Umpolung of methylenephosphonium ions in their manganese half-sandwich complexes and application to the synthesis of chiral phosphorus-containing ligand scaffolds

Article abstract of DOI:10.1002/anie.201501256

Half-sandwich manganese methylenephosphonium complexes [Cp(CO)₂Mn(η²-R₂PC(H)Ph)]BF₄ were obtained in high yield through a straightforward reaction sequence involving a classical Fischer-type manganese complex and a secondary phosphine as key starting materials. The addition of various nucleophiles (Nu) to these species took place regioselectively at the double-bonded carbon center of the coordinated methylenephosphonium ligand R₂P⁺C(H)Ph to produce the corresponding chiral phosphine complexes [Cp(CO)₂Mn(κ¹-R₂PC(H)(Ph)Nu)], from which the phosphines were ultimately recovered as free entities upon simple irradiation with visible light. The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer-type phosphine-NHC-phosphine ligand architectures. Reactivity trend reversal: Methylenephosphonium salts coordinated to a manganese center were found to exhibit "iminium-like" reactivity towards nucleophiles (see scheme). This unprecedented umpolung enables the instant formation of carbon-heteroatom bonds, highlighted by the synthesis of novel chiral pincer-type phosphine-NHC-phosphine ligands.

Full text of DOI:10.1002/anie.201501256

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201501256
German Edition: DOI: 10.1002/ange.201501256
Manganese Complexes
Umpolung of Methylenephosphonium Ions in Their Manganese
Half-Sandwich Complexes and Application to the Synthesis of Chiral
Phosphorus-Containing Ligand Scaffolds**
Dmitry A. Valyaev,* Oleg A. Filippov, Noꢀl Lugan,* Guy Lavigne, and Nikolai A. Ustynyuk
Abstract: Half-sandwich manganese methylenephosphonium
2
=
complexes [Cp(CO)2Mn(h -R2P C(H)Ph)]BF4 were obtained
in high yield through a straightforward reaction sequence
involving a classical Fischer-type manganese complex and
a secondary phosphine as key starting materials. The addition
of various nucleophiles (Nu) to these species took place
Scheme 1. Respective regioselectivities observed in the addition of
nucleophiles to iminium (left), methylenephosphonium salts (center),
and manganese methylenephosphonium complexes (right,
{Mp}=CpMn(CO)2).
regioselectively at the double-bonded carbon center of the
+
=
coordinated methylenephosphonium ligand R2P C(H)Ph to
produce the corresponding chiral phosphine complexes
1
[Cp(CO)2Mn(k -R2P C(H)(Ph)Nu)], from which the phos-
MeOꢀ)[3a,b,4a] at the electropositive phosphorus center
(Scheme 1). In attempts to explore the reactivity of manga-
ꢀ
phines were ultimately recovered as free entities upon simple
irradiation with visible light. The synthetic potential of this
umpolung approach is illustrated herein by the preparation of
novel chiral pincer-type phosphine–NHC–phosphine ligand
architectures.
nese methylenephosphonium complexes [Cp(CO)2Mn-
2
=
(h -R2P C(H)Ph)]BF4, readily available from half-sandwich
MnI Fischer carbenes and secondary phosphines as key
starting materials, we observed that the coordinated methyl-
enephosphonium moiety undergoes regioselective addition of
nucleophiles at the carbon center, thus mimicking the
reactivity mode of iminium salts (Scheme 1; in this scheme
and thereafter {Mp} = Cp(CO)2Mn). This key observation
inspired the development of an unconventional synthetic
methodology toward a wide variety of organophosphorous
derivatives, as illustrated herein by a simple route to a novel
family of chiral pincer-type phosphine–NHC–phosphine
ligands (NHC = N-heterocyclic carbene).
I
minium salts are of utmost importance in organic syn-
thesis,[1] owing in particular to their key role as intermediates
in many enantioselective organocatalytic transformations of
carbonyl compounds.[2] However, the synthetic utility of their
phosphorus analogues, methylenephosphonium ions,[3,4]
remains limited to
a
few examples of [2+2][4h,j] and
[2+4][4b,e,g] cycloaddition, the ene reaction,[4b,e] electrocyclic
cyclization with an arene ring,[4e,f] stereoselective rearrange-
ment to give phosphaalkenes,[4d] and the phospha-Stork
reaction.[4l] Whereas the reactivity of iminium salts is gov-
erned by the strong electrophilicity of the carbon atom
(Scheme 1), the methylenephosphonium salts undergo regio-
selective addition of nucleophiles (Nu = Fꢀ, Clꢀ, and
Earlier investigations in our laboratories had previously
+
ꢁ ꢀ
established that the cationic manganese carbyne [{Mp}
C
Ph]BPh4 ([2]BPh4) reacts sequentially with diphenylphos-
phine and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to give
the h1-phosphinocarbene complex 3a, which undergoes facile
CO insertion to form the h3-phosphinoketene complex 4a
(see structures in Scheme 2).[5] We have now found that
further protonation of 4a with HBF4·OEt2 leads to the
quantitative formation of the cationic complex [5a]+ with
a h2-methylenephosphonium ligand (Scheme 2).[6,7] This reac-
tion is a rare example of controlled CO deinsertion from
a ketene ligand in a mononuclear complex, a reaction
[*] Dr. D. A. Valyaev, Dr. N. Lugan, Dr. G. Lavigne
Laboratoire de Chimie de Coordination du CNRS (UPR 8241)
205 route de Narbonne, BP 44099, 31077 Toulouse Cedex 4 (France)
and
Universitꢀ de Toulouse
UPS, INPT, 31077 Toulouse Cedex 4 (France)
E-mail: dmitry.valyaev@lcc-toulouse.fr
noel.lugan@lcc-toulouse.fr
previously only explicitly induced by chloride abstraction
2
= =
within the complex [(tBu2PCH2PtBu2)(Cl)Ir(h -Ph2C C O)]
+ [8,9]
Homepage: http://www.lcc-toulouse.fr/lcc/spip.php?article24
=
to afford [(tBu2PCH2PtBu2)(CO)Ir CPh2] .
Dr. O. A. Filippov, Prof. Dr. N. A. Ustynyuk
A. N. Nesmeyanov Institute of Organoelement Compounds
(INEOS), Russian Academy of Sciences
A DFT study of the protonation-induced CO deinsertion
in 4a with CF3OH as a model of a strong acid revealed that
the reaction occurs through the initial formation of a hydro-
gen-bonded adduct involving the oxygen atom of the ketene
28 Vavilov Street, GSP-1, B-334, Moscow (Russia)
[**] Financial support from the CNRS and the PICS grant 4873 (CNRS)/
09-03-91060 (RFBR) is gratefully acknowledged. O.A.F. is grateful to
the Russian Scientific Foundation (grant 14-13-00801) for financial
support of DFT calculations. D.A.V. thanks the IDEX-UNITI for
a starting grant.
ligand, followed by proton transfer to the Cb atom with
+
ꢀ
concomitant Ca Cb bond cleavage to afford [5a] (see the
Supporting Information for full details). The calculated TS
energy of 3.8 kcalmolꢀ1 (SMD method, CH2Cl2) is consistent
with the experimental observation of a rapid transformation
4a![5a]+ at ꢀ808C.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501256.
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Scheme 2. Synthesis of manganese methylenephosphonium complexes
[5]+: i) BCl3, hexane, ꢀ808C; ii) R2PH/iPr2NEt, THF, ꢀ808C; iii) filtra-
tion through Al2O3, THF, ꢀ808C!RT; iv) extraction 1m aqueous
NaOH/toluene, RT; v) HBF4·OEt2, CH2Cl2, ꢀ808C!RT. Cy=cyclohexyl.
Figure 1. A perspective view of the cationic part of [5a]BF4 (ellipsoids
shown at the 30% probability level) and proposed limiting resonance
forms A and B for the bonded h2-methylenephosphonium ligand.
Contrary to the majority of free methylenephosphonium
salts, the solid methylenephosphonium complexes [5]+ are air-
stable and almost insensitive to moisture.[10] In solution,
however, they react cleanly with a variety of nucleophiles to
form the corresponding manganese k1-phosphine complexes
of the type 6–10 in very high yield, as illustrated for complex
[5a]+ in Scheme 3. Notably, complex [5a]+ was found to
ꢀ
(2.2405(7) ꢀ) compares well with the Mn P distance found in
the archetypal phosphine complex [Cp(CO)2Mn(PPh3)]
(2.236(3) ꢀ),[14] the Mn C bond (Mn1 C3 = 2.257(2) ꢀ)
was found to be significantly elongated relative to those
observed for h2-alkene analogues (CCDC average length of
2.17 ꢀ). Taken together, these structural features reflect
a contribution to the structure of [5a]+ of the limiting
resonance form B, in which a phosphorus lone pair pointing to
the metal atom is developing along with a benzyl-type
carbocation likely to be captured by incoming nucleophiles
Nu(H) to afford the isolated manganese phosphine com-
plexes 6–10 (Scheme 3). Although in the case of Clꢀ an
alternative reversible addition to the phosphorus atom in [5]+
ꢀ
ꢀ
ꢀ
+
ꢀ
ꢀ
to form the zwitterionic adduct [{Mp} C(H)Ph P Ph2Cl]
cannot be excluded within this model, the overall reaction
outcome is thermodynamically controlled owing to the
irreversible formation of the highly stable manganese phos-
Scheme 3. Reactions of manganese complex [5a]+ with nucleophiles.
Mes=mesityl (2,4,6-trimethylphenyl).
phine complex 7. For neutral nucleophiles, the initial for-
ꢀ
ꢀ
ꢀ
alkylate methanol in the absence of an added base under mild
conditions (room temperature, THF, 24 h) to afford 6. Further
reactions with other nucleophiles, either anionic (Clꢀ) or
neutral (2,4,6-trimethylaniline, PPh3, tetramethylthiourea),
were found to take place almost instantaneously in solution in
dichloromethane at room temperature. Clearly, all these
complexes result from the addition of the nucleophile to the
carbon atom of the h2-methylenephosphonium moiety, thus
revealing an inverse regioselectivity relative to that observed
for free methylenephosphonium salts (Nu = Fꢀ, Clꢀ, and
MeOꢀ)[3a,b,4a] and in the isolated case of a [W(CO)5] complex
mation of similar net cationic adducts [{Mp} C(H)Ph
+
+
ꢀ
P Ph2 Nu H] is even more unlikely.
With the aim to exploit the synthetic potential of complex
[5a]+, we next focused on its large-scale preparation. Its
precursor 4a was obtained on a 20 mmol scale from the
Fischer carbene 1, Ph2PH, and the Hꢁnig base and was next
used as the crude product in the final protonation step to
afford [5a]+ in 67% overall yield from 1 (Scheme 2). The
same procedure was successfully applied to the synthesis of
complex [5b]+ from Cy2PH, albeit in slightly reduced yield.
Having established an economically viable source of
manganese methylenephosphonium complexes,[15] we inves-
tigated their use for the synthesis of valuable organophos-
phorus products. Typically, the reaction of [5a]+ with N-
mesitylimidazole proceeded cleanly and almost instantane-
ously at room temperature in CH2Cl2 to give the phosphine
complex [11]+ (Scheme 4). Further irradiation of the resulting
solution with visible light in the presence of HBF4·OEt2
according to our previously reported procedure[16] produced
in 92% yield the free chiral phosphine [12]+ with a pendent
imidazolium moiety: the possible precursor of a chelating
phosphine–NHC ligand. Notably, access to this type of
(Nu = BF4 ).[11]
ꢀ
Looking for some clues regarding the novel reactivity
pattern of manganese methylenephosphonium complexes, we
performed X-ray diffraction analysis of [5a]+ (Figure 1).[12]
The h2-methylenephosphonium ligand in [5a]+ adopts the so-
called horizontal coordination mode typically found in h2-
alkene complexes in particular.[13] The interatomic C P bond
ꢀ
ꢀ
distance (P1 C3 = 1.735(2) ꢀ) within the coordinated meth-
ylenephosphonium ligand is intermediate between single and
ꢀ
double C P bonds, and very close to those observed in related
tungsten complexes.[6] Whereas the Mn1 P1 bond distance
ꢀ
2
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We further observed that the intermediate monoalkylated
imidazole complex 13a can be prepared in 84% yield under
more statistically favorable conditions, that is, by the slow
addition of a solution of [5a]+ to a solution containing
10 molar equivalents of imidazole (Scheme 5), thus opening
up a route to the synthesis of precursors of unsymmetrical
pincer-type ligands. Indeed, subsequent treatment of isolated
13a with complex [5b]+, followed by chromatographic
separation of the diastereoisomers, gave the complexes
[14a,b]+ in excellent yield, one of which was characterized
by X-ray diffraction (see Figure S2).[12] Photochemical depro-
tection under the usual conditions afforded precursors
[15ab]+ to unsymmetrical pincer-type ligands in about 80%
yield, thus illustrating the modular character of our synthetic
approach (Scheme 5).
Finally, to appraise the benefits of using imidazolium
compounds [15]+ as sources of unprecedented pincer-type
ligands, we treated rac-[15b]+ with [Rh(acac)(CO)2] (acac =
acetylacetonate). Monitoring of the reaction by IR spectros-
copy revealed the displacement of one carbonyl ligand by the
PCy2 moiety at room temperature, followed by the clean
conversion of this intermediate at 658C into the desired
pincer complex rac-[16]+ by the action of the acetylacetonate
ligand as an internal base.[19] The complex rac-[16]+ was
isolated in 90% yield and fully characterized by spectroscopic
methods and by X-ray diffraction (Figure 2).[12]
Scheme 4. Synthesis of the phosphine–NHC preligand [12]+: i) CH2Cl2,
RT, 5 min; ii) visible light, HBF4·OEt2, CH2Cl2, RT, 15 min, then
aqueous NaHCO3.
preligand even with a simple CH2 linker between the
phosphine and the imidazolium moieties would typically
require a tedious multistep synthesis with a moderate overall
yield.[17]
We next observed that the slow addition of one equivalent
of nonsubstituted imidazole to a solution of one of the
complexes [5]+ at room temperature resulted in the precip-
itation of imidazolium tetrafluoroborate along with the
formation of a binuclear manganese complex [14a]+ or
[14b]+ as an equimolar mixture of rac and meso isomers
(Scheme 5). The {Mp} moiety not only stabilizes the methyl-
enephosphonium salts and inverses the regioselectivity of the
nucleophilic addition, but also acts as a phosphine protecting
group and facilitates in particular the separation of the
diastereoisomers of [14]+ by column chromatography on
silica. Finally, photochemical deprotection of the diastereo-
merically pure complexes [14]+ (Scheme 5) afforded four
isomeric phosphine–imidazolium products [15]+ in approx-
imately 80% yield. The latter complexes are precursors to
a novel family of C2- and Cs-symmetric pincer-type phos-
phine–imidazolinylidene–phosphine ligands with single-
carbon-atom spacer between phosphine and NHC frag-
ments.[18] The relative configuration of the stereocenters in
[15]+ was assigned by X-ray diffraction studies for meso-
[15a]BF4 and rac-[14b]BF4 (see Figures S3 and S1, respec-
tively).[12]
In conclusion, we have observed an umpolung of the
methylenephosphonium cation when coordinated to a man-
ganese center. This reactivity offers a simple route to hybrid
backbone-chiral pincer-type phosphine–NHC–phosphine
ligands. The adaptation of our synthetic strategy to the
preparation of optically active manganese methylenephos-
phonium complexes and further applications of the original
phosphine–NHC ligands in homogeneous catalysis are under
investigation.
Scheme 5. Preparation of the pincer-type phosphine–NHC–phosphine preligands [15]+: i) slow addition of imidazole to [5]+, CH2Cl2, RT, 2–4 h;
ii) column chromatography on silica; iii) visible light, HBF4·OEt2, CH2Cl2, RT, 30–40 min, then aqueous NaHCO3; iv) slow addition of [5a]+ to
imidazole (10 equiv), CH2Cl2, RT, 4 h; v) [5b]+, CH2Cl2, RT, 10 min.
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Figure 2. A perspective view of the cationic part of the rhodium pincer-
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level).
Keywords: manganese · methylenephosphonium ligands ·
P ligands · photochemical demetalation · pincer ligands
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4
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[18] Several complexes with similar achiral pincer-type phosphine–
perimidinylidene–phosphine ligands were recently reported:
Received: February 9, 2015
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Manganese Complexes
D. A. Valyaev,* O. A. Filippov, N. Lugan,*
G. Lavigne,
N. A. Ustynyuk
&&&& — &&&&
Reactivity trend reversal: Methylene-
phosphonium salts coordinated to
by the synthesis of novel chiral pincer-
type phosphine–NHC–phosphine
ligands.
Umpolung of Methylenephosphonium
Ions in Their Manganese Half-Sandwich
Complexes and Application to the
Synthesis of Chiral Phosphorus-
Containing Ligand Scaffolds
a manganese center were found to exhibit
“iminium-like” reactivity towards nucleo-
philes (see scheme). This unprecedented
umpolung enables the instant formation
of carbon–heteroatom bonds, highlighted
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!