170
F. Glorius, K. Hirano
Scheme 10. Conjugate umpolung using different imine substrates (Sohn et al.
2005)
6
Conjugate Umpolung
of α-Substituted Cinnamaldehyde Derivatives
A particularly challenging class of substrates are α-substituted cinnamaldehyde derivatives. Under conditions optimized for the previously
mentioned reactions using IMes as the catalyst, the use of α-methyl cinnamaldehyde and trifluoroacetophenone did not give any of the desired
product. This can easily be understood when analyzing the structure
of the conjugate enamine of α-methyl cinnamaldehyde in the conjugated planar conformation. This planar arrangement is disfavored, due
to the steric demand of the mesityl groups that results in an unfavorable
steric interaction with the α-methyl group. Consequently, the size of
the imidazolium substituents was reduced, and thus the dimethyl substituted imidazolylidene IMe provided 10% of the desired lactone product.
F. Glorius, K. Hirano
Scheme 10. Conjugate umpolung using different imine substrates (Sohn et al.
2005)
6
Conjugate Umpolung
of α-Substituted Cinnamaldehyde Derivatives
A particularly challenging class of substrates are α-substituted cinnamaldehyde derivatives. Under conditions optimized for the previously
mentioned reactions using IMes as the catalyst, the use of α-methyl cinnamaldehyde and trifluoroacetophenone did not give any of the desired
product. This can easily be understood when analyzing the structure
of the conjugate enamine of α-methyl cinnamaldehyde in the conjugated planar conformation. This planar arrangement is disfavored, due
to the steric demand of the mesityl groups that results in an unfavorable
steric interaction with the α-methyl group. Consequently, the size of
the imidazolium substituents was reduced, and thus the dimethyl substituted imidazolylidene IMe provided 10% of the desired lactone product.