Au Nanoparticle-Catalyzed Insertion of Carbenes from α-Diazocarbonyl Compounds into Hydrosilanes

Article abstract of DOI:10.1021/acs.orglett.8b01638

Supported Au nanoparticles on TiO₂ catalyze the insertion of carbenes from α-diazocabonyl compounds into hydrosilanes. It is proposed that the transformation involves two modes of catalytic activation: formation of nucleophilic Au carbenes on the surface of nanoparticle via expulsion of N₂ and activation of the Si-H bond of hydrosilane on Au nanoparticle, followed by coupling of the chemisorbed species. No external ligands or additives are required, while the process is purely heterogeneous, thus allowing the recycling and reuse of the catalyst.

Full text of DOI:10.1021/acs.orglett.8b01638

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Au Nanoparticle-Catalyzed Insertion of Carbenes from
α‑Diazocarbonyl Compounds into Hydrosilanes
Marios Kidonakis and Manolis Stratakis*
Department of Chemistry, University of Crete, Voutes 71003, Heraklion, Greece
S
* Supporting Information
ABSTRACT: Supported Au nanoparticles on TiO2 catalyze the insertion of
carbenes from α-diazocabonyl compounds into hydrosilanes. It is proposed
that the transformation involves two modes of catalytic activation: formation
of nucleophilic Au carbenes on the surface of nanoparticle via expulsion of N2
and activation of the Si−H bond of hydrosilane on Au nanoparticle, followed
by coupling of the chemisorbed species. No external ligands or additives are
required, while the process is purely heterogeneous, thus allowing the recycling and reuse of the catalyst.
he chemistry of metal carbenes, organometallic com-
pounds bearing a neutral divalent carbon, has attracted
Note that the formal insertion of carbenes into hydrosilanes9 is
a well-known reactivity mode (Scheme 1) and is catalyzed by a
series of metals such as Rh(II),10 Rh(I),11 Cu(II),10a,12
Cu(I),13 Ir(III),14 Ag(I),15 Ru(II),16 and Fe(II).17
T
the interest of organic chemists in recent years.1 Several
examples of metal carbenes are known involving a variety of
cationic metals, among them not only ionic Au but metallic Au
as well. Thus, R2C: carbenes adsorbed on the surface of Au
powder have been suggested as intermediates in the
dimerization of diazoalkanes2 or α-diazoketones3 into alkenes
and of alkenyl gem-dibromides into cumulenes.4 In addition,
N-heterocyclic carbenes adsorbed on adatoms of the Au
surface have been recognized.5 Recently, Leyva-Perez and
Corma6 established that electron-rich carbenes from diazo-
acetates [AunC(H)COOR] can be formed on the surface of
supported Au nanoparticles (Aun) and were characterized
spectroscopically. DFT calculations revealed that the carbene
carbon atom bonds to two vicinal Au atoms, and there is an
additional stabilizing interaction between an ester oxygen atom
and a third vicinal Au atom. An exceedingly important feature
of these Au carbenes is that they have a Schrock-type
nucleophilic character through injection of electron density
from the Au nanoparticle to the carbene carbon atom. It was
established that these carbonyl group-stabilized electron-rich
Au nanoparticle-adsorbed carbenes lack any reactivity with
nucleophiles, such as alcohols, react slowly with electron poor
alkenes, and in general, do not undergo any insertion reactions.
Nucleophilic behavior was also shown in the so far only
reported example of a Au(III)−carbene complex,7 in contrast
to the metal carbenes of cationic Au(I), which are electrophilic
and react readily with nucleophiles.8
Scheme 1. Catalyzed Reaction Between α-Diazoesters and
Hydrosilanes
Treatment of a premixed solution of ethyl diazoacetate (1)
and 1.2 equiv of a hydrosilane (Et3SiH, ArMe2SiH, or
Me3SiOMe2SiH) in dry 1,2-dichloroethane (DCE) with Au/
TiO2 (1 mol %) for 40 min at 70 °C results in the formation of
α-silylacetates (1a−e) in >70% isolated yield (Figure 1).
Under the same conditions, and in the absence of hydrosilane,
quantitative dimerization into diethyl fumarate and diethyl
maleate occurs.6 The observation of insertion of a formal
carbene from ethyl diazoacetate (1) into the Si−H bond of
hydrosilanes urged us to examine the scope and limitations of
this transformation in a series of α-diazocarbonyl compounds
(Figure 1).18 Thus, aryl- or alkyl-substituted α-diazoesters
react smoothly, forming hydrosilylation products in very good
isolated yields. The only possible side pathway is the
replacement of the diazo group by two hydrogen atoms
(CN2 → CH2). This pathway depends on the amount of
moisture, and in nondried solvent, it may reach up to 30% in
relative yield. Note that in the literature there are only a few
reports of this reductive transformation using Pd/C-catalyzed
hydrogenation.19 An analogous reductive pathway, which
requires a Au hydride and a proton, has been also observed
in the nano Au-catalyzed reaction between hydrosilanes and π
While Corma’s work appeared in the literature,6 con-
currently, we were also working on the activation of α-
diazocarbonyl compounds by Au nanoparticles supported on
titania (Au/TiO2) and have confirmed their results. Indeed, α-
diazoesters cleanly and quantitatively undergo dimerization
into alkenes. Yet, in contrast to the conclusion that these
umpolung Au nanoparticle carbenes do not participate in
insertion reactions, if a hydrosilane is present, insertion into
the Si−H bond takes place to form α-silylesters in very good
yields within short reaction times and low catalyst loading.
Received: May 24, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01638
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of Au/TiO2,21 in α-diazoketones, the reaction occurs
exclusively on the carbene carbon atom.
The catalytic system is reusable with no deterioration of its
catalytic activity after three consecutive runs. Thus, after a
simple filtration, washing with the reaction solvent and drying
in the oven at 90 °C for 2 h, the catalyst can be used in a new
run as active as before, which is indicative of the pure
heterogeneous nature of the process. To the best of our
knowledge, this is the first example of heterogeneous catalytic
hydrosilylation of α-diazocarbonyl compounds. Additionally,
we emphasize that, with one exception,10b all previous known
analogous metal-catalyzed protocols require an excess of
hydrosilane (up to 4 equiv), while the addition of the diazo
compound into a solution of hydrosilane should be slow (using
a syringe pump) and over the reaction time to avoid
dimerization. In our case, we use almost stoichiometric
amounts of reactants and they are immediately mixed.
In contrast to hydrosilanes, 1,2-disilanes, silylboranes, or
diboranes, compounds that are readily activated by Au/TiO2
and perform addition on π systems22 do not undergo any
reaction with Au nanoparticle carbenes on the basis of our
attempts thus far.23 Instead, α-diazoesters readily dimerize6
under the reactions conditions or partially form hydrosilylation
products in the case of disilanes and silylboranes if traces of
moisture are present.
To draw mechanistic conclusions, we performed the kinetic
competition between an equimolar mixture of PhMe2SiH (1
equiv) and PhMe2SiD (1 equiv) and 0.4 equiv of ethyl
diazoacetate (1). The product analysis revealed a substantial
kinetic primary isotope effect of kH/kD = 4.8 0.4 (Scheme 2).
Scheme 2. Kinetic Isotope Effect Regarding the Insertion of
Ethyl Diazoacetate into Si−H/Si−D Bonds
It was calculated as an average of four measurements at ∼20%
insertion of the isotopically competing hydrosilanes (Support-
ing Information). This value is significantly larger than the
reported ones in analogous metal-catalyzed protocols, which
range between kH/kD = 1.1−1.6,13a,14a,17,24 and implies a
different mechanism of insertion.25 Additionally, the influence
of the electronic nature of aryl substituents on kinetics was
examined (Supporting Information). As a general trend,
donors at the para position of aryl-substituted diazoesters
and acceptors at the para position of aryldimethylsilanes
increase the reaction rate. The linear correlation of the
Hammett plots is not often ideal. We indicate, however, that in
heterogeneous Au nanoparticle-catalyzed reactions such data
should be treated with care because adsorption−desorption
phenomena of reactants on the catalyst’s surface, as well as the
support, largely affect the reaction’s kinetics.26 The measured
isotope effect and the profile of Hammett kinetics will be
further analyzed in the accompanying mechanistic discussion.
Regarding the proposed mechanism that is presented in
Scheme 3, we invoke two well-established activation modes by
Au nanoparticles (Aun): the insertion of the σ Si−H bond of
hydrosilanes on Aun to form intermediate I27 and the
Figure 1. Reaction of hydrosilanes with α-diazocarbonyl compounds
catalyzed by Au/TiO2.
systems if a proton source (water or alcohol) is present.20
Among the characteristics of the Au-catalyzed transformation
is the clean reaction between hydrosilanes and allyl ester 9
forming products 9a or 9b, without observing any intra-
molecular insertion13e on the double bond. α-Diazoketones
also undergo smooth insertion into hydrosilanes forming α-
silylketones in good yields (Figure 1). Notably, although
simple ketones readily undergo hydrosilylation in the presence
B
DOI: 10.1021/acs.orglett.8b01638
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Proposed Mechanism in the Au/TiO2-Catalyzed
Insertion of Carbenes from α-Diazoesters into Hydrosilanes
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: stratakis@uoc.gr.
ORCID
Manolis Stratakis: 0000-0002-0962-8297
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.K. thanks the ΕΛΙΔΕΚ foundation for financially
supporting his doctoral studies. ProFI (FORTH, Heraklion,
Greece) is acknowledged for obtaining the HRMS spectra of
the unknown compounds.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01638.
1H and 13C NMR of all products (PDF)
C
DOI: 10.1021/acs.orglett.8b01638
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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D
DOI: 10.1021/acs.orglett.8b01638
Org. Lett. XXXX, XXX, XXX−XXX