Stable o-Quinone Methide Complexes of Iridium: Synthesis, Structure, and Reversed Reactivity Imparted by Metal Complexation

o -Quinone methides are important intermediates in biochemistry and organic chemistry, partly because of their high reactivity: the simplest compound o -quinone methide ( 1 ) is unstable in condensed phases above approximately −100 °C. In contrast, here a general synthetic route to the first metal complex of o -quinone methide and complexes of several simple alkyl derivatives is reported. Coordination of 2-alkylphenols to [Cp*Ir(acetone) 3 ](BF 4 ) 2 and subsequent deprotonation with Et 3 N affords (η 5 -Cp*)Ir[η 5 -(2-alkyl)oxodienyl](BF 4 ) complexes 5 in 85−90% yield. Deprotonation of 5 with KO- t -Bu gives 81−96% yields of neutral o -quinone methide complexes Cp*Ir{η 4 -C 6 H 3 R 1 [ C(R 2 ) 2 ]O} [R 1 = R 2 = H ( 6a ); R 1 = Me, R 2 = H ( 6b ); R 1 = H, R 2 = Me ( 6c ); R 1 = i -Pr, R 2 = Me ( 6d )], in which the Cp*Ir fragment is coordinated in η 4 fashion to the two carbon−carbon double bonds of the six-membered ring. The remarkable stability of the complexes allows characterization of their structure and reactivity. The X-ray molecular structure of 6d and a series of 1D and 2D NMR studies on 6a and 6c are reported, showing the pronounced effects of Cp*Ir coordination to the o -quinone methide ligand, particularly a strong upfield 13 C chemical shift for the exocyclic carbon [ C (R 2 ) 2 ] of the uncoordinated carbon−carbon double bond. Although stable under argon at room temperature, Cp*Ir- o -quinone methide complexes 6 exhibited unusual reactivity toward acids or electrophiles; for instance treatment of 6a with 1 equiv of HBF 4 ·Et 2 O or I 2 lead to the oxodienyl complexes [Cp*Ir(η 5 -C 7 H 7 O)][BF 4 ] ( 5a ) or [Cp*Ir(η 5 -C 7 H 6 IO)][I] ( 8 ), respectively. Moreover, when complex 6a was treated with methyl propynoate, a new o -quinone methide complex ( 9 ) was obtained as a result of a coupling reaction between the electrophilic alkyne and the exocyclic carbon ( CH 2 ) of complex 6a . Finally, treatment of 6a with N -methylmaleimide gave the tricyclic iridium complex ( 11 ) as a result of an unprecedented [2+3] cycloaddition with part of the o -quinone methide complex 6a . The above reactions and 13 C NMR evidence show that in o -quinone methide complexes 6 the exocyclic carbon [ C (R 2 ) 2 ] is nucleophilic, opposite of what is reported for free, electrophilic o -quinone methides. The difference in reactivity is attributed to the Cp*Ir unit, which modifies dramatically the electronic properties of the o -quinone methide ligand.