Scheme 1 Proposed catalytic cycle.
was 45% (TON 5 23). The results show the C–C bond in MeCN
is cleaved catalytically. Seeking a more effective catalyst, we
examined the reaction in identical conditions for many iron
complexes, such as (C5R5)(CO)2FeMe (C5R5 5 C5H5, C5H4Me,
C5HMe4, C5Me5, C5H4(SiMe3), C5H4{P(O)(OMe)2}), (Indenyl)-
(CO)2FeMe, and (C5H5)(CO)2FeX (X 5 Cl, I, CH2Ph, H).
Consequently, we found that Cp(CO)2FeMe is an effective
catalyst. That reaction is shown in eqn. (6). The yield of
Et3SiCN was 72% and the TON is 36. Other organic products
in this reaction were CH4 and a small amount of Et3Si–SiEt3.
the reaction on changing the molar ratio of Et3SiH and MeCN.
Reaction of Et3SiH with a 10-fold molar excess of MeCN in the
presence of 0.83 mol% Cp(CO)2FeMe under photolysis for 24 h at
50 uC produced 99% yield of Et3SiCN base on Et3SiH
(TON 5 118). The highest TON (156) was obtained when
Et3SiH and a 10-fold molar excess of MeCN were photolyzed for
48 h at 50 uC in the presence of 0.30 mol% of Cp(CO)2FeMe.
In summary, we have established a new catalytic system
involving C–C bond cleavage in organonitrile. Furthermore, it
initially involves Si–NC and finally Si–CN bond formation. This
report is unprecedented in terms of (i) catalytic acetonitrile C–C
bond cleavage, (ii) a transition metal catalyst other than Ni for
organonitrile C–C bond activation, and (iii) catalytic silylcyanide
formation where the cyano group stems from organonitrile.
Efforts for expansion of this scope and elucidation of the detailed
mechanism are now under way in our laboratory.
cat: CpðCOÞ2FeMe
in THF, hv
Et3SiHzMeCN DCCDCCA Et3SiCNzCH4
(6)
The catalytic system found for acetonitrile C–C bond cleavage
was next applied to a catalytic arylcyanide C–C bond cleavage.
The results in Table 1 show that Cp(CO)2FeMe can also serve as a
catalyst for aryl–CN bond cleavage. The main product for
dicyanobenzene was cyanobenzene in the reaction condition. The
TONs were about 10 in all cases; they were similar to those
reported for Ni-catalyzed aryl cyanation of alkynes reported by
Hiyama et al.16
This work was supported by a Grant-in-Aid (No. 15205010)
and by a Grant-in-Aid for Science Research on Priority Areas
(No. 16033250, Reaction Control of Dynamic Complexes) from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan, and by the Yamada Science Foundation.
Notes and references
We propose the catalytic cycle shown in Scheme 1 for reaction
with MeCN. One CO ligand in Cp(CO)2FeMe is released by
photolysis to give Cp(CO)FeMe, which reacts with Et3SiH to
give Cp(CO)FeMe(H)(SiEt3). The successive reductive elimination
of CH4 yields Cp(CO)Fe(SiEt3). Then MeCN reacts with the
16e species to give Cp(CO)Fe(SiEt3)(g2-NCMe), which is
converted into Cp(CO)Fe(Me)(g1-CNSiEt3) according to eqn. (5).
Dissociation of Et3SiNC reproduces Cp(CO)FeMe to complete
the catalytic cycle. The released Et3SiNC isomerizes to Et3SiCN.
Cp(CO)Fe(SiEt3) produced in the catalytic cycle is expected to
react mainly with MeCN, but it may also react with Et3SiH that is
present in solution to give Cp(CO)FeH(SiEt3)2. Et3Si–SiEt3 may
be formed, if the two silyl groups are eliminated reductively from
the species. A small amount of the disilane was observed (see
above). The reaction with MeCN may become dominant, and the
catalytic cycle is expected to work more effectively if the reaction
of Cp(CO)Fe(SiEt3) with Et3SiH is suppressed. Thus we attempted
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