Synthesis and Identification of Aryl and Alkyl Gem-Dilithium Phosphido-Boranes: A Boost to the Chemistry of Phosphandiides

Article abstract of DOI:10.1002/chem.201800742

The synthesis and identification of unprecedented gem-dianionic phosphorus compounds, that is, gem-dilithium phosphido-boranes Li₂[RP?BH₃], with R=Ph or Cy, are reported in THF solution. These were obtained by double deprotonation of the corresponding primary phosphine-borane precursors RPH₂?BH₃. Their in-depth structural study, based on multinuclear (¹H, ⁶Li, ⁷Li, ¹¹B, ¹³C, ³¹P) mono- and bi-dimensional NMR analyses, indicates a strong influence of the phosphorus substituent on the structure of the gem-dianionic phosphorus structure; a monomeric arrangement was obtained when R=phenyl, whereas a cyclic oligomer was observed for R=cyclohexyl. These compounds represent a new type of useful reagent, and their access paves the way for the concept of “RP synthons” (i.e., RP^2? phosphandiides), likely to be the most flexible precursors of a variety of phosphorus targets.

Full text of DOI:10.1002/chem.201800742

A Journal of
Accepted Article
Title: Synthesis and Identification of Aryl and Alkyl Gem-Dilithium
Phosphido-Boranes: A Boost to the Chemistry of Phosphandiides
Authors: Anne Harrison-Marchand, Jie Guang, Romain Duwald,
Jacques Maddaluno, Hassan Oulyadi, Sami Lakhdar, and
Annie-Claude Gaumont
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201800742
Link to VoR: http://dx.doi.org/10.1002/chem.201800742
Supported by

Chemistry - A European Journal
10.1002/chem.201800742
COMMUNICATION
Synthesis and Identification of Aryl and Alkyl Gem-Dilithium
Phosphido-Boranes: A Boost to the Chemistry of Phosphandiides
a
a
a
a*
b
Jie Guang, Romain Duwald, Jacques Maddaluno, Hassan Oulyadi, Sami Lakhdar, Annie-Claude
b*
a
*
Gaumont, Anne Harrison-Marchand
Abstract: The synthesis and identification of unprecedented gem-
dianionic phosphorus compounds, i.e. gem-dilithium phosphido-
Y = R2C
Y = RP
Y
P
R
R
gem-dianion
R
boranes Li
2 3
[RP•BH ], with R = Ph or Cy, are reported in THF solution.
methandiide
phosphandiide
Those are obtained by double deprotonation of the corresponding
what if
i-Pr
i-Pr
P
Ph2P
PPh2
primary phosphine-borane precursors RPH
2
•BH
3
. Their in-depth
R = alkyl or aryl?
Si
1
6
7
11
13
31
S
S
2
ie with common R substituent
structural study, based on multinuclear ( H, Li, Li, B, C, P)
mono- and bi-dimensional NMR analyses, indicates a strong
influence of the phosphorus substituent on the structure of the
i-Pr
g
Mézailles
i-Pr
i-Pr
Driess3a
2
P(BH3)
i-Pr Ph2P
PPh2 i-Pr
gem-dianionic phosphorus structure: while
a
monomeric
Ph
Cy
2 Li
2 Li
N
N
arrangement is obtained when R = phenyl, a cyclic oligomer is
observed for R = cyclohexyl. These compounds represent a new
type of common handy reagents, and their access certainly
insufflates the long awaited dynamic to the concept of “RP synthons”
likely to be the most flexible precursors of a multitude of varied
phosphorus targets.
P
2
t-Bu Si
P(BH3)
3
i-Pr i-Pr
Liddle2h
Westerhausen3c
gem-dilithium phosphido-borane
Scheme 1. Examples of studied methandiides and phosphandiides: from very
specific R substituents to common aryl- and alkyl- phosphorus gem-dianions.
The rare examples of phosphandiides, from which structural
information has been obtained,[3] refer to solid state data and are
related to very specific R groups. The information brought by
these studies indicates that the second deprotonation is very
difficult and achieving effective stabilization of the final product
requires the presence of a main-group metallic cation and an
adjacent silicon atom to supply an empty d orbital. This
Due to the capacity of gem–dianions Y2- (Scheme 1) to deliver
complex molecular structures within a minimum of chemical
steps, an increasing interest has been devoted to these species
this last decade.[1-3] Among points raised in such studies is the
understanding of how these compounds react and what their
inherent advantages and limitations for practical reactions are.[1]
2-
constraint, in addition to the fact that the concerned RP
Numerous carbon-centred gem-dianions, i.e. methandiides R
Scheme 1, left), have been examined,[2] in contrast to the
2
C2-
compounds are poorly soluble in common organic solvents,
probably due to highly aggregated structures, narrows the scope
of the R substituent and thus, limits synthetic applications. The
complexation of a P-atom with a borane group being a general
(
2
-
phosphorus analogues, i.e. phosphandiides RP (Scheme 1,
right), that have received less attention.[3] A main reason could
be attributed to the weak stability of their precursors, primary
phosphorus species, that easily oxidize or decompose under
[6]
and easy strategy employed to stabilize phosphines, we
decided to overpass the problems mentioned above by taking
advantage of this to reach likely easier-to-handle primary
[
4]
ambient reaction conditions. Our interest in phosphination
reactions,[ which is another topic garnering attention in organic
chemistry, led us to initiate a program to investigate the
structures in solution of dianions derived from the bis
deprotonation of primary phosphine derivatives. In this context,
we report here the first structural identification in THF solution of
two representative gem-dianionic phosphorus compounds, i.e.
5]
phosphine-boranes (RPH •BH
dilithiated derivatives (Li [RP•BH
3
PhPH •BH and CyPH •BH , have been selected to exemplify
2
3
)
and their corresponding
2
3
]). Two RPH •BH i.e.
2
3
,
2
3
2
common aromatic and alkyl primary phosphine-boranes,
respectively. Both species, seldom considered in literature, have
nonetheless been easily prepared by mixing commercially
gem-dilithium
phosphido-boranes
Li
2
[RP•BH
3
]
with
R
2 2 3 2
available PhPH or CyPH with BH •SMe at 0 °C for 30 minutes,
corresponding to a phenyl (R = Ph) or a cyclohexyl (R = Cy)
group.
respectively (pages 3S to 5S in SI).
The double deprotonation of PhPH
2
3
•BH underwent quickly
and quantitatively using n-BuLi at –78 °C in THF. It afforded
Li [PhP•BH ] fully soluble and stable for several hours, even at
room temperature (pages 8S, 9S and 11S in SI). Reaching pure
Li [CyP•BH ] in THF proved more difficult, resulting in a mixture
of Li[CyPH•BH ], Li [CyP•BH ] and n-BuLi, even after a longer
2
3
[
[
a] Drs J. Guang, R. Duwald, J. Maddaluno, H. Oulyadi, A. Harrison-
Marchand
Normandie Université, UNIROUEN, INSA de Rouen, CNRS
Laboratoire COBRA (UMR 6014 & FR 3038)
2
3
3
2
3
76000 Rouen, France
time of contact (pages 10S and 11S in SI). Using stronger bases,
such as sec-BuLi and tert-BuLi, was not considered since these
E-mails: anne.harrison@univ-rouen.fr, hassan.oulyadi@univ-rouen.fr
b] Dr S. Lakhdar, Prof.Dr A.-C. Gaumont
Normandie Université, ENSICAEN, UNICAEN, CNRS
Laboratoire LCMT (UMR 6507 & FR 3038)
reactants are likely to deprotonate the α-CH group, even at –
[6,7]
78
°C.
While still using n-BuLi, we completed the
deprotonation, however under ultrasound at room temperature
for 15 min in toluene containing THF. Under these conditions,
the final solution is homogeneous, showing a good solubility of
1
4000 Caen, France
E-mail: annie-claude.gaumont@ensicaen.fr
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
2 3
Li [CyP•BH ] in a 10:1 toluene/THF solution (page 12S in SI).
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
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COMMUNICATION
The structural investigations of the two dilithiated phosphido-
2 3
borane Li [RP•BH ] freshly prepared above were conducted on
[
8]
the basis of the lithium NMR data with the aim of locating the
7
two lithium cations. Note that in the following, Li NMR is used
6
by default, but we also resorted to Li NMR (deprotonating the
phosphine-boranes with 6Li-labelled n-BuLi[5a]
strengthen the conclusion derived from the observations via Li
NMR or reduce the possibility of peak overlapping.[9]
)
to either
7
Previous work on the structure of a secondary lithium
phosphido-borane (Li[Ph P•BH
2 3
][5a]) showed that the lithium
atom is exclusively next to the borane group. This has led to
substantiating the dual reactivity observed for such species. We
wondered if this sole BH
Starting with Li [PhP•BH
3
---Li link remains within Li
2
[RP•BH
3
].
7
2
3
] in THF, the Li NMR spectrum
obtained at 20 °C shows a sole signal at δ7Li 0.88 ppm (page
6
1
3S in SI). Studying the Li congener did not provide any change,
Figure 2. 6Li- H HOESY spectrum of 6Li
1
2 3 8
[PhP•BH ] in THF-d at –40 °C.
nor did it when varying the temperature of the analysis from
0 °C to –90, however there was the expected change of the
move of the chemical shift (δ6Li 0.41 ppm, page 13S in SI),
Figure 1). Such observations put into evidence that the two
lithium cations of Li [PhP•BH ] are not distinguished by NMR in
THF, even at very low temperature.
2
(
2
3
2 3
Figure 3. The two possible locations of the Li cations of Li [PhP•BH ]
We then completed our study resorting to a 1H rDOSY
experiment designed to evaluate the aggregation/solvation state
of Li [PhP•BH
2 3
] in THF at –40 °C.[10] A diffusion coefficient-
formula weight correlation analysis in the presence of four
internal references (benzene = BEN, cyclooctene = OCT,
tetradecene = TDE and squalene = SQU) led us to determine a
molecular weight MWexp of 473 (Table 1 and page 14S in SI).
This value points toward either a penta-solvated monomer (MWth
494, –4.4% error) or a tri-solvated dimer (MWth 508, –7.0%
error).
Table 1. 1H Referenced DOSY: Theoretical (MWth) and experimental (MWexp
)
-
1
molecular weight (g.mol ) of internal references
+
putative Li
2 3
[PhP•BH ]
monomer (MWth = 134) and dimers (MWth = 268) in THF-d
8
(MWth = 80).
entry
Compound
BEN
MWth
78.1
110
MWexp
Err (%)
Figure 1. VT 6Li-NMR spectra of Li
1
2
3
4
5
78.6
111
190
418
+1
+1
–3
+2
2
[PhP•BH
3
] in THF-d
8
OCT
The fact that this Li signal corresponds to
a singlet
regardless of the temperature and the isotope suggests that
TDE
196
Li
2
[PhP•BH
3
] would not comprise P-Li bonds. Nevertheless, the
SQU
410
6
1
Li- H HOESY spectrum recorded at –40 °C (Figure 2) shows
that the Li signal strongly correlates with the BH
3
group, a result
Li
Li
2
[PhP•BH
[PhP•BH
3
]+4THF
]+5THF
414
494
+12.5
–4.4
2
3
similar with our previous observations on secondary phosphido-
boranes.[ and more modestly, but in a non-negligible way, also
with the ortho protons of the phenyl appendage. If these results
allow us to discard the option of a separated ion pair, two
possible dispositions of the lithium cations with respect to the
473
5a]
6
{Li
Li
2
[PhP•BH
[PhP•BH
3
]}
]}
2
+2THF
+3THF
428
+9.5
{
2
3
2
508
–7.0
Considering that there are four Li+ in a dimeric structure, it
seemed unlikely in the absence of significant steric hindrance,
that three THF molecules could suffice to fully stabilize the four
[
PhP•BH
P-Li scalar coupling: i) the two Li atoms are equivalent, being in
interaction with both the phenyl and the BH groups (Figure 3,
left); ii) one Li interacts with the phenyl and the other with the
BH , and they undergo a rapid exchange process (Figure 3, left).
3
] moiety are to be considered to fit with the absence of
3
cations.
Therefore,
the
penta-solvated
monomer
Li [PhP•BH
2
3
]•5THF should be the most likely species in THF.
3
Note that an accurate measurement of the solvation values by
DOSY techniques is, as for other organolithium species,[
limited by the dynamic of Li-THF interactions.
11]
At this stage, instrumental and technical limitations do not allow
deciding between these two options.
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Chemistry - A European Journal
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COMMUNICATION
With regards to the cyclohexyl congener, a VT 7Li-NMR
each other or with solvents. However, the low-field Li-signal is a
triplet, signifying that this lithium atom is surrounded by two
experiment (Figure 4) was carried out first on Li
2 3
[CyP•BH ] in the
1
0:1 Tol-d /THF-d solution. This time, two different Li peaks
8
8
equivalent P-atoms. Such information is in favour of
a
were identified below –20 °C with an integration ratio of one to
one. Interestingly, the downfield signal is a triplet with a 46 Hz
coupling constant. This value is consistent with the one-bond
{Li [CyP•BH ]} cyclic oligomer (Figure 6), which is an
2
3 n
hypothesis that is well-oriented to the general landscape of
lithiated species into solution where cyclic dimers and trimers
are commonplace.[13] Again, instrumental limitations preclude
coupling between Li and 31P (1J7Li-31P) reported by Reich (≈ 43-
7
PLi).[
12]
verifying this hypothesis by performing a Li{ P} experiment.
We therefore decided to repeat the VT experiment with a 6Li-
enriched Li [CyP•BH ] sample (page 15S in SI) to assert, at
least, that the lithium splitting pattern is really due to a Li-P
7
31
[14]
4
8 Hz) in the case of lithium diphenylphosphide (Ph
Thus, the structure of Li [CyP•BH ] in 10:1 Tol-d /THF-d
contain a P-Li bond observed for the first time in the context of
2
2
3
8
8
would
2
3
7
1
such compounds. A Li- H HOESY spectra obtained at –30 °C
Figure 5) shows that the upfield peak (a singlet) exhibits
through-space dipolar interactions with the BH hydrides.
1
1
(
coupling. Indeed, the ratio of J7Li-31P to J6Li-31P should be more
7
6
3
or less equal to the quotient of the gyromagnetic ratio of Li to Li
6
that is 2.64. We prepared Li
2
[CyP•BH
3
] by adding 0.5 equivalent
6
of CyPH
/THF-d
2
•BH
3
to n-Bu Li in the same solvents mixture (10:1 Tol-
). Again, two broad peaks showed up in the 6Li NMR
d
8
8
spectrum below –20 °C, the downfield peak becoming a sharp
triplet at —110 °C (page 15S in SI).[15] At this temperature, 1J6Li-
1P is found to be 17 Hz and thus, the ratio of 1J7Li-31P / 1J6Li-31P
=
3
2.70 is in fine agreement with our expectations. This further and
strongly supports the hypothesis of a P-Li-P bridging. To check if
the aggregation state would be influenced by the solvent, we
7
7
repeated the Li VT experiment with a Li
2 3
[CyP•BH ] sample in
pure THF-d (page 16S in SI). Similarly, the down-field Li peak is
8
1
2 3
a triplet with J7Li-31P = 44 Hz. Hence, Li [CyP•BH ] forms a cyclic
oligomer, even in pure THF (no monomer is observed while
increasing the THF-concentration) at least at low temperature.
2 3 n
Figure 6. Speculated {Li [CyP•BH ]} cyclic oligomer in THF.
Figure 4. VT 7Li-NMR studies of Li
] in 10:1 Tol-d
/THF-d
2
[CyP•BH
3
8
8
.
To conclude, we investigated the structure in solution of
dilithium phosphido-boranes Li [PhP•BH ] and Li [CyP•BH ].
2
3
2
3
This work shines light on an area of phosphorus chemistry that
has been completely ignored to date, although it opens the door
to a very promising key “RP synthon”, i.e. RP2- phosphandiides.
2
The borane complexation of common RPH primary phosphine
precursors, with R = Ph and Cy, presents the double advantage
of providing sufficient stability to the derived dianions so that
they can be easily handled, and to allow these dianions to be
fully soluble in the selected solvents. Structures in solution have
been investigated for both dilithiated compounds thanks to NMR
studies. Aromatic Li
monomer in THF while a cyclic oligomer would rather be formed
for Li [CyP•BH ] with a Li-P interaction observed for the first time
2 3
[PhP•BH ] is identified as a pentasolvated
2
3
for this class of lithium species. This structural study provides
robust foundations for the chemistry of a novel class of
phosphido-borane species that have synthetic potential in both
organic and inorganic synthesis, and thus lays the foundations
for new synthetic and theoretical investigations in phosphorus
chemistry. Preliminary results obtained in this context by
Figure 5. 7Li- H HOESY spectrum of Li
1
2
3 8 8
[CyPH•BH ] in 10:1 Tol-d /THF-d at –
30 °C.
trapping the above Li
complete conversion of the lithiated starting material, and ii)
intricated final mixtures, in which RPMe •BH is spoted. The
complexity of the results shows that, probably because of the
strong electron density at the phosphorus atom,
2 3
[RP•BH ] with MeI put into evidence i) a
Unfortunately, the aggregation/solvation of Li
2
[CyP•BH
3
] in
2
3
1
0:1 Tol-d /THF-d could not be examined running rDOSY
8
8
experiments because all H peaks are upfield and overlap with
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Chemistry - A European Journal
10.1002/chem.201800742
COMMUNICATION
phosphanediides do not behave as simple nucleophiles.
Mechanistic studies, including EPR spectroscopy, laser flash
photolysis and DFT calculations, are currently underway to
Org. Chem. 2016, 81, 8759–8769; See as well the closely related recent
work: f) M. M. Hansmann, R. Jazzar, G. Bertrand, J. Am. Chem. Soc.
2
016, 138, 8356-8359; g) S. Kundu, B. Li, J. Kretsch, R. Herbst-Irmer, D.
M. Andrada, G. Frenking, D. Stalke, H. W. Roesky, Angew. Chem. Int. Ed.
017, 56, 4219-4223.
2 3
confirm the unique reactivity of Li [RP•BH ].
2
[
5] See for instance a) G. Barozzino-Consiglio, P. Queval, A. Harrison-
Marchand, A. Mordini, J.-F. Lohier, O. Delacroix, A.-C. Gaumont, H.
Gérard, J. Maddaluno, H. Oulyadi, J. Am. Chem. Soc. 2011, 133,
Acknowledgements
The authors are thankful to the Labex SynOrg (ANR-11-LABX-
029) and the Région Normandie for having financed this project
6472−6480; b) I. Abdellah, E. Bernoud, J.-F. Lohier, C. Alayrac, L. Toupet,
0
C. Lepetit, A.-C. Gaumont, Chem. Commun. 2012, 48, 4088-4090; c) K.
Jouvin, R. Veillard, C. Theunissen, C. Alayrac, A.-C Gaumont, G. Evano,
Org. Lett. 2013, 15, 4592-4595; d) L. Routaboul, F. Toulgoat, J. Gatignol,
J-F Lohier, B. Norah, O. Delacroix, C. Alayrac, M. Taillefer, A.C. Gaumont,
Chem. Eur. J., 2013, 19 , 8760-8764. e) K. Rousée, X. Pannecoucke, A.-
C. Gaumont, J.-F. Lohier, F. Morlet-Savary, J. Lalevée, J.-P. Bouillon, S.
Couve-Bonnaire, S. Lakhdar, Chem. Commun. 2017, 53, 2048-2051; e) J.
Dupré, A.-C. Gaumont, S. Lakhdar, Org. Lett. 2017, 19, 694-697.
6] T. Imamoto, T. Oshiki, T. Onozawa, T. Kusumoto, K. Sato, J. Am. Chem.
Soc. 1990, 112, 5244-5252.
both in terms of wages (JG and RD) and material resources.
Normandie Université (HO, ACG and AHM) and CNRS (JM and
SL) are also thanked for financial support.
Keywords: primary phosphine • phosphandiide • phosphido-
borane • RP-synthon • NMR structure
[
[
[
[
[
1] M. Fustier-Boutignon, N. Mézailles, Top. Organomet. Chem. 2014, 47, 63-
27.
1
7] K. Izod, C. Wills, E. Anderson, R. W. Harrington, M. R. Probert,
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Neuburger, B. Spingler, Angew. Chem. Int. Ed. 1999, 38, 92-94; b) C. M.
Ong, D. W. Stephan, J. Am. Chem. Soc. 1999, 121, 2939-2940; c) A.
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8] The main goal here is to study the structure in solution and in situ of the
reactive species, which explains that solid state analyses by X-ray
crystallography is not here put forward as a priority. Indeed an NMR-
based analytical strategy warrants that the data obtained correspond to
the best possible description of the structures in solution, the closest and
most representative observation, which could be made in the reaction
media. By contrast, if solid state structures, and X-ray crystallographic
data in particular, are extremely useful and bring fundamental information
on structural arrangements, this analytical technique is not necessarily
representative of the situation in solution. It brings part of the information
but ignores the dynamic that takes place inevitably in the solvent.
However, with the aim to be as complete as possible, both
phosphanediide-boranes have been isolated as white solid powders with
the hope to isolate crystals by varying the solvent ratios and temperatures,
but we failed in this perspective. We did not pursue in this direction by
adding extra-ligands to chelate the lithium cation, a classical solution in
such situations, since we tried to stay as close as possible to standard
reaction conditions.
6385; f) L. Orzechowski, G. Jansen, S. Harder, Angew. Chem. Int. Ed.
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Murai, K. Takai, J. Am. Chem. Soc. 2015, 137, 114-117 ; without
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Engl. 1990, 29, 295-296; as well as recent related contributions by Stalke,
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[
9] 7Li results in larger coupling constants than 6Li in the same chemical
environment due to its larger gyromagnetic ratio (γ, 107 rad•s •T ): γ6Li
-1
-1
=
3
.9371 while γ7Li
=
10.3976. So 7Li NMR is ideal to observe the splitting
pattern. However, the line-width of 6Li spectra is narrower than that of 7Li
NMR, because of the smaller quadrupole moment of 6Li. Therefore, 6Li
NMR is selected for the samples containing different lithium atoms to
minimize peak overlapping.
[
3] a) M. Driess, H. Pritzkow, S. Martin, S. Rell, D. Fenske, G. Baum, Angew.
Chem. Int. Ed. Engl. 1996, 35, 986-988; b) M. Driess, S. Rell, H. Pritzkow,
R. Janoschek, Chem. Commun. 1996, 305-306; c) M. Westerhausen, M.
Krofta, A. Pfitzner, Inorg. Chem. 1999, 38, 598-599; d) M. Driess, U.
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[10] a) D. Li, I. Keresztes, R. Hopson, P. G. Williard, Acc. Chem. Res. 2009,
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8
854-8860.
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phosphines RPH in which the stabilization is governed by
[
12475-12479.
2
[
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available and should be built on demand.
stereoelectronic effects associated to R. These achievements have
significantly contributed to the renaissance of primary phosphine
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limitation for further developments. See a) N. J. Goodwin, W. Henderson,
B. K. Nicholson, J. Fawcett, D. R. Russell, Dalton Trans. 1999, 1785–
[
[
[
7
1793; b) B. Stewart, A. Harriman, L. J. Higham, Organometallics 2011, 30,
15] The difference between the signals observed for the 7Li and 6Li NMR
spectra at same temperature is most probably due to a same dynamic of
exchange observed at different resonance frequencies (192 MHz and 73
MHz for these two isotopes, respectively).
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Chemistry - A European Journal
10.1002/chem.201800742
COMMUNICATION
COMMUNICATION
Aryl and alkyl phosphandiides,
Jie Guang, Romain Duwald, Jacques
Maddaluno, Hassan Oulyadi, Sami
*
stabilized by complexation with
a
*
borane group, are synthesised and
their structure in THF-solution is
reported. A monomer is identified for
the gem-dilithium phosphido-boranes
Lakhdar, Annie-Claude Gaumont, Anne
Harrison-Marchand*
Page No. – Page No.
Synthesis and Identification of Aryl
and Alkyl Gem-Dilithium Phosphido-
Boranes: A Boost to the Chemistry of
Phosphandiides
Li
2
[PhP•BH
3
]
while
2 3
Li [CyP•BH ]
arranges as a cyclic oligomer. This
study highlights a new type of handy
reagents and opens the door to a new
concept of “RP synthons” likely to be
the most flexible precursors of
multitude of tertiary phosphines.
a
This article is protected by copyright. All rights reserved.