Boosting the nucleophilicity of phosphole lone pairs by isomerization

Article abstract of DOI:10.1021/om100423r

The phosphole isomer 4c can be obtained by nickelocene reduction complexation from the corresponding sulfide. It displays a ligating ability comparable to that of tris-tert-butylphosphine, and its [LPdCl₂] ₂ complex efficiently catalyzes the deboronation homocoupling of arylboronic acids.

Full text of DOI:10.1021/om100423r

Organometallics 2010, 29, 4785–4786 4785
DOI: 10.1021/om100423r
Boosting the Nucleophilicity of Phosphole Lone Pairs by Isomerization†
Ana Ciric and Franc-ois Mathey*
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences,
Nanyang Technological University, 21 Nanyang Link, Singapore 637371
Received May 5, 2010
Summary: The phosphole isomer 4c can be obtained by nick-
In fact, the lone pair of 4c appears at the same energy as the
lone pair of tri-tert-butylphosphine. Obviously, these phosp-
hole isomers deserved an experimental investigation.
Our starting products were the two isomeric sulfides 2b,c.
Compound 2b has already been described in the literature.2
The previously unknown 2c4 was made by acidic isomeriza-
tion of the corresponding phosphole sulfide following an
early protocol2 (eq 1).
elocene reduction complexation from the corresponding sul-
fide. It displays a ligating ability comparable to that of tris-
tert-butylphosphine, and its [LPdCl2]2 complex efficiently
catalyzes the deboronation homocoupling of arylboronic acids.
Due to their slight antiaromaticity,1 phosphole sulfides
such as 1a tend to isomerize to give products such as 2a with
one exocyclic double bond.2 Since phospholes are slightly
aromatic, the tendency is inverted for the trivalent species.
DFT calculations at the B3LYP/6-311þG(d,p) level perfectly
confirm these trends: 2a is more stable than 1a by 2.4 kcal
mol-1, whereas 4a is less stable than 3a by 3.5 kcal mol-1
.
The desulfurization of 2c by sodium in boiling toluene did
not give satisfactory results; thus, we decided to use a mild
technique relying on the reduction-complexation of P-sulfides
by nickelocene.5 Being driven by the complexation ability of
the P ligand, this method efficiently reduces 2, whereas it fails
with 1 (eq 2). The 1:1 P:Ni complexes thus obtained were fully
characterized,6 including X-ray crystal structure analyses.
A closer inspection of the theoretical data showed that the
lone pair at phosphorus is shifted to higher energy by 0.49 eV
upon isomerization of 3a into 4a. This destabilization of the
lone pair is probably due to an antibonding overlap with the
HOMO of the diene unit as shown in Figure 1, although
other factors such as geometry changes and hyperconjuga-
tion with the neighboring CH2 group might also intervene. It
must be stressed here that 2-phospholenes should also be
electron-rich ligands for reasons similar to those already
mentioned.3 When 4a is compared with 4c, the replacement
of Me by tBu leads to a further increase of the lone pair
energy by 0.11 eV.
The decomplexation was performed by KCN in H2O/
CH2Cl2. It takes several days for completion (eq 3).
As expected, the conditions are more drastic for 4c than for
4b, implying that the corresponding phosphole isomer 3c is a
† Part of the Dietmar Seyferth Festschrift. This paper is dedicated to
Professor Dietmar Seyferth as a leading contributor to main-group and
transition-metal chemistry.
(5) Mathey, F. J. Organomet. Chem. 1975, 87, 371. Mathey, F.;
Sennyey, G. J. Organomet. Chem. 1976, 105, 73.
(6) 5b: 31P NMR (CDCl3) δ 35.5; 1H NMR (CDCl3) δ 2.08 (s, Me),
2.97 (dd, 1H, JHH =17.4 Hz, JHP =10 Hz, CH2P), 3.52 (d, 1H, JHH =
*To whom correspondence should be addressed. E-mail: fmathey@
ntu.edu.sg.
(1) Nyulaszi, L.; Holloczki, O.; Lescop, C.; Hissler, M.; Reau, R. Org.
Biomol. Chem. 2006, 4, 996.
17.4 Hz, JHP=0 Hz, CH2P), 5.21 and 5.33 (2s, 2 ꢀ 1H, dCH2), 5.40 (s,
13C NMR (CDCl3) δ 16.70 (d, JCP = 11.5 Hz, Me), 38.61 (d, JCP
=
ꢀ
Cp), 6.14 (d, JHP=29.8 Hz, dCHP), 7.41 (s br, m,p-Ph), 7.91 (m, o-Ph);
(2) Mathey, F. Tetrahedron 1972, 28, 4171.
(3) Leca, F.; Lescop, C.; Toupet, L.; Reau, R. Organometallics 2004,
23, 6191. Leca, F.; Fernandez, F.; Muller, G.; Lescop, C.; Reau, R.; Gomez,
36.3 Hz, CH2P), 93.35 (s, Cp), 111.56 (d, JCP=8.6 Hz, dCH2), 128.64 (d,
JCP=10.5 Hz, CH(Ph)), 129.43 (d, JCP=43.9 Hz, dCHP), 130.96 (s, p-
CH(Ph)), 132.93 (d, JCP =12.4 Hz, CH(Ph)), 135.36 (d, JCP =40.1 Hz,
ipso-C(Ph)), 148.89 (s, dC), 153.70 (d, JCP=4.8 Hz, dC). 5c: 31P NMR
(CDCl3) δ 57.2; 1H NMR (CDCl3) δ 1.16 (d, JHP=15 Hz, tBu), 1.97 (s,
Me), 2.80 (dd, 1H, JHH=17.4 Hz, JHP=7.3 Hz, CH2P), 3.19 (d br, 1H,
JHH =17.4 Hz, JHP =0 Hz, CH2P), 5.14 and 5.20 (2s, 2 ꢀ 1H, dCH2),
5.34 (s, Cp), 5.97 (d, JHP=28.8 Hz, dCHP); 13C NMR (CDCl3) δ 16.57
(d, JCP = 10.5 Hz, Me), 27.41 (d, JCP = 3.8 Hz, Me(tBu)), 32.94 (d,
JCP = 31.6 Hz, CH2P), 34.58 (d, JCP = 22 Hz, C(tBu)), 93.27 (s, Cp),
110.43 (d, JCP = 7.7 Hz, dCH2), 128.14 (d, JCP= 38.3 Hz, dCHP),
149.50 (s, dC), 153.16 (s, dC).
ꢀ
ꢀ
M. Eur. J. Inorg. Chem. 2009, 5583.
(4) 2c: 31P NMR (CDCl3) δ 73.8; 1H NMR (CDCl3) δ 1.20 (d, JHP
=
17 Hz, tBu), 2.03 (s, Me), 2.81 (pseudo t, 1H, JHH ≈ JHP ≈ 17 Hz, CH2P),
3.15 (dd, 1H, JHH = 17.9 Hz, JHP = 5.5 Hz, CH2P), 5.16 (s br, 1H,
dCH2), 5.25 (s br, 1H, dCH2), 5.94 (d, JHP = 24.3 Hz, dCHP); 13C
NMR (CDCl3) δ 16.48 (d, JCP=16.2 Hz, Me), 24.69 (s, Me (tBu)), 33.90
(d, JCP =51.5 Hz, CH2P), 34.99 (d, JCP =50.6 Hz, C(tBu)), 112.02 (d,
JCP = 12.4 Hz, dCH2), 124.70 (d, JCP = 69.6 Hz, dCHP), 145.92 (d,
JCP =7.6 Hz, Cd), 156.42 (d, JCP =10.5 Hz, dC).
r
2010 American Chemical Society
Published on Web 06/07/2010
pubs.acs.org/Organometallics

4786 Organometallics, Vol. 29, No. 21, 2010
Ciric and Mathey
Figure 1. Highest occupied orbital (Kohn-Sham) of 4a.
significantly more powerful ligand than its phenyl analogue 3b.
Due to their high sensitivity to oxidation, these phosphole
isomers were only characterized by 1H, 13C, and 31P NMR.7
In order to confirm their high nucleophilicity, they were
Figure 2. X-ray crystal structure of the dimeric complex 6.
˚
Main bond lengths (A) and angles (deg): P1-Pd1 = 2.2094(5),
Pd1-Cl1 = 2.2775(6), Pd1-Cl2 = 2.4491(5), Pd1-Cl2A =
2.3277(5), C2-C3 = 1.445(4), C2-P1 = 1.808(2), C3-C6 =
1.396(3), C3-C4 = 1.459(4), C4-C5 = 1.400(3), C5-P1 =
1.800(2), C8-P1= 1.854(2); C5-P1-C2=93.54(12).
1
complexed with [W(CO)5]. It is known that the JP-W
coupling constant is directly correlated with the electron-
donating power of the P substituents: the higher the dona-
tion, the lower the coupling.8 In our case, the 1JP-W coupling
in the tungsten complex of 4c is 227.8 Hz vs 228.5 Hz for the
tBu3P complex and 245 Hz for the Ph3P complex. Both 4b
and 4c were reacted with PdCl2. Once again, their behaviors
are different (eq 4).
cross-coupling of p-chlorotoluene with phenylboronic acid
(eq 5).
The surprising result was that we got a great deal of
biphenyl produced by deboronation homocoupling. This
side reaction is known,11 but its extent is exceptionally high.
Thus, we were led to investigate more closely this transfor-
mation. The results are shown in eq 6.
All of these complexes were isolated and fully character-
ized, including X-ray crystal structure analyses. We give the
data for 6 in Figure 2.
Since the energy levels of the lone pairs of 4c and tris-tert-
butylphosphine are identical at the same level of theory, we
were led to check the catalytic properties of the palladium
complexes of 4c in the Suzuki cross-coupling of aryl chlorides
with arylboronic acids. Tris-tert-butylphosphine and, more
generally, bulky trialkylphosphine Pd complexes are known
to be exceptionally efficient as catalysts for the Heck and
Suzuki reactions.9,10 Using the conditions described in the
literature,10 we found that complex 7 indeed catalyzes the
It is clear that 6 deserves further investigation as a catalyst
for homocoupling and that other uses of 4c as a ligand for
catalytic reactions are possible.
Acknowledgment. We thank the Nanyang Technolog-
ical University in Singapore for financial support of this
work, Dr. Li Yong Xin for the X-ray crystal structure
analyses, and Amy Tan and Mohamed Feroz Shah for
their help during their HNRS projects.
(7) 4b: 31P NMR δ -16 ppm; 1H NMR (CDCl3) δ 2.03 (s, Me), 2.65
(m, 1H, CH2P), 3.10 (m, 1H, CH2P), 5.05 (s br, 1H, dCH2), 5.12 (s br,
1H, dCH2), 6.24 (d, JHP=38.5 Hz, dCHP), 7.32 (m, 3H, m,p-Ph), 7.45
(m, 2H, o-Ph); 13C NMR (CDCl3) δ 16.71 (Me), 33.82 (d, JCP=7.6 Hz,
CH2P), 107.81 (d, JCP = 3.8 Hz, dCH2), 128.45 (d, JCP = 7.6 Hz,
CH(Ph)), 129.09 (s, p-CH(Ph)), 132.53 (d, JCP = 20.0 Hz, CH(Ph)),
133.66 (d, JCP=13.3 Hz, dCHP), 140.61 (d, JCP=22.9 Hz, ipso-C(Ph)),
150.40 (s, β-Cd), 152.55 (s, β-Cd). 4c: 31P NMR δ 7.
Supporting Information Available: Text giving additional
spectral data and CIF files giving details of the X-ray crystal
structure analysis of compounds 5a,b and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
(8) Schumann, H.; Kroth, H.-J. Z. Naturforsch. 1977, 32b, 768.
Mercier, F.; Mathey, F.; Afiong-Akpan, C.; Nixon, J. F. J. Organomet.
Chem. 1988, 348, 361.
(9) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.
(10) Kirchhoff, J. H.; Dai, C.; Fu, G. C. Angew. Chem., Int. Ed. 2002,
41, 1945.
(11) Jin, Z.; Guo, S.-X.; Gu, X.-P.; Qiu, L.-L.; Song, H.-B.; Fang,
J.-X. Adv. Synth. Catal. 2009, 351, 1575.