Highly Functionalized Cyclohexenes Derived from Benzene: Sequential Tandem Addition Reactions Promoted by Tungsten

The
dihapto-coordination of benzene to the π-basic fragment
{TpW­(NO)­(PMe3)} (Tp = hydridotris­(pyrazolyl)-borate) enhances
the basicity of the arene ligand to the point that it can be protonated
with a mild Brønsted acid (diphenylammonium triflate; pKa ∼ 1). The resulting η2-benzenium complex reacts with a wide range of nucleophiles including
protected enolates, cyanide, amines, methoxide, and aromatic nucleophiles
to form 5-substituted 3,4-η2-1,3-cyclohexadiene complexes
in good yield (42–70%). These coordinated dienes were successfully
taken through a second protonation and nucleophilic addition with
a similar scope of nucleophiles (54–80%). The resulting cis-3,4-
and cis-3,6-disubstituted η2-cyclohexene complexes
were prepared with high regio- and stereocontrol, as governed by the
asymmetric nature of π-allyl intermediates. In some cases, a
diene linkage isomerization from 3,4-η2 to 1,2-η2 could be effected with a redox catalyst, and reactions of
the latter species led to cis-3,5-disubstituted cyclohexene products
exclusively. Oxidative decomplexation afforded the free cyclohexene
products in moderate yield (37–68%). Additionally, when a single
enantiomer of the chiral dearomatization agent was used, the elaborated
cyclohexenes were able to be synthesized in enantioenriched forms
(86–90% enantiomeric excess). Full characterization of 40 new
compounds is provided that includes two-dimensional NMR, IR, electrochemical
and in some cases crystallographic data.