Rhodium-Catalyzed Annulations of 1,3-Dienes and Salicylaldehydes/2-Hydroxybenzyl Alcohols Promoted by 2-Ethylacrolein

Article abstract of DOI:10.1002/adsc.201800796

A rhodium-catalyzed 2-ethylacrolein-promoted protocol enables the annulation reactions of 1,3-dienes with either salicylaldehydes or 2-hydroxybenzyl alcohols leading to 2-alkylchroman-4-ones with high regioselectivity. This research highlights the use of 2-ethylacrolein which probably serves as a tool of bidentate coordination to rhodium intermediates. Mechanistic studies reveal that the transformation proceeds through the 1,4-hydroacylation pathway to access unsaturated linear ketones with subsequent oxo-Michael addition. (Figure presented.).

Full text of DOI:10.1002/adsc.201800796

Title: Rhodium-Catalyzed Annulations of 1,3-Dienes and
Salicylaldehydes/2-Hydroxybenzyl Alcohols Promoted by 2-
Ethylacrolein
Authors: Hong-Shuang Li, Yang Xiong, and Guozhu Zhang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800796
Link to VoR: http://dx.doi.org/10.1002/adsc.201800796

10.1002/adsc.201800796
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Rhodium-Catalyzed Annulations of 1,3-Dienes and
Salicylaldehydes/2-Hydroxybenzyl Alcohols Promoted by 2-
Ethylacrolein
Hong-Shuang Li,a Yang Xiong,b Guozhu Zhangb,*
a
Institute of Pharmacology, School of Pharmaceutical Sciences, Taishan Medical University, 619 Changcheng Road,
Taian 271016, People’s Republic of China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Center for Excellence in
b
Molecular Synthesis, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road,
Shanghai 200032, People’s Republic of China
E-mail: guozhuzhang@sioc.ac.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
oxidative cyclization, which could favor either 1,2- or
1,4-hydroacylation depending on the aldehyde
component.[10] In view of the fact that only a few
examples have been described for transition-metal-
catalyzed C–C couplings of 1,3-dienes or their
precursors with aldehydes or alcohols to access
functionalized ketones,[7–10,11] it would be highly
desirable to develop novel methodologies to achieve
the conversion.
Abstract. A rhodium-catalyzed 2-ethylacrolein-promoted
protocol enables the annulation reactions of 1,3-dienes with
either salicylaldehydes or 2-hydroxybenzyl alcohols leading
to 2-alkylchroman-4-ones with high regioselectivity. This
research highlights the use of 2-ethylacrolein which
probably serves as a tool of bidentate coordination to
rhodium intermediates. Mechanistic studies reveal that the
transformation proceeds through the 1,4-hydroacylation
pathway to access unsaturated linear ketones with
subsequent oxo-Michael addition.
Keywords: Rhodium catalysis; 2-Ethylacrolein promoted;
1,3-Dienes; 1,4-Hydroacylation; 2-Alkylchroman-4-ones
The last decade has seen a speedy expansion of using
1,3-dienes as important building blocks in the
preparation of substantially more complex and value-
added molecules.[1] In this regard, the intermolecular
reductive couplings of 1,3-dienes to carbonyl
compounds or alcohols catalyzed by transition metals,
such as ruthenium,[2] nickel,[3] iridium,[4] and
titanium,[5] have been demonstrated as attractive and
powerful strategies with prominent regio-, diastereo-
and enantioselectivities. It is noteworthy that only two
examples of diene-carbonyl reductive couplings were
achieved relying on the rhodium catalysis system.[6] In
Figure 1. Representative Bioactive 2-Alkylchroman-4-
ones
2-Alkylchroman-4-ones, as conspicuous privileged
structures, have been found in numerous naturally
occurring products and medicinally relevant
compounds (Figure 1), possessing a broad range of
pharmaceutical activities.[12] Consequently, extensive
efforts have been devoted to the synthesis of this
scaffold[13] from readily available materials. For
example, the Glorius group[14] developed one example
of the Rh(III)-catalyzed annulation reaction of
salicylaldehyde with styrene to furnish a flavanone by
use of stoichiometric silver salt. Meanwhile, the
asymmetric hydrogenation using the chiral Ru–NHC
complex with subsequent selective oxidation to access
flavanones and chromanones was successively
reported in the same group.[15] Miura[16] and Stanley[17]
demonstrated the Rh(I)-catalyzed cyclization of
contrast
with
the
extensively
investigated
intermolecular reductive couplings, illustrations on the
1,3-diene-participated hydroacylation remain limited.
Since the pioneering work of Kondo and Mitsudo on
the Ru-catalyzed intermolecular hydroacylation of
1,3-dienes with (hetero)aromatic aldehydes,[7]
Krische[8] and Ryu[9] independently developed the Ru–
H-catalyzed diene hydroacylation to deliver 1,2-
addition products. In 2014, Dong and co-workers
demonstrated
an
interesting
Co-catalyzed
regioselective hydroacylation of 1,3-dienes by
1
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10.1002/adsc.201800796
Advanced Synthesis & Catalysis
salicylaldehydes to internal alkynes via tandem alkyne
hydroacylation and oxo-Michael addition. In 2016,
Yoshikai realized the Co(I)-catalyzed reductive
annulations using the same substrate partners in the
presence of a superstoichiometric amount of Zn.[18]
butadiene (2a). With the previous reported
acylrhodium intermediates stabilized by the bidentate
chelation of acrylamides in mind,[20] we tested various
α,β-unsaturated compounds to promote the model
reaction. Indeed, the best results were obtained when
Recently,
Rh-catalyzed
intermolecular
the
reaction
was
performed
under
the
hydroacylation of alkenes[19] with salicylaldehydes has
been exquisitely investigated to afford linear or
branched carbonyl products. Inspired by Tanaka’s
work on strong coordination of various acrylamides to
the rhodium intermediates,[20] we hypothesized that the
presence of an appropriate α,β-unsaturated compound
could promote the intermolecular couplings of
conjugated dienes to salicylaldehydes. To continue our
efforts in the difunctionalization of 1,3-dienes,[21]
herein we describe the successful rhodium-catalyzed
2-ethylacrolein-assisted couplings of 1,3-dienes with
either salicylaldehydes or 2-hydroxybenzyl alcohols
resulting in 2-alkylchroman-4-ones in a highly
regioselective manner.
Rh(COD)2SbF6/(p-Me-Ph)3P catalytic system using 2-
ethylacrolein (L1) as the ligand and KF as the additive,
providing the desired 2-ethylchroman-4-one 3aa in
77% yield (Table 1, entry 1). Replacing (p-Me-Ph)3P
with PPh3 delivered a slightly lower yield (Table 1,
entry 2). The reaction turnover is extremely dependent
on the ligand, and the yield of the product dropped
significantly to 13% without L1 (Table 1, entry 3). The
specific nature of the ligand was proved to be very
important, as the simpler acrolein (L2) could not
promote the annulation reaction at all (Table 1, entry
4). Cinnamaldehyde (L3) produced a yield similar to
that achieved with styrene (L4) (Table 1, entry 5–6). It
is noteworthy that the reaction assisted by 1-penten-3-
one (L5) was far less effective than that by methyl
acrylate (L6) (Table 1, entry 7–8). Additionally, 3-
Table 1. Optimization of the Reaction Conditions.[a]
methylcrotonaldehyde
(L7)
and
(E)-2-
methylcrotonaldehyde (L8) as structural analogues
showed totally different results from each other (Table
1, entry 9–10), and L8 gave a yield comparable to L1.
To our delight, when 2-hydroxybenzyl alcohol (1a′)
was exposed to the reaction conditions, the
corresponding product was afforded in 73% yield
(Table 1, entry 11), and as expected, the control
experiment revealed that the absence of L1 had a
detrimental impact on the reaction conversion (Table
1, entry 12).
Subsequently, the optimal reaction conditions were
utilized to investigate the scope of a variety of
commercially available salicylaldehydes for the
annulation reactions with 2a (Scheme 1). Initially,
salicylaldehyde derivatives substituted with electron-
donating groups such as alkoxy and alkyl on the
benzene ring proved to be good substrates with the
Rh(COD)2SbF6/(p-Me-Ph)3P catalytic couple to give
rise to the expected compounds (3ba–3fa) in moderate
to good yields; but unfortunately, the substrate with the
dimethylamino group at the meta position of the
hydroxyl group was not amenable to this
transformation. Actually, exposure of electron-neutral
deviation from the standard
conditions
yield[b]
(%)
77(74)[c]
71
13
14
62
61
7
66
entry
1
2
3
4
5
6
7
8
None
PPh3 instead of (p-Me-Ph)3P
Without L1
L2 instead of L1
L3 instead of L1
L4 instead of L1
L5 instead of L1
L6 instead of L1
L7 instead of L1
L8 instead of L1
None
9
7
73
73
58
10
11[d]
12[d]
Without L1
2-hydroxy-1-naphthaldehyde
to
the
reaction
[a] Unless otherwise noted, all reactions were carried out in
N2 atmosphere using 1a (0.3 mmol, 1 equiv) and 2a (2.5
equiv) in 1.5 mL of toluene.
conditions revealed that a good yield of 3ga was
obtained. Good compatibility with a host of functional
groups for the one-pot synthesis of 2-ethylchroman-4-
ones was observed, with trifluoromethoxy (3ha),
[b] Determined by 1H NMR analysis using diethyl phthalate
as the internal standard.
fluoro
(3ia–3ja),
chloro
(3qa–3ra)
and
[c] Isolated yield in parentheses.
trifluoromethyl groups (3ma) all being well-tolerated.
However, the substrates bearing bromo substituents
(3ka–3la, 3sa) were found to be subjected to
debromination. It is worth mentioning that a
chromanone 3na derived from methyl 3-formyl-4-
[d] 2-Hydroxybenzyl alcohol (1a′) was used as the substrate.
Reaction optimization (Table 1) was initially carried
out by coupling salicylaldehyde (1a) with 1,3-
2
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10.1002/adsc.201800796
Advanced Synthesis & Catalysis
product—rac-flindersiachromanone (3af) and its
regioisomer (3af′) in a combined yield of 64%.
hydroxybenzoate was provided in an excellent yield,
thus demonstrating the applicability of our technique.
Strikingly,
2,4-dihydroxybenzaldehyde
was
successfully coupled with 1,3-butadiene to deliver a
modest yield of the target product 3oa with a free
phenolic hydroxyl group. In general, the electronic
factor had no prominent effect on the transformation
of the salicylaldehydes. On the other hand, 2-
hydroxybenzyl alcohols (1p′–1t′), which were readily
prepared by reduction of the corresponding
salicylaldehydes by NaBH4/EtOH, were also tolerable
under the standard reaction conditions, regardless of
either electron-donating group (3pa) or electron-
withdrawing groups (3qa–3ta) on the benzene ring.
Scheme 2. Reaction Scope with Respect to 1,3-Dienes.
Scheme 1. Reaction Scope with Respect to
Salicylaldehydes.
During exploration of the scope of 1,3-dienes in the
annulation reactions, it was found that our protocol
could also be suitable for the functionalization of other
representative conjugated dienes (Scheme 2). For
example, alkyl-substituted 1,3-butadienes were
reacted with salicylaldehyde (1a) smoothly under the
optimized conditions to afford the exclusively
regioselective products (3ab–3ad) in moderate to
good yields. However, isoprene proceeded sluggishly
with a low yield of 2,2-dialkyl-substituted compound
(3ae) obtained. Likewise, this strategy could be
extended to the coupling reaction of terminal aryl-
substituted dienes with 2-hydroxybenzyl alcohol (1a′)
rather than salicylaldehyde (1a), furnishing a natural
Scheme 3. Investigation of Mechanism.
As shown in Scheme 3, indirect evidence was
assembled to elucidate the role of 2-ethylacrolein (L1)
in company with the catalytic mechanism. The
incompetent reaction of benzaldehyde (1u) with 1,3-
butadiene (2a) either in the presence or the absence of
2-ethylacrolein (L1) by 1H NMR analysis of the crude
3
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10.1002/adsc.201800796
Advanced Synthesis & Catalysis
mixture distinctly revealed the significance of a
hydroxy group[14,19e] in initiating the annulation
reaction (Scheme 3a). The cyclization of 1a to 2a
without L1 was discreetly studied (Scheme 3b),
isolating the product 3aa (9% yield), a saturated
ketone 4 (11% yield) and a pair of inseparable
unsaturated regioisomers 5 and 6 (51% combined
yield). These results indicated that the two isomers
were more likely to be the intermediates in the present
catalytic system and that the reactions proceeded in an
excellent linear selectivity. Moreover, employing 50
mol % of L1 in the transformation of salicylaldehyde
(1a) as well as 2-hydroxybenzyl alcohol (1a′)
increased the yield of 3aa to 77% and 73% (Table 1,
entry 1 vs entry 3, entry 11 vs entry 12), respectively,
which could highlight the bidentate chelation
assistance of L1.[20]
Finally, mechanistic studies on the kinetic isotope
effect (KIE) and the deuterium labeling experiments
are reported. The value of kH/kD = 2.2 from two parallel
reactions reveals that β-hydride or reductive
elimination might be involved in the turnover-limiting
step (Scheme 3c).[10] When the annulation is carried
out using deuterated salicylaldehyde (1a-D) as the
substrate with and without L1, about 90% deuterium
is incorporated at the terminal methyl group (Scheme
3d–e). In addition, detectable deuterium at other
aliphatic carbon atoms is also observed while
performing the reactions of deuterated 2-
hydroxybenzyl alcohol (1a′-D2), which might be
attributed to the Rh−D species generated in situ
(Scheme 3f–g).
undergoes transformation to obtain the seven-
membered rhodiumacycle II[10] with the chelation
assistance of L1.[20] The subsequent β-H elimination
delivers intermediate III, which upon reductive
elimination produces the linear unsaturated ketone IV
(1,4-hydroacylation pathway) and regenerates the
Rh(I) catalyst. Isomerization of IV under high
temperature gives rise to the more stable α,β-
unsaturated ketone V which is converted into the target
product 7 through the final intramolecular oxo-
Michael addition. In the case of deuterated 2-
hydroxybenzyl alcohol (1a′-D2), alkyloxyrhodium
intermediate[9,11a,23] initially forms with subsequent β-
H elimination, leading to the Rh–D species and the
aldehyde (1a-D). Note that the Rh–D species exert
double bond isomerization through iterative
hydrorhodation/β-H
elimination[9,11a,23b,23c]
that
accounts for the complex deuterium distribution on the
product.
In conclusion, efficient construction of 2-
alkylchroman-4-ones through rhodium-catalyzed 2-
ethylacrolein-promoted cyclization of 1,3-dienes to
either salicylaldehydes or 2-hydroxybenzyl alcohols
has been developed. The participation of 2-
ethylacrolein in the reaction might stabilize different
kinds of rhodium intermediates. An oxidative
cyclization mechanism is proposed to afford the 1,4-
hydroacylation product as the key intermediate. This
methodology features readily available materials,
good functional group tolerance, and high
regioselectivity. More insights into the catalytic
mechanism and the synthetic applications in the
bioactive pharmaceuticals are underway in our
laboratory.
Experimental Section
To an oven-dried sealed tube equipped with a stirrer bar was
added Rh(COD)2SbF6 (8.3 mg, 0.015 mmol, 5 mol %), tri(p-
tolyl)phosphine (9.1 mg, 0.03 mmol, 10 mol %), K2CO3 (8.3
mg, 0.06 mmol, 20 mol %), KF (17.4 mg, 0.3 mmol, 1.0
equiv), salicylaldehyde 1a (36.6 mg, 0.3 mmol, 1.0 equiv),
1,3-butadiene 2a (500 μL, 0.75 mmol, 2.5 equiv, ca. 1.5
mol/L in THF), and 2-ethylacrolein L1 (12.6 mg, 0.15 mmol,
50 mol %). Then dry toluene (1.5 mL, 0.2 M) was added.
After the mixture was stirred at room temperature for 15 min,
the resulting mixture was removed from the glove box, and
then stirred at 110 ℃ for 12 h. Upon completion of the
reaction, the mixture was concentrated and the residue was
purified by silica gel flash chromatography (hexane/EtOAc)
to afford the desired product 3aa.
Acknowledgements
We are grateful to NSFC-21772218, 21421091, XDB20000000,
the “Thousand Plan” Youth program, State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, and Key Research and
Development Program of Shandong Province, China
(2018GGX109010).
Scheme 4. Plausible Reaction Mechanism.
On the basis of the above observations and the
previous reports on the coupling reactions of 1,3-
dienes,[2g,3d,10,22] a tentative mechanism is proposed in
Scheme 4. First, the rhodium catalyst would trigger
oxidative cyclization between the aldehyde (1a-D) and
1,3-butadiene (2a) to form intermediate I, which
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10.1002/adsc.201800796
Advanced Synthesis & Catalysis
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5
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Advanced Synthesis & Catalysis
UPDATE
Rhodium-Catalyzed Annulations of 1,3-Dienes and
Salicylaldehydes/2-Hydroxybenzyl Alcohols
Promoted by 2-Ethylacrolein
Adv. Synth. Catal. Year, Volume, Page – Page
Hong-Shuang Li, Yang Xiong, Guozhu Zhang*
6
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