O-Quinone methides

Keywords Lewis acids, asymmetric reactions, tandem, tethered. Intramolecular reactions, o-quinodimethanes, o-quinone methides, befera-Dlels-Alder reactions... 

Uchida and Irie have reported a photochromic system based on ESIPT to an alkene carbon.82 They observed that vinylnaphthol 121 isomerizes to the ring-closed 123 when irradiated with 334 nm light ( = 0.20, Eq. 1.34). The reaction is photoreversible since irradiation of 123 (at400 nm) regenerates the starting vinylnaphthol. The authors proposed a mechanism in which ESIPT from the naphthol OH to the [3-alkenyl carbon gives intermediate o-quinone methide 122, which undergoes subsequent electrocyclic... 

Brousmiche, D. W. Wan, P. Photogeneration of an o-quinone methide from pyridoxine (vitamin B6) in aqueous solution. J. Chem. Soc., Chem. Commun. 1998, 491 -92. 

Nakatani, K. Higashida, N. Saito, I. Highly efhcient photochemical generation of o-quinone methide from Mannich bases of phenol derivatives. Tetrahedron Lett. 1997, 38, 5005-5008. 

Modica, E. Zanaletti, R. Freccero, M. Alkylation of amino acids and glutathione in water by o-quinone methide. Reactivity and selectivity. J. Org. Chem. 2001, 66, 41-52. 

Foster, K. L. Baker, S. Brousmiche, D. W. Wan, P. o-Quinone methide formation from excited state intramolecular proton transfer (ESIPT) in an o-hydroxystyrene. J. Photochem. Photobiol. A Chem. 1999, 129, 157-163. 

The prototype o-quinone methide (o-QM) and / -quinone methide (p-QM) are reactive intermediates. In fact, they have only been detected spectroscopically at low temperatures (10 K) in an argon matrix,1 or as a transient species by laser flash photolysis.2 Such a reactivity is mainly due to their electrophilic nature, which is remarkable in comparison to that of other neutral electrophiles. In fact, QMs are excellent Michael acceptors, and nucleophiles add very fast under mild conditions at the QM exocyclic methylene group to form benzylic adducts, according to Scheme 2.1.2a 3... 

Di Valentin, C. Freccero, M. Zanaletti, C. Sarzi-Amade, M. o-Quinone methide as alkylating agent of nitrogen, oxygen, and sulfur nucleophiles. The role of H-bonding and solvent effects on the reactivity through a DFT computational study, j. Am. Chem. Soc. 2001, 123, 8366-8377. 

Wang, H. Wang, Y. Han, K.-L. Peng, X.-J. A DFT study of Diels-Alder reactions of o-quinone methides and various substituted ethenes selectivity and reaction mechanism. J. Org. Chem. 2005, 70, 4910-4917. 

Lev, D. A. Grotjahn, D. B. Amouri, H. Reversal of reactivity in diene-complexed o-quinone methide complexes insights and explanations from ab initio density functional theory calculations. Organometallics 2005, 24, 4232 -240. 

Da Silva, G. Chen, C.-C. Bozzelli, J. W. Quantum chemical study of the thermal decomposition of o-quinone methide (6-methylene-2,4-cyclohexadien-l-one). J. Phys. Chem. A 2007, 111, 7987-7994. 

Amouri, H. Besace, Y. Bras, J. L. Vaissermann, J. General synthesis, first crystal structure, and reactivity of stable o-quinone methide complexes of Cp Ir. J. Am. Chem. Soc. 1998, 120, 6171-6172. 

Amouri, H. Vaissermann, J. Rager, M. N. Grotjahn, D. B. Rhodium-stabilized o-quinone methides synthesis, structure, and comparative study with their iridium congeners. Organometallics 2000, 19, 5143-5148. 

Amouri, H. Le Bras, J. Taming reactive phenol tautomers and o-quinone methides with transition metals a structure-reactivity relationship. Acc. Chem. Res. 2002, 35, 501-510. 

Dorrestijn, E. Pugin, R. Ciriano Nogales, M. V. Mulder, P. Thermal decomposition of chroman. Reactivity of o-quinone methide. J. Org. Chem. 1997, 62, 4804-4810. 

Amouri and coworkers also demonstrated that the nucleophilic reactivity of the exocyclic carbon of Cp Ir(T 4-QM) complex 24 could be utilized to form carbon -carbon bonds with electron-poor alkenes and alkynes serving as electrophiles or cycloaddition partners (Scheme 3.17).29 For example, when complex 24 was treated with the electron-poor methyl propynoate, a new o-quinone methide complex 28 was formed. The authors suggest that the reaction could be initiated by nucleophilic attack of the terminal carbon of the exocyclic methylene group on the terminal carbon of the alkyne, generating a zwitterionic oxo-dienyl intermediate, followed by proton transfer... 

These reactions clearly indicate that the exocyclic carbon of the complexed QM in these systems is nucleophilic in character, in contrast to its electrophilic nature in free o-quinone methides. The Cp Ir metal center stabilizes the mesomeric form in which the exocyclic carbon experiences high electron density (Scheme 3.18).29... 

In a similar approach, double deprotonation of r 6-coordinated ortho- and para-cresols, 30 and 31, with t-BuOK led to formation of stable r 4-coordinated p- and o-quinone methide complexes of manganese, 32 and 33 (Scheme 3.19).37... 

Stokes, S. M. J. Ding, F. Smith, R L. Keane, J. M. Kopach, M. E. Jervis, R. Sabat, M. Harman, W. D. Formation of o-quinone methides from T 2-coordinated phenols and their controlled release from a transition metal to generate chromans. Organometallics 2003,22, 4170-4171. 

From the synthetic investigations that have been described in the previous schemes, an appreciation of the mechanism for these reactions (methods A-H) has emerged in our group. However, the characteristics and exact nature of the o-quinone methide intermediate are still debated. Our past observations clearly indicate the cascade leading to the reactive species that behaves as an o-quinone methide should behave is... 

Sugimoto, H. Nakamura, S. Ohwada, T. Generation and application of o-quinone methides bearing various substituents on the benzene ring. Adv. Synth. Catal. 2007, 349, 669-679. 

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