Photochemical formation and chemistry of long-lived adamantylidene-quinone methides and 2-adamantyl cations.
Hydroxymethylphenols strategically substituted with the 2-hydroxy-2-adamantyl moiety, AdPh 8-10, were synthesized, and their photochemical reactivity was investigated. On excitation to the singlet excited state, AdPh 8 undergoes intramolecular proton transfer coupled with a loss of H(2)O giving quinone methide 8QM. The presence of 8QM has been detected by laser flash photolysis (CH(3)CN-H(2)O 1:1, tau = 0.55 s) and UV-vis spectroscopy. Singlet excited states of AdPh 9 and 10 in the presence of H(2)O dehydrate giving 9QM and 10QM. Photochemically formed QMs are trapped by nucleophiles giving the addition products (e.g., Phi for methanolysis of 8 is 0.55). In addition, the zwitterionic 9QM undergoes an unexpected rearrangement involving transformation of the 2-phenyl-2-adamantyl cation into the 4-phenyl-2-adamantyl cation (Phi approximately 0.03). An analogous rearrangement was observed with methoxy derivatives 9a and 10a. Zwitterionic 9QM was characterized by LFP in 2,2,2-trifluoroethanol (tau = 1 mus). In TFE, in the ground state, AdPh 10 is in equilibrium with 10QM, which allowed for recording the (1)H and (13)C NMR spectra of the QM. Introduction of the adamantyl substituent into the o-hydroxymethylphenol moiety increased the quantum yield of the associated QM formation by up to 3-fold and significantly prolonged their lifetimes. Furthermore, adamantyl substituent made the study of the alkyl-substituted quinone methides easier by LFP by prolonging their lifetimes and increasing the quantum yields of formation.
Basarić
Nikola
N
Department of Organic Chemistry and Biochemistry, Ruder Bosković Institute, Bijenicka cesta 54, 10000 Zagreb, Croatia.
Zabcić
Ivana
I
Mlinarić-Majerski
Kata
K
Wan
Peter
P
D000218:Adamantane / Q000032:analysis / Q000138:chemical synthesis / Q000737:chemistry*; D000818:Animals; D002384:Catalysis; D002412:Cations / Q000737:chemistry; D045563:Indolequinones / Q000032:analysis / Q000737:chemistry*; D007834:Lasers; D008027:Light; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D015394:Molecular Structure; D010777:Photochemistry; D010782:Photolysis; D011809:Quinones / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
D002412:Cations; D045563:Indolequinones; D011809:Quinones; C068040:quinone methide; C020855:adamantylidene; D000218:Adamantane