Activation of Si-H, B-H, and P-H bonds at a single nonmetal center

Article abstract of DOI:10.1002/anie.201005698

Carbenes more than measure up: Singlet stable carbenes compete with transition metals in the activation of enthalpically strong bonds, as shown by the cleavage of the E-H bonds of silanes, boranes, and phosphanes (see scheme). However, in contrast with th

Full text of DOI:10.1002/anie.201005698

Communications
DOI: 10.1002/anie.201005698
Carbenes
À
À
À
Activation of Si H, B H, and P H Bonds at a Single Nonmetal
Center**
Guido D. Frey, Jason D. Masuda, Bruno Donnadieu, and Guy Bertrand*
For many years, it was believed that only transition-metal
centers could activate small molecules and enthalpically
strong bonds. However, it has recently been shown that
several nonmetallic systems are capable of some of these
tasks.[1,2] For example, stable singlet carbenes can activate
CO,[3a] H2,[3b] and P4.[3c–e] Such reactions have long been known
for transition metals.[4,5] However, stable singlet carbenes can
also activate NH3;[3b] a much more difficult task for transition
metals.[6,7] The oxidative addition of hydrosilanes, hydro-
boranes, and hydrophosphines at vacant coordination sites of
transition metals are well-exemplified and are considered as
key steps in the transition-metal-catalyzed hydrosilylation,
hydroboration, and hydrophosphination of multiple bonds.[8]
À
Herein, we report the first examples of the activation of E H
bonds (E = Si, B, P) at a single nonmetal center.
On the basis of our successful results with H2,[3b] we began
À
our study with the activation of Si H bonds. Indeed, silanes
are similar to H2 in that they lack both nonbonding electron
pairs and p electrons. They can bind to various metal centers
À
to form stable Si H s complexes, which undergo subsequent
Scheme 1. Reaction of carbenes 1a,b and 5 with silanes.
oxidative addition.[4] To test the possible activation of Si H
À
bonds with carbenes, we treated the cyclic (alkyl)-
(amino)carbenes (CAACs) 1a and 1b[9] with primary, secon-
dary, and tertiary silanes.
4.21 ppm corresponding to the diastereotopic hydrogen
atoms of the SiH2 fragment. The structure of 2a was
confirmed by X-ray crystallography[10] (Figure 1, top),
whereas the presence of a triplet at d = 4.53 ppm and a
doublet at d = 4.08 ppm in the H NMR spectrum confirmed
the identity of adduct 2b.
CAACs 1a,b also reacted with (EtO)3SiH to afford 3a
(d.r. 3:1) and 3b in 64 and 73% yield, respectively. However,
when Ph2SiH2 was used, only the less bulky carbene 1b
The addition of phenylsilane to 1a and 1b occurred
readily at room temperature, and the corresponding adducts
2a,b were isolated in 91 and 83% yield, respectively
(Scheme 1). As expected, in the case of the enantiomerically
pure CAAC 1a, two diastereomers 2a,a’ were formed (in a
2:1 ratio), as shown by two singlets at d = À36.4 and
À29.3 ppm in the 29Si NMR spectrum. The 13C NMR spec-
1
À
trum revealed the loss of the carbene signal and a new C H
peak at d = 63.2 (2a) and 65.5 ppm (2b). The 1H NMR
spectrum of the major isomer 2a revealed a pseudotriplet at
d = 4.78 ppm (SiCH) and two doublets at d = 4.29 and
underwent insertion into the Si H bond (to give 4b in 65%
À
yield), and a reaction time of 16 hours at 808C was necessary
for the reaction to reach completion. Surprisingly, although it
has been shown that, in contrast to CAACs, N-heterocyclic
carbenes (NHCs) do not react with H2,[11] we found that
imidazolidin-2-ylidene 5[12] also reacted at room temperature
[*] Dr. G. D. Frey, Dr. J. D. Masuda,[+] B. Donnadieu, Prof. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry, University of California
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: guy.bertrand@ucr.edu
[+] Current Address: Maritimes Centre for Green Chemistry and
Department of Chemistry, Saint Mary’s University
Halifax, NS B3H 3C3 (Canada)
À
with phenylsilane to afford the Si H insertion product 6 in
88% yield (Figure 1, bottom).
The formation of 6 raises the question of the mechanism
À
of the activation of Si H bonds with carbenes. Why should
NHCs react with silanes although they are inert towards
hydrogen? The evident difference is the presence of low-lying
vacant orbitals in silanes. In other words, the observed
reactivity might be due to the Lewis acid character of silanes;
indeed, several NHC–SiX4 adducts are known.[13]
[**] We are grateful to the NSF (CHE-0808825) for financial support, the
Alexander von Humboldt Foundation for a Feodor-Lynen Fellowship
(G.D.F.), and the Natural Sciences and Engineering Research
Council of Canada for a Postdoctoral Fellowship (J.D.M.).
To test this hypothesis, we turned our attention to boranes,
the prototypical Lewis acids. As early as 1993, Kuhn et al.[14]
reported that imidazol-2-ylidenes (unsaturated NHCs) react
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005698.
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Figure 2. Solid-state structure of 7a (hydrogen atoms at the carbene
fragment have been omitted for clarity).
B(C6F5)3. They isolated [(C6H4O2)BPtBu2R][HB(C6F5)3].
Similarly, CAACs 1a,b reacted cleanly with pinacolborane
to afford the desired oxidative-addition products 8a,b after
2 hours at room temperature in 79 and 85% yield, respec-
tively. As observed with the silanes, when enantiomerically
pure 1a was used, two diastereomers 8a,a’ were formed (in a
44:56 ratio). Interestingly, although the saturated NHC 5 also
reacted with pinacolborane, a totally different and unex-
pected reaction took place, and the dimeric product 9 was
isolated in 89% yield. The structure of 9 was ascertained by a
single-crystal X-ray diffraction study (Figure 3). Although the
mechanism of the reaction leading to 9 is not yet clear, these
results demonstrate once again the very different chemical
behavior of NHCs and CAACs.
Figure 1. Solid-state structures of 2a (top) and 6 (bottom; hydrogen
atoms have been omitted for clarity, except at the former carbene
carbon atom and at the silicon atom).
with BH3 to afford the corresponding NHC–BH3 adducts.[15]
Similarly, cyclic (alkyl)(amino)carbenes (CAACs) 1a,b
reacted at room temperature with BH3 to give the corre-
sponding Lewis acid–base adducts 7a,b (in 96 and 84% yield,
À
respectively); no trace of B H bond activation was observed
(Scheme 2, Figure 2). However, Dureen and Stephan[16]
À
recently showed that the B H bond of more hydridic
catecholborane can be activated by a mixture of tBu3P and
Figure 3. Solid-state structure of 9 (hydrogen atoms at the aryl and
pinacol fragments have been omitted for clarity).
The results obtained for the reactions between boranes
and CAACs 1a,b suggest that, as hypothesized for the silane
substrates, the activation involves the Lewis acid character of
the substrate and the initial formation of Lewis acid–base
adducts, such as 7a,b. Then, if the hydrogen atom is hydridic
enough, a migration occurs from the main-group atom (Si or
B) to the former carbene center.
Carbenes are known to form Lewis acid–base adducts[17]
with other main-group elements. We therefore studied their
reactivity with primary and secondary phosphanes
(Scheme 3). Phenylphosphane reacted with CAACs 1a and
1b at room temperature to afford the oxidative-addition
products 10a and 10b as mixtures of four (1.0:1.4:1.3:1.8) and
two diastereomers (1.0:3.0) in 96 and 93% yield, respectively.
When sterically more demanding diphenylphosphane was
used, CAACs 1a,b were inert, but the smaller CAAC 1c
Scheme 2. Reaction of carbenes 1a,b and 5 with boranes.
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were isolated as white powders and recrystallized as indicated in the
Supporting Information.
Received: September 11, 2010
Published online: October 28, 2010
Keywords: bond activation · boranes · carbenes · phosphanes ·
.
silanes
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Scheme 3. Reaction of singlet carbenes 1a–c and 5 with phosphanes.
underwent a slow but clean reaction at room temperature to
give 11 in 73% yield. Lastly, we found that although NHC 5
did not react with diphenylphosphane, it underwent insertion
À
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of small-molecule transfer.
Experimental Section
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All manipulations were performed under an inert atmosphere of dry
argon by using standard Schlenk techniques. Dry, oxygen-free
solvents were used.
General procedure: A solution of the silane, borane, or phos-
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the carbene (1 mmol) in THF, toluene, or hexanes (5 mL). The
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