Internal B–O Bond-Facilitated Photoisomerization of Boranes: Ring Expansion Versus Oxyborane Elimination/Intramolecular Diels–Alder Addition

Abstract

Boron compounds (1–4) containing an internal B–O bond have been found to undergo facile multistructural transformations upon irradiation at 365 or 410 nm, generating rare 8-membered B,O-heterocycles (1c–4c). In addition, 2 and 3 also undergo an intramolecular Diels–Alder addition and oxyborane elimination concomitantly, via intermediates 2b/3b, producing 2d/3d. The pathways to isomer c and product d were found to be a thermal process and a photo process, respectively.

Internal B ← O Bond Facilitated Photo/Thermal Isomerization of Tetra-Coordinated Boranes

“…The B–O distance is 1.634(2) Å (Figure a). This value is significantly longer than those of the carbonyl-phenyl-supported borane analogues (1.60–1.62 Å), suggesting a rather weak B–O coordination in BO2 . Similar to the N∧C-chelate B­(naphthyl)­pyMes 2 analogue, this aldehyde-supported O∧C-chelate borane also adopts an asymmetric arrangement of the Mes groups.…”

“…Compared to benzothiazole and thiazole moieties, the less bulky carbonyl group enables the Ar unit to stay on this carbon and prohibits a further dearomatization reaction. Surprisingly, unlike the thermally stable naphthyl-pyridyl and carbonyl-phenyl chelated boron analogues, 21,27,29 BO1− BO4 exhibit not only photoresponse but also thermal reactivity. After heating at 60 °C for a period of time, all of the compounds quantitatively converted to identical isomerized products to those obtained via a photochemical pathway as evidenced by their NMR and HRMS characterization data (see the SI).…”

“…This inappropriate understanding was corrected by a recent investigation of a carbonyl-supported tetra-coordinated borane system (Chart a). These carbonyl-supported boranes can undergo facile multistructural transformation with higher photoreactivity compared to those of imine/azole-supported analogues, indicating a unique impact of the weak donor group on the isomerization pathway …”

“…These carbonyl-supported boranes can undergo facile multistructural transformation with higher photoreactivity compared to those of imine/azole-supported analogues, indicating a unique impact of the weak donor group on the isomerization pathway. 27 Lately, we have reported that for the original N∧C-chelate BMes 2 system, when the naphthalene unit is used as the chelate backbone, unexpected benzoborepin species are isolated as the major product from the photochemical reaction [Chart 1b(i) first stage]. The stable C−C bond in the naphthalene ring can be effectively cleaved by UV light.…”

“…In 2008, N∧C-chelate tetra-coordinated organoboranes, such as B­(ppy)­Mes 2 (ppy = 2-phenyl pyridine, Mes = mesityl), were found to display unique thermally reversible photoisomerization. From then on, consistent research effort from Wang, Wagner, Braunschweig, Jäkle, and Yamaguchi et al ,,− ,− has been given to not only expand the scope of photoactive organoboranes but also comprehensively understand their rare photoactive properties. For instance, diaryl chelate units were believed to be necessary for photochemical reactivity for a long time.…”