C O M M U N I C A T I O N S
Table 1. Summary of Catalytic Hydrosilation Results
contributions from the PNP ligand backbone. The natural charges
obtained from an NBO analysis suggest localization of the cationic
charge on Si. Past experimental and computational studies suggest
that this cationic charge localization at silicon promotes alkene
binding directly to the silicon center and lowers the activation
energy for subsequent alkene insertion into the Si-H bond.3a These
aspects of the electronic structure of 2 have been correlated with
an unusual reaction typesthe direct addition of the Si-H bond of
the silylene ligand to an olefin.
silane
alkene
product
yield (%)a
MesSiH3
1-hexene
styrene
cis-cyclooctene
1-hexene
styrene
1-hexene
styrene
1-hexene
styrene
H2SiMes(Hex)
70
77
68
15
34
56
59
47
51
H2SiMes(CH2CH2Ph)
H2SiMes(C8H15)
H2SiPh(Hex)
H2SiPh(CH2CH2Ph)
H2SiHex2
H2SiHex(CH2CH2Ph)
H2SiCy(Hex)
H2SiCy(CH2CH2Ph)
PhSiH3
HexSiH3
CySiH3
Compound 2 rapidly reacts with 1-hexene at ambient temperature
to afford [(PNP)(H)IrdSiHex(Mes)][B(C6F5)4] (3) as a dark blue
solid in quantitative yield. Analogously, addition of styrene or cis-
cyclooctene to 2 rapidly formed [(PNP)(H)IrdSiCH2CH2Ph(Mes)]-
[B(C6F5)4] (4) and [(PNP)(H)IrdSiC8H15(Mes)][B(C6F5)4] (5),
respectively. The 29Si NMR resonances for these silylene complexes
(3: 296 ppm; 4: 295 ppm; 5: 306 ppm) are slightly downfield-
a Yields determined by integration against an internal standard by 1H
NMR spectroscopy and/or GC-MS.
substrates suggests a “direct Si-H addition” process as reported
for [Cp*(PiPr3)H2)RudSi(H)Ph][B(C6F5)4].
These results are consistent with a general structure-activity
relationship for the reactivity of cationic H-substituted silylene
complexes with alkenes. Furthermore, this work suggests that the
transition metal fragment supporting the silylene ligand can exhibit
strong steric influences over the reactivity toward the olefin
substrate; this was not observed in the previous ruthenium system.
Further investigations of mechanisms for the iridium catalyst system
are still needed to elucidate how to optimize olefin hydrosilation
over silane redistribution catalysis via catalyst design.
2
shifted with respect to that of 2. The JSiH coupling constants for
3, 4, and 5 were found to be smaller than 10 Hz, which is consistent
with little to no significant Ir-H-Si interaction. The 1H, 13C NMR,
and 2D NMR spectra of these products reveal that the olefin addition
proceeds with g95% anti-Markovnikov regioselectivity. Thus, the
cationic iridium silylene complex
2
behaves similarly to
and
[Cp*(PiPr3)H2OsdSiH(Trip)][B(C6F5)4]
[Cp*(PiPr3)-
H2RudSiH(Ph)][B(C6F5)4] in its reactions toward olefins.2,3a,9b
The silylene complex 2 catalyzes the hydrosilation of both aryl-
and alkyl-substituted primary silanes (RSiH3) with a variety of
unhindered alkenes (Table 1). Hydrosilation runs were conducted using
primary silane substrates with an equimolar amount of alkene with a
5 mol % loading of 2 in bromobenzene-d5 at 60 °C, and gave a mixture
of silane products. The disubstituted silane products (RR′SiH2) are
inactive toward further hydrosilation. The hydrosilation catalysis
proceeds regioselectively with exclusive formation of anti-Markovni-
kov products, as in the analogous stoichiometric reactions.
Acknowledgment. This work was supported by the National
Science Foundation under Grant No. CHE-0649583. The authors
would also like to thank Dr. Kathleen Durkin and Dr. Jamin L.
Krinsky for help and training in the Molecular Graphics Facility.
Supporting Information Available: Experimental details for syn-
thetic procedures, characterization of new compounds, procedures for
catalytic runs, computational details, and complete list of authors for
ref 11. This material is available free of charge via the Internet at http://
pubs.acs.org.
In addition to hydrosilation catalysis, complex 2 was found to
be a good catalyst for the redistribution of silanes. For example,
phenylsilane in the absence of alkene was added to a 5 mol %
solution of 2 in bromobenzene-d5 in a J. Young NMR tube at
ambient temperature. The 1H NMR spectroscopy indicated the
formation of Ph2SiH2, PhSiH3 (1:0.3), and SiH4 gas, as identified
by comparisons with authentic samples. Addition of MesSiH3 to a
5 mol % solution of 2 in bromobenzene-d5 also produced Mes2SiH2
(approximately 6 mol %) after heating to 60 °C for 4 h. Therefore,
the variability of product yields and mixture of silane products
observed for hydrosilation depends on the ease of redistribution of
the silane substrate (e.g., PhSiH3 > MesSiH3) in a concurrent
catalytic process.13 Furthermore, silane redistribution catalysis was
not observed for the ruthenium catalyst, which therefore exhibits
higher yields for hydrosilation (<98%).
Previous studies on the mechanism of hydrosilation by
[Cp*(PiPr3)H2)RudSi(H)Ph][B(C6F5)4] indicate that hydrosilation
proceeds via direct insertion into the Si-H bond, in a process that
is analogous to hydroboration.14 This work represents the second
report of olefin hydrosilation catalyzed by a cationic transition metal
silylene complex. The two catalytic systems share two features in
common: only primary silanes serve as substrates and hydrosilation
proceeds with a very high degree of anti-Markovnikov selectivity.
However, complex 2 differs from the ruthenium catalyst in
exhibiting a greater sensitivity to the steric properties of the alkene.
Thus, no hydrosilation of 1-methylcyclohexene was observed after
heating to 60 °C for 2 days in bromobenzene-d5, and instead only
redistribution catalysis occurred as determined by 1H NMR
spectroscopy. Although several mechanisms for hydrosilation can
be envisioned for this system, the specificity for primary silane
References
(1) (a) Marciniec, B. ComprehensiVe Handbook on Hydrosilylation; Pergamon
Press: Oxford, NY, 1992. (b) Marciniec, B. Silicon Chem. 2002, 1, 155.
(c) Brook, M. A. Silicon in Organic, Organometallic, and Polymer
Chemistry; Wiley: New York, 2000. (d) Lewis, L. N.; Stein, J.; Gao, Y.;
Colborn, R. E.; Hutchins, G. Platinum Metals ReV. 1997, 41, 66–75. (e)
Roy, A. K. AdV. Organomet. Chem. 2008, 55, 1–59.
(2) Glaser, P. B.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 13640–13641.
(3) (a) Hayes, P. G.; Beddie, C.; Hall, M. B.; Waterman, R.; Tilley, T. D.
J. Am. Chem. Soc. 2006, 128, 428–429. (b) Waterman, R.; Hayes, P. G.;
Tilley, T. D. Acc. Chem. Res. 2007, 40, 712–719.
(4) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004, 23, 326–
328.
(5) (a) Gatard, S.; C¸ elenligil-C¸ etin, R.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
J. Am. Chem. Soc. 2006, 128, 2808–2809. (b) Fafard, C. M.; Adhikari, D.;
Foxman, B. M.; Mindiola, D. J.; Ozerov, O. V. J. Am. Chem. Soc. 2007,
129, 10318–10319.
(6) (a) Bailey, B. C.; Fan, H.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J.
J. Am. Chem. Soc. 2007, 129, 8781–8793. (b) Gerber, L. C. H.; Watson,
L. A.; Parkin, S.; Weng, W.; Foxman, B. M.; Ozerov, O. V. Organome-
tallics 2007, 26, 4866–4868.
(7) Fan, L.; Parkin, S.; Ozerov, O. V. J. Am. Chem. Soc. 2005, 127, 16772–
16773.
(8) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175–292.
(9) For reports of H-substituted iridium silylene complexes, see: (a) Simmons,
R. S.; Gallucci, J. C.; Tessier, C. A.; Youngs, W. J. J. Organomet. Chem.
2002, 654, 224–228. (b) Feldman, J. D.; Peters, J. C.; Tilley, T. D.
Organometallics 2002, 21, 4065–4075.
(10) Mork, B. V.; Tilley, T. D. J. Am. Chem. Soc. 2004, 126, 4375–4385.
(11) Frisch, M. J.; et al. Gaussian 03, revision D.01; Gaussian, Inc.: Wallington,
CT, 2004. A complete author list can be found in the Supporting
Information.
(12) Two rotamers were observed for (PNP)TidC[tBu(ArF)](X): Bailey, B. C.;
Huffman, J. C.; Mindiola, D. J. J. Am. Chem. Soc. 2007, 129, 5302–5303.
(13) See Supporting Information.
(14) Brown, H. C. Boranes in Organic Chemistry; Cornell University Press:
Ithaca, NY, 1972.
JA803332H
9
J. AM. CHEM. SOC. VOL. 130, NO. 29, 2008 9227