Nitrous oxide reduction-coupled alkene-alkene coupling catalysed by metalloporphyrins

Article abstract of DOI:10.1039/c3cc43912g

Porphyrin complexes of iron, manganese and cobalt catalysed reductive dimerization of alkenes with NaBH₄ under N₂O. The reaction system using Fe^III porphyrin generated an Fe^I porphyrin intermediate that reduced N₂O to regenerate Fe^III porphyrin.

Full text of DOI:10.1039/c3cc43912g

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Nitrous oxide reduction-coupled alkene–alkene
coupling catalysed by metalloporphyrins†
Cite this: DOI: 10.1039/c3cc43912g
Shunsuke Saito,a Hiro Ohtake,b Naoki Umezawa,a Yuko Kobayashi,a Nobuki Kato,a
Masaaki Hirobeb and Tsunehiko Higuchi*a
Received 24th May 2013,
Accepted 4th August 2013
DOI: 10.1039/c3cc43912g
www.rsc.org/chemcomm
Porphyrin complexes of iron, manganese and cobalt catalysed reductive
dimerization of alkenes with NaBH4 under N2O. The reaction system
using FeIII porphyrin generated an FeI porphyrin intermediate that
reduced N2O to regenerate FeIII porphyrin.
Scheme 1
Metalloporphyrins are versatile catalysts, e.g. for oxidation,1a
cyclopropanation and aziridination.1b These catalysts have
been extensively investigated, mainly as model compounds
for the active sites of peroxidases and cytochrome P450, i.e.,
natural oxidation catalysts. Molecular oxygen, peroxides and
various other oxidants have been examined as oxidants.2 We
have shown that the oxidation of alkanes, alkenes, aromatics
and N-carbonyl cyclic amines is efficiently catalysed by ruthenium
porphyrins with heteroaromatic N-oxides as oxidants.3 We therefore
considered that other molecules having an N–O bond might also
serve as oxidants for metalloporphyrin catalysts. Physico-chemically,
nitrous oxide (N2O) is a stable, CO2-like molecule. Huge amounts
of N2O are generated as a by-product of industrial processes such
as Nylon 6-6 and nitric acid production. However, there have
been few reports on utilization of N2O for organic synthesis, e.g.
metalloporphyrin-catalysed oxidation utilizing N2O.4 At present,
most industrially generated N2O is simply decomposed catalytically
to N2 and O2, since N2O itself is a potent contributor to global-
warming and an ozone-depleting substance. We and other groups
previously reported alcohol formation from alkenes with NaBH4
under O2 catalysed by porphyrin complexes of Mn,5 Rh,6 Co7 and
Fe.8 In some cases, small amounts of the dimerised alkene were also
formed.7,8 Here, we report a N2O reduction-coupled alkene coupling
reaction catalysed by metalloporphyrins (Scheme 1). Interestingly,
the alkene was reductively dimerised at room temperature when
N2O was used instead of O2 in the iron porphyrin-catalysed system,
i.e., the use of N2O completely suppressed oxidation.
a-Methylstyrene (1a) dimerised to afford mainly 2,3-dimethyl-
2,3-diphenylbutane (2a) with NaBH4 in the presence of iron meso-
tetraphenylporphyrin chloride (FeIII(TPP)Cl) (1 mol%) under N2O9
(Table 1). Addition of HOꢀ or MeOꢀ enhanced the reaction,
resulting in complete consumption of 1a (entries 1–3). Note that
almost all added HOꢀ should be converted into MeOꢀ in the
solvent (toluene–MeOH (1 : 1)) because the equilibrium of ꢀOH +
MeOH $ H2O + MeOꢀ lies far to the right (there is a ca. 300-fold
excess of MeOH over added HOꢀ and the pKa values of MeOH and
H2O are almost the same). The catalytic activity of iron meso-
tetrakis(pentafluorophenyl)porphyrin chloride (Fe(TPFPP)Cl) was
higher than that of Fe(TPP)Cl (entries 4 and 5). Small amounts of
inseparable oligomers of 1a were also formed and no oxygenated
products were found. The reaction did not proceed in the absence
of Fe(TPP)Cl or N2O.
Table 1 Reductive coupling of a-methylstyrene 1a with NaBH4 catalysed by iron
porphyrins under N2O
Entry
Catalyst
Base
Time (h)
Yield (%)
1
Fe(TPP)Cl
Fe(TPP)Cl
Fe(TPP)Cl
Fe(TPFPP)Cl
Fe(TPP)Cl
Mn(TPP)Cl
Co(TPP)Cl
Rh(TMP)CH3
Cyanocobalamin
Fe(TPP)Clb
KOH
Me4NOH
—
KOH
KOH
KOH
KOH
KOH
KOH
14
14
14
4
73
87
41
69
29
25
33
1.5
5
2
3a
4
5a
6a
7a
8a
9a
10b
4
a Graduate School of Pharmaceutical Sciences, Nagoya City University,
3-1 Tanabe-dori, Mizuho-ku, Nagoya 467-8603, Japan.
E-mail: higuchi@phar.nagoya-cu.ac.jp; Fax: +81 52 836 3435;
Tel: +81 52 836 3435
b Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
14
14
16
14
14
Me4NOH
69
Catalyst: 1 mol% unless otherwise noted. Solvent: toluene–MeOH (1 : 1).
a
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc43912g
TMP: meso-tetramesitylporphyrinato. Starting material 1a remained.
b
Catalyst: 0.05 mol%.
c
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Table
2
Reductive coupling of alkenes with the NaBH4–Fe porphyrin–
N2O system
Entry
Substrate
Base
Product, isolated yield
1
Me4NOH
Fig. 1 FT-IR spectra of the gas phase in the sealed reaction vessel. (a) Gas phase
three hours after mixing alkene 1a and NaBH4 in toluene–MeOH under N2O.
(b) Gas phase one hour after the addition of Fe(TPP)Cl (1 mol%) to the mixed
solution.
2
Me4NOH
NaOMe
3b
4b
5c
KOH
KOH
Reaction conditions: Fe(TPP)Cl (1 mol%), NaBH4 (2.0 eq.), base (0.1 eq.)
a
at r.t. for 14 h under N2O. 1 : 1 mixture of meso- and DL-form.
b
c
Fe(TPFPP)Cl (0.5 mol%) was used. NaBD4 was used instead of NaBH4.
Fig. 2 UV-Vis spectra (a) and EPR spectra (b) of the reaction mixture of 1a,
Fe(TPP)Cl, NaBH4 and Me4NOH in toluene–MeOH (1 : 1) under Ar. (a) [Fe(TPP)Cl]
15 mM at 298 K. (b) [Fe(TPP)Cl] 100 mM at 77 K.
Mn(TPP)Cl, Co(TPP)Cl and Rh(TMP)CH3 and cyanocobalamin
showed lower catalytic activity than Fe(TPP)Cl (entries 6–9). The use
of 0.05 mol% of Fe(TPP)Cl in the reaction of 1a afforded 2a in 69%
yield, i.e., the turnover number reached 1380 or more (entry 10).
In order to confirm this speculation, we analysed the reaction
As shown in Table 2, 1,1-diphenylethylene (1b) similarly gave spectroscopically. Fe(TPP) solution in the presence of NaBH4
the coupling product (2b). Coupling of styrene (1c) afforded 2c under Ar showed the spectrum of FeII(TPP) almost exclusively
as a 1 : 1 mixture of meso- and DL-form in low yield. Other types (Fig. S1(a), ESI†). The addition of 1a to the mixture completely
of alkenes were also examined as substrates of this reaction altered the UV-Vis spectrum, producing a split Soret band (lmax
system. Cyclohexene did not react (data not shown). On the 391 nm and 418 nm) (Fig. 2(a)). This spectrum indicates the
other hand, methyl methacrylate 1d afforded an isolable head- formation of [FeI(TPP)]ꢀ since the spectral pattern is in good
to-tail coupling product 2d and an inseparable mixture of accordance with the reported spectrum of [FeI(TPP)]ꢀ.10 When
oligomers, probably due to radical chain reaction. This reaction N2O was bubbled into the solution at 20 1C, the spectrum
can be regarded as a kind of telomerization, and presumably changed into that of FeII(TPP) (Fig. S1(b), ESI†).
involves a carbon radical intermediate.
Next, we observed the electron paramagnetic resonance
Intramolecular coupling was examined by using (N,N- (EPR) spectrum of a mixed solution of Fe(TPP)Cl, excess NaBH4,
dimethallyl)-p-toluenesulfonamide 1e, and the head-to-head and 1a under Ar at 77 K. Before the addition of alkene, the
ring-closed product 2e was obtained in 18% yield.
solution showed no signal, probably because EPR-silent
The use of NaBD4 instead of NaBH4 gave deuterated product FeII(TPP) was formed by the reduction of FeIII(TPP)Cl with
2a0, i.e., the terminal position of the alkene was deuterated. NaBH4. However, a distinct low-spin signal appeared after addition
This result suggests that an iron–hydride bond is formed and of 1a (Fig. 2(b)). The g values (g = 2.23 and 1.94) and line shape
the reaction proceeds via insertion of the C–C double bond into unambiguously indicated the formation of [FeI(TPP)]ꢀ species.11
the iron–hydride bond.
Consumption of N2O was confirmed by measuring the FT-IR disappear. These results indicate that [FeI(TPP)]ꢀ reduces N2O to N2
spectra of the gas phase (absorptions at 2212 and 2236 cmꢀ1 and O2ꢀ, regenerating FeIII(TPP). There has been one paper
Bubbling of N2O into the solution at 20 1C caused the signal to
)
over the reaction solution in the sealed reaction vessel, in order reporting N2O reduction by [FeI(TPP)]ꢀ species,12 based on the
to confirm the involvement and consumption of N2O (Fig. 1). electrochemistry (reduction potential) of the Fe(TPP) complex, but
N2O was completely consumed under the reaction conditions without spectroscopic support. In the present study, we obtained
(1a, NaBH4, Fe(TPP)Cl). On the other hand, consumption of direct spectroscopic evidence that [FeI(TPP)]ꢀ reacted with N2O.
N2O was not observed in the absence of Fe(TPP)Cl, but began
once Fe(TPP)Cl was added.
Previously our group found that the reaction of styrene and
Fe(TPP)Cl in the presence of NaBH4 affords the s-alkyl-Fe
This result indicates that FeII(TPP) is unable to reduce N2O porphyrin complex (Scheme 2).8b Therefore, 1a may form a
and a more reductive species is formed, because NaBH4 easily similar complex having an Fe–C bond. We propose the reaction
reduces iron(III)porphyrin to iron(II)porphyrin.
mechanism shown in Scheme 2 in light of the above results.
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the reaction intermediates spectroscopically and proposed a reac-
tion mechanism. The porphyrin complex of iron, which is the most
abundant transition metal, was the most effective among those
tested. This reaction system forms a carbon radical under mild
conditions (r.t., dark). FeI(Por)ꢀ, a highly reduced form of iron
porphyrin, was confirmed to be formed in this reaction. Thus, the
reaction enables both N2O consumption and synthesis of coupling
products, some of which would be useful for organic synthesis.
Since N2O may become the dominant ozone-depleting substance
emitted in the 21st century,16 the reaction system described here
might prove useful as a green process to decrease emissions of N2O,
perhaps in conjunction with large-scale synthetic applications.
This work was partly supported by Grants-in-Aid for Scientific
Research on Priority Areas (No. 19028054, 20037056, ‘‘Chemistry
of Concerto Catalysis’’) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Scheme 2 Proposed mechanism of the reductive coupling of alkenes with the
N2O–Fe(Por)–NaBH4 system.
Notes and references
It is known that both FeIII(Por)(OMe)2 and FeIII(Por)OMe are
formed in the presence of a 10-fold excess of MeOꢀ over
FeIII(Por)Cl in toluene–MeOH.13 Either FeIII(Por)(OMe)2 or
FeIII(Por)OMe would be reduced to FeII(Por) with NaBH4 and
a small amount of H–FeII(Por)ꢀ is presumably also formed
because NaBH4 can transfer both a hydride and one electron
to Fe(Por). Formation of H–FeII(Por)ꢀ has also been proposed in
electrochemical reduction of H2O catalysed by Fe(TPP).14 The
alkenyl group is inserted into the Fe–H bond to afford the
s-alkyl-Fe(Por) complex initially. The Fe–C bond of tert-alkyl-
Fe(Por) is homolytically cleaved to give the tert-alkyl radical and
FeI(Por)ꢀ at room temperature due to the higher stability of the
tert-alkyl radical as well as steric repulsion. The two radicals
afford the homo-coupling product, and FeI(Por)ꢀ reduces N2O
to N2 and H2O, regenerating FeIII(Por). Kojo et al. suggested that
NaBH4 and iron porphyrin form the hydrogen radical (Hꢁ).15
Hꢁ, which is formed by homolysis of the H–Fe bond of the
intermediate, attacks the alkene to afford the same radical as
that formed in Scheme 2. However, this alternative pathway is
considered unlikely because the UV-Vis spectrum of Fe(TPP)Cl
with NaBH4 in the absence of alkene under Ar showed only
FeII(TPP) species, not FeI(TPP). The role of ROꢀ has not been
clarified, but it may stabilise FeI(Por)ꢀ species against degradation
through protonation of the basic pyrrole nitrogens and/or facilitate
Fe–C bond cleavage by acting as an axial ligand.
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