Efficient Catalytic Effects of Lewis Acids in the 1,3-Dipolar Cycloaddition Reactions of Carbonyl Ylides with Imines

1,3-Dipolar cycloaddition reactions between imines and carbonyl ylides generated by tandem
intramolecular carbenoid-carbonyl cyclizations were found to be effectively catalyzed by Lewis acids
(10 mol %). The Rh2(OAc)4-catalyzed reactions of o-(methoxycarbonyl)-α-diazoacetophenone with
imines such as N-[2-(benzyloxy)benzylidene]aniline in the absence of Lewis acid gave no 1,3-dipolar
cycloaddition products, but rather the dimeric product of the corresponding carbonyl ylide. In
contrast, in the presence of Lewis acids such as Yb(OTf)3, the 1,3-dipolar cycloaddition reactions of
the corresponding 1-methoxy-2-benzopyrylium-4-olate proceeded smoothly with several imines,
giving in most cases exo-selectivity and no formation of the dimeric product. When Yb(OTf)3 was
used as a Lewis acid catalyst, a fundamental catalytic effect was also observed in the cycloaddition
reactions of imines with carbonyl ylides generated from 1-diazo-5-phenyl-2,5-pentanedione, 1-diazo-2,5-hexanedione and diazomethyl 2,3,4,5-tetrachloro-6-methoxycarbonylphenly ketone. This efficient
catalytic effect can be satisfactorily explained in terms of energetics of the cycloaddition in the
absence and the presence of Lewis acid by calculations using the ONIOM (B3LYP/6-31G(d):PM3)
method.