Synthesis of phospholes and 1,1′-biphospholes mediated by zirconacyclopentadienes and PBr3

Article abstract of DOI:10.1016/j.tetlet.2019.151388

The efficient synthesis of phospholes through the reactions of Grignard reagents with in situ-generated 1-bromophospholes has been developed. This method allows the easy substituent variation at the phosphorus atom of the phospholes. The 1,1′-biphospholes can also be conveniently prepared through Zn-promoted debromo-dimerization of 1-bromophospholes. The gold-phosphole complexes of LAuCl were also synthesized, which were found to act as efficient catalysts in gold-catalyzed [3+2] cycloaddition of ynamides with 4,5-dihydro-1,2,4-oxadiazoles.

Full text of DOI:10.1016/j.tetlet.2019.151388

Journal Pre-proofs
Synthesis of Phospholes and 1,1’-Biphospholes Mediated by Zirconacyclo
pentadienesand PBr3
Chunxiang Qin, Xin Xie, Gaonan Wang, Xueshun Jia, Yuanhong Liu
PII:
S0040-4039(19)31179-7
DOI:
https://doi.org/10.1016/j.tetlet.2019.151388
TETL 151388
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Tetrahedron Letters
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11 November 2019
Please cite this article as: Qin, C., Xie, X., Wang, G., Jia, X., Liu, Y., Synthesis of Phospholes and 1,1’-Biphospholes
MediatedbyZirconacyclopentadienesandPBr3 , Tetrahedron Letters (2019), doi:https://doi.org/10.1016/j.tetlet.
2019.151388
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Synthesis of Phospholes and 1,1’-Biphospholes Mediated by Zirconacyclopentadienes
and PBr3
Chunxiang Qin,† Xin Xie,*‡ Gaonan Wang,‡ Xueshun Jia*† and Yuanhong Liu*‡
†Department of Chemistry, Shanghai University, 99 Shang-Da Road, Shanghai 200444, P. R.
China. ‡State Key Laboratory of Organometallic Chemistry, Center for Excellence in
Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032,
People’s Republic of China. E-mail: yhliu@sioc.ac.cn
R1
R2
R3MgBr
R3
P
R1
R1
R2
R2
R2
R2
R2
Cp
Cp
R1
PBr3
Zr
R1
Br
P
R1 R1
P P
R2
R2
R2
R2
R1
cat. Zn
R1= alkyl, aryl, silyl
27 examples, up to 81% yield
direct and efficient
mild conditions
R1 R1
Abstract: The efficient synthesis of phospholes through the reactions of Grignard reagents
with in situ-generated 1-bromophospholes has been developed. This method allows the easy
substituent variation at the phosphorus atom of the phospholes. The 1,1’-biphospholes can
also be conveniently prepared through Zn-promoted debromo-dimerization of
1-bromophospholes. The gold-phosphole complexes of LAuCl were also synthesized, which
were found to act as efficient catalysts in gold-catalyzed [3+2] cycloaddition of ynamides
with 4,5-dihydro-1,2,4-oxadiazoles.
1

Key words: Zirconacycle; metallacycle transfer; phospholes; 1,1’-biphospholes
Phosphole derivatives represent one of the most important classes of the
phosphine-containing heterocycles, which have received much attention in organic functional
materials and coordination chemistry, such as organic light-emitting diodes (OLEDs),1
nonlinear optical devices,2 sensory materials,3 chiral metal-bis(azahelicene phosphole)
assemblies,4 and coordination-driven supramolecular assembly5 etc. They have also been used
as the ligands in transition metal catalysis such as hydro-formylation,6 hydrogenation,7 allylic
substitution,8 hydrosilylation,9 olefin/CO copolymerization10 and gold-catalyzed
cycloisomerization11 etc. Up to now, many synthetically useful methods for phospholes have
been developed. Among them, Fagan-Nugent route12 involving metallacycle transfer from
zirconacyclopentadienes to various main group heteroles including phospholes is one of the
most developed and significant tools, which allows the assembly of mono- or fused
phospholes with a wide structural diversity and in one-pot operation (Scheme 1A). It is
known that the nature of P-substituents in phospholes plays an important role in the stability
and aromaticity of phospholes due to the hyperconjugation of the exocyclic P-R -bond and
the dienic -system,13 which is also important in tuning the electronic properties and steric
hindrance of the whole molecule. Although the P-substituents can be modified by
Fagan-Nugent method through the use of different dichlorophosphines (RPCl2), usually, only
1-P-phenyl-phospholes were prepared with PhPCl2. This is possibly due to the low stability
and less availability of other dihalophosphines. Except for this route, there are only limited
2

methods for variation of P-substituents in Fagan-Nugent route, mainly including reductive
cleavage of the P-R bond (R = aryl, halide) by lithium metal to give a phospholyl anion
followed by reaction with electrophiles14(Scheme 1B). However, this method usually suffers
from the use of highly reactive lithium metal and/or restricted to special substrates. Early
works by Douglas15 and Mathey14b indicated that the reactions of zirconacyclopentadienes
with PX3 (X = Cl or Br) in CH2Cl2 afforded 1-halophospholes. The straightforward way for
introducing of different P-substituents would be through the nucleophilic substitution of
1-halophospholes. However, in most reports, the resulted 1-halophospholes were transferred
to phospholyl anions, and then were used for the preparation of P-R phospholes14b-e or
1,1’-biphospholes.16 It was known that 1-halophospholes generated through other methods
such as the reaction of dilithium derivative with phosphorus halides17 could react with
organolithium or Grignard reagents,18 to our surprise, direct substitution of 1-halophospholes
in Fagan-Nugent route is quite rare. To the best of our knowledge, when Grignard reagent
was used as the nucleophile, there is only one report concerning the reaction of alkynyl
Grignard reagents with 1-chlorotetraethylphosphole derived from zirconacycles leading to
alkynylphospholes (two examples).19 However, the reaction required tedious procedure
involving two times of solvent-change. In this paper, we describe a convenient method for the
synthesis of phospholes through the reactions of 1-halophospholes with Grignard reagents,
which allows the facile substituent variation at the phosphorus atom of the phospholes
(Scheme 1C). A zinc promoted formation of 1,1’-biphospholes is also described.
Scheme 1. Synthesis of phospholes and 1,1’-biphospholes
3

A) 1988, Fagan-Nugent:
R1
Cp2Zr
R1
R1
Ph P
R1
R1
R1
R1
R1
Cp2ZrCl2/n-BuLi
-78 oC to rt
PhPCl2
THF, rt
R1
R1
B) PX3/Li mediated formation of phospholes and 1,1'-biphospholes:
R1
R1
R1
Cp2Zr
R1
R1
X P
R1
R1
R1
R1
R1
R1
R1
R1
R2X
PX3
Li
Li+ P
R2
P
-
in CH2Cl2
X = Cl, Br
in THF
R1
R1
R1
R1 R1
P P
R1
R1
R1
R1
i) Li, ii) I2
R1 R1
C) This work: PBr3 mediated formation of phospholes and 1,1'-biphospholes:
R1
Cp2Zr
R1
R1
R1
R2
R2
R2
R2
R2
R2
R3MgBr
PBr3
Br
P
R3
P
in THF
R1
R1
R2
R2
R1 R1
R2
R2
direct and efficient
mild conditions
facile isolation
cat. Zn
P P
R1 R1
Zirconacyclopentadienes, which were easily prepared by reduction of Cp2ZrCl2 with
n-BuLi followed by addition of two equiv of alkynes,20 were chosen for the metallacycle
transfer reaction with PBr3. As illustrated in Scheme 2, treatment of zirconacyclopentadiene
o
1a with 1.1 equiv of PBr3 at 0 C resulted in the color change from red to orange during the
reaction going on. After the reaction mixture was stirred at 0 oC for 1 h, 1.5 equiv of PhMgBr
was added. The color of the reaction mixture turned to light yellow immediately, and a large
amount of precipitate could be observed as the reaction proceeded. After the workup,
4

1-phenylphosphole 3a was obtained in 75% yield. The results suggested that the
1-bromophosphole 2a was formed as an intermediate.14b Interestingly, a trace amount of
1,1’-biphosphole 4a was also observed in this reaction. Usually, 1-halophospholes were
prepared in CH2Cl2 from the pre-synthesized zirconacyclopentadienes,14b-d or through a
one-pot procedure which required the change of the solvent from THF to CH2Cl2 after the
generation of zirconacyclopentadiene.14e,19 The synthesis described here does not need the
solvent change in most cases, thus providing a convenient route to phospholes while avoiding
the tedious operation.
With the optimized reaction conditions in hand, the scope of the substrates including both
alkynes and Grignard reagents were examined. For aryl alkynes, electron-donating groups
such as p-Me, m-Me, p-tBu and p-MeO on the aryl rings were well compatible, leading to
3b-3e in 59-76% yields. An electron-withdrawing p-Br group, which is usually not tolerated
in transition-metal catalyzed reactions, was suited for this reaction, leading to 3f in 54% yield.
A heterocycle ring such as 2-thienyl group could also be efficiently introduced at the C2~C5
positions of the phosphole ring, giving 3g in 81% yield. The use of zirconacyclopentadienes
derived from alkyl alkynes such as 3-hexyne resulted in 50% yield of the product 3h. In the
case of trimethyl(prop-1-ynyl)silane, the product 3i was also formed smoothly in 42% yield as
a single regioisomer. Next, the effects of various Grignard reagents were investigated.
Aromatic Grignard reagents such as p-methoxyphenylmagnesium bromide provided 3j in
52% yield. Sterically hindered Grignard reagents such as o-methyl- or
o-methoxyphenylmagnesium bromides also reacted well in the reaction to furnish 3k-3l in
65-79% yields. The results indicated that the steric effect of the Grignard reagents has little
5

influence on the reaction. Vinyl and alkynylmagnesium bromides proved to be also suitable,
leading to 3m-3p in 36-54% yields. The reactivity of alkyl Grignard reagents were also tested.
Methyl-, ethyl-, and sterically encumbered isobutyl, cyclopentyl or even tert-butyl-substituted
Grignard reagents were all compatible with the reaction, giving 3q-3v in 33-69% yields. The
structure of 3q was confirmed by X-ray crystallography.21
Scheme 2. Synthesis of mono-phospholesa
6

R1
R1
R1
Cp2Zr
R1
i) 1.5 equiv R3MgBr
0 oC to rt , 3 h
R2
R2
R2
R2
R2
R2
1.1 equiv PBr3
0 oC, 1 h
R3
P
Br
P
ii) MeOH, rt, 20 min
R1
R1
1
2
3
Ph
Ph P
Ph
3a, 75%
Ar
Ph P
Ar
Ar
Ar
Ph P
Ar
Ph
Ar
Ar
Ar
Ar
Ar
Ar
Ph P
Ar
Ph
Ar = p-tBu-C6H4
Ar = p-Me-C6H4
Ar = m-Me-C6H4
3b, 59%
3c, 76%
3d, 71%
S
Ar
Ar
Ar
Et
Et
Ar
Ar
S
Ph P
Ph P
Ar
Ph P
Ph P
S
Ar
Et
Et
Ar
S
Ar = p-MeO-C6H4
3e, 71%
Ar = p-Br-C6H4
3f, 54%
3g, 81%
3h, 50%
TMS
Me
Ph
P
Ph
P
Ph
P
OMe
Ph
Ph
Ph
Ph
Ph
Ph
Ph P
MeO
Me
TMS
Ph
Ph
Ph
3j, 52%b
3k, 79%b
3l, 65%b
3i, 42%
Ph
P
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
P
P
Ph
P
Ph
Ph
Ph
Ph
Ph
Ph
3o, 52%b
3m, 54%
3n, 41%
3p, 36%c
Et
iBu P
Et
Ph
Ph
iBu P
Ph
Ph
Ph
Et
Ph
Me P
P
Et
Ph
Ph
3s, 59%d
Ph
Ph
Ph
Ph
3q, 69%b
3r, 54%
3t, 33%b
Ph
Ph
Ph
Ph
P
tBu P
Ph
Ph
Ph
Ph
3v, 53%b
3u, 46%
aIsolated yields. b3 equiv of R3MgBr was used. cPhenylethynylmagnesium bromide was
prepared by the reaction of 2.4 equiv of phenylacetylene and 2.2 equiv of ethylmagnesium
bromide. dAfter formation of zirconacycle, the solvent was removed, and DCM was added.
7

As mentioned above, a small amount of 1,1’-biphosphole could be observed during
the process of PBr3-mediated phosphole formation. 1,1’-Biphospholes were usually
prepared through the reaction of 1-halophosphole with a large excess of metallic
16d
lithium followed by oxidation with I2 or by the reaction of 1-phenylphosphole with
excess Li followed by the treatment with AlCl3 and I2.16a Nyulászi and Réau et al.
found that treatment with zirconacyclopentadiene with an excess PBr3 (ca. 2.7 equiv)
resulted in the formation of 1,1’-biphospholes after stirring the reaction mixture for a
long time (4 d).16d Inspired by our results, we next wish to develop a more efficient
procedure for the synthesis of 1,1’-biphospholes. The results are shown in Table 1.
Interestingly, when MeLi was added to the reaction mixture after formation of
1-bromophosphole, 1,1’-biphosphole 4a was formed as the major product (50%, Table
1, entry 1). To our delight, addition of 1.0 equiv of zinc also delivered 4a in 41% yield
o
at 50 C (entry 2). Decreasing the amount of zinc to 0.4 equiv afforded a slightly
higher yield (entry 4 vs. 2 and 3). Other reductants such as Mg, Fe or Mn gave 4a in
lower yields (entries 5-7). Increasing the reaction temperature to 65°C improved the
o
yield of 4a significantly (entry 8). A 77% yield could be achieved at 65 C by using
only 20 mol% of Zn (entry 10). Without Zn, the reaction could also proceed, and a
lower yield of 61% was observed (entry 12). The reaction mechanism is not clear yet.
Possibly a P-centered radical is formed under thermal conditions through homolysis of
the P-Br bond of 1-bromophosphole intermediate which dimerizes to give
1,1’-biphosphole. Zinc may play a role in accelerating the formation of the radical
species.
8

Table 1. Optimization of the Reaction Conditions for 1,1’-Biphospholesa
Ph
P
Ph
Cp2Zr
Ph Ph
P P
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1.1 equiv PBr3
0 oC ,1 h
i) additive, temp. 3 h
Br
ii) 10 equiv MeOH
rt, 20 min
Ph
Ph
1a
Ph Ph
4a
2a
Temp. (oC)
Yield (%)
Entry
Additive (equiv)
MeLi (1.0)
Zn (1.0)
Zn (0.5)
Zn (0.4)
Mg (0.4)
50
41
47
50
16
rt
1
2
3
4
50
50
50
50
5
6
7
8
50
50
65
65
Fe (0.4)
Mn (0.4)
35
30
78
Zn (0.4)
Zn (1.0)
Zn (0.2)
Zn (0.1)
76
77
9
10
65
65
11
71
61
12
-
-
65
rt
12
13
aIsolated yields.
Under the optimized reaction conditions (Table 1, entry 10), a series of
1,1’-biphospholes were synthesized (Scheme 3). The functionalities such as p-Me, m-Me,
p-tBu and p-OMe groups on the aryl rings were well tolerated (4b-4e). However, this
method is not compatible with alkyl-substituted zirconacyclopentadienes.
Scheme 3. Synthesis of 1,1’-Biphospholes
9

Ar
Cp2Zr
Ar
i) 0.2 equiv Zn dust
0 oC to 65 oC, 3 h
Ar Ar
P P
Ar
Ar
Ar
Ar
Ar
Ar
1.1 equiv PBr3
0 oC, 1 h
10 equiv MeOH
rt, 20 min
ii)
Ar Ar
1
4
4a, Ar = Ph, 77%
4d, Ar =p-tBuC6H4, 46%
4e, Ar = p-MeOC6H4, 51%
4b, Ar = p-MeC6H4, 76%
4c, Ar = m-MeC6H4, 65%
To demonstrate the synthetic utility of the phospholes obtained by the present
method, transformations of the alkynylphosphole 3o were performed. Treatment of 3o
with EtMgBr generated alkynyl Grignard reagent 5. This intermediate was transferred
to internal alkynes 6~8 through the reactions with various electrophiles such as PBr3
PPh2Cl and (CHO)n, respectively. All these products were obtained in good yields.
Scheme 4. Transformations of the alkynylphosphole 3o
Ph
P
Ph
P
Ph
Ph
Ph
Ph
1.0 or 1.2 equiv EtMgBr
THF, 0 oC to rt , 1 h
BrMg
5
Ph
Ph
3o
0.28 equiv PBr3
0 oC to rt, 2 h
1.5 equiv (CHO)n
rt, 2 h
1.5 equiv
Ph2PCl
rt, 2 h
P
P
Ph
Ph
Ph
P
Ph
Ph
P
Ph
Ph
P
Ph
Ph
P
P
OH
=
Ph
P
Ph
Ph
Ph
P
Ph
7, 79%
8, 69%
6, 78%
Phospholes are potentially useful as ligands in transition-metal-catalyzed reactions.
As our continuing interests in gold catalysis, we therefore attempted to synthesize
10

Au(I)-phosphole complexes and test its catalytic activity. As shown in Scheme 5,
LAuCl complex derived from P-phenyl (9a) or P-tBu (9b) phospholes were readily
prepared through their reactions with Me2SAuCl. These complexes were isolated as
air-stable solids in high yields.21 Next, to evaluate the catalytic performance of
phosphole-gold(I) complexes 9, the gold-catalyzed [3+2] cycloaddition of ynamides
with 4,5-dihydro-1,2,4-oxadiazoles22 was chosen as a model reaction. To our delight,
although these phosphole ligands are highly sterically hindered, their corresponding
gold complexes showed good catalytic reactivity for this [3+2] cyclization (Scheme 6).
Scheme 5. Synthesis of gold(I)-phosphole complex
Ph
Ph
Ph
Ph
Ph
Ph
Me2SAuCl
DCM, rt
Ph
Ph
AuCl
P
R1
P
R1
R1 = Ph (3a)
R1 = tBu (3v)
9a, R1 = Ph, 98%
9b, R1 = tBu, 87%
Scheme 6. Gold(I)-phosphole complex-catalyzed reaction
Me
N
Ms
Ph
N O
N
Me
catalyst (5 mol %)
DCE, 80 oC, 5 h
Ph
N
Ms
+
Ph
Ph
Ph
N
H
N
H
11 (1.2 equiv)
10
12
9a/AgSbF6, 79%
9b/AgSbF6, 82%
PPh3AuCl/AgSbF6, 64% (Ref. 20)
In summary, we have shown that phospholes could be synthesized through the reactions
of Grignard reagents with in situ-generated 1-bromophospholes. Especially, the method
11

provides a straightforward route for the introduction of different P-substituents in phospholes.
The 1,1’-biphospholes can also be conveniently prepared through Zn-promoted debromo-
dimerization of 1-bromophospholes. The gold-phosphole complexes of LAuCl were also
synthesized, which were found to act as efficient catalysts in gold-catalyzed [3+2]
cycloaddition of ynamides with 4,5-dihydro-1,2,4-oxadiazoles.
Acknowledgments
We thank the National Natural Science Foundation of China (Grant No. 21702224), the
National Key R&D Program of China (2016YFA0202900), and the Strategic Priority
Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000) and
Science and Technology Commission of Shanghai Municipality (18XD1405000) for financial
support.
Appendix A. Supplementary data
Experimental procedures, spectral data, X-ray crystallography of compounds CCDC-1952958
(3q), -1952959 (4a), -1952960 (7), and -1952957 (9a) are given in the Supporting
Information.
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supplementary crystallographic data for this paper.
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(22)Xu, W.; Wang, G.; Sun, N.; Liu, Y. Org. Lett. 2017, 19, 3307.
Graphic Abstract for
Synthesis of Phospholes and 1,1’-Biphospholes Mediated by Zirconacyclopentadienes
and PBr3
†Department of Chemistry, Shanghai University, 99 Shang-Da Road, Shanghai 200444, P. R.
China. ‡State Key Laboratory of Organometallic Chemistry, Center for Excellence in
Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032,
People’s Republic of China. E-mail: yhliu@sioc.ac.cn
R1
R2
R3MgBr
R3
P
R1
R1
R2
R2
R2
R2
R2
Cp
Cp
R1
PBr3
Zr
R1
Br
P
R1 R1
P P
R2
R2
R2
R2
R1
cat. Zn
R1= alkyl, aryl, silyl
27 examples, up to 81% yield
direct and efficient
mild conditions
R1 R1
Hights
 Highly efficient synthesis of phospholes through the reactions of Grignard reagents
with in situ-generated 1-bromophospholes
16

 Easy substituent variation at the phosphorus atom of the phospholes
 Formation of 1,1’-biphospholes through Zn-promoted debromo-dimerization of
1-bromophospholes
 Gold-phosphole complexes serve as efficient catalysts in [3+2] cycloaddition of
ynamides with 4,5-dihydro-1,2,4-oxadiazoles
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
17

Graphic Abstract for
Synthesis of Phospholes and 1,1’-Biphospholes Mediated by Zirconacyclopentadienes
and PBr3
†Department of Chemistry, Shanghai University, 99 Shang-Da Road, Shanghai 200444, P. R.
China. ‡State Key Laboratory of Organometallic Chemistry, Center for Excellence in
Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032,
People’s Republic of China. E-mail: yhliu@sioc.ac.cn
R1
R2
R3MgBr
R3
P
R1
R1
R2
R2
R2
R2
R2
Cp
Cp
R1
PBr3
Zr
R1
Br
P
R1 R1
P P
R2
R2
R2
R2
R1
cat. Zn
R1= alkyl, aryl, silyl
27 examples, up to 81% yield
direct and efficient
mild conditions
R1 R1
18