Iron-catalyzed cyclopropanation in 6 M KOH with in situ generation of diazomethane

Article abstract of DOI:10.1126/science.1218781

Diazomethane is a common and versatile reagent in organic synthesis whose broader use is generally impeded by its explosiveness and toxicity. Here we report that a simple iron porphyrin complex catalyzes the cyclopropanation of styrenes, enynes, and dienes under the demanding conditions [aqueous 6 molar potassium hydroxide (KOH) solution, open to air] necessary for the in situ generation of diazomethane from a water-soluble diazald derivative. A biphasic reaction medium arising from the immiscibility of the olefin substrates with water appears essential to the overall efficiency of the process. The work we describe highlights an approach to catalysis with untoward reactive intermediates, in which the conditions for their generation under operationally safe regimes dictate catalyst selection.

Full text of DOI:10.1126/science.1218781

Iron-Catalyzed Cyclopropanation in 6 M KOH with in Situ Generation
of Diazomethane
Bill Morandi and Erick M. Carreira
Science 335, 1471 (2012);
DOI: 10.1126/science.1218781
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REPORTS
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use of nonaqueous solutions almost completely
We compare the charge capacity and the
147, 420 (2000).
suppresses electroactivity (21). However, poly- capacitance per mass of these materials to those
pyrrole has acid-base properties leading to marked for polypyrrole/carbon composites [table 1 in
pH sensitivity of conductivity (22, 23). It is a (24)]—as analyzed for the three-electrode situa-
conceivable proton acceptor, but within this pH tion, which does not take counter electrode and
range conductivity is retained.
Polypyrrole may thus be a site for proton density and capacitance of Ppy(Lig) are higher
storage after oxidation and for retrieval upon than reported for most of these materials. Only
reduction of the quinone group.
We know the mass fraction of –OH groups in bons is close to the values reported here.
the lignosulfonate material and can calculate the We have demonstrated interpenetrating net-
fraction of phenol groups in the composite elec- works of lignosulfonate and polypyrrole that can
trode (21). In the lignosulfonate used, two of the be used for charge and energy storage. The use of
monomers (sinapyl and coniferyl alcohol) dom- the renewable biopolymer should lead to low-
inate, and we neglect the contribution from the cost electrodes with improved safety and non-
p-coumaryl alcohol group. These monomers could toxicity, operating in water. There is ample room
each contribute one quinone group, giving a for further developments to improve charge den-
maximum of 7% by weight of quinone in the sity and capacitance by searching through the
composite electrode. This gives us a value of universe of lignins.
11. B. Zinger, Synth. Met. 30, 209 (1989).
12. H. Yoneyama, Y. Ii, S. Kuwabata, J. Electrochem. Soc.
139, 28 (1992).
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137, 145 (2004).
14. H. K. Song, G. T. R. Palmore, Adv. Mater. 18, 1764
(2006).
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Lignans: Advances in Chemistry (CRC Press, Boca Raton,
FL, 2010).
16. G. Milczarek, Electroanalysis 19, 1411 (2007).
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19. C. Sasso, M. Fenoll, O. Stephan, D. Beneventi,
Bioresources 3, 1187 (2008).
20. C. Yang, P. Liu, Ind. Eng. Chem. Res. 48, 9498 (2009).
21. Materials and methods are available as supporting
material on Science Online.
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J. Phys. Chem. Solids 45, 427 (1984).
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Adv. Mater. 23, 3751 (2011).
polypyrrole combined with nanostructured car-
69 mAh/g. For the polypyrrole fraction, elemen-
References and Notes
1. P. Novák, K. Müller, K. S. V. Santhanam, O. Haas,
Chem. Rev. 97, 207 (1997).
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Commun. 1986(16), 1293 (1986).
tal analysis indicates a low amount of anionic
–
Acknowledgments: This work was supported by the Knut and
Alice Wallenberg Foundation, and O.I. is a Wallenberg Scholar.
We thank R. Gabrielsson, N. Solin, A. Elfving, and V. Andersson
for experimental support and discussion. The kind donation
of lignosulfonate samples from Borregaard LignoTech AS is
gratefully acknowledged. The authors have applied for a
Swedish patent on the class of materials reported here.
dopant species due to ClO4 , and we can estimate
a upper limit to this charge capacity of 40 mAh/g,
based on anion exchange only. Assuming that
polypyrrole can be charged to one charge per
four monomers (with some fixed counterions
carried on the lignosulfonate) and that cation ex-
change accounts for the ion flow, we obtain
values of 90 mAh/g. We did not observe mass
loss during oxidation of the electrode, so the
cation insertion mechanism must be a minor one.
The 50/50% composite material could therefore
store ~80 mAh/g, based on the 1:1 stoichiometry.
In measurements, we find charge densities at
lower but comparable numbers, 70 to 75 mAh/g,
for the thin-film electrode.
4. P. Novák, O. Inganäs, R. Bjorklund, J. Electrochem. Soc.
134, 1341 (1987).
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46, 5284 (2007).
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7237 (2008).
8. B. Piro et al., J. Chim. Phys. 95, 1522 (1998).
9. D. Haringer, P. Novák, O. Haas, B. Piro, M. C. Pham,
J. Electrochem. Soc. 146, 2393 (1999).
Supporting Online Material
www.sciencemag.org/cgi/content/full/335/6075/1468/DC1
Materials and Methods
Figs. S1 to S9
References (25–29)
11 October 2011; accepted 15 February 2012
10.1126/science.1215159
The theoretical ratio between charge capacity
in the lignosulfonate and in polypyrrole is be-
tween 1.75 and 0.8, depending on the method of
estimating polypyrrole capacity. Our experimen-
tal value is 1.4. However, as we must assume that
the charge capacity in polypyrrole is higher than
that of the oxidized lignosulfonate in order to
explain our experimental results, we must also
conclude that we used a large fraction of all
quinone groups incorporated in the polymer film.
Iron-Catalyzed Cyclopropanation in
6 M KOH with in Situ Generation
of Diazomethane
Bill Morandi and Erick M. Carreira*
The observation that a large fraction of the redox Diazomethane is a common and versatile reagent in organic synthesis whose broader use is
capacity of the lignosulfonate is accessible for generally impeded by its explosiveness and toxicity. Here we report that a simple iron porphyrin
electrochemistry is consistent with the almost complex catalyzes the cyclopropanation of styrenes, enynes, and dienes under the demanding
molecular miscibility of polypyrrole and ligno- conditions [aqueous 6 molar potassium hydroxide (KOH) solution, open to air] necessary for the
sulfonate, as also proven by the absence of nano- in situ generation of diazomethane from a water-soluble diazald derivative. A biphasic reaction
structure in electron microscopy (21). The density medium arising from the immiscibility of the olefin substrates with water appears essential to the
of the material is 1.4 g/cm3, which leads to a overall efficiency of the process. The work we describe highlights an approach to catalysis with
volumetric charge density of 100 mAh/cm3.
untoward reactive intermediates, in which the conditions for their generation under operationally
Self discharge (21) is a problem with these safe regimes dictate catalyst selection.
electrodes and will need further study. However,
we observe considerable differences of perform-
ance between different lignosulfonate compounds.
The fraction of phenolic groups varies widely in
n general, chemical processes requiring the inherent risk associated with the use of these
use of reactive intermediates, such as diazo- reaction partners because of their explosive and
alkanes, azides, and arene diazoniums, ex- toxic nature, despite the fact that in many instances
I
lignosulfonates, depending on origin and pro- perience limitations as a consequence of the these intermediates are readily accessible and
cessing of lignin. This means that there is room
for optimization of the Ppy(Lig) materials using
different sources of processed lignins with varying
loading, with varying charge densities, and with a
possibility to improve upon present results.
inexpensive. A particularly useful example is di-
azomethane, CH2N2, a valuable reagent available
to the synthetic chemist (1): It may be used in
esterification, dipolar cycloaddition, epoxidation,
aziridination, cyclopropanation, and carbonyl
Laboratory of Organic Chemistry, HCI H335, Wolfgang-Pauli-
Strasse 10, 8093 Zürich, Switzerland.
*To whom correspondence should be addressed. E-mail:
carreira@org.chem.ethz.ch
www.sciencemag.org SCIENCE VOL 335 23 MARCH 2012
1471

REPORTS
homologation (1). However, the hazardous nature are considered to be inherently safer in water (23) development. In an attempt to address the chal-
of this reagent is well appreciated (2–5). The un- as a consequence of its high specific heat capacity lenge, we screened a range of common metal
toward properties of diazomethane, combined and nonflammability. However, the identification complexes known for their ability to mediate car-
with its versatility, make the identification of of a catalyst compatible with aqueous 6 M KOH bene transfer with diazocompounds. We selected
safe protocols for its use an important task. The would be key to the successful development of the cyclopropanation of p-methoxy styrene to
development of user-friendly reaction conditions such a procedure.
test the catalytic activity of these complexes un-
for its generation in substoichiometric quantities The protocols for the generation of diazo- der the demanding conditions required for the
as part of a continuous process and subsequent methane define the boundary conditions under decomposition of 1 (24), a water-soluble diazald
use in catalytic transformations would be a wel- which a catalyst would have to perform cyclo- analog.
come development in synthesis. In this report, propanation reactions (Fig. 1). Any such catalyst
We started our investigations with the fol-
we disclose results toward that goal achieved by must be compatible with strongly oxidizing (diazald) lowing reaction conditions (Fig. 2): Catalyst
conducting an iron-catalyzed cyclopropanation and strongly alkaline conditions (6 M KOH) [5 mole % (mol %); 2.5 mol % in the case of rho-
reaction under demanding conditions (aqueous and water. Moreover, it was our goal at the out- dium dimers], p-MeO-styrene [1 equivalent (equiv.),
6 M KOH) that permit safer use of diazomethane, set to conduct the reaction open to air. The set limiting reagent], and 6 M KOH (25) were vig-
obviating its isolation, purification, and handling. of excessive demands placed on any catalyst that orously stirred in an open vial, and an aqueous
More than 100 years ago, von Pechman dis- is compatible defines a distinct approach to catal- solution of reagent 1 (3 equiv.) was added slowly
covered that the treatment of nitrosamides with ysis, in which untoward reactive intermediates over 4 hours to avoid any substantial buildup of
alkali produces diazomethane as a yellow gas are generated and used in situ, and in which the diazomethane, which could be directly consumed
(6, 7). Since this seminal report, many precursors conditions for their generation under operation- in a tandem manner in the cyclopropanation re-
of diazomethane have been developed that rely ally safe regimes dictate catalyst selection and action. The two most commonly used transition
on base-mediated decomposition of nitrosamide
derivatives in organic solvents, mostly diethyl
ether (1). The diazomethane formed in this way
Fig. 1. Tandem diazometh-
ane generation/iron-catalyzed
is typically co-distilled with the diethyl ether
with the use of a special apparatus (a diazald kit)
(8), requiring great care because of the explo-
sion risk during this manipulation. Historically,
N-nitroso-N-methylurea was one of the most com-
monly used reagents for the generation of di-
azomethane (9, 10). Unfortunately, this reagent
itself was later recognized as being explosive
(11) and highly carcinogenic (12), and its use
was therefore discouraged. This led to the de-
velopment of a class of more-stable, low-toxicity
N-methyl-N-nitrososulfonamide reagents. Among
them, N-methyl-N-nitroso-p-toluenesulfonamide
(diazald), a stable, commercially available com-
pound, shows low toxicity (median lethal dose
2.7 g per kilogram of body weight) as compared
to other diazomethane precursors (13). This search
for safer diazomethane precursors contrasts with
the scant research efforts dedicated to the devel-
opment of safer protocols for the use of the
generated diazomethane itself (14). A notable
exception is the recent development of approaches
involving continuous-flow chemistry (15–17).
We have recently become interested in the gen-
eration and use of trifluoromethyl diazomethane
in metal-carbene transfer processes (18–20), which
has led in turn to the development of a safer strat-
egy for the use of diazomethane in catalytic re-
actions. The approach we describe combines
base-mediated generation of diazomethane over
time from water-soluble diazald with an in situ
metal-catalyzed carbene transfer process in a
tandem manner (Fig. 1) (21, 22). We reasoned
that slow addition of the nitrosamide 1 to a mix-
ture of base, catalyst, and substrate in water would
avoid buildup of any appreciable concentration
of in situ–generated diazomethane, which would
instead be rapidly consumed by the metal cata-
lyst. Implementation of such a protocol would
minimize human exposure to the reagent as well
as considerably reduce the risk of uncontrollable
cyclopropanation avoids the
hazards of diazomethane
isolation. R, organic group.
decomposition. In this respect, chemical processes Fig. 2. Catalyst screening for the cyclopropanation reaction. OTf, trifluoromethanesulfonate.
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23 MARCH 2012 VOL 335 SCIENCE www.sciencemag.org

REPORTS
metals in catalytic cyclopropanation with diazo- sist decomposition under these basic conditions. used (entry 7), the addition of toluene (100 ml)
methane, copper (catalyst 2) and palladium (cat- We conducted the reaction of p-methoxystyrene was needed to dissolve the substrate and catalyst.
alyst 3) (26), were inactive under the conditions at a 2-mmol scale to furnish product (Table 1, Overall, the transformation shows broad scope
required for the diazomethane generation. To entry 2) in 91% yield.
our delight, a range of other transition metals Having a safe and practical protocol in hand,
and affords useful products.
This transformation overcomes many po-
(Fe, Co, Rh, and Ru) proved suitable, giving the we examined the scope of olefins that could be tential incompatibility concerns, such as catalyst
product cyclopropane as observed by 1H nuclear used under these conditions (Table 1) (30). Ter- deactivation or degradation by 6 M KOH, ir-
magnetic resonance analysis of the crude mix- minal aromatic cyclopropanes are important mo- reversible oxidation of the metal catalyst by
ture. Fe(TPP)Cl (TPP = 5,10,15,20-tetraphenyl- tifs in drug discovery (31). Styrene derivatives strongly oxidizing diazald, or OH insertion of
21H,23H-porphine) (10) (27) gave the highest bearing both electron-withdrawing and -donating the metal-carbene intermediate, involving di-
conversion, which is particularly gratifying in substituents gave the corresponding products in azomethane generation and the catalytic carbene
light of the recent interest in this inexpensive good yields. Disubstitution was tolerated (Table 1, transfer reaction in a single reaction vessel. We
and nontoxic metal (Fe) as an alternative to noble entry 6), and the chemistry could be extended to therefore performed some preliminary experi-
metal catalysts (28, 29). A turnover number of the preparation of versatile vinyl (entries 9 and 10) ments to gain information about the exact nature
600 was measured using 0.1 mol % of the com- and alkynyl cyclopropanes (entries 11 and 12) of this tandem process (Fig. 3A). First, a water-
plex, attesting to the ability of this complex to re- (32, 33). In the case where a solid substrate was soluble substrate (11) was probed under the stan-
dard reaction conditions and gave no product
formation. This indicates that the substrate needs
to be immiscible with water, creating a biphasic
Table 1. Scope of the tandem diazomethane generation/iron-catalyzed cyclopropanation.
reaction mixture. Second, Rh2(CH3COO–)4, awater-
soluble catalyst, gave no conversion, whereas
Rh2(CH3(CH2)6COO–)4, a lipophilic analog, gave
45% conversion. This demonstrates that the cat-
alyst needs to be dissolved in the organic phase
to induce reaction of the metal-carbene interme-
diate with the substrate. Third, the addition of
EtOH as cosolvent led to very low conversion.
Formation of a homogeneous reaction mixture is
thus deleterious to the process, illustrating the
importance of the phase separation. Based on
these observations, we believe that the biphasic
nature is responsible for the efficiency of the
reaction. The following working model is pro-
posed (Fig. 3B): The aqueous phase contains
the base and water-soluble reagent 1, whereas
the catalyst is dissolved in the substrate phase
(organic). Upon formation, diazomethane mi-
grates from the aqueous phase into the lipophilic
substrate phase (organic), where it reacts with
the catalyst to form a metal-carbene intermediate
that is trapped by the surrounding substrate that
is locally present in excess. This model explains
the insensitivity of the catalytic reaction to the
strong base and the strong oxidizing reagent that
are localized in the aqueous phase. It also pro-
vides a rationale for the preferred reaction of
the metal-carbene intermediate with the substrate
over OH insertion with the water present in excess.
The unexpected compatibility of metal-
carbene catalysis with the strong alkaline con-
ditions necessary for diazomethane generation
illustrates the power of this approach to catalysis.
Given the number of other noxious reagents that
are inexpensively prepared in aqueous media
(diazoniums, azides, and diazocompounds), we
believe that these results will have broader im-
plications for their use in catalysis, enabling the
development of safer protocols for the prepara-
tion of valuable molecules for materials science,
biology, and drug discovery.
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1. T. H. Black, Aldrichim. Acta 16, 3 (1983).
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*Conditions: FeTPPCl (2 mol%), 1 (3 equiv. added over 4 hours), substrate (0.22 mmol), 2 ml of 6 M KOH.
FeTPPCl (3 mol%), 1 (5 equiv. added over 7 hours), substrate (0.22 mmol), 3 ml of 6 M KOH. ‡Isolated yield of pure product.
†Conditions:
www.sciencemag.org SCIENCE VOL 335 23 MARCH 2012
1473

REPORTS
B
A
Working model:
Me
N
ON
SO2
Me
N
FeTPPCl (2 mol%)
6 M KOH
(1)
N2
+
ON
SO2
KOH
KOOC
KOOC
no product formed
H
H
H
H
COONa
11
3 equiv.
slow addition
COONa
H2O
Me
N
Organic phase
ON
SO2
N2
FeTPPCl
N2
catalyst (2.5 mol%)
6 M KOH
(2)
R
+
MeO
R
MeO
COONa
catalyst: Rh2(CH3COO-)4
Rh2(CH3(CH2)6COO-)4 45%
0%
3 equiv.
slow addition
Fig. 3. (A) Mechanistic studies. (B) Working model.
Me
N
ON
SO2
FeTPPCl (2 mol%)
6 M KOH/EtOH
+
(3)
MeO
12 % conversion
MeO
COONa
3 equiv.
slow addition
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www.sciencemag.org/cgi/content/full/335/6075/1471/DC1
Materials and Methods
Figs. S1 to S16
References (34, 35)
5 January 2012; accepted 14 February 2012
10.1126/science.1218781
Energy Capture from Thermolytic Solutions
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waters, but potential applications are currently limited to coastal areas and the need for a large
number of membrane pairs. Using salt solutions that could be continuously regenerated with waste
heat (≥40°C) and conventional technologies would allow much wider applications of salinity-gradient
power production. We used reverse electrodialysis ion-exchange membrane stacks in microbial
reverse-electrodialysis cells to efficiently capture salinity-gradient energy from ammonium bicarbonate
salt solutions. The maximum power density using acetate reached 5.6 watts per square meter of cathode
surface area, which was five times that produced without the dialysis stack, and 3.0 T 0.05 watts per
square meter with domestic wastewater. Maximum energy recovery with acetate reached 30 T 0.5%.
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0.5 M KOH under otherwise identical reaction conditions
and afforded only 20% conversion. Strongly basic
medium is thus required for the process.
icrobial fuel cell (MFC)–based technol- sources (1–3). Exoelectrogenic microorganisms
ogies are promising methods for direct can oxidize soluble organic matter, such as that
electrical power production from waste present in wastewater (4, 5), and release electrons
M
organic matter, wastewater treatment, and the to an electrode. Power densities with air-cathode
capture of salinity gradients in salt- and freshwater MFCs have reached 2.7 W/m2 by using optimized
solutions with equally sized electrodes (6) but are
lower when using complex organics or solutions
Department of Civil and Environmental Engineering, 212
Sackett Building, Pennsylvania State University, University
(0.26 to 0.45 W/m2) with ionic conductivities
typical of domestic wastewater (~1 mS/cm) (7–9).
Reverse electrodialysis (RED) is a process for
direct electricity production from salinity-gradient
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*To whom correspondence should be addressed. E-mail:
blogan@psu.edu
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