen atom onto the I+-
activated carbon-carbon triple bond via a 6-endo-dig cyclization
mode produces intermediate B. The iminium ion of B can be
trapped by an azide anion formed in situ from TMSN3 and H2O,
followed by the elimination of 4-methylbenzenesulfinic acid[5a] and
retroelectrocyclization to give O-transfer intermediate E. Finally, an
Scheme 5. Proposed mechanism
In conclusion, we have developed the O-transfer-
facilitated cyclizations of propargylamides with TMSN3 to
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selectively afford
Journal Name
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Rominger, W. Frey and J. W. Bats, Chem.–Eur. J., 2010, 16, 956; o) A. S. K.
Hashmi, A. M. Schuster, M. Schmuck and F. Rominger, Eur. J. Org. Chem.,
functionalized
tetrazoles
and
diiodomethylated dihydroimidazoles. OAT from amide group
to C≡C bond was realized via a NIS-triggered-cyclization/ring-
opening cascade pathway. This process enables the amide to
serve as a two-atom unit in the cyclizations. The resulting
products can be further transformed into the corresponding
heterocyclic carbaldehydes. We expect our findings can be
applicable to develop new reaction patterns of amides.
This work was supported by the NSFC (21172218 and
21572225). We also thank one reviewer for helpful suggestions
on revising the manuscript.
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