Frustrated Lewis pair-catalyzed double hydroarylation of alkynes with: N -substituted pyrroles

Article abstract of DOI:10.1039/c9cc08654d

Metal-free hydroarylation of alkynes with N-substituted pyrroles is shown to be most efficiently mediated by B(C₆F₅)₃ to yield 12 variants of dipyrrole-alkanes, a mono-hydroarylation product and a tetrahydroarylation produ

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DOI: 10.1039/C9CC08654D
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Frustrated Lewis pair-catalyzed double hydroarylation of alkynes
with N-substituted pyrroles
Jing Guo,a Odelia Cheong,b Karlee L. Bamford,a Jiliang Zhoua and Douglas W. Stephan*a
aReceived 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Metal-free hydroarylation of alkynes with N-substituted pyrroles is Fontaine et al.5 Soós et al. have described the reductive
shown to be most efficiently mediated by B(C6F5)3 to yield 12 etherification of aldehydes and ketones.6 Wasa and colleagues
varriants of dipyrrole-alkanes, a mono-hydroarylation product and have described the applications of FLPs in Mannich reactions,7
a tetrahydroarylation product of a bis-alkyne. These products were Conia-Ene-type cyclizations and C−H functionalizations.8
generally obtained in good to excellent yields (up to 95%). Control
experiments suggest a mechanism involving FLP addition of the
borane and pyrrole to alkyne.
R'
N
R'
N
N
Me
R'
[Ru] [Pt]
[Au] [Ag]
R'
N
Me
R
Frustrated Lewis pairs (FLPs) have garnered much attention in
the areas of inorganic, organic and polymer chemistry since
their discovery in 2006.1 The breadth of applications of the
concept of FLPs has expanded dramatically in the last decade.2
To date, the major finding to emerge from this concept has been
the development of metal-free hydrogenation.3 This area has
grown rapidly, not only in terms of substrate scope, but also in
sophistication, with the development of highly enantioselective
FLP catalysts. Another aspect of rapid growth has involved the
use of FLPs in the capture of a wide range of small molecules,
including alkynes, alkenes, dienes, CO2, CO, SO2, cyclopropanes,
N2O and NO, among a number of others.
(a)
or
RCCH +
R'
N
R
tBu
N
tBu
N
(b)
RCCH
B(C6F5)3
+
+
R
B(C6F5)3
R'
(c) This work
RCCH
R'
N
N
Lewis acid
+
or
R'
Me
R
R'
R
N
N
This unique reactivity arising from combinations of Lewis
acids and bases with
a substrate molecule, has been
Scheme 1 Recent methods for hydroarylation of alkynes with pyrroles and
demonstrated for a range of systems. Indeed, much of Lewis
acid catalysis can be understood as FLP chemistry where the
Lewis acid activates a molecule for nucleophilic attack by a
third, (Lewis) basic component. For example, the well-
established Piers protocol for borane mediated hydrosilylation
of ketones4 is now considered the first example of what has
come to be known as an FLP mechanism.
Despite the broad potential beyond hydrogenation, the
application of FLPs in organic chemistry is only beginning to
emerge. Perhaps the most significant finding in this realm to
date has been the discovery of FLP-mediated borylations by
the present work.
In considering the potential for application in new bond
formation, we note our earlier report of the stoichiometric
addition of nucleophiles to alkynes.9 We further exploited such
FLP addition to alkynes, demonstrating that boranes can
mediate the hydroamination of alkynes by sterically
encumbered amines.10 Subsequently, we reported borane
mediated cyclizations of propargyl esters affording routes to
unique allyl boron species.11 Similarly, Melen et al. exploited FLP
additions of propargyl amides, ureas, carbamates, and
carbonates to generate heterocycles.12
In conceiving new applications of FLP additions to alkynes
we noted that there is considerable interest in derivatizations
of heteroaromatic species, due to their roles in natural products
and pharmaceuticals.13 We also noted that while a limited
number of ruthenium14, 15, platinum16, silver and gold17 based
catalysts have been described to effect the double
a. Department of Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario M5S 3H6, Canada, E-mail: dstephan@chem.utoronto.ca
b. Department of Chemistry and Biological Chemistry, Nanyang Technological
University, 50 Nanyang Avenue, 639798 (Singapore)
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
X-ray data are deposited see CCDC 1963872-1963873.
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Table 2 Reactions of alkynes with N-t-butylpyrrole a
tBu
hydroarylation of alkynes with pyrroles (Scheme 1a), metal-free
routes have remained less explored.18 In 2010, our group
described the stoichiometric addition of pyrroles and B(C6F5)3
FLPs to alkynes affording the zwitterionic addition products of
the form (C4H3NtBu)C(R)=CH(B(C6F5)3) (Scheme 1 b).19 Herein,
we exploit our early finding of C-C bond formation via the action
of pyrrole/B(C6F5)3 based FLPs, to develop a facile protocol for
catalytic hydroarylations of alkynes, affording a range of
dipyrrole-alkanes (Scheme 1c). Mono-hydroarylated products
are obtained using poorer Lewis acids or sterically demanding
alkynes. The mechanism of these reactions is considered.
A 1:2.5 ratio of phenylacetylene and N-t-butylpyrrole in C6D6
did not react (Table 1, entry 1). Similarly, addition of a catalytic
quantity of the Lewis acid BF3∙Et2O to the alkyne-pyrrole
mixture had no impact (entry 2). The Brønsted acids TFA and
TfOH also gave no reaction while the combination of anhydrous
B(C6F5)3 with H2O (10 mol%) yielded only trace amounts of the
double hydroarylated product (see ESI). Further survey of Lewis
acids revealed that use of Ga(C6F5)3 as a catalyst yielded a
complex mixture of uncharacterized products (entry 3). InCl3
and GaCl3 furnished the mono-hydroarylated product,
PhC(CH2)(C4H3NtBu) 1a in 32% and 65% yields, respectively
(entries 4 and 5). Other Lewis acids including [Ph3C][B(C6F5)4] or
View Article Online
DOI: 10.1039/C9CC08654D
N
tBu
10 mol% B(C6F5)3
tBu
N
N
Me
R
+
CF3
8a
C6D6
tBu
N
R
1-7, 9-12
tBu
tBu
tBu
tBu
N
N
tBu
10 mol% B(C6F5)3
C6D6
N
+
Me
Me
N
N
13
Entry
Alkyne
Time
(h)
5
12
5
5
5
5
6
6
12
5
5
5
12
24
48
24
12
5
T
Product
Yield
(%)
92
94
76
86
89
89
89
(°C)
25
25
25
25
25
25
25
25
25
25
25
25
25
50
50
50
25
25
1
2 b
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
PhCCH
PhCCH
1
1
2
3
4
5
6
7
4-MeC6H4CCH
4-tBuC6H4CCH
4-ClC6H4CCH
4-BrC6H4CCH
4-CF3C6H4CCH
3-CF3C6H4CCH
2-CF3C6H4CCH
2-MeC6H4CCH
2-BrC6H4CCH
2,4-F2C6H3CCH
1-decyne
95
8a
c
64
c
d
tr
9
80
34
52
56
38
53
85
20
the phosphorus(III) dication [(terpy)PPh][B(C6F5)4]2 gave no
10
10
11
12
13
14e
reaction (entries 6, 7). Using 10 mol% B(C6F5)3, varied ratios of
phenylacetylene and N-t-butylpyrrole was subsequently
explored. Interestingly, with a 1:1 ratio of the reagents, a 35%
yield of PhCMe(C4H3NtBu)2 1 was observed (entries 8, 9).
Similarly, when the catalyst loading was decreased to 5 mol%
and the reagent ratio was increased to 1:2.5 ratio, the product
1 was obtained in 68% yield (entry 10). However, the same
mixture of phenylacetylene and N-t-butylpyrrole with 10 mol%
B(C6F5)3 afforded the double hydroarylated product 1 in 92%
yield (entry 11).
1-decyne
1-octyne
cyclopropylacetylene
1,4-diethynylbenzene
PhCCH
All reactions were performed with alkynes (0.10 mmol, 1.0 equiv.), N-t-
butylpyrrole (0.25 mmol, 2.5 equiv.) and B(C6F5)3 (10 mol%, 5.1 mg) in C6D6 (0.5
mL). b The reaction of phenylacetylene (0.51 g, 5 mmol) with N-t-butylpyrrole (1.54
g, 12.5 mmol) in the presence of 10 mol% B(C6F5)3 afforded 1.64 g of 1 in a 94%
isolated yield. c 7.4:1 mixture of bis and mono-hydroarylated product observed by
d
NMR spectroscopy; the products were not separable. only traces of the mono-
Table 1 Optimization of reaction conditions.a
tBu
e
hydroarylated product seen.
PhCMe(C4H3NAd)2.
N-adamantyl pyrrole was used to yield
N
tBu
Using the optimal reaction conditions, the reactivity of a
series of alkynes with N-t-butylpyrrole in C6D6 was investigated.
Double hydroarylation of phenylacetylene or alkynes with
electron-donating substituents in the presence of 10 mol% of
B(C6F5)3 proceeded in 5 hours at room temperature to give the
corresponding products RC6H4CMe(C4H3NtBu)2 (R = 4-Me 2, 4-
tBu 3) (Table 2, entries 3 and 4) in 76% and 86% yields,
respectively. Moreover, various alkynes with electron-
withdrawing substituents reacted to provide the corresponding
double hydroarylated products RC6H4CMe(C4H3NtBu)2 (R = 4-Cl
4, 4-Br 5, 4-CF3 6, 3-CF3 7) (Table 2, entries 5−8) in good to
excellent yield (89−95%) in 5−6 h. Employing the more sterically
demanding alkyne 2-CF3C6H4CCH as a substrate under standard
conditions, only the mono-hydroarylated product 2-
(CF3)C6H4C(CH2)(C4H3NtBu) 8a was detected even with
prolonged reaction time (12 h) (Table 2, entry 9). When the less
sterically demanding alkyne 2-MeC6H4CCH was used as a
substrate, a 7.4:1 ratio of double hydroarylated product 2-
MeC6H4CMe(C4H3NtBu)2 and mono-hydroarylated product 2-
N
10 mol% Cat.
C6D6, RT
Me
+
+
tBu
N
tBu
N
A
B
1a
1
Entrya
Cat.
-
BF3∙Et2O
Ga(C6F5)3
InCl3
A:B
Yield
(1 b)
0
0
0c
-
-
0
0
35
82
68
92
Yield
(1a b)
1
2
3
4
5
6
7
8
9
1:2.5
1:2.5
1:2.5
1:2.5
1:2.5
1:2.5
1:2.5
1:1
-
-
-
32
65
-
-
-
-
-
GaCl3
[Ph3C][B(C6F5)4]
[(terpy)PPh][B(C6F5)4]2
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
1:2
1:2.5
1:2.5
10d
11
-
a All reactions were performed with phenylacetylene (A, 0.10 mmol, 1.0 equiv.), N-
t-butylpyrrole (B, 0.25 mmol, 2.5 equiv.) and B(C6F5)3 (10 mol%, 5.1 mg) in C6D6 (0.5
mL) at 25 °C for 5 hours. Isolated yield. The resulting reaction mixture was
complex. d 5 mol% B(C6F5)3.
b
c
2 | J. Name., 2012, 00, 1-4
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MeC6H4(CH2)(C4H3NtBu) (Table 2, entry 10) was observed as catalysis. This thus limits hydroarylation to pyrrole / B(C6F5)3
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evidenced by 1H NMR data. In contrast, the alkyne 2-BrC6H4CCH that are FLPs. These results are also consistent with our
DOI: 10.1039/C9CC08654D
yielded only trace amounts of the mono-hydroarylated product previous work demonstrating stoichiometric C-C bond
with no sign of the bis-hydroarylated species (Table 2, entry 11). formation in FLP addition reactions of sterically encumbered
Nonetheless, the use of 2,4-F2C6H3CCH, afforded the double pyrroles and B(C6F5)3 to alkynes to afford zwitterionic species of
hydroarylated product 2,4-F2C6H3CMe(C4H3NtBu)2 9 in 80% the form (C4H3NtBu)C(Ar)=CH(B(C6F5)3).
yield (Table 2, entry 12). The alkyl-substituted alkynes 1-decyne,
tBu
1-octyne and cyclopropylacetylene also reacted with N-t-
butylpyrrole in the presence of B(C6F5)3 to give the products
tBu
N
N
tBu
N
C
10 mol%
(a)
Ph
Me
Ph
+
RCMe(C4H3NtBu)2 (R = C8H17 10, C6H13 11, C3H5 12) albeit in
reduced yield of 38−56% (Table 2, entries 13−16). Raising the
reaction temperature to 60 °C and lengthening the reaction
time did not improve these yields.
Remarkably, the tetra-hydroarylation transformation with
1,4-diethynylbenzene and N-t-butylpyrrole proceeds, affording
the tetra-hydroarylated product 2,4-C6H4(CMe(C4H3NtBu)2)2 13
in 53% yield (Table 2, entry 17). All products were isolated via
standard workup, and fully characterized spectroscopically. In
addition, compounds 5 and 13 were characterized by single
crystal X-ray crystallography (Figure 1). Gratifyingly, these
double hydroarylation reactions could also be performed on a
C6D6, rt, 5 h
tBu
N
Ph
B(C6F5)3
B
A
1
C
0.1 mmol
0.25 mmol
92% yield
tBu
N
tBu
Me
Ph
N
C
+
(b)
C6D6, rt, 5 h
tBu
N
1
0.05 mmol 0.10 mmol
88% yield
10 mol% B(C6F5)3
C6D6, rt, 12 h
tBu
N
Ph
(c)
+
no reaction
N
1a
tBu
B
gram-scale. For example,
a
reaction of 0.51
g
of
0.1 mmol
0.2 mmol
phenylacetylene with 1.54 g of N-t-butylpyrrole in the presence
of 10 mol% B(C6F5)3 afforded 1.64 g of 1 in a 94% isolated yield
(Table 2, entry 2).
Scheme 2 Control experiments involving the hypothetical intermediates C and 1a
under the standard reaction conditions.
In considering the mechanism of these hydroarylations
reactions, we speculated that the product of stoichiometric FLP
addition could be an intermediate in the present catalytic cycle.
To
probe
this
possibility,
the
zwitterion
(C4H3NtBu)C(Ph)=CH(B(C6F5)3) (C) was prepared by our
literature method.19 Compound C is known to undergo a 1,1-
carboboration on standing affording a bicyclic product,19
however immediate addition of stoichiometric N-t-butylpyrrole
gives the double hydroarylated product PhCMe(C4H3NtBu)2 in
88% yield (Scheme 2b). Moreover, 10 mol% of C catalyses the
double hydroarylation of phenylacetylene with N-t-butylpyrrole
providing 1 in same yield (92 %) after 5 hours at room
temperature (Scheme 2a) as described above. These
observations together with the inability of Brønsted acids to
catalyze these hydroarylations support the view that the Lewis
acid initiates addition of N-t-butylpyrrole and alkyne affording
C. The acidity of the pyrrole protons in this zwitterionic
intermediate C prompts proton migration to the acetylenic
carbon adjacent boron with concurrent rearomatization of the
pyrrole fragment, affording the intermediate D. A subsequent
addition of the pyrrole to the intermediate D affords an
intermediate E and proton migration yields the double
hydroarylated product with the liberation of borane for further
catalysis (Scheme 3, pathway 1).
Figure 1 POV-Ray depictions of the molecular structures of (a) 5 and (b) 13;
hydrogen atoms are omitted for clarity. Colour scheme: C: black, N: blue, Br: red-
brown.
Efforts to effect the corresponding B(C6F5)3 mediated
hydroarylation of phenylacetylene using furan, thiophene or
the unsubstituted pyrrole were unsuccessful. N-methyl pyrrole,
N-iso-propyl pyrrole and N-iPr3Si pyrrole yielded only trace
amounts of the double hydroarylated product, while use of N-
Boc pyrrole only led to deprotection of the pyrrole. However,
use of bulky N-adamantyl pyrrole afforded PhCMe(C4H3NAd)2
14 in 85% yield (Table 2 entry 18). These observations suggest
that the binding of donors to the Lewis acid B(C6F5)3 precludes
An alternative possibility involves the liberation of the
borane and the intermediate olefin. To probe this, the mono-
hydroarylated product 1a was isolated from the reactions of
InCl3 and GaCl3. Subsequent reaction with pyrrole in the
presence of B(C6F5)3 (Scheme 2c) showed no double-
hydroarylated product. Similarly, attempts to use 2-
vinylpyridine, styrene or 1,1-diphenylethylene as surrogates for
1a yielded none of the expected hydroarylated products. The
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-4 | 3
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2.
3.
D. W. Stephan, Science, 2016, 354, aaf7229.View Article Online
reaction of 1a with pyrrole in the presence of C or B(C6F5)3/C
showed no double hydroarylation product. These observations
discount this alternative mechanism (Scheme 3, pathway 2), in
favour of pathway 1. The formation of the mono-hydroarylated
product 8a is presumably the result of unfavourable steric
demands, precluding further substitution in the ortho-CF3-
substituted analogue of intermediate D.
(a) J. Lam, K. M. Szkop, E. Mosaferi and D. W. Stephan,
DOI: 10.1039/C9CC08654D
Chem. Soc. Rev., 2019, 48, 3592−3612; (b) W. Meng, X.
Feng and H. Du, Acc. Chem. Res., 2018, 51, 191-201.
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4.
5.
6.
7.
tBu
N
tBu
N
Ph
Ph
+
+
A
A
+
B
+
B
B(C6F5)3
1
1
B(C6F5)3
tBu
N
tBu
H
H
N
tBu
8.
9.
N
Ph
B(C6F5)3
H
H
H
tBu
Pathway 1
N
Pathway 2
H
H
H
B(C6F5)3
tBu
N
Ph
tBu
B(C6F5)3
Ph
E
N
H
H
E
tBu
H
N
Ph
B(C6F5)3
B(C6F5)3
+
H
C
tBu
tBu
N
H
N
10.
11.
B(C6F5)3
Ph
Ph
tBu
N
D
1a
Scheme 3 Proposed reaction pathways for the double hydroarylation of alkyne by
N-t-butylpyrrole.
Conclusions
12.
13.
In summary, we have developed a facile and efficient metal-
free method for the hydroarylation of alkynes with sterically
encumbered pyrroles affording the products 1-14.
Mechanistically these reactions proceed via an FLP addition
reaction of the Lewis acid B(C6F5)3 and the Lewis base pyrroles
to the alkyne. This methodology tolerates electron-rich and
electron-poor substituents on arylalkynes as well as alkyl
alkynes. The protocol employs mild reaction conditions and is
effective on gram-scales. The wider application of FLP and main
group Lewis acid reactivity in organic chemistry is a focal point
of our on-going efforts.
14.
15.
16.
17.
18.
Conflicts of interest
There are no conflicts to declare.
19.
20.
(a) S. Chitnis, F. Kirscher and D. W. Stephan, Chem. Eur. J.,
2018, 4, 6543 –6546; (b) R. J. Andrews, S. S. Chitnis and D.
W. Stephan, Chem. Commun., 2019, 55, 5599--5602.
Acknowledgements
NSERC of Canada is thanked for financial support. D.W.S is
grateful for the award of a Canada Research Chair and a Visiting
Einstein Fellowship at TU Berlin. K.L.B is grateful for the award
of an NSERC CGS-D scholarship.
Notes and references
1.
(a) D. W. Stephan, Acc. Chem. Res., 2015, 48, 306-316; (b)
D. W. Stephan, J. Am. Chem. Soc., 2015, 137, 10018-
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