1,4‐Dehydrogenation with a Two‐Coordinate Cyclic (Alkyl)(amino)silylene

Abstract

Cyclic (alkyl)(amino)silylene (CAASi) 1 has been found to successfully dehydrogenate 1,4‐dihydroaromatic compounds containing various substituents to afford the corresponding aromatic compounds. The observed high substrate generality proves 1 to be a potential 1,4‐dehydrogenation reagent for organic compounds. For the reaction with 9,10‐dimethyl‐9,10‐dihydroanthracene, silylene 1 activated not only benzylic C−H bonds but also aromatic C−H bonds to yield a silaacenaphthene derivative, which is an unprecedented reaction of silylenes. The results of the experimental and computational study of the reaction of CAASi 1 with 9,10‐dihydroanthracene and 1,4‐cyclohexadiene are consistent with the notion that 1,4‐dehydrogenation with CAASi 1 proceeds mainly through a stepwise hydrogen‐abstraction mechanism.

Intermolecular C−H Activation at the Allylic/Benzylic and Homoallylic/Homobenzylic Positions of Cyclic Hydrocarbons by a Stable Divalent Silicon Species

“…Hydrogen abstraction of cyclohexene by 1 (route a) to afford hydrosilyl radical INT1 and cyclohexenyl radical H was calculated to be endergonic [(Δ G (298.15 K)=+137.5 kJ mol −1 ]. This is accord with the fact that the reaction with cyclohexene takes 4 days for 1 to be consumed, which is longer than the time for the reaction of 1 with 1,4‐cyclohexadiene or 9,10‐dihydroanthracene [16 h, Δ G =+96.0 kJ mol −1 ( INT1 +cyclohexadienyl radical G ) and +109.8 kJ mol −1 ( INT1 +10‐hydro‐9‐anthrylradical)] [8a] . Subsequent radical recombination to afford 3 was considerably exergonic [Δ G =−61.3 kJ mol −1 ( 3 SR ) and −45.0 kJ mol −1 ( 3 SS )].…”

“…XRD analysis of 3 showed a disorder of the cyclohexenyl moiety roughly corresponding to the diastereomeric ratio of 3 (Figure S34 in the Supporting Information). Notably, the reaction of 1 with 1.0 equivalent of cyclohexene in heptane (0.13 m for 1 and cyclohexene in heptane) did not proceed, [8a] implying that a high concentration of the alkene is necessary for the reaction to proceed.…”

“…Previously, we have conducted thorough mechanistic studies for 1,4‐dehydrogenation of 1 with 1,4‐cyclohexadiene or 9,10‐dihydroanthracene to conclude that 1,4‐dehydrogenation is most likely to proceed through a stepwise mechanism involving the C−H activation at the double allylic position to generate hydrosilyl radical INT1 and cyclohexadienyl radical G (Figure 1), eventually affording 2 and benzene as the final products [8a] . Accordingly, dehydrogenation of cyclohexene was considered to proceed via similar radical intermediates: INT1 and cyclohexenyl radical H .…”

“…Furthermore, we attempted to trap the radical intermediates of the reaction. We have previously reported that the radical intermediates in the reaction of 1 with cyclohexadiene derivatives could not be trapped by radical scavengers such as 2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO), di‐ tert ‐butyl peroxide, and phenol, as 1 readily reacts with such compounds to form silylated products [8a] . However, we found that 1 does not react with 1,2‐dihydronaphthalene in a low concentration (0.13 m in [D 6 ]benzene), which led us to think that the dehydrogenation of 1,2‐dihydronaphthalene in the low‐concentration solution should provide the evidence for the presence of a more reactive radical intermediate.…”

“…Furthermore, concerted direct 1,2‐dehydrogenation to afford 2 and 1,3‐cyclohexadiene (route d) was also found to have a high activation free energy ( TS4 : Δ G =+178.1 kJ mol −1 ). We previously reported that the reaction of 2 with anthracene, which was predicted to afford benzylsilane F (Δ G =−16.6 kJ mol −1 ) via TS6 (Δ G =+173.6 kJ mol −1 ), did not proceed [8a] in similar conditions (150 °C in a sealed tube) suggesting that the concerted 1,2‐dehydrogenation ( TS4 ) and direct C−H insertion of 1 ( TS3 ) is less likely to proceed as these two reactions should require greater activation energy than the reaction of 2 with anthracene (Scheme 4).…”