Enhanced N-directed electrophilic C–H borylation generates BN–[5]- and [6]helicenes with improved photophysical properties

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...

Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles

“…Borenium cations are much more energetically accessible than borinium cations, and we have recently shown that [R 2 N=BY] + borinium cations are extremely high in energy and not feasible intermediates in C–H borylation. 32 If an S N 2-type mechanism is operative, the absence of any diphenylacetylene hydroboration starting from 9/11 can be attributed to the stronger binding of NTf 2 to boron via nitrogen relative to O-bound NTf 2 (as found in bis-borane 4 ), leading to a higher reaction barrier.…”

“…If the hydroboration reaction with 4 or 9/11 proceeds via an S N 1-type mechanism, then this would require dissociation of NTf 2 from boron and formation of borenium (e.g., [D] + from 4 ) or borinium (e.g., [R 2 NB–H] + from 9/11 ) cations. Borenium cations are much more energetically accessible than borinium cations, and we have recently shown that [R 2 NBY] + borinium cations are extremely high in energy and not feasible intermediates in C–H borylation . If an S N 2-type mechanism is operative, the absence of any diphenylacetylene hydroboration starting from 9/11 can be attributed to the stronger binding of NTf 2 to boron via nitrogen relative to O-bound NTf 2 (as found in bis-borane 4 ), leading to a higher reaction barrier.…”

Divergent Synthesis of Double Heterohelicenes as Strong Chiral Double Hydrogen-Bonding Donors

“…As the isoelectronic species of nonpolar CC unit, the polar B–N unit can keep the structural similarity but alter the molecular polarity. However, compared with other multiple helicenes, the B–N-containing ones are even more scarce arising from the lack of synthetic methods to incorporate B–N units …”

Synthesis of BN-Polyarenes by a Mild Borylative Cyclization Cascade

“…To date, the most widely used strategy for the preparation of polycyclic BN-arenes is the classical electrophilic borylation, which requires high temperatures. 6 , 7 As such, the development of mild and efficient approaches for the synthesis of polycyclic BN-aromatic compounds is highly desirable. 8 …”

“…Intramolecular activation of the alkyne by the boron atom initially coordinated to the amino group, after BCl 3 facilitated chloride abstraction, 19 cannot be completely ruled out. However, the formation of the required borenium cation would presumably require a significantly high energy, 7 and the fact that cyclization occurs at room temperature without any external additive makes the proposed mechanism more feasible. 20 …”