Acceptorless photocatalytic dehydrogenation for alcohol decarbonylation and imine synthesis

Article abstract of DOI:10.1002/anie.201203556

It has come to light: Renewed interest in conversions of highly oxygenated materials has motivated studies of the organometallic-catalyzed photocatalytic dehydrogenative decarbonylation of primary alcohols into alkanes, CO, and H2 (see scheme). Methanol, ethanol, benzyl alcohol, and cyclohexanemethanol are readily decarbonylated. The photocatalysts are also active for amine dehydrogenation to give N-alkyl aldimines and H2.

Full text of DOI:10.1002/anie.201203556

Angewandte
Chemie
DOI: 10.1002/anie.201203556
Photocatalysis
Acceptorless Photocatalytic Dehydrogenation for Alcohol
Decarbonylation and Imine Synthesis**
Hung-An Ho, Kuntal Manna, and Aaron D. Sadow*
Selective conversions of biorenewable materials for chemical
feedstock and energy applications require chemical control of
oxygen- and nitrogen-containing functional groups. Reduc-
tive catalytic transformations for their removal, such as
hydrodeoxygenation and hydrodenitrogenation, generally
require a terminal stoichiometric reductant such as hydro-
gen.[1] Alternative catalytic reactions which affect defunction-
alization could be important components of biomass utiliza-
tion strategies, particularly if these can be accomplished with
hydrogen and carbon efficiency.[2] Herein we show that
photocatalytic deoxygenation can be accomplished through
a tandem dehydrogenation/decarbonylation process without
sacrificial reagents (Scheme 1).
plexes catalyze conversions of ethanol into CO/CO2, H2, and
CH4 under basic conditions at 1508C.[7] Increased CO yield
and greater rates are achieved by irradiation.[7] Likewise,
alcohol decarbonylation generates CO for Pauson–Khand
reactions catalyzed by [{(dppp)RhCl(CO)}2] (dppp = bis(di-
phenylphosphino)propane).[8]
We envisioned photolysis as a method for catalyst
reactivation through CO dissociation. Based on the above
reports, several rhodium(I) compounds were screened as
photocatalysts with cyclohexanemethanol (CyCH2OH) as
a test substrate (Table 1). However, these rhodium(I) com-
Table 1: Catalysts investigated for photocatalytic decarbonylation of
cyclohexanemethanol (CyCH2OH).[a]
Entry
Catalyst
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
[{RhCl(C8H12)}2]
[{RhCl(C8H14)2}2]
[{RhCl(CO)2}2]
[{RhCl(CO)2}2]+dppm
[{RhCl(CO)2}2]+dppp
[Cp*Ir(CO)2]
24
24
24
24
72
24
24
72
24
72
72
24
0
0
0
0
1.6
0
[Cp*Rh(CO)2]
[Tp*Rh(CO)2]
0
36
>95
56
>95
0
Scheme 1. A dehydrogenation and decarbonylation sequence for the
deoxygenation of primary alcohols.
9
[ToMRh(CO)2] (1)[c]
[ToPRh(CO)2] (2)
[ToMRh(H)2CO] (3)
[ToMIr(CO)2] (4)
10
11
12
Furthermore, the overall conversions in Scheme 1 show
increased enthalpy content of the products versus reactants
(DHrxn > O) and the production of H2 and CO (syn-gas) as by-
products.[3] A related photocatalytic amine dehydrogenation
provides imines.
[a] Reaction conditions: CyCH2OH (0.09 mmol), catalyst (0.009 mmol),
benzene (0.7 mL), 450 W medium pressure Hg lamp. [b] Yield as
determined by NMR spectroscopy using cyclooctane as an internal
standard. [c] Similar yields are obtained in toluene, but only starting
material is observed when the solvent is CH2Cl2, THF, or CH3CN.
Although acceptorless alcohol dehydrogenations and
aldehyde decarbonylations have been described,[4–6] catalytic
alcohol decarbonylation typically requires a CO trap. Addi-
tionally, these two steps are rarely coupled in catalytic
processes because dehydrogenation catalysts are often inhib-
ited by the carbon monoxide product of decarbonylation.
Pioneering studies showed that rhodium(I) phosphine com-
pounds are not effective under UV light at room temperature
in neutral solutions (Table 1, entries 1–5). We then tested
À
compounds known for C H bond activation under photo-
chemical conditions. [Cp*M(CO)2] and [Tp*M(CO)2] (Cp* =
h5-C5Me5, M = metal, Tp* = tris(3,5-dimethylpyrazolyl)bo-
[9]
À
rate) react with C H bonds under mild irradiation. These
compounds were chosen because related systems mediate
stoichiometric decarbonylations. For example, [Cp*-
(PMe3)IrCl2] reacts with primary alcohols at 1358C to
afford [Cp*(PMe3)IrR(CO)]Cl (R = Me, Et, Ph),[10] while
photolysis of [Tp*Rh(1,3-C8H12)] in methanol gives
[Tp*Rh(H)2CO], H2, and 1,3-C8H12.[11] Also, a few iridium
pincer compounds react with alcohols to give decarbonylation
products.[12]
[*] Dr. H.-A. Ho, K. Manna, Prof. A. D. Sadow
Department of Chemistry and US DOE Ames Laboratory
Iowa State University
1605 Gilman Hall, Ames IA 50011 (USA)
E-mail: sadow@iastate.edu
[**] This research was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences through the Ames Laboratory (Con-
tract No. DE-AC02-07CH11358). Aaron D. Sadow is an Alfred P.
Sloan Fellow.
[Cp*M(CO)2] (M = Rh, Ir) compounds do not give
detectable conversion of CyCH2OH upon irradiation. Inter-
estingly, cyclohexane is observed upon photolysis of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203556.
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CyCH2OH in the presence of [Tp*Rh(CO)2]. GC analysis
confirmed that CO and H2 are formed in the reaction.
In these catalytic reactions, the sequence depicted in
Scheme 1 is supported by the detection of a small quantity of
cyclohexanecarbaldehyde (CyCHO) by 1H NMR spectrosco-
py and GC/MS. Additionally, aldehydes are decarbonylated
under the photocatalytic reaction conditions with precatalysts
1 or 3 (Table 3). In the absence of light, there is no conversion
of either the aldehyde or alcohol.
Furthermore,
a
compound prepared in our group,
[ToMRh(CO)2] (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenyl-
borate),[13] is an order of magnitude more effective than
[Tp*Rh(CO)2]. The related optically active [ToPRh(CO)2] (2;
ToP = tris(4S-isopropyl-2-oxazolinyl)phenylborate)[13] is less
active. [ToMRh(H)2CO] (3) is approximately three times
slower than 1, and the iridium analogue 4 is not active.
Compound 1 is a photocatalyst for the conversion of
a range of primary alcohols into hydrocarbons in good overall
yield (Table 2). The catalyst tolerates silyl, ether, phenyl, and
Table 3: Photocatalytic decarbonylation of aldehydes.[a]
Entry
Aldehyde
RH
t
[h]
Yield
[%][b]
1
2
3
4
6
6
6
6
89
87
92
89
Table 2: [ToMRh(CO)2]-catalyzed alcohol conversion into RH, H2, and
CO.[a]
Entry
Alcohol
RH
t
[h]
Yield
[%][b,c]
1
24
24
24
24
93
94
92
92
2[c]
3[d]
4
[a] Reaction conditions: aldehyde(0.09 mmol), 1 or 3 (0.0023 mmol),
benzene (0.7 mL), RT, 450 W medium pressure Hg lamp. [b] Yield as
determined by GC using cyclooctane as an internal standard.
5
6
24
24
88[e]
90 (85)
Apparent turnover rates for 1- and 3-photocatalyzed
RCHO decarbonylations are much greater than the
RCH2OH conversions. Thus, even though light is required
for catalysis, catalytic turnover frequencies also depend on the
interaction of the substrate and photoactivated catalyst.
Relatively small amounts of CO (ca. 1 atm) do not appear
to inhibit the catalysis, although conversion is slower at higher
CO pressures (5 atm).
For catalyst 1, photolytic CO dissociation gives
[ToMRh(CO)], and this is the necessary first step for both
alcohol and aldehyde activation processes (see below).
Photolysis of the catalyst 3 could give either [ToMRh(CO)]
or [ToMRhH2] as intermediates. Because 3 is less efficient than
1 for alcohol conversion, the reactive intermediate obtained
from 3 is likely not the same as the one from 1. That photolysis
of 1 and 3 give non-equivalent intermediates is also suggested
by the results of thermal experiments (see below). Addition-
ally, photolysis of 3 in the presence of CO affords only 3, and
dicarbonyl 1 could not be detected (Scheme 2). Thus,
[ToMRhH2], rather than [ToMRh(CO)], is implicated as the
7[f]
8
24
36
0
95
9
24
24
72
99
10
11
94
84 (81)
12
13
72
36
81
90 (87)
[a] Reaction conditions: alcohol (0.09 mmol), 1 (0.009 mmol), benzene
(0.7 mL), RT, 450 W medium pressure Hg lamp. [b] Yield as determined
by GC using cyclooctane as an internal standard. [c] Yield of isolated
product. [d] Used toluene as the solvent. [e] 5 mol% 1 gives 95% yield
after 41 h. [f] X=CO2Me, NO2, or Cl.
fluorophenyl groups under these reaction conditions. Ben-
zylic alcohol moieties, central components of lignin,[14] are
readily decarbonylated, and the aromatic groups and ether
linkages are tolerated. Notably, aliphatic alcohols are readily
decarbonylated, thus indicating that b-hydrogen elimination
of the presumed alkylrhodium intermediate is slower than
reductive dealkylation. Simple alcohols, such as methanol and
ethanol are also readily decarbonylated with 1 as the photo-
catalyst. The nitro- and chloroarene groups, as well as esters,
were not tolerated (Table 2, entries 7 and 8).
Scheme 2. Photochemical reductive elimination of H2 from
[ToMRh(H)2CO] (3) is disfavored versus CO dissociation.
2
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intermediate from catalyst 3 in the dehydrogenation and
decarbonylation.
of amines to imines, under oxidative conditions, has recently
attracted significant attention.[17] Therefore, we investigated
oxidant-free conversions of primary amines with our photo-
catalyst.
The dehydrogenation/decarbonylation sequence could
require either one or two photons. The proposed intermedi-
ate, [ToMRhH2], was sought to address this question.
[ToMRhH2(NCMe)] (5) was reacted with CyCH2OH
[Eq. (1)] under thermal conditions with the expectation that
Indeed, [ToMRh(CO)2] is a photocatalyst for the dehy-
drogenation of primary amines to imines (Table 4). Linear
and cyclic/branched aliphatic amines provide the correspond-
ing imines with good yield. Benzylamine and substituted
derivatives also afford the coupled imine products in high
yields (Table 4).
Table 4: Oxidant-free imine synthesis from various primary amines.[a]
Entry
Amine
Imine
Yield
[%][b,c]
acetonitrile dissociation would give [ToMRhH2]. The overall
reaction gives cyclohexane and 3 as the products of stoichio-
metric dehydrogenation and decarbonylation. Once 3 is
formed, no further conversion of CyCH2OH occurs. Addi-
tionally, CyCHO is not detected in the reaction mixture, thus
providing additional evidence for rate-limiting dehydrogen-
ation with subsequent decarbonylation.
=
1
2
n-C6H13NH2
n-C7H15NH2
n-C6H13N CH(nC5H11)
79
77
=
n-C7H15N CH(nC6H13)
3
4
5
85 (78)
86
92 (83)
These results indicate that a) CO dissociation in 1 and 3 is
required for alcohol dehydrogenation, b) aldehydes are more
reactive than alcohols toward [ToMRh(CO)] and [ToMRhH2],
and thus c) catalytic turnover is limited by a competition
between CO re-coordination and substrate activation. The
catalytic decarbonylation of primary alcohols by 3 requires
only one photon. In contrast, dehydrogenation of alcohol with
[ToMRhCO] forms [ToMRhH2(CO)], and therefore a second
photon is required for 1-catalyzed decarbonylation. These
conclusions can be summarized to include several concurrent
catalytic cycles (Scheme 3).
Dehydrogenation of primary amines would generate
aldimine products.[15] Following the reductive amination
sequence, these aldimines would react with a second equiv-
alent of a primary amine and, upon reduction, give the
alkylated products.[16] However, [ToMRhI] compounds are not
effective for imine hydrogenation, and the final products
might be imines and H2. The photocatalytic dehydrogenation
6
7
8
90
85
88
9
86
79
10
[a] Reaction conditions: amines (0.09 mmol), 1 (0.009 mmol), benzene
(0.7 mL), RT, 450 W medium pressure Hg lamp. [b] Yield as determined
by GC using cyclooctane as an internal standard. [c] Yield of isolated
product.
Thus, 1 is a photocatalyst for the conversion of both
primary alcohols and primary amines. In fact, we are not
aware of previous reports of oxidant-free photocatalytic
amine coupling. Additionally, these catalysts are effective
for decarbonylations of aliphatic aldehydes which are difficult
under thermal catalysis because of b-hydrogen elimination.[4]
Isoelectronic [Cp*M(CO)2] and [Tp*Rh(CO)2]are much
less active than 1. Because the compounds reported in the
À
literature are well known for photochemical C H bond
oxidative addition reactions, and those reactions are proposed
to involve electronically unsaturated [M]-CO intermediates,
one might expect that [ToMRh(CO)2] would be very effective
for photochemically activated alkane oxidative addition.
À
Instead, C H bond activation with 1 is less facile than in
the [Cp*M] and [Tp*M] systems (e.g., we have not yet found
À
evidence for aliphatic C H bond oxidative addition with 1).
Thus, the highly reactive [M]-CO and [M](H)2 intermediates
exhibit a wide range of reactivity toward alcohol, aldehyde,
and hydrocarbon substrates as the ancillaries are varied.
These results emphasize the need for new catalysts, for
conversion of oxygenates, which are distinct from catalysts for
Scheme 3. Proposed catalytic cycles for alcohol and aldehyde decar-
bonylation by 1 and 3.
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Received: May 8, 2012
Published online: && &&, &&&&
Keywords: alcohols · dehydrogenation ·
.
homogeneous catalysis · imines · photochemistry
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Communications
Photocatalysis
H.-A. Ho, K. Manna,
A. D. Sadow*
&&&& — &&&&
It has come to light: Renewed interest in
conversions of highly oxygenated materi-
als has motivated studies of the organo-
metallic-catalyzed photocatalytic dehy-
drogenative decarbonylation of primary
alcohols into alkanes, CO, and H2 (see
scheme). Methanol, ethanol, benzyl
Acceptorless Photocatalytic
Dehydrogenation for Alcohol
Decarbonylation and Imine Synthesis
alcohol, and cyclohexanemethanol are
readily decarbonylated. The photocata-
lysts are also active for amine dehydro-
genation to give N-alkyl aldimines and
H2.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!