Structural Dynamics and Stereoselectivity of Chiral Benzylideneamine N,C‐Chelate Borane Photo–Thermal Isomerization

Abstract

New chiral N,C‐chelate organoboron compounds based on benzylideneamines (bza) with the general formula of B(bza‐R)Mes2 (R=H or Me; Mes=mesityl) are reported. A chiral substituent group R‐ or S‐CH(CH3)Ph (Ph=phenyl) was introduced to the imine center, which imposed a previously unobserved pseudo‐ or axial‐chirality on the BMes2, creating distinct diastereomers. NMR spectroscopic studies established that the diastereomers undergo slow exchange in solution at ambient temperature. The chiral N,C‐chelate B(bza‐R)Mes2 molecules undergo photoisomerization in the same manner as their non‐chiral analogues, generating chiral BN‐cyclooctatriene (BN‐COT) derivatives. Most significantly, by tracking the photoisomerization with circular dichroism (CD) and 1H NMR spectra along with time‐dependent density functional theory (TD‐DFT) computational studies, the photoisomerization was established to proceed in a highly stereoselective manner, that is, one diastereomer converts exclusively to the corresponding diastereomer product in the photoreaction.

Synthesis and stereochemistry of chiral aza-boraspirobifluorenes with tetrahedral boron-stereogenic centers

“…2 for assignment of the bond lengths and bond angles.bThe tetrahedral character of the dihedral angle (THC DA ) was calculated based on six angles around the boron atom using the equation in ref. 22.…”

“…13 Due to the high stability of the B-C bonds, BC 4 boranes exhibit high stereochemical stability. 14 To date, chiral tetrahedral boranes with B-N, [15][16][17][18][19][20][21][22][23][24][25][26][27] B-O, [28][29][30][31] B-H, 32,33 and boron-heteroatom [34][35][36][37] bonds have been synthesized and the stereochemistry of the compounds has been studied extensively. For example, Toyota has reported stable chiral tetrahedral boranes with three B-C bonds and one B-N bond (BC 3 N), and examined the racemization process (Fig.…”

Recent Advances in π-Conjugated N^C-Chelate Organoboron Materials

“…Despite this fascinating process, the rearranged "dark" products significantly limit their usability for real-life applications [30]. In order to control such process and to increase photochemical stability, several modifications (such as modulation of electronic factors and steric congestion) have been suggested [31,32]. It is shown that the functionalities attached to the chelating/auxiliary units significantly affect the isomerization process [33].…”

“…A similar observation was made by Yam et al [37] Here, the authors merged photoactive diarylethene-functionalized N∧C chelated thienylpyridine with bisalkynyl borane complexes. In order to control such process and to increase photochemical stability, several modifications (such as modulation of electronic factors and steric congestion) have been suggested [31,32]. It is shown that the functionalities attached to the chelating/auxiliary units significantly affect the isomerization process [33].…”

Boron‐Based Enantiomerism

“…Thirdly the very recent enantioselective, catalytic creation of enantiomeric boron compounds 1 or ent ‐ 1 will be highlighted. Finally the chiroptical properties of enantiomerically pure boron complexes and their role as ‘’smart’ materials and devices will be presented [8] …”