Annelation of phosphole-substituted fischer carbene complexes by alkynes

Article abstract of DOI:10.1021/om4009795

The reaction of 1-phenyl-2-lithio-3,4-dimethylphosphole with W(CO) ₆ at -90 C followed by methyl triflate at room temperature gives the complex [(phosphol-2-yl)methoxycarbene]pentacarbonyltungsten (1). This complex reacts with sulfur to give the corresponding complex [methyl (phosphol-2-yl)thiocarboxylate]pentacarbonyltungsten (2), in which tungsten has migrated from the carbene to phosphorus. The reaction of 3 with alkynes at 90 C gives the corresponding phospholene-annelated cyclopentenones 5 without the need for a nickel catalyst. In one case, an intermediate vinylketene rearranges by an intramolecular Diels-Alder reaction to give the tricyclic compound 6. All of the products have been characterized by X-ray crystal structure analysis.

Full text of DOI:10.1021/om4009795

Article
pubs.acs.org/Organometallics
Annelation of Phosphole-Substituted Fischer Carbene Complexes
by Alkynes
Kim Hong Ng, Yongxin Li, Rakesh Ganguly, and Francois Mathey*
Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
S
* Supporting Information
ABSTRACT: The reaction of 1-phenyl-2-lithio-3,4-dimethylphosphole with W(CO)6 at −90 °C followed by methyl triflate at
room temperature gives the complex [(phosphol-2-yl)methoxycarbene]pentacarbonyltungsten (1). This complex reacts with
sulfur to give the corresponding complex [methyl (phosphol-2-yl)thiocarboxylate]pentacarbonyltungsten (2), in which tungsten
has migrated from the carbene to phosphorus. The reaction of 3 with alkynes at 90 °C gives the corresponding phospholene-
annelated cyclopentenones 5 without the need for a nickel catalyst. In one case, an intermediate vinylketene rearranges by an
intramolecular Diels−Alder reaction to give the tricyclic compound 6. All of the products have been characterized by X-ray
crystal structure analysis.
experiments, we managed to change the ratio between the free
and complexed species and to isolate the tervalent phosphole
carbene 1 (eq 1).
INTRODUCTION
■
In contrast to furans, thiophenes, and pyrroles, phospholes are
essentially nonaromatic1 as a result of the pyramidality of the
heteroatom. Their weak aromatic stabilization energy (ASE)2
mainly results from a σ*/π hyperconjugation between the
exocyclic P−R bond and the dienic system. From a practical
standpoint, this weak ASE means that access to functional
phospholes is far less simple than for the classical heteroles. In a
different field, Fischer carbene complexes have seen an extraor-
dinary development of their applications in organic synthesis.3
Combining the chemistry of Fischer carbene complexes with
phospholes was thus quite appealing, and our preliminary
results4 immediately showed unique interactions between the
phosphole dienic system and the carbene−metal bond with no
equivalent in the furan, thiophene, and pyrrole derivatives.5
In this report, we wish to describe additional examples of this
nonconventional chemistry, including an annelation of the
phosphole ring by reaction with alkynes.
At 316.94 ppm, the 13C carbenic chemical shift of 1 is prac-
tically identical with that of its P−W(CO)5 complex (3). The
X-ray crystal structure of 1 is shown in Figure 1. The structural
parameters of 1 and its P−W(CO)5 complex are quite similar.
The CW bond is slightly longer in 1 than in 3 (2.202(2) vs
2.173(4) Å), and the ring−carbene bond is slightly shorter
(1.462(3) vs 1.475(6) Å), but these data are probably not
significant.
In order to test the reactivity of the trivalent phosphorus in 1,
we performed the reaction with sulfur under mild conditions
(room temperature, overnight). To our surprise, tungsten shifted
from carbon to phosphorus and a thiocarboxylate was formed
(eq 2).
The reaction is quasi-quantitative. The thiocarbonyl group
of 2 appears at 204.6 ppm in the 13C NMR spectrum, and the
P−W coupling is 225 Hz. The X-ray crystal structure is shown
in Figure 2. The thiocarboxylate and the dienic system are prac-
tically coplanar (interplane angle 6.97°). We have found a
similar reaction in the literature involving sulfur, CO, and a
RESULTS AND DISCUSSION
■
In our preceding report, we described the phosphole-carbene
derivative as a P−W(CO)5 complex, but the 31P monitoring of
the crude reaction mixture also showed a peak corresponding
very probably to the free species. By using only 1 equiv of
W(CO)6 per phosphole instead of 2 equiv as in our previous
Received: October 7, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om4009795 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 2. X-ray crystal structure of thioester 2. Main distances (Å) and
angles (deg): P−W 2.5062(6), P−C11 1.820(2), P−C6 1.789(2),
P−C14 1.825(3), C6−C7 1.343(4), C7−C9 1.480(4), C9−C11
1.373(3), C11−C12 1.451(3), C12−S 1.635(3), C12−O6 1.352(3);
C6−P−C11 90.70(11), C11−C12−O6 109.3(2), C12−C11−P
121.16(17), O6−C12−S 122.34(19).
extension of this work, we decided to investigate the reaction of
3 with alkynes. Once again, compound 3 displays a chemistry
which is different from that of other heteroaryl derivatives of
Fischer carbene complexes. In all of the cases, the major final
product is a cyclopentenone (eq 3).
Figure 1. X-ray crystal structure of the phosphole-substituted Fischer
carbene complex 1. Main distances (Å) and angles (deg): P−C14
1.836(3), P−C8 1.820(2), P−C13 1.785(3), C8−C9 1.373(3),
C9−C11 1.470(4), C11−C13 1.347(4), C8−C6 1.462(3), C6−O6
1.327(3), C6−W1 2.202(2); C8−P−C13 90.04(11), O6−C6−C8
106.7(2), O6−C6−W1 130.62(17), C8−C6−W1 122.19(17).
All three cyclopentenones display a ketonic carbonyl in their
13C NMR spectrum: 5a, δ(13CO) 199.84, JC−P = 3.9 Hz; 5b,
δ(13CO) 200.94, JC−P = 3.6 Hz; 5c, δ(13CO) 200.08, JC−P = 3.9
Hz. All of these species have been characterized by X-ray crystal
structure analysis. The structure of 5a is depicted in Figure 3
as an example. The O−C−C−P dihedral angle is 75.84°. The
cyclopentenone lies on the phenyl side of the phosphole plane;
the Ph−P−C−CO dihedral angle is 22.75°. These cyclope-
ntenones are not the initial products of the reaction. The crude
Fischer carbene−Cr(CO)5 complex leading to a thiocarbox-
ylate and Cr(CO)6.6 Here, the intramolecular P-ligand plays the
role assigned to CO in the published work. This result shows
that the carbene unit is more reactive than the tricoordinate
phosphorus.
mixture contains intermediates 4: 4a, δ(31P) −2.25, JP−W
234 Hz; 4b, δ(31P) −3.33, JP−W = 238 Hz; 4c, δ(31P) −4.66,
P−W = 234 Hz. These intermediates are hydrolyzed during the
=
J
chromatographic purification. We propose the mechanism given
in eq 4 to rationalize these observations.
In our previous work, we described the curious reaction of
styrene with the P−W(CO)5 complex of 1 (3). As a logical
This proposed mechanism is exactly similar to that of a
cyclopentenone synthesis from alkenyl Fischer carbenes and
B
dx.doi.org/10.1021/om4009795 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 3. X-ray crystal structure of cyclopentenone 5a. Main distances (Å) and angles (deg): P−W 2.4985(6), P−C6 1.832(2), P−C12 1.794(2),
P−C20 1.860(2), C12−C13 1.325(4), C13−C15 1.525(3), C15−C17 1.537(3), C17−C18 1.349(3), C18−C19 1.480(3), C19−C20 1.509(3),
C19−O6 1.215(3); C12−P−C20 91.36(11), C18−C19−C20 107.54(18), C19−C20−P 112.17(16).
Figure 4. X-ray crystal structure of tricyclic compound 6. Main
distances (Å) and angles (deg): P−W 2.5045(18), P−C6 1.837(7),
alkynes as proposed recently by Barluenga.7 The only difference
P−C12 1.811(7), P−C17 1.822(7), C12−C13 1.329(10), C13−C15
1.496(9), C15−C17 1.562(10), C15−C18 1.546(9), C18−O6
1.429(9), C18−C20 1.505(10), C20−C22 1.341(11), C22−C29
1.493(10), C29−O7 1.209(9), C29−C17 1.504(10); C12−P−C17
90.7(3), C15−C17−C18 60.4(4), C18−C17−C29 105.4(6),
C17−C29−C22 107.5(6).
is that the synthesis of Barluenga needs a nickel(0) catalyst. It is
clear that this annelation reaction works with phospholes
because the ring is not aromatic, in contrast to those of
thiophenes, furans, and pyrroles. In the case of phenylpropyne,
the cyclopentenone 5b is accompanied by the secondary pro-
duct 6, which we have been able to isolate and to characterize
by X-ray crystal structure analysis (Figure 4). Some time ago,
due to the close proximity of a reactive diene and a carbenic
center.
Moser described what he calls the interrupted Dotz reaction, in
̈
which a Fischer carbene complex reacts with an alkyne to give a
vinylketene.8 On this basis, we propose the mechanism given in
eq 5 for the formation of 6.
EXPERIMENTAL SECTION
■
Oven-dried glassware (105 °C) was used and cooled under a nitrogen
atmosphere. All reactions were carried out with distilled dry solvents
and under a N2 atmosphere. Commercially available diphenylacety-
lene, 1-phenyl-1-propyne, and phenylacetylene were used without
purification. Silica gel (230−400 mesh) was used for the chromato-
graphic separations. NMR spectra were recorded on a JEOL ECA
400, JEOL ECA 400 SL, or Bruker BBFO2 400 MHz spectrometer.
The tricyclic compound 6 probably results from an intra-
molecular [4 + 2] cycloaddition between the vinylketene and
the phosphole double bond.
At this stage, it is clear that the unique chemistry of
phospholyl-substituted carbene complexes such as 1 and 3 is
C
dx.doi.org/10.1021/om4009795 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Diphenylacetylene (neat) was added, and the tube was sealed and
heated at 90 °C for 7 h. Upon completion of the reaction, CH2Cl2
was added to the crude mixture,; 31P NMR showed a major signal at
−2.25 ppm. Purification by column chromatography with a 4/1 mix-
ture of hexane and dichloromethane gave product 5a as a yellow oil
(0.0194 g, 55%).
1
31P NMR (CD2Cl2): δ 14.27 ppm (JP−W = 243.3 Hz). H NMR
(CD2Cl2): δ 1.56 (s, 3H, Me), 1.80 (s, 3H, Me), 3.83 (s, 1H, CH),
2
6.04 (d, JH−P = 36.0 Hz, 1H, CH-P), 6.09−6.11 (m, 2H, Ph),
6.90−6.94 (m, 2H, Ph), 7.00−7.08 (m, 3H, Ph), 7.35−7.38 (m, 3H,
Ph), 7.48−7.53 (m, 3H, Ph), 7.59−7.61 (m, 2H, Ph). 13C NMR
(CD2Cl2): δ 18.50 (d, 3JC−P = 11.6 Hz, Me), 25.24 (d, 3JC−P = 5.4 Hz,
1
Me), 64.98 (s, C−Me), 66.65 (d, JC−P = 19.4 Hz, C−H), 122.49
(d, JC−P = 42.6 Hz, P−CH), 133.10 (d, JC−P = 29.9 Hz, P−C(Ph)),
136.7 (s, =C−Ph), 140.51 (s, C−Ph), 160.13 (d, JC−P = 3.8 Hz, 
C−Me), 197.36 (d, JC−P = 7.4 Hz, W(CO)5 cis CO), 199.84 (d, JC−P
= 3.9 Hz, CO), 200.20 (d, JC−P = 23.0 Hz, W(CO)5 trans CO).
Exact mass: calcd for C32H23O6PW, 718.0742; found, 718.0783.
Cyclopentenone 5b and Tricyclic Product 6. The same
procedure as for 5a was used with 1-phenyl-1-propyne. The crude
mixture contains mainly two products at −3.33 (4b) and 5.0 ppm.
Elution with a mixture of hexane and dichloromethane (4/1) gave the
product at 5 ppm, which evolved overnight to give 6 (0.0028 g, 9%),
while 4b (−3.33 ppm) gave 5b (0.0118 g, 40%), which was eluted with
a 3/2 hexane/dichloromethane mixture.
All spectra were recorded at 298 K. Proton decoupling was applied
for 13C and 31P spectra. X-ray crystallographic analyses were performed
on a Bruker X8 APEX CCD diffractometer or a Bruker Kappa CCD
diffractometer.
Tervalent Phospholyl-Substituted Fischer Carbene Complex
1. 1-Phenyl-2-bromo-3,4-dimethylphosphole9 (1.00 g, 3.74 mmol) was
dissolved in THF (15 mL) in a Schlenk tube and cooled to −100 °C.
Then, n-BuLi (2.6 mL, 1.6 M in Hex, 4.11 mmol) was added dropwise
into the solution. The reaction mixture was stirred for 0.5 h at −100
°C. To the mixture was added W(CO)6 (1.3161 g, 3.74 mmol), and it
was then stirred at −90 °C for 0.5 h followed by −80 °C for 0.5 h. The
reaction mixture was slowly warmed to room temperature. Subse-
quently, methyl trifluoromethanesulfonate (0.42 mL, 3.74 mmol)
was added. The crude mixture was purified using chromatography at
−20 °C with a 4/1 hexane/DCM eluent mixture. A dark red oil of
compound 1 (0.3730g, 18%) was obtained before its P-W(CO)5
complex 3 (0.3284, 10%). Hexane was added, and pure red crystalline
1 was obtained at −25 °C.
Data for 5b are as follows. 31P NMR (CDCl3): δ 13.12 ppm (JP−W
=
1
241.8 Hz). H NMR (CDCl3): δ 0.92 (s, 3H, Me), 1.60 (s, 3H, Me),
1.64 (s, 3H, Me), 3.68 (s, 1H, C−H), 5.97 (d, 2JH−P = 35.7 Hz, 1H, 
CH−P), 7.04−7.07 (m, 2H, Ph), 7.38−7.54 (m, 8H, Ph). 13C NMR
3
(CD2Cl2): δ 8.14 (s, Me), 18.18 (d, JC−P = 11.6 Hz, Me), 24.58
(d, JC−P = 5.3 Hz, Me), 64.39 (s, C−Me), 65.90 (d, 1JC−P = 19.8 Hz,
3
C−H), 121.56 (d, JC−P = 42.2 Hz, P−CH), 132.33 (d, JC−P = 28.5
Hz, P−C(Ph)), 135.3 (s, PhCC−Me), 137.77 (s, C−Ph), 159.26
(d, JC−P = 3.8 Hz, C−Me), 196.59 (d, JC−P = 7.2 Hz, W(CO)5 cis
CO), 199.38 (d, JC−P = 23.1 Hz, W(CO)5 trans CO), 200.94 (d,
J
C−P = 3.6 Hz, CO). Exact mass: calcd fo rC27H21O6PW, 656.0585;
1
31P NMR (CD2Cl2): δ 13.66 ppm. H NMR (CD2Cl2): δ 2.09
(d, 4JH−P = 3.2 Hz, 3H, Me), 2.12 (dd, 4JH−P = 4.4 Hz, 3H, Me), 4.41
found, 656.0586.
Data for 6 are as follows. 31P NMR (CDCl3): δ 13.21 ppm (JP−W
=
2
(s, 3H, OMe), 6.73 (d, JH−P = 38.8 Hz, 1H, CH−P), 7.24−7.32
238.1 Hz). 1H NMR (CD2Cl2): δ 1.68 (s, 3H, Me), 2.31 (s, 3H, Me),
2.33 (s, 3H, Me), 3.69 (s, 3H, OMe), 6.07 (d, 2JH−P = 37.6 Hz, 1H, 
CH-P), 7.24−7.30 (m, 2H, Ph), 7.35−7.46 (m, 3H, Ph), 7.49−7.51
(m, 3H, Ph), 7.68−7.74 (m, 2H, Ph). 13C NMR (CDCl3): δ 12.49 (s,
3
(m, 5H, Ph). 13C NMR (CD2Cl2): δ 16.97 (s, Me), 18.28 (d, JC−P
=
1.9 Hz, Me), 68.99 (s, OMe), 129.23 (d, JC−P = 7.7 Hz, Ph), 130.37 (s,
Ph), 130.55 (d, JP−C = 14.0 Hz, P−C(Ph)), 134.13 (d, JP−C = 4.8 Hz,
P−CH), 134.37 (d, JC−P = 19.7 Hz, Ph), 143.61 (d, JC−P = 16.6 Hz,
P−C), 150.23 (d, JC−P = 7.7 Hz, C−Me), 167.56 (d, JC−P = 2.8 Hz,
C−Me), 198.39 (s, C−W(CO)5 cis CO), 205.22 (s, C−W(CO)5
trans CO), 316.94 (d, JC−P = 30.1 Hz, CW). Exact mass: calcd for
C19H15O6PW, 554.0116; found, 554.0115.
Me), 15.30 (s, Me), 18.87 (d, 3JC−P = 11.3 Hz, Me), 48.75 (d, 2JC−P
=
31.7 Hz, P−C), 59.59 (s, OMe), 70.14 (s, C−Me), 84.27 (s, C−OMe),
128.63 (d, JC−P = 45.3 Hz, P−CH), 132.69 (d, JC−P = 34.6 Hz, P−
C(Ph)), 138.86 (d, JC−P = 6.4 Hz, C−Me), 150.75 (s, Ph−C
CMe), 161.08 (s, PhCC−Me), 194.56 (d, JC−P = 3.3 Hz, CO),
196.86 (d, JC−P = 7.5 Hz, W(CO)5 cis CO), 199.51 (d, JC−P = 22.7 Hz,
W(CO)5 trans CO). Exact mass: calcd for C29H23O7PW, 698.0691;
found, 698.0698.
Sulfuration of 1. Red solid 1 (0.020 g, 0.036 mmol) was dissolved
in toluene (5 mL), and sulfur powder (0.0012g, 0.036 mmol) was
added. A small drop of N-methylimidazole was added, and the reaction
mixture was stirred overnight at room temperature. The mixture was
purified by flash chromatography with a 4/1 hexane/dichloromethane
mixture. Thiocarboxylate 2 (0.0696 g, 95%) was obtained as an orange
oil. Crystallization with pentane was achieved at −25 °C.
Cyclopentenone 5c. The same procedure as for 5a was used with
phenylacetylene. The crude mixture contained 4c (−4.66 ppm) and
5c. Chromatography as usual gave pure 5c (0.014 g, 42%).
1
31P NMR (CH2Cl2): δ 20.96 ppm (JP−W = 225.4 Hz). H NMR
4
1
31P NMR (CDCl3): δ 13.49 ppm (JP−W = 244.1 Hz). H NMR
(CDCl3): δ 2.28 (d, JH−P = 0.9 Hz, 3H, Me), 2.64 (s, 3H, Me), 4.02
2
(s, 3H, OMe), 6.68 (d, JH−P = 35.7 Hz, 1H, CH−P), 7.38−7.49
(CDCl3): δ 1.72 (s, 3H, Me), 1.77 (s, 3H, Me), 3.69 (s, 1H, C-H),
5.34 (s, 1H, C−H), 5.98 (d, 2JH−P = 35.2 Hz, 1H, CH−P), 7.17−
7.20 (m, 2H, Ph), 7.42−7.44 (m, 6H, Ph), 7.48−7.52 (m, 2H, Ph). 13C
3
(m, 5H, Ph). 13C NMR (CDCl3): δ 17.64 (d, JC−P = 6.7 Hz, Me),
17.98 (d, 3JC−P = 9.5 Hz, Me), 57.97 (s, OMe), 128.44 (s, Ph), 128.92
(d, JC−P = 68.0 Hz, P−C(Ph)), 129.07 (d, JC−P = 11.5 Hz, Ph), 131.09
(s, Ph), 132.25 (d, JC−P = 13.4 Hz, Ph), 137.04 (d, JC−P = 40.3 Hz,
P−CH), 144.15 (d, JC−P = 45.0 Hz, P−C−C(S)OMe), 150.02
(d, JC−P = 5.8 Hz, C−Me), 158.69 (d, JC−P = 14.4 Hz, C−Me), 196.51
(d, JC−P = 5.7 Hz, W(CO)5 cis CO), 198.67 (d, JC−P = 21.1 Hz,
W(CO)5 trans CO), 204.60 (d, JC−P = 16.3 Hz, CS). Exact mass:
calcd for C17H11O6PSW, 557.9524; found, 557.9539.
3
3
NMR (CDCl3): δ 18.46 (d, JC−P = 11.6 Hz, Me), 25.33 (d, JC−P
=
2
5.2 Hz, Me), 66.17 (s, C-Me), 67.16 (d, JC−P = 19.2 Hz, C−H),
122.89 (d, JC−P = 42.3 Hz, P−CH), 131.47 (s, C−H), 132.31
(d, JC−P = 30.2 Hz, P−C(Ph)) 135.89 (s, C−Ph), 158.25 (d, JC−P
=
3.8 Hz, C−Me), 196.85 (d, JC−P = 7.1 Hz, W(CO)5 cis CO),
199.52 (d, JC−P = 23.1 Hz, W(CO)5 trans CO), 200.08 (d, JC−P
=
Cyclopentenone 5a. Complex 34 (0.0426g, 0.049 mmol) was
placed in an oven-dried NMR tube under a stream of N2.
3.9 Hz, CO). Exact mass: calcd for C26H19O6PW, 642.0429; found,
642.0443.
D
dx.doi.org/10.1021/om4009795 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures giving NMR spectra of the compounds prepared in this
work and CIF files giving X-ray crystallographic data for
compounds 1, 2, 5a, and 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for F.M.: fmathey@ntu.edu.sg.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the Nanyang Technological University in
Singapore for the financial support of this work.
■
REFERENCES
■
(1) Recent reviews on phosphole chemistry: Zagidullin, A. A.;
Bezkishko, I. A.; Miluykov, V. A.; Sinyashin, O. G. Mendeleev Commun.
́
2013, 23, 117. Reau, R.; Dyer, P. W. Comprehensive Heterocyclic
Chemistry III; Elsevier: Oxford, U.K., 2008; Vol. 10, p 954. Quin, L. D.
Curr. Org. Chem. 2006, 10, 43. Mathey, F. Acc. Chem. Res. 2004, 37,
954.
(2) Cyranski, M. K.; Krygowski, T. M.; Katritzky, A. R.; Schleyer, P.
̃
v. R. J. Org. Chem. 2002, 67, 1333.
(3) de Meijere, A.; Schirmer, H.; Duetsch, M. Angew. Chem., Int. Ed.
2000, 39, 3964. Barluenga, J.; Santamaria, J.; Tomas, M. Chem. Rev.
2004, 104, 2259. Doetz, K. H.; Stendel, J., Jr. Chem. Rev. 2009, 109,
3227. de Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev.
2009, 253, 862.
(4) Ng, K. H.; Li, Y.; Ganguly, R.; Mathey, F. Organometallics 2013,
32, 2287.
(5) See, for example: Metelkova,
́ ́
R.; Tobrman, T.; Kvapilova, H.;
Hoskovskova, I.; Ludvik, J. Electrochim. Acta 2012, 82, 470. Lotz, S.;
́
van Jaarsveld, N. A.; Liles, D. C.; Crause, C.; Gorls, H.; Terblans, Y. M.
̈
Organometallics 2012, 31, 5371. van Jaarsveld, N. A.; Liles, D. C.; Lotz,
S. Dalton Trans. 2010, 39, 5777. Crause, C.; Gorls, H.; Lotz, S. Dalton
̈
Trans. 2005, 1649.
(6) Zheng, Z.; Chen, J.; Luo, N.; Yu, Z.; Han, X. Organometallics
2006, 25, 5301.
(7) Barluenga, J.; Barrio, P.; Riesgo, L.; Lop
́ ́
ez, L. A.; Tomas, M. J.
Am. Chem. Soc. 2007, 129, 14422.
(8) Moser, W. H.; Sun, L.; Huffman, J. C. Org. Lett. 2001, 3, 3389.
(9) Deschamps, E.; Mathey, F. Bull. Soc. Chim. Fr. 1992, 129, 486.
E
dx.doi.org/10.1021/om4009795 | Organometallics XXXX, XXX, XXX−XXX