Gold-Catalyzed Ring Expansion of Alkynyl Heterocycles through 1,2-Migration of an Endocyclic Carbon-Heteroatom Bond

Article abstract of DOI:10.1002/chem.201504165

A mild and efficient gold-catalyzed oxidative ring-expansion of a series of alkynyl heterocycles using pyridine-N-oxide as the oxidant has been developed, which affords highly valuable six- or seven-membered heterocycles with wide functional group toleration. The reaction consists of a regioselective oxidation and a chemoselective migration of an endocyclic carbon-heteroatom bond (favored over C-H migration) with the order of migratory aptitude for carbon-heteroatom bonds being C-S>C-N>C-O. In the absence of an oxidant, polycyclic products are readily constructed through a ring-expansion/Nazarov cyclization reaction sequence.

Full text of DOI:10.1002/chem.201504165

DOI: 10.1002/chem.201504165
Communication
&
Synthetic Methods
Gold-Catalyzed Ring Expansion of Alkynyl Heterocycles through
1,2-Migration of an Endocyclic Carbon–Heteroatom Bond
Ming Chen, Ning Sun, Wei Xu, Jidong Zhao, Gaonan Wang, and Yuanhong Liu*[a]
Abstract: A mild and efficient gold-catalyzed oxidative
ring-expansion of a series of alkynyl heterocycles using
pyridine-N-oxide as the oxidant has been developed,
which affords highly valuable six- or seven-membered het-
erocycles with wide functional group toleration. The reac-
tion consists of a regioselective oxidation and a chemose-
lective migration of an endocyclic carbon–heteroatom
bond (favored over CÀH migration) with the order of mi-
gratory aptitude for carbon–heteroatom bonds being CÀ
S>CÀN>CÀO. In the absence of an oxidant, polycyclic
products are readily constructed through a ring-expan-
sion/Nazarov cyclization reaction sequence.
Scheme 1. Gold-catalyzed 1,2-heteroatom migration reactions.
Transformations that involve 1,2-heteroatom migrations have
received considerable attention in recent years, since they are
highly attractive for the rapid construction of diversely func-
tionalized structures from easily available starting materials.[1–3]
In this regard, gold-catalyzed 1,2-heteroatom migration reac-
tions have been developed with remarkable improvements
due to their relatively mild reaction conditions, high efficien-
cies, and high selectivities. These reactions usually involve
metal vinylidene species, carbocations, metal carbenes, or elec-
trophilic metal p-complexes.[2] For example, Fürstner disclosed
a 1,2-iodine migration via formation of a metal vinylidene spe-
cies,[2a,b] and Gevorgyan reported a 1,2-halogen migration via
a halirenium intermediate.[2h] Silicon,[2i–l] tin,[2i,j] germanium,[2i,j]
sulfur,[2m–o] nitrogen,[2p] and oxygen[2p] groups have been shown
by several research groups to undergo 1,2-migration through
gold–carbene intermediates. Although much progress has
been achieved, most of these transformations proceed by 1,2-
migration along an open-chain system or on a ring framework
[Scheme 1, Eq. (1)], whereas 1,2-migrations of endocyclic CÀ
heteroatom bonds that lead to ring expansion are quite rare
[Scheme 1, Eq. (2)]. To our knowledge, among metal-catalyzed
rearrangement reactions, this type of reaction has only been
reported for gold-catalyzed 1,2-thio-migration of propargyl di-
thioacetals.[2n,o] The development of new methodologies for
such transformations would be of great interest for producing
one heterocycle from another. We recently described a gold-
catalyzed oxidative ring expansion of 2-alkynyl-1,2-dihydropyri-
dine and its analogues,[4] which proceeds by exclusive 1,2-mi-
gration of a vinyl or phenyl group. Inspired by this result, we
envisioned that a 1,2-heteroatom migration from two-hetero-
atom-bearing alkynyl heterocycles would be desirable. Herein,
we report a highly regioselective gold-catalyzed oxidative ring
expansion of alkynyl heterocycles through 1,2-migration of en-
docyclic CÀS, CÀN, and CÀO bonds, with pyridine N-oxide as
the oxidant.[5] Interestingly, in the absence of N-oxide, a poly-
cyclic ring system could be readily constructed by a gold-cata-
lyzed ring-expansion/Nazarov cyclization sequence [Scheme 1,
Eq. (3)].
Initially, we focused on the possible ring-expansion reaction
of N-CO2Me-protected 2-alkynyl-2,3-dihydrobenzo[d]thiazole
1a in the presence of various gold catalysts with pyridine N-
oxide as the oxidant (Table 1). To our delight, various common-
ly used gold catalysts, such as [PPh3AuNTf2], [PPh3AuSbF6],
[PPh3AuOTf], and [IPrAuNTf2] [IPr=1,3-bis(2,6-diisopropylphe-
nyl)imidazol-2-ylidene], catalyzed the desired transformation
efficiently, furnishing 2a in high yields.[6] The results implied
that the reaction proceeds through highly selective migration
of the endocyclic CÀX bond or heteroatom group; 1,2-S migra-
tion occurred preferentially and no 1,2-H or 1,2-N migration
took place. The higher migratory aptitude of the thio group
over H and N groups might attribute to the facile formation of
the thiiranium ion intermediate. We next examined the sub-
strate scope of this oxidative ring expansion to benzothiazines,
with 2 mol% [PPh3AuNTf2] as the catalyst. 1,2-S migration
[a] M. Chen, N. Sun, W. Xu, J. Zhao, G. Wang, Prof. Y. Liu
State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences,
345 Lingling Lu, Shanghai 200032 (P. R. China)
E-mail: yhliu@sioc.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504165.
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Encouraged by the above results, we next attempted to ex-
amine the possible 1,2-N migration of 2-alkynyl-2,3-dihydro-
1H-benzo[d]imidazoles 3. 1,2-N migration reactions are quite
rare in gold catalysis. Davies and co-workers recently reported
a gold-catalyzed synthesis of indenes accompanied by 1,2-mi-
gration of an exocyclic sulfonamide group through CÀN bond
cleavage of the ynamides.[2p] As expected, the ring expansion
of 3 to 1,4-dihydroquinoxaline 4 occurred efficiently, catalyzed
by 5 mol% [PPh3AuNTf2] at 808C, indicating that 1,2-N migra-
tion indeed took place during the process (Table 2). A wide
Table 1. Gold-catalyzed synthesis of benzo[b][1,4]thiazines.[a]
Table 2. Gold-catalyzed synthesis of 1,4-dihydroquinoxalines.[a]
[a] Yields refer to isolated products. [b] 1 mol% [PPh3AuNTf2]. [c] 5 mol%
[PPh3AuNTf2], 808C. [d] 5 mol% catalyst A and 2.0 equivalents of 8-meth-
ylquinoline N-oxide.
products were obtained exclusively in all cases and isolated in
good to excellent yields (Table 1). Besides the N-CO2Me-pro-
tected substrates, N-CO2iPr- and N-CO2Bn-protected substrates
also underwent the reaction smoothly to give 2b and 2c in
92% and 77% yields, respectively. For aryl-substituted alkynes,
both electron-deficient (p-F, p-Cl, p-CO2Me, and even strongly
electron-withdrawing p-NO2) and electron-rich aryl alkynes
[3,4,5-(MeO)3] were well suited, furnishing the corresponding
products 2d–h in 93–97% yields. Substrate possessing a 2-
thienyl group was efficiently converted into 2j in 97% yield.
The method also worked well with the substrate bearing an al-
kenyl group at the alkyne terminus, and the desired 2k was
obtained in 84% yield. In addition, alkyl-substituted alkynes
(e.g., phenylethyl, cyclopropyl) were efficiently transformed
into benzothiazines 2l and 2m in 83% and 87% yields, respec-
tively, at 808C. In the case of monocyclic methyl 2-(phenylethy-
nyl)thiazole-3(2H)-carboxylate 1n, no desired ring expansion
product was formed under the standard reaction conditions.
However, the reaction proceeded smoothly when [JohnPhos-
(MeCN)AuSbF6] (catalyst A) was used as the catalyst in the
presence of 8-methylquinoline N-oxide as the oxidant, furnish-
ing 2n in 83% yield. The structures of benzothiazine products
were unambiguously confirmed by X-ray crystal analyses of 2h
and 2k.[7]
[a] Yields refer to isolated products. [b] 2 mol% [PPh3AuNTf2]. [c] Major
isomer unknown.
range of aryl-, alkenyl-, and alkyl-substituted alkynes under-
went the reaction efficiently, leading to 1,4-dihydroquinoxa-
lines 4 in good to high yields within short reaction times. For
aryl-substituted alkynes, a wide variety of functional groups on
aryl rings were well tolerated. The electronic nature of the aryl
ring had little influence on the product yields. Ferrocenyl-sub-
stituted 3i also worked for this reaction, affording 4i in 61%
yield. The reactions with alkenyl- or alkyl-substituted alkynes
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were satisfactory, producing the desired products 4j–m in 72–
76% yields. Substrate 3n, with an ynamide moiety, was also
well suited, providing 4n in 69% yield. In addition, the reac-
tion could be applied to the synthesis of 7-membered diaze-
pine 4p in 84% yield. When 5-methoxy-substituted benzo[d]i-
midazole 3q was employed, a mixture of two regioisomers 4q
and 4q’ was formed in a combined yield of 93%, indicating
that both NCO2Me groups migrated during the reaction. Grati-
fyingly, the monocyclic 2,3-dihydro-1H-imidazole 3r was suc-
cessfully converted into the 1,4-dihydropyrazine 4r in 62%
yield. It should be noted that substrate 3s, in which one of the
N-CO2Me groups was replaced with an N-methyl group, did
not react cleanly. These results indicate that the protecting
group also plays an important role in this reaction. The practi-
cality of this method was demonstrated by gram-scale synthe-
sis of 2a and 4a (see Tables 1 and 2), which were obtained in
92% and 73% yields, respectively.
Scheme 3. Transformations of ring-expanded products.
To better understand the comparative migratory aptitudes
of O and NR groups, we next synthesized 2-alkynyl-2,3-dihy-
drobenzo[d]oxazole 5a. The gold-catalyzed oxidative reaction
of 5a afforded a mixture of ring-expanded products 6a and
6b in 4:1 ratio (Scheme 2). The results indicated that both the
with a low yield of 31% in the case of 12a. The results implied
that a gold–carbene intermediate might be generated during
the process. After much effort, we found that the use of
5 mol% [IPrAu(MeCN)SbF6] under dilute reaction conditions
(0.015m) gave the best results[6] and was applicable to a wide
range of substrates. Due to the instability of 12 in most cases,
it was further converted into the double-bond-isomerized
product 13 in the presence of basic Al2O3 (Table 3).[9] This reac-
tion could not be achieved in the cases of substrates 3 and 5.
The reaction showed excellent scope with respect to aryl al-
kynes and good to high yields were obtained in all cases. Het-
eroaryl substituents, such as 2-thienyl or 3-benzothienyl, could
also be incorporated into the polycyclic products.
Scheme 2. Gold-catalyzed ring-expansion of 2-alkynyl-2,3-dihydrobenzo-
[d]oxazole.
O and the NCO2Me group migrated during the reaction pro-
cess, and that NCO2Me has higher migratory aptitude than O.
Based on the above results, we summarized that the order of
migratory aptitude was CÀS>CÀN>CÀO under the present
reaction conditions.
We propose the following reaction mechanism for these
ring-expansion reactions (Scheme 4). The reaction is initiated
by regioselective attack of the N-oxide at the b-carbon of the
gold-activated alkyne to afford vinylgold intermediate 14. The
regioselectivity may be caused by the inductive effect of the
heteroatom groups,[10] which renders the b-carbon more elec-
trophilic. Nucleophilic attack of heteroatom X on the vinylgold
The ketone-functionalized products could be transformed
into a variety of valuable building blocks (Scheme 3). For ex-
ample, 2a could be deprotected efficiently to give 7. Michael
addition of 2a or 2h with Grignard reagent followed
by treatment with base afforded trans-3,4-dihydro-
2H-benzo[b][1,4]thiazine 8a or 8b as a single diaste-
reoisomer.[8] 4a could be converted into aromatized
quinoxaline 9 by treatment with FeCl3. Interestingly,
stirring of 4a in the presence of tBuNH2 in MeOH
produced fused tricyclic heterocycle 10 in 86% yield.
10 exhibited strong fluorescence in solution, which
might be ascribed to its nearly planar structure. In
addition, hydrogenation of 4a provided 11 in 67%
yield.
Table 3. Gold-catalyzed synthesis of polycyclic ring products 13.[a]
Considering the inherent nucleophilicity of the het-
eroatom, it may attack the gold-coordinated alkyne
directly to trigger a ring-expansion reaction. To inves-
tigate this point, the alkynyl S,N-heterocycle 1 was
treated with gold-catalyst only to probe the new pos-
sibility (Table 3). To our surprise, , catalysis by 5 mol%
[PPh3AuNTf2] afforded polycyclic product 12, albeit
[a] Yields refer to isolated products.
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Scheme 5. Control experiments.
In summary, we have developed a mild and efficient gold-
catalyzed oxidative ring-expansion of a series of alkynyl hetero-
cycles. These unprecedented tandem reactions feature regiose-
lective oxidation and chemoselective migration of an endocy-
clic CÀX bond, favored over that of CÀH, with the order of mi-
gratory aptitude being CÀS>CÀN>CÀO. In the absence of an
oxidant, polycyclic products could be constructed through
a ring-expansion/Nazarov cyclization sequence. The current
methodology may serve as a new and general protocol for the
synthesis of heterocycles from other heterocycles. Further in-
vestigations on the detailed reaction mechanism and applica-
tions of this chemistry are in progress.
Scheme 4. Proposed reaction mechanism.
moiety in 14 leads to three-membered intermediate 15, which
undergoes ring opening to give the desired products 2, 4, and
6 (path a). Alternatively (path b), gold–carbene 16 might be
generated through fragmentation; subsequent ring-expansion
followed by elimination of the metal furnishes the final prod-
ucts. Path c involves first the formation of gold–carbene inter-
mediate 18,[2n,o] followed by oxidation. In this oxidative ring ex-
pansion, gold–carbene species 16 may not be involved, since
if it were generated, an alkene product would possibly be
formed through competing and highly favorable 1,2-CÀH in-
sertion from the ring side, as indicated in many gold-catalyzed
reactions involving alkyl-substituted gold–carbene species.[5e,i,-
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant Nos. 21125210, 21421091, 21372244, and 21572256),
the Chinese Academy of Science, and the Major State Basic Re-
search Development Program (Grant No. 2011CB808700) for fi-
nancial support.
Keywords: alkynes · gold · heterocycles · homogeneous
catalysis · ring expansion
l,10a,11]
However, no such product was detected in our case. It is
also unlikely to involve the formation of gold–carbene 18 in
the course of oxidation. It was reported that, in the presence
of N-oxide, 1,2-CÀH insertion of gold–carbene intermediate
with an alkyl substituent was much faster than oxidation.[11] In
our reaction system, no alkene products were formed during
the oxidation of the alkyl-substituted alkynes, indicating that
this reaction pathway is less likely. Moreover, It was found that
the gold-catalyzed reaction of 1l in the absence of N-oxide af-
forded alkene 20, formed by 1,2-CÀH insertion of the gold–car-
bene intermediate 18 in 69% yield (Scheme 5). When the reac-
tion of alkyl alkyne 1l was carried out in the presence of less
than one equivalent of N-oxide as the oxidant, no alkene prod-
uct 20 was observed (Scheme 5).[12] If the reaction goes
through path c, 20 should be formed, since the reactions lead-
ing to both the oxidation product 2l and alkene 20 proceeds
through the common intermediate 18. Furthermore, when
alkene 20 was subjected to the reaction conditions given in
Scheme 5 in the presence of one equivalent of pyridine N-
oxide, no oxidation product 2l was observed. Therefore, it ap-
pears that path a is the most favorable pathway. In the ab-
sence of an oxidant, Nazarov cyclization of gold–carbene inter-
mediate 18 leads to the fused-ring products 12 (Scheme 4).
Our results indicate that the reaction pathway can be altered
by adding N-oxide.[5e]
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[12] The use of [PPh3AuNTf2] as the catalyst afforded similar results. Like-
wise, treatment of aryl-substituted alkyne 1a with 5 mol% of
[PPh3AuNTf2] and 0.5 equivalents of pyridine N-oxide in toluene at 808C
resulted in the formation of 2a alongside some remnant 1a, with no
Nazarov cyclization product observed. The results also indicate that
path c is disfavored. For details, see the Supporting Information.
Received: October 16, 2015
Published online on November 5, 2015
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