Isomerization of olefins triggered by rhodium-catalyzed C-H bond activation: Control of endocyclic β-hydrogen elimination

Article abstract of DOI:10.1002/anie.201500596

Five-membered metallacycles are typically reluctant to undergo endocyclic β-hydrogen elimination. The rhodium-catalyzed isomerization of 4-pentenals into 3-pentenals occurs through this elementary step and cleavage of two C-H bonds, as supported by deuterium-labeling studies. The reaction proceeds without decarbonylation, leads to trans olefins exclusively, and tolerates other olefins normally prone to isomerization. Endocyclic β-hydrogen elimination can also be controlled in an enantiodivergent reaction on a racemic mixture.

Full text of DOI:10.1002/anie.201500596

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201500596
German Edition: DOI: 10.1002/ange.201500596
ꢀ
C H Activation
ꢀ
Isomerization of Olefins Triggered by Rhodium-Catalyzed C H Bond
Activation: Control of Endocyclic b-Hydrogen Elimination**
Stephanie Y. Y. Yip and Christophe Aꢀssa*
Abstract: Five-membered metallacycles are typically reluctant
to undergo endocyclic b-hydrogen elimination. The rhodium-
catalyzed isomerization of 4-pentenals into 3-pentenals occurs
ꢀ
through this elementary step and cleavage of two C H bonds,
as supported by deuterium-labeling studies. The reaction
proceeds without decarbonylation, leads to trans olefins
exclusively, and tolerates other olefins normally prone to
isomerization. Endocyclic b-hydrogen elimination can also be
controlled in an enantiodivergent reaction on a racemic
mixture.
Five-membered metallacycles are important intermediates
of numerous catalytic processes, both in academic laborato-
ries and in large-scale industrial chemistry.[1] As shown in
experimental[2] and theoretical[3] studies, geometric con-
straints make these intermediates reluctant to undergo
endocyclic b-hydrogen (b-H) elimination, especially in the
case of square-planar complexes. However, and although
thorough experimental studies are still lacking, theoretical
studies suggest that five-membered metallacycles that are not
square-planar could undergo b-H elimination more easily.[4]
For example, recent calculations indicate that the rhodium-
catalyzed decarbonylation of 4-pentenals could occur by
reversible endocyclic b-H elimination of intermediate A (R =
H) (Figure 1).[5] Importantly, substrate decarbonylation is
a notorious problem during the hydroacylation of 4-pentenals,
especially in the case of a,a-disubstituted aldehydes (R ¼
H).[6–9] In contrast, we have found that the rhodium-catalyzed
isomerization of 4-pentenal 1 (R = Ph) into 3-pentenal 2
occurs without decarbonylation in 86% yield and in a highly
stereoselective fashion. The most efficient catalyst was
prepared with ligand L1,[10] whereas those prepared with
L2–L5 led to incomplete conversion and decarbonylation.
Figure 1. Rhodium-catalyzed isomerization of 4-pentenals into 3-pente-
nals by endocyclic b-H elimination. Ligands and charges are omitted
for clarity: Rh=[Rh((ꢁ)-L1)]BF4. Yield of isolated product 2.
endocyclic b-H elimination of A (R = Ph) (c), and final
reductive elimination (d).
Herein, we report a thorough study of the reaction
depicted in Figure 1, including deuterium-labeling experi-
ments that support the postulated mechanism and the
endocyclic b-H elimination of rhodacyclopentanone A.
Moreover, we also show that the isomerization is chemo-
selective for olefins that enable the formation of A, and that
olefins located elsewhere on the substrate remain intact under
the reaction conditions, even in the challenging case of
sensitive olefins normally prone to facile isomerization in the
presence of transition-metal catalysts,[11] including rhodium
catalysts.[12] Finally, we describe how the endocyclic b-H
elimination of rhodacyclopentanones can be prevented,
whereby each enantiomer of the racemic 4-pentenal under-
goes a distinct and enantioselective rearrangement when
treated with an enantiopure catalyst.
We found that the rhodium-catalyzed isomerization of
deuterated substrates 3 and 4 into compounds 5 and 6,
respectively, occurred smoothly with complete transfer of the
deuterium atom at the positions indicated in Scheme 1.
Transient intermediate 3-int was observed in the isomer-
ization of 3 into 5, indicating that step (b) in Figure 1 is
reversible.[13] No intermolecular transfer of the deuterium
atom was observed when 3 and 7 were treated with the
ꢀ
Hence, we assumed this isomerization to be triggered by C H
bond activation (a), and the catalytic cycle would be
completed by migratory insertion of the terminal olefin into
the rhodium-hydrogen bond thus engendered (b), followed by
[*] S. Y. Y. Yip, Dr. C. Aꢀssa
Department of Chemistry, University of Liverpool
Crown Street, L69 7ZD (UK)
E-mail: aissa@liverpool.ac.uk
[**] Financial support from EPSRC (DTA studentship to S.Y.Y.Y.) is
gratefully acknowledged. We thank Takasago Ltd for the generous
gift of ligand L1, Emilie Danckaert for preliminary investigations,
and Dr. Craig M. Robertson for X-ray crystallographic analysis of
compound (S)-23.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500596.
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Scheme 1. Deuterium-labeling experiments.
rhodium catalyst. Instead, 5 and 8 were obtained in 75% and
93% yield, respectively. The results of these experiments are
in good agreement with the intramolecular addition of an
acylhydridorhodium intermediate and the endocyclic b-H
elimination envisioned in Figure 1. In contrast to many
precedents, neither the reversible intermolecular addition of
[15]
a metal–hydride species[14] nor allylic C H activation can
ꢀ
account for the olefin isomerization examined herein.
We then explored the generality of the isomerization of 4-
pentenals into 3-pentenals with substrates 9a–9o and
observed in all cases the stereoselective formation of 10a–
10o as trans isomer only (Scheme 2). This exquisite trans-
selectivity could be explained by the fact that the hydrogen
atom highlighted by a gray disc in A (Figure 1) is the only one
in this conformer of the five-membered metallacycle which is
correctly positioned to develop an agostic interaction with the
metal prior to b-H elimination.[5,16] Hence, placing the cis
isomer of 10a under the reaction conditions led to only
limited isomerization into its trans isomer (Z/E = 4.7:1).
Monosubstituted 9i could be converted into 10i, albeit with
decomposition owing to the instability of both 9i and 10i,
even in the absence of catalyst.[17] Remarkably, other olefins,
such as the remote terminal olefin in 10k, an allylbenzene
(10l), an allylic ether (10m), an allylic amide (10n), and a 1,4-
enyne (10o), remained unaffected by the active rhodium
catalyst, although they are all susceptible to undergo metal-
catalyzed isomerization.[11,12]
Scheme 2. Scope of the chemo- and stereoselective isomerization of 4-
pentenals into (E)-3-pentenals. All of the yields are given for isolated
products as the average of two experiments; see the Supporting
Information. [a] Yield determined by 1H NMR spectroscopy in
[D6]acetone in the presence of an internal standard. [b] Room temper-
ature. TBS=tert-butyldimethylsilyl. Ts=p-tolylsulfonyl.
decarbonylated olefins as well. In view of the inertness of
11 and the notoriously low yields of formation of a,a-
disubstituted cyclopentanones in hydroacylation reactions,[7,8]
we were surprised to observe that the treatment of 15 with the
active rhodium catalyst led to the isolation of 17 in 97% yield
The 1,1-disubstituted olefin in 11 does not undergo
isomerization but sluggish intramolecular hydroacylation,
and 12 was isolated in low yield besides the recovered starting
material [Eq. (1)]. The isomerization of 1,2-bisubstituted
as
a single diastereomer, as confirmed by NOESY
(Scheme 3). Importantly, compound 16 could be isolated as
transient intermediate in the formation of racemic 17 at room
temperature: at 50% conversion, 16 accounts for 45% of the
mass balance. The strikingly different reactivity of 11 and 16
suggests that the 1,2-disubsituted olefin of 16 facilitates the
observed intramolecular hydroacylation. This tandem reac-
tion could also be observed with other substrates (see the
Supporting Information).
olefin 13 into 14 is reversible and placing either 13 or 14
under the reaction condition leads to the same 13/14 ratio
[Eq. (2)], although the reaction of 14 led to traces of
Further exploration of the scope of the reaction revealed
that the isomerization of 4-pentenals into 3-pentenals can be
prevented in favor of intramolecular hydroacylation. Thus,
2
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21. The absolute stereochemistry of (R)-19 was assigned by
comparison with similar compounds,[20] and X-ray crystallog-
raphy of ester (S)-23 confirmed the configuration of (S)-21.[21]
The absolute stereochemistry and enantiomeric purity of
(S,S)-20 were deduced by considering the mass balance and
the enantiomeric ratios of the other products obtained in this
reaction after full conversion, using the mathematical treat-
ment proposed by Horeau.[22] Significantly, we observed that
19 is formed more quickly than 20 and 21: at 10% conversion,
19 accounts for more than 8% of the mass balance whereas 20
and 21 account less than 2% together. Accordingly, the
behavior of (ꢁ)-18 in the presence of an enantiopure catalyst
is best described as a divergent reaction on a racemic mixture
(RRM),[23] whereby each enantiomer follows predominantly
a distinct reaction pathway, (R)-18 leading to (R)-19, and (S)-
18 leading to (S,S)-20 and (S)-21. In B, coordination of the
metal by the oxygen atom lone pair of the methoxy group
would prevent endocyclic b-H elimination, and B would
instead undergo exocyclic b-H elimination and thereby revert
to an acylhydridorhodium intermediate, which eventually
would lead to (R)-19 by intramolecular hydroacylation. In C,
similar coordination by the methoxy group and the release of
the steric repulsion between the pseudo-axial phenyl and
methyl groups after reductive elimination would promote the
formation of (S,S)-20. In D, the coordination site on the metal
necessary to the endocyclic b-H elimination would remain
available and this intermediate would eventually lead to (S)-
21. To the best of our knowledge, the enantioselective
reaction depicted in Figure 2 is the first example of a divergent
RRM of 4-pentenals. Although the details of the interaction
of (R)-L1 with the substrates are not known, a preliminary
investigation indicated that a simple kinetic resolution of (ꢁ)-
16 with [Rh((R)-L1)]BF4 (10 mol%) at room temperature led
to the isolation of (ꢀ)-17 in 50% yield (e.r. = 98:2) whilst
(+)-16 was recovered in 40% yield (e.r. = 99:1).
Scheme 3. Olefin isomerization followed by intramolecular hydroacyla-
tion. a) [Rh((ꢁ)-L1)]BF4 (10 mol%), acetone, 608C, 17 h. The 16/17
ratios were obtained for the reaction performed at room temperature.
placing (ꢁ)-18 under our optimized conditions led to cyclo-
pentanone (ꢁ)-19 and trace amounts of cyclobutanone (ꢁ)-20
as single diastereomer [Eq. (3)]. Presumably, coordination by
the methoxy group can prevent endocyclic b-H elimination by
blocking a coordination site on the metal.
Considering the rare examples of simple kinetic resolu-
tion in the rhodium-catalyzed hydroacylation of 4-pentenals
into cyclopentanones,[7b,c,18] and parallel kinetic resolution of
4-alkynals,[19] we were curious to evaluate the effect of an
enantiopure catalyst on the reaction of (ꢁ)-18. Using (R)-L1
as ligand, we obtained (R)-19 besides a mixture of (S,S)-20
and (S)-21 (Figure 2). Reduction of the latter compounds
enabled the separation of (R)-22 and the determination of its
enantiomeric purity, which is assumed to be the same for (S)-
In conclusion, we have identified several factors which
control the behavior of a key five-membered metallacycle
intermediate in the isomerization of 4-pentenals into 3-
pentenals in terms of chemo- and stereoselectivity. Endocyclic
b-H elimination of this intermediate enables the stereoselec-
tive formation of a trans olefin with exquisite control.
Alternatively, this elementary step can be prevented by
a coordinating group, in which case it is possible to observe
a specific reaction for each enantiomer of the racemic 4-
pentenal in the presence of an enantiopure catalyst.
ꢀ
Keywords: C H activation · isomerization · metallacycles ·
rhodium · b-H elimination
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Angew. Chem. Int. Ed. 2015, 54, 1 – 5
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Received: January 21, 2015
Revised: March 12, 2015
Published online: && &&, &&&&
4
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Angew. Chem. Int. Ed. 2015, 54, 1 – 5
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Angewandte
Chemie
Communications
ꢀ
C H Activation
Control of endocyclic b-H elimination of
a pivotal five-membered metallacycle
enables the rhodium-catalyzed isomer-
ization of 4-pentenals into 3-pentenals
without decarbonylation and leads to
trans olefins with exquisite selectivity.
Other sensitive olefins, even if prone to
isomerization, remain intact. Endocyclic
b-H elimination can also be prevented, in
which case an enantiodivergent reaction
on the racemic substrate was observed.
S. Y. Y. Yip, C. Aꢁssa*
&&&& — &&&&
Isomerization of Olefins Triggered by
Rhodium-Catalyzed C H Bond
Activation: Control of Endocyclic b-
Hydrogen Elimination
ꢀ
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!