Effects of α-tert-Butyl Group Substitution on the Reactivity and Dimerization Products of Furan-Based o-Quinodimethanes

Article abstract of DOI:10.1021/jo00088a050

Flash vacuum pyrolysis (FVP) of 2-neopentyl-3-furylmethyl benzoate (8) produces 3-methylene-2-(tert-butylmethylene)-2,3-dihydrofuran (5), the major product as shown by low-temperature 1H NMR spectroscopy.Upon warming to room temperature, 5 dimerizes giving mostly two stereoisomeric <4 + 2> dimers 11a and 11b in addition to a small amount of the <4 + 4> dimer 12.FVP of a mixture of the two <4 + 2> dimers 11a and 11b gives the thermodynamically more stable <4 + 4> dimer 12.The rate constants for the dimerization of 5 in solution at temperatures from -29 to +5 deg C were determined by 1H NMR spectroscopy.The rate constants and activation parameters (ΔH(excit.) = 10.8 kcal/mol, ΔS(excit.) = -28.8 eu) are very similar to those reported for the unsubstituted furan-based o-quinodimethane.FVP of (2-methyl-3-furyl)(tert-butyl)methyl benzoate (13) and (2-neopentyl-3-furyl)(tert-butyl)methyl benzoate (18) give as the major product, 2-methylene-3-(tert-butylmethylene)-2,3-dihydrofuran (6) and 2,3-bis(tert-butylmethylene)-2,3-dihydrofuran (7), respectively.Compunds 6 and 7, in contrast to 5, are stable at room temperature apparently because for each of these compounds a bulky tert-butyl group is on the more reactive methylene, the 3-methylene.These results offer further support for the mechanism for the dimerization of furan-based o-quinodimethanes which proceeds in two steps via a transient diradical intemediate.