Triplet Energy and π-Conjugation Effects on Photoisomerization of Chiral N,C-Chelate Organoborons with PAH Substituents

Abstract

Chiral, PAH substituted N,C-chelate boron compounds are systematically investigated to establish the effect of triplet energy and substitution position on their photoreactivity. They all undergo regioselective photoisomerization, forming new dark isomers with quantum efficiencies reflecting these various factors. New PAH fused 4bH-azaborepins are obtained via thermal isomerization of the dark isomers. These results further implicate a photoactive triplet state in the photoisomerization process and its utility in achieving rare PAH-fused azaborepin-like heterocycles.

Photochemical Generation of Chiral N,B,X‐Heterocycles by Heteroaromatic C−X Bond Scission (X=S, O) and Boron Insertion

“…Owing to the concomitant formation of two photoproducts, the quantum yields of isomerization could not be determined. The importance of the proximity of the boron center and the C2−S bond in 1 / 1 a and 2 / 2 a was confirmed by the photoreaction of 1′ (Scheme ), which follows that of A → C in Scheme ,. Therefore, the C2−B connectivity in 1 / 1 a and 2 / 2 a plays a critical role in facilitating the photogeneration of 1 b and 2 b , which is elucidated in the computed reaction pathway for 1 to 1 b .…”

“…This approach has several advantages, with the most prominent being the ability to access reactive intermediates from easily prepared and often air‐stable starting materials. Using this approach, we have recently unveiled a number of photoinitiated transformations available to N,C‐chelate boron compounds (e.g., A ; Scheme ), wherein readily accessible precursors were converted into novel B,N‐embedded systems such as azaboratabisnorcaradienes ( B ),, B,N‐1,4,6‐cyclooctatrienes, azaborabenzotropilidenes, and 4b H ‐azaborepins ( C ),, which all display interesting properties and would be very difficult to prepare by traditional synthetic procedures. In many of these examples, the photogenerated boriranes B act as reactive intermediates, enabling subsequent transformations to more stable isomers.…”

“…All compounds were fully characterized by 1 H, 13 C, and 11 B NMR spectroscopy and HRMS analysis. The crystal structures of 2 , 5 , 6 , and 1′ were determined by single‐crystal X‐ray diffraction, and display bond lengths around the B atom that are similar to those of A (see the Supporting Information) ,. These compounds all give rise to strong and broad absorption bands at λ =300–405 nm ( ϵ =8000–20 000 L mol −1 cm −1 ), and fluoresce weakly in the visible region (λ em =400–570 nm, Φ fl ≈0.05–0.30) owing to a charge transfer (CT) S 1 transition from the heterocycle (π, HOMO) to the chelate backbone (π * , LUMO)…”

Four-coordinate triarylborane synthesis via cascade B–Cl/C–B cross-metathesis and C–H bond borylation

“…Four-coordinate triarylboranes with three different substituents are interesting molecules, 7 , 18 yet their syntheses are very rare due to the great challenge of selectivity. How to realize their construction in a one pot strategy has become a very attractive challenge.…”

Multistep Photoisomerization of Dimesitylboron-Functionalized Stilbene Analogues

“…However, isolated and well-characterized DHPs are rarely reported in the literature . Photoisomerization of boranes and their derivatives has emerged as a promising way to generate rare boron-doped polycyclic structures. , Studies of borylated stilbenes have also been reported. , For example, Shi et al recently found that stilbene analogues containing a para -substituted donor–acceptor (D–A; Mes 2 B–Ph– p -NMe 2 ) unit can produce rather rare but stable 4a,4b-DHP intermediates (Scheme b) . To better understand the effect of the donor and/or acceptor on the photoreactivity of the stilbene analogues, we decided to synthesize Mes 2 B-functionalized stilbene derivatives with varying aryl substituents, as shown in Scheme c.…”