A new catalytic synthetic route to functionalized 1,2-azaborinines has been developed by the [2+2]/[2+4] cycloaddition reactions of di-tert-butyliminoboranes and alkynes in presence of a rhodium catalyst. The first examples of ferrocene-functionalized azaborinines have been synthesized using this strategy. Moreover, the regioselectivity of this reaction can be controlled by the formation of an intermediate rhodium 1,2-azaborete complex, which results in the isolation of the first azaborinine boronic ester. The isolation of an NH-containing BN isostere by elimination of isobutene from an N(tBu) group under thermolytic conditions has also been achieved. Theoretical studies give further insight into the formation of 1,2-azaborinines and the elimination of isobutene from the N(tBu) group.