"FeS"-assisted scission of strong bonds in phenoxydiphenylmethanes. Competition between hydrogen atom transfer and free radical rearrangement pathways

Model compound studies comparing rates of decomposition and product distrib utions from ortho- and para-phenoxydiphenylmethanes [(PhO)PhCH2Ph] suggest that hydrogen atom abstraction from the model compounds, to yield a benzyli c radical intermediate, competes with hydrogen atom transfer to the aryl ri ngs from the reduced "FeS" catalyst. A free-radical rearrangement pathway i nvolving o-phenoxydiphenylmethane, facilitated by the presence of the "FeS" catalyst, generated in situ from ferric oxyhydroxysulfate (OHS) and sulfur , leads to apparent Ar-OAr bond scission at temperatures significantly lowe r than expected for homolytic scission pathways. Thermolysis of the ortho i somer proceeds predominately through a pathway involving an intramolecular addition of the benzylic radical to the 1-position of the appended diphenyl ether, Ar-1-5 participation, forming a spirodienyl radical intermediate. S cission of the C-O bond, followed by hydrogen atom abstraction, yields ther mally labile o-(hydroxyphenyl)phenylmethane (oHPPM). Under the reaction con ditions, at 390 degrees C, tautomerism of oHPPM to the keto isomer followed by homolysis of the weak C-C bond in the keto intermediate yields diphenyl methane and phenol. To unambiguously demonstrate the importance of the free -radical rearrangement pathway, products from the thermolysis of o-(4-methy lphenoxy)diphenylmethane were quantitatively determined. Decomposition of t his labeled diaryl ether at 390 degrees C in 9,10-dihydrophenanthrene conta ining OHS/sulfur yields 4-methyldiphenylmethane and phenol as the major pro ducts. Catalytic decomposition of the corresponding para isomer, p-(4-methy lphenoxy)diphenylmethane, where the intramolecular free-radical rearrangeme nt pathway is hindered, shows that the rate of decomposition is significant ly slower than observed for the corresponding ortho isomer, and 4-methyldip henyl ether and toluene are the major products. The selectivity observed fo r the product distribution in the catalytic thermolysis of the para isomer is consistent with a reversible hydrogen atom transfer pathway from the "Fe S" catalyst.