Rhodium-Catalyzed C-O Bond Alkynylation of Aryl Carbamates with Propargyl Alcohols

Article abstract of DOI:10.1021/acs.orglett.8b00674

The rhodium-catalyzed alkynylation of aryl carbamates with propargyl alcohols is described. This methodology can provide aryl acetylenes from aryl carbamates via C-O bond activation. The use of propargyl alcohols as alkynylating agents allows the use of a variety of functional groups that are incompatible with organometallic nucleophiles. This reaction also serves to broaden the utility of a carbamate moiety as a convertible ortho directing group.

Full text of DOI:10.1021/acs.orglett.8b00674

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium-Catalyzed C−O Bond Alkynylation of Aryl Carbamates with
Propargyl Alcohols
Kosuke Yasui, Naoto Chatani,* and Mamoru Tobisu*
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: The rhodium-catalyzed alkynylation of aryl
carbamates with propargyl alcohols is described. This method-
ology can provide aryl acetylenes from aryl carbamates via C−O
bond activation. The use of propargyl alcohols as alkynylating
agents allows the use of a variety of functional groups that are
incompatible with organometallic nucleophiles. This reaction
also serves to broaden the utility of a carbamate moiety as a convertible ortho directing group.
Scheme 1. Alkynylation of Inert Phenol Derivatives
iven the widespread use of aromatic alkynes in organic
Gchemistry, developing methods to synthesize these
compounds continues to be a subject of great interest.1 The
Sonogashira-type cross-coupling reaction is among the most
powerful methods for the construction of a C(sp)−C(sp2)
bond using aryl halides and terminal alkynes.2 Considering that
phenols are abundant chemical feedstock,3 it is of great
synthetic value to develop catalytic systems that use phenol and
its derivatives instead of aryl halides. A classical way to use
phenols in cross-coupling reactions is to convert them to aryl
triflates, thereby activating the C(aryl)−O bond toward
oxidative addition. Despite the synthetic utility of aryl triflates,
these reactions can be costly and generate harmful waste
derived from the fluorine-based leaving group. Nonfluorinated
phenol derivatives such as ethers, esters, and carbamates have
recently emerged as a less expensive and more environmentally
benign alternative to aryl halides and triflates.4−7 Although
significant progress has been made in the catalytic trans-
formation of inert phenol derivatives, the use of Sonogashira
type cross-coupling has had limited success.5h,8,9 We have
previously described a nickel-catalyzed cross-coupling of
anisoles with alkynyl Grignard reagents (Scheme 1a),5h while
Uchiyama developed a nickel-catalyzed alkynylation of aryl
carbamates using alkynylaluminum reagents (Scheme 1b).8
Although these two methods pioneered the Sonogashira-type
reaction of inert phenol derivatives, the use of strong
organometallic nucleophiles limits functional group compati-
bility. Shi reported a unique Ni/Cu cocatalytic system that
allows the direct use of terminal alkynes in the alkynylation of
aryl carbamates, although only two specific substrates were
examined, and a detailed investigation was not carried out.9 In
the field of C(aryl)−O bond activation of inert phenol
derivatives, the vast majority of the reported reactions use
nickel as the catalyst.4,6 In contrast, we have developed several
rhodium catalysts that can activate inert C(aryl)−O bonds.7
Importantly, the use of rhodium enabled transformations that
were not possible with nickel catalysts. For example, the use of
rhodium allows cross-coupling with nonorganometallic re-
agents, such as arenes bearing a directing group,7d and
isopropanol as a hydride equivalent.7e
Mechanistic investigation of the latter reaction revealed that
the hydride incorporated at the ipso position of aryl carbamates
is derived from the β-hydrogen of the isopropanol and is
transferred through β-hydrogen elimination. We envisioned
that aryl carbamates would undergo C−O bond alkynylation in
the presence of our rhodium catalyst, as propargyl alcohols
serve as alkynylating reagents under rhodium(I) catalysis via β-
carbon elimination.10,11 We also expected that a broad range of
aryl carbamates could be used by avoiding the use of
organometallic alkynylating reagents.
Received: February 26, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00674
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Our study began by optimizing the conditions for the
reaction between 2-naphthyl carbamate 1a and alkynylating
reagents 2a−2g in the presence of a rhodium/NHC catalyst.
After systematic screening, the target alkynylation product 3a
was formed in 74% isolated yield when 2a was used as an
alkynylating reagent, with L3 as the ligand and K3PO4 as the
stoichiometric base (Table 1, entry 1).
The use of terminal alkyne 2b significantly decreased the
yield of 3a due to the formation of enyne byproducts, which are
generated through dimerization of 2b (entry 2).12 Propargyl
alcohols 2c, 2d, and 2e failed to give 3a efficiently, again due to
the formation of the enyne byproducts (entries 3−5). The use
of 2a completely suppressed the formation of the enyne
byproduct, presumably as its bulkiness prevented 2a from
reacting with an alkynylrhodium intermediate. A TIPS
protecting group is critical, as evidenced by the lack of a
product formed with TMS- and phenyl-substituted derivative 2f
and 2g, respectively, presumably due to oligomerization of 2f
and 2g (entries 6 and 7). Despite the limited scope of the
propargyl alcohols, the TIPS group can be easily removed from
the alkynylated products to give corresponding terminal
alkynes, which are amenable to further elaboration (vide
infra). Several other bases, such as KOAc, K2CO3, and
K2HPO4, gave inferior yields to that obtained with K3PO4
(entries 8−10). NHC ligands with an N-mesityl structure (such
as L1 and L2) promoted the reaction, with L3 exhibiting the
best activity (entries 11 and 12). Both methyl groups on the
imidazole ring and the methoxy groups of L3 are important for
enhancing the catalytic activity. The use of L4 or L5, however,
led to a significant reduction in yield of 3a (entries 13 and 14).
Increasing the ratio of the NHC ligand to rhodium to 2:1 did
not improve the yield of 3a (entry 15), which indicated that the
catalytically active species generated in situ is the rhodium
complex bearing one NHC ligand. The [RhCl(C2H4)(L3)]2
species could be observed in the catalyst solution by using 13C
NMR and HRMS (see Supporting Information (SI) for
details).13
a
Table 1. Optimization of the Reaction Conditions
GC yields [%]
entry
variation from above
3a
1a
2
1
none
75 (74)
18
0
0
2
2b instead of 2a
12
15
9
trace
3
2c instead of 2a
35
0
4
2d instead of 2a
25
trace
5
2e instead of 2a
22
7
0
6
2f instead of 2a
0
64
>95
39
trace
trace
3
0
With the optimized reaction conditions in hand, the scope of
the aryl carbamates was evaluated next (Scheme 2). Aryl
carbamates 1b and 1c underwent alkynylation without affecting
7
2g instead of 2a
0
0
8
KOAc instead of K3PO4
K2CO3 instead of K3PO4
K2HPO4 instead of K3PO4
L1 instead of L3
L2 instead of L3
L4 instead of L3
L5 instead of L3
2.0 equiv of L3
23
0
9
47
0
a
10
11
12
13
14
15
16
28
0
Scheme 2. Reaction Scope
20
0
53
2
0
trace
12
68
47
0
0
trace
0
73
[RhCl(cod)]2 instead of
[RhCl(C2H4)2]2
trace
>95
>95
a
Conditions: [RhCl(C2H4)2]2 (0.015 mmol), L·HCl (0.030 mmol),
KOtBu (0.033 mmol), base (0.90 mmol), 1 (0.30 mmol) and 2 (0.45
mmol) in toluene (1.0 mL) at 130 °C for 24 h. Yield in parentheses is
isolated yield.
a
Conditions: [RhCl(C2H4)2]2 (0.015 mmol), L3·HCl (0.030 mmol),
KOtBu (0.033 mmol), K3PO4 (0.90 mmol), 1 (0.30 mmol), and 2c
(0.45 mmol) in toluene (1.0 mL) at 130 °C for 24 h. Isolated yields of
alkynylated products are shown. Reacted at 120 °C. L5·HCl (0.030
b
c
d
e
mmol) was used instead of L3·HCl. At 110 °C. [RhCl(C2H4)2]2
(0.023 mmol), L3·HCl (0.045 mmol), and KOtBu (0.050 mmol) were
f
used. At 140 °C.
B
DOI: 10.1021/acs.orglett.8b00674
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the methoxy group, which can be reactive with nickel
catalysts.5g The use of 2a as an alkynylating reagent allows
the use of aryl carbamates bearing a carbonyl functional group,
such as esters (1d), ketones (1f−1i), and amides (1j), which
are incompatible with organometallic alkynylating reagents.
Although nickel-catalyzed α-arylations of ketones and amides
using aryl esters as an aryl donor have been reported,14 this
rhodium system did not produce such an α-arylation product,
even when aryl carbamates bearing an enolizable ketone (1i)
and amide (1j) were used. Sterically hindered 1-naphthyl
carbamate 1e provided the corresponding alkynylated product
in 75% yield. In addition, the use of more sterically congested
ortho benzoyl carbamate 1f was also possible by using smaller
ligand L5.15 In this transformation, a carbamate bearing a
fluorine (1g) is also compatible, whereas C−F bonds often
react in nickel-catalyzed cross-couplings of inert phenol
derivatives.5h,16 Moreover, heteroaromatic carbamates, such as
carbazole 1k and quinoline 1l, successfully afforded the
corresponding alkynylated products. This catalytic system
could be used for the derivatization of biologically active
phenol compounds, as exemplified by the synthesis of
alkynylated Harmol 3m.17,18
A plausible mechanism for this transformation is shown in
Scheme 4. Initially, rhodium complex A is generated in situ and
Scheme 4. Possible Mechanism
reacts with propargyl alcohol 2 to form rhodium alkoxide B. β-
Carbon elimination of B then forms alkynylrhodium C,
releasing ketone 6. The formation of intermediate C is
supported by the isolation and X-ray analysis of analogous
alkynylrhodium complexes.11g,23 Intermediate C mediates the
oxidative addition of the C−O bond in aryl carbamate 1 to
form rhodium(III) intermediate D, which provides the
alkynylated product 3 via reductive elimination with regener-
ation of the catalyst.
Because a carbamoyl group is known to be a powerful ortho
directing group in arene functionalization reactions,19 ortho-
substituted aryl alkynes can readily be accessed via an ortho C−
H functionalization/ipso alkynylation sequence of simple aryl
carbamates. For example, ortho lithiation of 1a by LiTMP,
followed by reaction with carbamoyl chloride, forms carbamate
1p, which can be converted into alkyne 3p (Scheme 3).
Scheme 3. Synthetic Applications
In summary, we have developed a rhodium-catalyzed
alkynylation of aryl carbamates using propargyl alcohols as
the alkynylating agents. The use of propargyl alcohols allows
this inert C−O bond alkynylation to be compatible with a
range of functional groups, such as ketones, esters, and amides,
which are incompatible with previously reported cross-
couplings using organometallic nucleophiles. This alkynylation
method enables the use of a carbamate directing group as a
handle for the synthesis of functionalized aromatic alkynes,
which serve as useful building blocks in organic synthesis.
Further studies investigating the catalytic transformation of
inert bonds using a rhodium/NHC system are underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00674.
Detailed experimental procedures and characterization of
products (PDF)
Similarly, ortho-acylated aromatic alkyne 3q was successfully
synthesized in a straightforward manner. Although our method
requires the use of TIPS-protected propargyl alcohols, the TIPS
group in the product can be easily removed and replaced with a
different group via the established methods. For example,
deprotection of 3q, followed by arylation under Sonogashira
conditions afforded diaryl alkyne 4. It should also be noted that
the resulting ortho acylated aromatic alkynes are valuable
precursors of a range of heterocycles.20 For example, the
reaction of 4 with hydrazine led to the construction of a
phthalazine skeleton,21 which can further serve as a diene
component in aza Diels−Alder reactions.22
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chatani@chem.eng.osaka-u.ac.jp.
*E-mail: tobisu@chem.eng.osaka-u.ac.jp.
ORCID
Naoto Chatani: 0000-0001-8330-7478
Mamoru Tobisu: 0000-0002-8415-2225
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.8b00674
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(e) Yasui, K.; Higashino, M.; Chatani, N.; Tobisu, M. Synlett 2017, 28,
2569.
(8) Ogawa, H.; Yang, Z.-K.; Minami, H.; Kojima, K.; Saito, T.; Wang,
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Org. Chem. 2001, 66, 1910. (b) Hu, H.; Yang, F.; Wu, Y. J. Org. Chem.
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ACKNOWLEDGMENTS
■
This work was supported by the Scientific Research on
Innovative Area “Precisely Designed Catalysts with Customized
Scaffolding” (16H01022) from MEXT, Japan. M.T. thanks
Tokuyama Science Foundation and Hoansha Foundation for
their financial support. K.Y. thanks the JSPS Research
Fellowship for Young Scientists. We also thank the
Instrumental Analysis Center, Faculty of Engineering, Osaka
University, for their assistance with HRMS.
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D
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