Dendralene synthesis: Rhodium(III)-catalyzed alkenyl C-H activation and coupling reaction with allenyl carbinol carbonate

Article abstract of DOI:10.1002/anie.201306754

[3]DendrAl(l)ene! A new synthesis of [3]dendralenes is based on a Rh ^III-catalyzed alkenyl C-H activation and coupling reaction with allenyl carbinol carbonates (see scheme; DG=directing group). A variety of [3]dendralenes with diverse substitution patterns are accessible with good efficiency. The reaction is highly stereoselective and compatible with different directing groups and numerous functional groups. Copyright

Full text of DOI:10.1002/anie.201306754

Angewandte
Chemie
DOI: 10.1002/anie.201306754
À
C H Activation
À
[3]Dendralene Synthesis: Rhodium(III)-Catalyzed Alkenyl C H
Activation and Coupling Reaction with Allenyl Carbinol Carbonate**
Honggen Wang, Bernhard Beiring, Da-Gang Yu, Karl D. Collins, and Frank Glorius*
Dedicated to Professor Henning Hopf
Dendralenes, also known as acyclic cross-conjugated poly-
enes, have long been neglected as an important class of highly
unsaturated hydrocarbons.[1] However, recent years have seen
growing interest in dendralene chemistry due to their unique
roles in polymer chemistry,[2] theoretical chemistry,[3] and
synthetic chemistry.[4] Not surprisingly, [3]dendralenes or
cross-conjugated trienes, the simplest dendralenes,[5] have
attracted the most attention.[2–4] Their importance has been
reflected by their occurrence as a motif in an increasing
number of natural products,[6] and, notably, their engagement
in diene-transmissive Diels–Alder reactions (DTDA),[7]
which facilitate the rapid construction of complex multicyclic
frameworks (Scheme 1).[8] So far, there are a handful of
procedures available for the synthesis of [3]dendralenes.[9]
The drastic conditions typically used and limited substitution
patterns call for more straightforward and efficient alterna-
tives.
III
À
Over the past few years, Rh -catalyzed direct C H
functionalization has emerged as a versatile tool to effect
various C C and C X bond-forming reactions.[10–12] We have
recently reported a RhIII-catalyzed allylation of aromatic
À
À
[13]
À
C H bonds using allyl carbonates as electrophiles.
The
reaction was believed to proceed by means of a novel olefin
insertion/b-oxygen elimination reaction mechanism (Sche-
me 2a).[14] Intriguingly, this type of reaction pathway has been
À
rarely exploited in C H activation reactions. In addition, we
developed an oxidative annulation of allenes with N-(piv-
aloyloxy)benzamides in the presence of a RhIII catalyst,
wherein the carborhodation of monosubstituted allenes
À
delivered an intermediate in which a C C bond has been
formed at the central carbon atom (Scheme 2b).[15] On the
basis of these two reactions, we speculated that the replace-
ment of allyl carbonates with allenyl carbinol carbonates
would lead to an analogous allene carborhodation/b-oxygen
elimination sequence, providing products that incorporate
a valuable diene functionality. Herein, we disclose a RhIII-
catalyzed synthesis of [3]dendralenes based on an alkenyl
À
C H activation and coupling reaction with allenyl carbinol
carbonates (Scheme 2c). The reaction was extended to
À
aromatic C H activation to generate diene-substituted
arenes with good efficiency.[16]
À
We focused our efforts on alkenyl C H functionalization
Scheme 1. [3]Dendralenes in natural products and synthesis.
due to the high value of cross-conjugated triene products and
the intrinsic challenges in their synthesis. We initiated our
study by examining the reaction of cinnamamide 5a with
branched 1-methyl-substituted allenyl carbinol carbonate 6a.
To our delight, we found that the reaction proceeded
smoothly in the presence of [Cp*Rh(MeCN)3](SbF6)2
(5.0 mol%) and PivOH (0.5 equiv) in CH2Cl2 at 608C,
giving 5-substituted [3]dendralene 7aa in 79% yield as
a single stereoisomer [Eq. (1)].[17] Furthermore, the reaction
was found to be scalable and practical as 71% yield was
obtained when the reaction was performed at a 5-mmol scale
without the exclusion of air and moisture.[18] It is worth
mentioning that acrylamides represent an important and
readily accessible class of building blocks.
[*] Dr. H. Wang, B. Beiring, Dr. D.-G. Yu, Dr. K. D. Collins,
Prof. Dr. F. Glorius
Universitꢀt Mꢁnster, Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.oc/glorius/
index.html
[**] We thank N. Kuhl, N. Schrçder, C. Nimphius, C. Weitkamp, and D.
Dietrich for experimental support. This work was supported by the
European Research Council under auspices of the European
Community’s Seventh Framework Program (FP7 2007–2013/ERC
grant agreement no. 25936) and by the Alexander von Humboldt
Foundation (D.-G.Y.). The research of F.G. has been supported by
the Alfried Krupp Prize for Young University Teachers of the Alfried
Krupp von Bohlen und Halbach Foundation.
The scope of the reaction was then explored.[19] As shown
in Table 1, a variety of acrylamides and coupling partners with
different functional groups and substitution patterns were
examined. Cinnamamides 5b and 5c bearing OMe and CF3
groups, respectively, gave the corresponding [3]dendralene
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306754.
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batch of catalyst and 6 was added. In all cases,
excellent stereoselectivities (> 20:1) were
observed.
Realizing the current method was limited to
the synthesis of 5-substituted [3]dendralenes, we
reasoned that the use of linear substituted allenyl
carbinol carbonates might provide 3-substituted
[3]dendralenes, thus leading to more diverse
products. As expected, the reaction of a-substi-
tuted 5 f with 8 gave the desired 9a in 28% yield,
with complete Z selectivity at the newly formed
double bond [Eq. (2)]. Surprisingly, the reaction of
a,b-disubstituted acrylamide 5d gave a higher
yield (64%), but with complete reversal of ste-
reoselectivity—an E isomer was formed instead
[Eq. (3); vide infra for rationale of stereoselectiv-
ity].
À
We also sought to investigate the C H func-
tionalization on aromatic rings. The reaction was
demonstrated to be a reliable method to introduce
diene functionality to benzamides 10 under modi-
fied reaction conditions with 15 mol% Cu(OAc)2
as an additive (Table 2).[12t] Cu(OAc)2 might serve
Scheme 2. Rational design on RhIII-catalyzed diene incorporation. DG=directing
group, Piv=pivaloyl.
products 7ba and 7ca in good yields. Interestingly, whereas
cinnamamide 5d with a 2-phenyl substitutent showed excel-
lent reactivity, 2-methyl cinnamamide 5e reacted much more
slowly, providing 7ea in comparatively much lower yield.
However, 2-methacrylamide 5 f was a good substrate for this
transformation, though only a moderate yield was obtained
due to decomposition of the product 7 fa. We would like to
highlight two examples: the 3-bromo-substituted acrylamide
5g underwent smooth reaction to give 7ga in 78% yield. The
bromo substituent provides a valuable and reliable handle for
further transformations. Fumaric acid derivative 5h, repre-
senting a highly electron-deficient alkene, also gave the
product without difficulty. Gratifyingly, the reaction was not
limited to substrates with tertiary amide directing groups. For
instance, secondary N-n-butyl (5i) and N-methyl (5j) amides
reacted with comparable efficiency. Importantly, it was found
that Weinreb amide 5k also gave a reasonable yield, thus
expanding opportunities for functional group manipulation.
The reaction also shows broad substrate scope in terms of
the allenyl carbinol carbonate partner. Thus, unsubstituted
(R5 = H) and alkyl- (R5 = alkyl) and aryl-substituted (R5 =
Ph) allenyl carbinol carbonates 6 all delivered the corre-
sponding products in good yields. In some cases, deactivation
of catalyst was observed. To ensure high conversion, a second
as an oxidant to regenerate the active RhIII catalyst, which is
deactivated by an unproductive pathway. Many functional
groups, including ester, trifluoromethyl, formyl, methoxy,
chloro, and bromo groups, were well tolerated. The use of less
hindered N,N-diethylbenzamide (10b) or N,N-dimethylbenz-
amide (10c) also assured a good yield.
Finally, to demonstrate the synthetic utility of the
[3]dendralene product, Diels–Alder reactions with different
dienophiles were conducted (Scheme 3). The reaction of 7aa
with N-methylmaleimide (12) resulted in the formation of
cyclohexene 13 in 81% yield, with a diastereoselectivity of
8:1. The reaction takes place exclusively at the less sterically
hindered, electron-rich diene moiety. Analogously, the use of
dimethyl acetylenedicarboxylate (14) furnished the cyclo-
hexadiene 15 in good yields.
À
A putative reaction pathway was proposed. The C H
activation is assisted by the amide directing group and PivOH
2
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Table 1: Reaction scope.[a]
Table 2: Reaction scope.[a]
[a] Reaction conditions: 10 (0.4 mmol), 6a (0.8 mmol), [Cp*Rh-
(MeCN)3](SbF6)2 (5.0 mol%), Cu(OAc)2 (15 mol%), PivOH (1.0 equiv),
CH2Cl2 (2 mL), 608C, 3–24 h, yield of isolated product. [b] Without
Cu(OAc)2, 6a (0.48 mmol) was used.
eclipsing strain are pronounced. With a sterically hindered
substrate, eclipsing strain dominates, and the E isomer forms
selectively. In contrast, allylic 1,3-strain becomes more
pronounced when a less bulky substrate was used. As
a result, the Z isomer was observed.[21]
[a] Reaction conditions: 5 (0.4 mmol), 6 (0.44 mmol), [Cp*Rh-
(MeCN)3](SbF6)2 (5.0 mol%), PivOH (0.5 equiv), CH2Cl2 (2 mL), 608C,
2-24 h, yields of isolated products. [b] A second batch of [Cp*Rh-
(CH3CN)3](SbF6)2 (5 mol%) and 6 (0.44 mmol) was used. [c] 408C.
In conclusion, we have developed a RhIII-catalyzed
À
alkenyl C H activation and coupling reaction with allenyl
carbinol carbonates. This method provides a novel and
straightforward synthesis of [3]dendralenes. A variety of
[3]dendralenes with diverse substitution patterns are acces-
sible with good efficiency. In addition, the reaction is highly
stereoselective and compatible with different directing groups
and numerous functional groups. We also demonstrated that
the title reaction is robust, practical, and scalable. The method
was successfully extended to the synthesis of diene-substi-
tuted arenes. We expect this new method to complement
existing methodology and find its application in [3]dendralene
chemistry.
(Scheme 4a). Allene coordination followed by regioselective
carborhodation leads to a p-allyl rhodium(III) complex A,
which is in equilibrium with the h1-allyl rhodium(III) complex
B. A subsequent formal b-oxygen elimination delivers the
final product and regenerates the RhIII catalyst.[20] The
stereoselectivity in the 3-substituted [3]dendralene products
8 deserves some explanation (Scheme 4b). Four different
RhIII complexes could be possibly formed after the carbo-
rhodation. Within these complexes, allylic 1,3-strain and
Scheme 3. Diels–Alder reaction of [3]dendralene 7aa.
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Received: August 1, 2013
Published online: && &&, &&&&
À
Keywords: [3]dendralenes · C C coupling ·
.
À
C H activation · dienes · trienes
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[17] No product was detected when the RhIII catalyst was not added.
[18] A catalyst loading of 7.5 mol% was used to drive the reaction to
higher conversion. See the Supporting Information for details.
[19] We screened for robustness (see the Supporting Information for
details); a) K. D. Collins, F. Glorius, Nat. Chem. 2013, 5, 597;
b) K. D. Collins, F. Glorius, Tetrahedron 2013, 69, 7817; c) N.
Kuhl, N. Schrçder, J. Wencel-Delord, F. Glorius, Org. Lett. 2013,
15, 3860.
[20] a) A. S. Tsai, M. Brasse, R. G. Bergman, J. A. Ellman, Org. Lett.
2011, 13, 540; b) C. Feng, D. Feng, T.-P. Loh, Org. Lett. 2013, 15,
3670; c) Z. Qi, X. Li, Angew. Chem. 2013, 125, 9165; Angew.
Chem. Int. Ed. 2013, 52, 8995.
[21] a) R. Hoffmann, Chem. Rev. 1989, 89, 1841.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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.
Angewandte
Communications
Communications
À
C H Activation
H. Wang, B. Beiring, D.-G. Yu,
K. D. Collins, F. Glorius*
&&&& — &&&&
[3]Dendralene Synthesis: Rhodium(III)-
À
Catalyzed Alkenyl C H Activation and
Coupling Reaction with Allenyl Carbinol
Carbonate
[3]DendrAl(l)ene! A new synthesis of
diverse substitution patterns are acces-
sible with good efficiency. The reaction is
highly stereoselective and compatible
with different directing groups and
numerous functional groups.
[3]dendralenes is based on a RhIII-
À
catalyzed alkenyl C H activation and
coupling reaction with allenyl carbinol
carbonates (see scheme; DG=directing
group). A variety of [3]dendralenes with
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!