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

Abstract

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](BF4)2 and subsequent deprotonation with Et3N affords (η5-Cp*)Ir[η5-(2-alkyl)oxodienyl](BF4) 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{η4-C6H3R1[C(R2)2]O} [R1 = R2 = H (6a); R1 = Me, R2 = H (6b); R1 = H, R2 = Me (6c); R1 = i-Pr, R2 = Me (6d)], in which the Cp*Ir fragment is coordinated in η4 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 13C chemical shift for the exocyclic carbon [C(R2)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 HBF4·Et2O or I2 lead to the oxodienyl complexes [Cp*Ir(η5-C7H7O)][BF4] (5a) or [Cp*Ir(η5-C7H6IO)][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 (CH2) 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 13C NMR evidence show that in o-quinone methide complexes 6 the exocyclic carbon [C(R2)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.