&
Electrosynthe sis
AS yste ms Approa ch to aO ne-Pot Electrochemica lW ittig
Olefination Av oiding the Use of Chemical Reductan to rS acrificial
Electrode
Biswar up Cha krabo rt y, Arseni Kostenko, Prashanth W. Menezes ,a nd Matth ia sD riess* [a]
Abstract : An unprecedente do ne-pot fully electr ochemically
driven Wittig olefina tion reaction system witho ut employing
ac hemical reductant or sacri ficial electrode materia lt or e-
generate triphenylp ho sphine (TPP) from triphenylphosphine
oxide (TPPO) and base-fr ee in situ formation of Wittig ylides,
is reported. Startin gf rom TPPO, the initial step of the phos-
phoryl P = Ob ond activation proceeds through alkylation
with RX (R = Me, Et ;X = OSO 2 CF 3 (OTf)), affording the corre-
sponding [Ph 3 POR] + X @ salts which undergo efficient electro-
reduction to TPP in the presence of as ubstoichiometric
amount of the Sc(OTf) 3 Lewi sa cid on aA g-ele ctrode .S ubse-
quen ta lkylation of TPP affords Ph 3 PR + which enables a
facile and efficient electrochemi cal in situ formatio n of the
correspondin gW ittig ylide under base-free conditio na nd
their direct use fo rt he olefination of vario us carbo ny lc om-
pounds. Th em echanism and, in particula r, the intriguing
role of Sc 3 + as mediator in the TPPO electroredu ction been
unco ve red by densit yf unctiona lt heory calculations.
Introduction
The Wittig olefination reaction (WOR) is one of the most
common synthetic ro utes to produce functional alkenes. [1] The
perhaps most prominen tc ase of WOR at industrial scale is the
production of vitamin A. [2] The latter affords as toichi ometric
amount of triphenylp hophine oxide (T PPO) resulting as ab y-
product in severa lt ons per year during the synthesis
(Scheme 1a ). Henceforth, reduction of organophosphi ne
oxides to the co rresponding phosphine (e.g. ,P h
3 P, TPP) is not
only of funda mental but also of utmost industrial interest. [3] In
addition, the complete remova lo fT PPO from reactio nm ix-
tures is often not straig ht forward. [4] Until now ,d eoxygena tion
to regenerate TPP can nearly exclus ively be achieved using
chemica lr eductant, for examp le, silanes, [3b, 5] boranes [3a, 6] and
aluminiu m( hydrides) [7] and therefore, the pursuit of other re-
ducing agents [8] or regenera tion processes has ga ined atten-
tion in the last few year s( Scheme 1a ). [3a, b] Considering that re-
generatin gT PP with exp ensive reduction agen ts is not av iable
solution ,a lterna tive approaches have been tested. [9] Among
them, electrochemical reduction of TPPO to TPP is ah ighly at-
tractive aim, but rem ains cumbersome. [10] Up to now ,e lectrore-
ducti o no fT PPO to TPP suffe rs from shortcomings that prevent
from its recycling on ac ommerciall yf easible scale. [1 1]
Schem e1 . (a) Chemical route of Wittig olefination reaction s( WOR )f ollowed
by chemical recyc ling of TPP Ot oT PP with phos gene (COCl 2 )a nd Al 8 at
130 8 Ca pplied by BASF SE for vitamin Ap roduction .( b) One -p ot electro-
chemical WO Rp rotoc ol via cathodic recyclin go fT PPO with ou ts acri ficial
electr od ea nd subsequent Witti gy lide regeneration and carbonyl olefin a-
tion.
[a] Dr .B .C hakra borty ,D r. A. Kostenk o, Dr .P .W .M eneze s, Prof. Dr .M .D riess
Department of Chem istry :M etalorga nics and Inorganic Materials
Te chnis che Universit - tB erlin, Straße des 17 Juni 135, Sekr .C 2
10623 Berlin (Germany )
E-mail :m atthias.driess@tu -berlin.de
Supporting informa tion and the ORCID identifica tion nu mber(s) for the
author(s) of this articl ec an be found under :
https ://doi.org/10.1002/chem. 202001654.
T 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of Creative Commons Attrib ution NonComme rc ial-
NoDerivs Lice nse, which permits use and distri bution in any medium, pro-
vided the origina lw ork is properly cited, the use is non-commercial and no
modifications or adap tations are made.
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Previou sa ttempts towards selectiv ee lectroreduct ive deoxy-
genatio no fT PPO remained unsatisfa ctory due to undesired
C @ Pb ond dissociation to Ph 2 P(O)H and benzene. [10] The most
promising approaches toward ss uccessful TPP regeneration
from TPPO proceed through activation of the P = Ob ond with
Lewis acids (LA) such as Me 3 SiCl, [1 1a, b] AlCl 3 [1 1a] durin ge
lectr oly-
sis and/or formation of dichlorophophora ne (Ph 3 PCl 2 ). [1 1c] Re-
cently ,b oron esters, [B(OAr) 3 ], have also been demonstrated to
act as suitable LAs to form aP h
3 P = O ! B(OA r) 3 adduct whic h
facilitates the rate-determinin gp hosphoryl P = Ob ond dissocia-
tion to form TPP and [{(ArO) 3 B} 2 O] 2 @ diborate. [1 1e] However ,i ts
moderate overall efficiency along with side-p roduct formation
and the difficulty to recover the boron ester from the dibora te
limits its suitability for TPP regeneration. [1 1e] An improvement
in TPP regenerati on has lately been achieved under mild elec-
trochemical condit ions, in the presence of AlCl 3 as LA and tet-
ramethylethylene diami ne as an additive. [1 1f ] However ,t he in-
trinsic dissolution of as acrificial Al electr ode by electro-corro-
sion and subsequent formatio no fA l
2 O 3 ,w hich ha sa high
energy demand for recycling ,l imits this approac hf or sustain-
able TPP electro-regeneration too. [1 1a, c, d, f]
On the other hand, and to the best of our knowledge ,t he
direct use of electr o-regenerated TPP and base-free electro-
chemica lP h
3 P = CR 2 ylide formation and subsequen tW OR in
the presence of ac arbonyl compound in ao ne-po tp rotocol is
currentl yu nknown.
Herein, we describe as ystems approac ht oa one-pot fu lly
electrochemically driven protoco lf or direct carbonyl olefina-
tion via WOR, combining the required subset of stoichiometric
reactions. This includes the recycling of TPPO to TPP in excel-
lent yields omitting any chemical reduct ant and subsequen ti n
situ electrosyn th esis of Wittig ylides in the absence of ab ase
(Scheme 1b ).
Results and Discussion
The starting poin ti st he phosphoryl P = Ob ond activation of
TPPO via O-alkyl ation to form the isolable [Ph 3 P(OR)](OTf)
phosphonium salts (R = Me ( 1 ); Et ( 2 )), using ROTf (OTf =
OSO 2 CF 3 )a ss ources of R + .S tructural (XRD), spectros copic
( 31 PN MR, IR) and cyclic voltamm etry ev idences in support of
the P = Ob ond activation via alkyl ation is also provided .I nt he
presenc eo fa substoichiometric amoun ts of Sc(OTf) 3 acting as
av ery effective redox-innocent Lewis -acid mediator ,t he elec-
trochemical conversion of [Ph 3 P(OR)] + to TPP could be achiev-
ed in 78 %( R = Me) and almos tq uantitative yield for R = Et, re-
spectively ,a long with alcohol (ROH) format ion. The mechanism
and striking role of Sc 3 + have been rationalized by mean so f
density functional calcula tions (DFT) (see below).
Methyl triflate (MeOTf) wa su sed as af acile source of Me + to
activate the P = Ob ond of TPPO, resulting in almost qu antita-
tive isolation of the methoxy-phosphonium triflate,
[Ph 3 P(OMe)](OTf) 1 (Figures S1–S4 in Supportin gI nforma tio n).
The weake ni ng of the P = Ob ond via methyla tion is evident by
the large downfield shift in the 31 PN MR signa lo f 1 vs. TPPO
( D d = 37.9 ppm) (Figure 1a ), which is furthe rs upported by
FTIR spectroscopy with ab athochromic shift of the n (PO)
stretching vibrati on mode at 11 91 cm @ 1 for TPPO (Figure S4).
Accordingly ,t he P @ Od istan ce obtained from the X-ray crystal
structure an alysis of 1 (Figure 1b inset ;T ables S1 and S2) is
about 0.08(1) a longer than that in TPPO. [12] The activation of
the P = Ob ond in TPPO via formation of 1 furt her leads to a
drastic decrease of the redox potentia la se vident from the
cyclic voltamm ograms recorded in aceton itrile solutions with
0.15 m tetrabut ylammoniu mh exafluorophosphate (TBA PF 6 ). A
pseudo-reversible one-elec tron-reduction of TPPO was ob-
served at 2.92 V( vs. Fc/Fc + ) [10d, 11 e, f] (Figure 1b ,b lack curve ).
The methylation of P = Ot oP @ OMe + causes an anodic shif to f
870 mV and irrever sible reduction of 1 (Figure 1b ,r ed curve )
which, in contras t, has not been previously achieved with
TPPO ! LA (LA :B (OAr) 3 and AlCl 3 )a dducts, apparently due to
weaker association of TPPO an dL A. [1 1e, f] Howe ve r, the estimat-
ed free energy change ( D G )d etermined by DFT calculations for
the one-electron-reducti on of [Ph 3 P(OMe)] + (Figure S5) is
about 63.7 kcal mol @ 1 preferable in comparison to TPPO, while
that of the TPPO ! AlCl 3 adduct is stabilized by only 9–10 kcal
mol @ 1 ,a sr eported recently . [1 1f ] Additional four different phos-
phine oxide s( R
3 PO) were selected bear ing phenyl and/o ra lkyl
substituents Ra ttached to the Pc ente ra nd using the same
methodology of phosphoryl P = Ob ond via alkyla ti on, corre-
spond in gm ethoxy- and/or ethoxy-phosphon ium salts
[R 3 P(OR)](OTf )w ere synthesized and isolated (see Suppo rting
Inform at ion, Figures S6–S17). In all cases and the shifts of their
31 PN MR signals and the redox potentia ls indicat ea similar
degree of P = Oa ctivation as observe df or TPPO (Figures S6–
S17, Ta bl eS 3).
The one-pot electr ochemical WOR was perform ed in ac us-
tomized two-compartmen tc ell equipped with Pt fo il as a
counter electrode and Ag foil as aw orking electrode (Pt( + + )//
( @ )Ag, geometri cs urfac ea rea 0.5 V 0.5 cm) separated by glass
frit (G4 porosity) (Figur eS 18 in Suppo rting Inform at ion) to
avoid furthe ro xidat ion (at the counter electrode) using 1
(0.032 m )a ta constant current of 4.5 mA (for 2h ,3 2.4 C
charge passed) followed by addition of MeOTf (0.048 m )a nd
benzaldehyde (PhCHO, 0.048 m )a nd subsequent electrolysis of
the mixture for additional 40 mins under similar electr ochemi-
Figure 1. Activation of the P = Ob ond in TPPO via methy lation with MeOTf
to form 1 .( a) Downfield shift of 37.9 ppm of the 31 PN MR signal of TPO
(bottom) vs. 1 (top). (b) An odic shift (870 mv) of the first electro nr educ ti on
of 1 with respect to TPO (electrochemical condition ;0 .15 m TBAPF 6 in CH 3 CN
with a0 .1 Vs @ 1 scan rate, glassy carb on (GC) as working, Pt as ac oun te r, Ag
as ap seudo reference electro de ). (b, inset) Molecula rs tructure of the cation
in 1 determine db ya single-crystal X-ray diffraction analysis ;h ydrogen
atoms of the phenyl rings are omitted for cl arity .
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cal conditio n( entry 1i nT able 1). In the latter case, only at race
amount ( < 10 %, entry 1i nT able 1) of the de sired olefin (sty-
rene, PhCH = CH 2 )w as produced (Fi gure S19a) ,p resum ably due
to the moderate TPP regenera tion from 1 (about 39 %y ields ;
Figure S19b) befor es ubsequent addition of MeOT fa nd
PhCHO. DFT calculation of the free ener gy change ass ociated
with the electr o-reduction of 1 suggests ap lausible pathway
(Scheme S1) and predicts that as equential two step electron
uptake by the methoxyph osphonium cation in 1 to the corre-
sponding anion via formatio no ft he neutral mono -radica li s
energetically favorab le. The later steps consist of two parall el
pathways, P-O and C @ Ob ond dissociation, with compar able
free energy changes ( D G ), @ 50.3 and @ 45.4 kcal mol @ 1 ,r espec-
tively (Scheme S1 in Supporting Information). Most likely ,t he
low selectivity towar ds TPP formation is presum ably due to
competitive P @ Oa nd C @ Ob ond dissociation. Notably ,a lkene
formation co uld not be achieved under an identical electro-
chemica lc ondit ion only with TPPO which further highlights
the significance of P = Ob ond activation via alkylatio n( Fig-
ure S20). To increas et he TPP regeneratio nf rom 1 ,S c( OTf) 3 was
employed as ar edox-innocent Lewis aci da nd MeO @ acceptor
to accelerate the rate-det ermining P-O dissociation step.
Sc(OTf) 3 has already been used in other LA -mediated organ ic
transformations [13] and for the stab ilization of high-valent oxo-
transition-metal intermediates. [14] In fact, addition of as ubstoi-
chiometric amount of Sc(OTf) 3 to solutions of 1 and subse-
quent el ectrolysis under identical conditions (entry 2i nT able 1,
in presence of MeOTf and PhCHO) resulted in ad rastic increase
in styrene form ation to 63 %y ields (Figures S21 and S22). The
stoichiometry of Sc 3 + (0.6 molar equiv al ents) was optimize db y
mean so ft he highest yield of TP Pe lectro-rege nerated from 1
(Figures S23–S26).
Ta ble 1. Screening of reaction condition s. (a) One-po tr eaction scheme for electrochemica lW OR using [Ph 2 RP(OR 1 )] + (R = Ph, Me, Et and R 1 = Me, Et) in the
presence of subst oi chiometric amounts of Lewis acid (LA), (b) different [Ph 2 RP(OR 1 )] + ,( c) alkyl electro philes RX, (d) carbonyl compounds, and (e) olefins
produced by electrochem ical WOR.
Entry [a] WE [b] LA [c] [Ph 2 RP(OR 1 )] + [d] Alkyl
electrophile [e]
Carbonyl
compound [f]
Olefin Yi eld
1 Ag –[ Ph 3 P( OMe)] + MeOTf PhCH O PhCH = CH 2 < 10 %
2 Ag g Sc 3 + [Ph 3 P( OMe)] + MeOTf PhCH O PhCH = CH 2 63 %
3 GC h Sc 3 + [Ph 3 P( OMe)] + MeOTf PhCH O PhCH = CH 2 41 %
4 Ag Yb 3 + i [Ph 3 P( OMe)] + MeOTf PhCH O PhCH = CH 2 33 %
5 Ag Sc 3 + [Ph 2 MeP( OM e)] + MeOTf PhCH O PhCH = CH 2 31 %
6 Ag Sc 3 + [Ph 2 EtP(OMe)] + j MeOTf PhCH O PhCH = CH 2 –
7 Ag Sc 3 + [Ph 3 P( OEt)] + EtOTf PhCH O PhCH = CHCH 3 67 %
8 Ag Sc 3 + [Ph 3 P( OMe)] + PhCH 2 Br PhCHO PhCH = CHPh 43 %
9 Ag Sc 3 + [Ph 3 P( OMe)] + MeOTf CyCH O CyCH = CH 2 57 %
10 Ag Sc 3 + [Ph 3 P( OMe)] + MeOTf FuCH O FuCH = CH 2 46 %
11 Ag Sc 3 + [Ph 3 P( OMe)] + MeOTf Ph 2 CO Ph 2 C = CH 2 22 %
[a] Reactio nc ondition s: all the experiments were conducted under N 2 atmospher e( glove box), 0.175 m TBAPF 6 as supporting electrolyte, dried CH 3 CN prior
to use as solven t( total volume 3m L), electrolysis in separated (by glass frit) two electrode cel ls etup at ac onstant curren t( chronop otentiometr y; CP at
4.5 mA) using aP tf oil as co unter electrode (0.5 V 0.5 cm workin ga rea). [b] workin ge le ctrod e( WE) (0.5 V 0.5 cm worki ng area) .[ c] highes ty ield of TPP and
WOR was achieved with 0.02 m (0.6 equiv) LA. [d] electr ol ysis (2 h) was conduc te dw ith 0.032 m [Ph 2 RP(OR 1 )] + in the prese nce of 0 .6 equiv LA and formation
of TPP was confirmed analyzing the sol ution by 31 P( 1 H) NMR. [e] 0.048 m alkyl halid ew as added to the working compartm ent after 2h of electroly si sa nd
stirre df or additiona l3 0m inutes to prepar et he phosphonium salt in situ. [f] 0.048 m aldehyde and/or keton ea dded further to the working compartment.
[g] Ag foil 0.1 mm thi ck conn ec ted to with copper wire and copper tape (under an identical co ndition Cu foil, Mg foil and Ni foam did not produce
alkene), [h] glassy carbo nr od (GC ;6 mm diameter). [i ]0 .02 m Yb(OTf) 3 ;o ther LA (Fe 3 + ,N i
2 + ,Z n
2 + and B(OPh) 3 )r emain ineffective for one-po tW OR.
[j] [R 3 POMe](OTf) (R = n Bu and n-oct) did not produce any olefin due to poor TPP regeneration (Figures S40 and S41).
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The efficiency of WOR was further tested with four different
potentia lw orking electrode ma terials :C ua nd Mg foils, Ni
foam (NF ,0 .5 V 0.5 cm, three-dimensional) and glass yc arbon
rod (GC, 6m md iameter). Under similar experimenta lc ondi-
tions, GC carbon rod (entry 3i nT able 1) delivers am oderate
yield (41 %) of styrene (Figure S27 and Figure S28) while Cu,
Mg foils and Ni foam rem ain ineffective in the one-pot WOR
due to ineffective TPP format ion and unwanted TPPO regener-
ation by C-OPPh 3 bond dissociation (Figures S29–S31 and
Ta ble S4 ). The highest efficiency on aA ge lectrode surface
could be ascribed to it sh igher conduc tivity whic hp lays an im -
portant role in this electr ochemical system. [15] Other transition-
metal ions (Fe 3 + ,N i
2 + ,Y b
3 + ,Z n
2 + )w ere also probe da sp oten-
tial LAs for electr ochemical WO R. In the presence of Yb 3 + ,a
conversion of ca. 33 %s tyrene was obtained (entry 4i nT able 1,
Figure sS 32 and S33 in Support ing Information). Unfavora bly ,
the desired electrochemical WOR could not occur in the pres-
ence of Fe 3 + ,N i
2 + ,Z n
2 + salts most likely due to the lower re-
ductio np otentials of Fe 3 + ( E Fe3 + /Fe = @ 0.04 V), Ni 2 + ( E Ni2 + /Ni =
@ 0.26 V) and Zn 2 + ( E Zn2 + /Zn = @ 0.76 V) [16] in comparison with 1
(one-ele ct ron-reduction at 2.04 Vv s. Fc/Fc + )( Figure sS 34-S35).
Thus, the high reduction potential of Yb 3 + ( E Yb3 + /Yb = @ 2.37 V)
and Sc 3 + ( E Sc3 + /Sc @ 2.03 V) [16] make st hem suitable LAs for elec-
trochemical reduction of 1 to TPP and its disposal for ao ne-
pot WOR. In contrast, the addition of B(OPh) 3 as LA does not
result in olefin formation due to relativel yl ow TPP regenera-
tion (ca. 20 %) from 1 under the applie de xperimental co ndi-
tions (Figure S36).
The displacement of Ph groups in 1 by alkyl substitu ents
has also as ignificant influence on the WOR efficiency :u sing
[Ph 2 MeP(OMe)](OTf), MeOT fa nd PhCHO lowers the yields of
styrene to 31 %( entry 5i nT able 1, Figures S37 and S38), while
monoethyl substitu tion on the Pa tom ([Ph 2 EtP(OMe)](OTf))
leads merely to unwanted (P)O @ Cb ond scission and formation
of Ph 2 EtP = O( entry 6i n, Ta ble 1, and Figures S39–S41 ). Notably ,
due to the highe rr eductio np otentials of fully alkyl-substituted
phosphine oxide s( n Bu 3 PO, n Oct 3 PO), electro-chemic al and/or
chemica lr eductio nr emain unfavo rable under the here applie d
conditions. [8b, 11 f] Although DFT calculations performed with
[ n Bu 3 P(OMe)] + revealed that the one-electron reduction of the
phosphonium cente ri se nergeticall yf avorable, no minim aw as
obtaine di nt he potential energy surface for the two-electron -
reduction to n Bu 3 P( Schem eS 2). Conver sely ,t he computed D G
value for the C @ Ob ond dissociation of [ n Bu 3 P(OMe)] + is en er-
getically favor able and as ac onsequence, n Bu 3 PO and n Oct 3 PO
were solely obtained during electroreduction of the respective
methoxyphosphon ium salts (Scheme S2) an d, henceforth,
alkyl-substituted meth oxy phosphonium salt sa re unsuitable
for electrochem ic al WOR.
Startin gf rom TPPO, the electrochemi cal WO Rc an also be
achieve dw ith other alkoxy-phosphonium salts, [Ph 3 P(OR)] +
(R = Et) and using various carbon yl compounds (entry 7–1 1i n
Ta ble 1). Accordingly ,e lectrolysis of solutions conta ining
[Ph 3 P(OEt)](OTf) ( 2 ), EtOTf and Ph CHO affords prop-1-enyl-ben-
zene in 67 %y ields (PhCH = CHCH 3 ,e ntry 7i nT able 1, inset c;
Figures S42–S44). Ve ry similarly ,a bout 43 %s tilbene (entry 8i n
Ta ble 1, Figures S45–S47), 57 %v inylcylo he xane (CyCH = CH 2 ,
entry 9i nT able 1, Figure S48), 46 %2 -v inalyfuran (FuCH = CH 2
entry 10 in Ta ble 1, Figur eS 49) and 22 %1 ,1-diphenyl ethene
(Ph 2 C = CH 2 ,e ntry 11 in Ta ble 1, Figure S50) could be prepared.
Howe ve r, electrochemical WOR does not occur with unactivat -
ed TPPO in the presence of Sc 3 + althoug ha week interaction
between “free” TPPO and Sc 3 + is suggested by 31 P( 1 H) NMR
and also observed for the one-elec tron redox potentia lo f
TPPO in the presenc eo fS c
3 + in CV (Figure S51). Th is control
experiment prove st hat both alkoxy-pho sphonium salt and
Sc 3 + are the two ke yc omponent sf or successful one-pot elec-
troch em ical WOR.
In the earlie rr eports of electr oreduction of TPPO to TPP ,t he
LAs such as B(OA r) 3 ,A lCl 3 and Me 3 SiCl act as sinks to trap O 2 @ ,
producing the very stable [{(OAr) 3 B} 2 O] 2 @ diborate, [1 1e] Al 2 O 3 , [1 1f ]
and (Me 3 Si ) 2 Od isiloxane, [1 1b] respec tively .I nc ontra st ,t he elec-
tro-re du ctive P @ Ob ond cleavage of 1 and 2 furnishes the re-
spectiv ea lcohols CH 3 OH (from 1 in 68 %y ields) and C 2 H 5 OH
(from 2 in 75 %y ields), respec tively (see Figures S52–S54). Core
level X-ray photoelectro ns pectroscopi c( XPS) analyses of crude
solids isolated from concentrated electr olyzed solution sw ere
performed to deter mi ne the valence state of scandium after
electrolysis. The bindin ge nergies ob tained for Sc 3p 1/2
(407.8 eV) in these solids wer ei dentica lt ot hat of fresh
Sc(OTf) 3 (Figure S55), proving that Sc remains in the same oxi-
dation state ( + 3) after the reaction. [17] Although, TPPO can co-
ordinate Sc 3 + and forms as table isolab le complex, [18] 31 P( 1 H)
NMR and ESI-mass spectra of am ixture of 1 and Sc(OTf) 3 indi-
cated the presence of “free” 1 in solution (Figures S56 and
S57).
How does the electr oreduction of the cation in 1 ,[ 1-OTf] + + ,
occur and what is the ro le of Sc 3 + in this process ?W ep ropos e
ap lausible mechanistic path way based on results of DFT calcu-
lations as depicte di nF igure 2. To explain the reduction of [ 1-
OTf] + + in the presence of Sc 3 + ,g eomet ry optimization sw ere
carried out at the B3L YP [19a, b–d] -D3 [20] level of theory with Stutt-
gart 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 ac-
count for the solven te ff ec to fa cetonitrile, single point calcula-
tions of the optimized structu res were performed using Polar-
izable Continuum Model (PCM). Th ec alculation sr evealed that
coordination of Sc 3 + with [ 1-OTf] + + is energeticall yd isfavored,
while ligation wit ha cetonitrile takes place rather easy to form
complex A .T he latter can release one of the coordinating ace-
tonitrile ligand st of orm the pentaco ordinated Sc complex A’
but the reaction is slightly endergonic by 4.1 kcal mol @ 1 (Fig-
ure 2a ). Notabl y, the one-electron-reducti on product of [ 1-
OTf] + + ,t hat is, the corresp onding radical [ 1-OTf] C ,h as ah ighly
exerg on ic coordinatio na ffinity towards A’ with 57.9 kcal mol @ 1 ,
affording the Sc-arene p -comp lex B (Figure 2b ); in fact, related
scandium-arene complexes have previously been reported. [23]
O- and P-coordinatio no f[ 1-OTf] C to Sc could lead to the
isomer B’ and B’ ’ ,r espectively (Figure 2b ), but they are less
stable than B .U pon formation of B ,t he methoxy group on
phosphorus can be transferred to the electrophilic CN carbon
atom of the ligate da ceto nitrile via the tr ansition-state TS(B-C)
to give the methoxy-acetimida te-substitute dS cc omplex C ;
the latter can under go ao ne-electron-reduction to form the
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anionic complex D at D G = @ 159.0 kcal mol @ 1 (Figure 2c ). D re-
leases a“ free” phosphine via TS(D-E) forming the complex E at
D G = @ 170.4 kcal mol @ 1 in which the methoxy-ace timida te
oxygen atom coordinates to the Sc cente r. Cleavage of the
NC @ OMe bond of the meth yl-acetimidate group via TS(E–F) re-
sults in formation of the methoxy-su bstituted scandium com-
plex F at @ 187.4 kcal mol @ 1 .P rotonation of F can resul ti nf or-
mation of methanol (Figure 2d ,a so bserved experimentally ,
see also Figure S52) along with regeneration of A .
After successful regen eration of TPP from 1 and 2 ,a ddition
of alkyl electrophile sR X( R = Me, Et, benzy l; X = OTf, Br) result-
ed in the almost quantitative for mation of the corre sponding
phosphonium salts, [TPP-R](X ), as evidenced by their downfield
31 Pc hemical shifts from d = @ 5.6 (TP P) to 21.4 ppm for [TPP-
Me] + in the 31 P( 1 H) NMR spectrum (Figure S58, see Figures
S44 af or [TPP-Et] + and S45 for [TPP-CH 2 Ph] + ). Shano et al. pro-
posed that aW ittig ylide can be form ed in situ through one-
electron-reduction of phosp honiu mc ations. [10b, 24] Up to now ,
no experimental evidenc ei ns upport of ylide formation was re-
ported .I nf act, electror eduction of in situ regener at ed methyl-
phosphonium triflate, [TPP -Me](OTf), from 1 (Figure S58) and/
or independen tly prepared (Figure S59) under similar reactio n
conditions affords the corresp onding Ph 3 P = CH 2 Wittig ylide as
prove nb yi ts characteristic 31 P( 1 H) NMR spectru m( Figure S60) ;
this confirms that the one-pot olefination descri bed herein
occurs via in situ Wittig ylide formation (Scheme S4). After
complete electr ochemical WOR with Ph 3 P = CR’ 2 (CR’ 2 = CH 2 ,
CHMe, CHPh )c a. 87 %T PPO is generated (Figure S61).
Conclusion
In summary ,w er eported an ovel and facile one- pot electro-
chemica ls trategy for Wittig olefination reactions direc tly recy-
cling TPPO by means of P = Op hosphoryl bond activat ion via
alkylation with RX (R = Me, Et ;X = OTf) to give [Ph 3 P(OR)](OTf),
which proved to be as uitab le pathway for electro-recycl ing of
TPP with high efficiency (80–9 8% )i nt he presence of Sc(OTf) 3
in acetonitrile solution. DF Tc alculations shed light on the cru-
cial role of Sc 3 + and aceto nitrile which both act as mediators
in the electroreductio no ft he P @ OR bond in [Ph 3 P(OR)](OTf )( 1
and 2 )t of orm TPP .I nterestingly ,t he formation of alcohol as a
value-added side product could be achieve d( and ex plained by
the DFT -propos ed mechanism), which distinguishe st his ap-
proach from previou sm ethods using chemica lr eductant s
(e.g. ,s ilanes, boranes) and sacrificial electrode materia l( e.g. ,
Al). Moreover ,a sep arated cell setup with Ag as aw orki ng
electrode and Pt as as upportin ge lectr ode does not sho w
anodic corrosion and/or electrode dissolution, representing a
highly sustainable one-pot approach for WOR. Furthermo re,
the electroreduction of [Ph 3 P(OR)] + to TPP and subsequen t
olefination reaction via in situ electrochemical Wittig reagent
formation coul db er ealized in ao ne-po tp rotocol with exten d-
Figure 2. Calculated mechanism of electro reduction of 1 to triphenylphosphine (TPP )m ediated by Sc 3 + .T he values in eV are show n in parentheses.
Chem. Eur .J . 2020 , 26 ,1 1829 –1 1834 www.chemeurj.org T 2020 The Authors. Published by Wiley-VCH GmbH 11 833
Chemistr y— A European Jo urnal
Full Paper
doi.org/10.1002/che m.202001654

ed scope of substra tes. The strategy prese nt ed herein could
pave the way to bulk scale electroc hemical WOR synthesis of
more functionalized alkenes.
Acknowledgements
Funded by the Fun ded by the Deutsc he Forschungsgemein-
schaft (DFG, German Research Foundatio n) under Germany’s
Excellence Strategy—EXC 2008–3 90540038 —U niSysCat. Au-
thors thank Dr .X iaoho iD eng for his help in performing some
preliminar ye xperiments .O pen access fu nding enabled and or-
ganized by Projekt DEAL.
Conflict of interest
The authors declare no conflict of interes t.
Keywords : bond activ ation · electrosynthesis · Lewis acid ·
phosphine oxide reduct ion · scandium complexes
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Manuscript received :A pril 6, 2020
Accepted manusc ript online :A pril 7, 2020
Ve rsion of reco rd online :A ugust 13, 2020
Chem. Eur .J . 2020 , 26 ,1 1829 –1 1834 www.chemeurj.org T 2020 The Authors. Published by Wiley-VCH GmbH 11 834
Chemistr y— A European Jo urnal
Full Paper
doi.org/10.1002/che m.202001654

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