Facile Phenylphosphinidene Transfer Reactions from Carbene–Phosphinidene Zinc Complexes

Article abstract of DOI:10.1002/anie.201703672

Phosphinidenes [R-P] are convenient P₁ building blocks for the synthesis of a plethora of organophosphorus compounds. Thus far, transition-metal-complexed phosphinidenes have been used for their singlet ground-state reactivity to promote selective addition and insertion reactions. One disadvantage of this approach is that after transfer of the P₁ moiety to the substrate, a challenging demetallation step is required to provide the free phosphine. We report a simple method that enables the Lewis acid promoted transfer of phenylphosphinidene, [PhP], from NHC=PPh adducts (NHC=N-heterocyclic carbene) to various substrates to produce directly uncoordinated phosphorus heterocycles that are difficult to obtain otherwise.

Full text of DOI:10.1002/anie.201703672

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Accepted Article
Title: Facile Phenylphosphinidene Transfer Reactions from Carbene-
Phosphinidene Zinc Complexes
Authors: Chris Slootweg, Tetiana Krachko, Mark Bispinghoff, Aaron
Tondreau, Daniel Stein, Matthew Baker, Andreas Ehlers, and
Hansjörg Grützmacher
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201703672
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Facile Phenylphosphinidene Transfer Reactions from Carbene-
Phosphinidene Zinc Complexes
Tetiana Krachko, Mark Bispinghoff, Aaron M. Tondreau, Daniel Stein, Matthew Baker, Andreas W. Ehlers,
J. Chris Slootweg,* and Hansjörg Grützmacher*
Dedicated to Professor Evamarie Hey-Hawkins on the Occasion
of her 60th Birthday
using free carbenes. Reaction of 1,3-dimethylimidazolium iodide with
(PPh)5 and sodium tert-butoxide in THF afforded 1 after a simple work-
up (removal of all volatiles in vacuo, extraction into toluene and
crystallization) as an orange solid in 95% yield. The molecular structure
of 1 (δ31P (C6D6) = –49.1 ppm; Scheme 1, right),8 when compared to the
Abstract: Phosphinidenes [R–P] are convenient P1 building blocks for the
synthesis of
a plethora of organophosphorus compounds. Thus far,
bulkier MeNHC=PMes reported by Hey-Hawkins et al. (δ31P (C6D6) = –
transition-metal complexed phosphinidenes have been used for their singlet
ground-state reactivity, promoting selective addition and insertion reactions.
One disadvantage of this approach is that after transfer of the P1 moiety to
the substrate a challenging demetallation step is required to provide the free
phosphine. Here we report a simple method which allows the Lewis acid-
promoted transfer of phenylphosphinidene, [PhP], from NHC=PPh adducts
(NHC = N-heterocyclic carbene) to various substrates to produce directly
uncoordinated phosphorus heterocycles that are difficult to obtain otherwise.
9
73.8 ppm),
displays
a
slightly elongated P1–C1 bond
[1.7917(14)/1.7911(15) vs. 1.768(4) Å], a more acute C1–P1–C6 angle
[101.30(6)/99.98(7) vs. 104.6(2)°], and a larger dihedral angle between
the imidazole plane and the P1–C6 bond [48.01(16)/50.42(15) vs.
32.5(4)°], indicating considerable ylide character.9 Beside the nature of
the carbene moiety,10 also the P-substituent has a marked influence on
the structure 11 and thus the δ31P chemical shift of the carbene-
phosphinidene adduct. MesNHC=PPh D12 (93%) and DippNHC=PPh10
(87%) were also conveniently prepared by this method, complementing
the known procedures for making carbene-phosphinidene adducts.13
Considerable efforts have been devoted to the development of
phosphinidene transfer agents 1 , 2 which do not require a metal. 3
Cummins and co-workers developed the unprotected dibenzo-7λ3-
phosphanorbornadiene A as a source of the transient [iPr2NP] that
undergoes a [1+4]-cycloaddition with 1,3-cyclohexadiene.4 Driess and
co-workers successfully transferred the parent phosphinidene, [HP],
from phosphasilene
B (Dipp = 2,6-diisopropylphenyl) to a N-
heterocyclic carbene, 5 and Weber et al. demonstrated the acyclic
carbene-phosphinidene adducts C (R = tBu, Cy, 1-Ad, Ph, Mes) to be
viable [RP] transfer agents to diphenylketene.6 Interestingly, Arduengo
and co-workers showed that the Lewis acid BPh3 induces formation of
cyclopolyphosphines (PPh)n (n = 3–5) from MesNHC=PPh D,7 but no
transfer reactions of the extruded phenylphosphinidene moiety have
been reported to date. This inspired us to target the sterically little
encumbered MeNHC=PPh (1; Scheme 1) and study its ability to transfer
[PhP] to suitable substrates in the presence of an appropriate Lewis acid.
Scheme 1. Synthesis of MeNHC=PPh (1; left) and molecular structure (right; hydrogen
atoms and C6H6 solvent molecule are omitted for clarity, one crystallographic
independent molecule is shown). Selected bond lengths [Å] and angles [°] (values for
the second molecule in square brackets): P1–C1 1.7917(14) [1.7911(15)], P1–C6
1.8157(16) [1.8132(14)], N1–C1 1.359(2) [1.3546(19)], N2–C1 1.3562(18) [1.356(2)];
C1–P1–C6 101.30(6) [99.98(7)], N1–C1–N2 104.82(11) [105.15(13)]; N2–C1–P1–C6
48.01(16) [50.42(15)].
Next, we targeted the synthesis of a Lewis acid adduct of 1 that can
controllably release phenylphosphinidene and, ideally, simultaneously
capture the free carbene. Treatment of 1 with BPh3, AlCl3, MgCl2 or
Zn(OAc)2 merely resulted in the direct formation of
cyclopolyphosphines, consistent with Arduengo’s observations.7,14 But,
slow addition of 0.5 equiv of ZnCl2 to a THF solution of 1 afforded zinc
adduct 2 (12·ZnCl2) as a pale yellow precipitate (76%; Scheme 2, top),
which is poorly soluble in common organic solvents. Single crystals
suitable for X-ray diffraction analysis were obtained from DME, which
unequivocally established the formation of a 2:1 complex of a carbene
phosphinidene adduct (Scheme 2, bottom).8, 15 Upon complexation,
significant structural changes occur: the central P–C bonds [1.818(4),
1.827(4) Å] become longer, the C–P–C angles more acute [100.2(2),
100.8(2)°] and the dihedral angles smaller [31.1(4), 42.9(4)°] (Scheme
2, bottom).8 Reaction of 1 with 1 equiv of ZnCl2 in THF afforded the
soluble 1:1 adduct 3, which was isolated as colorless crystals in 87%
yield (Scheme 2, top). The structure of 3 in the solid state shows a di-
zinc complex that can be related to the one of 2 by adding ZnCl2(THF)
to P2 such that one MeNHC=PPh ligand bridges via P2 two Zn centers,
while the other MeNHC=PPh unit takes a terminal position (Scheme 2,
bottom).8 Consequently, the phosphorus atoms are inequivalent, yet in
First, we developed a scalable, efficient one-pot synthesis of
carbene-phosphinidene adduct MeNHC=PPh (1) that avoids the need of
[a]
[b]
T. Krachko, Dr. A. W. Ehlers, Assoc. Prof. Dr. J. C. Slootweg
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904, PO Box 94157, 1090 GD Amsterdam, The Netherlands
E-mail: j.c.slootweg@uva.nl
M. Bispinghoff, Dr. A. M. Tondreau, Dr. D. Stein, M. Baker, Prof. Dr. H.
Grützmacher
Laboratory of Inorganic Chemistry, ETH Zürich, Vladimir-Prelog-Weg 1,
8093 Zürich, Switzerland
E-mail: hgruetzmacher@ethz.ch
[c]
Dr. A. W. Ehlers, Department of Chemistry, Science Faculty, University of
Johannesburg, PO Box 254, Auckland Park, Johannesburg, South Africa
Supporting information for this article is given via a link at the end of the
document.
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solution, even at –80 °C, only one δ31P resonance was observed in THF-
d8 at –88.1 ppm indicating that in solution 3 has a different structure
(either monomeric or a dimer with symmetric cyclic Zn2P2 core).
Scheme 3. Reactivity of 3 towards phenanthrene-9,10-quinone, diphenylketene, and
trans-chalcone.
Scheme 2. Synthesis of ZnCl2 complexes 2 and 3 (top) and molecular structures
(bottom) of 2 (DME solvent molecule omitted) and 3 (THF solvent molecule omitted).
Selected bond lengths [Å] and angles [°] for 2: Zn1–P1 2.3959(11), Zn1–P2 2.3867(11),
P1–C1 1.818(4), P1–C6 1.826(4), P2–C12 1.827(4), P2–C17 1.841(5); C1–P1–C6
100.2(2), C12–P2–C17 100.8(2); N2–C1–P1–C6 31.1(4), N4–C12–P2–C17 42.9(4). 3:
Zn1–P1 2.3793(14), Zn1–P2 2.4386(13), Zn2–P2 2.4110(14), P1–C1 1.819(4), P1–C6
1.832(4), P2–C12 1.818(4), P2–C17 1.833(4); N2–C1–P1–C6 33.3(4).
Reaction of di-zinc complex 3 with 4 equiv of diphenylketene
afforded phosphorus heterocycles 6 and 7 in a 2:3 ratio (δ31P (THF-d8):
–2.9 and 90.9 ppm, respectively; Scheme 3) together with NHC complex
5. Whereas 1,4,2-dioxaphospholane 7 was reported previously,6 1,3-
oxaphospholan-5-one 6 is new and only derivatives thereof lacking the
exocyclic C=C double bond are known.20,21 The molecular structure of
6 was unambiguously established by single crystal X-ray analysis
(Figure 1).8,16 It shows a five-membered heterocycle resulting from a
formal [1+2+2] cycloaddition, where the endocyclic P1–C9 [1.824(2)
Å] and exocyclic P1–C1 bond [1.826(2) Å] are of similar length, while
the endocyclic P1–C7 bond [1.902(2) Å] is significantly elongated, most
likely due to sterical hindrance of neighboring phenyl rings. Treatment
of 3 with only 2 equiv. of diphenylketene yielded after work-up 10 as an
off-white solid (82%, δ31P (THF-d8) = –16.6 ppm; Scheme 4), which
contained traces of 6 and 7. Single crystals suitable for X-ray diffraction
analysis were obtained from DME, which established the formation of a
1:1 adduct (Scheme 4, right).8 The structural parameters are in accord
with the formula shown in Scheme 4 [P1–C1 1.835(3) Å, P1–C12
1.854(3) Å, C12–O1 1.330(4) Å are single bonds, C12–C13 1.362(4) Å
is a double bond]. Calculations at ωB97X-D/6-31G(d,p)14 reveal that
nucleophilic attack of the phosphorus atom of 3 at the ketene’s carbonyl
carbon first gives adduct 9 (G = 11.0 kcal·mol-1; Ga = 13.1 kcal·mol-
1; Scheme 4, top left), which affords 10 after ZnCl2 transfer from P to O
(G = –18.3 kcal·mol-1; Ga = 13.1 kcal·mol-1). Furthermore, we
confirmed experimentally that intermediate 10 is able to react with either
the C=C or C=O double bond of another equiv of diphenylketene
forming 6 and 7, after extrusion of Zn complex 5.
The soluble Lewis adduct 3 was tested as phosphinidene transfer
agent with phenanthrene-9,10-quinone, diphenylketene and trans-
chalcone as suitable acceptors (Scheme 3). 16 Treatment of 3 with
phenanthrene-9,10-quinone in THF afforded phosphonite 4 as a pale
green solid (46%; δ31P (C6D6) = 183.3 ppm), which was characterized
by single-crystal X-ray diffraction analysis (Figure 1).8 Previously, 4 has
only been accessible via thermal fragmentation of the corresponding
phosphorane.17 Heterocycle 4 was devoid of zinc chloride, which was
transferred to the carbene affording NHC complex 5 as an insoluble,
colorless, crystalline solid.18 The molecular structure of 5 displays a one-
dimensional coordination polymer with unusual zigzag Zn−chloride
chains with almost identical Zn−Cl distances [Zn1−Cl2 2.3670(11),
Zn2−Cl2 2.3597(12) Å] and the NHC moiety positioned orthogonally to
the main chain [Zn1–C1 2.021(6) Å] (Figure 1).8,19
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1.838(4), P1–C12 1.854(3), O1–C12 1.330(4), C12–C13 1.362(4); O1–C12–C13
126.3(3).
Treatment of 3 with 2 equiv. of trans-chalcone afforded the rare
tricoordinate oxo-3-phospholene 8 as a single diastereomer in 80% yield
(δ31P (C6D6) = 133.9 ppm; Scheme 3), simply after filtering off
[
MeNHC·ZnCl2]n 5 (83%), extraction into toluene and crystallization.
Such an unprotected, five-membered heterocycle has only been prepared
once before, via two-step procedure using an electrophilic
a
phosphinidene complex followed by demetallation.22 The molecular
structure of 8 firmly established the phenyl rings to be in trans position
and shows that the third phenyl ring (on C3) is in conjugation with the
C1–C2 double bond in the ring [C2–C3–C10–C11 –3.2(3)°] (Figure 1).8
We resorted again DFT calculations to provide insight into the formation
of 8.14 In contrast to the ketene, where the carbonyl carbon is attacked
first, now the reactions start with coordination of trans-chalcone to 3 by
Zn–O bond formation, which selectively affords 8 after P–C bond
formation and subsequent ring closure by P–O bond formation and
elimination of (NHC)Zn complex 5 (see the Supporting Information).
Figure 1. Molecular structures of 4, 5 (only a fragment is shown), 6 and 8. Selected
bond lengths [Å] and angles [°] for 4: P1–O1 1.6859(19), P1–O2 1.675(2), P1–C1
1.827(3), C7–C8 1.347(4). 5: Zn1–Cl1 2.2179(18), Zn1–Cl2 2.3670(11), Zn1–Cl2a
2.3670(11), Zn2a–Cl2a 2.3597(12), Zn1–C1 2.021(6), N1–C1 1.329(8), N2–C1
1.377(8); 6: P1–C1 1.826(2), P1–C7 1.902(2), P1–C9 1.824(2), O1–C8 1.365(2), O1–
C9 1.415(2), O2–C8 1.197(2), C7–C8 1.526(3), C9–C22 1.340(3); 8: P1–O1 1.6636(13),
P1–C1 1.876(2), P1–C16 1.8241(18), O1–C3 1.400(2), C1–C2 1.503(3), C2–C3
1.325(3); C2–C3–C10–C11 –3.2(3).
In summary, the sterically little hindered carbene phosphinidene
adduct MeNHC=PPh allows the synthesis of new zinc complexes, of
which the soluble Lewis adduct
3
selectively transfers
a
phenylphosphinidene fragment providing access to uncoordinated
phosphorus heterocycles. The driving force for these reactions is likely
the formation of the insoluble coordination polymer [MeNHC·ZnCl2]n 5,
which explains why only ZnCl2 proved to be efficient to date. Highly
reactive or unstable main group fragments can be stabilized by NHCs,23
however, their transfer to other substrates has been very rarely
observed.24 The Lewis acid promoted transfer reaction may help to
develop this chemistry further.
Acknowledgements
This work was supported by the European Union (Marie Curie ITN
SusPhos, Grant Agreement No. 317404). Prof. Dr. David Scheschkewitz
is gratefully acknowledged for fruitful discussions.
Scheme 4. Synthesis of intermediate 10 (left) and molecular structure of 10 (right;
hydrogen atoms, one disordered DME molecule, and toluene solvent are omitted for
clarity). Selected bond lengths [Å] and angles [°] for 10: P1–C1 1.835(3), P1–C6
Keywords: adducts • N-heterocyclic carbenes • phosphinidenes •
Lewis acids • phosphorus heterocycle
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Entry for the Table of Contents
COMMUNICATION
Tetiana Krachko, Mark Bispinghoff, Aaron
M. Tondreau, Daniel Stein, Matthew Baker,
Andreas W. Ehlers, J. Chris Slootweg,* and
Hansjörg Grützmacher*
Page No. – Page No.
Facile Phenylphosphinidene Transfer
Reactions from Carbene-Phosphinidene
Zinc Complexes
Phosphinidene transfer: ZnCl2 promoted phosphinidene transfer reactions of the sterically
unencumbered carbene-phosphinidene adduct MeNHC=PPh to various substrates (S) are
demonstrated. These unprecedented reactions provided access to new uncomplexed
phosphorus heterocycles, which are difficult to obtain otherwise.
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