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B.G.M. Rocha et al. / Journal of Catalysis 309 (2014) 79–86
undertaken (Table 1). When the reactions were completed, the
subsequent workup afforded the carbene species cis-[PtCl2{-
C(N(H)N=CR2R3)=N(H)R1}(CNR1)] (7–15) in good (75–85%) isolated
yields. The reaction between cis-[PtCl2(CNtBu)2] (16) and any of 4–
6 in CHCl3 even under prolonged reflux furnished no isolable car-
bene species. Only a mixture of the starting materials along with
small amounts of yet unidentified decomposition products was
identified after reflux for 2d.
The prepared aminocarbene complexes 7–15 were character-
ized by elemental analyses (C, H, N), ESI+–MS, IR, 1D (1H, 13C{1H})
and 2D (1H,1H-COSY, 1H,13C-HMQC/1H,13C-HSQC, 1H,13C-HMBC)
NMR spectroscopies. In addition, the structures of three species
(11, 13, and 14) were elucidated by single-crystal X-ray diffraction.
For detailed characterization of aminocarbene species 7–15, see
Supplementary information.
The crystallographic data and processing parameters for 11, 13,
and 14 are summarized in Table 1S (see Supplementary Informa-
tion), while the corresponding molecular structure plots can be
found in Figs. 1 and 1S, and bond lengths and angles are given in
Table 2. As far as the most significant structural features are con-
cerned, it is worth mentioning that in all three structures, the me-
tal centers adopt square-planar geometries (s4 in the 0.061–0.077
range), and the carbene ligands {C(N(H)N=CR2R3)=N(H)R1} are in
the cis position with respect to the unreacted isocyanide. Further-
more, the carbene moiety is roughly planar and the angles includ-
ing the C-carbene atoms (i.e., N–N–Ccarbene, N–Ccarbene–N and
Scheme 1. Acyclic diaminocarbenes (ADCs) versus N-heterocyclic carbenes (NHCs)
and generation of the [M]-ADC from the corresponding [M]-CNR species.
in Suzuki–Miyaura [37,40–42] and Sonogashira [39] couplings,
while the corresponding platinum species were not active. Addi-
tionally, inspection of literature data led to the conclusion that
the reported ADC-based catalytic systems contain palladium (in
case of cross-coupling) [1,2] or gold (in case of cyclization reac-
tions) [1,2] in the core of the catalyst, and only rare examples of
other metals, e.g. nickel [47], copper [48], ruthenium [22], or rho-
dium [15], are known. To the best of our knowledge, no use of plat-
inum-ADCs in catalysis has been demonstrated beforehand.
In order to verify whether metal-ADC species and, in particular,
platinum-ADCs can be employed as catalysts for the hydrosilyla-
tion of terminal alkynes, we prepared several new aminocarbene
derivatives (via the addition of hydrazones to platinum-bound iso-
cyanides) and evaluated their catalytic properties. Consequently,
we report herein on the first metal–ADCs catalysts for the hydrosi-
lylation of alkynes and, also, on the first application of platinum-
ADCs in catalysis. The results of our study are disclosed in the sec-
tions that follow.
C
carbene–N–C) range from 111.5(19) to 132.1(16), therefore sustain-
ing their sp2 hybridization. The two C–N bonds of the carbene frag-
ments are equal (in 13) or differ insignificantly (ca. 0.04–0.06 Å in
11 and 14).
2. Results and discussions
2.2. Application of platinum-aminocarbene complexes as catalysts for
hydrosilylation of terminal alkynes
2.1. Synthesis and characterization of the platinum-aminocarbene
complexes
In the last decade, metal complexes featuring ADC ligands have
been successfully employed as catalysts for several cross-coupling
(e.g. Suzuki–Miayura, Heck, and Sonogashira reactions) and some
cyclization reactions. Recent comprehensive reviews written by
two of us [1] and by Slaughter [2] survey the accumulated experi-
mental data indicating that M-ADCs species have never been pre-
viously employed as catalysts for hydrosilylation and, in
particular, in the hydrosilylation of terminal alkynes with organo-
silanes. In pursuit of our on-going research on novel efficient cata-
lytic systems [37,39–41], we decided to evaluate the catalytic
properties of the aminocarbene complexes from this study in the
hydrosilylation of terminal alkynes.
As a model system, we have chosen the reaction of phenylacet-
ylene with triethylsilane furnishing a mixture of vinyl silanes
(Scheme 3) employed previously for catalytic studies on hydrosily-
lation of terminal alkynes [33]. As it was previously reported [33]
for the hydrosilylation reaction catalyzed by platinum compounds,
no formation of (Z)-triethyl(styryl)silane (b-(Z) product) was ob-
served [33]. Accordingly, in all our trials, the hydrosilylation reac-
The reaction between the platinum(II)-isocyanide cis-[PtCl2(-
CNR1)2] [R1 = cyclohexyl (Cy) 1, 2,6-Me2C6H3 (Xyl) 2] complexes
and H2N–N=CPh2 (4) furnishing aminocarbene species 7 and 10,
correspondingly, was previously reported by some of us [37]. In
the current study, we extended the scope of this coupling to the
new platinum-isocyanide compound cis-[PtCl2(CNR1)2] (R1 = 2-Cl-
6-MeC6H3 3) and also to the hydrazones H2N–N=CR2R3 [R2/
R3 = 9H-fluorenyl 5; R2 = H, R3 = 2-(OH)C6H4 6] that were not previ-
ously explored in this reaction.
We observed that the coupling between the equimolar amounts
of 1–3 and 4–6 (in all possible combinations) proceeded smoothly
under reflux in chloroform (Scheme 2). The reaction rate varied
with the type of the starting materials used (in the previously re-
ported coupling of 1 and 2 with 4 [37], all additions proceeded
with a similar rate), i.e., bulky hydrazones and sterically hindered
isocyanides, required longer reaction time. Therefore, the optimi-
zation of the time for each of isocyanide ligand/hydrazone pair,
upon monitoring of the reaction mixture by IR spectroscopy, was
tion furnished
a mixture of triethyl(1-phenylvinyl)silane (a
Scheme 2. Coupling between cis-[PtCl2(CNR1)2] (1–3) and H2N–N = CR2R3 (4–6) affording aminocarbene complexes 7–15.