Fine-Tuning Strain and Electronic Activation of Strain-Promoted 1,3-Dipolar Cycloadditions with Endocyclic Sulfamates...

The ability to achieve predictable control over the polarization of strained cycloalkynes can influence their behavior in subsequent reactions, providing opportunities to increase both rate and chemoselectivity. A series of new heterocyclic strained cyclooctynes containing a sulfamate backbone (SNO-OCTs) were prepared under mild conditions by employing ring expansions of silylated methyleneaziridines. SNO-OCT derivative 8 outpaced even a difluorinated cyclooctyne in a 1,3-dipolar cycloaddition with benzylazide. The various orbital interactions of the propargylic and homopropargylic heteroatoms in SNO-OCT were explored both experimentally and computationally. The inclusion of these heteroatoms had a positive impact on stability and reactivity, where electronic effects could be utilized to relieve ring strain. The choice of the heteroatom combinations in various SNO-OCTs significantly affected the alkyne geometries, thus illustrating a new strategy for modulating strain via remote substituents. Additionally, this unique heteroatom activation was capable of accelerating the rate of reaction of SNO-OCT with diazoacetamide over azidoacetamide, opening the possibility of further method development in the context of chemoselective, bioorthogonal labeling.