Photodriven Transfer Hydrogenation of Olefins

Article abstract of DOI:10.1002/ejoc.201403021

An improved practical method for the photodriven diimide reduction of olefins was investigated. This catalyst-free procedure proceeds at ambient temperature, utilizes air as oxidant and a lower hydrazine loading, and produces inert nitrogen gas as the sole byproduct. Several functional groups were tolerated, and in some cases, the reaction was chemoselective. Challenging substrates such as cinnamate ester derivatives and trans-stilbene were reduced in excellent yields. The small amount of UVA rays emitted from a household compact fluorescent light bulb was proposed to enable the cis/trans isomerization of the diimide and to promote the loss of hydrogen from the diimide.

Full text of DOI:10.1002/ejoc.201403021

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403021
Photodriven Transfer Hydrogenation of Olefins
Dasheng Leow,*[a] Ying-Ho Chen,[a] Tzu-Hang Hung,[a] Ying Su,[a] and Yi-Zhen Lin[a]
Dedicated to the memory of Professor Carlos F. Barbas III
Keywords: Diimides / Hydrazine / Reduction / Hydrogen transfer / Alkenes
An improved practical method for the photodriven diimide
reduction of olefins was investigated. This catalyst-free pro-
selective. Challenging substrates such as cinnamate ester
derivatives and trans-stilbene were reduced in excellent
cedure proceeds at ambient temperature, utilizes air as oxi- yields. The small amount of UVA rays emitted from a house-
dant and a lower hydrazine loading, and produces inert
nitrogen gas as the sole byproduct. Several functional groups
were tolerated, and in some cases, the reaction was chemo-
hold compact fluorescent light bulb was proposed to enable
the cis/trans isomerization of the diimide and to promote the
loss of hydrogen from the diimide.
Introduction
The hydrogenation of olefins is one of the most impor-
tant organic transformations. Traditional methods rely on
heterogeneous[1] and homogeneous[2] transition-metal cata-
lysts. There are concerns on issues such as chemoselectivity
(cleavage of benzyl group) and alkene migration. On the
other hand, metal-free transfer hydrogenation by using di-
imide, which is generated in situ from hydrazine, offers com-
plementary reactivity.[3] The history of this reaction can be
traced back a century ago when it was first discovered by
Hanusˇ in 1905.[4a] However, its synthetic applications re-
mained relatively unexplored until the 1960s. Several groups
independently established the important role of diimide and
expanded its utility in organic synthesis.[4b–4e]
Diimide reduction is one of the greener transfer-hydro-
genation reactions. Hydrazine has a high hydrogen content
(12.5%). It is cheap and abundantly available (0.142 USD
per gram, Sigma–Aldrich). The reaction’s only byproduct is
gaseous N2. Owing to these advantages, there has been re-
Prabhu’s group disclosed a guanidinium salt catalyzed
newed interest in the development of new catalytic systems system [Equation (2)].[8] They proposed that diimide was
for the diimide reduction reaction in recent years. Imada, activated by the catalyst through double hydrogen bonding.
Naota, and co-workers employed synthetic flavins as cata- Recently, the Jamison and Kappe groups developed con-
lysts for the oxidation of hydrazine [Equation (1)].[5,6] Sub- tinuous-flow technologies for the reduction of olefins
sequently, several other groups also contributed to this through hydrogen transfer from diimide.[9] In particular,
area.[7]
Kappe and co-workers used hydrazine for their technology
[Equation (3), conditions A].[9b] A couple of teams found
that if both the reaction temperature and the amount of
hydrazine were increased, the reduction also proceeded
smoothly without any catalyst [Equation (3), condi-
tions B].[10] However, the substrate scope was narrower than
that of the catalytic examples. Other miscellaneous catalysts
have also been explored.[11]
[a] Department of Chemistry
National Tsing Hua University
101, Sec 2, Kuang-Fu Rd., Hsinchu 30013, Taiwan, ROC
E-mail: dsleow@mx.nthu.edu.tw
http://chem-en.web.nthu.edu.tw/bin/home.php
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403021.
Eur. J. Org. Chem. 2014, 7347–7352
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D. Leow, Y.-H. Chen, T.-H. Hung, Y. Su, Y.-Z. Lin
SHORT COMMUNICATION
Noncatalytic processes often require long reaction times, Table 1. Reaction optimization.[a,b]
high temperature, and an oxygen atmosphere. A large excess
amount of hydrazine is also needed because of dispropor-
tionation and decomposition of diimide to N2. Although
there are many reports on increasing the rate of hydrazine
oxidation, few describe improvements in the efficiency of
the diimide hydrogenation step.[8] Diimide is generated as
an equal mixture of both cis and trans isomers (Figure 1).
They undergo slow interconversion, as the trans isomer is
more stable than the cis isomer by approximately
8 kcalmol–1.[3] However, experimental observations indicate
that the cis isomer is the reactive species.[4b–4e]
Figure 1. Physical properties of trans- and cis-diimide.
[a] Unless otherwise noted, the reactions were performed with
The photoisomerization of diimide is well studied.[12,13]
The maximum absorption for trans-diimide is 355 nm.[13c]
We hypothesize that the rate of isomerization will increase
under photoirradiation. Photocatalysis has already been
utilized in synthetic organic chemistry.[14,15] Currently, this
methyl cinnamate (1a, 0.12 mmol) and N2H4·H2O (2.0 equiv.) in
solvent (1.0 mL) at ambient temperature under irradiation of a
light source under a specific atmosphere for 18 h. [b] Yield was de-
termined by analysis of the unpurified reaction mixture by
1H NMR spectroscopy by using MeNO2 as an internal standard.
[c] N2H4·H2O (2.5 equiv.) was used. [d] Reaction temperature
area is under intense development.[16,17] Herein, we disclose peaked at 35 °C. [e] Reaction was conducted at 35 °C. See the
Supporting Information for details.
the transfer hydrogenation of olefins accelerated by the
isomerization of diimide with irradiation from household
compact fluorescent light (CFL) bulbs [Equation (4)].
found to reach a maximum of 35 °C (Table 1, Entries 4 and
5). If the same reaction was heated to 35 °C in the dark,
the yield remained low. This proved that light was essential
(Table 1, Entry 7). Using light-emitting diodes (LEDs) with
specific ranges of wavelengths such as blue (460–490 nm)
Results and Discussion
The noncatalytic diimide reduction of cinnamate esters is and green (520–550 nm) with no UV emission led to lower
still not efficient.[6b,8,10a,11d] Therefore, we commenced our yields (Table 1, Entries 8 and 9). However, irradiation with
studies on the transfer hydrogenation of methyl cinnamate a longer-wavelength 4 W UV lamp (365 nm) led again to
(1a) with hydrazine under household CFL irradiation an enhancement in the yield (Table 1, Entry 10).
(Table 1). A quick screening of solvents revealed that
Under these optimized conditions, an array of syntheti-
MeCN was most favorable (Table 1, Entry 1; Table S1). The cally useful cinnamate derivatives 1a–l were tested (Table 2).
reaction yield was enhanced under an O2 atmosphere with Generally, the photostimulated transfer hydrogenation of
irradiation of 6500 K CFL (Table 1, Entry 2) as well as cinnamate esters afforded saturated esters 2a–l in excellent
2700 K CFL (Table S2, Entry 3). The reaction slowed down yields. However, the reaction yield was low (20%) if the
if argon was used (Table 1, Entry 3). It is likely that di- aromatic ring was replaced by an aliphatic straight chain
oxygen from air is responsible for the oxidation of hydraz- such as an n-butyl group. This indicated that steric and elec-
ine to diimide. By increasing the amount of N2H4 (from 2 tronic effects of the olefin were important factors in the
to 2.5 equiv.), an optimal yield was obtained (Table 1, En- diimide reduction, although this was not unique to our sys-
try 4). Both 6500 and 2700 K CFL emit the majority of tem. Dimethyl maleate and maleimides were also tested, but
light in the visible region with a small amount of UVB they gave significant amounts of the corresponding cyclic
(280–315 nm), UVA (315–400 nm), and infrared (Ͼ700 nm) hydrazides. Cinnamaldehyde and chalcone gave the desired
radiation. They could be used interchangeably, but the reduced products along with imine, diimine, and pyrazole
latter was chosen (Table 1, Entry 5).
side products.[10b]
The reaction slowed down significantly in the absence of
Another important class of olefins is the styrene deriva-
light (Table 1, Entry 6). The heating effect of the lamp was tives, which are electronically neutral relative to cinnamate
also investigated. The reaction temperatures under both ester derivatives. Many noncatalytic examples in the litera-
6500 and 2700 K CFL irradiation were measured and were ture reported modest yields for the diimide reduction of
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Photodriven Transfer Hydrogenation of Olefins
Table 2. Scope of cinnamate ester derivatives.[a,b]
Table 3. Scope of the styrene derivatives.[a,b]
[a] Unless otherwise noted, the reactions were performed with styr-
ene 3 (0.12 mmol) and N2H4·H2O (2.5 equiv.) in THF (1.0 mL) un-
der irradiation with a 23 W CFL for 18 h. [b] Yield of isolated
product. [c] N2H4·H2O (2 equiv.), 12 h; then N2H4·H2O (1 equiv.),
12 h. [d] As a result of the volatility of the product, the yield was
determined by 1H NMR spectroscopy; total recovered materials
(starting material and product; some were evaporated off during
concentration) in parentheses.
Table 4. Diverse range of unsaturated substrates.[a,b]
[a] Unless otherwise noted, the reactions were conducted with 1
(0.12 mmol) and N2H4·H2O (2.0 equiv.) in MeCN (1.0 mL) under
irradiation with a 23 W CFL for 12 h; then, N2H4·H2O (1.0 equiv.)
was added, and the mixture was stirred for 12 h. [b] Yield of iso-
lated product. [c] N2H4·H2O (2.5 equiv.), 18 h. See the Supporting
Information for details.
styrene derivatives in air.[8,10] A diversity of styrene deriva-
tives was examined (Table 3). The optimized solvent was
THF (Table S5). Gratifyingly, the styrene reduction reac-
tion was also accelerated by light to full consumption of
the starting material at ambient temperature in air (see the
Supporting Information for details). Bulky substrates such
as 2-substituted styrene 3a gave the desired product in excel-
lent yield. This method was also applicable to cyclic styrene
3h, although the internal double bond was sterically
hindered.
We then explored a wide range of olefins as well as the
functional-group tolerance under our reaction conditions
(Table 4). Generally, other than cinnamate esters, amide 5a
and alcohol 5b were compatible with our reaction condi-
tions. These are also sterically more demanding substrates [a] Unless otherwise noted, the reactions were conducted with ole-
containing internal double bonds.[8] Quinone 5c was
fin 5 (0.12 mmol) and N2H4·H2O (2.0 equiv.) in MeCN (1.0 mL)
under irradiation of a 23 W CFL for 12 h; then N2H4·H2O
successfully reduced to hydroquinone 6c. Stilbene trans-5d
has remained a problematic substrate requiring harsh con-
(1.0 equiv.) was added, and the mixture was stirred for 12 h.
[b] Yield of isolated product. [c] N2H4·H2O (2.5 equiv.), 18 h. [d] As
ditions for the diimide reduction.[7a,8,11d] However, under
a result of the volatility of the product, the yield was determined
by 1H NMR spectroscopy; total recovered materials (starting mate-
our optimized conditions, it was reduced smoothly in good
rial and product; some were evaporated off during concentration)
in parentheses.
yields. It also gave a better yield than its cis counterpart,
which is well known in the literature.[3a] Substrates 5e and
5f containing allylic heteroatom groups reacted smoothly
to give saturated products in excellent yields. Notably, the
For dioxane 6g, diimide reduction occurred with excel-
benzyloxycarbonyl (Cbz) group in amine 6f remained lent trans selectivity, in accord with the literature.[18] The
intact.
selectivity was attributed to the anisotropy effect induced
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D. Leow, Y.-H. Chen, T.-H. Hung, Y. Su, Y.-Z. Lin
SHORT COMMUNICATION
by the oxygen atoms, as previously proposed. An isolated process. Notably, the isolation of deuterated hydrazine,
olefin was also an effective substrate, as demonstrated by which is explosive, could be avoided. It was generated
6h. The observed cis selectivity is similar to that obtained in situ simply by prestirring N2H4·H2O in a mixture of
in traditional Pd/C reductions or in iron-catalyzed reductive MeCN and D2O. Similarly, the reaction proceeded with
couplings with the use of silanes.[19] The alkyne analogue of 95% yield under 4 W UVA irradiation under an air atmo-
5e was also tested, but the reduction did not stop at the sphere in 10 h.
alkene stage; as such, a mixture of the alkene and the fully
saturated product was obtained. Unlike Lindlar’s catalyst,
this reaction was not chemoselective towards alkene forma-
tion. The alkyne analogue of 5e was fully reduced to the
saturated alkane if hydrazine (4 equiv.) was used. Last but
not least, sterically encumbered estrone derivative 8 bearing
two vicinal all-carbon quaternary centers was isolated in
85% yield with a diastereomeric ratio of 5:1 in favor of the
Scheme 2. Selective cis-1,2-deuteration of olefin 9.
cis isomer (Scheme 1).
From these experimental observations,
a plausible
mechanism is that the minute amount of UVA emitted from
the household CFL at a distance of 2 cm is sufficient to
induce the cis/trans isomerization[12,13] of the small amount
of diimide present in the solution (Scheme 3). Then, re-
duction of the olefin proceeds with cis-diimide through a
six-membered transition state. At this stage, we also cannot
rule out the possibility that the UVA irradiation promotes
the stepwise loss of hydrogen from diimide and thus acceler-
ates the reduction step.[3c,20] More in-depth studies will be
needed to elucidate the reaction mechanism.
Scheme 1. Photodriven diimide reduction of estrone derivative 7.
Ester of Feist’s acid 6i contained a sensitive cyclopropyl
group that survived the reaction. From this result, we could
exclude any radical mechanism for the photodriven diimide
reduction reaction. To gain further insight into the mecha-
nism, preliminary kinetic studies were conducted (see Fig-
ure S2). The kobs for the photodriven and nonlight (control)
reactions were 1.62ϫ10–4 and 4.45ϫ10–5 –1 s–1, respec-
m
tively. The overall order of the reaction is two.
Scheme 3. Plausible mechanism by photoinduced isomerization of
diimide.
The half-life of diimide in the gas phase at room tem-
perature is several minutes.[3c] Therefore, it is likely that a
small amount of diimide is present in the solution at any
given time. The photochemistry of diimide in the gas phase
was studied at wavelengths of 310–405 nm. Theoretical
studies predicted the stepwise loss of hydrogen in the
photolysis of cis-diimide.[3c,20] We observed that the yield
Conclusions
We developed an efficient method that utilizes a photo-
was also improved under irradiation with a long-wavelength driven strategy for the diimide reduction of olefins at ambi-
4 W UV lamp (365 nm; Table 1, Entry 10), even achieving ent temperature. Currently, many noncatalytic processes use
an 80% yield under 8 h (Table S4, Entry 12) compared to a pure oxygen as the oxidant, which is not safe for large-scale
37% yield for irradiation under a CFL (Figure S2).
applications. Our new protocol is operationally easy and
We also determined the stereochemistry of the photoin- obviates the need for pure oxygen gas. Among the noncata-
duced diimide reduction of a disubstituted alkyne, diphen- lytic processes, the amount of hydrazine is also reduced,
ylacetylene, which is a challenging substrate.[10b] Upon which makes this reaction less hazardous, more practical,
treatment of this substrate with N2H4·H2O (4 equiv.) under and cost-effective. This reaction tolerates several functional
4 W UVA irradiation in air for 64 h, cis-stilbene and bi- groups and is chemoselective in some cases. We also over-
benzyl were obtained in 18 and 25% yield, respectively. No came several challenging substrates that contain internal
trans-stilbene was observed. Hence, the reduction might double bonds, including cinnamate derivatives and trans-
proceed through a concerted mechanism involving a cis-di- stilbene. We proposed that the reaction proceeds through
imide intermediate.[10b] 1,2-Deuteration of olefin 9 was per- irradiation of diimide with a small amount of UVA from a
formed, and the reduction proceeded in a cis fashion in household CFL. This causes cis/trans isomerization of di-
99% yield under an air atmosphere over 10 h (Scheme 2).[6] imide and promotes the loss of hydrogen from diimide. Fur-
The relative stereochemistry was determined by comparison ther development of continuous-flow technology can poten-
with the literature data,[6a,6c] and this result reaffirmed the tially lead to large-scale applications. Mechanistic studies
cis selectivity of the photoinduced diimide reduction are currently underway in our laboratory.
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Photodriven Transfer Hydrogenation of Olefins
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details and copies of the1H NMR and 13C NMR
spectra.
Experimental Section
General Procedure for the Photodriven Transfer Hydrogenation of
Olefins
Method A: A 15 cmϫ2 cm, 47 mL Pyrex tube with a polytetra-
fluoroethylene (PTFE) lined screw cap equipped with a magnetic
stir bar was charged with olefin 1 or 5 (0.12 mmol, 1.0 equiv.) and
N2H4·H2O (12 μL, 0.24 mmol, 2.0 equiv.). CAUTION: Hydrazine
is an extremely toxic chemical and should be handled in a well-
maintained fume hood! MeCN (HPLC grade, 1.0 mL) was used to
wash down the solids on the sides of the wall. The tube was then
capped and placed approximately 2 cm from the light source (23 W
2700 K CFL). After stirring for 12 h, N2H4·H2O (6.0 μL,
0.12 mmol, 1.0 equiv.) was added, and the mixture was kept for
another 12 h. Then, the crude mixture was filtered through a short
pad of silica gel. EtOAc (3ϫ 2 mL) was used for washing. The
filtrate was concentrated in vacuo, and the resulting residue was
purified by column chromatography or preparative TLC (hexanes/
EtOAc). This method can be used for olefins 1b, 1d, 1e, 1g, 1i, 1j,
5a, 5b, 5d, and 5h.
Acknowledgments
We gratefully acknowledge the National Tsing Hua University
(NTHU) and the Ministry of Science and Technology of Taiwan
(102-2113-M-007-017-MY2) for financial support. We also thank
Vijaykumar H. Thorat for preliminary observations.
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Method B: A 15 cmϫ2 cm, 47 mL Pyrex tube with a PTFE-lined
screw cap equipped with a magnetic stir bar was charged with
olefin 1, 5, or 7 (0.12 mmol, 1.0 equiv.) and N2H4·H2O (15 μL,
0.30 mmol, 2.5 equiv.). CAUTION: Hydrazine is an extremely toxic
chemical and should be handled in a well-maintained fume hood!
MeCN (HPLC grade, 1.0 mL) was used to wash down the solids
on the sides of the wall. The tube was then capped and placed
approximately 2 cm from the light source (23 W 2700 K CFL).
After stirring for 12 h, the crude mixture was filtered through a
short pad of silica gel. EtOAc (3ϫ 2 mL) was used for washing.
The filtrate was concentrated in vacuo, and the resulting residue
was purified by column chromatography or preparative TLC
(hexanes/EtOAc). This method can be used for olefins 1a, 1c, 1f,
1h, 1k, 5c, 5e–g, 5i, and 7.
Method C: A 15 cmϫ2 cm, 47 mL Pyrex tube with a PTFE-lined
screw cap equipped with a magnetic stir bar was charged with styr-
ene 3 (0.12 mmol, 1.0 equiv.) and N2H4·H2O (12 μL, 0.24 mmol,
2.0 equiv.). CAUTION: Hydrazine is an extremely toxic chemical
and should be handled in a well maintained fume hood! THF (HPLC
grade, 1.0 mL) was used to wash down the solids on the sides of
the wall. The tube was then capped and placed approximately 2 cm
from the light source (23 W 6500 K CFL). After stirring for 12 h,
N2H4·H2O (6.0 μL, 0.12 mmol, 1.0 equiv.) was added, and the mix-
ture was kept for another 12 h. Then, the crude mixture was filtered
through a short pad of silica gel. Et2O (3ϫ 2 mL) was used for
washing. The filtrate was concentrated in vacuo, and the resulting
1
residue was used for H NMR spectroscopy analysis. This method
can be used for olefins 3c and 3h.
[12] H. Suginome, CRC Handbook of Organic Photochemistry and
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Method D: A 15 cmϫ2 cm, 47 mL Pyrex tube with a PTFE-lined
screw cap equipped with a magnetic stir bar was charged with styr-
ene 3 (0.12 mmol, 1.0 equiv.) and N2H4·H2O (15 μL, 0.30 mmol,
2.5 equiv.). CAUTION: Hydrazine is an extremely toxic chemical
and should be handled in a well maintained fume hood! THF (HPLC
grade, 1.0 mL) was used to wash down the solids on the sides of
the wall. The tube was then capped and placed approximately 2 cm
from the light source (23 W 6500 K CFL). After stirring for 18 h,
the crude mixture was filtered through a short pad of silica gel.
Et2O (3ϫ 2 mL) was used for washing. The filtrate was concen-
trated in vacuo, and the resulting residue was purified by column
chromatography or preparative TLC (hexanes/EtOAc). This
method can be used for olefins 3a, 3b, and 3d–g.
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Received: July 31, 2014
Published Online: October 15, 2014
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