The first alkyl(silyl)palladium complexes: Formation by oxidative addition of silacyclobutanes to palladium complexes, reductive elimination, and other reactivities relevant to catalysis

Article abstract of DOI:10.1021/om970186j

1,1-Diphenyl- and 1,1-dimethylsilacyclobutanes reacted with Me₂Pd(dmpe) (dmpe = 1,2-bis(dimethylphosphino)ethane) or Pd(PhCH=CH₂)(dmpe) to give 2,2-diphenyl- and 2,2-dimethyl-1-pallada-2-silacyclopentane complexes, the diphenyl complex being characterized by X-ray analysis. Treatment of the diphenyl complex with acetylenes or 1,2-disilacyclopentane induced reductive elimination to regenerate the parent 1,1-diphenylsilacyclobutane. A dihydrosilane or a silacyclobutane reacted with the diphenyl complex to afford a 1,3-bis(hydrosilyl)propane or a 1,5-disilacyclooctane, respectively.

Full text of DOI:10.1021/om970186j

3246
Organometallics 1997, 16, 3246-3248
Th e F ir st Alk yl(silyl)p a lla d iu m Com p lexes: F or m a tion
by Oxid a tive Ad d ition of Sila cyclobu ta n es to P a lla d iu m
Com p lexes, Red u ctive Elim in a tion , a n d Oth er
Rea ctivities Releva n t to Ca ta lysis
Yoshifumi Tanaka,† Hiroshi Yamashita,‡ Shigeru Shimada,‡ and
Masato Tanaka*,†,‡
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305, J apan, and National
Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, J apan
Received March 6, 1997X
Summary: 1,1-Diphenyl- and 1,1-dimethylsilacyclobu-
tanes reacted with Me2Pd(dmpe) (dmpe ) 1,2-bis(di-
methylphosphino)ethane) or Pd(PhCHdCH2)(dmpe) to
give 2,2-diphenyl- and 2,2-dimethyl-1-pallada-2-silacy-
clopentane complexes, the diphenyl complex being char-
acterized by X-ray analysis. Treatment of the diphenyl
complex with acetylenes or 1,2-disilacyclopentane in-
duced reductive elimination to regenerate the parent 1,1-
diphenylsilacyclobutane. A dihydrosilane or a silacyclo-
butane reacted with the diphenyl complex to afford a
1,3-bis(hydrosilyl)propane or a 1,5-disilacyclooctane,
respectively.
sufficiently stable to allow their manipulation. This
paper reports the chemistry of 1-pallada-2-silacyclopen-
tane complexes, which are the first alkyl(silyl)palladium
species.
In one example Me2Pd(dmpe) (2a , 0.09 mmol; dmpe
) 1,2-bis(dimethylphosphino)ethane) and 1,1-diphenyl-
silacyclobutane (1a , 0.27 mmol) were mixed in C6D6 (0.3
mL), and the solution was heated in a sealed NMR tube
1
at 60 °C for 7 h. Monitoring of the reaction by H, 13C,
29Si, and 31P NMR spectroscopy revealed the formation
of a 1-pallada-2-silacyclopentane complex (3a , 90% NMR
yield)7 and allylmethyldiphenylsilane (4a , 80% NMR
1
yield) (eq 1). The H NMR spectrum displayed a very
Although silicon-carbon bonds usually are unreactive
toward transition-metal complexes, those of silacyclobu-
tanes (1) are exceptions. They are reactive due to their
ring strain.1 Quite a few reactions of 1 that are cata-
lyzed by transition-metal complexes have been reported,
such as ring-opening polymerization,2 dimerization,3
cross-dimerization with disilanes,4 and cycloaddition
reactions5 with acetylenes and allenes. 1-Metalla-2-
silacylopentanes are believed to be involved as inter-
mediates in these catalytic reactions. Indeed, 1-ferra-6
and 1-platina-2-silacyclopentane3 complexes have been
isolated. In a broader view, the chemistry of 1-metalla-
2-silacyclopentanes is important in its own right; these
complexes provide a rare opportunity to study the
reactivities of alkyl(silyl)metal species, a very important
class of catalytic intermediates, which are not always
weak signal at δ 0.23, suggesting that methane had
been generated. The structure of 4a was confirmed by
GC-MS of the reaction mixture. The mixture was
evaporated in vacuo, and the residue was recrystallized
from toluene-hexane to give 3a as pale yellow crystals
(32.5 mg, 75%). The structure of 3a was unambiguously
confirmed by X-ray diffraction, thus verifying that
oxidative addition of the Si-C bond in the ring system
had taken place. As the ORTEP8 drawing (Figure 1)
shows,9 3a is a square-planar complex, the deviation
from planarity being very small.10 The Pd-Si bond
distance (2.341(2) Å) is within the range of those
reported in the literature (2.33-2.43 Å).11 The Pd-P
bond trans to the silicon (2.351(2) Å) is significantly
longer than the other Pd-P bond trans to the carbon
(2.291(2) Å). This is due to the strong trans influence
† University of Tsukuba
‡ National Institute of Materials and Chemical Research
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
(1) The strain energies of silacyclobutane, silacyclopentane, and
silacyclohexane are 102.5, 20.1, and 13.0 kJ mol-1, respectively. See:
Gordon, M. S.; Boatz, J . A.; Walsh, R. J . Phys. Chem. 1989, 93, 1584.
(2) (a) Weyenberg, D. R.; Nelson, L. E. J . Org. Chem. 1965, 30, 2618.
(b) Nametkin, N. S.; Ushakov, N. V.; Vdovin, V. M. Vysokomol. Soedin.,
Ser. A 1971, 13, 29; Chem. Abstr. 1971, 74, 88325w. (c) Cundy, C. S.;
Eaborn, C.; Lappert, M. F. J . Organomet. Chem. 1972, 44, 291 and
references cited therein. (d) Poletaev, V. A.; Vdovin, V. M.; Nametkin,
N. S. Dokl. Akad. Nauk SSSR 1973, 208, 1112; Chem. Abstr. 1973,
79, 19191r. (e) Finkel’shtein, E. Sh.; Ushakov, N. V.; Pritula, N. A.;
Andreev, E. A.; Plate, N. A. Izv. Akad. Nauk SSSR, Ser. Khim. 1992,
223; Chem. Abstr. 1992, 116, 256191f. (f) Ushakov, N. V.; Yarysheva,
A. Yu.; Tal’roze, R. V.; Finkel’shtein, E. Sh.; Plate, N. A. Dokl. Akad.
Nauk 1992, 325, 964; Chem. Abstr. 1993, 118, 102788k. (g) Bialecka-
Florjanczyk, E.; Ganicz, T.; Stanczyk, W.; Sledzinska, I. Polimery
(Warsaw) 1993, 38, 424; Chem. Abstr. 1994, 121, 36351w. (h) Liao, C.
X.; Chen, M. W.; Sun, L.; Weber, W. P. J . Inorg. Organomet. Polym.
1993, 3, 231.
(7) Compounds 3a ,b showed satisfactory NMR and/or analytical
data (see the Supporting Information). 1H, 29Si, and 31P NMR spectral
data (C6D6) for 3a are as follows: 1H NMR δ 0.64 (d, J ) 7.3 Hz, 6H,
PMe), 0.66-0.95 (m, 4H, PCH2CH2P), 0.79 (d, J ) 5.9 Hz, 6H, PMe),
1.80 (t, J ) 6.6 Hz, 2H, CH2Si), 2.39-2.53 (m, 2H, CH2), 2.56-2.68
(m, 2H, CH2), 7.15-7.33 and 7.92-7.97 (each m, 3H and 2H, C6H5);
29Si NMR δ 47.1 (dd, J PSi ) 15.9 and 174.6 Hz); 31P NMR δ 10.6 (d,
J PP ) 9.6 Hz, J PSi ) 174.6 Hz), 14.2 (d, J PP ) 9.6 Hz).
(3) Yamashita, H.; Tanaka, M.; Honda, K. J . Am. Chem. Soc. 1995,
117, 8873.
(4) Reddy, N. P.; Hayashi, T.; Tanaka, M. Chem. Commun. 1996,
1865.
(5) Sakurai, H.; Imai, T. Chem. Lett. 1975, 891. Takeyama, Y.;
Nozaki, K.; Matsumoto, K.; Oshima, K.; Utimoto, K. Bull. Chem. Soc.
J pn. 1991, 64, 1461.
(6) Cundy, C. S.; Lappert, M. F. J . Chem. Soc., Chem. Commun.
1972, 445.
(8) J ohnson, C. K. ORTEP, a FORTRAN Thermal-Ellipsoid Plot
Program for Crystal Structure Illustrations; Report ORNL-3794; Oak
Ridge National Laboratory: Oak Ridge, TN, 1970.
S0276-7333(97)00186-6 CCC: $14.00 © 1997 American Chemical Society

Communications
Organometallics, Vol. 16, No. 15, 1997 3247
Sch em e 1
F igu r e 1. ORTEP drawing of 3a . C(6) is disordered, re-
sulting in large thermal parameters. Selected bond lengths
(Å) and angles (deg): Pd(1)-Si(2), 2.341(2); Pd(1)-P(3),
2.351(2); Pd(1)-P(4), 2.291(2); Pd(1)-C(7), 2.129(7): Si(2)-
Pd(1)-P(3), 174.0(1); Si(2)-Pd(1)-P(4), 100.1(1); Si(2)-
Pd(1)-C(7), 80.0(2); P(3)-Pd(1)-P(4), 85.0(1); P(3)-Pd(1)-
C(7), 95.2(2); P(4)-Pd(1)-C(7), 175.9(2).
nation to result in the formation of 4, (iii) extrusion of
a methane molecule to generate Pd(0) species, and (iv)
reaction of the resulting Pd(0) species with a second
molecule of 1 to give 3. Accordingly, a Pd(0) complex
having a more labile ligand, such as an olefin complex,
is expected to react more readily with 1 to form the five-
of a silyl ligand as compared with an alkyl ligand.
Complex 3a was unstable to air in solution but only
slightly decomposed in air in the crystalline state.
1,1-Dimethylsilacyclobutane (1b) reacted similarly
with 2a at 60 °C over 100 h to generate another five-
membered alkyl(silyl)palladium complex (3b)7 in 60%
yield along with allyltrimethylsilane (4b, 60%). How-
ever, before total consumption of 1b, slight decomposi-
tion of 3b started to form Pd(dmpe)212 to a small extent,
indicating that 3b is thermally less stable than 3a .
Complex 3b could not be isolated in a pure form. The
thermal instability of 3b presumably is associated with
reductive elimination to re-form 1b.13
1
membered-ring complex. Indeed, as monitored by H,
13C, and 29Si NMR spectroscopy, Pd(η2-PhCHdCH2)-
(dmpe) (2b, 0.025 mmol) reacted with 1a or 1b (1 equiv)
in C6D6 (0.25 mL) even at room temperature to give,
within 10 min, 3a or 3b in quantitative yield (eq 2).
Even though the formation of 3b in the solution was
quantitative, its isolation was not successful.14
As illustrated in Scheme 1, the foregoing reactions
forming compounds 3 can be best explained by a
sequence comprising (i) metathesis of Me2Pd(dmpe) with
the Si-C bond of 1 to generate methyl[3-(methyldior-
ganylsilyl)propyl]palladium species, (ii) â-hydride elimi-
Palladium complexes are able to catalyze the reac-
tions of silacyclobutanes with acetylenes to give silacy-
clohexenes and/or vinyl(allyl)silanes, and the catalysis
is envisioned to proceed via 1-pallada-2-silacyclopentane
intermediates.5 However, when 3a (0.06 mmol) was
allowed to react with diphenylacetylene (5a , 1.5 equiv)
in C6D6 (0.3 mL) at room temperature for 30 min in a
sealed NMR tube, neither the corresponding silacyclo-
hexene nor vinyl(allyl)silane was formed. NMR spectro-
scopic analysis of the resulting mixture revealed that
1a was formed in ∼12% yield along with a (diphenyl-
acetylene)palladium dmpe complex (∼13%).15 Contin-
(9) Crystal data for 3a : colorless transparent prism, 0.40 × 0.25 ×
0.15 mm3; C21H32P2PdSi; FW ) 479.0; monoclinic, space group P21/a,
a ) 15.939(3) Å, b ) 10.083(3) Å, c ) 14.834(3) Å, â ) 105.08(2)°, V )
2302.0(9) Å3, Z ) 4; Dcalcd ) 1.38 g cm-3; µ(Mo KR) ) 9.85 cm-1; Mac
Science MXC18 diffractometer; ambient temperature, Mo KR radiation
(λ ) 0.710 73 Å); 3.0° < 2θ < 55.0°, ω-2θ scan; 258 parameters; 4700
observed reflections (F > 3.0σ(F)). The structure was solved by direct
methods and refined by a full-matrix least-squares procedure to yield
the final residuals of R ) 0.044 and Rw ) 0.075. The non-hydrogen
atoms were refined anisotropically. The hydrogen atoms were gener-
ated at idealized calculated positions. All hydrogen atoms were then
included in the calculations but not refined. All calculations were
performed using the Crystan GM crystallographic software package.
(10) The high thermal parameters for C(6) suggest that C(6) is
disordered.
(11) (a) Pan, Y.; Mague, J . T.; Fink, M. J . Organometallics 1992,
11, 3495. (b) Murakami, M.; Yoshida, T.; Ito, Y. Organometallics 1994,
13, 2900. (c) Suginome, M.; Oike, H.; Ito, Y. Organometallics 1994,
13, 4148. (d) Murakami, M.; Yoshida, T.; Kawanami, S.; Ito, Y. J . Am.
Chem. Soc. 1995, 117, 6408. (e) Suginome, M.; Oike, H.; Shuff, P. H.;
Ito, Y. Organometallics 1996, 15, 2170. (f) Shimada, S.; Tanaka, M.;
Shiro, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 1856.
(14) The deterioration of the complex during the evaporation (0.1
Torr/room temperature) of the solvent appeared to be induced by
removal of 1b extruded by spontaneous reductive elimination, sug-
gesting the reversibility of the oxidative-addition-reductive-elimina-
tion processes under the conditions. In the resulting mixture were
found Pd(dmpe)2 by 1H NMR and a black precipitate (presumably
metallic palladium).
(12) Broadwood-Strong, G. T. L.; Chaloner, P. A.; Hitchcock, P. B.
Polyhedron 1993, 12, 721.
(15) 1H NMR: δ 0.95-1.17 (m, 16H, PMe and PCH2CH2P), 6.92-
8.04 (m, 10H, C6H5). 31P NMR: δ -7.7. These NMR data are consistent
with those of an authentic sample of (PhCtCPh)Pd(dmpe) generated
by the reaction of Pd(η3-CH2dCHCH2)(η5-C5H5), diphenylacetylene, and
dmpe in C6D6. See: Krause, J .; Bonrath, W.; Po¨rschke, K. R. Orga-
nometallics 1992, 11, 1158. Since the 1H NMR signals for PMe and
PCH2CH2P of (PhCtCPh)Pd(dmpe) were distinctly separated from
those for PCH2CH2P of 3a (0.66-0.95 ppm), the yield of (PhCtCPh)-
Pd(dmpe) was readily evaluated by integration.
(13) Me2Pd(PPh2Me)2 also reacted with 1a (2 equiv) in C6D6. at 60
°C for 1 h to give 4a (80% based on Pd), Pd(PPh2Me)4, and a black
precipitate (presumably metallic palladium). However, the correspond-
ing cyclic complex was not found by NMR in the reaction mixture,
suggesting its thermal instability. Me2Pd(PMe3)2 behaved very simi-
larly to give 4a (90% based on Pd) when treated with 1a at 60 °C for
30 min.

3248 Organometallics, Vol. 16, No. 15, 1997
Communications
volved in several catalytic reactions of 1 (vide supra).2-5
In this context, the reactivities of complex 3a with those
substrates relevant to the catalyses were briefly exam-
ined. Methylphenylsilane (6, 0.24 mmol) reacted with
3a (0.06 mmol) in C6D6 (0.3 mL) in a sealed NMR tube
even at room temperature, and the 1,3-bis(hydrosilyl)-
propane compound 7 was formed in 70% yield after 24
h (eq 4). This result is in good agreement with the ring-
opening reaction of silacyclobutanes with hydrosilanes.2a
Ta ble 1. Rea ction s of 1-P a lla d a -1-sila cyclop en ta n e
Com p lex 3a w ith Acetylen es 5a
RCtCR′ (5)
temp (°C)/time (h)b yield of 1a (%)c
PhCtCPh (5a )
60/0.5
60/3
60/3
80/12
66 (12)
30 (4)
16 (0)
10 (0)
30
PhCtCH (5b)
PhCtCMe (5c)
nPrCtCnPr (5d )
100/12
CHtCCOOEt (5e)
MeOOCCtCCOOMe (5f)
room temp/5.5
room temp/5.5
73 (20)
80 (54)
a
Complex 3a (0.06 mmol) was treated in a sealed NMR tube
with 5 (0.09 mmol) in C6D6 (0.3 mL), first at room temperature
for 0.5 h, and then under the conditions given in the table.
b
Reaction conditions after the beginning 0.5 h. c 1H NMR yield.
Figure in parentheses indicate yields after the beginning 0.5 h at
room temperature.
ued reaction at 60 °C for 30 min increased the yields of
1a and the complex to 66 and 67% respectively, but the
yields remained unchanged thereafter (eq 3). The reac-
The reaction of 3a (0.06 mmol) with 1b (0.24 mmol)
in C6D6 (0.3 mL) in a sealed NMR tube also proceeded
at 100 °C to afford a 1,5-disilacyclooctane (8, 44%) in
30 h (eq 5). This also provides a good model for the
dimerization of silacyclobutanes.3
tion was very clean; the starting 3a was still found (34%)
in the reaction mixture, and no other byproducts were
formed in substantial amounts. The results strongly
suggest that the incomplete conversion to 1a , i.e., 66%
yield of 1a at the 30 min reaction time and thereafter, is
due to attainment of an equilibrium under the conditions.
Other acetylenes reacted similarly with 3a to give 1a
(Table 1). Very interestingly, however, the reaction rate
and the yield very much depended on the structure of
the acetylenes. For instance, 4-octyne (5d ) was very
reluctant to induce reductive elimination; 1a was not
We have reported palladium-catalyzed selective Si-
C/Si-Si cross-metathesis between a silacyclobutane and
a 1,2-disilacyclopentane to form a cross-dimer.4 When
1,1,2,2-tetramethyl-1,2-disilacyclopentane (0.066 mmol)
was added to 3a (0.06 mmol) in C6D6 (0.3 mL) in a
sealed NMR tube, nearly quantitative formation of a bis-
(silyl)complex (9) was observed after 24 h at room
temperature, together with 1a (97%) arising from
reductive elimination. Thus, the reaction was unable
to provide a good model for the catalysis.
1
found by H NMR at all below 60 °C, and heating at 80
°C for 12 h resulted in only a 10% yield of 1a . The yield
remained as low as 30% even after heating to 100 °C
over 12 h. On the other hand, treatment of 3a with
dimethyl acetylenedicarboxylate (5f) at room tempera-
ture for 30 min already gave 1a in 54% yield. The yield
increased over 6 h to 80% and remained unchanged for
an additional 42 h. These results, inclusive of those with
other acetylenes shown in Table 1, clearly indicate that
electron-withdrawing groups bound to the acetylenic
carbon increase the reactivity in the reductive elimina-
tion both kinetically and thermodynamically. Similar
observations have been reported for a cis-alkyl(silyl)-
bis(phosphine)platinum complex.16 A mechanism that
involves dissociation of a phosphine ligand trans to the
silyl group and coordination of an acetylene to the coor-
dination site prior to the extrusion of an alkylated silane
has been proposed as a major pathway. The similarity
between the platinum and our palladium cases tempts
us to consider essentially the same mechanism. How-
ever, taking into account the difficulty of dissociation
of one of the phosphorus atoms of the dmpe ligand, it
may be premature to extend further discussion along
these lines.17 Five-coordinate species proposed in a sim-
ilar π-acid-induced reductive elimination from a dial-
kyl(bipyridyl)nickel18 complex also has to be considered.
Besides the reaction with acetylenes, 1-metalla-2-
silacyclopentane complexes are envisioned to be in-
Ack n ow led gm en t. We are grateful to the J apan
Science and Technology Corporation (J ST) for partial
financial support through the CREST program. Y.T.
thanks Prof. Wataru Ando for helpful discussions.
Su p p or tin g In for m a tion Ava ila ble: Text giving proce-
dures to generate authentic samples of Pd(dmpe)2 and
(PhCtCPh)Pd(dmpe) and characterization data for 3a , 3b, 4a ,
4b, 7, 8, and 9 and tables giving full details of the crystal
structure analysis for 3a (8 pages). Ordering information is
given on any current masthead page.
OM970186J
(17) On the basis of our observation,11 reductive elimination from a
T-shaped intermediate generated via dissociation of one of the phos-
phorus atoms of the dmpe ligand is also plausible. Such a mechanism
is more generally accepted for the reductive elimination from cis-
dialkylpalladium species. See: Yamamoto, A. Organotransition Metal
Chemistry: Fundamental Concepts and Applications; Wiley-Inter-
science: New York, 1986; p 240. (b) Crabtree, R. H. The Organometallic
Chemistry of the Transition Metals, 2nd ed.; Wiley-Interscience: New
York, 1988; p 151.
(16) Ozawa, F.; Hikida, T.; Hayashi, T. J . Am. Chem. Soc. 1994,
116, 2844.
(18) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J . Am. Chem. Soc. 1971,
93, 3350.