Highly Colored Boron-Doped Thiazolothiazoles from the Reductive Dimerization of Boron Isothiocyanates

Abstract

Reduction of (CAAC)BBr₂ (NCS) (CAAC=cyclic alkyl(amino)carbene) in the presence of a Lewis base L yields tricoordinate (CAAC)LB(NCS) borylenes which undergo reversible E/Z-isomerization. The same reduction in the absence of L yields deep blue, bis(CAAC)-stabilized, boron-doped, aromatic thiazolothiazoles resulting from the dimerization of dicoordinate (CAAC)B(NCS) borylene intermediates.

Keywords: DFT; aromaticity; borylenes; fused thiazaboroles; reduction.

Scheme 1

Synthesis and isomerization of CAAC–NHC‐stabilized (E)‐ and (Z)‐isothiocyanatoborylenes. Dip=2,6‐diisopropylphenyl.

Figure 1

Crystallographically derived molecular structures of a) (Z)‐2^Me (only one of the two molecules present in the asymmetric unit is represented) and b) (E)‐2^Cy. ^[38] Atomic displacement ellipsoids drawn at 50 % probability level. Ellipsoids on the CAAC ligand periphery and hydrogen atoms omitted for clarity. Selected bond lengths (Å) in blue, selected angles (°) in red.

Figure 2

a) Crystallographically derived molecular structure of 3^Me. ^[38] Ellipsoids on the CAAC ligand periphery and hydrogen atoms omitted for clarity. Selected bond lengths (Å) in blue, selected angles (°) in red. b) Plots of HOMO and LUMO of 3^Me and 4 at the OLYP/TZP level of theory. c) Low‐energy fully delocalized π orbital of 3^Me and [5^Me]²⁺. d) ACID plot of 3^Me (isosurface: 0.025).

Scheme 2

Reductive cyclization of 1^R in the absence of a Lewis base.