Reactivity differences between 2,4- and 2,5-disubstituted zirconacyclopentadienes: A highly selective and general approach to 2,4-disubstituted phospholes

Article abstract of DOI:10.1039/c3dt51158h

Mixtures of 2,4- and 2,5-disubstituted zirconacyclopentadienes were obtained by the reductive dimerisation of terminal alkynes using the Cp ₂ZrCl₂/lanthanum system. Reactions of dihalophosphines with these mixtures afforded selectively the corresponding 2,4-disubstituted phospholes and 1,4-disubstituted butadienes. A new series of phospholes was characterized by multi-nuclear NMR spectroscopy and X-ray analysis. A possible explanation for the observed selectivity was obtained from X-ray studies and DFT analysis of the intermediate zirconacyclopentadienes. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt51158h

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Reactivity differences between 2,4- and 2,5-
disubstituted zirconacyclopentadienes: a highly
selective and general approach to 2,4-disubstituted
phospholes†
Cite this: Dalton Trans., 2013, 42, 10997
Guillaume Bousrez,a Florian Jaroschik,*a Agathe Martinez,a Dominique Harakat,a
Emmanuel Nicolas,b Xavier F. Le Goffb and Jan Szymoniak*a
Mixtures of 2,4- and 2,5-disubstituted zirconacyclopentadienes were obtained by the reductive dimeri-
sation of terminal alkynes using the Cp2ZrCl2/lanthanum system. Reactions of dihalophosphines with these
mixtures afforded selectively the corresponding 2,4-disubstituted phospholes and 1,4-disubstituted buta-
dienes. A new series of phospholes was characterized by multi-nuclear NMR spectroscopy and X-ray ana-
lysis. A possible explanation for the observed selectivity was obtained from X-ray studies and DFT analysis
of the intermediate zirconacyclopentadienes.
Received 3rd May 2013,
Accepted 31st May 2013
DOI: 10.1039/c3dt51158h
www.rsc.org/dalton
rarely described. In most cases, mixtures of products were
obtained which could not be further exploited.6 Only the very
Introduction
Zirconacyclopentadienes are important intermediates in bulky tris(trimethylsilyl)silyl group afforded the 2,5-disubsti-
organic synthesis, including transition-metal catalysis and tuted zirconacyclopentadiene as the sole product.7 A switch
macrocyclisation reactions.1 Since the first synthesis of tetra- to the bis(indenyl)zirconocene allows the 2,5-selective coup-
phenylzirconacyclopentadiene in 1961,2 several synthetic ling of terminal alkynes.6b In 2006, the reductive dimerisation
pathways have been explored.3 Especially, the emergence of of terminal alkynes using a Mischmetall generated zircono-
low-valent zirconocene precursors, such as the Negishi, Taka- cene(II) equivalent was described.8 These reactions yielded
hashi or Rosenthal reagents, has contributed considerably to clean mixtures of 2,4- and 2,5-disubstituted zirconacyclo-
their development.3b–d These reagents reductively couple pentadienes as shown by hydrolysis experiments. To date, no
internal alkynes with a large variety of substituents. The further reaction involving 2,4-disubstituted zirconacyclopenta-
regioselectivity can be influenced by bulky trimethylsilyl or t- dienes has been reported.
butyl groups which preferentially couple in the alpha-posi-
Among the numerous transformations of zirconacyclopenta-
tion.4a,b On the other hand, pentafluorophenyl or mesityl dienes, the synthesis of 5-membered heterocycles has
groups show selectivity for the beta-position.4c,d The hydro- attracted much interest. The so-called Fagan–Nugent route
zirconation/methylation pathway developed by Buchwald is involves the reaction of electrophilic main group halides based
an interesting alternative as it gives ready access to trisubstituted on boron, phosphorus, tin and others with zirconacyclopenta-
complexes, whereas the former reagents are especially suit- dienes.9 We focused our attention on the synthesis of new
able for tetrasubstituted compounds.5 However, most phospholes, as they have recently become important building
reagents cannot be employed in the coupling of two terminal blocks in the fields of catalysis, coordination chemistry,
alkynes and disubstituted zirconacyclopentadienes have been material sciences and medicinal chemistry.10 In addition,
phospholide anions are interesting alternatives to cyclopenta-
dienide ligands in organometallic chemistry.11 In view of these
numerous applications, it seemed surprising that not all sub-
stitution patterns of phospholes have been investigated: the
majority of phospholes are tetrasubstituted, with 2,5- and
3,4-disubstituted phospholes being also readily accessible.12
In contrast, very few examples of 2,4-disubstituted phos-
pholes are known and no general synthetic route to such
compounds has been reported.13 Such compounds may offer
aInstitut de Chimie Moléculaire de Reims, UMR CNRS 7312, Université de Reims,
BP 1039, 51687 Reims, France. E-mail: florian.jaroschik@univ-reims.fr,
jan.szymoniak@univ-reims.fr; Fax: +33 326913166; Tel: +33 326913244
bLaboratoire “Hétéroéléments et Coordination”, Ecole Polytechnique and CNRS,
91128 Palaiseau Cedex, France
†Electronic supplementary information (ESI) available: NMR, DFT and crystallo-
graphic data. CCDC 926967–926969. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3dt51158h
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different steric and electronic properties, which could be [M + H]: calc. (m/z) 425.0847; found (m/z) 425.0853. Attempts
exploited in ligand design for catalytic and organometallic to obtain a correct elemental analysis failed.
purposes.
We report here a highly selective and general synthesis of
2,4-disubstituted phospholes using the Fagan–Nugent route in
combination with the reductive coupling of terminal alkynes
by the lanthanum generated zirconocene(II). An X-ray study
and DFT calculations on the intermediate zirconacyclopenta-
dienes offer some insights with respect to the observed
selectivity.
Synthesis of 1,4-diiodo-1,3- and 1,4-diphenylbutadienes 2a and
2b
A Schlenk tube was loaded with zirconocene dichloride
(Cp2ZrCl2) (292 mg, 1.0 mmol), lanthanum (93 mg,
0.66 mmol) and THF (5 mL). The resulting mixture was stirred
vigorously at room temperature until
a deep red color
appeared. At this stage, phenylacetylene (0.21 mL, 2 mmol)
was added to the reaction mixture. After 3 h, iodine (570 mg,
2.25 mmol) was added portionwise at room temperature and
the reaction was stirred for 24 h. The resulting brown solution
with a yellow precipitate was quenched with an aqueous
Na2S2O3 solution (1 M, 5 mL) and water (25 mL) at room temp-
erature. The aqueous layer was extracted with Et2O (3 × 25 mL),
and the organic phases were combined, washed with brine,
dried over MgSO4, and concentrated under vacuum. The red
residue was purified by flash column chromatography on silica
gel using petroleum ether as an eluent, yielding 2a and 2b in
25% (113 mg, 0.25 mmol) and 18% (82 mg, 0.18 mmol),
respectively. NMR data of 2a and 2b are in accordance with the
literature data.16
Experimental part
All reactions were conducted under an argon atmosphere
using standard Schlenk techniques and an argon-filled
Jacomex BS531-type dry box. Tetrahydrofuran and diethylether
were collected under argon from a PURSOLV MD-3 (Innovative
Technologies Inc.) solvent purification unit. Zirconocene
dichloride was purchased from Strem Chemicals. Alkynes were
purchased from Aldrich and Alfa Aesar or synthesized from
the corresponding aldehydes according to reported literature
procedures.14 Lanthanum ingot was purchased from Aldrich
and Strem Chemicals and freshly filed in the dry box prior to
use. 1H, 13C, 19F, 29Si and 31P NMR spectra were recorded in
CDCl3, unless specified, on a 250 MHz Bruker Avance I
spectrometer equipped with a QNP probe and a 500 MHz
Bruker Avance III spectrometer equipped with a BBFO+ probe.
Chemical shifts are reported in delta (δ) units, expressed in
parts per million (ppm). One and two dimensional experi-
ments, including NOESY and HOESY (1H–31P) experiments,
were performed for the NMR assignments of phospholes.
High resolution ESI-MS spectra were recorded on a hybrid
tandem quadrupole/time-of-flight (Q-TOF) instrument, equipped
with a pneumatically assisted electrospray (Z-spray) ion source
(Micromass, Manchester, UK) operated in positive mode.
High resolution EI-MS spectra were obtained on a GCT-TOF
mass spectrometer (Micromass, Manchester, UK) with an EI
source.
Optimised procedure for 1,2,4-triphenylphosphole 3a
A Schlenk tube was loaded with Cp2ZrCl2 (584 mg, 2.0 mmol),
lanthanum (186 mg, 1.3 mmol) and THF (10 mL). The result-
ing mixture was stirred vigorously at room temperature until a
deep red color appeared. At this stage, phenylacetylene
(0.42 mL, 4.0 mmol) was added to the reaction mixture and
the stirring was continued until complete disappearance of the
alkyne as shown by TLC. Then the optimized amount of
dichlorophenylphosphine (0.14 mL, 1.0 mmol) was added at
−78 °C. After slowly warming to room temperature, the reac-
tion mixture was stirred for 18 h. After that time, petroleum
ether (20 mL) was added to the brown solution and the solu-
tion was filtered over a short column of basic aluminum oxide
using petroleum ether–ethyl acetate 8 : 2 as the eluent. The
solvent was evaporated and the crude residue was purified by
flash column chromatography on silica gel using petroleum
Synthesis of 2,5-diphenylzirconacyclopentadiene 1b
A Schlenk tube was loaded with Cp2ZrCl2 (292 mg, 1.0 mmol), ether to yield phosphole 3a in 70% yield (218 mg, 0.70 mmol).
lanthanum (93 mg, 0.66 mmol) and THF (5 mL). The resulting Crystals suitable for X-ray analysis were obtained by recrystalli-
mixture was stirred vigorously at room temperature until a sation from diethyl ether. 1H NMR (500 MHz, CDCl3): 7.08 (dd,
deep red color appeared. At this stage, phenylacetylene JP–H = 40.0 Hz, JH–H = 1.5 Hz, 1H), 7.22–7.29 (m, 3H), 7.31 (d,
(0.21 mL, 2.0 mmol) was added to the reaction mixture and JH–H = 8.0 Hz, 2H), 7.34–7.38 (m, 3H), 7.43 (d, JH–H = 7.0 Hz,
stirring continued for 3 h. The solution was filtered to remove 2H), 7.46 (d, JP–H = 7.5 Hz, 2H), 7.61 (d, JH–H = 8.0 Hz, 2H),
insoluble LaCl3 salts. Then the solvent was reduced to 2 mL 7.66 (dd, JP–H = 12.5 Hz, JH–H = 1.5 Hz, 1H), 7.73 (d, JH–H = 7.0
under vacuum and put in the fridge at 4 °C. From this solution Hz, 2H). 13C NMR (125 MHz, CDCl3): 126.6 (d, JP–C = 1.3 Hz,
a crop of dark red crystals of 1b suitable for X-ray diffraction CH), 126.8 (d, JP–C = 9.5 Hz, CH), 127.5 (CH), 128.0 (CH), 128.2
was obtained. The 13C NMR data are in agreement with the (CH), 128.8 (CH), 128.8 (d, JP–C = 6.6 Hz, CH), 128.9 (CH),
analogous complex [(C5H4Me)2Zr(PhC(CH)2CPh)].15 1H NMR 129.7 (d, JP–C = 1.5 Hz, CH), 130.8 (d, JP–C = 9.9 Hz, Cq), 131.9
(250 MHz, C6D6): 5.91 (s, 10H), 7.03–7.06 (m, 5H), 7.13–7.14 (d, JP–C = 10.3 Hz, CH), 134.0 (d, JP–C = 19.5 Hz, CH), 136.5 (d, JP–C
(m, 2H), 7.27–7.33 (m, 5H). 13C NMR (62.5 MHz, C6D6): 112.5 = 15.9 Hz, Cq), 137.0 (d, JP–C = 3.0 Hz, Cq), 150.4 (d, JP–C = 7.8 Hz,
(CH), 119.6 (CH), 128.7 (CH), 129.8 (CH), 130.1 (CH), 130.5 Cq), 153.8 (d, JP–C = 2.0 Hz, Cq). 31P NMR (200 MHz, CDCl3): 11.3.
(CH), 130.7 (CH), 149.9 (C), 192.7 (C). HRMS (ESI) for C26H23Zr HRMS (EI) for C22H17P: calc. (m/z) 312.1068; found (m/z) 312.1068.
10998 | Dalton Trans., 2013, 42, 10997–11004
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iodine. The 1,4-diiodobutadienes 2a and 2b were obtained in a
1 : 1 ratio according to the crude NMR spectrum. Full conver-
sion was observed without the addition of a copper catalyst.26
Compounds 2a and 2b were isolated in low 25% and 18%
yields, respectively, due to partial decomposition on silica gel.
2a was obtained previously only in trace amounts by the
titanium-catalysed coupling of phenylacetylene.16
Selective synthesis of 2,4-disubstituted phospholes
Further transformation of diiodide 2a via iodine/lithium
exchange followed by addition of dichlorophenylphosphine
yielded the first 2,4-diphenylsubstituted phosphole 3a in 65%
yield (eqn (1)).12a
Scheme 1 Hydrolysis and iodonolysis of disubstituted zirconacyclopentadienes.
X-ray crystallography
ð1Þ
Single crystals were coated in Paratone-N oil and mounted on
a loop. Data were collected at 150.0(1) K on a Nonius Kappa
CCD diffractometer using a Mo Kα (λ = 0.71070 A) X-ray source
and a graphite monochromator. All data were measured using
In order to simplify the procedure and circumvent the di-
phi and omega scans. The crystal structures were solved using iodide compounds, the addition of one equivalent of PhPCl2 to
SIR 97 and refined using SHELXL97.17,18
diphenylzirconacyclopentadienes 1a–c was investigated. Surpris-
ingly, this did not lead to a mixture of the expected 1,2,4-and
1,2,5-triphenylphospholes, but after workup 1,2,4-triphenyl-
phosphole 3a and 1,4-diphenyl-1,3-butadiene 4a were obtained
in a 1 : 1 ratio (Table 1). The products could be separated by
column chromatography and identified by multinuclear NMR
spectroscopy. Further improvement was achieved when only 0.55
equivalent of dichlorophenylphosphine was added, as the for-
mation of by-products due to an unreacted phosphorous reagent
could be avoided. Phosphole 3a was isolated as a pale-yellow,
relatively air-stable solid in 70% yield based on the phosphorous
DFT analysis
Calculations were performed with the Gaussian03 and
Gaussian09 suite of programs.19 The B3LYP functional20 was
used for computation of monomers and NBO calculations as
implemented in Gaussian03,20 and the B97D functional for
computation of the tetramer, as implemented in Gaussian09.21
The lanld2z basis set and associated core potential were
used for Zr atoms,22 and the main group elements (C and H)
were described with the 6-31G basis set.23 All basis sets were
used as implemented in the program. Stationary points were
identified as having no imaginary frequency. NBO analyses
were performed using NBO 5.9 as implemented in
Gaussian03.24
Table 1 Synthesis and 31P NMR data of 2,4-disubstituted phospholes
Reductive dimerization of terminal alkynes
The reductive dimerisation of phenylacetylene using the
lanthanum generated zirconocene(II) yielded a dark red solu-
tion of a mixture of diphenylzirconacyclopentadienes 1a–c, as
evidenced by ESI-MS, X-ray crystallography (see below) and
quenching experiments (Scheme 1). Hydrolysis of this mixture
1a–c showed that the ratio of the 1,3- and 1,4-disubstituted
butadiene isomers was 1 : 1, whereas the 2,3-isomer was
present in lower than 5% yield according to NMR analysis.‡
Trace amounts of benzene derivatives were also present.25 The
observed ratio for the butadienes is close to the one reported
by Erker et al. obtained with a zirconocene–diene complex and
phenylacetylene.6a However, in our case no side-products
arising from the zirconocene precursor were observed. This
result was further confirmed by the reaction of 1a–c with
Entry
R
R′
Ratioa 3/4 Yieldb 3 31P NMRc
a
b
c
d
e
f
g
h
i
Ph
Ph
Ph
Ph
Ph
1/1
1/1
4/1
2/1
2/1
2/1
1/1
1/1
2/1
1/1
70 (35)
61 (30)
46 (39)
48 (34)
36 (26)
24 (17)
57 (29)
63 (32)
47 (33)
72 (36)
40 (20)
11.3
10.8
12.3
11.9
10.6
11.7
13.4
8.1
p-CH3–(C6H4)
p-Cl–(C6H4)
p-F–(C6H4)
p-OMe–(C6H4) Ph
2-Naphthyl
2-Thienyl
n-Pentyl
t-Butyl
Me3Si
Ph
Ph
Ph
Ph
Ph
Ph
1.4
31.9
26.0
j
k
c-C6H11 1/1
a Determined by 1H NMR from the crude reaction mixture; b Isolated
yield under optimised conditions with respect to phosphorous reagent
and in parentheses with respect to zirconium reagent; c In CDCl3.
‡The ratio 2a/2b = 1/2.5 reported in 2006 (ref. 8) was based on GC analysis and
did not take into account the facile homo-Diels Alder reaction of the butadienes.
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reagent. This methodology was then extended to a large variety
of phospholes carrying aryl 3b–f, heteroaryl 3g, alkyl 3h and 3i
and trimethylsilyl 3j groups. An initial reaction was carried out
with 1.0 equivalent of PhPCl2 to determine the ratio between
phosphole 3 and butadiene 4. Based on this result, in a second
reaction the optimised amount of dichlorophosphine was added
to prepare the phospholes 3. The isolated yields vary between 72
and 24% (Table 1) and in all cases, only the 2,4-disubstituted
phospholes were obtained. The substituents R on the terminal
alkyne have a limited influence on these ratios. The only excep-
tion is the p-chlorophenyl substituent which clearly favors the
formation of 2,4-disubstituted phosphole 3c. In order to further
investigate this behaviour, the reaction with other electrophilic
phosphorous reagents was examined. PhPBr2 has been described
as being more reactive than PhPCl2.27 However, when this
reagent was used in the reaction with 1a–c, again only com-
pound 3a was formed. Heating at 60 °C for 15 h did not influ-
ence the reaction outcome. Reaction of 1a–c with
dichlorocyclohexylphosphine afforded only the 2,4-diphenyl-
phosphole 3k.
In order to ensure that the observed reactivity did not origi-
Scheme 2 Competition experiment between di- and tetrasubstituted zircona-
nate from the employed zirconocene source Cp2ZrCl2/La, an cyclopentadienes.
experiment was conducted using phenylpropyne. As expected,
in this case both phospholes 3l and 3m were formed in a 3 : 1
Table 2 Selected NMR data of some 2,4-disubstituted phospholes
ratio (eqn (2)).28
ð2Þ
R
1H (Hα, Hβ)a,b
13C (Cα, Cα′, Cβ, Cβ′)a,c
Finally, a competing experiment between di- and tetra-
substituted zirconacyclopentadienes coming from phenylacetylene
(1a–c) and from phenylpropyne (1d–e) was performed.
Addition of 0.5 equivalent of PhPCl2 to this mixture led to the
exclusive formation of phosphole 3a. Further addition of 0.5
equivalent of PhPCl2 afforded a mixture of 3l and 3m, which
further increased with increasing phosphorous reagent. The
2,5-disubstituted phosphole was not observed (Scheme 2).
Ph (3a)
7.08 (40.0), 7.66 (12.5) 128.0 (0), 153.8 (2.0),
131.9 (10.3), 150.4 (7.8)
p-OMe–(C6H4) (3e) 6.91 (38.0), 7.56 (12.5) 124.5 (0), 153.3 (1.8),
130.2 (10.0), 150.0 (7.6)
2-Thienyl (3g)
t-Butyl (3i)
Me3Si (3j)
6.88 (38.5), 7.43 (11.5) 124.9 (0), 147.0 (4.4),
130.5 (9.1), 143.8 (7.8)
6.24 (41.0), 6.75 (15.0) 122.7 (1.3), 161.8 (7.1),
130.9 (10.8), 165.8 (9.1)
7.43 (40.5), 7.29 (18.5) 149.4 (10.8), 150.3 (26.1),
147.4 (10.1), 154.8 (5.5)
2
3
a In CDCl3; b in parentheses JPH and JPH coupling constants; c in
Characterisation of 2,4-disubstituted phospholes
parentheses 1JPC and 2JPC coupling constants.
Multinuclear NMR spectroscopy. The 31P NMR shifts of
phospholes 3a–g are only slightly influenced by the aryl substi-
tuents going from 10.6 ppm for the methoxy group to
13.4 ppm for the thienyl group. Compared to the corres-
X-ray analysis
ponding 1,2,5-triphenylphosphole 5a,29 compound 3a displays Single crystals of 3a suitable for X-ray analysis were obtained
a downfield shift of 10 ppm. In the H NMR spectra the main by crystallisation from diethyl ether at 4 °C (Fig. 1). Compound
1
2
feature of phospholes 3 is the large JP–H coupling (40 Hz) 3a displays many similar features compared to the 2,5-diphenyl
between phosphorus and the hydrogen on the alpha ring analogue 5a (Table 3).31,32 The pyramidal phosphorus atom
carbon atom.30 In the 13C NMR spectra, the JC–P lies 0.065(1)
Å below a least-square fitted phosphole
1
coupling depends on the substitution, ranging from 0 to 4 Hz plane, compared to 0.048(1) Å for 5a. The P–C bonds are in the
for hydrogen and aromatic substituents to 26 Hz for the tri- range of single bonds, with a slight difference between the
methylsilyl group. Further differences in the NMR shifts unsubstituted (1.787(2) Å) and the phenyl-substituted (1.815(2)
between differently substituted phospholes are summarized in Å) ring carbons. The double bonds in the phosphole ring
Table 2.
(1.360(2) Å and 1.355(2) Å) are localised. The phenyl rings are
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Fig. 2 Molecular structure of 2,5-diphenylzirconacyclopentadiene 1b (50%
probability ellipsoids); only one of the two complexes is shown. A THF molecule
and hydrogen atoms are omitted for clarity. Selected bond distances (Å) and
angles (°): Zr(1)–C(7) 2.244(2), Zr(1)–C(10) 2.248(2), Zr(1)–C(21) 2.465(3), Zr(1)–
C(24) 2.523(2), C(7)–C(8) 1.359(3), C(8)–C(9) 1.474(3), C(9)–C(10) 1.360(3), C(7)–
Zr(1)–C(10) 86.74(7).
in the unit cell. Bond lengths and angles involving zirconium
are similar to other zirconacyclopentadienes, such as for
example in Cp2Zr(C4H2(TTMS)2) and Cp2Zr(C4Ph4).7,34 The
most important feature of 1b is the nearly planar phenyl–zircon-
acyclopentadiene–phenyl system with twist angles ranging
from 5.33° to 16.91°. This is in stark contrast to tetrasubsti-
tuted zirconacyclopentadienes where the phenyl rings in the 2
and 5 positions are bent at 40–60° due to steric crowding; for
example, in Cp2Zr(C4Ph4) the twist angles are 48.69° and
54.34°.34
Fig. 1 Molecular structures of 3a and 5a (50% probability ellipsoids). Hydro-
gen atoms are omitted for clarity.
Table 3 Selected bond distances (Å) and angles (°) of 3a and 5a
3a
5a
P1–C1
P1–C4
P1–C17
C1–C2
C2–C3
C3–C4
C1–P1–C4
C1–P1–C17
C4–P1–C17
C1–C2–C3–C4
1.787(2)
1.815(2)
1.828(2)
1.360(2)
1.466(2)
1.355(2)
90.72(7)
105.91(7)
104.16(6)
0.15(25)
1.823(2)
1.824(2)
1.836(2)
1.357(2)
1.439(2)
1.359(2)
91.34(8)
104.33(7)
105.49(7)
0.37(25)
When the isolated complex 1b was reacted with PhPCl2 in
THF-d8, no reaction was observed.
DFT analysis
Based on the X-ray structure of 1b, DFT calculations using the
Gaussian03 suite of programs were carried out. Initially, the
B3LYP functional was used, in conjunction with the 6-31G
basis set for all main-group elements and the lanl2dz basis set
for Zr. It was shown that the structure of 1b could not be cor-
rectly calculated. A deviation in the dihedral angles of 10–20°
was observed. Variation of basis sets and functionals did not
improve the results. A closer look at the crystal structure
revealed that intermolecular interactions (π-stacking) lead to a
tetrameric structure, i.e. four zirconacyclopentadienes form an
independent tetramer. This structure could be correctly repro-
duced using the B97D functional, which takes into account
dispersion effects for short distance interactions (Table 4).
In solution, it seems however more likely that the mono-
meric structure is prevalent. The 2,4-diphenylzirconacyclo-
pentadiene 1a was therefore also calculated with the B3LYP
functional and an NBO analysis of 1a and 1b was performed.
slightly more twisted with respect to the phosphole plane (7.08(10)°
and 8.34(8)°) compared to 4.63(8)° and 7.99(7)° for 5a.
Disubstituted vs. tetrasubstituted zirconacyclopentadienes
It was previously observed that bulky alkyl ligands in the alpha
position of zirconacyclopentadienes can hinder the formation
of phospholes.33 However, the inertness of 2,5-diarylzircona-
cyclopentadienes towards PhPCl2 was very intriguing as tetra-
substituted zirconacyclopentadienes carrying aryl groups in
the 2 and 5 positions react readily with PhPX2 as previously
reported and shown above.27 In order to gain more insight
into this selective transformation, we focused next on the
intermediate zirconacyclopentadienes.
X-ray study
X-ray quality crystals of 1b were obtained from a THF solution For comparison reasons the tetrasubstituted complexes 1d and
of a mixture of 1a–c at 4 °C (Fig. 2). Compound 1b contains 1e arising from phenylpropyne were also calculated (Table 5,
two zirconacyclopentadiene complexes and one THF molecule Fig. 3 and 4).
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Table 4 Comparison of X-ray data and DFT calculations on 1b
Dihedral anglea
X-ray dataa
Monomerb
Tetramerc
C5–C8/C35–C38 [°]
C7–C10/C33–C36 [°]
C9–C12/C31–C34 [°]
16.91/5.33
0.86/0.11
12.89/13.3
25.75
1.15
25.77
13.33/5.56
1.34/0.21
13.33/7.91
a Two distinct complexes in a unit cell; b B3LYP//6-31G (H, C) and
lanl2dz (Zr); c B97D//6-31++G (H), 6-31G (C) and lanl2dz (Zr).
Table 5 Comparison of calculateda dihedral angles (°) of 1a,b,d,e
Angle 1
Angle 2
Angle 3
2,4-Ph,3,5-H (1a)
2,5-Ph,3,4-H (1b)
2,4-Ph,3,5-Me (1d)
2,5-Ph,3,4-Me (1e)
+35.44
+25.75
+89.95
+54.91
+2.90
+1.15
−1.39
+1.90
+31.43
+25.76
−56.62
+54.91
a B3LYP//6-31G (H, C) and lanl2dz (Zr).
These calculations led to the following observations: (i) the
HOMOs of all complexes are on the zirconacyclopentadiene
backbones; however, the energy differences are too small to
explain the observed selectivity; (ii) an important difference in
the twist angles of the phenyl groups is observed between 1b
(25.75°) and 1e (54.91°); (iii) the unsubstituted α-carbon in 1a
has a significantly more negative charge (−0.63) than the
corresponding phenyl-substituted carbons in 1b (−0.41) and
1e (−0.42) and the methyl-substituted carbon in 1d (−0.38).
This difference in charge – combined with the least steric hind-
rance around zirconium – is in agreement with intermediate
1a being more reactive compared to 1b, d, e and in accordance
with the above competition experiment. However, the com-
plete inertness of 1b towards dihalophosphines seems to be
mainly related to steric hindrance arising from the small tilt
angle of the phenyl groups.
Fig. 3 Optimised geometries of zirconacyclopentadienes 1a, 1d, 1b and 1e
showing the HOMO.
Conclusions
In conclusion, a straightforward one-pot synthesis of a large
series of 2,4-disubstituted phospholes has been described
making use of the difference in reactivity between 2,4- and 2,5-
disubstituted zirconacyclopentadienes towards electrophilic
phosphorous reagents. An X-ray study and DFT analyses of the
intermediate zirconacyclopentadienes point towards steric
reasons and only slight electronic influences to explain this
Fig. 4 NBO analysis and HOMO energies of 1a, 1b, 1d and 1e.
selectivity. Further studies to extend this methodology to other chemistry of the new phosphole building blocks are currently
5-membered heteroles as well as the investigation of the under way.
11002 | Dalton Trans., 2013, 42, 10997–11004
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Acknowledgements
We thank the CNRS, the Université de Reims and the Region
Champagne-Ardenne for financial support and the platform
PlAneT for technical support. The assistance of Miss Carine
Machado (mass spectrometry) is acknowledged.
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