Steric bulk around catalytic centers is one of the most important factors determining catalytic reactivities and selectivities. This paper reports the synthesis of structurally diverse 5-iodo-3-methyl-1,2,3-triazolium salts and the evaluation of their catalytic activities as halogen-bonding donors for the aza-Diels-Alder reaction of 2-siloxy-1,3-butadiene with imines. We found that steric hindrance around an iodide atom in halogen-bonding donors significantly impacted catalytic efficiency.

7) Compound 3 and 3-III provided the same product, 9. While 3-III very likely proceeded through II followed by the vinyl migration, 3 may have undergone a carbonium ion type rearrangement, at least as probably as a carbenoid (II) type reaction. Experimental evidence to provide a clear distinction between the two pathways in the case of 3 is not available at present. For further discussion, see ref 8.(8) M. Sakai, . H. Westberg, H. Yamaguchi, and S. Masamune, ibid., 93, 4611 (1971).