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

Stereoselective formation of boron-stereogenic organoboron derivatives

Four-coordinate organoboron derivatives present interesting chemical, physical, biological, electronical, and optical properties. Given the increasing demand for the synthesis of smart functional materials based on chiral organoboron compounds, the exploration of stereoselective synthesis of boron-stereogenic organo-derivatives is highly desirable. However, the stereoselective construction of organoboron compounds stereogenic at boron has been far less studied than other elements of the main group due to configurational stability concerns. Nowadays, these species are no longer elusive and configurationally stable compounds have been highlighted. The idea is to show the potential of the stereoselective building of the four-coordinate boron centre and encourage future endeavors and developments in the field.

Design and synthesis of stable four-coordinated benzotriazole-borane with tunable fluorescence emission

A new class of stable four-coordinated benzotriazole-borane compounds was developed via gold-catalyzed alkyne hydroboration. The application of polymeric (BH₂CN)_n reagent gave the formation of cyano-amine-boranes (CAB) complexes with less basic N-heterocyclic amines and anilines. Various new CABs were investigated in catalytic hydroboration to synthesize N–B cycles. The 1,2,3-benzotriazoles were identified as the only feasible N-source, giving the four coordinated borane N–B cycles (BTAB) in excellent yields (up to 90%) with good functional group tolerability. This new class of polycyclic N–B compounds showed excellent stability toward acid, base, high temperature, and photo-irradiation. The facile synthesis, excellent stability, strong and tunable fluorescence emission make BTAB interesting new fluorescent probes for future chemical and biological applications.

A new class of benzotriazole-boranes was developed via gold-catalyzed alkyne hydroboration. The facile synthesis, excellent stability, strong and tunable fluorescence emission make BTAB new fluorescent probes for chemical and biological applications.

Multistep Photoisomerization of Dimesitylboron-Functionalized Stilbene Analogues

Dimesitylboron-functionalized stilbene derivatives have been found to undergo an unusual regioselective photoisomerization upon irradiation at 365 nm. Using NMR to follow the photoreaction, the structures of key reaction intermediates and the final products were established. This photoisomerization occurs in four steps: trans-cis isomerization, Diels-Alder reaction, di-π-methane rearrangement, and ring opening with [1,3]-H migration. This results in the formation of a rare structure with three fused five-membered rings and a six-membered one.

Synthesis and Reactivity of Cyclic Borane-Amidine Conjugated Molecules Formed by Direct 1,2-Carboboration of Carbodiimides with 9-Borafluorenes

Efficient 1,2-carboboration reactions to the C=N bond of carbodiimides with 9-borafluorenes, which give rise to cyclic borane-amidine conjugates with a seven-membered BNC₅ ring, are reported. The resulting cyclic borane-amidine conjugates can be hydrolyzed into an acyclic bifunctional biaryl compound carrying both boronic acid and amidine groups, rendering the utility of the two-step protocol for the synthesis of multi-functionalized molecular systems with a potential as a supramolecular building block. Furthermore, the conjugated structure of the cyclic boron-amidine compounds can be changed upon alkylation of the boron atom that increases the coordination number of boron. The combination of Lewis acid (borane) and conjugated base (amidine) provides rich structural diversity of heteroatom-containing π-conjugated systems.

Photoisomerization of PtII Complexes Containing Two Different Photochromic Chromophores: Boron Chromophore versus Dithienylethene Chromophore

In order to examine competitive photoisomerization, a series of novel photochromic Pt^II molecules that contain both dithienylethene (DTE) and B(ppy)Mes₂ units (ppy=2-phenylpyridine, Mes=mesityl) were successfully synthesized and fully structurally characterized. Their photochromic properties were examined by UV/Vis, emission and NMR spectroscopy. It was found that the DTE unit in all three compounds is the preferred photoisomerization site, exhibiting reversible photochromism with irradiation. The B(ppy)Mes₂ unit does not undergo photoisomerization in these molecules, but likely enhances the photoisomerization quantum efficiency of the DTE moiety through the antenna effect. Extended irradiation with UV light leads to the rearrangement of the ring-closed isomers of DTE. TD-DFT computational studies indicate that the DTE photocyclization proceeds via a triplet pathway through an efficient energy transfer process.

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

To develop a simple and efficient synthetic method for four-coordinate triarylboranes, we herein describe a tandem highly selective B-Cl/C-B cross-metathesis of two of the same or different arylboranes and C-H bond borylation to synthesize four-coordinate triarylboranes with a broad substrate scope. By switching substituent groups of the target molecules, different emission wavelengths can be achieved from 467 nm to 583 nm with aggregation-induced emission (AIE) properties.

Boron-based donor-spiro-acceptor compounds exhibiting thermally activated delayed fluorescence (TADF)

Four boron-based donor-spiro-acceptor compounds, composed of different donor moieties and borylated 2-phenylpyridines as the acceptor, were studied. Their intense photoluminescence in the solid state can be tuned by changing the donor and long emission lifetimes on the microsecond scale indicate thermally activated delayed fluorescence (TADF).

Aryl Insertion vs Aryl-Aryl Coupling in C,C-Chelated Organoborates: The "Missing Link" of Tetraarylborate Photochemistry

The photoreactivity of 9-borafluorene-based, C,C-chelated organoborates was investigated. Unlike the related tetraarylborates, the charge-transfer transitions imparted by the biphenyl chelate lead to selective insertion of one aryl substituent into the endocyclic B-C bond of the 9-borafluorene moiety, resulting in the formation of boratanorcaradienes. This photoreaction likely proceeds according to a Zimmerman rearrangement, which is analogous to one of the initially proposed mechanisms for tetraarylborates and provides additional insight into these long-debated photochemical reactions.

Reaction of B2 (o-tol)4 with CO and Isocyanides: Cleavage of the C≡O Triple Bond and Direct C-H Borylations

The reaction of highly Lewis acidic tetra(o-tolyl)diborane(4) with CO afforded a mixture of boraindane and boroxine by the cleavage of the C≡O triple bond. ¹³ C labeling experiments confirmed that the carbon atom in the boraindane stems from CO. Simultaneously, formation of boroxine 3 could be considered as borylene transfer to capture the oxygen atom from CO. The reaction of diborane(4) with ^t Bu-NC afforded an azaallene, while the reaction with Xyl-NC furnished cyclic compounds by direct C-H borylations.

Functional conjugated pyridines via main-group element tuning

The functional properties arising from a combination of main-group elements with pyridine-based organic conjugated scaffolds are highlighted.