Organometallics
Communication
a
also compatible. The reaction is not limited to aromatic
alkynes, and heteroaromatic substrates also participated in this
reaction. When bis(2-thienyl)acetylene (1i) was used,
benzothiophene was obtained in 41% yield. However,
Pt(L5)(dvtms) did not show any catalytic activity for the
cyclodimerization of aliphatic alkynes and 2-phenyl-1-trime-
thylsilylacetylene.
Table 1. Platinum-Catalyzed Cyclodimerization of 1a
In order to gain insight into mechanistic aspects of this
cyclodimerization reaction, deuterium labeling experiments
were carried out. The platinum−silylene-catalyzed reaction of
2,6,2′,6′-tetradeuteroditolylacetylene (1b-d4) under the stand-
ard reaction conditions led to the formation of a naphthalene
derivative with deuterium being incorporated at the 4-position
(i.e., 2b-d4) in addition to the positions where deuterium
atoms were originally located (Figure 2a). Although the
deuterium content at the 4-position was 66%,13 the majority of
the cleaved deuterium atom is incorporated into the product,
indicating that the cleaved hydrogen atom migrates in an
intramolecular manner rather than via an intermolecular
proton transfer pathway. In terms of the reaction rate,
deuterium labeling had no significant effect on the yield at
the early stage of the cyclodimerization reaction (Figure 2b),
which indicates that the turnover-determining step of this
reaction is not the C−H bond cleavage step. We next
examined the effect of the substituents on the cyclodimeriza-
tion reaction by comparing the cyclodimerization yields for 1a,
1c, 1d, and 1e at 1 and 3 h (Figure 2c). The yields for alkynes
1a and 1c were significantly lower than those for alkynes 1d
and 1e, and the reaction of 1d was complete within 3 h,
revealing that this cyclodimerization is strongly accelerated by
electron-withdrawing groups.
b
entry
[Pt]
yield of 2a (%)
1
2
3
4
5
6
7
8
Pt(L5)(dvtms)
Pt(ICy)(dvtms)
72
0
0
0
0
0
0
0
Pt(IMes)(dvtms)
Pt(IPr)(dvtms)
Pt(SIPr)(dvtms)
Pt2(dvtms)3 (Karstedt’s catalyst)
PtCl2(MeCN)2
Pt(L5)(dvtms) + 20 mol % Py·HBr
a
Reaction conditions: 1a (0.20 mmol), [Pt] (0.010 mmol), and 4-
MeTHP (0.50 mL) in a screw-capped vial under N2 for 20 h.
b
Determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloro-
ethane as an internal standard.
A possible mechanism is shown in Figure 3. First, ligand
exchange between dvtms and two alkynes forms Pt(alkyne)2
complex A. The ortho C−H bond of alkyne 1a then oxidatively
adds to the platinum center, with an alkyne serving as an ortho
directing group,11,12,14 to provide Pt(II) hydride intermediate
B. The insertion of a second alkyne into the aryl−platinum
bond forms alkenylplatinum complex C, which leads to the
formation of seven-membered platinacycle D via intra-
molecular trans hydrometalation.15 Finally, reductive elimi-
nation from D affords naphthalene 2a and regenerates Pt(0).
In this reaction, silylene L5 serves as an efficient ligand. The
silylene ligand L5 is a strong σ donor, as evidenced by its
Tolman electronic parameter (TEP) value of 2027 cm−1,
which is lower than the TEP values for typical NHC ligands
(e.g., 2037 cm−1 for ICy).6 Nevertheless, the Pt−vinyl bond
distance in Pt(L5)(dvtms) (2.123−2.151 Å) is longer than
that in Pt(NHC)(dvtms) (2.114−2.132 Å), probably because
of the greater Pt → ligand π back-donation with the silylene
ligand compared with the NHC ligand.6 In addition, the silicon
center of L5 is tricoordinate because of the coordination of the
imine, rendering the silicon center more sterically demanding
than typical NHCs. These electronic and steric features of
silylene L5 would be expected to facilitate the dissociation of
dvtms and reductive elimination. We confirmed that Pt(L5)-
(dvtms) and Pt(ICy)(dvtms) release dvtms at similar rates
upon the addition of 1a, indicating that the initial ligand
exchange step is not specifically accelerated by L5 (see the
observed in this reaction (Figure 2b), the turnover-limiting
step of this catalytic cycle is most probably the reductive
elimination step, which would be expected to be facilitated by
silylene ligand L5 as discussed above. This conclusion is also in
reported that π-acidic Au(I) complexes can catalyze the
cyclodimerization of push−pull alkynes.9 Therefore, we first
considered the initiation of the reaction by electrophilic Pt(II)
complexes generated in situ. However, this possibility was
excluded since electrophilic Pt(II) complexes such as
[PtCl2(MeCN)2] did not show any catalytic activity (entry
7). It was also reported that Rh(I) complexes, in the presence
of a proton source, can generate a Rh(III) hydride as a key
reactive species, which initiates the dimerization of alkynes by a
hydrorhodation process.8a,b To verify the possibility that such a
metal hydride species is involved in our platinum system, the
reaction was conducted in the presence of 20 mol % Py·HBr.
However, the reaction was completely blocked when a proton
source was added (entry 8). Therefore, it is unlikely that the
reaction is catalyzed by a platinum hydride species that is
generated by the reaction of Pt(L5)(dvtms) with residual
water.
The scope of the platinum−silylene-catalyzed cyclodimeri-
zation of diphenylacetylene derivatives was next investigated
(Table 2). Substrates bearing electron-donating groups, such
as methyl (1b) or methoxy groups (1c), were applicable to this
reaction, although the yields were relatively low compared with
that for electron-neutral alkyne 1a because of the formation of
indene derivative 312 as a byproduct. In contrast, electron-
deficient alkynes, such as that bearing trifluoromethyl groups
(1d), were efficiently cyclodimerized without the formation of
an indene-type byproduct. Halogen atoms, including fluorine
(1e) and chlorine (1f), were tolerated, with the corresponding
cyclodimerization products being formed in 87% and 63%
yield, respectively. Acetyl (1g) and cyano (1h) groups were
B
Organometallics XXXX, XXX, XXX−XXX
Innocent, Jean
