Consecutive O?S/N?S Bond Cleavage in Gold-Catalyzed Rearrangement Reactions of Alkynyl N-Sulfinylimines

Article abstract of DOI:10.1002/anie.202100207

Gold-catalyzed reactions of alkynyl N-sulfinylimines were used to produce the corresponding 2H-azirines possessing sulfenyl and acyl groups at the 3-position of the azirine ring in good to excellent yields. These reactions involved internal transfer of the sulfinyl oxygen atom to form a thiooxime intermediate tethered to an α-oxo gold carbene moiety. Subsequent insertion of the carbene into the N?S bond resulted in ring construction.

Full text of DOI:10.1002/anie.202100207

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Transition-Metal Catalysis
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À
Consecutive O S/N S Bond Cleavage in Gold-Catalyzed
Rearrangement Reactions of Alkynyl N-Sulfinylimines
Hiroki Tashiro, Masahiro Terada, and Itaru Nakamura*
Abstract: Gold-catalyzed reactions of alkynyl N-sulfinyl-
imines were used to produce the corresponding 2H-azirines
possessing sulfenyl and acyl groups at the 3-position of the
azirine ring in good to excellent yields. These reactions
involved internal transfer of the sulfinyl oxygen atom to form
a thiooxime intermediate tethered to an a-oxo gold carbene
À
moiety. Subsequent insertion of the carbene into the N S bond
resulted in ring construction.
p-Lewis acidic metal catalyzed skeletal rearrangements
are an attractive method for the synthesis of multifunction-
alized heterocycles,[1] which are useful in pharmaceutical and
materials science, because the rearrangements enable the
rapid construction of complex molecular skeletons in an
efficient manner. These rearrangements involve the cleavage
of the s-bonds of functional groups through an intramolecular
nucleophilic attack on either an p-activated alkyne moiety or
a metal-carbene carbon generated from an alkyne.[2,3] Over
the years, oxime ethers have served as an intriguing functional
group for skeletal rearrangement reactions involving the
[4]
À
À
=
cleavage of C O, N O, and C N bonds. Remarkably, the
reactions have resulted in the effective synthesis of a variety
of multisubstituted heterocycles possessing nitrogen and
oxygen atoms, such as isoxazoles,[4a,b] pyridine-N-oxides,[4c]
and isoquinolines[4f] (Scheme 1a), which are much less
accessible by other conventional methods.[4] In sharp contrast,
however, thiooximes (related sulfur analogues, sulfenyl-
imines) have remained unexplored in p-Lewis acidic catalysis
to afford the corresponding heterocycles possessing nitrogen
and sulfur atoms either within the ring structure or as
substituents on the ring. Unlike the reactions of oximes,
thiooxime reactions are probably hampered by low accessi-
bility and instability of thiooximes in comparison to oximes as
well as the potential deactivation of the soft Lewis acid
catalysts by the sulfur atom.[5] To overcome these hurdles, we
envisioned that thiooximes would be catalytically generated
by oxygen transfer from sulfinylimines, which are readily
accessible by the condensation of carbonyls and sulfinamides
Scheme 1. p-Lewis acidic metal-catalyzed skeletal rearrangement.
and stable under ambient conditions (Scheme 1b). According
to the Au-catalyzed internal oxygen transfer reactions of
sulfoxides,[6] the oxygen atom of the sulfinylimine would be
transferred to the p-activated alkyne at a proper position to
generate thiooxime A with an a-oxo gold carbene moiety.
À
Subsequently, insertion of the carbene into the thiooxime N
S bond would construct a quaternary carbon that is bound to
sulfur, nitrogen, and carbonyl.[7] Thus, the transformation
À
formally involves the consecutive a-addition of the O S bond
À
and b-addition of the N S bond of the sulfinyl group into the
À
C C triple bond. In addition, our strategy affords N- and S-
containing heterocycles with multiple substitutions through
the favorable design of the starting material. Herein, we
report that the Au-catalyzed reactions of alkynyl N-sulfinyl-
imines 1 afforded corresponding S-functionalized azirines 2 in
good to excellent yields through the consecutive cleavage of
[8]
À
À
O S/N S bonds [Eq. (1)].
[*] Prof. Dr. I. Nakamura
Research and Analytical Center for Giant Molecules
Graduate School of Science, Tohoku University
6-3 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8578 (Japan)
E-mail: itaru-n@tohoku.ac.jp
H. Tashiro, Prof. Dr. M. Terada
Department of Chemistry
Graduate School of Science, Tohoku University
6-3 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8578 (Japan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202100207.
The initial hypothesis was proven by the reaction of
alkynyl N-(p-toluene)sulfinylketimine 1a in the presence of
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a catalytic amount of IPrAuNTf2 [IPr: N,N’-bis(2,6-diisopro-
pylphenyl)imidazol-2-ylidene] in 1,2-dichloroethane (DCE)
at 808C for 1 h (Table 1, entry 1), which afforded correspond-
ing 2H-azirine 2a in 65% yield, along with thiooxime 3a
(6%), 1,2-diketone 4a (7%), and di-p-tolyl disulfide 5a
(15%) as byproducts. Initial studies for screening various p-
Lewis acids indicated that an IPr ligand on AuI was effective,
whereas gold(I)–phosphine complexes were less efficient and
other metal complexes were ineffective for the present
reaction (see the Supporting Information). Because
IPrAuNTf2 partially decomposed the desired product 2a
(low mass balance), gold complexes bearing two neutral
ligands were evaluated (entries 2–4). To our delight, the use of
triazole-bound Au complex [IPrAu(TA-Me)NTf2], in which
the Au atom was ligated with 1-methyl-1H-benzotriazole
(TA-Me), was effective to obtain 2a with an improved yield
(entry 4).[9] It should be noted that lower mass balance was
observed at lower reaction temperatures (entries 5 and 6).
The chemical yield was further improved by carrying out the
reaction at a lower concentration of 1a (0.1 M, entry 7).
Next, the optimized reaction conditions (Table 1, entry 7)
were applied to various sulfinylimines 1 (Scheme 2). Aryl-
substituted sulfinylimines 1b–h were efficiently converted
into desired products 2b–h in good yields, irrespective of their
electronic character. In particular, bulky aryl groups, such as
ortho-tolyl (1g) and mesityl (1h), were tolerated on the sulfur
atom without decelerating the reaction. In addition, readily
available tert-butylsulfinylimine 1i was converted into desired
azirine 2i in an acceptable yield. Various aryl groups (1j–m)
and normal-alkyl group (1n) were compatible at the alkyne
Table 1: Optimization of the reaction conditions.[a]
Scheme 2. Gold-catalyzed reactions of 1b–u. The reactions of
1 (0.2 mmol) were conducted in the presence of [IPrAu(TA-Me)NTf2]
(0.01 mmol) in DCE (0.1 M, 2.0 mL) at 808C for 1–4 h. Yields are for
the isolated product.
Entry
Catalyst
T [8C]
t [h]
2a [%][b]
terminus (R1). The electronic nature of substituent R2 at the
ketimine was not affected in the present reaction (1o versus
1p). The 2-furyl group was tolerated as a heteroaryl sub-
stituent. In addition, alkyl groups, such as methyl and n-
propyl, were also compatible at R2. In contrast, the reaction of
aldimine 1u did not give the desired product; 1u was
decomposed under the reaction conditions. The chemical
structure of the azirine was confirmed using spectroscopic
methods including the X-ray crystallographic analysis of 2q
(see the Supporting Information).[10]
1
2
3
4
[IPrAuNTf2]
80
80
80
80
60
40
80
1
24
7
1
4
65
74
72
82
73
59
87 (82)
[IPrAu(PPh3)NTf2]
[IPrAu(Lut)NTf2]
[IPrAu(TA-Me)NTf2]
[IPrAu(TA-Me)NTf2]
[IPrAu(TA-Me)NTf2]
[IPrAu(TA-Me)NTf2]
5[c]
6[d]
7[e]
24
3
[a] Reaction conditions: 1a (0.2 mmol), catalyst (0.01 mmol), DCE
(0.5 M), 808C. [b] The yield was determined by NMR spectroscopy using
CH2Br2 as the internal standard. The yield of the isolated product is in
parentheses. [c] The reaction was carried out at 608C. [d] The reaction
was carried out at 408C. [e] Reaction mixture concentration: 0.1 M DCE.
Tf =trifluoromethanesulfonyl, Lut=2,6-lutidine, TA-Me=1-methyl-1H-
benzotriazole.
To gain an insight into the mechanism of the present
consecutive reaction, we conducted two control experiments.
First, the reaction was carried out using a mixture of
2
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substrates 1a and 1o, which should be equally reactive, to
afford corresponding products 2a and 2o. The lack of any
crossover products, such as 2d and 2v (Scheme 3a, analyzed
by high-resolution mass spectrometry) indicates that the
sulfenyl group migrates intramolecularly. Second, the reac-
tion was carried out using enantiomerically enriched substrate
(R)-1a (96% ee, Scheme 3b). The nearly racemic mixture of
the corresponding product (4% ee) suggests that the reaction
does not preserve the chirality of the substrates.
Based on the above results, we propose a mechanism[11]
for the present reaction as illustrated in Scheme 4. First, the
cationic Au catalyst coordinates with substrate 1 to form p-
complex 6.[12] This is followed by 6-endo cyclization by
nucleophilic attack by the oxygen atom on an electrophilically
activated alkyne moiety. The resulting vinylgold species 7[13]
À
undergoes O S bond cleavage, driven by the donation of
electrons from the Au atom, producing Z thiooxime Z-8,
which is directly bound to an a-oxo gold carbene.[14]
Subsequently, thiooxime intermediate Z-8 takes two plausible
routes to form product 2. In path a, the carbene insertion into
À
the thiooxime N S bond is triggered by a nucleophilic attack
of the sulfur atom, followed by ring contraction of the
thiazete–gold enolate (9 or 10) to give azirine 2. Alternatively
(path b), intermediate Z-8 undergoes a three-step sequence
to produce 2: 1) Z/E isomerization of the thiooxime,[15]
2) nucleophilic attack of the nitrogen atom of thiooxime E-
8, and 3) rearrangement of the sulfenyl group with regener-
ation of the Au catalyst. Preliminary DFT studies suggest that
Scheme 4. Proposed mechanism.
the former route via four-membered cyclic intermediate 7
(path a) is more likely than the latter (path b), which involves
an energetically unfavorable migration of the sulfenyl group
of the azirinium intermediate due to severely strained
transition state (12) with a bicyclo[1.0.0] structure. This is
further supported by computational studies, indicating that
the ring contraction is initiated from Au O-enolate 10 (see the
Supporting Information). Additionally, computations of the
intrinsic reaction coordinate (IRC) for the ring contraction
process from 10 to 2 indicated that the initial elongation of the
À
N S bond would be followed by torsion of the four-
À
membered ring and subsequent formation of the C N bond
after the saddle point (see the Supporting Information).
According to DFT calculations, the ring contraction is the
rate-determining step of the present transformation. This is in
good agreement with the experimental observation that a low
reaction temperature is related to a low mass balance
(Table 1, entries 4–6), presumably through the degradation
of substrate 1 via such reactive intermediates as 9, 10, and
11.[16] Further mechanistic investigations are underway in our
laboratory.
Finally, 2-sulfenyl-2H-azirine 2d was derivatized into
corresponding 4-sulfenyloxazole 13d in good yield with the
use of imidazole as the nucleophilic reagent [Eq. (2)].[17,18] It
is noteworthy that 2H-azirines 2 can serve as an efficient
intermediate for multisubstituted oxazole derivatives that can
further be transformed into important components in bioac-
tive compounds and functionalized materials.[19]
Scheme 3. Mechanistic studies.
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À
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group and a carbonyl group from readily accessible alkynyl-
À
À
sulfinylimines through a consecutive O S/N S bond cleavage
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Acknowledgements
This research was supported by JSPS KAKENHI Grant
Number JP20H02731, Grant-in-Aid for Scientific Research
(B). We thank Prof. Ilya D. Gridnev for a valuable discussion
on DFT calculations.
Conflict of interest
=
Chem. Sci. 2019, 10, 5283 – 5289; For C N bond cleavage, m) I.
The authors declare no conflict of interest.
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Keywords: alkynes · carbenes · gold · heterocycles ·
rearrangement
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Manuscript received: January 6, 2021
Accepted manuscript online: February 16, 2021
Version of record online: && &&, &&&&
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Communications
Transition-Metal Catalysis
H. Tashiro, M. Terada,
I. Nakamura*
&&&& — &&&&
À
À
Consecutive O S/N S Bond Cleavage in
Gold-Catalyzed Rearrangement Reactions
of Alkynyl N-Sulfinylimines
À
À
The O S and N S bonds of alkynyl N-
sulfinylimines were cleaved in gold-cata-
lyzed skeletal rearrangement reactions to
afford S-functionalized 2H-azirines in
good to excellent yields (see scheme).
The reactions proceed via a thiooxime
intermediate directly bound to an a-oxo
gold carbene moiety.
6
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