Lewis Acidic Telluronium Cations: Enhanced Chalcogen-Bond Donor Properties and Application to Transfer Hydrogenation Catalysis

We
describe the synthesis and structures of o-C6F4(TeMes)2 (1) and o-C6F4(TeArF)2 (2, ArF = 3,5-(CF3)2C6H3)), two new bifunctional tellurides featuring
an electron-withdrawing backbone. While 2 resisted methylation, 1 reacted with Me3O·BF4 in CH2Cl2 to afford o-C6F4(TeMes)­(TeMeMes) ([3]+), a mixed-valent
telluride/telluronium cation isolated as a tetrafluoroborate salt.
Although attempts to methylate the second telluride have been unsuccessful,
[3]+ readily catalyzes the hydrogenation of
2-phenyl-quinoline with Hantzsch ester. Comparison with simple telluronium
cations including [ArF2TeMe]+ and
[MesArFTeMe]+ confirms that the catalytic activity
of these compounds originates from the presence of a tetravalent,
cationic tellurium center.