Synthesis of cyclooctyne-sulfamates by the Nicholas cyclisation

Abstract

Here, we present the Nicholas cyclisation as an alternative, simple synthetic method for producing cyclooctyne-sulfamates as promising SPAAC reagents. Co₂(CO)₆-complexes of both monocyclic and benzene-fused cyclooctyne-sulfamates were prepared using the Nicholas cyclisation. After cobalt deprotection, the non-fused cyclooctyne-sulfamate SNO-Me₂ remained stable and underwent SPAAC with benzyl azide (k = 0.0163 M⁻¹ s⁻¹) readily. In contrast, the benzene-fused cyclooctyne-sulfamates B-SNO and B-SNO-Me₂ were found to be kinetically unstable. The reactivity and stability of monocyclic and benzene-fused cyclooctyne-sulfamates were predicted and compared using DFT calculations. The crucial influence of benzene-fusion and propargylic methyl groups on geometry, electronic structure, stability and reactivity was demonstrated.