Thermally-generated reactive intermediates: trapping of the parent ferrocene-based o-quinodimethane and reactions of diradicals generated by hydrogen-atom transfers

Abstract

<p>The first part of this dissertation addresses the thermal reactions of ferrocene-based reactive intermediates. Ferrocenocyclobutene is prepared by the flash vacuum pyrolysis (FVP) of the N-amino-2-phenylaziridine hydrazone of 2-methylferrocenealdehyde. We propose that FVP of the hydrazone gives 2-methylferrocenyl carbene which rearranges to ferrocenocyclobutene by C-H insertion. Heating ferrocenocyclobutene and N-phenylmaleimide (NPMI) in phenyl ether at 200°C for 30 h gives two stereoisomeric 1:1 adducts of NPMI and the parent ferrocene-based o-quinodimethane, which is generated by ring opening of ferrocenocyclobutene;In the second section of this dissertation, we have observed a series of novel hydrocarbon rearrangements. For example, the FVP of o-allyltoluene at 0.1 Torr (700-900°C) gives 2-methylindan and indene as the major products, accompanied by o-propenyltoluene. We propose that these products are formed by intramolecular hydrogen-atom transfer occurs generating a diradical intermediate which undergoes coupling (2-methylindan) or intramolecular disproportionation (o-propenyltoluene). Indene is formed by secondary pyrolysis of 2-methylindan. Similarly, FVP of o-methallyltoluene under similar conditions gives 2,2-dimethylindan as the primary product. [delta]H\ddagger values for formation of the diradical intermediates were estimated to be ca. 39-47 kcal mol[superscript]-1. Solution-phase thermolysis of o-methallyltoluene leads primarily to 1-(o-tolyl)-2-methylpropene, probably by a radical-chain mechanism;FVP of 2-methyl-2[superscript]'-vinylbiphenyl affords 9-methyl-9,10-dihydrophenanthrene, which fits our proposed mechanism. However, FVP of 2-(o-methylbenzyl)styrene gives mainly anthracene and 1-methylanthracene, possibly through an o-quinodimethane intermediate. Extension of this cyclization reaction to phenol derivatives is successful with o-allylphenol and o-(2-methylallyl)phenol, which afford dihydrobenzofuran derivatives, presumably by the hydrogen transfer/diradical coupling mechanism we have proposed.</p>