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 -1 00 degrees C. In contrast, here a general synthetic route to the first meta l complex of o-quinone methide and complexes of several simple alkyl deriva tives is reported. Coordination of 2-alkylphenols to [Cp*Ir(acetone)(3)](BF 4)(2) and subsequent deprotonation with Et3N affords (eta(5)-Cp*)Ir[eta(5)- (2-alkyl)oxodienyl](BF4) complexes 5 in 85-90% yield. Deprotonation of 5 wi th KO-t-Bu gives 81-96% yields of neutral o-quinone methide complexes CP*Ir {eta(4)-C6H3R1[=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 frag ment is coordinated in eta(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 st ructure of 6d and a series of 1D and 2D NMR studies on 6a and sc are report ed, showing the pronounced effects of Cp*Ir coordination to the o-quinone m ethide ligand, particularly a strong upfield C-13 chemical shift for the ex ocyclic carbon [=C(R-2)(2)] of the uncoordinated carbon-carbon double bond. Although stable under argon at room temperature, Cp*Ir-o-quinone methide c omplexes 6 exhibited unusual reactivity toward acids or electrophiles; for instance treatment of 6a with 1 equiv of HBF4 . Et2O or I-2 lead to the oxo dienyl complexes [CP*Ir(eta(5)-C7H7O)][BF4] (5a) or [Cp*Ir(eta(5)-C7H6IO)][ I] (8), respectively. Moreover, when complex 6a was treated with methyl pro pynoate, 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] c ycloaddition with part of the o-quinone methide complex 6a. The above react ions and C-13 NMR evidence show that in o-quinone methide complexes a the e xocyclic 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 p roperties of the o-quinone methide ligand.