Origins of Diastereoselectivity in the Addition of Enoxysilanes to Vinyl Diazonium Salts.

The addition of enoxysilanes to vinyl diazonium ions occurs with varying levels of diastereoselectivity. To understand the origins of the stereoselectivity, we studied these transformations using density functional theory (DFT) calculations. The selectivity stems from a stabilizing cation-π interaction that orients the nucleophile and the diazonium ion.

Lewis-Acid-Catalyzed Oxa-Michael Addition to Give α-Diazo-ß-alkoxy Carbonyls and Tetrahydro-3H-furo[3,4-c]pyrazoles.

The conjugate addition of alcohols to vinyl diazonium ions formed via Zn(OTf)2-catalysis gives α-diazo-ß-alkoxy carbonyls. The diazo group is retained in this reaction, and this process is an efficient way to couple a reactive partner to the diazo fragment. As an example, we disclose that addition of allyl alcohols provides tetrahydro-3H-furo[3,4-c]pyrazoles via an addition/cycloaddition sequence. This two-step sequence provides good yields and good diastereoselectivity of these sterically hindered pyrazoline scaffolds with up to three quaternary centers and four stereogenic centers. These products can be elaborated to cyclopropane-fused tetrahydrofurans upon liberation of nitrogen. The reaction conditions are mild, operationally simple, and avoid the use of expensive transition metal catalysts.