Room temperature iron catalyzed transfer hydrogenation usingn-butanol and poly(methylhydrosiloxane)

Article abstract of DOI:10.1039/d0gc04175k

Reduction of carbon-carbon double bonds is reported using a three-coordinate iron(ii) β-diketiminate pre-catalyst. The reaction is believed to proceedviaa formal transfer hydrogenation using poly(methylhydrosiloxane), PMHS, as the hydride donor and a bio-alcohol as the proton source. The reaction proceeds well usingn-butanol and ethanol, withn-butanol being used for substrate scoping studies. Allyl arene substrates, styrenes and aliphatic substrates all undergo reduction at room temperature. Unfortunately, clean transfer of a deuterium atom usingd-alcohol does not take place, indicating a complex catalytic mechanism. However, changing the deuterium source tod-aniline gives close to complete regioselectivity for mono-deuteration of the terminal position of the double bond. Finally, we demonstrate that efficient dehydrocoupling of alcohol and PMHS can be undertaken using the same pre-catalyst, giving high yields of H₂within 30 minutes at room temperature.

Full text of DOI:10.1039/d0gc04175k

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Room temperature iron catalyzed transfer
hydrogenation using n-butanol and poly
(methylhydrosiloxane)†
Cite this: Green Chem., 2021, 23,
2703
Thomas G. Linford-Wood,
Nathan T. Coles
and Ruth L. Webster
*
Reduction of carbon–carbon double bonds is reported using a three-coordinate iron(II) β-diketiminate
pre-catalyst. The reaction is believed to proceed via a formal transfer hydrogenation using poly(methyl-
hydrosiloxane), PMHS, as the hydride donor and a bio-alcohol as the proton source. The reaction pro-
ceeds well using n-butanol and ethanol, with n-butanol being used for substrate scoping studies. Allyl
arene substrates, styrenes and aliphatic substrates all undergo reduction at room temperature.
Unfortunately, clean transfer of a deuterium atom using D-alcohol does not take place, indicating a
complex catalytic mechanism. However, changing the deuterium source to D-aniline gives close to com-
plete regioselectivity for mono-deuteration of the terminal position of the double bond. Finally, we
demonstrate that efficient dehydrocoupling of alcohol and PMHS can be undertaken using the same pre-
catalyst, giving high yields of H2 within 30 minutes at room temperature.
Received 9th December 2020,
Accepted 10th March 2021
DOI: 10.1039/d0gc04175k
rsc.li/greenchem
specific area of interest, developing chemistry that allows
selective functionalization. In the case of TH, with an appro-
Introduction
The implementation and manipulation of carbon–carbon priate catalyst or substrate or TH reagent, regioselective mono-
double bonds is of fundamental importance in organic syn- deuteration is feasible: this is simply not possible using D2.
thesis. The reduction of unsaturated carbon–carbon bonds is Since some of the earliest studies on Sn-catalyzed TH of acti-
one such transformation. This chemistry is dominated by tran- vated functional groups (aldehydes and ketones) in refluxing
sition metal-mediated reduction in the presence of H2.1,2 EtOH, and alkene and nitro group reduction over Pd/C under
There are countless examples of homogeneous and hetero- similar conditions,15 much of the research in the area of TH
geneous catalysts undertaking the reduction of CvC bonds, mediated by alcohols and silanes is dominated by copper cata-
under a range of reaction conditions3,4 and H2 pressures. An lysis,16 in particular alkyne semi-hydrogenation,17,18 whereas
alternative to the use of a gaseous reductant is the use of trans- iron-catalyzed transformations have relied on H2-release from
fer hydrogenation (TH), which is traditionally undertaken a silane19 or formic acid.20
using the conditions developed by Meerwein, Ponndorf and
We recently reported on the ability of a well-defined iron(II)
Verley in the 1920s.5,6 In order to develop the original chem- β-diketiminate complex (1) to undertake transfer hydrogen-
istry for application in CvC reduction, there have been many ation of unactivated carbon–carbon double bonds
adaptations, including alternative catalysts7,8 to the traditional (Scheme 1a).23 The reaction is rapid and mild; proceeding
aluminium alkoxide system, chiral variants9 and a move away smoothly at room temperature within minutes. Although an
from isopropanol (IPA) as the hydrogen atom source in TH. elegant method to reduce unsaturated systems and regioselec-
Although H2 gas is impossible to compete with in terms of tively introduce a single deuterium atom, our initial disclosure
atom economy, the use of TH reagents,10–12 such as IPA or relies on benzene solvent (with C6D6 used for ease of in situ
other alcohols, amines, boranes and silanes proffer the oppor- analysis), pinacol borane (HBpin) as a hydride source and an
tunity to develop chemistry that does not rely on gas handling, amine (such as PhNH2 or nBuNH2) as a proton source. We felt
develop detailed mechanistic understanding13,14 that may not that the sustainability credentials could be readily improved by
be viable if high pressures of H2 are required and, our own moving to the use of sustainable hydride and proton sources.
Early indicators from our published work suggested that a
change in proton source would be viable, since we had some
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
positive TH results using o-allyl-phenol and propargyl alcohol
E-mail: r.l.webster@bath.ac.uk
as proton-containing substrates. Ideally, the reaction would
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
also not rely on C6D6 as the reaction solvent. We herein report
d0gc04175k
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Table 1 Optimization of TH
PMHS
(equiv.)/1
Proton
Conv.a, Spec. yieldb,
Entry source
(mol%)
Solvent
%
%
1
2
3
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
1/10
2/10
3/10
3/5
3/5
3/5
3/10
3/5
3/10
3/5
C6D6
C6D6
C6D6
C6D6
C6D6
Tol-d8
2-MeTHF n.d.
CPME
—
60
85
>99
>99
>99
>99
52
76
>99
99
90
82
9
16
84
71
n.d.
n.d.
n.d.
32
4
70
85
74
58
0
0
0
4
5c
6
7
8
9
n.d.
n.d.
n.d.
39
2
0
10
11d
12d
13d
14d
15d
16
17
18
19
20e
21f
22g
23h
—
D-(−)-Fructose 1/10
D-(+)-Glucose
Sucrose
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
PMHS
PMHS
PMHS
—
1/10
1/10
D-(−)-Fructose 3/10
36
6
D-(+)-Glucose
nBuOH
nBuOH
EtOH
3/10
3/5
—/5
—/5
—/5
3/5
1.5/5
—/—
—/5
>99
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
Scheme 1 (a) Previous work on TH using pre-catalyst 1 was fast and
mild, with reactions complete in minutes at RT. However, the chemistry
relied on non-renewable reagents: HBpin and amines such as aniline or
n-butylamine. (b) This work focuses on the development of a more sus-
tainable and environmentally benign route to alkene TH using alcohols
and poly(methylhydrosiloxane) (PMHS, a waste product from the silicone
industry21,22).
MeOH
nBuOH
nBuOH
nBuOH
—
—
PMHS
PMHS
<5
1
a Determined by H NMR spectroscopy based on consumption of allyl-
benzene. n.d. = not determined. b Determined by 1H NMR spec-
troscopy using 7.9 µL (0.1 mmol) DCE internal standard. c 16 h.
d J-Young NMR-tube agitated at 10 rpm. e 4 equiv. nBuOH. f 3 equiv.
nBuOH. g No catalyst. h 29% isomerization to β-methylstyrene also
obtained.
on our pursuit of a more benign transfer hydrogenation
methodology.
However, solvent free conditions, where PMHS is used to solu-
bilize the other reagents, gives good yield of propylbenzene at
Results and discussion
We initiated our studies by optimizing the TH of allylbenzene both 10 mol% and 5 mol% loading of 1 (entries 9 and 10).
using poly(methylhydrosiloxane) (PMHS) as the hydride donor Moving on to investigate more sustainable alternatives to
(Table 1). PMHS has been used extensively with iron pre-cata- PhNH2, we found that sugars are not suitable for the trans-
lysts to undertake hydrosilylation24 of carbonyl compounds, formation; this may in part be due to lack of solubility in the
followed by hydrolysis to give the free alcohol,25–34 in the reaction medium (entries 11 to 15). Pleasingly, nBuOH, which
reduction of nitriles35 and amides36,37 in dehydroxylation38 or is biorenewable, gives 85% spectroscopic yield of propylben-
tandem hydrosilylation/isomerization.39 Enthaler has reported zene in conjunction with 5 mol% 1 and PMHS at RT in 24 h.
elegant examples of alkene synthesis via alkyne reduction Notably iBuOH tends to be the key biofuel in the automotive
mediated by silanes.40,41 However, application of PMHS in TH industry and therefore this process would not impact demands
of carbon–carbon bonds42–45 is, to the best of our knowledge, for this fuel.46,47 Increasing the loading of nBuOH leads to
unreported.
Gratifyingly,
a
1 : 1 : 1
ratio
of complete loss of TH reactivity due to competing dehydrocou-
allylbenzene : PhNH2 : PMHS gives good conversion and spec- pling of the silane and alcohol, leading to H2 production
troscopic yield to propylbenzene at RT using 10 mol% 1 (entry (entries 20 and 21). H2 and Fe catalyst do not generate propyl-
1). Increasing the loading of PMHS further increases the spec- benzene (vide infra). The catalyst is essential for TH to take
troscopic yield, with 3 equivalents of PMHS generating a quan- place (entry 22). The proton source is necessary for efficient
titative amount of propylbenzene. In our search for a greener formation of propylbenzene (entry 23). 1 is sensitive due to its
and safer alternative to C6D6, we found that key renewable sol- low coordinate environment but it is pleasing to note the
vents such as 2-methyl-THF and cyclopentyl methyl ether ability of 1 to catalytically turnover alcohols, particularly when
(CPME) lead to a drastic reduction in yield (entries 7 and 8). one considers the oxophilic nature of Fe(II).48
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Following this optimization process, we moved on to inves- yields observed at RT for m- and o-Me (2c and 2d) and p-CF3
tigate substrate scope for this transformation (Scheme 2). It is and p-F substrates (2f and 2g). However, increasing the reac-
worth noting that during the preparation of this manuscript tion temperature to 60 °C increases the yield of product, for
an elegant Fe-catalyzed hydrogen atom transfer (HAT) method example 68% 2c is obtained after 24 h. Pleasingly, styrene sub-
was published by Kattamuri and West.49 That work used strates, which appear to be more challenging substrates for Fe-
PhSiH3 (2 equiv.), EtOH as the solvent, co-catalytic PhSH catalyzed HAT chemistry and are, as yet, unproven substrates
(10 mol%) in the presence of Fe(acac)3 (10 mol%) under very for iron-catalyzed silane/alcohol TH, react well under the stan-
mild conditions. The transformation works well for a wide dard reaction conditions. Spectroscopic yields ranging from
range of substrates, but struggles with styrenes, an area where 44% to 82% are obtained for unsubstituted, p-OMe, p-CF3 and
our catalytic system operates well.
p-Ph substrates (2h, 2j, 2k, 2m). There is no clear link between
Allyl arenes respond well to the TH reaction, with unsubsti- electronics and reactivity for substrates, with p-F styrene only
tuted, p-Me and p-OMe substrates (2a, 2b and 2e) all giving giving 31% 2l. The effect of sterics are as expected, with o-OMe
high yield of product. Steric hindrance and electron withdraw- styrene giving 43% 2n. The transformation responds well to
ing groups do lower reactivity, as exemplified by the reduced scale-up, with a 10-fold increase in scale (to 2.5 mmol) giving
full conversion of styrene starting material and 85% isolated
yield of 2h. Scale-up allows for facile isolation of the products.
The by-product of the reaction is poly(butoxymethylsilane), a
non-volatile compound that the reduced organic product is
readily distilled away from. However, to demonstrate flexibility
in isolation methods, scale-up synthesis of 2j is achieved with
full conversion and 69% isolated yield via flash chromato-
graphy. trans-β-Methylstyrene is a poor substrate for TH, but
1,1-disubstituted styrenes are active giving 76% 2p and 63% 2q
at RT. Unactivated alkenes, such as but-3-en-1-ylbenzene and
cyclohexene give 71% and 68% of butylbenzene (2r) and cyclo-
hexane (2s) respectively. Scale-up of the cyclohexene reduction
(2.5 mmol) gives 74% isolated yield. Note the high isolated
yield achieved from scale-up and distillation (2h and 2s) com-
pared to the spectroscopic yield obtained on a small scale.
Similarly, the isomers of hexene all give good yield of hexane
(2t), this is in vast contrast to our previous report of TH using
an amine and HBpin which only gave 28% and 48% conver-
sion to product for cis-2-hexene and cis-3-hexene respectively.23
Unfortunately, internal alkynes do not react well with ≥90%
starting material recovered. Substrates which carry an acidic
proton, such as terminal alkynes and alkenes that are functio-
nalized with carboxylic acids, do not react as desired and
instead form bonds with PMHS (forming an (ethynyl)silane
and silyl acetate respectively). No reaction is observed with
alkenyl substrates such as allyl acetate, allyl 2,2,2-trifluoroace-
tate, cinnamyl chloride, pent-4-en-2-one, 1,2-epoxy-5-hexene:
only unreacted starting material is observed. Acrylonitrile
undergoes side-reactions, presumably forming poly(acryloni-
trile). α,β-Unsaturated aldehydes, such as cinnamaldehyde,
undergo preferential reduction of the carbonyl, forming cinna-
myl alcohol in 65% yield.
Interestingly, compared to our previous studies using
HBpin and nBuNH2, which undergoes reaction gelling that we
use as a qualitative measure of reaction completion, which was
subsequently linked to the formation of extended Lewis acid/
base macrostructures,23 reactions using amines or alcohols
with a silane do not gel. This is because the silane cannot
form a Lewis acid adduct with the Lewis basic alcohol (or
amine). The fact that the reactions take longer (hours using
silane versus minutes using HBpin), there is opportunity for
the silane to act as a reducing agent, taking Fe(II) to Fe(I)
Scheme 2 Substrate scope for TH of alkenes and alkynes using PMHS
and nBuOH, catalyzed by 1. Values in brackets relate to 2.5 mmol or
b 0.5 mmol scale reactions and the isolated yield is reported.
a
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in situ and generating an η2-alkene complex,50,51 which then
undertakes a competitive double bond isomerization process.
We note that lower yielding allyl arene substrates show a
higher percentage of isomerized product (i.e. trans-
β-methylstyrenyl product). For example, in addition to 2c, the
allyl-m-methylbenzene substrate also gives 26% isomerized
product, likewise allyl-p-(trifluoromethyl)benzene produces
34% isomerized product as well as 36% 2f.52 It is important to
note that no starting material is observed in the crude reaction
mixtures where the reagent can be isomerized; there are
clearly competing reaction processes taking place. High yield-
ing reactions e.g. the formation of 2a and 2e show little or no
isomerized product. Based on the lack of TH reactivity
obtained using trans-β-methylstyrene (2o) there is no surprise
that once isomerization takes place to form trans-
β-methylstyrenyl compounds the TH process stalls. In short,
we believe two competing catalytic cycles are viable with sub-
strates that can be isomerized.
These results are also reflected in deuterium-labeling using
nBuOD53,54 (Scheme 3a) where attempted mono-deuteration of
1-hexene gives a 60 : 40 ratio of hexane-2-d/hexane-3-d : hexane-
1-d. All d-atoms were accounted for using an internal standard,
thereby demonstrating high levels of deuteration for all sub-
strates.55 Dropping the loading of PMHS to 1 equiv. gives
almost identical levels of deuterium incorporation (compare
Schemes 3a and b), but this comes at the expense of yield com-
pared to that obtained using the standard reaction con-
ditions.55 Our previous work using HBpin/PhND2 gave 8 : 92
(Scheme 3c). With PMHS and nBuOD allyl benzene gives
15 : 49 : 36 of (propyl-1-d) : (propyl-2-d) : (propyl-3-d)benzene
while styrene gives a ratio of 37 : 63 (ethyl-1-d) : (ethyl-2-d), the
latter is not dissimilar to the results obtained using PhND2
and HBpin (24 : 76 ratio).23 Determined to provide selective
mono-deuteration under sustainable conditions we continued
to use PMHS as the hydride donor, but swapped D-alcohol for
PhND2 (Scheme 3d). Pleasingly this system gives similar deu-
teration ratios to HBpin/PhND2 or a modest improvement. For
example styrene is preferentially deuterated at the terminal
position (17 : 83 (ethyl-1-d) : (ethyl-2-d)benzene). Having noted
these modest improvements in selectivity with a silane/amine
system, we investigated the labeling selectivity obtained using
Ph2SiH2 and PhND2 (Scheme 3e). Pleasingly we observe com-
plete terminal deuteration with 1-hexene and allyl benzene,
but once again styrene gives only moderate preference for
terminal deuteration. In the case of styrene, we believe that
electronics are influencing the regioselectivity, potentially by
Scheme 3 Mono-deuteration studies comparing the selectivity
obtained using nBuOD and PhND2 using PMHS, HBpin or Ph2SiH2.
b
a 10 mol% 1, 0.26 mmol PhND2, 0.26 mmol HBpin, RT, 16 h. 5 mol% 1,
0.26 mmol PhND2, 0.26 HBpin/Ph2SiH2, RT, 16 h.
forming a mixture of linear and branched iron-alkyl species for a standard reaction (Fig. 1a). As can be seen, a maximum
during the catalytic cycle. Finally, Ph2SiH2 and nBuOD 0.023 bar H2 is released for the standard reaction ( , 5 mol%
(Scheme 3f) give similar results to those obtained using PMHS 1, 0.25 mmol allyl benzene, 0.25 mmol nBuOH, 0.5 mL PMHS,
and PhND2 indicating that it is possible to obtain good term- RT). If dehydrocoupling of PMHS and nBuOH occurs with
inal deuteration selectivity using one sustainable proton or 100% yield of H2 then the maximum possible pressure of H2
hydride source but using both PMHS and nBuOD compromise in the reaction vessels used for TH is 0.645 bar. Even an
the selectivity.
attempted reduction of 0.25 mmol allyl benzene using
To further probe whether double bond reduction is taking 10 mol% 1 and 20 bar H2 in 0.5 mL C6D6 does not give any
place predominantly by a TH process (as opposed to hydrogen- product. Attempted activation of 1 using 0.5 mL PMHS and
ation by H2) we monitored gas evolution as a function of time reaction of allyl benzene with 2 bar H2 gives 5% 2a; this is the
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after 7 minutes. Two factors may be at play. Firstly, the steric
bulk of nBuOH is limiting, where the reaction is more efficient
with smaller proton sources and secondly the difference in pKa
with MeOH and EtOH < nBuOH, the latter being closer in pKa
to amines, which we know from our previous disclosure are
very efficient TH reagents. This reflects in the reduced ability
of MeOH to act as a TH reagent: dehydrocoupling is highly
competitive during the TH of allylbenzene (Table 1, entry 19).
Although the potential wt% of H2 that can be released is
smaller than H3N·BH3, it is still interesting to note that up to
5.7 wt% can be released from the industrial by-product PMHS
and a bio-alcohol using iron pre-catalyst 1.56
Finally, although detailed mechanistic studies have thus far
been limited by the nature of the reaction (PMHS as a solvent,
necessity for stirring and gas evolution) where any deviation
from this may lead to kinetics results that are not representa-
tive of the reaction being undertaken, we postulate a catalytic
cycle for TH that involves a completely redox-neutral process
(Fig. 2). Activation of 1 involves formation of an iron-alkoxide
complex,57,58 a σ-bond metathesis type process then takes
place in a similar mode to that already reported by our
group.23 The resultant iron hydride (which may exist as a
dimer either on-cycle or off-cycle)59 then undergoes insertion,
and finally protonolysis to release the product. The reaction
using PhND2 as the deuterium source leads to higher selecti-
vity for terminal deuteration. We, qualitatively, link this to the
rate of protonolysis, with amine-mediated reactions going to
completion faster than those mediated by alcohols. With this
in mind, it may be that there is a change in the rate-limiting
step when alcohols are used, so there is more opportunity to
equilibrate between the branched and linear insertion product
prior to a slower rate of protonolysis. The competing catalytic
cycles (isomerization and dehydrocoupling) can become domi-
Fig. 1 (a) H2 gas evolution during TH of allyl benzene using 1 equiv.
nBuOH ( ) or 2 equiv. nBuOH ( ); (b) dehydrocoupling studies using
PMHS (0.5 mL) and 1 mmol of MeOH ( ), EtOH ( ) or nBuOH ( ), moni-
toring the change in H2 pressure as a function of time.
same result obtained in the absence of H2 (see Table 1, entry
23). The same reaction but using 0.25 mmol nBuOH instead of
PMHS does not lead to the formation of 2a. To summarize the
deuterium labelling and H2-mediated reduction results: the
reaction is not a simple hydrogenation by H2.
Dehydrocoupling becomes competitive with TH at higher
alcohol loadings ( , 0.5 mmol nBuOH used, 0.083 bar released
after 4 h, Fig. 1a). We therefore proceeded to explore the ability
of 1 to dehydrocouple PMHS and alcohols in the absence of an
alkene. Even though reactions have been scaled up, there is
still a 10- to 25-fold increase in the quantity of gas released in
these alkene-free reactions. This further supports the hypoth-
esis that H2 release and uptake is not responsible for alkene
reduction. A greater quantity of H2 is released from the reac-
tion using 1 mmol MeOH as the proton source, which in turn
is greater than 1 mmol EtOH and nBuOH (Fig. 1b). Based on
the overall change in pressure, we calculate the number of
mmol of H2 released is 0.88 (MeOH), 0.97 (EtOH) and 0.73
(nBuOH) which gives conversions of 88%, 97% and 73% for
MeOH, EtOH and nBuOH respectively. Initial release of H2
from the nBuOH reaction is slower than those using EtOH and
MeOH. The EtOH and nBuOH reactions are 90% complete
Fig. 2 Postulated catalytic cycle for alkene TH, which is believed to
proceed via a redox neutral Fe(II) cycle. Reagent dependent competing
(redox neutral) dehydrocoupling cycles and (redox active) double bond
after 15 minutes, whereas the MeOH reaction is 90% complete isomerization are also at play and can affect TH outcomes.
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nant under certain circumstances. For example, isomerization
competes when there is a double bond that can isomerize to
form a more stable, conjugated species. This isomerization
process is aided by the slower rate of TH using PMHS/alcohol
(compared to HBpin/amine) which facilitates access to the Fe
(I) species necessary for isomerization.51 For dehydrocoupling,
if TH is too challenging, or if the concentration of hydride and
proton source become too great, then hydrogen release chem-
istry dominates and isomerization stalls.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
Danila Gasperini is thanked for some preliminary reaction
optimization. The EPSRC is thanked for funding.
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