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

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.

One-Pot Synthesis of 2,5-Dihydrosiloles and Their Silole-Annulated Analogs Starting from Alkynylsilanes with a Terminal Alkynyl Group

“…We recently published an efficient high-yield synthesis of fused oligosiloles (silahelicenes) in the reaction of vinyloligo­(alkynylidenesilylene)­s with 9-BBN-H . Obviously, the reaction sequence presented in the current study can also be employed for the building of oligocyclic compounds, although with a different location of the double bond in the first silole cycle compared to the compounds obtained from vinylsilanes in the previous study.…”

“…The initial 1,2-hydroboration of the terminal double bond may be considered as a key step in the reaction sequence, which allows the introduction of the boryl group under the mild conditions and eases the subsequent intramolecular 1,1-carboboration of the triple bond. This approach has also allowed a highly efficient synthesis of silahelicenes, where the boryl group introduced by hydroboration has initiated a cascade of up to seven sequenced intramolecular carboboration reactions . The present study describes a new and efficient variation of the hydroboration/carboboration concept that selectively leads to 2,5-dihydrosiloles and their fused analogs in which the dialkynylsilanes bearing the terminal alkynyl fragment are used as the starting substrates.…”

Alkyne‐Based Syntheses of Carbo‐ and Heterohelicenes

“…It should be noted that similar, but not radical, approach for the synthesis of the helicene‐like molecules consisting of fused silacyclopentadiene rings has been recently reported [77] . The ring‐by‐ring knitting of a linear oligo(alkynilydenesilylene) precursors 82 , bearing a vinyl group on the terminal silicon atom, has been achieved by heating with 9‐borabicyclononane in the benzene solution (Scheme 25).…”

“…Formation of a rigid polyheterocyclic skeleton starts from 1,2‐hydroboration of the terminal double bond of 82 ( 82 → 84 ), which then continues with a cascade of intramolecular 1,1‐carboboration reactions ( 84 → 85 ) accompanied with the closure of a new silole ring after each step affording the target silahelicenes with, currently, up to seven condensed silole rings in excellent yields. On authors opinion, [77] once anchored in the molecule, the dialkylboryl fragment could be regarded as being a catalytic center, which remains unchanged after each individual cyclization step.…”

Alkyne 1,1‐Hydroboration to a Reactive Frustrated P/B‐H Lewis Pair

“…The1 ,1-hydroboration may be viewed as as pecial variant of the more common 1,1-carboboration reaction, [5] am ore and more important alternative method of preparation of alkenylboranes. [6,7] Scheme 1s hows ar ecent 1,1-hydroboration example from our group [4] which made use of the reactive HB(C 6 F 5 ) 2 borane [8] as the specific reagent for the alkyne to alkenyl borane conversion. Ther esulting 1,1-hydroboration product 3 is afrustrated Lewis pair (FLP) [9] and it underwent av ariety of typical FLP reactions.…”