Electron and hydrogen self-exchange of free radicals of sterically hindered tertiary aliphatic amines investigated by photo-CIDNP

• Published in: Beilstein journal of organic chemistry Vol. 9; no. 1; pp. 437 - 446
• Main Authors: Goez, Martin, Frisch, Isabell, Sartorius, Ingo
• Format: Journal Article
• Language: English
• Published: Germany Beilstein-Institut 26.02.2013
• Subjects: amines; Chemistry; CIDNP; electron transfer; free radicals; Full Research Paper; hydrogen transfer; ketones; kinetics; photochemistry; self-exchange; CIDNP; free radicals; electron transfer; photochemistry; self-exchange; amines; ketones; kinetics; hydrogen transfer
• ISSN: 1860-5397; 1860-5397
• DOI: 10.3762/bjoc.9.46

The photoreactions of diazabicyclo[2,2,2]octane (DABCO) and triisopropylamine (TIPA) with the sensitizers anthraquinone (AQ) and xanthone (XA) or benzophenone (BP) were investigated by time-resolved photo-CIDNP (photochemically induced dynamic nuclear polarization) experiments. By varying the radical-pair concentration, it was ensured that these measurements respond only to self-exchange reactions of the free amine-derived radicals (radical cations DH (•) (+) or α-amino alkyl radicals D (•) ) with the parent amine DH; the acid-base equilibrium between DH (•) (+) and D (•) also plays no role. Although the sensitizer does not at all participate in the observed processes, it has a pronounced influence on the CIDNP kinetics because the reaction occurs through successive radical pairs. With AQ, the polarizations stem from the initially formed radical-ion pairs, and escaping DH (•) (+) then undergoes electron self-exchange with DH. In the reaction sensitized with XA (or BP), the polarizations arise in a secondary pair of neutral radicals that is rapidly produced by in-cage proton transfer, and the CIDNP kinetics are due to hydrogen self-exchange between escaping D (•) and DH. For TIPA, the activation parameters of both self-exchange reactions were determined. Outer-sphere reorganization energies obtained with the Marcus theory gave very good agreement between experimental and calculated values of ∆G (‡) 298.