Tetrahedron Letters
Mild conversion of propargylic alcohols to
a
in ionic liquids (ILs); a new ‘metal free’ life for the Rupe
rearrangement
,b-unsaturated enones
⇑
Ganesh C. Nandi, Benjamin M. Rathman, Kenneth K. Laali
Department of Chemistry, University of North Florida, 1 UNF Drive, Jacksonville, FL 32224, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A mild and selective transition metal free protocol for the conversion of propargylic alcohols to cyclic and
Received 7 August 2013
Revised 3 September 2013
Accepted 10 September 2013
Available online 18 September 2013
acyclic a,b-unsaturated enones via the Rupe rearrangement is reported. The method utilizes the Brønsted
acidic ionic liquid [BMIM-SO3H][OTf] as catalyst and [BMIM][PF6] as solvent and offers the potential for
recycling and reuse of the IL solvent. The feasibility to synthesize bicyclic fused cyclopentenone deriva-
tives via a Rupe ? Aldol ? Nazarov sequence utilizing this protocol has also been demonstrated.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Conjugated enones
Propargylic alcohols
Metal free Rupe rearrangement
Imidazolium ionic liquids
Recycling/reuse
Rupe–Aldol–Nazarov sequence
Synthesis of
a
,b-enones and
a
,b-enals via formal isomerization
nation of iminophosphorane-phosphane ligands to [ReBr(CO)5]
was shown to be quite active for the Rupe isomerization under
microwave irradiation in ionic liquid solvent.13 Except for the
works reported in Refs. 7 and 13, the primary focus of the afore-
mentioned studies has been the Meyer–Schuster reaction.
of propargylic alcohols represents an attractive, atom-economical,
approach that provides direct access to these highly versatile syn-
thetic building blocks.1–3 Propargylic cations are key intermediates
involved in these transformations. Whereas trapping (by H2O) of
the mesomeric allenyl cations
(
c
-substitution) leads to the
Despite considerable progress that has been achieved through
transition-metal catalysis, there is a need to develop readily avail-
able, low cost, methods that also offer potential for recycling and
reuse. In relation to our continuing interest in the utility of ionic
liquids as catalysts and solvents for synthetic carbocation and
onium ion chemistry,14 and our more recent focus on generation
and synthetic utility of tamed propargylic cations,15 we report here
on a transition metal free, IL-based, method for the selective con-
Meyer–Schuster rearrangement products (enones or enals), b-
elimination (when applicable) triggers the Rupe rearrangement,
forming an enyne and subsequently the enone. These mechanistic
pathways are outlined in Scheme 1.
The traditional protocols employed in the Meyer–Schuster and
Rupe reactions typically involve the use of large quantities of
strong Brønsted acids, and consequently suffer from low chemose-
lectivity and poor yields.4 The utility of Nafion-H as a solid acid cat-
alyst for the Rupe rearrangement was shown in representative
cases.5 During the last decade a number of transition metal-based
methods were discovered for the conversion of propargylic alco-
hols to enones and enals. These include rhodium/BINAP complex,6
allyl-ruthenium(II) complex/TFAH,7 rhenium(V)-oxo complex,8
and an oxovanadate complex under microwave.9 A number of
gold-catalyzed processes have also been reported including
version of propargylic alcohols to
Rupe reaction.
a,b-unsaturated enones via the
Optimization studies were performed by using 1-ethynylcyclo-
hexanol 1a as a test substrate (Table 1). Whereas acceptable iso-
lated yields of the enone could be obtained after 3 days at room
temperature, optimal conversions were achieved at 50 °C in less
than an hour by employing 30 mol % of [BMIM-SO3H][OTf].
These conditions were then adopted in a subsequent survey of
the scope of the reaction (Table 2).16 Monitoring the progress of
the reactions (TLC) showed that tweaking of the reaction times
was necessary depending on the substrates in order to achieve
optimal conversions. It is noteworthy that competing Meyer–
Schuster products were not detected in these reactions. Reaction
of propargylic alcohol 1e resulted in the expected enone 2e along
[(NHC)AuCl]AgX,10 Ph3PAuNTf2,11 and
a triazole-coordinated
Au(I) complex.12 A carbonyl-rhenium complex formed by coordi-
⇑
Corresponding author. Tel.: +1 904 620 1503; fax: +1 904 620 3535.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.