Nonbenzenoid aromaticity of 1-phosphafulvenes: Synthesis of phosphacymantrenes

Article abstract of DOI:10.1039/d0dt03934a

The coordination chemistry of 1-phosphafulvenes was investigated by employing their [6 + 4] adducts or α-C2-bridged biphospholes as a precursor. Unbridged phosphacymantrenes arise from 1-phosphafulvenes via proton abstraction. α-C2-bridged biphosphacymantrenes are probably yielded by the reductive coupling of 1-phosphafulvene with Mn2(CO)10. The coordination behavior of 1-phosphafulvenes is comparable to that of pentafulvenes, which again demonstrates the phosphorus-carbon analogy in low-coordinate organophosphorus chemistry.

Full text of DOI:10.1039/d0dt03934a

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Nonbenzenoid aromaticity of 1-phosphafulvenes:
synthesis of phosphacymantrenes†
Cite this: Dalton Trans., 2021, 50,
476
Yanjie Liu, Rongqiang Tian, * Zheng Duan * and François Mathey
Received 17th November 2020,
Accepted 10th December 2020
DOI: 10.1039/d0dt03934a
rsc.li/dalton
The coordination chemistry of 1-phosphafulvenes was investigated However, the bulky Mes* group hampered the investigation of
by employing their [6 + 4] adducts or α-C2-bridged biphospholes phosphafulvene chemistry. We have demonstrated that in situ
as a precursor. Unbridged phosphacymantrenes arise from 1-phos- generated unhindered 1-phosphafulvenes can act as 2π, 4π, or
phafulvenes via proton abstraction. α-C2-bridged biphosphacy- 6π precursors in cycloaddition chemistry7 (Scheme 1c). The
mantrenes are probably yielded by the reductive coupling of unusual addition of 1-phosphafulvene to benzoquinone was
1-phosphafulvene with Mn2(CO)10. The coordination behavior of discovered by the same in situ generation strategy,8 but the for-
1-phosphafulvenes is comparable to that of pentafulvenes, which mation of phosphametallocenes by coordination of 1-phospha-
again demonstrates the phosphorus-carbon analogy in low-coor- fulvene with transition metals has never been explored.
dinate organophosphorus chemistry.
Phosphametallocene, the η5 coordination of phospholide9
with a transition metal cation, behaves as a strong π acceptor
and weak σ donor, closer in behavior to CO than to ordinary
phosphines.10 The coordination chemistry of phosphaferro-
cenes has been widely explored and has been successfully
applied in homogeneous catalysis.11 In comparison, less atten-
tion has been paid to phosphacymantrene (phospholylmanga-
nese tricarbonyls) derivatives, which are more stable as com-
pared to phosphaferrocenes in air. Notably, methylcymantrene
is stable enough to be used as a lead-free antiknock derivative
in gasoline and related fuels.12 Thus, it is interesting to
prepare phosphacymantrene from 1-phosphafulvene.
Pentafulvenes are cyclic isomers of benzene with nonbenze-
noid aromaticity and are unique in their rich and varied cyclo-
addition chemistry as well as organometallic chemistry.1
Pentafulvenes can act as 2π, 4π, or 6π systems in cycloaddition
and have been recognized as valuable synthons for the con-
struction of polycycles and natural products1b,2 (Scheme 1a).
Another remarkable feature of pentafulvenes is the π electron
shift from the exocyclic olefin to the endocyclic aromatic Cp
ring,3 which displays a wide array of coordination modes to
metals. In particular, the reductive dimerization of fulvenes
readily gives access to ansa-metallocene derivatives of Ca for
It is known that 1-phosphafulvene 1 reacts with 2H-phosp-
hole to give a [6 + 4] cycloadduct 2 in low yield due to the dis-
sociation of the adduct.7 The easy dissociation of 2 suggests
that it can be used as a precursor of phosphafulvene.
transmetalation
(Scheme 1b).
to
numerous
transition
metals1c,4
However, the chemistry of phosphafulvenes, the phospha-
analogues of pentafulvenes, has been virtually unexplored.
This is probably because of the high reactivity of the CvP
bond that stems from weak π-bond strength (45 kcal mol−1 in
HPvCH2 vs. 65 kcal mol−1 for CH2vCH2).5 With kinetic stabi-
lization by the bulky 2,4,6-tri-tert-butylphenyl group (Mes*),
Yoshifuji prepared the first isolable 1,3,6-triphosphafulvene.6
College of Chemistry, Green Catalysis Center, International Phosphorus Laboratory,
International Joint Research Laboratory for Functional Organophosphorus Materials
of Henan Province, Zhengzhou University, No. 100 Science Avenue, Zhengzhou,
Henan, 450002, China. E-mail: tianrq@zzu.edu.cn, duanzheng@zzu.edu.cn
†Electronic supplementary information (ESI) available: X-ray crystal structure
analysis of compounds. CCDC 2009638 (5a); 2021606 (5b); 2009639 (7b),
2009640 (8a) and 2011029 (9). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/d0dt03934a
Scheme 1 The cycloaddition and coordination chemistry of pentaful-
venes and 1-phosphafulvenes.
476 | Dalton Trans., 2021, 50, 476–479
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The reaction of 1 with 1 equivalent of decacarbonyldiman-
ganese Mn2(CO)10 was carried out at 140 °C in a heavy wall
pressure tube (Scheme 2). The 31P NMR signals (δ −25.0,
−29.5, and −35.8 ppm) of the crude reaction mixture show
that the reaction gives three products, with a typical high-field
position, similar to the reported examples of phosphacyman-
trenes.13 The reaction of 2H-phosphole with Mn2(CO)10 gave
3,4-dimethyl-2-phenylphosphacymantrene 3. The NMR data of
3 were consistent with the literature report.14 The structure of
4a was deduced from mass, 1H, and 13C NMR spectroscopy.
Multiple peaks between 3.40 and 3.44 ppm in the 1H NMR
spectrum are assigned to CH2. The 13C NMR spectrum of 4a
shows a characteristic phospholyl carbon at 115.3, 116.1,
Fig. 1 X-ray crystal structures of 5a (left) and 8a (right). All hydrogen
atoms are omitted for clarity; the level set for thermal ellipsoids of all
atoms is 30%. Main bond lengths (Å) and angles (°): 5a: Mn1–P1 2.3898
(9), C4–P1 1.771(3), C7–P1 1.786(3), Mn1–phospholyl centroid 1.7800(5),
Mn2–P2 2.3835(10), C27–P2 1.775(3), C30–P2 1.779(3), Mn2–phospholyl
centroid 1.7742(5); C4–P1–C7 90.20(15), C16–C7–P1 125.4(2), C27–P2–
C30 90.23(15), C39–C30–P2 126.0(2). 8a: Mn1–P1 2.4025(13), Mn2–P1
2.1326(12), C6–P1 1.747(4), C9–P1 1.744(4), Mn1-phospholyl centroid
1.7781(7), Mn2–P2 2.3773(13), C18–P2 1.796(4), C21–P2 1.800(4), Mn2–
phospholyl centroid 1.7584(6); Mn1–P1–Mn2 137.43(6), P1–Mn2–P2
93.84(4), C6–P1–C9 93.42(19), C18–P2–C21 89.54(19).
1
119.0, and 119.9 ppm with a very high JC–P coupling constant
(1JC2–P = 57.0 Hz, 60.8 Hz). α-C2-bridged biphosphacymantrene
5a was isolated in a 33% yield. The structure of 5a was unam-
biguously established by X-ray crystallographic analysis (Fig. 1,
left). The two phosphacymantrene rings are in a head-to-tail
disposition with the two Mn(CO)3 moieties having the same
orientation and in the para-position of the C–C bridge in order
to minimize steric crowding. The observation of products 3, 4a
and
Phosphacymantrene 4a originates from 1-phosphafulvene 1
and abstracts H from 2H-phosphole during the reaction, along
5a
implies
a
radical
based
mechanism.
•
with the formation of 3. A similar proton abstraction process
was observed in the reductive coupling of dimethylfulvene.15
Since the reaction with 2 will give 3 as a co-product, the
reaction was then explored with a facilely prepared α-C2-
bridged biphospholes 6.16 The versatile cycloaddition of
1-phosphafulvenes has been revealed by employing biphosp-
holes 6 as precursors.7 As shown in Scheme 3, the reaction of
biphospholes 6a with 1 equivalent of Mn2(CO)10 readily gave
four phosphacymantrene derivatives. Besides the same pro-
ducts 4a and 5a from 1 as shown in Scheme 2, the reaction
with 5a offered additional C2-bridged phosphacymantrenes 7a
and 8a. The structure of 7a was deduced by comparing the
NMR data of 7a and 7b, as well as the X-ray crystal structure of
7b (Fig. 2, right). The structure of 8a was definitively estab-
lished by performing an X-ray study (Fig. 1, right). In 7a and
8a, the phospholyl ligand acts not only as a six-electron η5-
Scheme 3 Coordination chemistry of 6 with Mn2(CO)10
.
Fig. 2 X-ray crystal structures of 5b (left) and 7b (right). All hydrogen
atoms are omitted for clarity; the level set for thermal ellipsoids of all
ligand towards a single metal but also as an eight-electron atoms is 30%. Main bond lengths (Å) and angles (°): 5b: Mn1–P1 2.3723
η5,η1-ligand with the phosphorus lone pair providing
additional two electrons. The incorporation of phosphorus
(8), C7–P1 1.786(3), C10–P1 1.776(3), Mn1–phospholyl centroid 1.7834
(4), Mn2–P2 2.3752(8), C19–P2 1.780(3), C22–P2 1.776(3), Mn2–phos-
pholyl centroid 1.7771(4); C7–P1–C10 89.84(13), C36–C7–P1 123.50(19),
C19–P2–C22 89.54(13), C37–C19–P2 123.8(2). 7b: Mn1–P1 2.3989(8),
Mn2–P1 2.1499(7), C6–P1 1.754(2), C9–P1 1.761(3), Mn1–phospholyl
centroid 1.7952(5), Mn2–P2 2.3746(7), C12–P2 1.779(2), C15–P2 1.783
(2), Mn2–phospholyl centroid 1.7651(4); C6–P1–C9 92.75(12), C12–P2–
C15 89.31(11), P1–Mn2–P2 111.80(3).
into fulvene backbones provides an extra two-electron coordi-
nation site. The two phospholyl ligands are in a head-to-tail
disposition for 7a and a head-to-head disposition for 8a.
When p-MeOC6H4 was replaced by 2-pyridinyl, the reaction
with 6b 17 produced similar products. The X-ray structure of 5b
Scheme 2 Coordination chemistry of 2 with Mn2(CO)10
.
(Fig. 2, left) shows that the stereochemistry is different from
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that of 5a, which agrees with the NMR data. The X-ray crystal
structure of 7b (Fig. 2) demonstrates that the two phosphacy-
mantrene rings are on the same side of the bridge and
approximately parallel with a head-to-tail disposition. The two
Mn(CO)3 moieties are in opposite directions. The structure of
8b was deduced from the p-MeOC6H4 analogy.
The distances between Mn and the η5-phospholyl centroid
in 7 and 8 are in the range of 1.758–1.795 Å, which are similar
to those in the phosphacymantrenes18 and bridged cyman-
trene derivatives.19 In the case of 7b, the distances between
Mn and the η5-phospholyl centroid are quite different (Mn1–
phospholyl centroid = 1.795 Å, Mn2–phospholyl centroid =
1.765 Å). The η1-coordination of P1 to Mn2 reduces the elec-
tron density of the phospholide ring of P1 which weakens the
η5-coordination with Mn1 and shows
a longer distance
Scheme 4 Plausible mechanism.
(1.795 Å) than that of 5b (1.783 Å). Meanwhile, the η1-coordi-
nation of P1 to Mn2 increases the back donation from Mn2 to
the phospholide ring of P2 and reinforces the coordination
effect, reflecting the short distance between Mn2 and the
phospholyl centroid (1.765 Å). The IR spectra of all products
were collected. The infrared CO stretching frequencies of the
products are comparable to those of the known
phosphacymantrenes.20
phafulvene. (3) The coordination with Mn(CO)3 gives phospha-
cymantrene radical 11. (4) There are two possible routes to
afford the unbridged product 4: (a) a H
affords unbridged product 4 or (b) 11 reacts with the
species to give anionic phosphacymantrene intermediate 12
which is converted into 4 after the H+ abstraction. (5) The
radical dimerization of 11 provides bridged products 5, 7, and
8. However, we cannot exclude the formation of 5, 7, and 8 by
the P–P bond cleavage of 2 or 6 with Mn2(CO)10. (6) The 1,6-
radical addition of 11 to 1-phosphafulvene 1 yields the unex-
pected product 9.
•
abstraction from 11
Mn(CO)5
•
Furthermore, we obtained a single crystal of 9 occasionally,
which contains three 1-phosphafulvene moieties. The structure
of 9 also suggests the reaction of 6 with Mn2(CO)10 via 1-phos-
phafulvenes. Unfortunately, we did not observe the 31P NMR
signals corresponding to 9 in the crude reaction mixture, and
no other data are available at present for 9 except the X-ray
analysis results (Fig. 3).
It was known that the reductive coupling of pentafulvenes
with metals afforded ansa-metallocene complexes.4,21 The for-
mation of proton abstraction product 4 and the formation of 9
suggest a mechanism involving 1-phosphafulvene. A plausible
mechanism was proposed (Scheme 4) on the basis of experi-
mental results and literature reports on the reductive dimeriza-
tion of pentafulvenes. (1) Upon heating, 1-phosphafulvene 1 is
obtained by either retro-[6 + 4] cycloaddition of 2 or the dis-
sociation of biphosphole 6. (2) A radical anion intermediate 10
In conclusion, we have demonstrated the remarkable
coordination chemistry of manganese carbonyls with 1-phos-
phafulvene which was generated in situ. α-C2-bridged bipho-
sphacymantrenes were obtained probably via the reductive
coupling of 1-phosphafulvenes. Unbridged phosphacyman-
trenes 4 were formed by the abstraction H
or H+. This is the
•
first illustration of nonbenzenoid aromaticity of 1-phosphaful-
venes. The coordination chemistry of fulvenes was replicated
in their phospha-analogues successfully, which highlighted
the analogy between phosphorus and carbon in low-coordinate
“phosha-organic” chemistry.22 Finally, we believe that these
phosphacymantrenes have potential applications in coordi-
nation and catalytic reactions based on their good stability and
π-acceptor property.
is formed by transferring an electron from Mn(CO)5 to 1-phos-
•
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 3 Molecular structure of 9. All hydrogen atoms are omitted for
clarity; the level set for thermal ellipsoids of all atoms is 30%. Main bond
lengths (Å) and angles (°): Mn1–P1 2.3886(14), C44–P1 1.781(4), C47–P1
1.786(4), Mn1–phospholyl centroid 1.7874(8), Mn1–P2 2.1809(11), Mn2–
P2 2.4099(12), C12–P2 1.759(4), C15–P2 1.765(4), Mn2–phospholyl cen-
troid 1.7866(9), C31–P3 1.800(4); P1–Mn1–P2 90.58(5), C44–P1–C47
89.0(2), C12–P2–C15 91.99(19).
This work was supported by the National Natural Science
Foundation of China (21302174, 21672193, 21272218), the
Ministry of Industry and Information Technology of the
People’s Republic of China (Z135060009002), the 111 Project
(D20003), the Key Research Projects of Colleges and
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Universities in Henan Province (No. 20A150040) and the
Zhengzhou University of China (JC2020530190).
7 Z. Hu, R. Tian, K. Zhao, Y. Liu, Z. Duan and F. Mathey, Org.
Lett., 2017, 19, 5004.
8 N. Shen, Y. Liu, R. Tian, Z. Duan and F. Mathey,
Chin. J. Org. Chem., 2019, 39, 2277.
9 (a) L. Xu, Y. Chi, S. Du, W.-X. Zhang and Z. Xi,
Angew. Chem., Int. Ed., 2016, 55, 9187; (b) S. Du, J. Yin,
Y. Chi, L. Xu and W.-X. Zhang, Angew. Chem., Int. Ed.,
2017, 56, 15886; (c) S. Du, W.-X. Zhang and Z. Xi,
Organometallics, 2018, 37, 2018; (d) S. Du, Z. Chai,
J. Hu, W.-X. Zhang and Z. Xi, Chin. J. Org. Chem., 2019, 39,
2338.
10 (a) F. Mathey, J. Organomet. Chem., 2002, 646, 15;
(b) D. Carmichael and F. Mathey, Top. Curr. Chem., 2002,
220, 27.
11 (a) G. C. Fu, Acc. Chem. Res., 2006, 39, 853; (b) Y. Hao,
R. Tian, D. Wu, Z. Duan and F. Mathey, Chin. J. Org. Chem.,
2018, 38, 277.
12 D. P. Hollrah and A. M. Burns, Oil Gas J., 1991, 89, 86.
13 F. Mathey, A. Mitschler and R. Weiss, J. Am. Chem. Soc.,
1978, 100, 5748.
14 F. Mercier, S. Holand and F. Mathey, J. Organomet. Chem.,
1986, 316, 271.
15 (a) H. Schwemlein and H. H. Brintzinger, J. Organomet.
Chem., 1983, 254, 69; (b) P.-J. Sinnema, P. J. Shapiro,
B. Höhn, T. E. Bitterwolf and B. Twamley, Organometallics,
2001, 20, 2883.
16 Z. Hu, Z. Li, K. Zhao, R. Tian, Z. Duan and F. Mathey, Org.
Lett., 2015, 17, 3518.
17 Y. Liu, K. Zhang, R. Tian, Z. Duan and F. Mathey, Org. Lett.,
2020, 22, 6972.
18 M. Ogasawara, Y. Y. Tseng, M. Uryu, N. Ohya, N. Chang,
H. Ishimoto, S. Arae, T. Takahashi and K. Kamikawa,
Organometallics, 2017, 36, 4061.
19 K. Kamikawa, Y.-Y. Tseng, J.-H. Jian, T. Takahashi and
M. Ogasawara, J. Am. Chem. Soc., 2017, 139,
1545.
20 F. Nief, F. Mathey and L. Ricard, Organometallics, 1988, 7,
921.
21 (a) K. M. Kane, P. J. Shapiro, A. Vij, R. Cubbon and
A. L. Rheingold, Organometallics, 1997, 16, 4567;
(b) J. J. Eisch, X. Shi and F. A. Owuor, Organometallics,
1998, 17, 5219.
Notes and references
1 (a) H. Hopf, in Cross Conjugation: Modern Dendralene,
Radialene and Fulvene Chemistry, ed. H. Hopf and
M. S. Sherburn, Wiley, New York, 2016, pp. 1–440;
(b) P. Preethalayam, K. S. Krishnan, S. Thulasi, S. S. Chand,
J. Joseph, V. Nair, F. Jaroschik and K. V. Radhakrishnan,
Chem. Rev., 2017, 117, 3930; (c) P. J. Shapiro, Coord. Chem.
Rev., 2002, 231, 67; (d) K. Strohfeldt and M. Tacke, Chem.
Soc. Rev., 2008, 37, 1174; (e) R. Gleiter, C. Bleiholder and
F.
Rominger,
Organometallics,
2007,
26,
4850;
(f) R. Beckhaus, A. Lützen, D. Haase, W. Saak, J. Stroot,
S. Becke and J. Heinrichs, Angew. Chem., Int. Ed., 2001, 40,
2056; (g) H. Ebert, V. Timmermann, T. Oswald, W. Saak,
M. Schmidtmann, M. Friedemann, D. Haase and
R. Beckhaus, Organometallics, 2014, 33, 1440.
2 (a) J. Barluenga, S. Martinez, A. L. Suárez-Sobrino and
M. Thomás, J. Am. Chem. Soc., 2002, 124, 5948;
(b) S. S. Bhojgude, T. Kaicharla, A. Bhunia and A. T. Biju,
Org. Lett., 2012, 14, 4098; (c) Z.-L. He, H.-L. Teng and
C.-J. Wang, Angew. Chem., Int. Ed., 2013, 52, 2934;
(d) C. Ebner and E. M. Carreira, Angew. Chem., Int. Ed.,
2015, 54, 11227.
3 (a) K. Hafner, K. H. Häfner, C. König, M. Kreuder, G. Ploss,
G. Schulz, E. Sturm and K. H. Vöpel, Angew. Chem., Int. Ed.
Engl., 1963, 2, 123; (b) M. L. Peterson, J. T. Strnad,
T. P. Markotan, C. A. Morales, D. V. Scaltrito and
S. W. Staley, J. Org. Chem., 1999, 64, 9067; (c) T. L. Andrew,
J. R. Cox and T. M. Swager, Org. Lett., 2010, 12, 5302.
4 (a) A. Recknagel and F. T. Edelmann, Angew. Chem., Int.
Ed., 1991, 30, 693; (b) M. Rieckhoff, U. Pieper, D. Stalke
and F. T. Edelmann, Angew. Chem., Int. Ed., 1993, 32, 1079;
(c) M. Kessler, S. Hansen, C. Godemann, A. Spannenberg
and T. Beweries, Chem. – Eur. J., 2013, 19, 6350.
5 (a) M. W. Schmidt, P. N. Truong and M. S. Gordon, J. Am.
Chem. Soc., 1987, 109, 5217; (b) P. v. R. Schleyer and
D. Kost, J. Am. Chem. Soc., 1988, 110, 2105.
6 S. Ito, H. Sugiyama and M. Yoshifuji, Angew. Chem., Int.
Ed., 2000, 39, 2781.
22 F. Mathey, Angew. Chem., Int. Ed., 2003, 42, 1578.
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