Effects of .alpha.-tert-butyl group substitution on the reactivity and dimerization products of furan-based o-quinodimethanes
作者:Walter S. Trahanovsky、Yihchuan Jason Huang、Mankit Leung
DOI:10.1021/jo00088a050
日期:1994.5
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 H-1 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 OC were determined by H-1 NMR spectroscopy The rate constants and activation parameters (Delta H-double dagger = 10.8 kcal/mol, Delta S-double dagger = -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-dihydro (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 S-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.