R. Xu, C. Cai
CatalysisCommunications107(2018)5–8
Table 2
withdrawing or electron-donating groups reacted smoothly under the
optimized conditions, producing corresponding vinylsilanes with sa-
tisfactory yields. Furthermore, m-substituted and o-substituted styrene
derivatives were both compatible with this reaction (Table 2, 3h and
3i), and the halo-substituted vinylsilanes were conductive to further
functionalization. In addition, 2, 4, 6-trimethylstyrene was also suitable
for this transformation (Table 2, 3j). Meanwhile, the protocol was ef-
fective on sterically hindered α-phenylstyrene (Table 2, 3k). Our pro-
tocol was also suitable for heteroaromatic alkenes and aliphatic olefins,
affording corresponding silylation products in moderate yield (Table 2,
3l–3n). To test some sensitive groups of styrene derivatives under such
oxidative conditions, we tried 4-NH2, 4-COOCH3 and 4-CN styrene, but
only the ester and nitrile were suitable for this transformation (Table 2,
3o and 3p).
Substrate scope for silylation.a
To further study the possible reaction mechanisms for the silylation,
control experiments under the standard reaction conditions were car-
ried out. The product (3a) was not detected in the presence of radical
inhibitors, 2, 2, 6, 6- tetramethyl-1-piperidinyloxy (TEMPO) or buty-
lated hydroxytoluene (BHT), indicating that the current reaction in-
cludes a radical process. A possible mechanism outlined in Scheme 2
was proposed for the present process. Initially, in the presence of Fe2+
species, DTBP is split into Fe3+ (Ot-Bu) and the t-BuO% radical. The
reacts t-BuO% reacts with HSiEt3 to lead to the SiEt3% radical and t-
BuOH, and then addition across styrene affords benzylic radical A.
Single-electron-transfer (SET) between benzylic radical and Fe3+ (Ot-
Bu) gives benzylic cation B, Fe2+ species and t-BuO−. Finally, β-H
elimination of benzylic cation B with t-BuO− gives desired product 3a
and t-BuOH. And the FeCl2 was regenerated to reinitiate the reaction
cycle.
3. Conclusions
aReaction conditions: FeCl2 (0.1 mmol), olefins (0.5 mmol), HSiEt3 (2.5 mmol), DTBP
(1.0 mmol), t-BuOH (3 mL), at 100 °C for 24 h under Ar. Isolated yields.
In conclusion, we have developed an efficient Fe-catalyzed direct
CeH silylation of styrene and its derivatives using silanes with per-
oxides for selective synthesis of vinylsilanes. This approach is highly E-
selective and produced important classes of vinylsilanes in good yields.
This protocol tolerates a wide variety of functional groups, and pro-
poses an inexpensive, non-toxic and environmentally benign character.
Moreover, the preliminary mechanistic studies disclose that the reac-
tion may proceed via a radical pathway. Further investigations and
studies are being directed toward understanding the reaction mechan-
istic and the application to other substrates for the synthesis of natural
bioactive products.
functional group tolerance of our protocol. A series of styrene deriva-
tives (1b–1j) with triethylsilane (2a) were subjected to the optimized
reaction conditions (20 mol% FeCl2, DTBP, t-BuOH, 100 °C), the cor-
responding vinylsilanes (Table 2, 3b–3j) were obtained in moderate to
good yields as shown in Table 2. p-Substituted styrene derivatives
containing electron-donating groups (Table 2, 3b–3d) worked slightly
better than that containing electron-withdrawing groups (Table 2,
3e–3g). In general, p-substituted styrene derivatives with electron-
Scheme 2. Proposed reaction mechanism.
7
Xu, Rui
