Arylative Allenol Cyclization via Sequential One-pot Enzyme & Palladium Catalysis

Article abstract of DOI:10.1002/cctc.202001619

The one-pot combination of halogenation biocatalysis and Suzuki-type cross coupling enables the direct arylative cyclization of allenic alcohols with boronic acids. This modular approach to unsaturated five-membered O-heterocycles proceeds in an aqueous emulsion with air as terminal oxidant. Here, the enzymatic oxidative activation of simple halide salts acts as traceless ring-closure-inducing event to trigger the subsequent C?C coupling. With the original protocol merging soluble proteins and a homogeneous SPhos-based palladium catalyst as a template, a novel heterogeneous nanobiohybrid was developed. Consisting of an oxidase matrix hosting small spherical palladium nanoparticles, this enzyme-metal hybrid exhibits catalytic competence for both the biocyclization as well as the C?C bond-forming cross coupling, underlining the potential of this new techniques for streamlining chemoenzymatic approaches.

Full text of DOI:10.1002/cctc.202001619

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Arylative Allenol Cyclization via Sequential One-pot Enzyme &
Palladium Catalysis
Authors: Janne M Naapuri, Gustav Åberg, Jose M Palomo, and Jan
Deska
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To be cited as: ChemCatChem 10.1002/cctc.202001619
Link to VoR: https://doi.org/10.1002/cctc.202001619
01/2020

10.1002/cctc.202001619
ChemCatChem
FULL PAPER
Arylative Allenol Cyclization via Sequential One-pot Enzyme &
Palladium Catalysis
Janne M. Naapuri,[a] Gustav A. Åberg,[a] Jose M. Palomo,[b] and Jan Deska*[a]
[a]
MSc Janne M. Naapuri, BSc Gustav A. Åberg, Prof. Dr. Jan Deska
Department of Chemistry
Aalto University
Kemistintie 1, 02150 Espoo (Finland)
E-mail: jan.deska@aalto.fi
Web: www.deskalab.com
[b]
Dr. Jose M. Palomo
Department of Biocatalysis
Institute of Catalysis (CSIC)
c/ Marie Curie 2, 28049, Madrid (Spain)
Supporting information for this article is given via a link at the end of the document.
Abstract: The one-pot combination of halogenation biocatalysis and
Suzuki-type cross coupling enables the direct arylative cyclization of
allenic alcohols with boronic acids. This modular approach to
unsaturated five-membered O-heterocycles proceeds in an aqueous
emulsion with air as terminal oxidant. Here, the enzymatic oxidative
activation of simple halide salts acts as traceless ring-closure-
inducing event to trigger the subsequent C-C coupling. With the
original protocol merging soluble proteins and a homogeneous
a enzymatic allene halocyclization (2017)
chloroperoxidase
X
glucose oxidase
NaX, glucose, air
R''
R'''
R
R
R''
aqueous emulsion
OH
O
R'
R'''
R'
b traceless activation/cross coupling (this work)
SPhos-based palladium catalyst as
a
template,
a
novel
heterogeneous nanobiohybrid was developed. Consisting of an
oxidase matrix hosting small spherical palladium nanoparticles, this
enzyme-metal hybrid exhibits catalytic competence for both the
biocyclization as well as the C-C bond-forming cross coupling,
underlining the potential of this new technique for streamlining
chemoenzymatic approaches.
R''
R'''
enzymatic halocyclization
+
Ar
R
Pd-catalyzed cross coupling
OH
R
R''
R'
aqueous emulsion
O
+
R'
R'''
Ar-B(OH)2
P
Introduction
[Pd]
OMe
The combination of multiple catalytic transformations in a single
reaction vessel is an important objective for efficient and
sustainable synthesis. These one-pot reaction designs allow for
more economic process chains by eliminating waste production
and time consuming work-up and purification steps.[1] In the field
of biocatalysis, single vessel cascade reactions have been widely
implemented due to the ease of incorporating several enzymatic
transformations together.[2] Here, the biocatalysts’ natural affinity
towards similar process conditions, tolerance of a wide range of
functional groups, and minimal disturbance towards other
enzymes in the mixture are clearly rather unique properties
compared to other traditional catalysts. The progress in protein
engineering and directed evolution has drastically increased the
desirability of biocatalysis for industrial applications in recent
MeO
from homogeneous
metal co-catalysis...
...to multifunctional metal-enzyme
nanobiohybrids
Scheme 1. From biocatalytic to chemoenzymatic synthesis of substituted
O-heterocycles.
in organic solvents, which on the other hand are most commonly
the media of choice in chemocatalytic processes. Several
approaches have been successfully designed to alleviate this
problem, such as enzyme immobilization or the use of deep
eutectic solvents, ionic liquids or colloidal media.[5]
With the aim to broaden the application range of biocatalysis
by enzymatic interpretations of traditional organic synthesis
approaches, we recently designed a biocatalytic halocyclization
of allenic alcohols and carboxylates[6] as a more sustainable
alternative to the classical methods of halogenation. Taking
advantage of haloperoxidases, an enzyme class, that has found
wide use in both halogenation[7] and oxidation[8] reactions, the
process relies on an enzymatic cascade featuring
years.[3] Combining biocatalysis in
a one-pot fashion with
chemocatalysis is an attractive concept as it merges the
advantages of the two fields. Even though many such applications
have emerged recently, the approach suffers from daunting
challenges as the process conditions for chemo- and biocatalysis
are often very different.[4] The most apparent issue is the opposing
requirement regarding reaction media as few enzymes stay active
1
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10.1002/cctc.202001619
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FULL PAPER
chloroperoxidase from C. fumago (CPO) and glucose oxidase
from A. niger (GOx) to catalyze the formation of halodihydrofurans
while only consuming mild stoichiometric reagents sodium halide
and D-glucose (Scheme 1a). A critical requirement for the reaction
was the incorporation of a non-ionic surfactant, resulting in an
emulsified reaction medium. As the vinyl halide handle produced
in the bi-enzymatic halogenative cyclization is an obvious target
for further chemical modifications through e.g. cross coupling
reactions, we envisioned that this process would represent an
interesting premise for generally investigating these colloidal
conditions in the context of one-pot chemo/biocatalytic
applications. Palladium catalyzed cross couplings and related
reactions have been established as some of the most versatile
and efficient tools for synthesis since being first developed in the
1970’s.[9] Recently, the Pd cross coupling reactions have also
found applications in one-pot chemoenzymatic approaches,
increased by addition of an inorganic base prior to supplementing
the mixture with Na2PdCl4, a monodentate phosphine ligand and
phenylboronic acid as coupling partner. The first attempts using
triphenylphosphine as the ligand for the Pd catalyst unfortunately
resulted in a low yield of phenylated product 3a (Table 1, entry 1).
Nevertheless, the bromodihydrofuran 2a was recovered and the
combined yield of almost 90% of 2a and 3a indicate that no severe
degradation was taking place. Unsurprisingly, the nature of the
phosphine ligands had an important influence, and both XPhos
Table 1. Optimization of cross coupling parameters.
enzymatic
halocyclization
system
palladium-based
Suzuki
system
one-pot reaction
without isolation
CPO, GOx
NaBr, Glc
Pd, base
PhB(OH)2
Ph
Br
including combinations with alcohol dehydrogenase[10]
,
Ph
transaminase[11] and tryptophan halogenase[12]. These works
have shown Pd-catalyzed Suzuki-Miyaura reactions to be
compatible with water and biocatalytic processes, inspiring us to
combine this C-C coupling with the enzymatic allenol
halocyclization to produce complex arylated dihydrofurans in
sequential and modular one-pot strategy (Scheme 1b). In addition
to the more established homogeneous chemoenzymatic catalysis,
recent developments in the field of nanotechnology have also
made it possible to produce dual-active enzyme-metal
nanoparticle biohybrids with great potential for heterogeneous
one-pot catalysis.[13] In these nanobiohybrids, the enzyme acts as
the support during the transition metal nanoparticle formation,
while at the same time also retaining enzymatic activity. The
enzyme-metal nanoparticle biohybrids have so far been used as
dual-active heterogeneous catalysts in hydrolytic/reductive
cascade reactions, as well as in a dynamic kinetic resolution of
amines.[14] In this study, we explored this technique to produce
oxidase/Pd nanoparticle biohybrids and evaluated them as
multifunctional catalysts in the one-pot halocyclization and cross-
coupling approach.
OH
aqueous
emulsion
r.t., 2.5 h
aqueous
emulsion
T, 3 h
O
O
Ph
Ph
rac-1a
rac-2a
rac-3a
Entry[a]
Pd source
(mol%)
ligand
base
(eq.)
T
(°C)
2a[b]
(%)
3a[b]
(%)
1
2
Na2PdCl4 (5)
Na2PdCl4 (5)
Na2PdCl4 (5)
Na2PdCl4 (5)
Na2PdCl4 (5)
Na2PdCl4 (5)
Pd(OAc)2(5)
Na2PdCl4 (1)
Na2PdCl4 (5)
Na2PdCl4 (5)
Na2PdCl4 (5)
Na2PdCl4 (5)
Na2PdCl4 (5)
Na2PdCl4 (5)
PPh3
4
K2CO3 (10)
K2CO3 (10)
K2CO3 (10)
K2CO3 (10)
K2CO3 (10)
K2CO3 (10)
K2CO3 (10)
K2CO3 (10)
K2CO3 (50)
K2CO3 (2)
80
80
80
80
80
80
80
80
80
80
80
60
80
80
68
25
4
19
60
82
71
11
0
3
5
4
6
16
78
89
2
5
7
6
8
7
5
83
82
47
33
88
42
84
85
8[c]
9
5
7
5
32
55
0
10
11
12
13[d]
14[e]
5
5
K3PO4 (10)
K3PO4 (10)
K3PO4 (10)
K3PO4 (10)
Results and Discussion
5
44
0
Allenic building blocks offer a unique reaction scope and have
thus found broad application as versatile intermediates in organic
synthesis.[15] As a starting point for our investigation on the
sequential one-pot approach, the biocatalytic halocyclization of
allenic alcohol 1a was performed using the conditions found
optimal in our previous studies.[6] In this first step, GOx serves as
catalyst for a controlled release of hydrogen peroxide into the
solution as aerial oxygen is reduced. The H2O2 is subsequently
consumed by CPO while catalyzing the oxidation of bromide into
electrophilic hypobromite species, which triggers a 5-endo-trig
bromocyclization of the allenol. As the cyclization step was found
to be fairly sensitive regarding changes in the reaction medium,
the emulsion comprising of a 1:1 mixture of n-heptane and citrate
buffer (pH 4.5) with 10 mM Brij 35 as the detergent (a non-ionic
polyoxyethylene lauryl ether, also marketed as Brij L23) was also
adopted for the one-pot investigation. The palladium-mediated
cross coupling with phenylboronic acid was chosen as the
benchmark reaction for optimizing the conditions of the enzyme-
metal-catalyzed sequence. Here, after the initial biocatalytic
conversion at room temperature, the reaction medium pH was
5
5
0
PCy2
PCy2
OMe
PCy2
OMe
SO3–Na+
iPr
iPr
XPhos (4)
MeO
SPhos (5)
MeO
sSPhos (6)
iPr
PPh2
NMe3+ TfO–
PCy2
SO3– Na+
7
8
[a] Conditions: (i) 8.0 µL chloroperoxidase (C. fumago, 37 U/µL), 2.6 mg glucose
oxidase (A. niger, 19 U/mg), 100 μmol allene, 300 μmol NaBr, 200 μmol D-
glucose, 100 μmol Brij 35, 5 mL heptane, 5 mL citrate buffer (0.1 M, pH 4.5),
room temperature, 2.5 h; (ii) 1.5 µL catalase (M. lysodeikticus, 131 U/µL), 15
min; (iii) Pd catalyst (5 mol%), ligand (15 mol%), 300 μmol phenylboronic acid,
base, T, 3 h. [b] Yields determined by quantitative 1H NMR against dimethyl
sulfone as standard. [c] 18h reaction time. [d] No catalase addition. [e] Argon
atmosphere during cross coupling.
2
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10.1002/cctc.202001619
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FULL PAPER
(4) and SPhos (5) performed better (Table 1, entries 2 & 3), with
SPhos giving desired arylation product 3a with 82% yield over the
two consecutive steps. In their work on successfully combining
Suzuki-Miyaura cross couplings in a one-pot design with the
bromination by tryptophan halogenase in aqueous medium, the
group of Sewald opted to use a water-soluble ligand (sulfonated
SPhos or sSPhos, 6) as their optimal system did not involve any
cosolvents.[12a] In contrast, the water-soluble phosphine ligands 6-
8 (Table 1, entries 4–6) did not perform better than their non-water
soluble counterparts in our colloidal biphasic medium. Across all
tested reactions, approximately 10% of material is lost, possibly
due to formation of more polar side products that are likely to
remain in the aqueous layer in the workup of the emulsion system.
isolated yield of 3a also underline the good comparability with the
NMR yields used in the initial screening. Although the α-
monosubstituted products 3b-f were also furnished in good yields
over the two steps, these transformations fell noticeably short of
the efficiency achieved previously for the isolated
bromocyclization part (>80%), indicating stability issues in the
elevated temperature and basic conditions of the cross coupling
reaction. The very high yield of 3g, lacking any potentially labile
allylic C-H bonds, strongly supports this theory. The most
palpable drop in synthetic efficiency was observed for the
products 3h-i carrying substituents in the vinylic position. Here,
the added steric hinderance likely diminishes the conversion in
the Pd-catalyzed reaction, resulting in mediocre overall yields.
Despite involving an emulsified reaction medium stabilized
by the non-ionic surfactant, the enzymatic bromocyclization of
allenic alcohol 1a could be combined in one-pot with
phenylboronic acid cross coupling under fairly normal conditions
for a Suzuki-Miyaura reaction. The optimal conditions resulted in
a high yield of 88% over the two reaction steps, with the cross
coupling reaction involving potassium phosphate as the base at a
temperature of 80 °C (Table 1, entry 11). Full conversion could be
reached within 3 hours of reaction time using 5 mol% palladium
catalyst in the transformation. The nature of the palladium (II)
source, (Na2PdCl4 vs Pd(OAc)2) had no major effect (Table 1,
entry 3 vs entry 7). Using just 1 mol% of the catalyst also results
in full conversion and similar yield, yet demanded a longer
reaction time of 18 h (Table 1, entry 8). Generally, a substantial
amount of the inorganic base was required to overcome the citrate
buffer, with a smaller amount substantially decelerating the
process (Table 1, entry 10). Interestingly, when exceeding a
certain level of the base concentration, destabilization of the
colloidal medium and formation of larger globules was observed,
also resulting in deceleration of the reaction (Table 1, entry 9). In
the biocatalytic first step of the process, hydrogen peroxide is
released steadily as a by-product in GOx-catalyzed glucose
oxidation to avoid deactivation of the CPO caused by excess
H2O2.[16] To eliminate the potential lingering hydrogen peroxide
causing issues in the cross-coupling step, catalase from
Micrococcus lysodeikticus was supplemented to the mixture
between the steps. However, omitting this catalase addition
(Table 1, entry 13) had minimal effect on the process, indicating
that the H2O2 is not a major issue, and is likely already exhausted
by the CPO. Similarly, a protective argon atmosphere to prevent
potential oxidative side reactions during the cross coupling step
turned out non-essential (Table 1, entry 14), underlining the
robustness of the process. The change of the traceless activation
agent to NaCl or NaI resulted in significantly lower overall yields
(w/ NaCl: 6%; w/ NaI: 48%).
1) air, D-glucose, NaBr
glucose oxidase
chloroperoxidase;
R'
R
catalase, r.t., 4 h
R''
OH
R
R'
R''
O
2) PhB(OH)2, K3PO4
Na2PdCl4, SPhos
80 °C, 16 h
1
3
Ph
Ph
Ph
O
O
O
3c, 56%
Cl
3a, 85%
3b, 62%
Ph
Ph
Ph
O
O
3e, 58%
O
OMe
3d, 60%
3f, 50%
Ph
Ph
OH
Ph
O
O
O
3g, 88%
3h, 45%
3i, 53%
Scheme 2. Structural variation of the allenic coupling partner.
Allenol 1a was subsequently also chosen as model
substrate for the investigation on the scope of the other coupling
partner, the boronic acid (Scheme 3). Eliminating the previously
experienced challenges regarding steric hindrance and allylic
lability, most tested boronic acids performed splendidly. A wide
selection of substitution patterns on aromatic boronic acids was
tolerated, giving excellent yields for simple methyl-substituted
species (3j & 3k) and both activating (3l & 3m) and deactivating
(3n) substituents on the arene providing the corresponding
dihydrofurans with high selectivity in the one-pot process. Within
this series, only para-chlorophenylboronic acid posed an outlier,
offering a lower yield of 48% for 3o. Although no major secondary
biphenyl-type product could be detected, further activation of the
chloroarene by the highly reactive SPhos-Pd system could
account for slightly inferior performance. In contrast to the very
effective vinyl-aryl couplings, achieving furyl or olefin substitutions
was more challenging. Acceptable yields of 3p-q using cis-
propenyl- or 2-furylboronic acid as the cross coupling partner
could only be reached after further optimization of the protocol
and via continuous addition of excess boronic acid solution.
Gratifyingly, the optimized protocol translated very well to
structural variations both on the allene and boronic acid part of
the transformation and the one-pot process was successfully
applied to generate a wide range of substituted dihydrofurans in
mostly good to excellent isolated yields (Schemes 2 & 3). As
stereochemistry was not the focus of this study, all chiral
substrates were used as in optically inactive form, yielding
racemic heterocycles. Addressing the scope of allenol substrates
first, a comparative study was conducted relative to the fully α-
substituted 1a (Scheme 2). For this model substrate, the excellent
of 85% of phenyldihydrofuran 3a, compared to the yields achieved
in the individual halocyclization step (85% to 2a),[6] is synonymous
with a quantitative conversion in the cross coupling step. The 85%
3
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10.1002/cctc.202001619
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air, D-glucose, NaBr
glucose oxidase
chloroperoxidase;
catalase, r.t., 4 h
rendering a hybrid material with a dual functionality as it retains
the enzyme's activity while adding catalytic properties of the metal
nanoparticle. Among the various applications of nanobiohybrids,
a lipase-palladium hybrid has been reported as excellent mediator
of Suzuki cross couplings.[14] Considering these results, we aimed
to apply this concept in order to perform the previously introduced
1)
R
Ph
OH
O
Ph
RB(OH)2, K3PO4
Na2PdCl4, SPhos
80 °C, 16 h
2)
1a
3
one-pot sequential process with
a tailored multifunctional
nanobiohybrid. From the two enzymes, chloroperoxidase and
glucose oxidase, the latter was chosen as support as GOx does
not depend on any metal cofactors and would therefore suffer less
interference from the added metal ions. To our delight, GOx
induced the formation of Pd nanoparticles smoothly and without
the necessity of any external reducing agents.[13] SEM analysis of
the thus obtained hybrid revealed that the formed precipitate
O
O
O
Ph
Ph
Ph
3a, 85%
3j, 84%
3k, 90%
constituted an aggregate with
a mesoporous amorphous
MeO
F3C
O
superstructure composed by palladium atoms dispersed into an
organic (GOx) matrix (Fig 1a). X-ray diffraction (XRD) analysis
confirmed the exclusive formation of Pd(0) species on the hybrid
(Fig. 1b), and the ICP-OES analysis determined a Pd content on
the hybrid of 40% (w/w). TEM analysis showed the formation of
very small spherical crystalline Pd nanoparticles in the range of
2 nm (Fig. 1c-d).
O
O
O
O
Ph
Ph
Ph
3l, 84% a
3m, 72%
3n, 83%
Cl
O
O
O
Ph
Ph
O
Ph
3p, 29% b
3q, 76% b
3o, 48%
Scheme 3. Structural variation of the boronic acid coupling partner. [a] 10 eq.
RB(OH)2. [b] 15 eq. RB(OH)2 (0.3 M in H2O) continually added via syringe pump
over 15 h.
In order to further increase the synthetic utility of the bio-
metal-coupled protocol in terms of introduced functionalities, a
series of alternative cross coupling approaches were tested as
secondary modules to complement the halocyclization. While
initial attempts to implement Stille- and Heck-type couplings have
so far remained unproductive, the substitution of the boronic acids
by phenylacetylene, under otherwise identical conditions, gave
rise to a successful halocyclization/Sonogashira sequence. The
corresponding alkynylated dihydrofuran 3r could thus be obtained
after the one-pot transformation in a high yield of 76% (Scheme
4).
Figure 1. Characterization of the GOx/Pd nanobiohybrid: a) SEM, b) XRD
pattern, c) TEM, d) Pd nanoparticles size distribution.
Ph
1) CPO, GOx, NaBr,
Glc, air, r.t., 4 h
The specifically tailored GOx/Pd nanobiohybrid consisting
of the oxidase matrix and the palladium nanoparticles was
subsequently evaluated for its catalytic performance in either of
the two individual reactions. Substituting the GOx from the original
halocyclization protocol by the nanobiohybrid in presence of CPO,
glucose and NaBr indeed led to consumption of allenol 1a and
formation of O-heterocyclic products in an encouraging overall
yield of 76%. In addition to the expected bromodihydrofuran 2a,
however, also minor amounts of the non-halogenated
5-membered heterocycle 9 were observed (Scheme 5a). Though
undesired in the context of the present study, this kind of allenol
cycloisomerization, more commonly attributed to homogeneous
coinage metal catalysis,[17] has been described for heterogeneous
nanoparticle catalysis previously,[18] and could represent an
interesting alternative application target in future investigations of
palladium-based nanobiohybrids. Even more gratifyingly, also the
2) Na2PdCl4, SPhos, K3PO4
phenylacetylene, 80 °C, 16 h
Ph
OH
O
Ph
aqueous emulsion
1a
3r, 76%
Scheme
4.
Chemoenzymatic
alkynylation
through
one-pot
halogenation/Sonogashira coupling.
An intrinsic challenge in chemoenzymatic strategies, where
proteins and transition metal species are combined, lies in the
potential mutual deactivation of the individual catalysts. As one
possible solution, recently, the concept of nanobiohybrids has
received significant attention. Herein, protein-based biocatalysts
act as support during the formation of metal nanoparticles,
4
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Caldariomyces fumago (CPO), 37 kU/mL, Merck KGaA; glucose oxidase
type II from Aspergillus niger (GOx), 19 kU/g, Merck KGaA; glucose
oxidase from Aspergillus niger (GOx), Novozymes; catalase from
Micrococcus lysodeikticus, 131 kU/mL, Merck KGaA. Synthesized
products were purified by column chromatography over silica gel
(Macherey-Nagel MN-Kieselgel 60, 40-60 µm, 240-400 mesh) or by
distillation. Reactions were monitored by thin layer chromatography (TLC)
Suzuki coupling between the vinyl bromide intermediate 2a and
3,5-xylyl boronic acid was catalyzed effectively by the same
enzyme-metal hybrid. The arylated dihydrofuran 3j was thus
obtained in 81% yield, underlining the great potential of the
nanobiohybrid concept as tool to streamline chemoenzymatic
approaches. So far, early attempts to implement the GOx/Pd
nanobiohydrid in a true one-pot conversion from allenols to
arylated dihydrofurans have exposed certain challenges related
to potential deactivation/poisoning effects, giving overall yields
within a 20-30% range. A detailed follow-up study on the
preparation and composition of the GOx/Pd assembly related to
a more robust hybrid catalyst is currently under way.
carried out on precoated silica gel plates (Merck, TLC Silica gel 60 F254
)
using UV light and KMnO4-solution for visualization. NMR, IR and HRMS
data of the synthesized compounds were recorded at the Department of
Chemistry of Aalto University. 1H and 13C NMR spectra were recorded with
a Bruker Avance NEO 400 (1H 400.13 MHz, 13C 100.62 MHz). The
chemical shifts are reported in parts per million (ppm) in relation to non-
deuterated chloroform signal (1H NMR: δ = 7.26; 13C NMR: δ = 77.16).
Quantitative 1H NMR were recorded using delay time D = 30 s and 30 °
pulse, and are reported in relation to dimethyl sulfone internal standard
signal (δ = 3.00, s, 6 H). Infrared spectra were recorded at room
temperature on a Bruker Alpha ECO-ATR FT-IR-Spectrometer, absorption
bands are reported in wave numbers [cm-1]. High resolution mass
spectrometry was performed on an Agilent 6530 (Q-TOF) mass
spectrometer. Enzyme-metal hybrids were synthesized and characterized
at the CSIC Institute of Catalysis and Petroleum Chemistry (ICP-CSIC).
Inductively coupled plasma optical emission spectroscopy (ICP-OES) and
X-ray diffraction (XRD) data was recorded by the technical staff at ICP-
CSIC. Transmission electron microscopy (TEM) was done using a JEOL
JEM-2100F field emission electron microscope. Scanning electron
microscopy (SEM) was done using a Hitachi TM-1000 tabletop microscope.
JASCO V-730 UV-visible spectrophotometer was used for enzymatic
activity assays. Lyophilization of the hybrids was done using a Telstar
LyoQuest laboratory freeze-dryer.
a
GOx/Pd nanobiohybrid
CPO, NaBr, Glc
Br
Ph
+
OH
aqueous emulsion
r.t.
O
O
Ph
Ph
1a
2a, 69%
9, 7%
b
GOx/Pd nanobiohybrid
3,5-(Me)2PhB(OH)2
Br
aqueous emulsion
80 °C
O
O
Ph
Ph
2a
3j, 81%
Representative procedure for the one-pot enzymatic halocyclization
and palladium-catalyzed cross coupling: 2,2,5-trimethyl-3,5-
diphenyl-2,5-dihydrofuran (3a). n-Heptane (5 mL) was added to a
solution of Brij 35 (123 mg, 100 µmol) in an aqueous citrate buffer (5 mL,
0.1 M, pH 4.5) to form an emulsion with vigorous stirring. Sodium bromide
(31 mg, 300 µmol), D-(+)-glucose monohydrate (39 mg, 200 µmol), allenic
alcohol 1a (18.5 mg, 100 µmol) and chloroperoxidase from Caldariomyces
fumago (8.0 µL, 37 U/µL) were all added to the emulsion. Lastly, glucose
oxidase from Aspergillus niger (2.6 mg, 19 U/mg) was added to start the
reaction. The mixture was stirred vigorously under air at room temperature
for 2.5 hours. Catalase from Micrococcus lysodeikticus (1.5 µL, 131 U/µL)
was added to decompose any excess of hydrogen peroxide, and the
mixture was stirred for 15 minutes. Tribasic potassium phosphate (209 mg,
1 mmol), phenylboronic acid (37 mg, 300 µmol), SPhos (6.2 mg, 15 µmol)
and lastly sodium tetrachloropalladate (1.5 mg, 5 µmol) were all added to
the emulsion. The mixture was heated to 80 °C and stirred vigorously for
3 hours. The reaction was allowed to cool down, and acetonitrile (6 mL)
was added to enable phase separation. The mixture was extracted with n-
pentane (4 x 20 mL). The combined organic layers were washed with brine,
dried over sodium sulfate and concentrated under reduced pressure.
Purification by flash chromatography (SiO2, n-pentane/diethyl ether 80:1)
provided 2,2,5-trimethyl-3,5-diphenyl-2,5-dihydrofuran 3a as a clear oil
(22.2 mg, 84 μmol, 85% yield). Rf (n-heptane/ethyl acetate 10:1) = 0.46.
1H NMR (400 MHz, CDCl3): δ [ppm] = 7.54-7.49 (m, 2H), 7.42-7.21 (m,
8H), 6.14 (s, 1H), 1.72 (s, 3H), 1.63 (s, 3H), 1.51 (s, 3H). 13C NMR (100
MHz, CDCl3): δ [ppm] = 147.5, 145.4, 134.3, 130.1, 128.5, 128.3, 127.8,
127.4, 126.7, 125.0, 88.8, 87.7, 31.0, 29.8, 28.8. FT-IR (neat, ATR): ν
[cm−1] = 3058 (w), 2973 (m), 1600 (w), 1494 (m), 1444 (m), 1363 (m), 1066
(m), 854 (m), 757 (s), 695 (s). HRMS (ESI): m/z [M+H]+ calcd. for C19H21O:
265.1587, found: 265.1589.
Scheme 5. Nanobiohybrid with combined oxidase and transition metal reactivity,
catalyzing both halocyclizations and Suzuki-type cross couplings.
Conclusion
Despite considerable inherent challenges to overcome, the
combination of chemo- and biocatalytic techniques in streamlined
one-pot processes is attracting attention following the emergence
of
powerful
enzyme
engineering
techniques.
The
chemoenzymatic sequential halocyclization and cross coupling
methods described in this work overcome the issues of
incompatibility by introducing colloidal biphasic reaction media,
allowing for the highly efficient conversion of allenic alcohols to
densely substituted 2,5-dihydrofurans. The robustness and
applicability of biocatalytic halogenations was perfectly
exemplified by combinations with both Suzuki and Sonogashira
couplings in homogeneous one-pot catalysis. Further, potential
for performing chemoenzymatic multicatalytic reactions under
heterogeneous catalysis conditions by taking advantage of the
recently introduced enzyme-metal nanoparticle biohybrid concept
opens interesting possibilities for advanced process development.
The GOx/Pd nanobiohybrid presented here was successfully
applied as catalyst for both individual reaction steps and will serve
as inspiration for further optimization in the area.
Experimental Section
Acknowledgements
General remarks: All reactions carried out in dry solvents were performed
under argon atmosphere using anhydrous conditions. Anhydrous solvents
were acquired from an MBraun MB-SPS800 drying system. Commercially
available reagents were used without further purification unless otherwise
noted. Enzymes were obtained from: chloroperoxidase from
We gratefully acknowledge financial support by the Academy of
Finland (grant numbers 298250 & 324976, JD), and the COST
action CA15106 (CHAOS). Furthermore, this work was supported
5
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10.1002/cctc.202001619
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FULL PAPER
Enzyme Microb. Technol. 2016, 95, 242-247; c) D. Lopez-Tejedor, B. de
las Rivas, J. M. Palomo, Molecules 2018, 23, 2358.
by the Spanish Government and the Spanish National Research
Council (CSIC) (grant no CSIC-PIE PI20184291), the Spanish
Government (Project AGL2017-84614-C2-2-R) and the Ministry
of Education, Youth and Sports of the Community of Madrid and
the European Social Fund (PEJD-2017PRE/SAL-3762). We
thank Dr. Martinez from Novozymes for the gift of GOx.
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6
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Entry for the Table of Contents
Chemoenzymatic Arylative Allene Cyclization
Ph
OH
+
Ph
B(OH)2
O
air, NaBr
glucose
Through a chemoenzymatic one-pot approach merging halogenation biocatalysis with palladium-mediated cross coupling, allenic
alcohols are converted to densely substituted O-heterocycles in an aqueous emulsified reaction medium. In addition to the modular
synthetic methodology based on homogeneous catalysts, a tailor-made enzyme-metal nanobiohybrid was developed to effectively
combine individual catalytic modules in a single heterogenous catalyst.
7
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