Highly Regio- and Stereoselective Catalytic Synthesis of Conjugated Dienes and Polyenes

Article abstract of DOI:10.1021/jacs.8b05421

Conjugated dienes and polyenes are typically synthesized by sequential introduction of C=C bonds. Here, we report a practical and scalable, catalytic dienylation that is highly regio- and stereoselective for both C=C bonds. The reaction is enabled by a stereoselective palladium-catalyzed cross-coupling that is preceded by a regioselective base-induced ring opening of readily available sulfolenes. The dienylation reaction is particularly useful for the synthesis of synthetically challenging dienes containing cis double bonds. We also show that the reaction can serve as a synthetic platform for the construction of conjugated polyenes.

Full text of DOI:10.1021/jacs.8b05421

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Communication
Highly Regio- and Stereoselective Catalytic
Synthesis of Conjugated Dienes and Polyenes
Vu T. Nguyen, Hang Dang, Hoang H. Pham, Viet D. Nguyen,
Carsten Flores-Hansen, Hadi D. Arman, and Oleg V. Larionov
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05421 • Publication Date (Web): 24 Jun 2018
Downloaded from http://pubs.acs.org on June 24, 2018
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Highly Regio- and Stereoselective Catalytic Synthesis of Con-
jugated Dienes and Polyenes
Vu T. Nguyen,‡ Hang T. Dang,‡ Hoang H. Pham, Viet D. Nguyen, Carsten Flores-Hansen, Hadi D. Ar-
man, and Oleg V. Larionov*
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Department of Chemistry, The University of Texas at San Antonio, San Antonio, Texas 78249, United States
Supporting Information Placeholder
ABSTRACT: Conjugated dienes and polyenes are typically
synthesized by sequential introduction of C=C bonds. Here, we
report a practical and scalable, catalytic dienylation that is highly
regio- and stereoselective for both C=C bonds. The reaction is
enabled by a stereoselective palladium-catalyzed cross-coupling
that is preceded by a regioselective base-induced ring opening of
readily available sulfolenes. The dienylation reaction is particular-
ly useful for the synthesis of synthetically challenging dienes
containing cis double bonds. We also show that the reaction can
serve as a synthetic platform for the construction of conjugated
polyenes.
Scheme 1. Synthesis of conjugated dienes and polyenes
Conjugated dienes and polyenes are key structural motifs of many
natural products and pharmaceuticals.1 They also play central
roles in organic synthesis2 and materials science.3 Construction of
linear conjugated -systems presents unique synthetic challenges,
since every C=C bond has to be produced stereoselectively and
with substituents introduced in a regioselective manner. In order
to control the regio- and stereoselectivity, a number of catalytic
cross-coupling methods have been developed that attach dienyl
groups in a sequence of several steps, for example by a sequential
appendage of two C=C units (Scheme 1, part A).4,5 A reaction that
enables direct appendage of a C4 unit will allow for a more con-
vergent and efficient synthesis of conjugated dienes and polyenes.
However, the development of this synthetic platform has been
impeded by the lack of reactions that produce both C=C bonds in
a regio- and stereoselective manner, and the lack of dienylation
reagents that are stable and readily available in a variety of substi-
tution patterns. Existing methods, for example, Pd-catalyzed ary-
lation reactions of substituted conjugated dienes, produce mix-
tures of regio- and stereoisomers and suffer from low yields and
narrow scope.6 Although dienylboronic acids have been explored
for dienylation, they are unstable and difficult to prepare.7 Aro-
matic sulfinates have been recently introduced as a new class of
stable nucleophiles for Pd-catalyzed biaryl couplings.8 However,
stereoselective cross-coupling reactions of vinylic sulfinates have
remained elusive.
We report herein a general and practical reaction that enables a
regio- and stereoselective and scalable synthesis of substituted
conjugated dienes and polyenes by dienylation with sulfolenes
that form sulfinates under the reaction conditions (Scheme 1).
Sulfolenes are air- and moisture-stable compounds that are readily
accessible in a convergent manner and from simple precursors.9,10
Although sulfolenes can be -alkylated and then converted to
dienes by heating at elevated temperatures, typically 150–200 C
or by flash vacuum pyrolysis, this stepwise approach is unselec-
tive, inefficient, incompatible with many functional groups, and
cannot be used to introduce aryl and vinyl substituents.10 We en-
visioned that a combination of a regio- and stereoselective base-
induced ring opening of sulfolenes (e.g., sulfolene 1 to sulfinate 2,
Scheme 1, part B) and a subsequent desulfitative Pd-catalyzed
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coupling (Scheme 1, part C) would result in a direct, regio- and
The generality of the coupling reaction with sulfolene 1 was ex-
stereoselective dienylation. Indeed, the sulfolene ring opening
reaction proceeds regio- and stereoselectively and affords sul-
finates 3 from 2-substituted sulfolenes 4, and sulfinates 5 from 3-
substituted sulfolenes 6.11 However, dienylsulfinate salts may
isomerize (e.g., as for sulfinates 7) at higher temperatures or in
the presence of catalytic amounts of base.11f Thus, the catalyst
will play the key role in determining the stereoselectivity of the
C–C bond-forming step.
amined with a number of bromo- and chloro(hetero)arenes (Table
2).
Table 2. Scope of the Reaction with Sulfolene 1a
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Table 1. Optimization of Reaction Conditionsa
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entry ligand
base/additive
solvent
yield,b
%
E/Z
ratio
1
2
dppe
dppf
CH3OK
CH3OK
CH3OK
CH3OK
dioxane
dioxane
dioxane
THF
12
24
29
26
32
99c
61
56
59
18
62
0
1.5:1
1.3:1
>30:1
>30:1
>30:1
>30:1
>30:1
11:1
1:2
3
dppbz
dppbz
4
5
dppbz CH3OK/K2CO3 dioxane
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dppbz CH3OK/K2CO3
dppbz CH3OK/K2CO3
dppbz CH3ONa/K2CO3
dppbz CH3OK/Na2CO3
dppbz CH3OK/Cs2CO3
dppbz t-BuOK/K2CO3
THF
toluene
THF
7
8
9
THF
10
11
12
THF
8:1
THF
1:1.5
–
dppbz
K2CO3
THF
a
Reaction conditions: 4-bromobenzonitrile (8) (1 mmol), sul-
folene 1 (2 mmol), Pd(OAc)2 (5 mol %), ligand (8 mol %), base
(1.6 mmol), additive (2 mmol), solvent (8 mL), 110  C, 14 h.
dppe = 1,2-bis(diphenylphosphino)ethane, dppf = 1,2-bis(di-
phenylphosphino)ferrocene, dppbz = 1,2-bis(diphenylphosphino)-
b
benzene.
Determined by 1H NMR spectroscopy with 1,4-
dimethoxybenzene as an internal standard. c Isolated yield.
In initial experiments, a catalytic system consisting of Pd(OAc)2
and a bisphosphine ligand in the presence of potassium methoxide
showed promising activity in the model reaction of 4-
bromobenzonitrile (8) and sulfolene 1 (Table 1, entries 1–3). 1,2-
Bis(diphenylphosphino)benzene, dppbz, proved to be a particular-
ly suitable ligand. The reaction favored E-diastereomer 9 with
high E-diastereoselectivity (>30:1, entry 3). Although dioxane
and tetrahydrofuran appeared to be equally suitable solvents (en-
tries 3 and 4), a remarkable difference was observed when the
reaction was carried out in the presence of potassium carbonate
(entries 5 and 6). While only marginal improvement in the yield
was observed in dioxane, a nearly quantitative conversion to E-
diene 9 took place in tetrahydrofuran, and the product was isolat-
ed in 99% yield and with >30:1 E/Z ratio. The reaction was less
efficient in other solvents, (e.g., toluene, entry 7). In addition,
other combinations of basic additives (e.g., sodium and cesium
carbonates) and alkoxide bases (e.g., sodium methoxide or potas-
sium tert-butoxide) afforded product 9 in lower yields and with
lower E/Z ratios (entries 8–11). No reaction occurred in the ab-
sence of an alkoxide base (entry 12), indicating that deprotonation
of 3-sufolene 1 is an important step in the catalytic process.
a
Reaction conditions for small scale reactions: haloarene (1
mmol), sulfolene 1 (2 mmol), Pd(OAc)2 (5 mol %), dppbz (8 mol
%), CH3OK (1.6 mmol), K2CO3 (2 mmol), THF (8 mL), 110 C,
14–36 h. Bromoarenes were used, and E/Z ratio >30:1, unless
otherwise specified. b KOtBu (2.6 equiv.) was used as a base.
A variety of functional groups were tolerated in substituted bro-
mobenzenes (9-17), including acetamide 10. The E-1,3-butadienyl
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Table 3. Scope of the Reaction with Substituted Sulfolenesa
moiety was also successfully appended to naphthalenes (18-20)
and phenanthrene (21). A primary amino group was tolerated
(19), indicating that anilines can be good substrates for the cou-
pling reaction. The conjugated dienes were in all cases produced
as single E-diastereomers or with high E-diastereoselectivity.
Nitrogen-containing heterocyclic conjugated dienes (22-34) of the
pyridine, quinoline, isoquinoline, and acridine series were also
produced in high yields. 2- and 4-Chloropyridines and quinolines,
as well as a substituted 4-chloroacridine were readily converted to
dienes 22-27. 2,4-Dichloropyridine afforded diene 22 with the
dienyl group in the C2 position. Similarly, diene 28 was formed
as the only product from 4-chloro-6-bromoquiniline. The reaction
can also be used to install two dienyl moieties, e.g., as in product
29. The reaction with 2- and 4-haloazines proceeded cleanly, and
sulfones that could potentially be formed by the SNAr reaction
with the intermediate sulfinate 2 were not observed. Other hetero-
cyclic conjugated dienes with benzothiophene, benzofuran, thio-
phene, and benzothiazole (35-39) were also successfully prepared.
In addition, uracil-derived diene 40 was produced in 53% yield.
The reaction was also tested with 1-bromocyclohexene, and triene
41 was isolated in 67% yield.
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Similarly, two cholesterol- and stigmasterol-derived products 42
and 43 that contain a conjugated tetraene unit were produced,
both in 54% yield from the corresponding dienyl bromides. These
results demonstrate that the dienylation with sulfolenes can also
be used for the synthesis of conjugated polyene systems.5
We next proceeded to explore the regio- and stereoselectivity of
the dienylation with substituted sulfolenes 44-51 (Table 3). Good
yields were observed for products 52-54 derived from 3,4-
disubstituted sulfolene 44. E-Diastereomers were produced in
these cases. For conjugated dienes 52 and 53, formation of small
amounts of isomers 52a and 53a that are produced by the hydro-
gen migration from the proximal methyl group to the benzylic
carbon was also observed. The ratio of conjugated dienes 52 and
53 to the isomers was 13 : 1 and 15 : 1, respectively. 2-Substituted
sulfolenes 45-47 were examined next. Due to the regioselectivity
of the base-mediated sulfolene ring opening, 1,4-disubstituted
conjugated dienes were expected to be formed in the ensuing Pd-
catalyzed coupling reaction with haloarenes. Indeed, 1,4-
disubstituted (1E,3E)-dienes 55-64 were produced in good yields
in line with the mechanistic model. The reaction was E-
diastereoselective with respect to the newly-formed styrene moie-
ty. The cyclopropyl ring in products 59 and 60 remained unaffect-
ed.
3-Substituted sulfolenes 48 and 49 also reacted highly regioselec-
tively and in line with the expected pattern of the ring opening.
However, in this case the arylation proceeded with Z-
diastereoselectivity. For example, (1Z)-conjugated dienes 65-69
were produced in good yields from isoprene- and myrcene-
derived sulfolenes 48 and 49. Similarly, 3,5-disubstituted sul-
folene 50 produced (1Z)-conjugated dienes 70-75 in good yields
and in line with the proposed model of regioselectivity. Choles-
terol- and stigmasterol-derived products 74 and 75 that contain a
disubstituted conjugated tetraene unit were produced in 92 and
72% yields, respectively. The reaction of 2-bromonaphthalene
with sulfolene 50 was carried out with 2 mol% Pd, and diene 72
was produced in 80% yield, indicating that lower Pd catalyst load-
ings can also be used. The reaction with 1,1,3-trisubstituted sul-
folene 51 was also Z-selective, in line with the reactivity of other
3-substituted sulfolenes (e.g., products 76-78). While potassium
methoxide was the base of choice for most of sulfolenes, potassi-
um tert-butoxide afforded conjugated dienes with higher yields
and diastereoselectivity in reactions with 3-substituted sulfolenes
49 and 51. Use of potassium tert-butoxide also led to higher
yields of acetamide 10, indicating that other alkoxide bases can be
used for reaction optimization.
a For reaction conditions see footnote to Table 2. X = Br, and E/Z
or Z/E ratio >30:1, unless otherwise specified. b KOtBu as a base.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
Financial support by the Welch Foundation (AX-1788), the NSF
(CHE-1455061), NIGMS (SC3GM105579), and Max and Minnie
Tomerlin Voelcker Fund is gratefully acknowledged.
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The dienylation reaction with 3-substituted sulfolenes 48-51 is
particularly advantageous, because it introduces an aryl group at
the more substituted double C–C bond, and it affords the syntheti-
cally challenging (1Z)-conjugated dienes. The syntheses of prod-
ucts 23, 30, 35, 36, and 72 were readily carried out on gram
scales. The structures of products 24, 64 and 73 were confirmed
by X-ray crystallography.
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dienylsulfinate intermediates 7 (Scheme 1). No reaction was ob-
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prene, respectively), indicating that cheletropic cycloelimination
of sulfur dioxide from sulfolenes is not involved in the catalytic
process. Similarly, no reaction was observed, when the sulfone
that had been produced by the substitution of 4-chloro group in
4,7-dichloroquinoline with sulfinate 2 was used instead of the
haloarene and sulfolene 1 (see Scheme S1 in the SI). This result
suggests that the reaction does not proceed through formation of
sulfones. However, diene 9 was formed in a nearly quantitative
yield and with exclusive E-diastereoselectivity, when sulfolene 1
and potassium methoxide were replaced with potassium sulfinate
(K+·Z-2). Further, when sulfolene 1 was heated at 110C with
potassium methoxide in THF, rapid formation of potassium sul-
finate (K+·Z-2) was observed (95% conversion in 10 min). Taken
together, these results indicate that the reaction proceeds through
formation of dienylsulfinates 7. The isomerization of sulfinate
K+·Z-2 to the E-diastereomer K+·E-2 was slower (7 : 1 Z/E ratio
after 1 h at 110 C in THF). This result may suggest that the E-
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(Scheme 1).
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Experimental procedures; characterization data (PDF)
Crystallographic data for 24 (CIF)
Crystallographic data for 64 (CIF)
Crystallographic data for 73 (CIF)
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AUTHOR INFORMATION
Corresponding Author
oleg.larionov@utsa.edu
Author Contributions
‡These authors contributed equally
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