Cyclohexa-1,4-dienes in transition-metal-free ionic
transfer processes
Sebastian Keess and Martin Oestreich *
Safe- and convenient-to-handle surrogates of hazardous chemicals are always in demand. Recently
introduced cyclohexa-1,4-dienes with adequate substitution fulfil this role as El
+
/H
equivalents in
B(C
6
F
5
)
3
-catalysed transfer reactions of El–Htop- and s-donors (C]C/C^C and C]O/C]N).
Surrogates of Si–H/Ge–H, H–H and even C–H bonds have been designed and successfully applied to
ionic transfer hydrosilylation/hydrogermylation, hydrogenation and hydro-tert-butylation, respectively.
These processes and their basic principles are summarised in this Minireview. The similarities and
differences between these transfer reactions as well as the challenges associated with these
transformations are discussed.
Concept
Transfer processes represent a practical strategy for performing
challenging bond formations or avoiding handling hazardous
reagents. Limited mainly to transfer hydrogenation
1
for a long
time, this technique has recently emerged as a powerful
approach for the application of various toxic, ammable/
explosive and/or gaseous chemicals that have otherwise only
been rarely used in synthetic chemistry.
2
The aptitude of adequately substituted cyclohexa-1,4-dienes
Ito engage in ionic transfer reactions as synthetic equivalents
of El
+
/H
(El ¼Si,
3
Ge,
4
H,
5
tBu
6
) was demonstrated by our
laboratory during the last years (Scheme 1). The underlying
concept relies on the ability of diene Ito transiently form ion
pair III
+
[HB(C
6
F
5
)
3
]
by B(C
6
F
5
)
3
-mediated hydride abstraction
from the bisallylic methylene group (I/III
+
)
7
and subse-
quently release electrofuge El
+
; aromatisation to furnish the
respective arene is exploited as the driving force (Scheme 1,
top). The fate of Wheland complex III
+
was shown to be
dependent on the nature of the attached El group, following
divergent pathways: El–H release and subsequent activation by
B(C
6
F
5
)
3
or direct delivery of electrofuge El
+
to substrate V
(Scheme 1, bottom, grey pathways). Transfer hydrosilylation (El
¼Si)
3
or hydrogermylation (El ¼Ge)
4
were shown to pass
through two interdependent catalytic cycles, liberating the
Sebastian Keess (born in 1988 in
Wuppertal/Germany) studied
chemistry at the RWTH Aachen
University (2008–2013),
including a research internship
at the University of York (2012).
He obtained his bachelor's
degree with Dieter Enders (2011)
and his master's degree with
Magnus Rueping (2013). His
education was funded by
a Deutschlandstipendium sup-
ported by the Bayer Science &
Education Foundation (2012–2013). Aer a ve-month internship
at Bayer Pharma AG (Wuppertal/Germany), he moved to Berlin
where he currently pursues graduate research in the group of
Martin Oestreich at the Technische Universit¨
at Berlin.
Martin Oestreich (born in 1971
in Pforzheim/Germany) is
Professor of Organic Chemistry
at the Technische Universit¨
at
Berlin. He received his diploma
degree with Paul Knochel (Mar-
burg, 1996) and his doctoral
degree with Dieter Hoppe
(M¨
unster, 1999). Aer a two-
year postdoctoral stint with
Larry E. Overman (Irvine, 1999–
2001), he completed his habili-
tation with Reinhard Br¨
uckner
(Freiburg, 2001–2005) and was appointed as Professor of Organic
Chemistry at the Westf¨
alische Wilhelms-Universit¨
at M¨
unster
(2006–2011). He also held visiting positions at CardiffUniversity
in Wales (2005) and at The Australian National University in
Canberra (2010).
Institut f¨
ur Chemie, Technische Universit¨
at Berlin, Strasse des 17. Juni 115, 10623
Cite this: Chem. Sci.,2017,8, 4688
Received 13th April 2017
Accepted 20th May 2017
DOI: 10.1039/c7sc01657c
rsc.li/chemical-science
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hydrosilane and hydrogermane, respectively (III
+
/El–H+
RC
6
H
5
,lecycle), followed by B(C
6
F
5
)
3
-catalysed El–H bond
activation. The thus-formed h
1
-adduct IV
8
then participates in
the reduction of C–C multiple bonds (right cycle).
9,10
Conversely,
transfer hydrogenation
5
and hydro-tert-butylation
6
proceed by
direct transfer of the electrofuge El
+
from Wheland interme-
diate III
+
onto substrate Vto eventually furnish adduct VII aer
hydride reduction by [HB(C
6
F
5
)
3
]
(III
+
+V/VI
+
/VII).
11
Consistent with this dichotomy, liberation of the El–H func-
tionality from Ioccurs even in the absence of a Lewis-basic
substrate for hydrosilanes/hydrogermanes (El ¼Si and Ge)
3,4
while degradation of the H–H and C–H surrogates (El ¼H and
tBu) proceeds only slowly at room temperature.
5,6
The illustrated concept forms the foundation for all devel-
oped transition-metal-free ionic transfer processes using
cyclohexa-1,4-dienes Ias transfer reagents. This Minireview
summarises the recent advances in these transformations. It
outlines and discusses the challenges and limitations as well as
the differences and similarities of the individual transfer
processes.
Transfer reagents
The successful implementation of cyclohexa-1,4-dienes Iin the
different transfer reactions, i.e., transfer hydrosilylation/
hydrogermylation, transfer hydrogenation and transfer hydro-
tert-butylation, required deliberate modication of the substi-
tution pattern of the cyclohexa-2,5-dien-1-yl unit (Fig. 1).
Unsubstituted cyclohexa-2,5-dien-1-ylsilanes 1and
-germanes 2cleanly transform into the corresponding hydro-
silane or hydrogermane and benzene at room temperature
when treated with B(C
6
F
5
)
3
(Fig. 1, top).
3,4
Essential for this
transformation to proceed is sufficient hydridic character of
the bisallylic C(sp
3
)–Hbondin1due to hyperconjugation with
the C(sp
3
)–Si bond and the associated stabilisation of the
resulting low-energy Wheland complex III
+
[HB(C
6
F
5
)
3
]
(cf.
Scheme 1),
12
as supported by computational studies by Sakata
and Fujimoto.
9
Comparable stabilisation from the C(sp
3
)–Ge
bond is expected to facilitate the release of hydrogermanes
from surrogates 2. Conversely, dihydrogen surrogates 3are
devoid of this stabilisation and require electron-donating
substituents at C1/C5 (3a,R
1
¼Me, R
2
¼H)
5a
or C1/C3/C5
(3b,R
1
¼R
2
¼Me)
5b
to lend stabilisation to the resulting
high-energy Wheland intermediates III
+
[HB(C
6
F
5
)
3
]
(middle),
as well as to suppress undesired reaction pathways, e.g.,
dihydrogen release or cationic heterodimerisation of reac-
tants. While unsubstituted cyclohexa-1,4-diene (3c)favoured
side reactions in the transfer hydrogenation of alkenes cata-
lysedbytheLewisacidB(C
6
F
5
)
3
,
5b
Brønsted acids such as
Tf
2
NH were shown to selectively mediate transfer hydrogena-
tion from this surrogate.
5c,13
Likewise, adjustment of the
substitution pattern at the cyclohexa-2,5-dien-1-yl core was
necessary for the design of the transfer reagents 4for the
B(C
6
F
5
)
3
-catalysed transfer hydro-tert-butylation (bottom).
6
Another substituent “ipso”to the tert-butyl group in 4had to be
introduced to avoid competing proton release from that
position.
Scheme 1 Reaction of cyclohexa-1,4-dienes with B(C
6
F
5
)
3
in the
absence (top) and presence (bottom) of p-basic substrates.
Fig. 1 Substituted cyclohexa-1,4-dienes as synthetic equivalents of
hydrosilanes/hydrogermanes (top), dihydrogen (middle) and isobutane
(bottom).
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Transfer hydrosilylation/
hydrogermylation
Simonneau and Oestreich introduced cyclohexa-1,4-dienes as
reagents in ionic transfer processes
14
and provided the proof-of-
principle for the concept outlined above by employing surrogate
1a as an equivalent of gaseous Me
3
SiH (Fig. 2, le).
3b
Surrogates
of various other hydrosilanes, e.g., functionalised (EtO)
3
SiH,
3d
were prepared and successfully tested in B(C
6
F
5
)
3
-catalysed
ionic transfer hydrosilylations (not shown).
3a
The development
of solid 1b as an easy-to-handle surrogate of pyrophoric and
explosive SiH
4
disclosed a rare strategy for the use of mono-
silane in organic synthesis.
3e,15
Likewise, related 2a and 2b are
surrogates of Et
3
GeH and gaseous MeGeH
3
(Fig. 2, right) that
enabled the rst examples of transfer hydrogermylation.
4
The ionic transfer hydrosilylation and hydrogermylation of
p-basic substrates, i.e., alkenes and alkynes,
16
proved to be
applicable to a wide range of unfunctionalised derivatives
(Scheme 2).
3b,d,4
Both transfer processes proceeded at room
temperature using catalytic amounts of the Lewis acid and
a slight excess of surrogate 1a or 2a in CH
2
Cl
2
or 1,2-F
2
C
6
H
4
,
respectively. Terminal (/5–14), i.e., mono- and 1,1-disubsti-
tuted, as well as 1,2-disubstituted (/15–16) and trisubstituted
(/17–18) alkenes were compatible with the transfer protocols
and furnished tetraorganosilanes and -germanes in high yields.
Reduction of an internal electronically unbiased alkyne selec-
tively yielded the product of trans-addition (/(Z)-19–(Z)-20)
whereas transfer hydrogermylation of electronically biased ethyl
3-phenylpropiolate proceeded selectively with cis addition, and
the ester group was perfectly compatible (not shown).
16b
The exo
selectivity in the reduction of norbornene (/15) and norborna-
2,5-diene (/16) and predominant cis diastereoselectivity in the
hydrosilylation of 1-methylcyclohexene (not shown) as well as
the absence of products of radical cyclisation in the hydro-
germylation of an acyclic 1,6-diene (not shown) conrmed the
ionic nature of the mechanism for both transfer reactions.
4,9
The regioselective formation of 17 and 18 emphasises the fav-
oured formation of a benzylic (secondary) carbocation over
a tertiary. A discrepancy in the performance of both surrogates
1a and 2a was observed in the reduction of functionalised
substrates (grey box). Allyltriethylsilane reacted cleanly in the
transfer hydrogermylation (/21) whereas only decomposition
was observed when subjected to the setup of the transfer
hydrosilylation (not shown). Acetophenone yielded alcohol 22
as product of hydrosilylation,
17
but hydrogermylation of a,b-
unsaturated esters and ketones furnished products with
untouched carboxyl (/23) and carbonyl (/24) groups,
respectively.
A systematic study was recently reported by Oestreich and co-
workers that provides comprehensive insight into the parame-
ters that govern the transfer hydrosilylation.
3d
The analysis
includes surrogates 1with modied electronic and steric
properties, fully or partially uorinated triarylboranes as well as
representative p- and s-basic substrates.
18
Selected data of this
study are summarised in Fig. 3. Cyclohexa-1,4-diene 1a reacts
readily with p-donor 25 at room temperature while elevated
Fig. 2 Representative cyclohexa-1,4-dienes as surrogates of hydro-
silanes (left) and hydrogermanes (right).
Scheme 2 Transfer hydrosilylation and hydrogermylation of C–X
multiple bonds using surrogates of Me
3
SiH and Et
3
GeH.
a
Performed at
90 C.
Fig. 3 Interplay between surrogate structure and reactivity.
a
Surrogate
1e fully consumed.
b
Partial deoxygenation to styrene.
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temperatures are required to split the Lewis acid/base adduct of
s-donor 26 and the borane catalyst (column 1). Derivative 1c
with a methyl group ipso to the departing silicon group was less
reactive than parent 1a (column 2), as was surrogate 1d bearing
an extended psystem (column 3). Introduction of +M substit-
uents as in 1e signicantly increased the reactivity due to an
enhanced hydricity of the bisallylic methylene group, yielding
quantitative conversion of s-basic acetophenone (26) at room
temperature within minutes (column 4). Conversely, the s-
donating methoxy substituents in resorcinol-derived 1e
outcompete p-basic 1,1-diphenylethylene (25) for the transfer of
the silicon electrophile, and only cleavage of the ether groups of
1e was observed (column 4).
Simonneau and Oestreich were able to further advance this
strategy by introducing 1b as a surrogate of SiH
4
, a silane that is
rarely used by synthetic chemists due to the associated safety
issues.
3e
Later, this approach was unsuccessfully tested to access
the related monogermane GeH
4
, and surrogate 2b as equivalent
of MeGeH
3
was prepared instead.
4
Both surrogates 1b and 2b
were shown to liberate SiH
4
and MeGeH
3
, respectively, upon
treatment with catalytic amounts of B(C
6
F
5
)
3
followed by n-fold
hydrosilylation or 3-fold hydrogermylation of typical alkenes
(Scheme 3). Monohydro- (/27,30,31), dihydro- (/28,32) and
tetraalkyl-substituted silanes (/29) became accessible depen-
dent on the steric demand of the alkene; the degree of substi-
tution at the silicon atom can usually not be controlled by the
stoichiometry of the reagents. However, for 1,1-diphenylethylene
(25), reversal of the chemoselectivity, that is the formation of 31
over 32, was achieved by adjustment of the stoichiometry and
the use of di(cyclohexa-2,5-dien-1-yl)silane instead of 1b (grey
box). Also, this method allowed for the mild preparation of
tetraalkyl-substituted germanes (/33–37).
Transfer hydrogenation
Kihara and co-workers introduced cyclohexa-1,4-diene (3c)as
the dihydrogen source in the Lewis acid-catalysed reduction of
dithioacetals to the corresponding suldes (not shown).
19,20
Later, the research group of Gandon reported a gallium(III)-
assisted transfer hydrogenation of alkenes using the same
hydrogen donor 3c (Scheme 4).
13,21
Their protocol was appli-
cable to 1,1-disubstituted (/38), 1,2-disubstituted (/39)as
well as trisubstituted acyclic (/40–41) and cyclic (/42–46)
alkenes and tolerated ketone (/39) or ester (/41) function-
alities. Tetrasubstituted alkenes or those without an aryl
substituent were unreactive (not shown).
Aryl-substituted alkynes participated in a cascade
hydroarylation/transfer hydrogenation sequence catalysed by
the same gallium(III) complex with cyclohexa-1,4-diene (3c)as
reductant to afford dicyclic (/47–49) as well as tricyclic (/50)
products in high yields (Scheme 5).
13
The formation of penta-
cyclic 51 gave a signicantly lower yield. Although the mecha-
nism of the gallium(III)-assisted transfer hydrogenation has not
been studied in detail yet, an ionic process was proposed for the
dihydrogen transfer (not shown).
13a
Later, it was demonstrated
that the transformations depicted in Schemes 4 and 5 work
equally well with an indium(III) complex (not shown).
13b
Chatterjee and Oestreich disclosed the B(C
6
F
5
)
3
-catalysed
ionic transfer hydrogenation of imines and related heteroarenes
employing substituted cyclohexa-1,4-dienes 3a or 3b as the
dihydrogen source.
5a
Later, Grimme and Oestreich showed that
this transfer process also works with alkenes and conrmed the
mechanism by quantum-chemical calculations.
5b
The catalytic
cycle commences with rate-limiting Lewis acid-mediated
hydride abstraction from surrogate 3a or 3b to give ion pair
VIII
+
[HB(C
6
F
5
)
3
]
in low concentration (Scheme 6, lecycle).
High-energy Wheland intermediate VIII
+
acts as a strong
Scheme 3 Transfer hydrosilylation/hydrogermylation of alkenes with
surrogates of monosilane or methylgermane.
a
Dicyclohexa-2,5-dien-
1-ylsilane was used as the surrogate. Scheme 4 Gallium(III)-assisted transfer hydrogenation of alkenes.
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Brønsted acid and protonates s-orp-basic substrate IX to
furnish ion pair XI
+
[HB(C
6
F
5
)
3
]
together with stoichiometric
arene X. Dihydrogen release from VIII
+
in the presence of
a Lewis-basic substrate was excluded. Conversely, subsequent
hydride transfer from [HB(C
6
F
5
)
3
]
to the carbenium ion in XI
+
to regenerate B(C
6
F
5
)
3
concomitant with the formation of
product XII was proposed to compete with reversible dihydrogen
liberation in the case of imines (grey pathway).
22
The preference
of either pathway depends on the electrophilicity of the carbon
atom of the iminium ion intermediate as well as the basicity of
the imine nitrogen atom. The formation of highly Brønsted-
acidic Wheland intermediate VIII
+
in the course of the Lewis
acid-mediated transfer hydrogenation inspired Chatterjee and
Oestreich to investigate potentially competing Brønsted-acid
catalysis. As part of these studies, these authors successfully
showed that reasonably strong Brønsted acids such as Tf
2
NH are
equally able to initiate transfer hydrogenation
23
from cyclohexa-
1,4-dienes 3a or 3b by catalytically generating the same Wheland
intermediate VIII
+
(right cycle).
5c
It seems plausible that
protonation of substrate IX occurs from either Brønsted acids
VIII
+
or Tf
2
NH to furnish intermediate XIII
+
[Tf
2
N]
.Inthe
absence of the borohydride [HB(C
6
F
5
)
3
]
, cyclohexa-1,4-diene 3a
or 3b steps in as the hydride donor for the reduction of
XIII
+
[HB(C
6
F
5
)
3
]
, thereby closing the catalytic cycle.
20g
The transfer hydrogenation of imines requires forcing reac-
tion conditions, i.e., 125 C and 10 to 15 mol% of the catalyst,
and is limited to certain protecting groups at the nitrogen atom
to secure optimal steric shielding, sufficient Lewis basicity and
stability (Scheme 7).
5a,c
The protocol is compatible with differ-
ently functionalised ketimines (/52–55) and aldimines (/57–
61) and tolerated electron-withdrawing substituents
(/54,55,59–61) and even ortho substitution (/59). A
Scheme 5 Gallium(III)-assisted hydrogenative cyclisation of alkynes.
a
2.4 equiv. of 3c used.
Scheme 6 Catalytic cycles for Lewis and Brønsted acid-catalysed
transfer hydrogenation of imines (X ¼NPG) and alkenes (X ¼CH).
Scheme 7 Lewis and Brønsted acid-catalysed transfer hydrogenation
of imines and nitrogen-containing heteroarenes.
a
Messy reaction.
b
2.6
equiv. of surrogate 3a used.
c
Prepared by reductive amination.
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cyclohexanone-derived imine was completely unreactive (not
shown), and a 4-anisyl-substituted ketimine showed only
moderate reactivity in the presence of the Brønsted acid Tf
2
NH
and no reactivity when subjected to catalysis with B(C
6
F
5
)
3
(/56), likely due to lower hydride affinity of the respective
iminium ion intermediate. Nitrogen-containing heterocycles
participated well in the B(C
6
F
5
)
3
-catalysed transfer hydrogena-
tion affording 62–64 in high yields.
The transfer hydrogenation of p-basic alkenes proceeds
equally well with B(C
6
F
5
)
3
or Tf
2
NH under mild reaction condi-
tions using 5.0 mol% of the catalyst at room temperature
(Scheme 8).
5b,c
The transfer process, however, requires an addi-
tional methyl group in the bisallylic position of the cyclo-
hexadienyl group where the hydride abstraction occurs to
prevent undesired side reactions, that is liberation of dihydrogen
and heterodimerisation of cationic intermediates. The method
can be applied to a wide range of 1,1-disubstituted alkenes
(/38,65–71) and works also with trisubstituted derivatives
(/42). 1,1-Diarylalkenes furnished the corresponding alkanes
38,65 and 66 in high yields, irrespective of the electronic prop-
erties of the arene. a-Alkyl-substituted styrenes as well as 1,1-
dialkylalkenes required sterically demanding substituents, e.g.,
an isopropyl (/67) or a cyclohexyl group (/68–69), to prevent
thermoneutral cationic dimerisation as observed for 70 and 71.
Transfer hydro-tert-butylation
Keess and Oestreich introduced 3-tert-butyl-substituted
cyclohexa-1,4-dienes 4as transfer reagents in the transfer
hydro-tert-butylation of alkenes.
6
This methodology represents
an unprecedented approach to install tertiary alkyl groups at
carbon frameworks
24
but competing reaction channels that
could not be completely suppressed still limit its synthetic utility.
The transfer of the tert-butyl group proceeds smoothly at
room temperature with only little excess of transfer reagent 4,
yielding quantitative conversion of the alkene (Scheme 9).
6
Extensive optimization of the reaction conditions using 1,1-
diphenylethylene (25) as model substrate could not fully prevent
the formation of byproducts 75a and 76a (column 1). Elec-
tronically modied 1,1-diarylalkenes 72 and 73 were also tested
but favoured the formation of the byproducts 75 and 76 to an
Scheme 8 Lewis acid- and Brønsted acid-catalysed transfer hydro-
genation of alkenes.
Scheme 9 Transfer hydro-tert-butylation of 1,1-diarylalkenes.
Scheme 10 Proposed catalytic cycle for the transfer hydro-tert-
butylation.
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even greater extent (columns 2 and 3). The inuence of the
surrogate structure, namely the substituent “ipso”to the tert-
butyl group, is profound, resulting in superior selectivities for
surrogate 4a (R ¼Ph) compared to 4b (R ¼vinyl). In the latter
case, separation of the stoichiometrically formed arene
byproduct is conveniently achieved as styrene polymerises
under the reaction conditions.
The proposed catalytic cycle that rationalises the pathways for
byproduct formation commences with the B(C
6
F
5
)
3
-triggered
abstraction of a bisallylic hydride from surrogate 4to furnish
Wheland complex XV
+
[HB(C
6
F
5
)
3
]
(Scheme 10), followed by
transfer of either the tert-butyl cation (XV
+
/XVII
+
,lecycle) or
a distal proton (XV
+
/XVIII
+
, right cycle) to alkene XVI; stoi-
chiometric liberation of gaseous isobutene likely accounts for
the latter pathway. The carbenium ion in XVIII
+
is eventually
reduced by borohydride [HB(C
6
F
5
)
3
]
to afford byproduct 75,
thereby closing the catalytic cycle. Likewise, intermediate XVII
+
can either directly collapse and form the desired alkane 74 or
rst transfer a proton from the bposition in XVII
+
to another
molecule of alkene XVI and form byproducts 75 and 76 aer
hydride transfer from [HB(C
6
F
5
)
3
]
(grey pathway).
Outlook
The recent advances in transition-metal-free ionic transfer
processes using substituted cyclohexa-1,4-dienes as transfer
reagents hint its great promise. While still at the early stages of
development, we believe that these transformations are about to
emerge as useful synthetic tools. Particularly, unleashing small
reactive molecules such as SiH
4
from cyclohexa-1,4-dienes by
straightforward treatment with a Lewis-acid catalyst could also
prove valuable for inorganic chemists.
Transfer hydrosilylation is feasible for several p-ands-donors
with a variety of hydrosilane surrogates, particularly of SiH
4
and
(EtO)
3
SiH. Lack of chemoselectivity and, hence, functional-group
tolerance is the obvious limitation of this method. That problem
is less pronounced in the related transfer hydrogermylation.
Issues in the transfer hydrogenation such as hetero- and homo-
dimerisation of the reactants have been successfully addressed
by judicious choice of the substituents at cyclohexa-1,4-diene
core. The substrate scope for both C]N and C]C bonds is
however still relatively narrow. The transfer of a tert-butyl group
is currently the biggest challenge. While it represents promising
precedence for the transfer of carbon electrofuges, the surrogate
synthesis still remains unsolved. The design of new (short)
synthetic routes and extension to other stabilised carbenium
ions as departing groups will hopefully allow for more efficient
transfer hydroalkylation reactions in the future.
On the basis of the knowledge gained from these efforts, we
will continue improving the existing procedures and devise new
El
+
/H
equivalents. We also hope that our ndings serve as an
inspiration for others in the eld.
Acknowledgements
This research was supported by the Deutsche For-
schungsgemeinscha(Oe 249/11-1). Parts of the ndings
summarised herein were funded by the Humboldt Foundation
and the Cluster of Excellence “Unifying Concepts in Catalysis”
of the Deutsche Forschungsgemeinscha(EXC 314/2). M. O. is
indebted to the Einstein Foundation (Berlin) for an endowed
professorship. M. O. thanks Dr Antoine Simonneau and Dr
Indranil Chatterjee for their enthusiasm and commitment.
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