Electronically tunable compact trialkylphosphines: SMAPs-bridged bicyclic phosphines

Article abstract of DOI:10.1246/cl.2006.294

A series of silicon-constrained monodentate trialkylphosphines SMAPs bearing a Si substituent with diverse electronic natures were synthesized. DFT calculations and NMR measurements indicated that these compounds constitute a class of electronically tunab

Full text of DOI:10.1246/cl.2006.294

294
Chemistry Letters Vol.35, No.3 (2006)
Electronically Tunable Compact Trialkylphosphines:
SMAPs-bridged Bicyclic Phosphines
Atsuko Ochida, Shinichiro Ito, Takahiro Miyahara,
Hajime Ito, and Masaya SawamuraÃ
Division of Chemistry, Graduate School of Science,
Hokkaido University, and PRESTO, JST, Sapporo 060-0810
(Received December 27, 2005; CL-051576; E-mail: sawamura@sci.hokudai.ac.jp)
A series of silicon-constrained monodentate trialkylphos-
S
P
1) TfOH (8 equiv.)
CH2Cl2
25 °C, 15 h
S
P
S
P
X
phines SMAPs bearing a Si substituent with diverse electronic
natures were synthesized. DFT calculations and NMR measure-
ments indicated that these compounds constitute a class of
electronically tunable trialkylphosphines. The tunable range
of donor power overlaps those of tri(tert-butyl)phosphine and
monoalkyldiarylphosphines.
I
(5 equiv.)
•
Pd2(dba)3 CHCl3
(1 mol%)
KOAc (2 equiv.)
NMP
Si
Si
H
Si
2) LiAlH4 (10 equiv.)
Et2O
–5 °C, 24 h
(3)
88%
(slow addition of 3)
X
2a
P
Si
2b: X = 4-NMe2 (50 °C, 10 h, 63%)
2c: X = 4-OMe (100 °C, 4 h, 55%)
2d: X = 4-Me (100 °C, 5 h, 51%)
2e: X = 4-Cl (50 °C, 4 h, 34%)
2f: X = 4-CF3 (100 °C, 3 h, 33%)
2g: X = 3,5-(CF3)2 (rt, 4 h, 20%)
Modification of the properties of a metal complex with elec-
tronically tunable ligands can be a powerful means for the devel-
opment of a catalyst with specific properties. Such ligands also
are useful for mechanistic research and the development of func-
tional materials. To gain an electronic effect without limitation
by accompanying steric effects, a modifying group must operate
at the position distal to the metal-coordinating center. This is
possible with ligands of a sp2-hybridized donor atom such as
pyridine derivatives or with those of a sp3-hybridized donor
atom having an aromatic substituent such as an arylphosphine.1
On the other hand, electronic tuning of trialkylphosphines,
which are a class of ligands with strong ꢀ-donor ability, is not
possible with existing compounds. We report here that the sili-
con-constrained bicyclic phosphines SMAPs (1a–1g, Figure 1)2
bearing Si substituents with varied electronic natures constitute a
class of electronically tunable trialkylphosphines. DFT calcula-
tions indicated that the tunable range of the donor power over-
laps those of (t-Bu)3P and RAr2P (R: alkyl, Ar: aryl).
SMAP derivatives with a substituted phenyl group on the
bridgehead silicon were synthesized through a hydrosilane-type
compound H-SMAP sulfide (3), a pivotal compound of diverse
reactivity. The hydrosilane 3 was obtained as an air-stable crys-
talline material from Ph-SMAP sulfide (2a) in 88% yield through
protodesilation with TfOH followed by reduction with LiAlH4
(Scheme 1). Note that the Si–Ph bond in Ph-SMAP sulfide
was stable against protodesilation when compared to that in acy-
clic phenylsilane PhSiBu3. While the latter was totally cleaved
on treatment with 1.1 equiv. of TfOH in CH2Cl2 at 0 ꢀC for
3 h, the former needed 8 equiv. of TfOH and a reaction time of
15 h at 25 ꢀC for 100% cleavage. The low reactivity of Ph-SMAP
sulfide likely is due to instability of the leaving non-planar
Si2Cl6 (4 equiv.)
benzene
reflux, 3 h
X
1b: 4-Me2N-Ph-SMAP (76%)
1c: 4-MeO-Ph-SMAP (79%)
1d: 4-Me-Ph-SMAP (94%)
1e: 4-Cl-Ph-SMAP (90%)
1f: 4-CF3-Ph-SMAP (92%)
1g: 3,5-(CF3)2-Ph-SMAP (85%)
Scheme 1. Synthesis of SMAP derivatives with a substituted
phenyl group at the bridgehead silicon atom (1b–1g).
bridgehead silyl cation.
The palladium-catalyzed hydrosilane–iodoarene coupling
developed by Masuda was applied to H-SMAP sulfide (3) to af-
ford a series of SMAP sulfides (2b–2g).3 The yield of the cross-
coupling product depended on the iodoarenes and the reaction
conditions. Generally, the yield was improved (20–63%) by
slow addition of a solution of hydrosilane 3 into a solution of
the catalyst and iodoarene. Reduction of the phosphine sulfides
with Si2Cl6 afforded the corresponding phosphines (Ar-SMAPs,
1b–1g). These phosphines are air-stable, crystalline, colorless
solids that do not produce a noxious phosphine odor.
DFT calculations [B3LYP/6-31G(d,p)] indicated that the
electronic character of the P-lone pair of the SMAP derivatives
is strongly influenced by the distal Si substituents.4 We used
the molecular electrostatic potential minimum Vmin (kcal/mol)
value associated with the phosphine lone pair region as a quan-
titative measure of donor power (Table 1).5 A larger negative
Vmin value corresponds to a stronger electron-donating ability
of the phosphine. As reported previously, Ph-SMAP (1a) is com-
parable with Me3P in donor power (Table 1, Entries 7 and 8).2
Calculations indicated that the donor powers of 4-MeO-Ph-
SMAP (1c) and 4-Me-Ph-SMAP (1d) apparently were stronger
than those of Me3P and Ph-SMAP (1a), and comparable with
those of Bu3P and (i-Pr)3P (Entries 3–8). The para-Me2N group
exerts a much larger effect, increasing the donor ability of the
aniline-type derivative 4-Me2N-Ph-SMAP (1b) even more than
(t-Bu)3P, which is one of the ꢀ-donor ligands possessing the
P
X = H: Ph-SMAP (1a)
X = 4-Me2N: 4-Me2N-Ph-SMAP (1b)
X = 4-MeO: 4-MeO-Ph-SMAP (1c)
X = 4-Me: 4-Me-Ph-SMAP (1d)
X = 4-Cl: 4-Cl-Ph-SMAP (1e)
Si
X = 4-CF3: 4-CF3-Ph-SMAP (1f)
X = 3,5-(CF3)2: 3,5-(CF3)2-Ph-SMAP (1g)
X
Figure 1. Structures of SMAP family of compounds (1).
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.3 (2006)
295
Table 1. Vmin (kcal/mol) for various phosphines and 1J(31P,
744
740
736
732
728
724
720
716
712
77Se) (Hz) for phosphine selenides
g
1J(31P,77Se)
e
f
Vmim
kcal molÀ1
Entry SMAP
R3P
a
/Hz
c
1
2
4-Me2N-Ph-SMAP (1b)
À46:58
À45:48a
À44:47a
À44:30
À43:86
À43:71a
À43:14
À43:02a
À40:86
À40:41a
À39:85
À37:77
729.3
n.d.
d
(t-Bu)3P
(i-Pr)3P
b
3
n.d.
4
4-MeO-Ph-SMAP (1c)
4-Me-Ph-SMAP (1d)
734.2
733.0
717.1
735.4
713.5
738.5
n.d.
5
J
1
6
Bu3P
7
Ph-SMAP (1a)
Bu3P(=Se)
Me3P(=Se)
8
Me3P
9
4-Cl-Ph-SMAP (1e)
10
11
12
13
Me2PhP
–48
–46
–44
V
–42
–40
–38
–36
4-CF3-Ph-SMAP (1f)
739.1
742.2
n.d.
3,5-(CF3)2-Ph-SMAP (1g)
(kcal/mol)
min
a
ˇ
MePh2P
S36.76
Figure 3. A plot of 1J(31P,77Se) vs Vmin of the phosphine deriv-
atives. The correlation line is associated with 1a–1g/4a–4g.
aData taken from Ref. 5.
nides increase by 12.9 Hz upon examination of the selenide
(4b) possessing the strongest donating aryl-SMAP (1b) to that
(4g) of the weakest donating one (1g). Figure 3 demonstrates
–36
–38
–40
–42
1g
1
that J(31P,77Se) values for SMAP selenides 4a–4g show good
linear correlation with the theoretical measures (Vmin’s).
Figure 3 also shows that the values for Me3P=Se and Bu3P=Se
are far below the correlation line for 4a–4g. This drastic devia-
tion is attributable to the change in steric environment around the
P center.
In conclusion, the SMAP series of compounds with various
Si substituents were synthesized. The donor properties of the
P-lone pair varied over a wide range depending upon the
electronic nature of the Si substituent, with retention of the steric
demand around the phosphorus center. These characteristics
should allow SMAPs to find diverse utility in mechanistic
studies of organometallic reactions and in the development of
efficient catalytic reactions.7
1f
1e
V
1a
1d
–44
1c
–46
1b
–48
–1
–0.5
0
0.5
1
σ
Figure 2. A plot of Vmin (kcal/mol) vs Hammett’s substituent
constants ꢀ’s for SMAP derivatives 1a–1g. A value two-fold
greater than the ꢀ value of m-CF3 was used for 1g.
References and Notes
1
a) E. Shirakawa, K. Ishii, T. Tsuchimoto, Chem. Commun.
2004, 2752. b) M. Qadir, T. Mochel, K. K. Hii, Tetrahedron
¨
strongest donor power (Entries 1 and 2). In contrast, substitution
at the para-position with an electron-withdrawing group decreas-
ed donor power; the donating power of 4-Cl-Ph-SMAP (1e) and
4-CF3-Ph-SMAP (1f) were comparable with monoaryl-dialkyl-
phosphine Me2PhP (Entries 9–11). Donor power of 3,5-(CF3)2-
Ph-SMAP (1g) is as weak as that of MePh2P (Entries 12 and 13).
Figure 2 shows that the substitution effect at the para-position
of Ph-SMAP on Vmin is proportional to Hammett’s substituent
constant ꢀ.
2000, 56, 7975.
A. Ochida, K. Hara, H. Ito, M. Sawamura, Org. Lett. 2003, 5,
2671.
a) M. Murata, K. Suzuki, S. Watanabe, Y. Masuda, J. Org.
Chem. 1997, 62, 8569. b) W. Gu, S. Liu, R. B. Silverman,
Org. Lett. 2002, 4, 4171.
For MO analysis of the 1,4-diazabicyclo[2.2.2]octane sys-
tem, see: E. Heilbronner, K. A. Muszkat, J. Am. Chem.
Soc. 1970, 92, 3818.
C. H. Suresh, N. Koga, Inorg. Chem. 2002, 41, 1573.
D. W. Allen, B. F. Taylor, J. Chem. Soc., Dalton Trans.
1982, 51.
Although we have been unsuccessful in finding a dramatic
ligand substituent effect in relation to catalysis, we have
confirmed that the SMAP ligands can be used as Me3P-like
ligands and have found noticeable substituent effects in
some transition metal-catalyzed reactions. Details are forth-
coming.
2
3
4
The 1J(31P,77Se) measurements for phosphine selenides
provided experimental evidence for the substituent effect at
the bridgehead silicon atom on phosphine donor power. The se-
lenides were prepared by heating a mixture of the phosphine and
5
6
7
1
selenium in C6D6 in an NMR tube, and the J(31P,77Se) values
were measured in situ. It is commonly accepted that the stronger
1
the donor power is, the smaller the J(31P,77Se) value.6 Values
for SMAP selenides (4a–4g) and Me3P=Se and Bu3P=Se are
1
listed in Table 1. The J(31P,77Se) values for the SMAP sele-
Published on the web (Advance View) March 5, 2006; DOI 10.1246/cl.2006.294