Arylation of diazoesters by a transient N-H insertion organocascade

Article abstract of DOI:10.1002/anie.201304921

It takes two: A unique organocatalyzed cascade for the unsymmetric double arylation of α-nitrodiazoesters is described. This organocascade features the strategic use of carbene-activating anilines in conjunction with a urea catalyst, thus allowing for the

Full text of DOI:10.1002/anie.201304921

Angewandte
Chemie
DOI: 10.1002/anie.201304921
Organocatalysis
À
Arylation of Diazoesters by a Transient N H Insertion
Organocascade**
Tyler J. Auvil, Sonia S. So, and Anita E. Mattson*
Cascade catalysis is a powerful tool demonstrated both by
nature and chemists alike to generate complex products
The geminal diaryl functionality is frequently found in
bioactive target molecules, and several drugs already com-
mercially available to treat a variety of diseases contain this
[
1]
through sequential catalytic processes. These processes
benefit from the efficiency of catalyzing multiple steps in
a single flask for the rapid construction of molecular complex-
ity without the necessity of intermediate isolation. Autotan-
dem catalytic methods are cascade processes employed to
promote two or more distinct reactions with a single cata-
[
5]
structural motif. Specific examples include the antidepres-
sant Zoloft, and anticancer agents Femara and Etoposide.
Aware of the therapeutic significance and synthetic chal-
lenges associated with preparing unsymmetrical geminal
[
6]
diaryl frameworks, we focused the development of transient
insertion organocascade catalysis on arylation reactions of
diazo compounds. The working hypothesis was that an
appropriate amine could trigger urea-catalyzed arylation
[2]
lyst. Inspired by the demonstrated power of autotandem
[
3]
organocascade catalysis, we sought to invoke a fundamen-
tally novel sequence taking advantage of catalytic transient
carbene activation. More specifically, we envisioned activat-
[7]
reactions of diazo compounds (1) through transient NÀH
[4]
[8,9]
ing a species having carbene character by a urea-catalyzed
insertion intermediates
(4; Scheme 2). To the best of our
insertion reaction with an appropriate carbene activator to
generate a transient intermediate which would then enter
into a second distinct cycle for further functionalization
(
Scheme 1). Successful development of this process would
Scheme 2. Double arylation of nitrodiazoesters through transient NÀH
insertion organocascade catalysis.
Scheme 1. Transient carbene activation cascade catalysis.
knowledge, cascade reactions proceeding through transient
carbene insertion intermediates had not been reported at the
onset of our investigations. The testing of the feasibility of our
hypothesis began with investigating anilines as carbene
afford a straightforward organocatalytic route to synthetically
challenging structural motifs from simple starting materials,
and also expose the potential of transient carbene insertion
reactions as innovative tools in organocatalytic method
development. Herein, we detail our breakthrough discoveries
in urea-catalyzed transient NÀH insertion reactions for the
[
10]
activators of nitrodiazoesters.
Our studies began with the preparation of 2a by the
double arylation of ethyl nitrodiazoacetate (1a) with 5-
methoxyindole and 5-bromoindole in the presence of
[
11,12]
controlled functionalization of nitrodiazoesters.
20 mol% of the boronate urea 3a
and an aniline activator
(Table 1). Our first attempt afforded a disappointing 12% of
the desired adduct 2a when aniline was employed as the
carbene activator (entry 1). Further examination of the
reaction led us to identify participation of the aniline activator
in the diarylation reaction. Next, we turned to p-substituted
anilines, activators less likely to be involved in undesired
arylation pathways. Indeed, the strategic choice of activators
proved fruitful and we were delighted to find that activation
of 1a with p-anisidine gave rise to 73% of 2a (entry 2).
Additional tuning of the aniline led us to 4-fluoroaniline as an
even better carbene activator (entry 3), potentially because of
its better leaving ability. The necessity of an aniline activator
able to undergo NÀH insertion with the diazoester 1a is clear:
[
*] T. J. Auvil, S. S. So, Prof. Dr. A. E. Mattson
Department of Chemistry and Biochemistry
The Ohio State University
100 W. 18th Ave., Columbus, OH 43210 (USA)
E-mail: mattson@chemistry.ohio-state.edu
Homepage: http://mattson.group.chemistry.ohio-state.edu/
[
**] The Ohio State University Department of Chemistry and Biochem-
istry and Donors of the American Chemical Society Petroleum
Research Fund are gratefully acknowledged for supporting this
research. Prof. J. Fox (University of Delaware) is thanked for
insightful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304921.
a 0% yield of 2a was observed when 2,6-dimethylaniline, an
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[
a]
Table 1: Study of aniline activators in transient NÀH insertion catalysis.
activated iminoglyoxylate then generates the transient NÀH
[13]
insertion intermediate 4a. In Cycle II, the a-amino ester is
activated through dual binding with the urea catalyst,
conceivably through the complex D. The urea-assisted sub-
stitution of the N-bound 4-fluoroaniline with 5-bromoindole
then generates the desired double arylated ester product 2a
upon aromatization, thus introducing 3a and 4-fluoroaniline
[
c]
Entry
1
2
3
4
0
5
[14,15]
back into the catalytic cycle.
In support of the proposed reaction pathway, the enan-
Ar-NH2
none
0
tioenriched transient NÀH insertion intermediate 4a was
isolated and subjected to the reaction conditions
[
b]
Yield [%]
12
73
78
[
13]
(Scheme 4).
From this experiment we found that 4a is
[
[
a] Reactions performed at 2m in DCE. [b] Yield of isolated product.
c] 0% of 2a was generated in the absence of 3a.
aniline too sterically encumbered to undergo NÀH insertion,
was tested as the activator (entry 4). From the results of two
critical control experiments, we concluded that the urea
catalyst and aniline activator working in combination are
uniquely able to effect the double arylation of 1a. Specifically,
the first control experiment resulted in no desired 2a when an
NÀH insertion activator was excluded from the reaction
Scheme 4. Urea-catalyzed arylation of the enantioenriched 4a.
system (entry 5). Similarly, in the second control, none of 2a
was observed if the boronate urea 3a was excluded from the
optimized reaction conditions (entry 3).
A plausible organocascade for the unsymmetrical diary-
lation of nitrodiazoesters, depicted in Scheme 3, takes
advantage of the urea catalyst (3a) in two sequential roles:
converted into a racemic mixture of 2a in 91% yield under
the influence of 10 mol% 3a. Importantly, the urea is
essential for this reaction: no reaction was observed if the
urea catalyst was excluded. The loss of enantiomeric excess in
1
) nitrodiazoester activation for NÀH insertion/multicompo-
nent coupling and 2) a-amino ester activation for arylation. In
the first cycle, 1a is activated by 3a and 4-fluoroaniline, thus
affording the NÀH insertion adduct B through the hydrogen-
the conversion of 4a into 2a suggests that an S 1-like
N
mechanism is operating for the conversion of D into E.
Additional control experiments conducted in our laboratory
suggest the stereocenter in the di-a-arylester 2 is stable to the
bonded complex A. Upon liberation of nitrous acid, B is
converted into the iminoglyoxylate C. The arylation of the
Scheme 3. Plausible reaction pathway for the unsymmetric double arylation of nitrodiazoesters through a transient NÀH insertion intermediate.
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reaction conditions, thus further supporting an S 1-like
sequence, albeit longer reaction times were required for
N
reaction pathway as opposed to an S 2 and subsequent
step 2. o-Iodoaniline gave rise to 56% of 2o after 72 hours at
608C. Additionally, pyrroles operated moderately well in the
cascade sequence (2q,r).
The reaction also proved tolerant of a wide range of
nitrodiazoesters (1; Table 2). Sterically demanding nitrodi-
azoacetates (1b–d) were well tolerated, thus providing the
N
[
16]
racemization.
Stoichiometric amounts of inexpensive 4-
fluoroaniline were necessary to prevent the generation of
undesired symmetrical a-diarylated esters, which were
formed when using catalytic quantities of the carbene
activator. However, the feasibility of transient insertion
cascades requiring only catalytic amounts of the carbene
activator is a promising avenue for future studies not
featuring double functionalization.
[
a]
Table 2: Diazoester substrate scope.
The autotandem catalytic double arylation of 1a was
easily extended to a remarkably wide array of substrates to
give rise to the di-a-aryl esters 2 in excellent yields
(
Scheme 5). Unsymmetrical double indolylation was effi-
[
b]
Entry
1
Diazoester
1
Product
Yield [%]
88
1b
2s
2
3
1c
1d
2t
75
63
2u
4
1e
1 f
2v
60
68
[
c]
5
2w
[a] Reactions performed at 2m in DCE in step 2. [b] Yield of isolated
product. [c] 2w isolated as a 1:1 mixture of diastereomers.
products 2s–u in up to 88% yield. Substituted benzyl esters
were also easily incorporated into the double arylation
sequence. For instance, the product 2v was prepared in
6
0% yield from the diazoester 1e (entry 4). The (À)-menthol-
derived nitrodiazoester 1 f gave rise to 2w in 68% yield as
a 1:1 mixture of diastereomers.
In summary, we have discovered that the intermolecular
double arylation of nitrodiazoesters is achieved through
a unique urea-catalyzed transient NÀH insertion organo-
Scheme 5. Substrate scope for the double arylation of ethyl nitro-
diazoacetate. [a] Reactions performed at 2m in DCE in step 2. See
the Supporting Information for details [b] Yield of isolated products.
[
a,b]
cascade. The reaction can accommodate a remarkable
number of anilines and indoles, thus enabling the efficient
preparation of a large family of unsymmetric a-diaryl esters.
The proposed reaction pathway invokes the urea catalyst in
two distinct, sequential catalytic cycles: NÀH insertion and a-
[c] 72 h reaction time in step 2. DCE=1,2-dichloroethane.
ciently achieved in up to 97% yield when indoles were used as
both the first and second nucleophiles (2a–e). Anilines were
also easily incorporated into the diarylation reaction. Exam-
ples of indoles working in combination with anilines afforded
the products 2 f–j in excellent yields of up to 98% yield. A
variety of anilines, as both the first and second nucleophiles,
are excellent reactants for this coupling reaction as well. Even
sterically encumbered, electron-rich anilines were well tol-
erated in the diarylation reaction. For instance, 3,5-dimeth-
oxyaniline gave rise to 98% of 2n. Select halogenated anilines
were capable of being integrated into the double arylation
arylation. We anticipate transient insertion catalysis may be
a fruitful new direction in organocatalysis. Ongoing inves-
tigations in our laboratory are focused on exploring the
possibility of enantioselective transient NÀH insertion catal-
ysis.
Received: June 7, 2013
Revised: July 12, 2013
Published online: && &&, &&&&
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Keywords: carbenes · diazo compounds · insertion ·
organocatalysis · synthetic methods
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4
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Communications
Organocatalysis
T. J. Auvil, S. S. So,
A. E. Mattson*
&&&& — &&&&
Arylation of Diazoesters by a Transient
NÀH Insertion Organocascade
It takes two: A unique organocatalyzed
cascade for the unsymmetric double
arylation of a-nitrodiazoesters is de-
scribed. This organocascade features the
strategic use of carbene-activating ani-
lines in conjunction with a urea catalyst,
thus allowing for the synthesis of phar-
maceutically attractive a-diarylesters
through a transient NÀH insertion pro-
cess.
6
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