A Desulfonylative Approach in Oxidative Gold Catalysis: Regiospecific Access to Donor-Substituted Acyl Gold Carbenes

Article abstract of DOI:10.1002/anie.201504511

Donor-substituted acyl gold carbenes are challenging to access selectively by gold-promoted intermolecular oxidation of internal alkynes as the opposite regioisomers frequently predominate. By using alkynyl sulfones or sulfonates as substrates, the oxidative gold catalysis in the presence of substituted pyridine N-oxides offers regiospecific access to acyl/aryl, acyl/alkenyl, and acyl/alkoxy gold carbenes by in situ expulsion of sulfur dioxide. The intermediacies of these reactive species are established by their reactivities, including undergoing further oxidation by the same oxidant, cyclopropanation of styrenes, engaging in a [3+2] cycloaddition with α-methylstyrene, and conversion into dienones. Accept it: A desulfonylative approach was developed to regiospecifically access these underexplored acyl gold carbenes from either alkynyl aryl/alkenyl sulfones or alkynyl sulfonate substrates. The reactivities of these donor- and acceptor-substituted carbenes are examined.

Full text of DOI:10.1002/anie.201504511

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504511
German Edition: DOI: 10.1002/ange.201504511
Carbenes
A Desulfonylative Approach in Oxidative Gold Catalysis:
Regiospecific Access to Donor-Substituted Acyl Gold Carbenes
Hongyi Chen and Liming Zhang*
Abstract: Donor-substituted acyl gold carbenes are challeng-
ing to access selectively by gold-promoted intermolecular
oxidation of internal alkynes as the opposite regioisomers
frequently predominate. By using alkynyl sulfones or sulfo-
nates as substrates, the oxidative gold catalysis in the presence
of substituted pyridine N-oxides offers regiospecific access to
acyl/aryl, acyl/alkenyl, and acyl/alkoxy gold carbenes by in situ
expulsion of sulfur dioxide. The intermediacies of these
reactive species are established by their reactivities, including
undergoing further oxidation by the same oxidant, cyclo-
propanation of styrenes, engaging in a [3+2] cycloaddition
with a-methylstyrene, and conversion into dienones.
accessed selectively based on structural biases and on
optimizations of catalyst, oxidant, and other reaction con-
ditions. For example, in our previous work,[6] under optimal
reaction conditions, differences in steric bulk at the two ends
of the alkyne were used to achieve selective oxygen delivery
to the less hindered C(sp) (e.g., 1; Scheme 1b), and with
electronically biased internal alkynes oxidation occurs selec-
tively at the end which best accommodates the developing
positive charge upon gold coordination, that is, proximal to
aryl, alkenyl, and N-amido groups (e.g., 2 and 3;[7] Sche-
me 1b)[5l-o,6] and distal to electron-withdrawing groups (e.g.,
4; Scheme 1b).[4f, 5e,f] There is, however, no viable approach to
generating a-oxo gold carbenes selectively with regiochemis-
tries opposite to the structurally preferred ones, let alone
regiospecifically. Hence, the potentially rich reactivities of the
minor regioisomeric gold carbenes could not be readily
explored. Herein, we disclose a solution to this challenge with
regard to arylalkyne, enyne, and alkoxyalkyne by using
a desulfonylative strategy, and explore the reactivities of
these regiospecifically generated donor-substituted acyl car-
benes, including acyl/aryl, acyl/alkenyl, and acyl/alkoxy gold
carbenes. Notably, most of these types of carbenes[5a] studied
so far are generated either from the corresponding diazo
precursors[,3j] or by oxidation of activated aryl/alkenyl-termi-
nated ynamides, instead of typical internal alkynes.[5b,o]
G
old-catalyzed intermolecular oxidation of alkynes[1] has
become an increasingly popular approach to accessing highly
electrophilic a-oxo gold carbene intermediates since the first
report in 2010[2] (Scheme 1a). This strategy permits ready
explorations of these reactive intermediates without resorting
to hazardous and potentially explosive diazo ketone precur-
sors,[3] and thereby facilitates the development of versatile
synthetic methods.[2,4,5] While terminal alkynes are most
frequently oxidized to terminal gold carbene intermediates,
regioselective oxidations of internal alkynes can be challeng-
ing, and often only one of the two regioisomers can be
Our approach is shown in Scheme 2. While arylalkynes
and enynes would often lead to selective generation of the
a-oxo gold carbene A over its regioisomer B, which is an
acceptor- and donor-substituted gold carbene, it is envisioned
that B could be generated exclusively from a different class of
substrates, namely, alkynyl sulfones (5). Hence, upon its
ꢀ
coordination to a cationic gold(I) catalyst, the C C bond,
polarized by the electron-withdrawing sulfonyl group, should
Scheme 1. A) Oxidative gold catalysis: a facile, non-diazo access to
a-oxo gold carbenes. B) Examples of regioselective oxidative gold
catalysis with the optimized ratios shown.
[*] H. Chen, Prof. Dr. L. Zhang
Department of Chemistry and Biochemistry, University of California
Santa Barbara, CA (USA)
E-mail: zhang@chem.ucsb.edu
Homepage: http://www.chem.ucsb.edu/~zhang/index.html
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504511.
Scheme 2. A desulfonylative approach toward exclusive generation of
the typically minor a-oxo gold carbene intermediates.
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be attacked by a N-oxide regiospecifically at the b-C(sp).
Such a process, followed by redox rearrangement, would lead
to the dual acceptor-substituted gold carbene intermediate D.
The carbene moiety is likely to be highly electrophilic, and
could react intramolecularly with the aryl/alkenyl group on
the other side of the sulfonyl group, thus leading to the
formation of the episulfone intermediate E. Alternatively,
this intermediate could be formed directly from the initial
adduct C, that is, the precursor to the carbene D, by a 3-exo-
trig cyclization.[5p] The desulfonylative fragmentation of E[8]
would then afford only the a-oxo gold carbene B. It is
noteworthy that B is a donor-substituted acyl gold carbene
and should display a reactivity which is characteristically
different from those without donor substitution.
most effective (entry 8), and 7a was isolated in 80% yield. It
is notable that 7a could not be generated cleanly by gold-
catalyzed double oxidation[5k,12] of the corresponding
À
1-methyl-4-(prop-1-yn-1-yl)benzene, as facile 1,2-C H inser-
tions by the isomeric gold carbenes of type A would lead to
the formation of enone side products.[6,13]
We explored the scope of this desulfonylative double
oxidation chemistry. As shown in Table 2, the p-tolyl group of
5a could be replaced with either an electron-rich 4-methox-
yphenyl (entry 1) or a slightly electron-deficient 4-bromo-
phenyl (entry 2). The reaction yields correlated well with the
trend of the benzene ring reacting with electrophiles and are
consistent with the envisioned reaction mechanism. When the
alkynyl terminal methyl group was replaced with a n-butyl
group in 5d (entry 3), the desired 1,2-diketone 7d was formed
in only 19% yield. Instead, the sulfonylcyclopentanone 7d’
was isolated in 54% yield. This side product must be the result
As shown in Table 1, we set out to validate the above
design and discover optimal reaction conditions by using the
readily prepared[9] alkynyl sulfone 5a as the substrate. It is
anticipated that the gold carbene of type B could be further
oxidized by the same N-oxide to deliver the corresponding
[4h]
À
of C H insertion
by an acyl/sulfonyl-substituted gold
carbene of type D. It was envisioned that a more-electron-
rich phenyl ring in D could accelerate its desulfonylative
rearrangement, thereby minimizing side reactions. Indeed,
[10]
1,2-diketone 7a. When the Gagosz catalyst Ph3PAuNTf2
was employed with DCE as the solvent and 2,6-dichloropyr-
idine N-oxide (6a) as the oxidant, the desired product 7a was
indeed formed, albeit in a low 15% yield (entry 1). Subse-
quent catalyst screenings revealed that the N-heterocyclic-
carbene-based catalyst IPrAuNTf2 performed noticeably
better (entries 2–5). Given our concern over potential chlo-
ride abstraction[11] of the solvent DCE by highly electrophilic
gold carbene intermediates of type C, we used PhCF3 instead.
Much to our delight, the reaction yield was improved to
a decent 72% (entry 6). Our ensuing screenings of the
N-oxides revealed large variations of reaction efficiencies
(entries 7–10). Among them, 2-tert-butyl-4-chloropyridine
N-oxide (6c), first reported by Gagosz,[5i] proved to be the
Table 2: Scope of desulfonylative double oxidation.[a]
Entry
5
7
Yield
[%]
1
2
5b
5c
7b 96
7c 59
19
3
5d
5e
5 f
(54)[b]
Table 1: Optimization of reaction conditions for double oxidation.[a]
4
7e 69[c]
5[d]
7 f 65[c]
Entry
L
Solvent N-Oxide Yield [%][b] Conv. [%]
1
2
3
4
5
6
7
8
9
10
PPh3
DCE
6a
6a
6a
6a
6a
6a
6b
6c
6d
6e
15
8
57
40
100
93
45
100
61
100
91
96
6[d]
5g
5h
7g 67
7h 78
(2,4-tBu2PhO)3P DCE
Mor-DalPhos
tBuMe4XPhos
IPr
IPr
IPr
IPr
IPr
IPr
DCE
DCE
DCE
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
10
17
22
72
32
80[c]
8
7
8[d]
5i
7i 60
20
[a] All reactions were run in sealed vials. [b] Determined by 1H NMR
analysis using diethyl phthalate as an internal reference. [c] Yield of
isolated product. DCE=1,2-dichloroethane, IPr=1,3-bis(diisopropyl-
phenyl)imidazol-2-ylidene, M.S.=molecular sieves, Tf =trifluorometha-
nesulfonyl.
9[d]
5j
7j 75
7k 87
10[d]
5k
[a] The reactions were run using the optimized reaction conditions (i.e.,
Table 1, entry 8), and the yield of isolated product is reported. [b] Yield of
À
À
the C H insertion product 7d’. [c] C H insertion product <5%. [d] 6a
used as oxidant.
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with the anisyl sulfone substrate 5e, the double oxidation
lecular cyclopropanation. The reaction scope was then
À
displayed a much improved yield, while the C H insertion
examined. As shown (8b–e), the aryl sulfones 5 with different
aryl groups, including furan-2-yl (8d), underwent the reaction
smoothly. The terminal substituents of the alkyne could be
either a phenyl (8e) or a cyclopropyl (8 f). Our attempt to
vary the styrene led to successful reaction with 4-bromostyr-
ene (8g) and 4-tert-butylstyrene (8h), but in the case of
4-methoxystyrene, polymerization occurred during the reac-
tion. Other electron-rich alkenes, such as ethyl vinyl ether,
were also not suitable.
With a-methylstyrene as a solvent, a stepwise, formal
[3+2] cycloaddition occurred, thus affording the dihydrofuran
9 in a serviceable yield (Table 3).[5a] This divergent reactivity
can be understood by considering that the benzylic position of
the styrene can better accommodate positive charge and has
increased steric hindrance, both of which disfavor the
expected concerted cyclopropanation reaction.
product was formed in less than 5% yield (entry 4). Similarly,
a cyclohexyl group at the alkyne terminus was allowed
(entry 5). In addition, the alkyne terminus could accommo-
date a cyclopropyl (entry 6), a phenyl (entry 7), and a furan-2-
yl (entry 8) group, the last two of which allowed regiospecific
generation of diaryl-substituted a-oxo gold carbenes by
simply using appropriate aryl alkynyl sulfone substrates.
Besides phenyl-based sulfones, a trans-b-styryl sulfone (5j;
entry 9) and a furan-2-yl sulfone (5k; entry 10) underwent the
reaction without incident, thus affording the diketone prod-
ucts 7j (75%) and 7k (87%), respectively. Of note, in some
cases (entries 5,6, and 8–10), 2,6-dichloropyridine N-oxide
(6a) was more efficient than 6c.
While these results are consistent with acyl gold carbene
ꢀ
intermediates of type B, the fact that the C C bond is
oxidized into a symmetric 1,2-dicarbonyl moiety prevented us
from unequivocally establishing the regiochemistry of these
carbenes, despite the difficulty in formulating an alternative
mechanism for the formation of its isomer A. Liu and co-
workers[14] previously demonstrated that that carbenes
related to B, which are generated by intramolecular alkyne
oxidation, can undergo cyclopropanation reactions with
styrenes. When the oxidation of the alkynyl sulfone 5a was
performed in the presence of styrene, the intended cyclo-
propanation reaction indeed occurred, and the cyclopropyl
ketone 8a was formed selectively in 80% yield upon isolation
(Table 3). This result offers strong support for the regiospe-
cific generation of gold carbenes of the type B, and reveals
that the pyridine byproduct does not interfere the intermo-
The success with the b-styryl sulfone 5j (Table 2, entry 9)
encouraged us to examine other alkenyl sulfones. As shown in
Equation (1), the 2-methylprop-1-en-1-yl sulfone 5l under-
went the gold-catalyzed oxidative desulfonylation, but the
isolated product was the trans-dienone 10, instead of the
corresponding diketone. Our attempt to trap the alkenyl acyl
gold carbene intermediate with styrene was futile. This result
highlights the facile nature of the carbene to undergo E1-type
elimination under the reaction conditions, as outlined in the
equation. With the cyclohexen-1-yl sulfone 5m as the
substrate, the expected dienone product 11 was isolated,
along with the bicyclic furan 12 [Eq. (2)]. The formation of
the latter product should involve a 4p electrocyclic ring
closure, which is in line with the related work reported by the
group of Liu[5b] and ourselves.[15]
Table 3: Cyclopropanation with styrenes: Scope.[a]
8a (80%)
8d (57%)
8b (77%)
8e (76%)
8c (51%)
To further investigate the desulfonylative carbene rear-
rangement, we surmised that instead of an aryl or alkenyl
group an alkoxy might behave in a similar manner, thereby
leading to the generation of a new type of donor/acceptor-
8 f (54%)[b]
8g (83%)
8h (67%)
9 (57%)[b]
[a] All reactions run in vials. The oxidant was introduced by syringe pump
over a 12 h period, and the product d.r.>95:5. [b] 6b was used as the
oxidant, and a-methylstyrene as solvent.
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Acknowledgements
We acknowledge support from the NSF (CHE-1301343), the
NIH shared instrument grant (S10OD012077) for a 400 MHz
NMR instrument, and the NSF MRSEC (DMR-1121053) and
the NSF (CNS-0960316) for the Center for Scientific Com-
puting from the CNSI and MRL.
Keywords: carbenes · gold · oxidation · sulfur ·
synthetic methods
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11775–11779
Angew. Chem. 2015, 127, 11941–11945
substituted gold carbenes, that is, the acyl/alkoxy gold
carbenes F [Eq. (3)]. When the alkynesulfonates 13 were
subjected to the reaction, the a-ketoesters 14, apparently
resulting from further oxidation of the gold carbene inter-
mediates, were indeed formed, albeit with moderate yields in
both cases. In these cases, 6a was a better oxidant than 6c as
slower reactions and lower yields were observed with the
latter (e.g., 2 days, ca. 14% yield). Notably, F could not be
possibly derived from the alkynyl ethers 15 by direct oxidative
gold catalysis because of their opposite polarized nature, even
though the same diketone products might be formed from
these electron-rich alkynes under the same gold catalysis via
isomeric gold carbene intermediates.
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In summary, we have implemented a novel approach to
achieving regiospecific generation of donor-substituted acyl
gold carbenes. These intermediates are either often formed as
minor isomers, or cannot not be generated at all from the
corresponding donor-substituted alkynes because the polar-
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Chemie
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Received: May 19, 2015
Revised: July 4, 2015
Published online: August 12, 2015
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