Catalytic conjunctive cross-coupling enabled by metal-induced metallate rearrangement

Article abstract of DOI:10.1126/science.aad6080

Transition metal catalysis plays a central role in contemporary organic synthesis. Considering the tremendously broad array of transition metal-catalyzed transformations, it is remarkable that the underlying elementary reaction steps are relatively few in

Full text of DOI:10.1126/science.aad6080

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| REPORTS
consistent with all the experimental evidence.
We expect the same-strength isotope effect if
the 2D crystals are combined with other pro-
ton conductors based on oxides (13, 25–28),
and the separation factor should be even larger
for proton-conducting media with stronger
hydrogen bonds; for example, in fluorides (28).
The above explanation allows for several ob-
servations about proton transport through 2D
crystals. First, it partially explains the disagree-
ment between the experiment (5) and theory
(5, 8–10) in the absolute value of EH for graphene:
Zero-point oscillations reduce the activation
barrier by ≈0.2 eV compared to theoretical val-
ues. We speculate that the remaining differences
[<20% in the case of (8)] may be accounted for
by considering other effects of the surrounding
media (for example, two-body processes involv-
ing a distortion of the electron clouds by protons
residing at the Nafion-graphene interface). Sec-
ond, the experiments confirm that hydrogen
chemisorption to 2D crystals is not the limiting
step in the transfer process because, otherwise,
the isotope effect would be different for hBN and
graphene. Third, the described sieving mechanism
implies a ≈30 for tritium-hydrogen separation.
Fourth, it is quite remarkable that zero-point
oscillations, a purely quantum effect, can still
dominate room-temperature transport properties
of particles 4000 times heavier than electrons.
The observed large a compares favorably with
sieving efficiencies of the existing methods for
hydrogen isotope separation (15–20). The high
proton conductivity exhibited by graphene and
boron nitride monolayers, comparable to that
of commercial Nafion films (5, 22), makes them
potentially interesting for such applications.
In this respect, the increasing availability of
graphene grown by chemical vapor deposition
(CVD) (29, 30) provides a realistic prospect of
scaling up the described devices from micro-
meter sizes to those required for industrial uses.
Indeed, although micromechanical cleavage al-
lows 2D membranes of highest quality, the ap-
proach is not scalable. As a proof of concept, we
repeated the mass spectrometry measurements
using centimeter-sized membranes made from
CVD graphene and achieved the same a ≈ 10 (fig.
S7). Notably, this shows that macroscopic cracks
and pinholes present in CVD graphene do not
affect the efficiency, because hydrons are electro-
chemically pumped only through the graphene
areas that are electrically contacted (22). Fur-
thermore, we estimate the energy costs asso-
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ciably lower than costs of the existing enrichment
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fundamentally simple and robust sieving mech-
anism, potentially straightforward setups, and
the need for only water at the input without the
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ACKNOWLEDGMENTS
This work was supported by the European Research Council,
Lloyd’s Register Foundation, the Graphene Flagship, the Royal
Society, the Air Force Office of Scientific Research, and the Office
of Naval Research. M.L.-H. acknowledges a Ph.D. studentship
provided by the Consejo Nacional de Ciencia y Tecnología
(Mexico). The reported results are included in a patent application
“Graphene-Proton Transport,” which is mostly based on (5).
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SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/351/6268/68/suppl/DC1
Materials and Methods
Figs. S1 to S7
References (31–49)
8 July 2015; accepted 18 November 2015
10.1126/science.aac9726
22. See supplementary materials on Science Online.
ORGANIC CHEMISTRY
Catalytic conjunctive cross-coupling
enabled by metal-induced
metallate rearrangement
Liang Zhang, Gabriel J. Lovinger, Emma K. Edelstein, Adam A. Szymaniak,
Matteo P. Chierchia, James P. Morken*
Transition metal catalysis plays a central role in contemporary organic synthesis.
Considering the tremendously broad array of transition metal–catalyzed transformations,
it is remarkable that the underlying elementary reaction steps are relatively few in
number. Here, we describe an alternative to the organometallic transmetallation step
that is common in many metal-catalyzed reactions, such as Suzuki-Miyaura coupling.
Specifically, we demonstrate that vinyl boronic ester ate complexes, prepared by
combining organoboronates and organolithium reagents, engage in palladium-induced
metallate rearrangement wherein 1,2-migration of an alkyl or aryl group from boron to the
vinyl a-carbon occurs concomitantly with C–Pd s-bond formation. This elementary
reaction enables a powerful cross-coupling reaction in which a chiral Pd catalyst merges
three simple starting materials—an organolithium, an organoboronic ester, and an
organotriflate—into chiral organoboronic esters with high enantioselectivity.
rganoboronic acids and their derivatives
are widely available and broadly useful
starting materials for organic synthesis
(1). In addition to being environmentally
benign and generally inexpensive, these
tion. The most commonly practiced such reaction
is the transition metal–catalyzed Suzuki-Miyaura
cross-coupling reaction between organic electro-
philes and organoboron compounds (2). In broad
strokes, the mechanism of the Suzuki-Miyaura re-
action involves a sequence of (i) oxidative addition
between a metal catalyst and the electrophile, (ii)
transmetallation with the organoboron reagent,
and (iii) reductive elimination of the C-C bonded
product (3). Here, we used an alternative path-
way to the organoboron transmetallation step.
The overall putative catalytic cycle enables a class
of organoboron cross-coupling that we term “con-
junctive cross-coupling” (Fig. 1A) because it merges
O
reagents exhibit a near-ideal balance of stability
and reactivity. Although chemically and config-
urationally stable, organoboronic esters engage
in a broad array of carbon-carbon and carbon-
heteroatom bond-forming processes upon activa-
REFERENCES AND NOTES
1. J. S. Bunch et al., Nano Lett. 8, 2458–2462 (2008).
2. S. P. Koenig, L. Wang, J. Pellegrino, J. S. Bunch, Nat.
Nanotechnol. 7, 728–732 (2012).
Department of Chemistry, Boston College, Chestnut Hill, MA
02467, USA.
*Corresponding author. E-mail: morken@bc.edu
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Fig. 1. Metal-induced metallate shift as a strategy
for catalytic reaction design. (A) The catalytic conjunc-
tive cross-coupling process. (B) Common metallate shift
to saturated carbon centers requires a leaving group. L,
ligand. (C) Metallate shift to unsaturated carbons is often
activated by addition of an external electrophile. M, metal;
E, electrophile. (D) A metallate shift promoted by a metal
complex can serve as an alternative to a transmetalla-
tion process. (E) Proposed catalytic cycle for conjunc-
tive cross-coupling. Ar, aryl group.
Catalytic Enantioselective Conjunctive Cross-Coupling
RO
OR
OR
catalyst
Li
B
C(sp2)-X
R1
B
+
+
C(sp2)
R1
OR
simple, monofunctional starting materials
versatile chiral organoboronic ester product
efficient reaction with high enantioselectivity
electrophile-induced
1,2-metallate shift to sp3 centers
1,2-metallate shift to sp2 centers
X
L
E+
L
A
L
E
B
B
L
A
R
L2B
L2B
-X
R
R
R
"ate" complex
A=C,N,O
metal-induced 1,2-metallate rearrangement
MLn
MLn
L2B
L2B
R
R
M
R
B
Ar-X
P
P
P
Ar
X
L2
Pd
M X
II
P
Ar
R
trans-
metallation
Pd
P
Ar
Pd
conjunctive coupling
cycle
P
P
Pd
R
P
or
B
I
P
Ar
L2
III
Pd
BL2
R
P
P
Ar
P
Pd
metal-induced
metallate
V
BL2
R
IV
Ar
R
Ar
rearrangement
Suzuki-Miyaura
products
or
conjunctive
coupling
Ar
two nucleophilic reagents into one during the course
of the reaction. Overall, the conjunctive cross-coupling
constructs chiral products by merging three simple
starting materials: an organolithium reagent, an
organoboronic ester, and an organic electrophile.
The reaction, which adds to recent catalytic alkene
carboboration reactions (4, 5), establishes two new
carbon-carbon bonds and forms a stereogenic
center bearing a useful organoboronic ester with
high enantioselectivity.
Organoboronic esters are known to participate
in a wide range of reactions that occur by stereo-
specific 1,2-metallate rearrangements (6, 7). The
preponderance of these reactions occur with a
four-coordinate anionic boron-centered “ate” com-
plex (Fig. 1B) that rearranges by a 1,2-carbon shift
from boron to an adjacent sp3-hybridized electro-
philic center (designated “A”) bearing an attached
leaving group (X). The stereoelectronic requirements
of the metallate shift dictate an anti-periplanar
arrangement of the migrating carbon atom (R)
and the leaving group such that the overall process
is stereoretentive at R but stereoinvertive at A (8).
These processes are common (9) for transforma-
tions where the “A” group is a carbon, nitrogen, or
oxygen atom and less common but still estab-
lished for sulfur and phosphorus (10, 11). Because
of the stereospecific nature of the above-described
processes, stereoselectivity in metallate shifts is
generally subject to substrate control (12), although
Jadhav and Man reported an example with selec-
tivity dictated by a chiral catalyst (13). Metallate
rearrangement from boron to sp (14) and sp2 (15)
hybridized carbons often require the addition of
an external electrophilic activator (Fig. 1C), and
generally the 1,2-metallate shift is followed by
elimination to reestablish unsaturation (16).
In this context, we considered that in place of
stoichiometric electrophilic activating agents,
p-acidic late transition metals might similarly
promote the 1,2-metallate shift of alkenyl boronates
(Fig. 1D) and that the resulting chiral organo-
metallic intermediate might be used in subse-
quent bond-forming processes.
sidered that an electrophilic palladium complex
(II, Fig. 1E), generated by oxidative addition
of I with an organic electrophile, might induce
1,2-migration in a vinyl boronate-derived ate complex
(III→IV) and establish a new C–C bond and a
boron-substituted stereogenic center. Subsequent
reductive elimination (IV→I) would serve to estab-
lish a second C–C bond, release the product, and
concomitantly furnish a reduced Pd complex that
might continue a catalytic cycle. The net reaction,
in this cycle, is aligned with important work from
Murakami, who studied reactions of alkynyl boron
ate complexes (17). Although p-acidic late tran-
sition metal complexes are well known to activate
alkenes for nucleophilic attack, nucleopalladation
with Pd(II) complexes generated by oxidative ad-
dition reactions are less common (18, 19). For the
envisioned process to be successful, several key
questions emerged: Would the Pd(II) aryl com-
plexes be sufficiently p-acidic to facilitate metal-
late rearrangement (III→IV)? Would common
direct transmetallation (III→V) dominate the re-
action and furnish Suzuki-Miyaura products?
In one embodiment of catalysis based on metal-
induced 1,2-metallate rearrangements, we con-
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Me Me
Fig. 2. Scope of the catalytic
conjunctive cross-coupling
reaction starting from vinyl
boronic esters. Yields represent
isolated yield of purified material
and are an average of two
Me
Me
Me Me
1% Pd(OAc)2
1.2% (SpSp)-L1
R2OTf
Li
Ph
O
O
MeO
P
Fe
OH
B
Me
Me
NMe2
2
O
R1
O
R2
Ph
B
R1
+
THF, 60 °C,14 h
then NaOH, H2O2
P
OMe
experiments. The absolute
R1 Li
NMe2
(SpSp)-L1
2
configuration was determined by
anomalous dispersion x-ray and
by chromatographic comparison
to known compounds. *VinylB(neo)
was replaced with vinylB(pin);
†reaction conducted at 80°C. er
was determined by chiral
supercritical fluid chromatography.
OAc, acetate; Me, methyl group;
Ph, phenyl group; OTf, trifluoro-
methanesulfonate; OTBS, tert-
butyldimethylsiloxy.
OH
OH
OH
OH
OH
Ph
Ph
Ph
Me3Si
Ph
Ph
Ph
n-Bu
hexyl
i-Pr
1: 83% yield
97:3 er
3: 77% yield
98:2 er
4: 74% yield
98:2 er
5*: 48% yield
88:12 er
2: 66% yield
93:7 er
Me
OMe
CF3
Me
OH
OH
OH
OH
HO
Ph
Ph
Ph
Ph
Ph
†
†
Me
Me
6: 68% yield
97:3 er
7: 84% yield
97:3 er
OMe
8: 51% yield
9 : 51% yield
10 : 44% yield
94:6 er
96:4 er
70:30 er
CHO
OMe
OMe Ph
O
O
OH
OH
OH
OH
OH
Ph
Ph
Ph
Ph
Ph
11: 49% yield
12: 83% yield
13: 68% yield 14*: 73% yield
96:4 er 95:5 er
15*: 86% yield
98:2 er
98:2 er
97:3 er
OTBS
OH
OH
OH
OH
OH
heptyl
Ph
hexyl
Ph
Ph
Ph
CO2Me
16*: 73% yield
17*: 74% yield
18*: 50% yield
19*: 46% yield
20*: 41% yield
87:13 er
94:6 er
89:11 er
93:7 er
90:10 er
Could facial selectivity in olefin binding render
the migration (III→IV) enantioselective? Last,
would b-hydrogen elimination in intermediates
such as IV compete with reductive elimination?
To begin our studies of the conjunctive cross-
coupling reaction, we selected phenyl triflate as
the electrophile, anticipating that the outer-sphere
triflate anion would leave open a coordination site
on Pd for alkene binding (Fig. 1E, III). In an effort
to minimize steric penalties that might inhibit
metallate shift, we opted for an unsubstituted vinyl
group in construction of the boron ate complex.
Similarly, on the basis of recent studies from
Mayr and Aggarwal (20, 21), we selected a neo-
pentylglycolato ligand for boron because the ate
complexes derived from this ligand (versus others
readily available) were determined to be the most
nucleophilic. Last, in an effort to favor reductive
elimination relative to b-hydrogen elimination in
putative intermediate IV, we selected ligands with
large bite angles for the palladium complex. Initial
experiments with achiral bidentate ferrocenyl
1,1′-diphosphines were promising and suggested that
the conjunctive cross-coupling could indeed operate.
A survey of a number of chiral diphosphine struc-
tures revealed that Josiphos (22) and MandyPhos
(Solvias AG, Kaiseraugst, Switzerland) (23) are par-
ticularly effective ligand classes, with the MandyPhos
ligand L1 providing an outstanding level of enantio-
selectivity and very good catalyst efficiency in the
reaction.
ever, isolation of the organoboron product itself
is also possible (the organoboron precursor to
alcohol 1 was isolated in 76% yield by silica gel
chromatography). As shown in Fig. 2, both aryl
and alkyl groups proved competent migrating ele-
ments in conjunctive coupling reactions. In cases
where the yield of product is low, analysis of the
reaction mixture before oxidation showed that
by-products generally consist of recovered organo-
boronic ester (likely generated during workup by
protonolysis of one carbon ligand from the ate
complex) and direct Suzuki-Miyaura products.
Primary and secondary alkyl groups migrate, as
do functionalized alkyl appendages [e.g., (trime-
thylsilyl)methyl]. Conjunctive couplings were also
effective for both electron-rich and electron-poor
electrophiles. Moderately encumbered electrophiles,
such as ortho-substituted arenes, can engage in the
reaction, although the more highly substituted
2,6-dimethylphenyl derivative suffered from lower
selectivity. With respect to preparation of chiral
hydrocarbon frameworks, a range of substituted
alkenyl triflates participate, and the configuration
of the product alkene directly reflects that of the
precursor electrophile.
In optimizing the reaction conditions, we found
tetrahydrofuran (THF) to be the most effective sol-
vent. Therefore, when the migratory R group was
appended to boron by addition of hydrocarbon
solutions of organolithium reagents to the neopentyl
glycol–derived vinyl boronic ester vinylB(neo)
(Fig. 2), the resulting ate complexes were evapo-
rated to dryness and redissolved in THF before
reaction. The efficiency of the reaction was also
greatly diminished by chloride, bromide, or iodide
ions, an obstacle easily overcome by the continued
use of aryl and alkenyl triflates as the electro-
phile. For coupling of alkenyl triflate electrophiles,
the reaction selectivity was markedly enhanced
with boron ate complexes derived from pinaco-
lato ligands in place of the neo-pentylglycol de-
rivative [e.g., product 15 is formed in 53% yield
and 82:18 enantiomeric ratio (er) when using the
neo-pentylglycol ligand], whereas for aryl triflates,
the neo-pentylglycol ligand furnished higher selec-
tivity [1 formed in 94% yield, 93:7 er using B(pin)
group]. With these features optimized, the scope of
the conjunctive coupling was explored with an array
of ate complexes and electrophiles. During these
experiments, it was most convenient to oxidize
the product organoboronic ester to the derived
alcohol by treatment with NaOH and H2O2; how-
The ate complex could be generated either by
addition of an organolithium reagent to vinylB(neo)
(Fig. 2) or by addition of vinyllithium to organo-
boronic esters (Fig. 3). Considering the broad
72 1 JANUARY 2016 • VOL 351 ISSUE 6268
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| REPORTS
Me
Me
Fig. 3. Scope of the catalytic con-
junctive cross-coupling reaction
starting from alkyl or aryl boronic
esters. Yields represent isolated
yield of purified material and are an
average of two experiments. Cy,
cyclohexyl group.
O
O
Me Me
Me
Me
2% Pd(OAc)2
2.4% (SpSp)-L1
PhOTf
R1
B
Ph
MeO
P
Li
Fe
OH
Me
Me
NMe2
2
O
O
Ph
Ph
B
+
R1
R1
THF, 60 °C,14 h
then NaOH, H2O2
Li
P
OMe
NMe2
(SpSp)-L1
2
OH
OH
OH
OH
OH
Ph
Ph
Ph
Ph
Ph
n-Bu
i-Bu
Ph
Cy
3: 76% yield
98:2 er
21: 59% yield
1: 84% yield
97:3 er
22: 80% yield
23: 93% yield
99:1 er
95:5 er
99:1 er
OH
OH
OH
OH
MeO
MeO
Ph
Ph
Ph
Ph
MeO
F3C
Cl
24: 77% yield
25: 83% yield
27: 66% yield
OMe
26: <5% yield
94:6 er
97:3 er
97:3 er
OH
OH
Me OH
OH
Me OH
Ph
Ph
Ph Me
Ph
Ph
N
Me
Ph2N
Me
Me
31: 67% yield 32: 73% yield
28: 75% yield
29: 86% yield
30: 86% yield
95:5 er
95:5 er
95:5 er
97:3 er
72:28 er
Fig. 4. Synthesis and
mechanistic studies.
array of organoboronic esters that are already
available for use in common cross-coupling pro-
cesses, the latter strategy is particularly enabling.
In this context, we found that the presence of
lithium halide salts, even at 1 to 2 mole percent
(mol %) loading, markedly erodes conjunctive
coupling efficiency. Thus, to perform the reaction
with highest efficiency with 2 mol % catalyst load-
ing, it is critical that halide-free vinyllithium be
used. To prepare this reagent, we developed a
procedure that involved lithium-halogen exchange
with n-BuLi (Bu, butyl group) in hexane, followed
by low-temperature recrystallization of pure vinyl-
lithium. When these precautions are taken, con-
ducting the reaction as in Fig. 3 allowed the scope
of the migrating group to be surveyed more com-
pletely and revealed that, although reactions re-
quiring migration of very electron-deficient groups
are still a challenge, electron-neutral and electron-
rich arenes can engage in the migration, as can
those that are more highly substituted (i.e., 2,6-
disubstituted arenes). We also found that, with
5 mol % catalyst loading, vinyllithium prepared
as above but without recrystallization was ef-
fective (e.g., 3 formed in 69% yield, 98:2 er) and
that vinyllithium prepared by lithium-tin exchange
(24) could be used directly (2 mol % catalyst, 1
formed in 83% yield, 97:3 er).
1% Pd(OAc)2
1.2% (SpSp)-L1
OMe
MeO
MeO
B(neo)
OTBS
OMe
(A) Synthesis of com-
bretastatin by conjunc-
tive cross-coupling.
(B) The stereochemical
course of metal-induced
metallate rearrange-
ment. dr, diastereomeric
ratio.
OH
TfO
OTBS
OMe
MeO
MeO
then NaOH
H2O2
+
33
80% yield
95:5 er
Li
OMe
OH
OMe
OH
MeO
MeO
(n-Bu)4NF
combretastatin
97% yield
OMe
1% Pd(OAc)2
1.2%(SpSp)-L1
Li
OH
D
TfO
B(pin)
D
H
THF, 60 °C,14 h
then NaOH, H2O2
34: 57% yield
82:18 er, >20:1 dr
(pin)B
Ph
PdLn
H
B(pin)
D
H
D
H
B
A
Ph
To probe the utility of conjunctive cross-
coupling to practical organic synthesis, we were
attracted to the natural product (–)-combretastatin
(25), a member of a family of cytotoxic stilbene-
derived natural products that bind b-tubulin (Fig.
4A). Although many synthetic methods have
facilitated the construction of combretastatins (26),
alternative processes may engage different starting
materials and thereby provide access to distinct
analogs. In the case of conjunctive cross-coupling,
the requisite boronic ester and electrophile are
readily available and, as depicted in Fig. 4A, are
readily converted to the coupled product in an
efficient and highly selective fashion. Removal
of the silicon protecting group furnished the target
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| REPORTS
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68, 3397–3405 (2003).
ACKNOWLEDGMENTS
Experimental data are available in the associated Supplementary
Materials. This research was supported in part by the NIH,
National Institute of General Medical Sciences (GM 64451), and by
Boston College. Metrical parameters for the crystal structure of
compound 1 are available free of charge from the Cambridge
Crystallographic Data Centre under accession number
CCDC 1437520.
The mechanism of the conjunctive coupling
reaction is the subject of ongoing investigations,
although the substrate scope (Figs. 2 and 3) gives
clues about the process. The observation that
electron-deficient arenes are less prone to migra-
tion is consistent with the mechanistic hypothesis
put forward in Fig. 1E. According to this hypothesis,
formation of IV would likely be stereochemistry
determining, and, in line with this prediction,
the selectivity of the reaction depends not only
on the ligand framework but also on the organo-
boronic ester ligand (pinacol versus neo-pentylglycol),
the migrating group, and the electrophile. In
addition to these observations, one preliminary
experiment sheds important light on the nature
of the metal-induced metallate rearrangement that
appears to underlie the conjunctive coupling process.
As depicted in Fig. 4B, when the reacting ate
complex was constructed from stereochemically
defined deuterium-labeled vinyllithium (27) and
phenylB(pin), the (1R,2R) stereoisomer of the con-
junctive coupling product was formed in >20:1
diastereoselection (82:18 er). Although other in-
terpretations are possible, should the mechanism
be in line with that proposed in Fig. 1E and re-
ductive elimination occur with retention of con-
figuration at carbon (a reasonable assumption), the
observed stereochemical outcome in Fig. 4B is
consistent with anti-migration of the arene group
to a Pd-olefin complex (Fig. 1D). Such an outcome
is reminiscent of nucleometallation reactions
that do not involve preassociation of the migrating
group and the metal center (28).
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/351/6268/70/suppl/DC1
Materials and Methods
Tables S1 to S7
References (30–57)
12 October 2015; accepted 18 November 2015
10.1126/science.aad6080
MICROBIAL ENGINEERING
Self-photosensitization of
nonphotosynthetic bacteria for
solar-to-chemical production
Kelsey K. Sakimoto,1,2 Andrew Barnabas Wong,1,2 Peidong Yang1,2,3,4
*
Improving natural photosynthesis can enable the sustainable production of chemicals.
However, neither purely artificial nor purely biological approaches seem poised to realize the
potential of solar-to-chemical synthesis. We developed a hybrid approach, whereby we
combined the highly efficient light harvesting of inorganic semiconductors with the high
specificity, low cost, and self-replication and -repair of biocatalysts. We induced the
self-photosensitization of a nonphotosynthetic bacterium, Moorella thermoacetica, with
cadmium sulfide nanoparticles, enabling the photosynthesis of acetic acid from carbon dioxide.
Biologically precipitated cadmium sulfide nanoparticles served as the light harvester to
sustain cellular metabolism.This self-augmented biological system selectively produced acetic
acid continuously over several days of light-dark cycles at relatively high quantum yields,
demonstrating a self-replicating route toward solar-to-chemical carbon dioxide reduction.
We anticipate that many other transition metal–
catalyzed reactions might also be reengineered to
incorporate metal-induced metallate rearrange-
ments, thereby providing distinct strategies for
catalytic enantioselective synthesis.
he necessity of improving the natural mech-
anisms of solar energy capture for sustain-
able chemical production (1) has motivated
the development of photoelectrochemical
devices based on inorganic solid-state mate-
Several inorganic-biological hybrid systems
have been devised: semiconductor nanoparticles
with hydrogenases to produce biohydrogen (7),
long wavelength–absorbing nanomaterials to
improve the photosynthetic efficiency of plants
(8), and whole cells with photoelectrodes for CO2
fixation (9, 10). Whole-cell microorganisms are
favored to facilitate the multistep process of CO2
fixation and can self-replicate and self-repair (11).
Furthermore, bacteria termed “electrotrophs” can
undergo direct electron transfer from an electrode
(12). However, traditional chemical synthesis of
the semiconductor component often requires high-
purity reagents, high temperatures, and complex
microfabrication techniques. Additionally, the
integration of such foreign materials with biotic
systems is nontrivial (13). Many reports have
shown that some microorganisms induce the
precipitation of nanoparticles (14), producing
an inherently biocompatible nanomaterial under
mild conditions.
REFERENCES AND NOTES
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T
rials (2). Although solid-state semiconductor light
absorbers often exceed biological light harvesting
in efficiency (3), the transduction of photoexcited
electrons into chemical bonds (particularly toward
multicarbon compounds from CO2) remains chal-
lenging with abiotic catalysts (4, 5). Such catalysts
struggle to compete with the high-specificity, low-
cost material requirements and the self-replicating,
self-repairing properties of biological CO2 fixation
(6). Thus, a viable solution must combine the best
of both worlds: the light-harvesting capabilities
of semiconductors with the catalytic power of
biology.
1Department of Chemistry, University of California–Berkeley,
Berkeley, CA 94720, USA. 2Materials Sciences Division,
Lawrence Berkeley National Laboratory, Berkeley, CA 94720,
USA. 3Department of Materials Science and Engineering,
University of California–Berkeley, Berkeley, CA 94720, USA.
4Kavli Energy NanoSciences Institute, Berkeley, CA 94720,
USA.
Although photosynthetic organisms can pre-
cipitate semiconductor nanoparticles, their meta-
bolic pathways are arguably less desirable than
those of their nonphotosynthetic counterparts.
Although gene modification of phototrophs has
19. D. Bruyère, D. Bouyssi, G. Balme, Tetrahedron 60, 4007–4017
(2004).
*Corresponding author. E-mail: p_yang@berkeley.edu
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