Solid Acid-Catalyzed Dehydration of Pinacol Derivatives in Ionic Liquid: Simple and Efficient Access to Branched 1,3-Dienes

Article abstract of DOI:10.1021/acscatal.7b00066

The selective dehydration of pinacol derivatives to branched 1,3-dienes is extremely challenging because of the predominance of pinacol rearrangement. Herein, we successfully achieve this goal by employing a recyclable solid acid/ionic liquid catalyst system. The dehydration of alkyl- and cycloalkyl-derived diols in an Amberlyst-15/[Emim]Cl system afforded the corresponding 1,3-dienes in good yields, while a Nafion/[Emim]Cl system was demonstrated to be a better catalyst system for the dehydration of aryl-substituted substrates. Our protocol features straightforward and simple access to branched 1,3-dienes, high chemoselectivity, a recyclable catalyst system, a facile separation of dienes just by decantation, and a broad substrate scope.

Full text of DOI:10.1021/acscatal.7b00066

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Article
Solid Acid-Catalyzed Dehydration of Pinacol Derivatives in Ionic
Liquid: A Simple and Efficient Access to Branched 1,3-Dienes
Yancheng Hu, Ning Li, Guangyi Li, Aiqin Wang, Yu Cong, Xiaodong Wang, and Tao Zhang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00066 • Publication Date (Web): 02 Mar 2017
Downloaded from http://pubs.acs.org on March 2, 2017
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ACS Catalysis
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Solid AcidꢀCatalyzed Dehydration of Pinacol
Derivatives in Ionic Liquid: A Simple and Efficient
Access to Branched 1,3ꢀDienes
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Yancheng Hu,† Ning Li, †,‡,* Guangyi Li,† Aiqin Wang,†,‡ Yu Cong,† Xiaodong Wang,† and Tao Zhang†,‡,
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†State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, China.
‡iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
ABSTRACT: The selective dehydration of pinacol derivatives to branched 1,3ꢀdienes is extremely
challenging due to the predominance of pinacol rearrangement. Herein, we successfully achieve this goal
by employing a recyclable solid acid/ionic liquid catalyst system. The dehydration of alkyl and cycloalkyl
derived diols in Amberlystꢀ15/[Emim]Cl system afforded the corresponding 1,3ꢀdienes in good yields,
while Nafion/[Emim]Cl was demonstrated to be a better catalyst system for the dehydration of aryl
substituted substrates. Our protocol features a straightforward and simple access to branched 1,3ꢀdienes,
high chemoselectivity, a recyclable catalyst system, a facile separation of dienes just by decantation, and
broad substrate scope.
KEYWORDS: branched 1,3ꢀdienes, pinacol dehydration, ionic liquid, solid acid, Amberlystꢀ15, Nafion,
[Emim]Cl
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ACS Catalysis
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Introduction
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1,3ꢀDienes are key structural motifs in a variety of natural products and bioactive molecules.1
Moreover, due to their high reactivity in many reactions (such as DielsꢀAlder cycloaddition,
polymerization, etc.), 1,3ꢀdienes have found wide applications in organic synthesis2 and polymer science.3
Thus, to date, tremendous methods have been developed for their synthesis. The conventional approaches
include transitionꢀmetal catalyzed crossꢀcouplings,4 olefination of unsaturated carbonyl compounds
(Horner–Wadsworth–Emmons reaction for instance)5 and enyne metathesis (Scheme 1).6 However, these
methods often suffer from highꢀcost transition metal catalysts, toxic reagents and solvents, harsh reaction
conditions and a multiꢀstep process. Besides, most of them focus on the synthesis of linear dienes, while
the selective formation of branched dienes is cumbersome. Consequently, the exploration of new
synthetic routes to branched 1,3ꢀdienes, that fulfill the criteria of green chemistry, is highly demanded.
The dehydration of diols is arguably the simplest and most straightforward access to 1,3ꢀdienes. The
past decade has witnessed substantial achievements in the gasꢀphase dehydration of linear diols (such as
1,3ꢀ, 2,3ꢀ and 1,4ꢀbutanediol) towards 1,3ꢀbutadiene.7 In contrast, the dehydration of branched vicꢀdiols to
highly substituted dienes still lags behind due to the predominance of pinacol rearrangement. Pinacol
derivatives, as an intriguing class of vicꢀdiols, can be easily synthesized through a lowꢀvalentꢀmetal
mediated reductive coupling8 or electrolytic reduction9 of carbonyl compounds, that is, the soꢀcalled
pinacol coupling. Very recently, Rueping et al. reported a sustainable visibleꢀlightꢀinduced pinacol
coupling of ketones or aldehydes to diols.10 With these wellꢀestablished synthetic methods in hand, we
herein develop a recyclable solid acid/ionic liquid system for the selective dehydration of pinacol
derivatives to various branched 1,3ꢀdienes (Scheme 1). This approach features a straightforward and
simple access to branched 1,3ꢀdienes, high chemoselectivity, a recyclable catalyst system, a facile
separation of dienes just by decantation, and broad substrate scope.
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ACS Catalysis
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Scheme 1. Synthesis Strategy of 1,3ꢀDienes.
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Conventional approaches:
R1
R
R
Ru cat.
Z
X
R
R1
linear dienes
R1
+
enyne
metathesis
Z = H, B(OH)2,
Bpin, MgBr,
ZnBr, ZrCp2Cl
X = I, Br, H
cross-coupling
RCH2PO(OEt)2
HWE reaction
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R1
O
This work:
OH
R
HO
R
R
high chemoselectivity
Solid acid
[Emim]Cl
simple access to branched 1,3-dienes
recyclable catalytic system
facile separation
R
R = Alkyl, Ar,
Cycloalkyl
Results and Discussion
At the outset, we chose the dehydration of pinacol as the model reaction. It is wellꢀknown that this
process typically involves two competitive pathways: 1) 1,2ꢀelimination of H2O to form 2,3ꢀ
dimethylbutadiene (i.e. 2a in Scheme 2). 2) pinacol rearrangement to form pinacolone (i.e. 3a in Scheme
2). In the past decades, a broad range of acid catalysts including Brønsted acids,11 Lewis acids,12 solid
acids13 and supercritical water14 have been used in the dehydration of pinacol. However, almost all of the
reactions exclusively proceeded via a rearrangement pathway to produce 3a. The selective dehydration of
pinacol to 2a was rarely reported.15 To increase the diene selectivity, we investigated the influence of
solvents on the H2SO4ꢀcatalyzed pinacol dehydration. The results are illustrated in Table 1. Under
solventꢀfree condition, the dehydration gave 3a as the single product (Table 1, entry 1). Similar results
were obtained when we used nonꢀpolar solvent (such as toluene) and protic solvent (such as water) (Table
1, entries 2 and 3). In contrast, 2a was obtained as the main product when the reaction was carried out in
some polar aprotic solvents such as dimethyl sulfoxide (DMSO) and Nꢀmethylꢀ2ꢀpyrrolidone (NMP)
(Table 1, entries 4 and 5). In view of the fact that ionic liquids are a new class of polar aprotic solvents
that are endowed with strong electrostatic force,16 we surmised that ILs could drastically increase the
diene selectivity. Just as anticipated, 2a was afforded in 85% carbon yield with a high chemoselectivity
when the dehydration was conducted in [Emim]Cl (Table 1, entry 6). However, the reactions in
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