Facile regio- and stereoselective metal-free synthesis of all-carbon tetrasubstituted alkenes bearing a C(sp3)-F unit via dehydroxyfluorination of morita-baylis-hillman (MBH) adducts

Article abstract of DOI:10.1021/ol501855m

Highly E-selective all-carbon tetrasubstituted alkenes with a C(sp ³)-F unit have been synthesized through a dehydroxyfluorination of Morita-Baylis-Hillman (MBH) adducts which can be readily prepared from α,β-unsaturated carbonyl compounds and α-keto esters. A variety of subsequent transformations afforded monofluoromethyl substituted heterocycles in high yields.

Full text of DOI:10.1021/ol501855m

Letter
pubs.acs.org/OrgLett
Facile Regio- and Stereoselective Metal-Free Synthesis of All-Carbon
Tetrasubstituted Alkenes Bearing a C(sp3)−F Unit via
Dehydroxyfluorination of Morita−Baylis−Hillman (MBH) Adducts
Shinobu Takizawa,* Fernando Arteaga Arteaga, Kenta Kishi, Shuichi Hirata, and Hiroaki Sasai*
The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka, Ibaraki-shi, Osaka 567-0047, Japan
S
* Supporting Information
ABSTRACT: Highly E-selective all-carbon tetrasubstituted alkenes with
a C(sp3)−F unit have been synthesized through a dehydroxyfluorination
of Morita−Baylis−Hillman (MBH) adducts which can be readily
prepared from α,β-unsaturated carbonyl compounds and α-keto esters.
A variety of subsequent transformations afforded monofluoromethyl
substituted heterocycles in high yields.
Scheme 1. Facile, Regio- and Stereoselective Synthesis of
All-Carbon Tetrasubstituted Alkenes Bearing a C(sp3)−F
Unit
lkenes represent one of the most versatile and widespread
classes of organic compounds.1 In current organic
A
chemistry, tetrasubstituted alkenes have received considerable
attention owing to their high potential when incorporated in
liquid crystals, molecular devices, and pharmaceutical com-
pounds.2 Recent efficient approaches for their construction3
mainly involve the use of transition metal intermediates via
carbometalation of alkynes, or olefin metathesis. However, the
regio- and stereocontrol of the tetrasubstituted alkenes is a key
issue to be addressed in these methodologies.3
however, the regio- and stereocontrol is not always favorable
because the fluorination steps could proceed through an SN2′,
SN2, SNi′, and/or SN1 mechanism.5a,i,t,u Byproducts resulting
from carbonium type rearrangements and dehydration are also
occasionally observed.5a We envisioned that the dehydroxy-
fluorination of 1 would proceed via a five-membered cyclic
orthoester-like9 intermediate A (Scheme 1), by interaction
between two closely located carbonyl groups, followed by the
smooth addition of fluoride to the double bond under the SN2′
mechanism, to predominantly afford E-alkenes 2.
We started our study evaluating the reaction of MBH adduct
1a as a model substrate with diethylaminosulfur trifluoride
(DAST), which is a commonly used reagent for the
dehydroxyfluorination of allylic alcohols (Table 1).5a Initial
attempts yielded the desired 2a in high conversion with high E-
stereoselectivity (entries 1−3). The corresponding hydroxyl-
fluoro exchanged regioisomer 3a, generated via an SN1-type
process, was not obtained. Reducing the reaction temperature
(entries 1 and 3) or decreasing the amount of DAST (entry 2)
led to no significant improvement. When using bis(2-
methoxyethyl)aminosulfur trifluoride (Deoxo-Fluor) as the
fluoride source,10 2a was obtained in a higher yield (75%),
compared with DAST, maintaining high stereoselectivity (E:Z
= >20:1), as shown in entry 4. Although solvent such as CHCl3,
toluene, and THF (entries 5 and 8) gave a complex mixture or
Fluorine-containing compounds are gaining much attention
in material sciences and medicinal chemistry because the
inclusion of a F atom into organic molecules dramatically
changes the physical, chemical, and biological properties, for
example, lipophilicity, bioavailability, and metabolic stability.4
Alkenes with a C(sp3)−F unit, allylic fluorides, have been
identified as important building blocks for the preparation of
biologically active compounds,4,5 and transition-metal-mediated
allylic fluorination has been recognized as a common and
straightforward method for their construction. However, this
method has proven to be challenging because of the dual
reactivity profile of fluoride (nucleophile and base), leading to
undesired products.5e−h,j,k,n,o,r,s Moreover, the practical use of
toxic and expensive transition metal catalysts is best avoided to
prevent residual metal impurities. Herein, we report a facile,
nonmetallic synthesis of all-carbon tetrasubstituted alkenes 2
bearing a C(sp3)−F unit from readily available allylic tertiary
alcohols 1 by Deoxo-Fluor-mediated dehydroxyfluorination
(Scheme 1).
The Morita−Baylis−Hillman (MBH) reaction is one of the
most useful and atom economical carbon−carbon bond
forming reactions between the α-position of an electron-
deficient alkene, such as an enone, and the sp2 carbon atom of
an aldehyde.6 Highly functionalized tertiary alcohols (MBH
adducts) 1 are readily obtained from an MBH reaction using
ketones such as an α-keto ester.7,8 Dehydroxyfluorination of 1
via an SN2′ mechanism would readily provide all-carbon
tetrasubstituted and vicinal fluorinated alkenes 2 (Scheme 1);
Received: June 27, 2014
© XXXX American Chemical Society
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Letter
a
Table 1. Optimization for the Synthesis of Tetrasubstituted
Alkenes
Scheme 2. Substrate Scope of the Dehydroxyfluorination
a
fluoride source
conversion
yield
b
b
entry
(equiv)
solvent
CH2Cl2
temp (°C)
(%)
(%)
c
1
2
3
4
DAST (1.8)
DAST (1.1)
DAST (1.8)
0 to 25
>99
55
d
CH2Cl2
(ClCH2)2
CH2Cl2
0
82
50
54
75
−20 to 0
0
>99
>99
Deoxo-Fluor
(1.8)
5
Deoxo-Fluor
(1.8)
CHCl3 or
toluene
0
>84
95
trace
73
6
Deoxo-Fluor
(1.8)
(ClCH2)2
1,4-dioxane
THF
0
7
Deoxo-Fluor
(1.8)
10
0
>99
83
64
8
Deoxo-Fluor
(1.8)
20
c
9
Deoxo-Fluor
(1.8)
diglyme
diglyme
diglyme
0
96
96
10
11
Deoxo-Fluor
(1.8)
−10
0
87
80
54
Deoxo-Fluor
(1.5)
67
a
The reaction was performed for 1 h under N2 in the indicated solvent
(0.2 M), DAST (diethylaminosulfur trifluoride); Deoxo-Fluor (bis(2-
b
methoxyethyl)aminosulfur trifluoride). Determined by 1H NMR
c
using 1,3,5-trimethoxybenzene as an internal standard. Isolated yield.
d
Reaction was performed for 5 h.
a
The reaction was performed for 1 h under N2 in diglyme (0.2 M).
b
Isolated yields after column chromatography. Performed at −20 °C
low yield of 2a, the use of diglyme had a beneficial impact,
resulting in a 96% isolated yield of 2a (entry 9). A lower
temperature (entry 10) or decreased amount of Deoxo-Fluor
(entry 11) resulted in lower yields.
for 1 h. c Performed at −40 °C for 1 h in CH2Cl2. d Determined by 1H
e
f
NMR. Stirred for 2 h. Stirred for 17 h, >90% conversion.
Having optimized the dehydroxyfluorination conditions, we
then investigated the substrate scope (Scheme 2). When
alcohols bearing an electron-withdrawing group on the aryl unit
1b−e (R3 = 4-Br, 4-Cl, 4-CF3, or 3-Cl−C6H4) were utilized, the
corresponding alkenes 2b−e were obtained in 74%−91% yields
with excellent stereoselectivities (E:Z = >20:1). However, the
alcohols with an electron-donating moiety on the aryl unit 1f−
h (R3 = 4-Me-C6H4, 4-MeO-C6H4, or 1-Np) afforded alkenes
2f−h in lower yields, presumably due to a competitive benzyl
carbocation stabilization effect preventing the SN2′ fluorination.
Trifluoromethyl, methyl ester, and benzoyl groups 1i−j and 1s
(R2 = CO2CH2CF3, CO2Me, or COPh) were easily introduced
and conserved after the olefination giving 2i−j and 2s. Alcohols
1k−o derived from acrylates and acyclic or cyclic enones (R1 =
CO2R or COR) gave the corresponding alkenes 2k−o in 70%−
92% yields with excellent E-stereoselectivities. Cyano-substi-
tuted alcohol 1p (R1 = CN) gave 2p in high regioselectivity
(E:Z = 13:1).11 Notably, the scope was further expanded by the
use of aliphatic systems 1q,r (R3 = Cy or Me), which were
found to smoothly deliver high yields of the alkenes 2q,r
without formation of any byproducts. The E-geometry of the
tetrasubstituted alkenes was unambiguously determined by X-
ray crystallographic analysis of 2q (Figure 1).12,13
Figure 1. ORTEP drawing of tetrasubstituted alkene 2q.
Scheme 3. Reaction of 1t and 1u with Deoxo-Fluor
carbonyl group under the optimized reaction conditions.
Interestingly, the desired tetrasubstituted alkenes 2 were
obtained in low or poor yields (2t: 14%; 2u: trace), whereas
the hydroxyl-fluoro exchanged products 3 and the dehydrated
products 4 were mainly formed (3t: 43%, 4t: 33%; 3u: 26%, 4u:
24%), respectively.
It was clear that substrates 1 possessing two carbonyl groups
were effective in the regio- and stereoselective transformation.
To clarify the role of these carbonyl groups in the reaction, we
examined substrates 1t and 1u (Scheme 3), possessing one
B
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The sterically and thermodynamically unfavorable E-selective
allylic fluorination mechanism has not been unequivocally
established;14 however, these results, along with previous
reports,5a,i,t,u,9,15 may be in agreement with a five-membered
cyclic orthoester-like intermediate directing the observed E-
stereoselectivity, as shown in Scheme 4. The OH of MBH
Finally, epoxidation of 2p gave oxirane 8, bearing two
tetrasubstituted carbon stereogenic centers, as a single
diastereomer in good yield.19
In summary, we have developed a facile method for the
preparation of alkenes bearing four different C-substituents
with high regio- and stereocontrol. A highly challenging
C(sp3)−F bond was also generated as part of the present
synthetic method. Furthermore, the alkenes possessing a
fluoromethyl group were successfully converted into structur-
ally appealing 3-, 4-, or 5-membered heterocycles, exhibiting
their importance as synthetic building blocks. Further
investigations into the reaction mechanism and scope, as well
as application to the synthesis of biologically active compounds,
are currently underway.
Scheme 4. Plausible Reaction Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and compound characterization data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
adduct 1 reacts with the sulfur moiety of Deoxo-Fluor resulting
in int-I. Due to the proximity of the two carbonyl groups, the
formation of an orthoester-like intermediate int-II is possible.16
The allylic fluorination most likely proceeds through int-IIa via
a favorable six-membered transition state in a concerted
manner to predominantly form the E-alkene, although a
stepwise process through int-IIb would also be possible.
To demonstrate the significant synthetic utility of the
tetrasubstituted alkenes 2 as building blocks, several trans-
formations were carried out (Scheme 5). Base-mediated
*E-mail: taki@sanken.osaka-u.ac.jp.
*E-mail: sasai@sanken.osaka-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Advanced Molecular Trans-
formations by Organocatalysis” from The Ministry of
Education, Culture, Sports, Science and Technology
(MEXT), Japan, the CREST project of the Japan Science and
Technology Corporation (JST), and Advanced Catalytic
Transformation Program for Carbon Utilization (ACT-C) of
JST. We acknowledge the technical staff of the Comprehensive
Analysis Center of ISIR, Osaka University (Japan).
Scheme 5. Synthetic Transformations of Alkenes 2
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