Helicenes Built from Silacyclopentadienes via Ring‐by‐Ring Knitting of the Helical Framework

Ring‐by‐ring: a new family of helicenes built from silole units has been obtained by employing a novel highly efficient one‐pot protocol. The boron atom initially introduced to the precursor molecule by 1,2‐hydroboration reacts sequentially with each triple bond by a 1,1‐carboboration mechanism resulting in the formation of a silole ring after each step.

Abstract

Despite the apparent diversity of the protocols developed for the synthesis of helicenes, they essentially follow the same strategy: the closure of one, or several, internal rings in a key step. Herein, we report the synthesis of a new family of the heterohelicenes consisting of fused silacyclopentadiene rings formed via a facile and novel process. The treatment of oligo(alkynilydenesilylene) precursors of type H₂C=CH−(SiMe₂−C≡C)_n −R (n=3–7), bearing a vinyl group on the terminal silicon atom, with 9‐borabicyclononane leads first to 1,2‐hydroboration of the terminal double bond which then continues with a cascade of intramolecular 1,1‐carboboration reactions accompanied with the closure of a new silole ring after each step affording the target silahelicenes with, currently, up to seven condensed silole rings and with excellent yields. According XRD analysis, the seven fused silole rings of the heptacyclic compound 11 b form an almost complete turn of a helix. The presented one‐pot sequence of reactions is the first example of ring‐by‐ring knitting of a helical framework starting from easily available linear precursors.

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