Gold-Catalyzed Alkynylative Meyer-Schuster Rearrangement

Article abstract of DOI:10.1021/acs.orglett.0c01596

By applying the "interplay"mode, which consolidates two key reactivity modes of gold catalysis, namely π-activation mode and cross-coupling mode, the first alkynylative Meyer-Schuster rearrangement is designed and successfully implemented. The current protocol gives straightforward access to enynones, a highly valuable building block, from easily available propargyl alcohol feedstocks. Control experiments suggest an Au(III) catalyst triggers the Meyer-Schuster rearrangement, whereas monitoring the reaction with ESI-HRMS provided strong evidence in favor of a key alkynylgold(III) intermediate which supports the proposed "interplay"scenario.

Full text of DOI:10.1021/acs.orglett.0c01596

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Letter
Gold-Catalyzed Alkynylative Meyer−Schuster Rearrangement
Somsuvra Banerjee, Shivhar B. Ambegave, Ravindra D. Mule, Beeran Senthilkumar,* and Nitin T. Patil*
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ABSTRACT: By applying the “interplay” mode, which consol-
idates two key reactivity modes of gold catalysis, namely π-
activation mode and cross-coupling mode, the first alkynylative
Meyer−Schuster rearrangement is designed and successfully
implemented. The current protocol gives straightforward access
to enynones, a highly valuable building block, from easily available
propargyl alcohol feedstocks. Control experiments suggest an
Au(III) catalyst triggers the Meyer−Schuster rearrangement,
whereas monitoring the reaction with ESI-HRMS provided strong evidence in favor of a key alkynylgold(III) intermediate which
supports the proposed “interplay” scenario.
Scheme 1. Conceptual Roadmap toward Alkynylative
Interception of Meyer-Schuster Rearrangement
ver the last two decades, homogeneous gold catalysis has
Oestablished itself as reliable approach in modern organic
synthesis to activate C−C multiple bonds, capitalizing on the
tunable soft π-acid nature of gold salts toward intra- or
intermolecular attack by nucleophiles (“π-activation mode”,
Scheme 1A, path I).1 Under such homogeneous conditions,
the cross-coupling reactions were nontrivial for gold catalysts,
unlike its other late-transition-metal counterparts, since the
comparatively high oxidation potential2 of Au(I)/Au(III)
redox couple restricted gold’s redox activities. Later, such
gold-catalyzed cross-coupling reactions were realized by
employing sacrificial oxidants,3 visible-light photoredox
catalysis,4 or rational ligand design5 which enabled access to
the Au(I)/Au(III) catalytic cycle (“cross-coupling mode”,
Scheme 1A, path II). While exploration of both of the
activation modes, i.e., paths I and II, is still being actively
pursued, concurrent research endeavors have also witnessed
considerable attention on merging the two modes of reactivity
(“interplay mode”, Scheme 1A, path III).6 In this context, the
research teams of Toste, Zhang, Russell, Glorius, and a few
others have deployed the “interplay mode” in executing gold-
catalyzed 1,2-difunctionalization reactions of C−C multiple
bonds.7 However, all such reactions are either based on the use
of sacrificial oxidants or highly reactive aryl diazonium salts.
Overcoming these limitations, our group has recently inducted
a unique approach to the interplay mode, involving a ligand-
enabled cross-coupling mode and the typical π-activation mode
of gold complexes, to perform the very first gold-catalyzed 1,2-
diarylation of alkenes.8 Even though the reactivities and
selectivities exhibited by this interplay mode of gold catalysis
have been unique as compared to other transition metals in
almost all of the cases, the application of this mode of gold
catalysis has remained confined to 1,2-difunctionalization
reactions of C−C multiple bonds.7,8 We believe that there
exists a lot of potential for the “interplay mode” in unlocking
a
a
BI = benziodoxolone.
Received: May 10, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01596
© XXXX American Chemical Society
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A

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novel reactivities beyond 1,2-difunctionalization, which are
challenging and yet unachieved.
Table 1. Reaction Development
One such reactivity which poses a significant challenge is the
alkynylative Meyer−Schuster rearrangement. It is to be noted
that the Meyer−Schuster rearrangement, although a century
old transformation, is very crucial for organic synthesis because
it converts readily available propargyl alcohols into α,β-enones
in an atom-economical fashion.9 To broaden the utility of this
classical process in synthetic organic chemistry, the “inter-
cepted” Meyer−Schuster rearrangement, in which the vinyl-
metal intermediate in the rearrangement is intercepted with
various coupling partners to obtain highly useful and versatile
α-functionalized enones, holds significant merit and has
emerged as a highly important protocol (Scheme 1B).10
However, the corresponding alkynylative interception, termed
as the “alkynylative” Meyer−Schuster rearrangement (Z =
alkynyl), has remained elusive so far. Although the term
“alkynylative Meyer−Schuster rearrangement” was originally
coined by Reddy and co-workers, their efforts to develop the
same was unsuccessful.11 In their case, instead of a Meyer−
Schuster rearrangement, the reaction between alkynols and
terminal alkynes under Pd catalysis resulted in syn-
alkynopalladation12 (Int I′) via [1,2]-migratory insertion
from Int I (Scheme 1C, eq a). Such migratory insertions are
dominant in regular organotransition-metal complexes and
clearly illustrate the difficulties of achieving an alkynylative
Meyer−Schuster rearrangement using conventional transition-
metal catalysis.
a
entry
1
variation of the “standard condition”
yield of 3a (%)
none
<5
39
44
36
31
69
14
17
68
73
NR
69
b
2
L2AuCl/AgSbF6
L3AuCl/AgSbF6
Ph3PAuNTf2
[Au]1SbF6
[Au]2OTf
b
3
b
4
b
5
b
6
7
b
AuCl3
PicAuCl2
b
8
b
9
7 mol % of [Au]2OTf
1.3 equiv of 2a was used
without [Au] cat.
temp raised to 60 °C
DCE used as a solvent
c
10
d
11
c
,
d
d
12
c
,
13
25
c
a
b
Isolated yields. Instead of AuCl/L1 as catalyst system. [Au]2OTf
was used as catalyst in place of AuCl/L1. 1.3 equiv of 2a. NR = no
d
reaction.
On the contrary, an alkynylgold(III) complex such as Int II
is supposed to be reluctant to undergo migratory insertion,
considering the reactivity of organogold species being
frequently complementary to that of analogous organo-
transition-metal complexes (Scheme 1C, eq b).13 We therefore
hypothesized that the inherent π-activation capability of Au-
complexes would trigger a [1,3]-OH shift in Int II required for
Meyer−Schuster rearrangement14 furnishing Int II′, which
would further undergo reductive elimination to lead to the
desired α-alkynylated enones. Because of our continuous
involvement in gold-catalyzed alkynylations with ethynylben-
ziodoxolones (EBXs),6,15 we believed such alkynylgold(III)
complex (Int II) can be obtained in situ utilizing EBXs, which,
by design, can function as an oxidant as well as an alkyne
surrogate. Thus, the idea of applying the interplay mode of
gold catalysis, consisting of an EBX-enabled cross-coupling
mode and the π-activation mode, in achieving the challenging
task of alkynylative Meyer−Schuster rearrangement is
conceived and herein, we report the successful implementation
of the same. The reaction gives selective straightforward access
to (E)-enynones, provides moderate to excellent yields and is
highly general with respect to both alkynols and EBXs. It is
necessary to reiterate the fact that such enynones or 1,3-enyne
systems are well-recognized to be highly valuable synthetic
building blocks.16,17
and 3). Even with Ph3PAuNTf2, where “NTf2” is weakly
coordinating, the yield was 36% (entry 4). We therefore
surmised that (P,N) ligand systems with stronger “N”-
coordination might be the key in achieving desired out-
comes.5,8 Gold catalysts were screened accordingly (entries 5
and 6), wherein [Au]2OTf, to our delight, stood out with 69%
yield (entry 6) and even with decreased catalyst loading the
yield could be maintained at 68% (entry 9). Finally, utilizing
1.3 equiv of 2a delivered us the desired product in 73% yields
(entry 10). Notably, variation of Au catalysts, temperature, and
solvents did not give us any improvements (entries 7−8, 12,
13) while the omission of [Au] completely halts the reaction
(entry 11).
Having optimized the reaction conditions (Table 1, entry
10), we began to evaluate the scope of various alkynols 1 in the
alkynylative Meyer−Schuster rearrangement keeping the EBX
(2a) constant (Scheme 2). First, several aryl alkynols (R1 = Ar;
R2 = H) were tested against 2a, and it was observed that all
such alkynols were well-tolerated to afford products 3b, 3d−n
in up to 83% yields irrespective of their steric or electronic
nature. However, a stronger electron-donating −NMe2 group
was found to be incompatible, giving a complex reaction
mixture (3c). Substrates appended with sterically encumbered
1-naphthyl (1o) and 9-anthracenyl (1p) moieties efficiently
underwent alkynylative Meyer−Schuster reaction to provide
the corresponding enynones in 65 and 77% yields, respectively.
Intriguingly, the alkynylated product 3q could be exclusively
produced from alkynol 1q over a potentially competing
Rautenstrauch rearrangement.18 Aryl alkynols with varied R3
groups (1r−v) could be successfully engaged in the reaction to
form enynones 3r−u in good yields (58−73%), whereas with
1v the yield suffered a little (45%). Acyclic and cyclic tertiary
substituted alkynols also performed well under the reaction
On the basis of our prior experience15a with alkynylation
reactions involving Au(I)/Au(III) catalysis, initial studies into
the new alkynylative Meyer−Schuster rearrangement were
performed with alkynol 1a and TIPS-EBX 2a in MeCN at 50
°C, employing AuCl (10 mol %)/L1 (15 mol %) as the catalyst
system (Table 1, entry 1). However, the reaction conditions
did not give us any leads, with 1a remaining unreacted. A
significant improvement of the outcome up to 44% yields is
seen when we switched from bidentate (N,N) ligand system
(L1) to hemilabile (P,N) ligand systems (L2−L3) (entries 2
B
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Scheme 2. Scope of Alkynylative Meyer−Schuster
Scheme 3. Mechanistic Studies
a,b
Reaction
(1a to 3a′), indicating the fact that formation of Int IIIa (and
consequently Int IIIa′) is improbable under the reaction
conditions. On the contrary, since with Au(III) catalysts21 the
reaction concerned can obtain moderate yields of the blank
Meyer−Schuster product (3a′), it is likely that first the EBX
must undergo oxidative addition to [Au]2OTf to generate an
intermediate alkynylgold(III) complex which is capable of
performing [1,3]-OH shift (Meyer−Schuster rearrangement)
on 1a. Further probing the reaction with mass spectrometry
(Scheme 3b) revealed the formation of alkynyl(Ph3P)+ (Int
IVa, m/z = 443.2327) which may have resulted out of a
reductive elimination from a phosphine-ligated alkynylgold-
(III) intermediate.22
The mechanistic studies mentioned above are in agreement
with our mechanistic hypothesis described in Scheme 1. First,
the oxidative addition to the linear two-coordinate Au(I)
complex, [Au]2OTf, proceeds with 2a to afford the putative
Int Va (Scheme 4).22 Thereafter, it proceeds to coordinate
a
b
Reactions performed with 0.10 mmol 1 and 0.13 mmol 2. Isolated
c
d
yields. Complex mixture obtained. The alkynol 1a remained
unreacted.
conditions and led to tetrasubstituted enone products (3w−x,
55−62%), outcompeting any Rupe rearrangement pro-
ducts.9c,19 Further, heteroarenes were also remarkably
tolerated, as demonstrated by the synthesis of furan, thiophene,
pyridine, indole, and isatin derivatives of enynones (3y−ad) in
good to excellent yields (62−90%). The chemoselectivity of
the reaction is clearly marked when the thiophene and indole-
tethered alkynols 1aa and 1ac furnished exclusively the desired
enynones 3aa and 3ac, respectively, instead of their regular C-2
alkynylated products.20 In order to apply this protocol for the
late-stage alkynylation of biologically important molecules, we
performed the title reaction on menthol-based alkynol 1ae
which was indeed functionalized with moderate efficiency
under the standard reaction conditions (3ae, 50%). We next
set out to explore the scope with respect to various EBXs using
the aryl alkynol 1a as partner. Accordingly, alkynol 1a were
treated with TBDMS-EBX (2b), TBDPS-EBX (2c), and Ph-
EBX (2d), respectively, under the standard reaction con-
ditions. While the reaction worked well with 2b and 2c, giving
rise to products 3af and 3ag in 68 and 77% yields, respectively,
it failed to deliver the desired enynone 3ah upon using Ph-EBX
(2d). Of note, the X-ray crystallographic analysis for 3b
unequivocally confirmed the (E)-selectivity of the reaction.
Next, a series of control experiments were designed to gain
deeper mechanistic insights (Scheme 3). First, we wanted to
ascertain whether the oxidative addition concerned takes place
at the stage of [Au]2OTf or at the stage of vinylgold(I) species
Int IIIa (Scheme 3a). Interestingly, we observed that
[Au]2OTf fails to catalyze the blank Meyer−Schuster reaction
Scheme 4. Proposed Mechanistic Rationale
with the alkynol 1a to generate the key intermediate (Int IIa)
which is now able to activate the alkyne toward Meyer−
Schuster rearrangement leading to Int II′a. Finally, a reductive
elimination from Int II′a delivers the desired enynone 3a and
regenerates the active catalyst [Au]2OTf.
In summary, we have accomplished the first alkynylative
Meyer−Schuster rearrangement, which was not successful
previously under Pd catalysis. The current reactivity has been
achieved by harnessing the potential of the “interplay mode” of
gold catalysis, which integrates the π-activation mode and an
EBX-enabled cross-coupling mode. This mechanistic paradigm
is further corroborated with our mechanistic investigation
involving typical control experiments and mass-spectrometry
C
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studies that indicate the formation of a key alkynylgold(III)
intermediate. The reaction offers a straightforward and
exclusive access to a wide range of highly valuable(E)-
enynones, barring the formation of any undesired enone side
product. The reaction reported herein again proved that the
“interplay mode” of gold catalysis should find important
implications in discovering new reactivities which are not
possible by other transition metals.
necessary laboratory support provided by Dr. Senthilkumar
in CSIR-NCL to conduct the research work when Dr. Patil
moved to IISER Bhopal.
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AUTHOR INFORMATION
Corresponding Authors
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■
Beeran Senthilkumar − Division of Organic Chemistry, CSIR-
National Chemical Laboratory, Pune 411 008, India; Academy
of Scientific and Innovative Research, Ghaziabad 201 002,
India; Email: b.senthilkumar@ncl.res.in
Nitin T. Patil − Department of Chemistry, Indian Institute of
Science Education and Research Bhopal, Bhauri, Bhopal 462
066, India; orcid.org/0000-0002-8372-2759;
Email: npatil@iiserb.ac.in
Authors
Somsuvra Banerjee − Division of Organic Chemistry, CSIR-
National Chemical Laboratory, Pune 411 008, India; Academy
of Scientific and Innovative Research, Ghaziabad 201 002,
India
Shivhar B. Ambegave − Department of Chemistry, Indian
Institute of Science Education and Research Bhopal, Bhauri,
Bhopal 462 066, India
Ravindra D. Mule − Division of Organic Chemistry, CSIR-
National Chemical Laboratory, Pune 411 008, India; Academy
of Scientific and Innovative Research, Ghaziabad 201 002,
India
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01596
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Generous financial assistance by the Science and Engineering
Research Board (SERB), New Delhi (File Nos. EMR/2016/
007177 and DIA/2018/000016) is gratefully acknowledged.
S.B., S.B.A., and R.D.M. thank UGC and CSIR for the award of
Research Fellowships. We heartily acknowledge all the
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