Nitrogenated lewis bases 4 + 2 cycloadditions

In copyrolytic reactions of the aminosilylenes with unsaturated ketones or imines (heterodienes) we mainly obtained isomeric mixtures. The chemo- and regioselectivity of main- and byproducts can be explained with multistep-cycloadditions. We assume a primary Lewis acid-base interaction between the lone electron pair of the heteroatom (oxygen or nitrogen) and the electron gap at silylene, which is followed by a [2+l]-cycloaddition and a radical ring-opening ring-closure reaction. 

The [4-1-3] cycloaddition between 2-aminofuran and oxyallyl cations, followed by base-induced elimination of the resulting adducts, was used in the synthesis of 3-aminotropones <05TL8475>. A chiral Lewis acid-catalyzed [4t-3] cycloaddition between furans and nitrogen-stabilized oxyallyl cations derived from allenamides was developed. As depicted below, the Cj-symmetric salen-based ligand is the most effective in promoting this asymmetric cycloaddition reaction <05JA50>. 

A -Methyleneamine equivalents can be generated from 1,3,5-trisubstituted hexahydro-1,3,5-triazines in the presence of a Lewis acid (LA) and used for aminomethylation reactions with various nucleophiles at the -position of amines (Scheme 16). A -Methyleneaniline equivalents, as shown by 110 and 111, result from the breaking of all three carbon-nitrogen bonds of l,3,5-triphenylhexahydro-l,3,5-triazine 109. They have characteristics of imine and iminium ions similar to Mannich bases and can be used for various aminomethylation reactions. However, there have been no reports of using A -methyleneamine equivalents as azadienes, which may react with olefins via [47t-F2Jt] cycloaddition to afford 1,2,3,4-tetrahydroquinolines 112 <2000T5259>. 

Cycloadditions. The Lewis acidic nature of AgN03 enables activating conplexation with alkynes, allenes, and alkenes, thus permitting various cycloadditions to proceed. These Ag-catalyzed electrophilic cychzations can either occur under oxygen or nitrogen attack leading to the corresponding furan- or pyrrolidine-based heterocycles. ... 

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