F. Wang, D.C. Neckers / Journal of Organometallic Chemistry 665 (2003) 1Á
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6
3
Fig. 2. Conversion from the photoactivated hydrosilylation reaction of 1-hexyne and Et3SiH. Procedure, 10 min irradiation followed by 1 h dark
reaction, then a subsequent irradiation of the reaction mixture for another 10 min (molar ratio of 1-hexyne:silane:Pt(acac)2 is 1:1:10ꢀ3; the
irradiation period is indicated by the dark squares).
Table 1
Conversion 1-hexyne with different silanes immediately after 20 min
irradiation at 350 nm (molar ratio of 1-hexyne:silane:Pt(acac)2 is
Product distribution from the photoactivated reaction
of Et3SiH and 1-hexyne catalyzed by Pt(acac)2 is the
same as in the thermal reactions catalyzed by Karstedt
1:1:10ꢀ4
)
catalyst or CODPtCl2 b:aꢂ
/
89:11 [15]. If the concentra-
Silane
Converison (%)
Ph3SiH
97.25
ClMe2SiH
98.41
(EtO)3SiH
33.42
Et3SiH
13.44
tion of catalyst in the reaction mixture is increased or
irradiation time increased, product distribution does not
change. In every case, the b-trans isomer is the major
product. This is consistent with results from other
platinum complex catalyzed hydrosilylation reactions
though it is different from the rhodium complex
catalyzed reactions.
hexyne and diphenylacetylene are complete. The relative
reactivity of the silanes that follow is Ph3SiHꢀ
ClMe2SiHꢀ(EtO)3SiHꢀEt3SiHꢀEt2SiH2. PhSiH3
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and triisopropylsilane are unreactive. This is consistent
with the trend for the hydrosilylation reaction of alkenes
except for the case of Ph3SiH [7]. Triphenylsilane is the
least reactive silane in the hydrosilylation of alkenes [7].
In the thermal process, the rate of addition of silane to
alkynes has been reported to be much faster than that
with alkenes [24]. Likewise we found that the rate of the
photoactivated hydrosilylation of alkynes is faster than
that of the photoactivated hydrosilylation of alkenes.
The competition between the hydrosilylation of an
alkyne with the hydrosilylation of an alkene was studied
by irradiating a degassed mixture of Et3SiH [one part]
with 1-hexyne [one part] and styrene [one part] for 10
min. The hydrosilylation of 1-hexyne occurs with the
same rate as it does in the absence of styrene, but no
hydrosilylation of styrene was observed.
Product distributions after 98% conversion of reac-
tant are summarized in Table 2. The ratio of the b-trans
isomer to the a isomer is decreased for the internal
alkynes in comparison with terminal alkynes. Concen-
trations of alkyne or silane do not significantly effect
product distribution. Upon increasing the ratio of
triethysilane to 2-hexyne to 2:1, the ratio of products,
b-trans:a also increased.
We found that if we increased the molar ratio of silane
to alkyne to 2:1 but kept the concentration of Pt(acac)2
the same, no further hydrosilylation of the products
occurred. However, isomerization of trans-1-(triethylsi-
lyl)-1-hexene formed in the reaction of 1-hexyne with
Et3SiH with Pt(acac)2 (molar ratio 1:2:10ꢀ4) after
irradiation of 20 min is shown in Fig. 3. Studies of
these isomerization reactions will be reported elsewhere.
We propose that the mechanism of the Pt(acac)2
catalyzed photoactivated hydrosilylation of an alkyne is
similar to that of the photoactivated hydrosilylation of
an alkene [1]. Irradiation of Pt(acac)2 either with Et3SiH
or with 1-hexyne for 10 min followed by immediate
addition of the other reactant gave the same result in the
subsequent dark reaction as did the irradiation of
Et3SiH, 1-hexyne and Pt(acac)2 in the same flask at
the same time though the reaction rate is slower (Fig. 4).
This can be explained were the reactive species formed
upon irradiation composed of both alkyne and silane
coordinated to platinum.
In a previous study of the photoactivated hydrosilyla-
tion of ethylene, we isolated the five-coordinated
platinum
complex
Pt(hfac)2(h2-C2H4)
1,1,1,5,5,5-hexafluoro-2,4-pentanedionato] [25]. This
[hfacꢂ
/