10.1021/ja971612v.s001
Patrick J. Baesjou
Patrick J.
Baesjou
Willem L. Driessen
Willem L.
Driessen
Ger Challa
Ger
Challa
Jan Reedijk
Jan
Reedijk
<i>Ab Initio</i> Calculations on 2,6-Dimethylphenol and
4-(2,6-Dimethylphenoxy)-2,6-dimethylphenol. Evidence of an
Important Role for the Phenoxonium Cation in the
Copper-Catalyzed Oxidative Phenol Coupling Reaction
American Chemical Society
1997
mechanism
charge
Ab Initio Calculations
dimer
DMP
phenoxy substituent results
singlet cationic state
phenol
Reaction Ab initio
para carbon
singlet cation
1997-12-24 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/_i_Ab_Initio_i_Calculations_on_2_6-Dimethylphenol_and_4-_2_6-Dimethylphenoxy_-2_6-dimethylphenol_Evidence_of_an_Important_Role_for_the_Phenoxonium_Cation_in_the_Copper-Catalyzed_Oxidative_Phenol_Coupling_Reaction/3666150
<i>Ab initio</i> unrestricted HartreeāFock calculations with
a 6-31G* basis set were performed on 2,6-dimethylphenol (DMP or monomer) and
4-(2,6-dimethylphenoxy)-2,6-dimethylphenol (dimer) to gain more
insight
into the mechanism of the copper-catalyzed oxidative phenol coupling
reaction. Atomic charges were determined
for the phenol, phenoxyl radical, phenolate anion and phenoxonium
cation (both the singlet and triplet state) of both
species. Calculations for the monomer show that the only aromatic
carbon atom bearing a partial positive atomic
charge is the <i>para</i> carbon of the cation in the singlet
state. In the case of the dimer, the presence of a
<i>p</i>-phenoxy
substituent results in a partial positive charge for the
<i>para</i> carbon of the first aromatic ring in all states, but
this
charge is significantly higher for the singlet cation. In the
singlet cationic state, the <i>para</i> carbon of the first
aromatic
ring is therefore the site most susceptible to nucleophilic aromatic
substitution. This result strongly supports a
proposal
for the mechanism of the copper-catalyzed oxidative phenol coupling, in
which dinuclear phenolate-bridged
copper(II) species act as intermediates affording phenoxonium cations
after a double one-electron transfer.
Furthermore, the charges in the second aromatic ring of the dimer
are hardly, -if at all, influenced by the overall
electronic state of the molecule. The charge in the first ring
cannot pass the ether bond. This result, combined
with
the observation that the <i>para</i> carbon of the first aromatic
ring of the singlet cation bears a large positive charge,
favors a mechanism where quinone ketals are formed as intermediates
during the coupling of oligomeric phenols.