C O M M U N I C A T I O N S
Table 1. Data for The σ-Bond Silyl Cross-Metathesis of
TMSCtCH with RCtCH Catalyzed by Complex 3 in Refluxing
THF
Scheme 2. Plausible Mechanism for the Disproportionation of
TMSCtCH Promoted by Complex 3 (only a half molecule of the
organometallic complex is presented for clarity)
R in
time
(h)
TMSC
t
CTMS
TMSC
tCR
run
RC
t
CH
TMSC
tCH:RC
tCH
(%)
(%)
1
iPr
1:4
20
44
64
139
38
61
14.1
7.8
6.2
40.2
59.8
68.5
93.6
3.4
2.6
2
tBu
2:1
24.1
28.6
26.3
3.7
11.4
90
Table 2. Data for the Cross-Metathesis Reaction of TMSCtCR′
with RCtCH Catalyzed by Complex 3
R in RC
t
CH
CR
TMSC
:RC
t
CR
′
time
(h)
TMSC
t
CR
TMSC
t
CH
A plausible mechanism for metathesis of a silylalkyne catalyzed
by complex 3 is presented in the Scheme 2. The first step of the
catalytic cycle involves the rapid reaction of the starting complex
3 with the silyl acetylene to form the corresponding uranium
acetylide complex A and 2 equiv of butane. Complex A reacts in
a four-centered transition state with an additional TMSCtCH
cleaving the Si-C bond, forming the internal alkyne, TMSCt
CTMS, and the corresponding U-CtCH complex as presented in
complex B. The consecutive protonolysis reaction of complex B
with another molecule of TMSCtCH regenerates complex A and
forms the obtained acetylene HCtCH. A similar mechanism can
be depicted for the cross-metathesis of TMSCtCR (R ) H, TMS,
CH3) with terminal alkynes. In conclusion, we have shown a unique
structure-reactivity relationship in an oxophilic ansa-organoactinide
complex. The high coordinative unsaturation at the metal center
was found to be the leading feature for higher reactivities serving
to design better M-O-M catalytic heterogeneous chemistry.
run
R′
in TMSC
t
′
tCH
(%)
(%)
1
R′ ) TMS, R ) iPr
R′ ) TMS, R ) tBu
R′ ) TMS, R ) nBu
1.62:2.94
1.62:7.40
26
66
66
90
23
45
66
44
4
32.2
45.9
56.4
62.4
53.8
74.6
88.8
100
61.2
100
14.7
12.0
13.2
12.1
13.9
12.7
5.6
0
19.4
0
2
3
1.62:14.02
4
5
R′ ) TMS, R ) CH3 1.62:7.79
R′ ) TMS, R ) Ph
1.62:9.23
23
45
92
6
R′ ) CH3, R ) tBu
5.4:9.89
13.6
16.8
TMSCtCR, TMSCtCH, and HCtCH (eq 3). To obtain better
conversions, we have found that the terminal alkynes must be in
excess amounts compared to the amount of TMSCtCTMS.
Acknowledgment. This research was supported by the Israel
Science Foundation, administrated by the Israel Academy of Science
and Humanities under Contract 1069/05.
3
TMSCtCH + RCtCH y z TMSCtCTMS +
THF
TMSCtCR + HCtCH (2)
Supporting Information Available: Crystallographic data of
complex 2. Complete Experimental Section, including the synthesis
and characterization of complex 3 and detailed procedures for the
catalytic reactions. This material is available free of charge via the
3
TMSCtCTMS + RCtCH y z TMSCtCH +
THF
TMSCtCR + HCtCH (3)
The data for the diverse cross-metathesis results are summarized
in Table 2. The cross-metathesis of CH3CtCH with TMSCtCTMS
yields selectively only the product TMSCtCCH3. Surprisingly, n-
BuCHdCH2 or CH3CtCCH3 did not react with TMSCtCTMS.
These results implicate that, in the σ-bond metathesis reaction, the
terminal hydrogen of the terminal alkyne is indispensable. It is
important to point out that only a Si-C bond cleavage and a C-C
bond formation are involved in the metatheses processes. This has
also been confirmed by the fact that no metathesis reaction between
the internal alkyne 2-butyne with a range of terminal alkynes is
observed.
However, when the internal alkyne, TMSCtCCH3, was used
as the starting material (source of TMS group) and reacted with
tBuCtCH, the cross-metathesis products tBuCtCTMS and CH3Ct
CH were obtained, although the reaction progress was very slow.
From the activity exhibited by the different alkynes used as a source
of the TMS group, we have found that the reactivity toward the
cross-metathesis reaction is as follows: TMSCtCTMS > TMSCt
CCH3. In addition, the reactivity of the terminal alkynes follows
the order of the following: TMSCtCH > PhCtCH > CH3CtCH
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n
t
i
> BuCtCH > BuCtCH > PrCtCH.
JA063443X
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9351