Stabilising fleeting intermediates of stilbene photocyclization with amino-borane functionalisation: the rare isolation of persistent dihydrophenanthrenes and their [1,5] H-shift isomersElectronic supplementary information (ESI) available. CCDC 1590110, 1590111, 1590113, 1816881, 1590195, 1590116, 1816882, 1590114, 1590115, 1590112, 1590109 and 1811865. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc00560e

The key intermediate, 4a,4b-dihydrophenanthrene (DPH), involved in the photocyclization of stilbene and derivatives is known to be unstable, and is therefore poorly characterized/understood. We have found that functionalising stilbenes with NMe 2 and BMes 2 groups can greatly enhance the stability of 4a,4b-DPHs, allowing quantitative isolation and full characterization of these rare species. Furthermore, we discovered that the new amino-borane decorated 4a,4b-DPHs can undergo thermal [1,5] H sigmatropic shift, forming isomers 4a,10a-DPHs. Both 4a,4b-DHPs and 4a,10a-DHPs are stable towards air and moisture, while only the former were found to undergo oxidative dehydrogenation upon irradiation at 365 nm under air, yielding brightly blue/green fluorescent NMe 2 and BMes 2 functionalised phenanthrene analogues. Control studies established that the trans -Mes 2 B-Ph-NMe 2 unit is responsible for the stability of these isolated 4a,4b-DHPs and their [1,5]-H shift isomers. Amino-borane functionalization is the key for isolating rare air-stable dihydrophenanthrene intermediates in stilbene photocyclization.