Insight into the mechanism of carbonyl hydrosilylation catalyzed by brookharts cationic iridium(iii) pincer complex

Article abstract of DOI:10.1021/ja503254f

New experimental findings suggest partial revision of the currently accepted mechanism of the carbonyl hydrosilylation catalyzed by the iridium(III) pincer complex introduced by Brookhart. Employing silicon-stereogenic silanes as a stereochemical probe results in racemization rather than inversion of the configuration at the silicon atom. The degree of the racemization is, however, affected by the silane/carbonyl compound ratio, and inversion is seen with excess silane. Independently preparing the silylcarboxonium ion intermediate and testing its reactivity then helped to rationalize that effect. The stereochemical analysis together with these control experiments, rigorous multinuclear NMR analysis, and quantum-chemical calculations clearly prove that another silane molecule participates in the hydride transfer. The activating role of the silane is unexpected but, in fact, vital for the catalytic cycle to close.

Full text of DOI:10.1021/ja503254f

Communication
pubs.acs.org/JACS
Insight into the Mechanism of Carbonyl Hydrosilylation Catalyzed by
Brookhart’s Cationic Iridium(III) Pincer Complex
Toni T. Metsanen, Peter Hrobarik,* Hendrik F. T. Klare, Martin Kaupp, and Martin Oestreich*
́
̈
Institut fur Chemie, Technische Universitat Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
̈
̈
S
* Supporting Information
Scheme 1. Carbonyl Hydrosilylation Catalyzed by Lewis Acids
(LAs) B(C6F5)3 (1) or Pincer Complex 2+
ABSTRACT: New experimental findings suggest partial
revision of the currently accepted mechanism of the
carbonyl hydrosilylation catalyzed by the iridium(III)
pincer complex introduced by Brookhart. Employing
silicon-stereogenic silanes as a stereochemical probe results
in racemization rather than inversion of the configuration
at the silicon atom. The degree of the racemization is,
however, affected by the silane/carbonyl compound ratio,
and inversion is seen with excess silane. Independently
preparing the silylcarboxonium ion intermediate and
testing its reactivity then helped to rationalize that effect.
The stereochemical analysis together with these control
experiments, rigorous multinuclear NMR analysis, and
quantum-chemical calculations clearly prove that another
silane molecule participates in the hydride transfer. The
activating role of the silane is unexpected but, in fact, vital
for the catalytic cycle to close.
ctivation of Si−H bonds with Lewis acids is the initiation
1
Astep of a number of catalytic processes. The mechanisms of
these reactions are, however, often not thoroughly understood.
In a recent highlight article,2 we discussed the similarities of two
widely used catalysts known to activate the Si−H bond, B(C6F5)3
(1)3 and cationic iridium(III) pincer complex 2+[B(C6F5)4]−4
(Scheme 1, top). Si−H bond activation with either 1 or 2+ is
believed to commence with η1-σ coordination of the Si−H bond
of 3 to the Lewis acidic center (3 → 4, Scheme 1, bottom),5,6
followed by nucleophilic attack of a Lewis base at the silicon
atom. For carbonyl hydrosilylation (5 → 9), it is the oxygen atom
of 5 that acts as the nucleophile, and the net reaction is a formal
transfer of a silicenium ion onto the carbonyl group (4 → 6 with
1, left cycle and 4 → 7 + 8 with 2+, right cycle).3b,4,6,7 The nature
of the hydride “complex” generated in the silyl transfer step
critically depends on the Lewis acid catalyst used. With neutral 1,
an ate complex, [HB(C6F5)3]−, is released that forms an ion pair
with the silylcarboxonium ion (6, left cycle). Conversely, cationic
2+ transforms into the neutral iridium(III) dihydride 8, likely to
be separated from silylcarboxonium ion 7 (7 + 8, right cycle).6
For the catalysis with B(C6F5)3 (1), it was shown both
experimentally3b,7 and computationally8 that ion pair 6 under-
goes rapid hydride transfer to produce 9 and regenerate catalyst
1. However, such in-depth analysis of the fate of the
corresponding intermediates 7 + 8 remains to be conducted.9
Kinetic measurements by Brookhart and co-workers4 indicated
that the carbonyl hydrosilylation catalyzed by 2+ is first order in
silane 3 and zero order in carbonyl compound 5. These findings
were inconclusive, though, as to whether the silyl or the hydride
transfer is the rate-determining step. Also, the involvement of
silicon cation intermediates cannot be excluded on the basis of
these results.
Our laboratory elucidated the mechanism of B(C6F5)3-
catalyzed carbonyl hydrosilylation using a silicon-stereogenic
silane as a mechanistic probe.7 The observed inversion of the
configuration at the silicon atom unambiguously showed the
transfer of the silicon group to obey the SN2-Si mechanism, as
proposed earlier by Piers and co-workers (4 → 6, left).3b
Moreover, the absence of racemization precluded the release of a
silicon cation from complex 4 or ion pair 6 and lent further
support for rapid hydride transfer (6 → 9, left). We envisioned
that a related analysis of the catalysis with the iridium(III)
POCOP complex 2+[B(C6F5)4]− would provide similar insight
[POCOP = 2,6-bis(di-tert-butylphosphinito)phenyl].
We knew from the B(C6F5)3-mediated Si−H bond activation
that the steric situation around the silicon atom is crucial (t-Bu
group not tolerated),7 and we therefore chose cyclic i-Pr-
substituted silane (SiS)-3a10a (e.r. = 97:3). We were then
Received: April 1, 2014
Published: May 2, 2014
© 2014 American Chemical Society
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delighted to find that catalyst 2+ promoted the reaction of rigid
(SiS)-3a and an equimolar amount of acetophenone (5),
affording ether 9a with poor diastereocontrol (cyclic scenario,
Table 1, columns 2 and 3). The reaction was slow, requiring 20 h
bond in (SiS,RS)-9b furnished (SiR)-3b (e.r. = 36:64) with
opposite absolute configuration. While the degree of racemiza-
tion is certainly substantial, the remaining enantioenrichment of
(SiR)-3b is evidence of the rate of racemization being dependent
on the silane/carbonyl compound ratio. Partial racemization of
the recovered silane (SiS)-3b confirms the reversibility of the
silane coordination as well as the silyl transfer.
Table 1. Two-Step Stereochemical Analysis with Silicon-
a b
,
Stereogenic Silanes (SiS)-3a and (SiS)-3b
The experiments with the silicon-stereogenic silanes provide
valuable information but do not explain the entirely different
behavior of (SiS)-3a and (SiS)-3b. These seemingly inconclusive
results again made us compare the catalytic cycles with 1 and 2+
as catalysts (cf. Scheme 1, bottom). It is the nature of the
silylcarboxonium intermediates that distinguishes these pro-
cesses, with the B(C6F5)3 catalysis passing through an ion pair
known to immediately react (6 → 9, left). Less information is
available on the hydride transfer from the iridium dihydride 8 (7
+ 8 → 9, right). Brookhart proposed that either neutral 8 would
directly donate a hydride, forming cationic 2+ (without Do
molecule) or, alternatively, the hydride transfer would be assisted
by solvent coordination (11 with ArCl = C6H5Cl or o-Cl2C6D4,
Figure 1).4,12 In addition to the solvent, both reactants, silanes 3
and carbonyl compound 5, are likely to coordinate to the iridium
center (12 and 13, Figure 1).
c
d
(SiS)-3 (1 equiv)
(SiS)-3 (4 equiv)
e
e
scenario
cyclic with (SiS)-3a
e.r.
yield (%)
e.r.
yield (%)
f
f
9a
recovered 3a
reformed 3a
10
55:45
−
86
0
55:45
94:6
86
95
99
86
49:51
79
69
48:52
51:49
51:49
acyclic with (SiS)-3b
f
f
9b
59:41
−
90
0
62:38
74:26
36:64
46:54
89
85
86
93
recovered 3b
reformed 3b
10
Figure 1. Conceivable hydride donors for the reduction of the
silylcarboxonium ion intermediate.
48:52
85
99
49:51
a
b
Hydrosilylation according to General Procedure 1. Reductive
c
To identify the hydride source, we independently synthesized
intermediates 7 and 8 (cf. Scheme 1, right). The silylcarboxo-
nium ion 7d13 was generated from benzene-stabilized silicenium
cleavage according to General Procedure 2. Reaction time 20 h.
d
e
Reaction time 30 min. Enantiomeric ratios determined by HPLC
f
analysis using chiral stationary phases; e.r. = S/R. Diastereomeric ratio
14
ion [Et3Si(C6D6)]+[B(C6F5)4]− (derived from the corre-
determined by GLC analysis.
sponding silane 3d) and acetophenone (5, 1 equiv) in o-Cl2C6D4
(Scheme 2, top). We then conducted a series of control
experiments (Schemes 2A−D, bottom), and clean formation of
for completion. Using excess (SiS)-3a dramatically accelerated
the reaction, producing the same result in minutes rather than
hours; unconsumed (SiS)-3a was recovered without any
racemization (cyclic scenario, Table 1, columns 4 and 5). To
our surprise, the reductive cleavage of the Si−O bond with
DIBAL−H (proceeding with retention at the silicon atom11)
yielded both reformed 3a and alcohol 10 essentially in racemic
form within the experimental error.
1
freshly prepared 7d was confirmed by H NMR spectroscopy
prior to each run. Treatment of 7d with dihydride 8 produced a
remarkable result: 8 that was originally assumed to be the hydride
source4,9 is reluctant to transfer a hydride onto the carbon atom
of 7d! Instead, immediate hydride transfer onto the silicon atom
of 7d occurred, as indicated by formation of 2+[B(C6F5)4]− and
14d+[B(C6F5)4]− (Scheme 2A). The backward reaction is thus
favored over closing the catalytic cycle at short reaction times,
despite the fact that the forward reaction, i.e., the reduction (7d
→ 9d), is more exoenergetic by about 64 kJ/mol (see Figure S1
in the SI for free energies calculated at the B3LYP-D3(BJ)/ECP/
6-31++G(d,p) level using an SMD solvation model). That hints
at a somewhat higher reaction barrier for the reduction as
compared to the backward reaction associated with the cleavage
of the Si−O bond in 7d. Attempts to find the transition states on
the electronic energy surface for the hydride transfer from 8 to
the carbon or silicon atom of 7d were not successful, suggesting a
process without a potential energy barrier for both the forward
and backward reactions. The experimental observation could
hence be related to a higher entropy barrier for the reduction,
connected also with ion pair dissociation (not taken into account
in our DFT simulations). Then, addition of silane 3d (1 equiv)
resulted in instantaneous consumption of 7d as well as any
This outcome stands in stark contrast to the catalysis with
B(C6F5)3 (1),7 and it was, at least at this stage, not meaningful.
To rule out any detrimental effect of the rigid cyclic structure of
(SiS)-3a, we repeated the same set of experiments with acyclic i-
Pr-substituted silane (SiS)-3b10b (e.r. = 97:3). Again, the reaction
time was dependent on the amount of (SiS)-3b used relative to 5,
and results were discouraging with an equimolar ratio (acyclic
scenario, Table 1, columns 2 and 3). The setup with excess (SiS)-
3b was, however, markedly different from the previous ones
(acyclic scenario, Table 1, columns 4 and 5). Recovered (SiS)-3b
was now partially racemized, indicating that unconsumed (SiS)-
3b (e.r. = 74:26) had been involved in the catalysis. It must be
noted here that neither cationic 2+ (iridium monohydride) nor
1
neutral 8 (iridium dihydride) racemizes (SiS)-3b, while H/2H
scrambling of deuterium-labeled Me2PhSiD (3c-d1) is fast with
2+ and slow with 8 (see the Supporting Information (SI) for
details). More importantly, the reductive cleavage of the Si−O
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fast equilibrium with the originally proposed silane adduct 13,
iridium(V) silyl trihydride 1917 after oxidative addition of the Si−
H bond to the iridium(III) center of 13, and iridium(III)
monohydride 2012 after reductive elimination of dihydrogen
(Scheme 3; see the SI for NMR measurements, crystallographic
Scheme 2. Identification of the Hydride Source in the
Reduction of Silylcarboxonium Ion 7
Scheme 3. Revised Catalytic Cycle, a Two-Silicon Cycle
([B(C6F5)4]− as Counteranion Omitted for Clarity)
acetophenone (5) released earlier (Scheme 2B). The expected
silyl ether 9d formed along with the corresponding silyl enol
ether 15d.4,15
To probe the reactivity of the proposed acetophenone adduct
12 (Figure 1), we treated the coexisting mixture of
silylcarboxonium ion 7d and dihydride 8 with excess
acetophenone (5, 2 equiv), yet no reaction was observed. The
crucial role of the silane was again demonstrated by the addition
of excess 3d (3 equiv). As expected, reduction proceeded
immediately (7d → 9d), but the silyl ether reacted further to
finally yield ethylbenzene (9d → 16, Scheme 2C). Not
surprisingly, addition of 8 (5 mol %) and 3d (1 equiv) to freshly
prepared 7d afforded solely 16 (Scheme 2D). Hence, the
reduction does not stop at the alcohol oxidation level when
silanes 3 act as reductants.16 We must conclude from this that,
despite being the most available hydride source, free silanes 3
cannot fulfill the role of the hydride donor in the real catalytic
process using 2+[B(C6F5)4]−, since only traces of deoxygenation
are observed. The difference between the experiments shown in
Scheme 2C,D and the catalytic setup is that the equimolar ratio of
7d and 8 is out of balance, and with excess 7d relative to 8,
reduction by 3 becomes kinetically competent. It is also clear
from the reaction between 7d and 8 that the neutral dihydride 8
alone cannot donate that hydride (Scheme 2A), and
coordination of neither solvent (as in 11) nor carbonyl
compound (as in 12) to 8 is able to mediate the hydride transfer
(Scheme 2C).
characterization of 20c and 20d, and detailed bonding analysis).
The assignment of the resonance signals (cf. Scheme 3) was
guided by our state-of-the-art relativistic four-component DFT
calculations of NMR shifts18 (Tables S4 and S5 in the SI) and by
the elegant work of Heinekey and co-workers on related borane
complexes.19 Based on our control experiments (cf. Scheme 2)
and computed hydricities of iridium POCOP complexes present
in the reaction mixture with excess 3 (cf. Figure 1 and Table S6 in
the SI), either 13 or 19, with more ionic metal−hydrogen bond
character and hydricity as compared to 8, is de facto the active
hydride donor. We cannot decide either experimentally or
computationally whether 13 or 19 ultimately acts as the hydride
source, but the elusive iridium(V) complex 19 appears somewhat
more likely because adduct formation alone, as in 11 and 12, is
not sufficient. The reaction could be classified as a silane-
mediated hydride transfer. The final hydride shift step is also
reversible, as 14+ slowly racemizes enantioenriched 9d (see the
SI for racemization experiments).
On the basis of these findings, we propose a revised catalytic
cycle (Scheme 3) where the initial Si−H bond activation is in
agreement with the previously proposed mechanism.4,6 The
silane 3 is reversibly coordinated by cationic monohydride 2+
(concomitant with liberation of the donor) to form the η1-σ
complex 14+. Nucleophilic substitution with the carbonyl oxygen
atom of 5 through transition state 17‡ yields the silylcarboxo-
nium ion 7 with inversion at the silicon atom along with neutral
dihydride 8. Racemization then occurs at this stage by reversible
The key actor in the hydride transfer is silane 3 that is required
to boost the hydride donor strength of iridium(III) dihydride 8.
NMR experiments on a mixture of 8 and 3 show that these are in
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formation of siliconium ion 18 that emerges from coordination
of another molecule of 5 to 7. The net result is exchange of the
silicenium ion fragment between two molecules of 5. A related
transfer process was recently considered by Oro and co-workers
in a hydrosilylation of C−C triple bonds,20 and a similar
mechanism was reported by Brookhart and co-workers in the
ether cleavage with silanes.12
Dr. Elisabeth Irran for the X-ray analyses and Francis Forster for
his experimental contributions (both TU Berlin).
REFERENCES
■
(1) For an authoritative review, see: Corey, J. Y. Chem. Rev. 2011, 111,
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(2) Robert, T.; Oestreich, M. Angew. Chem., Int. Ed. 2013, 52, 5216−
5218 and references cited therein.
The revised catalytic cycle also explains the results obtained
with the silicon-stereogenic silanes (SiS)-3a and (SiS)-3b (Table
1). At low silane concentration, i.e., using equimolar amounts of
3 (both scenarios, Table 1, columns 2 and 3), the hydride transfer
is much slower than racemization through siliconium ion 18.
Conversely, excess (SiS)-3b greatly enhances the hydride transfer,
and its increased rate is reflected in the inversion at the silicon
atom of reformed (SiR)-3b (acyclic scenario, Table 1, columns 4
and 5). Recovered (SiS)-3b is partially racemized, and that is a
clear indication of the reversibility of the silyl transfer and the Si−
H bond activation (7b → 17b‡ → 14b+ → (SiS)-3b). The
situation is different in the cyclic scenario with (SiS)-3a (Table 1,
columns 4 and 5). Neither inversion of reformed rac-3a nor
partial racemization of recovered (SiS)-3a is detected. We reason
that both hydride transfers, onto the carbon atom of 7a (forward)
or onto the silicon atom of 7a (backward), are hampered by the
steric bulk of rigidified (SiS)-3a. Consequently, 7a → 9a is again
outcompeted by formation of 18, and 7a → 17a‡ → 14a+ is even
slower.
(3) (a) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440−
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(4) Park, S.; Brookhart, M. Organometallics 2010, 29, 6057−6064.
(5) Brookhart and co-workers were able to resolve the molecular
structure of 4 derived from 2+[B(C6F5)4]− by X-ray analysis. However,
quantum-chemical calculations showed that there is only a slight energy
difference between η1-σ and η2-σ coordination of the Si−H bond in 3 to
the iridium(III) center of 2+: Yang, J.; White, P. S.; Schauer, C. K.;
Brookhart, M. Angew. Chem., Int. Ed. 2008, 47, 4141−4143.
(6) During the preparation of this manuscript, a quantum-chemical
investigation of the carbonyl hydrosilylation catalyzed by pincer
complex 2+[B(C6F5)4]− appeared in the literature. This work
demonstrates that the SN2-Si mechanism found for the catalysis with
B(C6F5)3 (1)7 also applies to 2+[B(C6F5)4]−, thereby verifying η1-σ as
the reactive coordination mode:4 Wang, W.; Gu, P.; Wang, Y.; Wei, H.
Organometallics 2014, 33, 847−857.
(7) Rendler, S.; Oestreich, M. Angew. Chem., Int. Ed. 2008, 47, 5997−
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(8) Sakata, K.; Fujimoto, H. J. Org. Chem. 2013, 78, 12505−12512.
(9) The aforementioned quantum-chemical study6 indeed comprises
an analysis of both the silyl and the hydride transfer steps. However,
some kinetically relevant POCOP complexes were not considered in the
latter.
(10) For the preparation of silicon-stereogenic silanes, see: (a) Rendler,
S.; Oestreich, M.; Butts, C. P.; Lloyd-Jones, G. C. J. Am. Chem. Soc. 2007,
129, 502−503. (b) Klare, H. F. T.; Oestreich, M.; Ito, J.-i.; Nishiyama,
H.; Ohki, Y.; Tatsumi, K. J. Am. Chem. Soc. 2011, 133, 3312−3315.
(11) Oestreich, M.; Auer, G.; Keller, M. Eur. J. Org. Chem. 2005, 184−
195.
The fact that B(C6F5)3 (1) and iridium complex 2+[B-
(C6F5)4]− catalyze the same set of transformations2 had
prompted us to compare the individual mechanisms of action
of these fundamentally different Lewis acids. Carbonyl hydro-
silylation was chosen as a model reaction, and the basic steps of
the catalysis with 1 were fully understood at the outset of the
present investigation.3b,7,8 Conversely, hydride transfer in the
catalysis by 2+ had been unclear, and we demonstrated here that
the originally assumed hydride source4,9 is not the active
reductant. The hydride donor strength of that iridium(III)
dihydride is greatly enhanced by η1-σ coordination or even
oxidative addition of another molecule of the silane. The overall
transformation is a two-silicon rather than a one-silicon cycle.
These findings are likely to have implications for related
processes.2
(12) Yang, J.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2008, 130,
17509−17518.
(13) Prakash, G. K. S.; Bae, C.; Rasul, G.; Olah, G. A. J. Org. Chem.
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(14) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993,
260, 1917−1918.
(15) (a) Involvement of silyl enol ethers in the catalytic cycle was
−
excluded (see the SI for details). (b) Complexes 2+BF4 and 8 were
ASSOCIATED CONTENT
recently shown to be active in alcohol dehydrogenation: Polukeev, A. V.;
Petrovskii, P. V.; Peregudov, A. S.; Ezernitskaya, M. G.; Koridze, A. A.
Organometallics 2013, 32, 1000−1015.
(16) Silanes are known to rapidly deoxygenate silylcarboxonium ions:
(a) Reference 3b. (b) Kira, M.; Hino, T.; Sakurai, H. Chem. Lett. 1992,
■
S
* Supporting Information
Experimental details; characterization, crystallographic, and
1
quantum-chemical calculation data; H, 13C, and 29Si NMR
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
555−558. (c) Muther, K.; Oestreich, M. Chem. Commun. 2011, 47,
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334−336.
(17) Park, S.; Brookhart, M. J. Am. Chem. Soc. 2012, 134, 640−653.
1
(18) For the important role of spin−orbit relativistic effects on H
AUTHOR INFORMATION
■
́ ́ ́
NMR hydride shifts, see, for example: (a) Hrobarik, P.; Hrobarikova, V.;
Corresponding Authors
peter.hrobarik@tu-berlin.de
martin.oestreich@tu-berlin.de
Meier, F.; Repisky, M.; Komorovsky, S.; Kaupp, M. J. Phys. Chem. A
́
́
́ ́ ́
2011, 115, 5654−5659. (b) Hrobarik, P.; Hrobarikova, V.; Greif, A. H.;
Kaupp, M. Angew. Chem., Int. Ed. 2012, 51, 10884−10888.
(19) Hebden, T. J.; Denney, M. C.; Pons, V.; Piccoli, P. M. B.; Koetzle,
T. F.; Schultz, A. J.; Kaminsky, W.; Goldberg, K. I.; Heinekey, D. M. J.
Am. Chem. Soc. 2008, 130, 10812−10820.
Notes
The authors declare no competing financial interest.
(20) It was shown that acetone acts as a shuttle to deliver the silicenium
ion to the C−C triple bond: Iglesias, M.; Sanz Miguel, P. J.; Polo, V.;
ACKNOWLEDGMENTS
■
Fernan
2013, 19, 17559−17566.
́ ́
dez-Alvarez, F. J.; Perez-Torrente, J. J.; Oro, L. A. Chem.−Eur. J.
This research was supported by the Cluster of Excellence
Unifying Concepts in Catalysis of the Deutsche Forschungsge-
meinschaft (EXC 314/2). M.O. is indebted to the Einstein
Foundation (Berlin) for an endowed professorship. We thank
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