A Systems Approach to a One-Pot Electrochemical Wittig Olefination Avoiding the Use of Chemical Reductant or Sacrificial Electrode

Article abstract of DOI:10.1002/chem.202001654

An unprecedented one-pot fully electrochemically driven Wittig olefination reaction system without employing a chemical reductant or sacrificial electrode material to regenerate triphenylphosphine (TPP) from triphenylphosphine oxide (TPPO) and base-free in situ formation of Wittig ylides, is reported. Starting from TPPO, the initial step of the phosphoryl P=O bond activation proceeds through alkylation with RX (R=Me, Et; X=OSO₂CF₃ (OTf)), affording the corresponding [Ph₃POR]⁺X^? salts which undergo efficient electroreduction to TPP in the presence of a substoichiometric amount of the Sc(OTf)₃ Lewis acid on a Ag-electrode. Subsequent alkylation of TPP affords Ph₃PR⁺ which enables a facile and efficient electrochemical in situ formation of the corresponding Wittig ylide under base-free condition and their direct use for the olefination of various carbonyl compounds. The mechanism and, in particular, the intriguing role of Sc³⁺ as mediator in the TPPO electroreduction been uncovered by density functional theory calculations.

Full text of DOI:10.1002/chem.202001654

Accepted Article
Accepted Article
Title: A Systems Approach to a One-Pot Electrochemical Wittig
Olefination Avoiding the Use of Chemical Reductant or Sacrificial
Electrode
Authors: Matthias Driess, Biswarup Chakraborty, Arseni Kostenko, and
Prashanth W. Menezes
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To be cited as: Chem. Eur. J. 10.1002/chem.202001654
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10.1002/chem.202001654
Chemistry - A European Journal
FULL PAPER
A Systems Approach to a One-Pot Electrochemical Wittig Olefination
Avoiding the Use of Chemical Reductant or Sacrificial Electrode
Biswarup Chakraborty,a Arseni Kostenko,a Prashanth W. Menezes,a Matthias Driessa*
Abstract: An unprecedented one-pot fully electrochemically driven
Wittig olefination reaction system without employing a chemical
reductant or sacrificial electrode material to regenerate
triphenylphosphine (TPP) from triphenylphosphine oxide (TPPO) and
base-free in situ formation of Wittig ylides, is reported. Starting from
TPPO, the initial step of the phosphoryl P=O bond activation proceeds
through alkylation with RX (R = Me, Et; X = OSO2CF3 (OTf), affording
the corresponding [Ph3POR]+ X- salts which undergo efficient
electroreduction to TPP in the presence of a sub-stoichiometric amount
of the Sc(OTf)3 Lewis acid on a Ag-electrode. Subsequent alkylyation of
TPPO proceed via activation of the P=O bond with Lewis acids
(LA) such as Me3SiCl[11a, 11b], AlCl3[11a] during electrolysis and/or
formation of dichlorophophorane (Ph3PCl2).[11c] Recently, boron
esters, [B(OAr)3], have also been demonstrated to act as suitable
LAs to form a Ph3P=OB(OAr)3 adduct which facilitates the rate-
determining phosphoryl P=O bond dissociation to form TPP and
[{(ArO)3B}2O]2- diborate.[11e] However, its moderate overall
efficiency along with side-product formation and the difficulty to
recover the boron ester from the diborate limits its suitability for
TPP regeneration.[11e] An improvement in TPP regeneration has
lately been achieved under mild electrochemical conditions, in the
presence of AlCl3 as LA and tetramethylethylene diamine as an
additive.[11f] However, the intrinsic dissolution of a sacrificial Al
electrode via electro-corrosion and subsequent formation of Al2O3,
which has a high energy demand for recycling, limits this approach
for sustainable TPP electro-regeneration too.[11a, 11c, 11d, 11f]
TPP affords Ph3PR+ which enables
a facile and efficient
electrochemical in situ formation of the corresponding Wittig ylide
under base-free condition and their direct use for the olefination of
various carbonyl compounds. The mechanism and, in particular, the
intriguing role of Sc3+as mediator in the TPPO electroreduction been
uncovered by Density Functional Theory calculations.
The Wittig olefination reaction (WOR) is one of the most
common synthetic routes to produce functional alkenes.[1] The
perhaps most prominent case of WOR at industrial scale is the
production of vitamin A.[2] The latter affords a stoichiometric
amount of triphenylphophine oxide (TPPO) resulting as a by-
product in several tons per year during the synthesis (Scheme
1a). Henceforth, reduction of organophosphine oxides to the
corresponding phosphine (e.g., Ph3P, TPP) is not only of
fundamental but also of utmost industrial interest.[3] In addition,
the complete removal of TPPO from reaction mixtures is often not
straightforward.[4] Until now, deoxygenation to regenerate TPP
can nearly exclusively be achieved using chemical reductant e.g.,
silanes,[3b, 5] boranes[3a, 6] and aluminum (hydrides)[7] and therefore,
the pursuit of other reducing agents[8] or regeneration processes has
gained attention in the last few years (Scheme 1a).[3a, 3b] Because
regenerating TPP with expensive reduction agents is not a viable
solution, alternative approaches have been tested.[9] Among them,
electrochemical reduction of TPPO to TPP is a highly attractive
aim, but remains cumbersome.[10] Up to now, electroreduction of
TPPO to TPP suffers from shortcomings that prevent from its
recycling on a commercially feasible scale.[11]
Previous attempts towards selective electroreductive
deoxygenation of TPPO remained unsatisfacory due to undesired
C-P bond dissociation to Ph2P(O)H and benzene.[10] The most
promising approaches towards successful TPP regeneration from
Scheme 1. (a) Chemical route of Wittig olefination reations (WOR) followed by
chemical recylcing of TPPO to TPP with phosgene (COCl2) and Al0 at 130 0C
applied by BASF SE for vitamin A production. (b) One-pot electrochemical
WOR protocol via cathodic recycling of TPPO without sacrificial electrode and
subsequent Wittig yilde regeneration and carbonyl olefination.
[a]
Dr. B. Chakraborty, Dr. A. Kostenko, Dr. P. W. Menezes, and
Prof. Dr. M. Driess,
Department of Chemistry: Metalorganics and Inorganic
Materials, Technische Universität Berlin,
On the other hand, and to the best of our knowledge, the direct use
of electro-regenerated TPP and base-free electrochemical
Ph3P=CR2 ylide formation and subsequent WOR in the presence
Straße des 17 Juni 135, Sekr. C2, 10623 Berlin, Germany
E-mail: matthias.driess@tu-berlin.de
Supporting information for this article is available on the WWW
under http://
1
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10.1002/chem.202001654
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of a carbonyl compound in a one-pot protocol is currently
unknown.
and irreversible reduction of
contrast, has not been previously achieved with TPPO
1
(Fig. 1b, red curve) which, in
LA (LA:
Herein, we describe a systems approach to a one-pot fully
electrochemically driven protocol for direct carbonyl olefination
via WOR, combining the required subset of stoichiometric
reactions. This includes the recycling of TPPO to TPP in excellent
yields omitting any chemical reductant and subsequent in situ
electrosynthesis of Wittig ylides in the absence of a base (Scheme
1b).
B(OAr)3 and AlCl3) adducts, apparently due to weaker association
11f]
of TPPO and LA.[11e,
However, the estimated free energy
change ( G) determined by Density Functional Theory (DFT)
∆
calculations for the one-electron-reduction of [Ph3P(OMe)]+ (Fig.
S5) is about 63.7 kcal mol-1 preferable in comparison to TPPO,
while that of the TPPOAlCl3 adduct is stabilized by only 9-10
kcal mol-1, as reported recently.[11f] Additional four different
phosphine oxides (R3PO) were selected bearing phenyl and/or
alkyl substituents R attached to the P center and using the same
methodology of phosphoryl P=O bond via alkylation,
corresponding methoxy- and/or ethoxy-phosphonium salts
[R3P(OR)](OTf) were synthesized and isolated (see Supporting
Starting point is the phosphoryl P=O bond activation of TPPO
via
O
-alkylation to form the isolable [Ph3P(OR)](OTf)
); Et ( )), using ROTf (OTf =
phosphoium salts (R = Me (
1
2
OSO2CF3) as sources of R+. Structural (XRD), spectroscopic (31P
NMR, IR) and cyclovoltammometric evidences in support of the
P=O bond activation via alkylation is also provided. In the
presence of a sub-stoichiometric amounts of Sc(OTf)3 acting as a
very effective redox-innocent Lewis-acidic mediator, the
electrochemical conversion of [Ph3P(OR)]+ to TPP could be
achieved in 78% (R = Me) and almost quantitative yields for R =
Et, respectively, along with ROH alcohol formation. The
mechanism and striking role of Sc3+ have been rationalized by
means of Density Functional Calculations (DFT) (see below).
Information; Figs. S6-S17). In all cases and the shifts of their 31
P
NMR signals and the redox potentials indicate a similar degree of
P=O activation as observed for TPPO (Figs. S6-S17, Table S3).
The one-pot electrochemical WOR was perfomed in a
customized two-compartment cell equipped with Pt foil as a
counter electrode and Ag foil as a working electrode (Pt(+)//
(-)Ag, geometric surface area 0.5 x 0.5 cm) separated by glass frit
(G4 porosity) (Fig. S18 in Supporting Information) to avoid further
oxidation (at the counter electrode) using 1(0.032 M) at a constant
current of 4.5 mA (for 2 h, 32.4 C charge passed) followed by
addition of MeOTf (0.048 M) and benzaldehyde (PhCHO, 0.048
M) and subsequent electrolysis of the mixture for additional 40
mins under similar electrochemical condition (entry 1 in Table 1).
In the latter case, only a trace amount (<10%, entry 1 in Table 1)
of the desired olefin (styrene, PhCH=CH2) was produced (Fig.
S19a), presumably due to the moderate TPP regeneration from
1
(about 39 % yields; Fig. S19b) before subsequent addition of
MeOTf and PhCHO. DFT calculation of the free energy change
associated with the electro-reduction of
pathway (Scheme S1) and predicts that a sequential two step
electron uptake by the methoxyphophonium cation in to the
corresponding anion via formation of the neutral mono-radical is
energetically favorable. The later steps consist of two parallel
pathways, P-O and C-O bond dissociation, with comparable free
energy changes (∆
G), -50.3 and -45.4 kcal mol-1, respectively
1 suggests a plausible
Figure 1 Activation of the P=O bond in TPPO via methylation with MeOTf to
form 1. (a) Downfield shift of 37.9 ppm of the 31P NMR signal of TPO (bottom)
vs. 1 (top). (b) Anodic shift (870 mv) of the first electron reduction of 1 with
respect to TPO (electrochemical condition; 0.15 M TBAPF6 in CH3CN with a 0.1
Vs-1 scan rate, glassy carbon (GC) as working, Pt as a counter, Ag as a pseudo
reference electrode). (b inset) Molecular structure of the cation in 1 determined
by a single-crystal X-ray diffraction analysis; hydrogen atoms of the phenyl rings
are ommited for clarity.
1
(Scheme S1 in Supporting Information). Most likely, the low
selectivity towards TPP formation is presumably due to
competitive P-O and C-O bond dissociation. Notably, alkene
formation could not be achieved under an identical
electrochemical condition only with TPPO which further
highlights the significance of P=O bond activation via alkylation
Methyl triflate (MeOTf) was used as a facile source of Me+ to
activate the P=O bond of TPPO, resulting in almost quantitative
isolation of the methoxy-phosphonium triflate, [Ph3P(OMe)](OTf)
1
(Figs. S1-S4 in Supporting Information). The weakening of the
P=O bond via methylation is evident by the large downfield shift
in the 31P NMR signal of
vs. TPPO (∆δ = 37.9 ppm) (Fig. 1a),
(Fig. S20). To increase the TPP regeneration from 1, Sc(OTf)3 was
employed as a redox-innocent Lewis acid and MeO- acceptor to
accelarate the rate-determining P-O dissociation step. Sc(OTf)3
has already been used in other LA-mediated organic
transformations[13] and for the stabilization of high-valent oxo-
transition-metal intermediates.[14] In fact, addition of a sub-
1
which is further supported by FTIR spectroscopy with a
bathochromic shift of the ν(PO) stretching vibration mode at 1191
cm-1 for TPPO (Fig. S4). Accordingly, the P-O distance obtained
from an X-ray crystal structure alnalysis of
S1, S2) is about 0.08(1) Å longer than that in TPPO.[12] The
activation of the P=O bond in TPPO via formation of further
1 (Fig. 1b inset; Tabs.
stoichiometric amount of Sc(OTf)3 to solutions of
1 and
subsequent electrolysis under identical conditions (entry 2 in Table
1, in presence of MeOTf and PhCHO) resulted in a drastic increase
in styrene formation to 63% yields (Figs. S21 and S22). The
stoichiometry of Sc3+ (0.6 molar equivalents) was optimized by
1
leads to a drastic decrease of the redox potential as evident from
the cyclic voltammograms recorded acetonitrile solutions with
0.15 M tetrabutylammonium hexafluorophosphate (TBAPF6). A
means of the highest yield of TPP electro-regeration from
S23-S26).
1
(Figs.
pseudo-reversible one-electron-reduction of TPPO was observed
11e, 11f]
at 2.92 V (vs. Fc/Fc+)[10d,
(Fig. 1b, black curve). The
methylation of P=O to P-OMe+ causes an anodic shift of 870 mV
2
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The efficiency of WOR was further tested with four different
potential working electrode materials: Cu, Mg foils, Ni foam (NF,
0.5x0.5 cm, three-dimensional) and glassy carbon rod (GC, 6 mm
diameter). Under similar experimental conditions, GC carbon rod
(entry 3 in Table 1) delivers a moderate yield (41%) of styrene
(Fig. S27 and Fig. S28) while Cu, Mg foils and Ni foam remain
ineffective in one-pot WOR due to ineffective TPP formation and
unwanted TPPO regeneration by C-OPPh3 bond dissociation (Figs.
S29-S31 and Table S4). The highest efficiency on a Ag electrode
surface could be ascribed to its higher conductivity which plays an
important role in this electrochemical system.[15] Other transition-
metal ions (Fe3+, Ni2+, Yb3+, Zn2+) were also probed as potential
LAs for electrochemical WOR. In the presence of Yb3+, a
conversion of ca. 33% styrene was obtained (entry 4 in Table 1,
Fig. S32-S33 in Supporting Information). Unfavorably, the desired
electrchemical WOR could not occur in the presence of Fe3+, Ni2+,
Zn2+ salts most likely due to the lower reduction potentials of Fe3+
(EFe3+/Fe; -0.04 V), Ni2+ (ENi2+/Ni; -0.26 V) and Zn2+ (EZn2+/Zn; -0.76
V)[16] in comparison with
1 (one-electron-reduction at 2.04 V vs
Fc/Fc+) (Figs. S34-S35). Thus, the high reduction potential of Yb3+
(EYb3+/Yb -2.37 V) and Sc3+ (ESc3+/Sc -2.03 V)[16] makes them
suitable LAs for electrochemical reduction of
1 to TPP and its
disposal for a one-pot WOR. In contrast, the addition of B(OPh)3
as LA does not result in olefin formation due to relatively low TPP
regeneration (ca. 20%) from
conditions (Fig. S36).
1
under the applied experimental
The displacement of Ph groups in
1
by alkyl substituents has
also a significant influence on the WOR efficiency: using
[Ph2MeP(OMe)](OTf), MeOTf and PhCHO lowers the yields of
styrene to 31% (entry 5 in Table 1, Fig. S37-S38), while monoethyl
substitution on the P atom ([Ph2EtP(OMe)](OTf)) leds merely to
unwanted (P)O-C bond scission and formation of Ph2EtP=O (entry
Table 1. Screening of reaction conditions; (a) one-pot reaction scheme for electrochemical WOR using [Ph2RP(OR1)]+ (R= Ph, Me, Et and R1
= Me, Et) in presence of sub-stoichiometric amounts of Lewis acid (LA), (b) different [Ph2RP(OR1)]+ (c) alkyl electrophiles RX, (d) carbonyl
compounds, and (e) olefins prodced by electrochemical WOR.
Alkyl
electrophilee
MeOTf
MeOTf
MeOTf
MeOTf
MeOTf
MeOTf
EtOTf
PhCH2Br
MeOTf
MeOTf
MeOTf
Carbonyl
compoundf
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
CyCHO
FuCHO
Ph2CO
Entrya
WEb
LAc
[Ph2RP(OR1)]+d
Olefin
Yield
1
2
3
4
5
6
7
8
9
Ag
Agg
GCh
Ag
Ag
Ag
Ag
Ag
Ag
Ag
Ag
-
[Ph3P(OMe)]+
[Ph3P(OMe)]+
[Ph3P(OMe)]+
[Ph3P(OMe)]+
[Ph2MeP(OMe)]+
[Ph2EtP(OMe)]+j
[Ph3P(OEt)]+
[Ph3P(OMe)]+
[Ph3P(OMe)]+
[Ph3P(OMe)]+
[Ph3P(OMe)]+
PhCH=CH2
PhCH=CH2
PhCH=CH2
PhCH=CH2
PhCH=CH2
PhCH=CH2
PhCH=CHCH3
PhCH=CHPh
CyCH=CH2
FuCH=CH2
Ph2C=CH2
<10%
63%
41%
33%
31%
-
67%
43%
57%
46%
22%
Sc3+
Sc3+
Yb3+i
Sc3+
Sc3+
Sc3+
Sc3+
Sc3+
Sc3+
Sc3+
10
11
aReaction conditions: all the experiments were conducted under N2 atmosphere (glove box), 0.175 M TBAPF6 as supporting electrolyte, dried
CH3CN prior to use as solvent (total volume 3 mL), electrolysis in separated (by glass frit) two electrode cell setup at a constant current
(chronopotentiometry; CP at 4.5 mA) using a Pt foil as counter electrode (0.5 x 0.5 cm working area). bworking electrode (WE) (0.5 x 0.5 cm
d
working area). c highest yield of TPP and WOR was achieved with 0.02 M (0.6 equiv.) LA. electrolysis (2h) was conducted with 0.032 M
[Ph2RP(OR1)]+ in the presence of 0.6 equiv. LA and formation of TPP was confirmed analyzing the solution by 31P(1H) NMR. e0.048 M alkyl
halide was added to the working compartment after 2h of electrolysis and stirred for additional 30 minutes to prepare the phosphonium salt in
situ. f0.048 M aldehyde and/or ketone added further to the working compartment. gAg foil 0.1 mm thick connected to with copper wire and
copper tape (under an identical condition Cu foil, Mg foil and Ni foam didn't produce alkene), hglassy carbon rod (GC; 6 mm diameter). i0.02
j
M Yb(OTf)3; other LA (Fe3+, Ni2+, Zn2+ and B(OPh)3) remain ineffective for one-pot WOR. [R3POMe](OTf) (R = n-Bu and n-oct) didn’t
produce any olefin due to poor TPP regeneration (Figs. S40-41)
3
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10.1002/chem.202001654
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6 in, Table 1, and Figs. S39-S41). Notably, due to the higher
reduction potentials of fully alkyl-substituted phosphine oxides
CV (Fig. S51). This control experiment proves that both alkoxy-
phosphonium salt and Sc3+ are the two key components for
successful one-pot electrochemical WOR.
In the earlier reports of electroreduction of TPPO to TPP, the
LAs such as B(OAr)3, AlCl3 and Me3SiCl act as sinks to trap O2-,
producing the very stable [{(OAr)3B}2O]2- diborate, [11e] Al2O3,[11f]
and (Me3Si)2O disiloxane,[11b] respectively. In contrast, the electro-
(
n
Bu3PO,
remain unfavorable under the here applied conditions.[8b,
Although DFT calculations performed with [
Bu3P(OMe)]+
nOct3PO), electro-chemical and/or chemical reduction
11f]
n
revealed that the one-electron- reduction of the phosphonium
center is energetically favorable, no minima was obtained in the
potential energy surface for the two-electron-reduction to
(Scheme S2). Conversely, the computed G value for the C-O
bond dissociation of [
Bu3P(OMe)]+ is energetically favorable and
as a consequence, Bu3PO and Oct3PO were solely obtained
during electroreduction of the respective methoxyphosphonium
salts (Scheme S2) and, henceforth, alkyl-substituted methoxy
phosphnium salts are unsuitable for electrochemical WOR.
Starting from TPPO, the electrochemical WOR can also be
achieved with other alkoxy-phosphonium salts, [Ph3P(OR)]+ (R =
Et) and using various carbonyl compounds (entry 7-11 in Table 1).
n
Bu3P
reductive P-O bond cleavage of
1
and
2
furnishes the respective
in
∆
alcohols CH3OH (from in 68 % yields) and C2H5OH (from 2
1
n
75% yields), respectively (see Figs. S52-S54). Core level X-ray
photoelectron spectroscopic (XPS) analyses of crude solids
isolated from concentrated electrolyzed solutions were performed
to determine the valence state of scandium after electrolysis. The
binding energies obtained for Sc 3p1/2 (407.8 eV) in these solids
were identical to that of fresh Sc(OTf)3 (Fig. S55), proving that Sc
remains in the same oxidation state (+3) after the reaction.[17]
Although, TPPO can coordinate Sc3+ and forms a stable isolable
n
n
Accordingly,
electrolysis
of
solutions
containing
complex,[18] 31P(1H) NMR and ESI-mass spectra of a mixture of
1
[Ph3P(OEt)](OTf) (
2
), EtOTf and PhCHO affords prop-1-enyl-
and Sc(OTf)3 indicated the presence of ‘free’
S56-S57).
How does the electroreduction of the cation in
occur and what is the role of Sc3+ in this process? We propose a
plausible mechanistic pathway based on results of DFT
calculations as depicted in Figure 2. To explain the reduction of [
- OTf]+ in the presence of Sc3+, geometry optimizations were
carried out at the B3LYP[19a, 19b-d]-D3[20] level of theory with
Stuttgart RSC 1997 valence basis set and effective core potential[21]
for Sc and 6-31G(d,p) basis set[22] for all other atoms. To account
for the solvent effect of acetonitrile, single point calculations of the
1
in solutions (Figs.
benzene in 67% yields (PhCH=CHCH3, entry 7 in Table 1, inset c;
Figs. S42-S44). Very similarly, about 43 % stilbene (entry 8 in
Table 1, Figs. S45-S47), 57% vinylcylohexane (CyCH=CH2, entry
9 in Table 1, Fig. S48), 46% 2-vinalyfuran (FuCH=CH2 entry 10
in Table 1, Fig. S49) and 22% 1,1-diphenylethene (Ph2C=CH2,
entry 10 in Table 1, Fig. S50) could be prepared. However,
electrochemical WOR does not occur with unactivated TPPO in
the presence of Sc3+ although a week interaction between ‘free’
TPPO and Sc3+ is suggested by 31P(1H) NMR and also observed for
the one-electron redox potential of TPPO in the presence of Sc3+ in
1
, [1 - OTf]+
,
1
Figure 2. Calculated mechanism of electroreduction of 1 to triphenylphosphine (TPP) mediated by Sc3+. The values in eV are
shown in parentheses.
4
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10.1002/chem.202001654
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optimized structures were performed using Polarizable Continuum
Model (PCM). The calculations revealed that coordination of Sc3+
with [1 - OTf]+ is energetically disfavoured, while ligation with
electrode and Pt as a supporting electrode does not show anodic
corrosion and/or electrode dissolution, representing a highly
sustainable one-pot approach for WOR. Furthermore, the
electroreduction of [Ph3P(OR)]+ to TPP and subsequent olefination
reaction via in situ electrochemical Wittig reagent formation could
be realized in a one-pot protocol with extended scope of substrates.
The strategy presented herein could pave the way to bulk scale
electrochemical WOR synthesis of more functionalized alkenes.
acetonitrile takes place rather easy to form complex
A. The latter
can release one of the coordinating acetonitrile ligands to form the
pentacoordinated Sc complex A′ but the reaction is slightly
endergonic by 4.1 kcal mol-1 (Fig. 2a). Notably, the one-electron-
reduction product of [1 - OTf]+, that is, the corresponding radical
[
1 - OTf]., has a highly exergonic coordination affinity towards A’
with 57.9 kcal mol-1, affording the Sc-arene
-complex (Fig. 2b);
in fact, related scandium-arene complexes have previously been
reported.[23] O- and -cordination of [1 - OTf]· to Sc could lead to
the isomer B′ and B″, respectively (Fig. 2b), but they are less
stable than . Upon formation of , the methoxy group on
π
B
Acknowledgements: Funded by the Funded by the Deutsche
Forschungsgemeinschaft
(DFG,
German
Research
P
Foundation) under Germany´s Excellence Strategy – EXC
2008 – 390540038 – UniSysCat. Authors thank Dr. Xiaohoi
Deng for his help in performaing some preliminary
experiments.
B
B
phosphorus can be transferred to the electrophilic CN carbon atom
of the ligated acetonitrile via the transition-state TS(B-C) to give
the methoxy-acetimidate-substituted Sc complex
undergo a one-electron-reduction to form the anionic complex
at ΔG = -159.0 kcal mol-1 (Fig. 2c).
releases a ‘free’ phosphine
via TS(D-E) forming the complex
at ΔG = -170.4 kcal mol-1 in
C; the latter can
Conflict of interest: The authors declare no conflict of interest.
D
D
E
Keywords: Phosphine oxide Reduction; Scandium Complexes;
Lewis Acid; Bond Activation; Electrosynthesis
which the methoxy-acetimidate oxygen atom coordinates to the Sc
center. Cleavage of the NC-OMe bond of the methyl-acetimidate
group via TS(E-F) results in formation of the methoxy-substituted
[1] a) G. Wittig, G. Geissler, Liebigs Ann. Chem. 1953, 580, 44-57; b)
G. Witttig, U. Schöllkopf, Chem. Ber. 1954, 87, 1318-1330; c) G.
Wittig, H. Pommer, Patent DBP954247, Germany, 1956; d) B. E.
Maryanoff, A. B. Reitz, Chem. Rev. 1989, 89, 863-927; e) D. G.
Gilheany, Chem. Rev. 1994, 94, 1339-1374; f) P. A. Byrne, D. G.
Gilheany, Chem. Soc. Rev. 2013, 42, 6670-6696; g) D. Hermeling, P.
Bassler, P. Hammes, R. Hugo, P. Lechtken, H. Siegel, Patent 5527966,
scandium complex
result in formation of methanol (Fig. 2d, as observed
experimentally, see also Fig. S52) along with regeneration of
After successful regeneration of TPP from and , addition of
F F can
at -187.4 kcal mol-1. Protonation of
A.
1
2
alkyl electrophiles RX (R = Me, Et, benzyl; X = OTf, Br) resulted
in the almost quantitative formation of the corresponding
phosphonium salts, [TPP-R](X), as evidenced by their downfield
δ
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10.1002/chem.202001654
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FULL PAPER
Fully electrified: For the first time, a
electrochemical system for a one-pot
Wittig olefination reaction (WOR) is
Biswarup Chakraborty, Arseni
Kostenko, Prashanth W. Menezes, and
Matthias Driess*
reported, which includes
a
very
Page No. – Page No.
efficient recyling of triphenylphosphine
from triphenylphosphine oxide waste
and subsequent carbonyl olefinations
via in situ base-free Wittig ylide
A One-Pot Electrochemical Strategy
to Wittig Olefinations Avoiding the
Use of Chemical Reductant or
Sacrifical Electrode
formation,
avoiding
chemical
reductants or sacrificial electrodes.
7
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