Abstract
Helicenes are chiral polycyclic aromatic hydrocarbons (PAHs) of significant interest, e.g. in supramolecular chemistry, materials science and asymmetric catalysis. Herein an enhanced N-directed electrophilic C–H borylation methodology has been developed that provides access to azaborine containing helicenes (BN–helicenes). This borylation process proceeds via protonation of an aminoborane with bistriflimidic acid. DFT calculations reveal the borenium cation formed by protonation to be more electrophilic than the product derived from aminoborane activation with BBr3. The synthesised helicenes include BN-analogues of archetypal all carbon [5]- and [6]helicenes. The replacement of a CC with a BN unit (that has a longer bond) on the outer helix increases the strain in the BN congeners and the racemization half-life for a BN–[5]helicene relative to the all carbon [5]helicene. BN incorporation also increases the fluorescence efficiency of the helicenes, a direct effect of BN incorporation altering the distribution of the key frontier orbitals across the helical backbone relative to carbo-helicenes.
Highlights
The synthesis of the BN helicenes reported to date requires forcing conditions, for example, A is formed by heating the amine precursor with excess BBr3 at 220°C.4b these reactions proceed via ill-defined
In the systems studied it provides better outcomes than that achieved by the more established synthetic approach - activating aminoboranes with BBr3 and heating. This is attributed to the greater electrophilicity of borenium cations generated by aminoborane protonation relative to borenium cation equivalents generated by BBr3 coordination to N of the aminoborane
This HNTf2 mediated C-H borylation methodology enables access to azaborines otherwise challenging to access
Summary
The replacement of CC for isoelectronic BN units generates superficially similar molecules, but the BN congeners often have distinct properties. Due to this phenomenon BN incorporation into organic materials has received significant attention, with it an established approach to modify physical and optoelectronic properties. This approach has been applied to helicenes, with BN incorporation within the helical backbone of [4]helicenes (e.g. A and B, Fig. 1) found to improve the fluorescence efficiency relative to the all carbon [4]helicene. the incorporation of BN units into the helical backbone of the higher [n]helicenes (n ≥ [5], necessary for configurational stability) is underdeveloped, with no simple BN analogues of the higher helicenes, [5]- and [6]helicene (C and D), reported to date to our knowledge (all examples published to date are extended BN-helicenes, e.g. E). While four coordinate at boron BN units and other boron units have been incorporated into the backbone of higher helicenes, these do not contain helical cores isoelectronic to helicenes C and D.The scarcity of BN-isosteres of the higher helicenes is presumably due to the synthetic challenge of incorporating BN units into these strained PAHs. The replacement of CC for isoelectronic BN units generates superficially similar molecules, but the BN congeners often have distinct properties.. The replacement of CC for isoelectronic BN units generates superficially similar molecules, but the BN congeners often have distinct properties.1 Due to this phenomenon BN incorporation into organic materials has received significant attention, with it an established approach to modify physical and optoelectronic properties.. Due to this phenomenon BN incorporation into organic materials has received significant attention, with it an established approach to modify physical and optoelectronic properties.3 This approach has been applied to helicenes, with BN incorporation within the helical backbone of [4]helicenes (e.g. A and B, Fig. 1) found to improve the fluorescence efficiency relative to the all carbon [4]helicene.. Methods to form BN-PAHs predominantly build on Dewar’s seminal work in which -N(R)H substituted PAHs are functionalised via electrophilic C-H borylation using haloboranes, often in the presence of an additional Lewis acid (e.g. AlCl3 ) and/or a base. Notable work extending Dewar’s approach to access BN-containing higher helicenes comes from Hatakeyama and co-workers who reported benzo-fused aza (and oxa) bora-helicenes, e.g. E, and multi-helicenes. the synthesis of the BN helicenes reported to date requires forcing conditions, for example, A is formed by heating the amine precursor with excess BBr3 at 220°C.4b these reactions proceed via ill-defined
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