Cationic silane δ-complexes of ruthenium with relevance to catalysis

Article abstract of DOI:10.1021/ja101583m

Hydrosilylation of carbonyls catalyzed by 2 goes via intermediate formation of cationic silane σ-complexes 4 which undergo nucleophilic abstraction of the silylium cation studied by DFT calculations.

Full text of DOI:10.1021/ja101583m

Published on Web 04/09/2010
Cationic Silane σ-Complexes of Ruthenium with Relevance to Catalysis
Dmitry V. Gutsulyak,† Sergei F. Vyboishchikov,*,‡ and Georgii I. Nikonov*,†
Chemistry Department, Brock UniVersity, 500 Glenridge AVenue, St. Catharines, Ontario L2S 3A1, Canada, and
Institut de Qu´ımica Computacional, Campus de MontiliVi, UniVersitat de Girona, 17071 Girona, Catalonia, Spain
Received February 23, 2010; E-mail: gnikonov@brocku.ca
Table 1. Hydrosilylation and Coupling Reactions Catalyzed by 2a
Silane σ-complexes attract a lot of attention as possible inter-
mediates in metal catalyzed transformations of organosilanes,
Substrate
Silane
Time
Conv.
Products
although only in a few instances their involvement in catalytic
process has been established directly.1 Most of the isolated silane
σ-complexes are neutral species met in η2- or η3-coordination
modes.2 In contrast, isolable cationic silanes σ-complexes are very
rare,3,4 and virtually nothing is known about substituent effects on
the Si-H interaction in these species.
We have recently reported carbonyl hydrosilylation catalyzed
by the iron complex [Cp(Me2PriP)Fe(NCMe)2]+ (1).5 Thinking that
the analogous ruthenium chemistry will be more effective due to
the larger size of the metal, we opted to study the catalytic chemistry
of compounds [Cp(R3P)Ru(NCMe)2]+. This led us to the discovery
of a series of surprisingly stable cationic silane σ-complexes which
exhibit unexpected substituent effects.
1
PhC(O)H
HSiMe2Ph 30 min 100%
PhCH2OSiMe2Ph
PhCH2OSiEt3
HSiEt3
36 h,
82%
50 °C
H2SiMePh 20 h
H3SiPh
100%
60%
PhCH2OSiHMePh
PhCH2OSiH2Ph +
(PhCH2O)2SiHPh
C6H11OSiMe2Ph
PhCH(OSiMe2Ph)
Me
4 h
20 h
99%
2
3
Cyclohexanone HSiMe2Ph 18 h
PhC(O)Me
100%
30%
HSiMe2Ph 24 h
4
5
MeC(O)Et
MeC(O)OEt
HSiMe2Ph 24 h
HSiMe2Ph 24 h
11%
100%
(silane)
s-BuOSiMe2Ph
EtOSiMe2Ph
6
7
PhCOOH
EtOH
HSiMe2Ph 20 min 100%
HSiMe2Ph 5 min 100%
PhCOOSiMe2Ph
EtOSiMe2Ph
EtOSiEt3
i-PrOSiMe2Ph
t-BuOSiMe2Ph
PhOSiMe2Ph
PhNHSiMe2Ph
HOSiMe2Ph +
O(SiMe2Ph)2
HSiEt3
HSiMe2Ph 30 min 100%
HSiMe2Ph 1 h 100%
50 h
100%
The compound [Cp(Pri3P)Ru(NCMe)2]+BAF- (2, BAF )
B(C6H5)4) has been found to catalyze a variety of hydrosilylation
reactions under mild conditions and more effectively than 1.6
Rewardingly, the corresponding BF4- derivative (2′) showed a very
similar catalytic activity and due to its lower cost is a preferable
catalyst. Of several silanes screened in the hydrosilylation of
benzaldehyde, the fastest rate was observed for HSiMe2Ph (Table
1). Hydrosilylation of ketones is sluggish (entries 2-4) and does
not reach completion for bulkier ketones (entries 3-4). Noteworthy,
complex 2 also catalyzes the hydrosilylation of ethyl acetate (entry
5), which is known to be a very inert substrate.7,8,9b In spite of
this, the reaction with benzoic acid (entry 6) stops at the silyl ester.
Fast silane alcoholysis (entries 7-10), aminolysis (entry 11),10 and
hydrolysis (entry 12) were observed.11 Slow catalytic dehalogena-
tion of CDCl3 by HSiMe2Ph also takes place.12 In contrast, CH2Cl2
and chlorobenzene proved unreactive.
8
9
i-PrOH
t-BuOH
10 PhOH
11 PhNH2
12 H2O
HSiMe2Ph 30 min 100%
HSiMe2Ph 10 min 100%
HSiMe2Ph 10 min 100%
a Catalyst load 3-5%, substrate/silane ratio 1:1, room temperature,
CDCl3.
satellites in the 1H NMR spectra (Table 2). Unexpectedly, the
J(Si-H) values showed a V-type dependence on the electron-
withdrawing ability of the SiR3 group, first decreasing from 53 Hz
for R3 ) Cl3 to 45 Hz for R3 ) ClMe2 and then increasing to 50
Hz for R3 ) Me2Ph. The reduced stability of 4 and unusual behavior
of their Si-H coupling constants suggests significantly reduced
back-donation from the cationic Ru center,2 so that the variation
of J(Si-H) is mainly affected by the change of hybridization at
Si.2a,14 For comparison, in neutral complexes 6 the J(Si-H) shows
the opposite trend.13 The PPh3 derivatives 5 are more labile and
demonstrate increased J(Si-H) values in comparison with their
Pri3P analogues, signifying a further decrease of back-donation from
the less electron-rich metal.
A DFT study of complexes 4 revealed η2-silane coordination
without any Si · · ·N interligand interactions.15 The calculated Si-H
coupling constants, although systematically lower (Table 2), cor-
relate well with experimental values.16 Surprisingly, there is no
direct correlation between the magnitude of Si-H coupling and
the Si-H distance: complex 4g having the shortest Si-H bond
(1.898 Å) shows almost the same calculated J(Si-H) as that for
To establish the mechanism of hydrosilylation, stoichiometric
reactions were attempted. EXSY NMR showed fast exchange
between coordinated and free nitrile, suggesting that NCMe
dissociation is a reasonable first step. There is no coordination of
carbonyl substrates even when the reaction is monitored in neat
carbonyl. In contrast, reactions of 2 and its PPh3 analogue,
[Cp(Ph3P)Ru(NCMe)2]+BAF-, with silanes easily give silane
σ-complexes [Cp(R′3P)Ru(NCMe)(η2-HSiR3)]+ (4 and 5 in eq 1;
R3 ) Cl3 (a), MeCl2 (b), Me2Cl (c), H2Ph (d), HMePh (e), PhMe2
(f)). Compounds 4 and 5 were characterized by NMR and IR
spectroscopy. The stability of these species is significantly reduced
in comparison with related neutral compounds Cp(Pri3P)Ru(Cl)(η2-
HSiR3) (6).13 Nevertheless, complexes 4a (R ) SiCl3) and 4b (R
) SiMeCl2) are remarkably stable, surviving several days in CDCl3
solutions. For less stable derivatives the NMR spectra were recorded
in neat silane.
To evaluate the extent of Si-H activation by the cationic Ru
center, Si-H coupling constants were measured from the 29Si
† Brock University.
‡ Universitat de Girona.
9
5950 J. AM. CHEM. SOC. 2010, 132, 5950–5951
10.1021/ja101583m  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Experimental and Calculated J(Si-H) Values (in Hz) and
Calculated Si-H Bond Lengths (in Å) for Silane σ-Complexes 4-6
on silylium ion abstraction in 4, by a carbonyl substrate followed
by hydride transfer to the silyloxy carbenium ion.
a
b
c
d
e
f
Acknowledgment. This work was supported by NSERC, OPIC,
and MCINN. We thank CFI and OIT for a generous equipment
grant.
4 (exp.)
(calc.)
5
53
-34.8
59
33
1.996
45
-19.9
48
45
1.980
45
-17.8
47
50
1.947
48
48
50
-33.2a
-
-
39
-
-
-
-
6
46
Supporting Information Available: Experimental and computa-
tional details. This material is available free of charge via the Internet
at http://pubs.acs.org.
Si-H
1.898a
a Calculated data for complex 4g with R3 ) Me3.
Scheme 1. Ionic Hydrosilylation and Dissociative Ojima-Type
References
Pathways Calculated by DFTa
(1) (a) Lachaize, S.; Sabo-Etienne, S; Donnadieu, B.; Chaudret, B. Chem.
Commun. 2003, 214. (b) Yang, J.; White, P. S.; Brookhart, M. J. Am. Chem.
Soc. 2008, 130, 17509.
(2) (a) Nikonov, G. I. AdV. Organomet. Chem. 2005, 53, 217–309. (b) Lin, Z.
Chem. Soc. ReV. 2002, 31, 239. (c) Corey, J. Y.; Braddock-Wilking, J.
Chem. ReV. 1999, 99, 175. (d) Kubas, G. J. Metal Dihydrogen and s-Bond
Complexes; Kluwer Academic/Plenum: New York, 2001. (e) Crabtree, R. H.
Angew. Chem., Int. Ed. Engl. 1993, 32, 789.
(3) Labile, NMR characterized: (a) Luo, X.-L.; Crabtree, R. H. J. Am. Chem.
Soc. 1989, 111, 2527. (b) Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J.
Inorg. Chim. Acta 1999, 294, 240. (c) Scharrer, E.; Chang, S.; Brookhart,
M. Organometallics 1995, 14, 5686. (d) Fang, X.; Huhmann-Vincent, J.;
Scott, B. L.; Kubas, G. J. J. Organomet. Chem. 2000, 609, 95. (e) Fang,
X.; Scott, B. L.; John, K. D.; Kubas, G. J. Organometallics 2000, 19, 4141.
(4) Isolated, X-ray characterized: (a) Freeman, S. T. N.; Lemke, F. R.; Brammer,
L. Organometallics 2002, 21, 2030. (b) Yang, J.; White, P. S.; Schauer,
C. K.; Brookhart, M. Angew. Chem., Int. Ed. 2008, 47, 4141.
(5) Gutsulyak, D. V.; Kuzmina, L. G.; Howard, J. A. K.; Vyboishchikov, S. F.;
Nikonov, G. I. J. Am. Chem. Soc. 2008, 130, 3732.
21
a Gibbs energies are given in kcal ·mol-1
.
(6) For recent reviews on hydrosilylation of carbonyls, see: (a) Carpentier,
J.-F.; Bette, V. Curr. Org. Chem. 2002, 6, 913. (b) Riant, O.; Mosterfai,
N.; Courmarcel, J. Synthesis 2004, 18, 2943.
(7) For hydrosilylation of esters, see: (a) Mao, Z.; Gregg, B. T.; Cutler, A. R.
J. Am. Chem. Soc. 1995, 117, 10139. (b) Ojima, I.; Kogure, T.; Kumagai,
M. J. Org. Chem. 1977, 42, 1671. (c) Igarashi, M.; Mizuno, R.; Fuchikami,
T. Tetrahedron Lett. 2001, 42, 2149. (d) Berc, S. C.; Kreutzer, K. A.;
Buchwald, S. L. J. Am. Chem. Soc. 1991, 113, 5093. (e) Berc, S. C.;
Buchwald, S. L. J. Org. Chem. 1992, 57, 3751.
(8) Dioumaev, V. K.; Bullock, R. M. Nature 2003, 424, 530.
(9) For metal-free reactions with a similar mechanism, see: (a) Parks, D. J.;
Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090. (b) Parks,
D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440. (c) Gevorgyan, V.;
Rubin, M.; Benson, S.; Liu, J.-X.; Yamamoto, Y. J. Org. Chem. 2000, 65,
6179. (d) Scott, V. J.; Celenligil-Cetin, R.; Ozerov, O. V. J. Am. Chem.
Soc. 2005, 127, 2852.
(10) For dehydrogenative coupling of silanes with amines and ammonia, see:
(a) Nametkin, N. S.; Chernysheva, T. I.; Gevenyan, M. L.; Lashenko, I. N.;
Pritula, N. A. IzV. Akad. Nauk SSSR, Ser. Khim. 1972, 10, 2230. (b) Wang,
W. D.; Eisenberg, R. Organometalics 1991, 10, 2222. (c) Biran, C.; Blum,
Y. D.; Glaser, R.; Tse, D. S.; Youngdahl, K. A.; Laine, R. M. J. Mol.
Catal. 1988, 48, 183. (d) Blum, Y.; Laine, R. M. Organometallics 1986,
5, 2081. (d) Kono, H.; Ojima, I. Org. Prep. Proc. Int. 1973, 5, 135.
(11) For the coupling of silanes with water as a source of H2, see: (a) Ison,
E. A.; Corbin, R. A.; Abu-Omar, M. M. J. Am. Chem. Soc. 2005, 127,
11938. (b) Corbin, R. A.; Ison, E. A.; Abu-Omar, M. M. Dalton Trans.
2009, 2850.
(12) For selected examples of catalytic dehalogenation of haloalkanes and arenes
by hydrosilanes, see: (a) Yang, J.; Brookhart, M. AdV. Synth. Catal. 2009,
351, 175. (b) Karstedt, D.; Bell, A. T.; Tilley, T. D. Organometallics 2006,
25, 4471. (c) Petersen, A. A.; McNeill, K. Organometallics 2006, 25, 4938.
(d) Esteruelas, M. A.; Herrero, J.; Oliva´n, M. Organometallics 2004, 23,
3891, and references therein.
(13) Gutsulyak, D. V.; Kuzmina, L. G.; Howard, J. A. K.; Vyboishchikov, S. F.;
Nikonov, G. I. Organometallics 2009, 28, 2655.
(14) This trend can be rationalized in terms of a combination of reduced back-
donation from metal and reduced Si 3s character in the Si-H bond in the
less chlorinated species.
complex 4a with the longest Si-H bond (1.996 Å). The rationale
for this anomaly is that the Si-H bond in 4a, although more
activated than that in 4g, has increased Si 3s character. Previously,
such unusually increased Si-H coupling has been found only in
complexes with interligand hypervalent interactions and agostic
Si-H bonds.17
1H EXSY spectra of complexes 4 do not show any exchange
with free nitrile. However, a reaction of 4b with 15 equiv of acetone
leads to the bis(nitrile) 2 and a new labile mono(phosphine) silane
σ-complex (J(H-Si) ) 40 Hz), which may suggest a usual Ojima
type mechanism of hydrosilylation based on nitrile dissociation,
η2-carbonyl coordination, and silyl migration.18 However, DFT
COSMO calculations taking into account solvent effects show that
acetone addition to [Cp(Me3P)Ru(η2-HSiMe3)]+ gives only the η1-
OdCMe2 derivative [Cp(Me3P)Ru(η2-HSiMe3)(η1-OCMe2)]+ which
resists silyl migration to the oxygen atom. For an alternative
mechanism, the ionic hydrosilylation (Scheme 1),8,19 we found that
direct Me3Si+ transfer to the carbonyl is the rate-determining step
(∆‡G°298 ) 18.6 kcal ·mol-1). This is followed by a low-barrier
(∆‡G°298
)
4
kcal · mol-1
)
hydride transfer from
Cp(Me3P)Ru(NCMe)(H) to [Me2C-O-SiMe3]+ to form the σ-com-
plex Cp(Me3P)Ru(NCMe)(η1-HsCMe2sOsSiMe3)+, which easily
dissociates Me2HCsOSiMe3. Similar pathways were suggested for
borane-catalyzed hydrosilylation,9a for dialkyl ether cleavage with
silanes mediated by a cationic Ir complex,1b and for silane
alcoholysis on electrophilic metal centers.20
(15) Significant Si · · · Cl bonding was found in neutral complexes 6 (ref 13).
(16) Experimental Si-H coupling constants are given in absolute values,
calculated J(H-Si) are negative due to negative gyromagnetic ratio of Si.
(17) (a) Dubberley, S. R.; Ignatov, S. K.; Rees, N. H.; Razuvaev, A. G.;
Mountford, P.; Nikonov, G. I. J. Am. Chem. Soc. 2003, 125, 644. (b)
Ignatov, S. K.; Rees, N. H.; Tyrrell, B. R.; Dubberley, S. R.; Razuvaev,
A. G.; Mountford, P.; Nikonov, G. I. Chem.sEur. J. 2004, 10, 4991.
(18) (a) Ojima, I.; Kogure, T.; Kumagai, M.; Horiuchi, S.; Sato, T. J. Organomet.
Chem. 1976, 122, 83. (b) Ojima, I. The Hydrosilation Reaction. In The
Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.;
Wiley: New York, 1989; Chapter 25.
Further support for this mechanistic proposal was found in the
dependence of silane alcoholysis on the nucleophilicity of alcohol
(Table 1, entries 7-10) suggesting that alcohol attack on the silane
ligand in 4f is the rate-determining step.
In conclusion, we found evidence that hydrosilylation of carbonyl
substrates catalyzed by 2 goes via intermediate formation of cationic
silane σ-complexes 4 and 5, whose Si-H coupling constants suggest
significantly reduced back-donation from the cationic ruthenium
center in comparison with neutral analogues. DFT studies of the
mechanism of hydrosilylation favored a reaction pathway based
(19) Du, G.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2005, 129,
5180.
(20) (a) Kubas, G. J. AdV. Inorg. Chem. 2005, 56, 127. (b) Bu¨hl, M.; Mauschick,
F. T. Organometallics 2003, 22, 1422.
(21) For a slightly more elaborated scheme, see Supporting Information.
JA101583M
9
J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010 5951