Dynamic covalent chemistry (DCC) allows to construct diverse organic architectures in high yields from simple precursors by driving the reaction towards thermodynamically favored products. The feasibility to synthesize various three-dimensional (3D) porous organic cages using DCC has been realized by exploiting multitopic building blocks. Current design approaches adopt the heuristic method, relying on geometry and angle of the precursors to predict final architectures. However, subtle structural changes in precursors can instigate ‘emergent behavior’ that result in unanticipated and ill-defined products. Here we investigate the effects of precursor arm length on cage formation utilizing alkyne metathesis (AM), which is a DCC method that provides structurally rigid linear acetylene linkages with good chemical stability. Our research has demonstrated that a rigid tritopic precursor (Pa) with pre-organized alternating up-down conformation can favorably direct the formation of tetrahedral cage (Ta) in near-quantitative yield via AM. While the cage is presumed to be the thermodynamic sink, we revealed that the cage is kinetically trapped. Here, we further demonstrate that an additional phenylene spacer in tritopic precursor (Ext-Pa) can provide an extended tetrahedral cage (Ext-Ta) with altered reaction pathways. In contrast to the AM of Pa that yielded a kinetically trapped single product, Ta, the AM of Ext-Pa resulted in a mixture of Ext-Ta and complex oligomers. This suggests that a small increase in flexibility and length of the precursor can drastically influence the reaction pathways in multitopic dynamic systems. We hypothesize that the extended arms have accentuated effects from the vertex and cause the connecting points to deviate from the intended geometry and angles. Different reaction conditions and investigation of intermediates will be discussed along with computational studies to underline the influence of molecular rigidity and size of the arms on cage forming pathways. We anticipate that these studies will provide insights to approaching rational design strategies for constructing more complex 3D cages.

Alkyne metathesis to delineate the effect of building block size on organic cage synthesis

253rd National Meeting (2017)

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