W. Trahanovsky, Yih-chuan Jason Huang, M. Leung
May 1, 1994
Citations
0
Influential Citations
18
Citations
Journal
Journal of Organic Chemistry
Abstract
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 1 H 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 o C were determined by 1 H NMR spectroscopy. The rate constants and activation parameters (ΔH * =10.8 kcal/mol, ΔS * = -28.8 eu) are very simlar 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-nutylmethylene)-2,3-dihydrofuran (6) and 2,3-bis(tert-butylmethylene)-2,3-dihydrofuran (7), respectively. Compounds 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 intermediate