Reduction of Benzonitriles via Osmium–Azavinylidene
Intermediates Bearing Nucleophilic and Electrophilic Centers
Version 2 2019-06-20, 14:13
Version 1 2019-06-19, 19:17
Posted on 2019-06-20 - 14:13
The
reduction of the NC bond of benzonitriles promoted by OsH6(PiPr3)2 (1)
has been studied. Complex 1 releases a H2 molecule
and coordinates 2,6-dimethylbenzonitrile to afford the tetrahydride
OsH4{κ1-N-(NCC6H3Me2)}(PiPr3)2 (2), which is thermally stable toward the insertion
of the nitrile into one of the Os–H bonds. In contrast to 2,6-dimethylbenzonitrile,
benzonitrile and 2-methylbenzonitrile undergo insertion, via Os(η2-NCR) intermediates, to give the azavinylidene derivatives
OsH3(NCC6H4R)(PiPr3)2 [R = H (3) or Me
(4)]. The analysis by means of computational tools (EDA–NOCV)
of the bonding situation in these compounds suggests that the donor–acceptor
nature of the osmium azavinylidene bond dominates over the mixed electron-sharing/donor–acceptor
and pure electron-sharing bonding modes. The N atom is strongly nucleophilic,
whereas one of the hydrides is electrophilic. In spite of the different
nature of these centers, the migration of the latter to the N atom
is kinetically prevented. However, the use of water as a proton shuttle
allows hydride migration, as a consequence of a significant decrease
in the activation barrier. The resulting phenylaldimine intermediates
evolve by means of orthometalation to give OsH3{κ2-N,C-(NHCHC6H3R)}(PiPr3)2 [R
= H (5) or Me (6)]. The presence of electrophilic
and nucleophilic centers in 3 confers upon it the ability
to activate σ-bonds, including H2 and pinacolborane
(HBpin). The reaction with the latter gives OsH3{κ2-N,C-[N(Bpin)CHC6H4]}(PiPr3)2 (7).
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Babón, Juan C.; Esteruelas, Miguel A.; Fernández, Israel; López, Ana M.; Oñate, Enrique (2019). Reduction of Benzonitriles via Osmium–Azavinylidene
Intermediates Bearing Nucleophilic and Electrophilic Centers. ACS Publications. Collection. https://doi.org/10.1021/acs.inorgchem.9b01013
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AUTHORS (5)
JB
Juan C. Babón
ME
Miguel A. Esteruelas
IF
Israel Fernández
AL
Ana M. López
EO
Enrique Oñate