Palladium-catalyzed substitution of (coumarinyl)methyl acetates with C-, N-, and S-nucleophiles

Article abstract of DOI:10.3762/bjoc.8.133

The palladium-catalyzed nucleophilic substitution of (coumarinyl)methyl acetates is described. The reaction proceeds though a palladium ?-benzyl-like complex and allows for many different types of C-, N-, and S-nucleophiles to be regioselectively added to the biologically active coumarin motif. This new method was utilized to prepare a 128-membered library of aminated coumarins for biological screening.

Full text of DOI:10.3762/bjoc.8.133

Palladium-catalyzed substitution of
(coumarinyl)methyl acetates with C-, N-,
and S-nucleophiles
Kalicharan Chattopadhyay1,2, Erik Fenster2, Alexander J. Grenning1
and Jon A. Tunge*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1200–1207.
1Department of Chemistry, University of Kansas, 1251 Wescoe Hall
Drive, Lawrence, KS 66045-7528, USA and 2KU Chemical
Methodologies and Library Development Center of Excellence,
University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas
66047, USA
doi:10.3762/bjoc.8.133
Received: 26 April 2012
Accepted: 26 June 2012
Published: 27 July 2012
Email:
This article is part of the Thematic Series "Recent developments in
chemical diversity".
Jon A. Tunge* - tunge@ku.edu
* Corresponding author
Guest Editor: J. A. Porco Jr.
Keywords:
© 2012 Chattopadhyay et al; licensee Beilstein-Institut.
License and terms: see end of document.
benzylation; catalysis; coumarin; chemical diversity; decarboxylative;
palladium; substitution
Abstract
The palladium-catalyzed nucleophilic substitution of (coumarinyl)methyl acetates is described. The reaction proceeds though a
palladium π-benzyl-like complex and allows for many different types of C-, N-, and S-nucleophiles to be regioselectively added to
the biologically active coumarin motif. This new method was utilized to prepare a 128-membered library of aminated coumarins for
biological screening.
Introduction
Coumarins are privileged chemical motifs found in many Substituted methylcoumarins, including aminomethyl-
natural products and drug molecules [1-15]. Because of their coumarins are important biologically active motifs (Figure 1)
biological significance, there have been many classic and [6-15]. Substitution of methylcoumarins to form compounds
modern methods developed for the synthesis of these useful akin to 2 typically utilizes the corresponding halomethyl-
core structures [16-24]. Due to the ambiphilic nature of the coumarin and highly stabilized nucleophiles or amines [6-15].
heterocyclic ring of coumarin, this core-structure undergoes a Due to the sensitivity and toxicity of related benzyl halides,
diverse array of coupling reactions, such as halogenations [25], there has been interest in catalytically activating hydroxy-
cycloadditions [26-32], conjugate additions [33-37] and tran- methylarene and heteroarenes (e.g., benzyl alcohol derivatives)
sition-metal-catalyzed C–H activation/coupling reactions toward reactions with nucleophiles [45-55]. In this realm, we
[38-44].
[53,55] and others [54] have focused efforts on catalyzing
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Beilstein J. Org. Chem. 2012, 8, 1200–1207.
Table 1: Reaction development for the coupling of in situ generated
enolates and coumarin π-benzyl complexes.
Entry
Catalyst
Solvent
Conv. (%)
Figure 1: Representative biologically active aminomethylcoumarins.
1
2
3
4
5
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd2dba3, dppf
toluene
THF
68
70
MeCN
MeCN
95 (88% yield)
benzylic substitutions with less-stabilized (DMSO pKa ~ 20–30)
nucleophiles through decarboxylative coupling. In the present
context, we hypothesized that a broad diversity of nucleophiles
could be added to the (coumarinyl)methyl core through palla-
dium-catalyzed couplings of hydroxymethylcoumarin deriva-
88
10
Pd2dba3, BINAP MeCN
tives. Herein we report that hydroxymethylcoumarin deriva- tions (5 mol % Pd(PPh3)4, MeCN, rt, Table 1, entry 3), a variety
tives of type-1 undergo selective nucleophilic substitution at the of enolate nucleophiles could be selectively coupled with
exo-methyl position with C-, N-, and S-based nucleophiles by coumarin electrophiles (Scheme 2). For example, acetone,
using palladium(0) as a catalyst (Scheme 1). In addition, given acetophenone and 1,1,1-trimethylacetone enolates can all be
the known biological activity of aminomethylcoumarins, we generated and coupled with the coumarin electrophile. Impor-
prepared a 128-member library of aminated coumarins using tantly, selective monobenzylation was achieved for each
rapid automated synthesis.
example. Regarding the coumarin moiety, the decarboxylative
coupling was compatible with a variety of simple substitutions,
including methoxy (2c,d,g,h), chloro (2g,h) and naphthyl (2e)
coumarins.
Scheme 1: Approach to diversely substituted coumarins.
Results and Discussion
To begin investigating the diversification of hydroxymethyl-
coumarins, we chose to investigate their decarboxylative
couplings of enolates. We have previously shown that decar-
boxylative benzylation (DcB) is a useful method for the addi-
tion of less-stabilized enolate anions to a benzyl functionality
[46,53-55]. Thus, we envisioned being able to add various
ketone enolates to the exo-methyl position of the coumarin core
by this method (Table 1). Screening of reaction conditions
showed that, in contrast to common decarboxylative benzyla-
tion conditions (Table 1,entries 1 and 2) under which nonpolar
solvents give the best DcB, the selective mono-alkylation of the
enolate proceeds in highest yield in acetonitrile (2a, Table 1,
entry 3). Bidentate ligated palladium complexes gave mixed
results: the Pd/dppf complex (Table 1,entry 4) catalyzed the
reaction smoothly, while little product was seen on using the
Pd/BINAP complex (Table 1,entry 5). Under the best condi-
Scheme 2: Scope of the decarboxylative coupling.
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Beilstein J. Org. Chem. 2012, 8, 1200–1207.
While the decarboxylative coupling worked well for the
couplings of relatively nonstabilized enolate nucleophiles, we
wished to find a more universal starting material that would
allow a broader range of nucleophiles to be coupled to the
coumarin core in an intermolecular fashion. Such a method was
deemed necessary for rapid diversification and chemical library
synthesis. Thus, we chose to investigate the Pd-catalyzed
benzylic substitution reaction of (coumarinyl)methyl acetates
with various nucleophiles. In the forthcoming sections, the
coupling of this coumarin template to arylboronic acids [56-61],
amines [47] and arylsulfinates is described [62]. Moreover,
compared to related methods for palladium-catalyzed benzylic
substitution that require specialized ligands [45,47,60-62], we
report that coumarin π-benzyl formation is easily achieved with
simple PPh3 ligated palladium.
Table 2: Palladium-catalyzed coupling of coumarinyl acetate and
phenylboronic acid.
Entry
Solvent
T (°C)
Base
Conv. (%)
1
2
3
4
5
6
7
MeOH
MeOH
MeOH
toluene
THF
65
65
65
90
65
90
90
K2CO3
KF
65
59
65
63
63
85
45
K3PO4
K2CO3
K2CO3
K2CO3
–
dioxane
dioxane
Regarding the development of the Suzuki-like coupling reac-
tion of (coumarinyl)methyl acetate and arylboronic acids, we
screened reaction conditions for the coupling of coumarin 1a
with phenylboronic acid (Table 2). We were pleased to find that
the reaction progressed reasonably well to 3a under various With the optimal conditions in hand, we next tested the scope of
conditions. For example, inorganic bases such as K2CO3, the reaction using various boronic acid and coumarinyl acetates
K3PO4, and KF all effected the reaction equally well in (Scheme 3). Regarding the boronic acid coupling partner, modi-
methanol. A brief screen of solvents showed that the nonpolar fications including fluoro and alkyl substitution were tolerated.
aprotic solvent 1,4-dioxane gave the highest yields of the In addition to aryl boronic acids (3a–c), the reaction was also
coupling product (Table 2, entry 6). Lastly, in the absence of extended to couplings of vinyl boronic acids having varied elec-
added base, the acetate generated upon π-benzyl formation also tronic properties (3d–e). As before, various simple substitu-
promoted the desired coupling, albeit in lower yield over the tions and electronic changes were tolerated on the coumarin
time frame allowed (12 h, Table 2, entry 7).
core.
Scheme 3: Scope of Suzuki-coupling of coumarinyl acetates.
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Beilstein J. Org. Chem. 2012, 8, 1200–1207.
Scheme 4: Coupling of (coumarinyl)methyl acetates with N- and
S-nucleophiles.
Scheme 5: Scope of the coumarinyl acetate and aryl sulfinate coupling
reaction.
In addition to the above Suzuki couplings, acetoxymethyl-
coumarins were found to undergo palladium-catalyzed substitu- tolerated various simple electronic (4b,c,e) and alkyl (4a,c,d)
tion with sulfinates and secondary amines under the previously substitution patterns.
developed palladium-catalysis conditions (Scheme 4). Impor-
tantly, related aminomethylcoumarins have been shown to have Since (coumarinyl)methylamines are known to possess interest-
significant biological activity [6-15].
ing biological activity, the scope of the amination was investi-
gated in somewhat more detail. Regarding the scope of the
Regarding the coupling reaction between aryl sulfinates and the amine and coumarinyl acetate coupling reaction, dialkyl (5a–c)
coumarinyl acetate, the reaction could be performed without the and the cyclic amines pyrrolidine (5d–5f), pyrazine (5g), and
addition of an external base, since the aryl sulfinate is adminis- morpholine (5h) were competent coupling partners (Scheme 6).
tered as its anion (Scheme 5). Phenyl (4a–c) and tolyl (4d–e) Primary amines were also compatible coupling partners,
sulfinates were viable coupling partners, giving the product although under these conditions, the reaction never went to full
sulfones in good yield. As previously noted, the coumarin core conversion (5i–k).
Scheme 6: Scope of the coumarinyl acetate and amine coupling reaction.
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Beilstein J. Org. Chem. 2012, 8, 1200–1207.
As mentioned previously, we were devising this approach to
coumarin substitution not only for the sake of new chemical
methodology, but also because this approach could easily lead
to rapid library synthesis. As aminomethylcoumarins are a bio-
logically active chemical motif [6-15], we set out to produce a
128-member library of aminated coumarins using our method,
with the goal of making unique, biologically active molecules
(Figure 2). To begin, an 8 × 16 library (coumarin C1–8 × amine
A1–16) of aminomethylcoumarins was devised for preparation
by using a Chemspeed SLT100 automated synthesizer. For
automated synthesis, the procedure and workflow was modi-
fied somewhat for optimal yield. Specifically, the loading of Pd
was lowered to 3 mol % and the quantity of amine was raised to
1.5 equivalents to ensure complete conversion. Using the work-
flow shown in Figure 2 for the reaction of coumarin C3 with
diallylamine A4, followed by filtration through a silica SPE,
produced product C3A4 in 74% yield in a test case. In addition,
the product was determined to contain 2% PPh3 impurity. Thus,
it was concluded that the entire library should be purified by
mass-directed fractionation to ensure high-quality compounds
for biological screening.
Regarding the library, coumarins and amines were chosen with
diverse structures and properties in order to access the most
chemical space within the aminomethylcoumarin family
(Figure 2). The coumarins utilized had various electronic prop-
erties and substitution patterns. A polyaromatic coumarin was
also screened (C4). Similarly, a diverse array of amines was
chosen. Aside from simple dialkylamines, cyclic and
heteroatom-containing amines were utilized. Amines with
various functional groups, such as amides, aromatics, olefins,
and alkyne substitution patterns, were also incorporated in this
library. The automated library synthesis had a success rate of
>85%, with the desired products isolated in high purity [63].
Taking center cuts by using mass-directed fractionation ensured
that compounds were isolated in >95% purity; however, purity
was achieved at some expense to the isolated yields, which
were typically lower (5–75%) than those achieved in batch
reactions. While most compounds were obtained in sufficient
quantity for biological screening, the yields were variable
(Figure 3). In particular, coumarins C6, C2 and C8 provided
lower average yields than the other coumarin cores. Thus, it
appears that 6,7-substitution of the coumarin core is somewhat
Figure 2: Library planning for amine (A) and coumarin (C) coupling
partners.
problematic. In addition, analysis of the library yields indicates Conclusion
that N-arylpiperazines A8 and A9 in addition to propargyl- In conclusion, we have developed a simple and general strategy
amine A15 were the most problematic. Perhaps unsurprisingly, for the palladium-catalyzed substitution of coumarins selec-
the amine that provided the highest yields and highest success tively at an exomethyl position. This approach allowed the
rate was diallyl amine, which was used for initial validation of coupling of various C-,N-, and S-based nucleophiles under mild
the Chemspeed method (see above). In addition, piperidine and conditions. We also utilized this approach in the synthesis of a
the simple open-chain dialkyl amines also provided good yields 128-member chemical library appropriate for biological
and high success rates.
screening.
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Beilstein J. Org. Chem. 2012, 8, 1200–1207.
Figure 3: Results for the synthesis of a 128-member library of aminated coumarins by using the Chemspeed SLT100 automated synthesizer. Puri-
fied by an automated preparative reverse-phase HPLC (Waters 2767 Mass Directed Fractionation) detected by UV (270 nm). Purity was determined
by reverse-phase HPLC (Waters Alliance 2795 system) with peak area (UV) at 214 nm.
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(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.8.133
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