Photochemical generation and the reactivity of o-naphthoquinone methides in aqueous solutions.

Abstract

Irradiation of 3-hydroxy-2-naphthalenemethanol (3a) and 2-hydroxy-1-naphthalenemethanol (4a) results in efficient (Phi(254) = 0.17 and 0.20) dehydration and the formation of isomeric naphthoquinone methides, 2,3-naphthoquinone-3-methide (1) and 1,2-naphthoquinone-1-methide (2), respectively. In aqueous solution, naphthoquinone methides 1 and 2 undergo rapid hydration to regenerate starting materials (tau(H2O) (1) = 7.4 ms and tau(H2O) (2) = 4.5 ms at 25 degrees C). The hydration reaction is strongly catalyzed by the hydroxide ion but shows acid catalysis only at pH < 1. Reactive intermediates 1 and 2 can be intercepted by other nucleophiles, such as the azide ion (k(N3)(1) = 2.0 x 10(4) M(-1) s(-1) and k(N3)(2) = 3.0 x 10(4) M(-1) s(-1)) or thiol (k(SH)(1) = 2.2 x 10(5) M(-1) s(-1) and k(SH)(2) = 3.3 x 10(5) M(-1) s(-1)). Ethyl vinyl ether readily reacts with 1 and 2 (k(DA)(1) = 4.1 x 10(4) M(-1) s(-1) and k(DA)(2) = 6.0 x 10(4) M(-1) s(-1)) to produce Diels-Alder adducts in excellent yield. o-Naphthoquinone methides 1 and 2 were also generated by photolysis of 3-ethoxymethyl- (3b) and 1-(ethoxymethyl)-2-naphthols (4b), as well as from (2-hydroxy-3-naphthyl)methyl- (3c) and [(2-hydroxy-1-naphthyl)methyl] trimethylammonium iodides (4c). Laser flash photolysis of 3a,b and 4a,b allows the detection of short-lived (tau(25 degrees C) approximately 12 micros) precursors of naphthoquinone methides 1 and 2. On the basis of the precursor reactivity and the results of DFT calculations, 2H-naphthoxete structure was assigned to these species.

• Authors: Selvanathan Arumugam, Vladimir V Popik
• Journal: Journal of the American Chemical Society (J Am Chem Soc) Vol. 131 Issue 33 Pg. 11892-9 (Aug 26 2009) ISSN: 1520-5126 [Electronic] United States
• PMID: 19650661 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't, Research Support, U.S. Gov't, Non-P.H.S.)

Chemical References

• 1,2-naphthoquinone-1-methide
• 2,3-naphthoquinone-3-methide
• Buffers
• Naphthoquinones
• Perchlorates
• Solutions
• Water

Topics

• Buffers
• Hydrogen-Ion Concentration
• Isomerism
• Kinetics
• Naphthoquinones (chemistry)
• Perchlorates (chemistry)
• Photochemical Processes
• Quantum Theory
• Solutions
• Water (chemistry)