Radiation-induced C—C bond cleavage in 1,2-diarylethanes as model compounds of coal. Part 2.—Pulse and steady-state radiolysis of 1,2-di(pyren-1-yl)ethane in tetrahydrofuran, dimethoxyethane and toluene in the presence of sodium dihydridobis(2-methoxyethoxy)aluminate

Abstract

1,2-Di(pyren-1-yl)ethane (1,2-DPE) has been used as a model for coal to study the C—C bond cleavage of the ethano linkage in the radiolysis of solutions containing NaAlH₂(OCH₂CH₂OCH₃)₂[NaAlH₂(OR)₂]. Transient species were investigated by pulse radiolysis of solutions of 1,2-DPE, 1-methylpyrene (1-MP) and pyrene (P) in THF, DME and toluene in the absence and the presence of NaAlH₂(OR)₂. In the presence of the latter stabilized and long-lived radical anion/sodium cation pairs of 1,2-DPE, 1-MP and P were generated even in the non-polar solvent toluene. The reaction mechanisms differ substantially for the ether solvents (THF or DME) and solutions in toluene. In the case of the ether solvents (SH) NaAlH₂(OR)₂ acts as a scavenger for solvent radical cations SH^˙+ and solvent cations SH(H⁺) generated through ionization. This produces stabilized sodium cation/electron pairs (Na⁺, e^–_s) which reduce the aromatic substrates to their radical anion/sodium cation pairs. The prevailing species generated in the radiolysis of solutions in toluene are electronically excited molecules of the solvent and the substrate from which the excited triplet molecules are preferentially reduced to radical anions by NaAlH₂(OR)₂ in an electron-transfer process. Steady-state radiolysis (⁶⁰Co γ-rays) of solutions of 1,2-DPE in THF, DME or toluene containing NaAlH₂(OR)₂ resulted in the C—C bond cleavage of the ethano linkage. This is attributed to the unstable dianion (1,2-DPE^2–, 2Na⁺) formed in two successive reductions via(1,2-DPE^˙–, Na⁺). UV–VIS spectroscopy and product analysis showed that the radiation-induced reduction of 1,2-DPE, 1-MP and P requires lower doses in THF and DME than in toluene. However, reactions of (1,2-DPE^˙–, Na⁺) with THF and DME to form substrate–solvent addition products limit C—C bond cleavage (16% in THF and 32% in DME). The inertness of toluene towards such side reactions makes the radiation-induced cleavage process of 1,2-DPE in this solvent (despite of the higher dose required) much more efficient than in THF and DME, yielding after hydrolysis up to 52% 1-MP. Radiation-induced reduction of polycyclic arenes in the NaAlH₂(OR)₂–toluene system is very useful for investigating the radical anions of these arenes by UV–VIS and EPR spectroscopy, since essentially no other interfering species are formed.