Abstact and Biography

The (mostly) unexpected reactions of [n]cumulenes

Rik R. Tykwinski

Department of Chemistry, University of Alberta, Edmonton, Canada

E-mail: rik.tykwinski@ualberta.ca

Molecules composed of a contiguous sequence of three or more double bonds, [n]cumulenes, are linear molecules with a structure similar to polyynes.¹ The pi-electron systems of polyynes and cumulenes are, however, very different. The conjugated framework of polyynes features two orthogonal and degenerate pi-electron systems, while the two orthogonal pi-systems of [n]cumulenes are non-degenerate. Thus, the electronic properties of [n]cumulenes are quite different from those of polyynes. At an infinite length, the effect of endgroups become irrelevant, and [n]cumulenes would provide a possible form of the hypothetical sp-hybridized carbon allotrope carbyne. Noteworthy, bond length alternation (BLA) approaches zero in “cumulenic” carbyne. Polyynes provide the other possible form of carbyne, with a larger BLA at infinite length.   

On the road to model carbyne, we have developed syntheses toward both odd and even [n]cumulenes (Figure 1a) that offer model compounds to help to approximate the properties of carbyne, often in comparison to polyynes.² We, and others, have shown that polyynes bearing sterically encumbered endgroups are stable to lengths of over 40 sp-carbon atoms. Conversely, and perhaps surprisingly, [n]cumulenes with analogous substitution can be chemically unstable at the length of three sp-carbons for even cumulenes ([4]cumulenes) and four sp-carbons for an odd cumulene ([5]cumulenes). Furthermore, even [n]cumulenes are substantially less stable than odd [n]cumulenes. The most common decomposition pathway involves an efficient dimerization reaction to give a [4]radialene as the major product, and these transformation can be observed in solution, the solid-state, and in melts.4 Our recent advances in synthesis of [n]cumulenes and our understanding of their reactivity will be presented and discussed.

References

1. Wendinger, D.; Tykwinski, R. R. Acc. Chem. Res. 2017, 50, 1468.

2. Bühringer, M. U.; Padberg, K.; Phleps, M. D.; Maid, H.; Placht, C; Neiss, C.; Ferguson, M. J.; Görling, A.; Tykwinski, R. R. Angew. Chem. Int. Ed. 2018, 57, 8321.

3. Gao, Y.; Tykwinski, R. R. Acc. Chem. Res. 2022, 55, 3616-3630.

4. Johnson, M. A.; Meckes, J. A.; Zhou, Z.; Kuwatani, Y.; Ferguson, M. J.; Iyoda, M.; Petrukhina, M. A.; Tykwinski, R. R. J. Phys. Org. Chem. 2023, 36, e4454.