Photochemical addition of methanol to stilbenes

A New Perspective on the Binding Power of an Electron

“…Subjecting ( Z) ‐β‐[D 1 ]‐styrene to the standard reaction conditions (Scheme ) afforded bibenzyl (±)‐[D 1 ]‐ 27 as a single diastereoisomer,41 confirming the stereospecificity observed for methoxyarylation of ( E )‐β‐methylstyrene. The net anti ‐addition across the olefin precludes intermediacy of a benzylic carbocation or radical, and is consistent with, but cannot differentiate between, two possible scenarios: 1) anti ‐oxyauration, followed by arylation with retention of configuration, or 2) syn ‐oxyauration, followed by arylation with inversion 42…”

Photochemistry of the Stilbenes in Methanol. Trapping the Common Phantom Singlet State

“…Laarhoven and co-workers irradiated t -St in CH 3 OD and found a significant increase in the carbene insertion/direct addition ratio, ϕ e c /ϕ e d on changing λ exc , from 0.6 ± 0.1 at 360 nm to >3 at 185 nm. There was also an increase in the threo/erythro ratio, k t / k e , of the 1,2-addition product from 0.7 at 360 nm to 1.8 at 254 nm.…”

“…In addition to insertion to methanol, Laarhoven and co-workers established that the carbene undergoes rearrangement back to ground state stilbenes. 16 Zwitterionic twisted intermediates, first proposed by Dauben and co-workers to explain the stereospecific photocyclization of trans-3-ethylidenecyclooctene 50 and the photoaddition of methanol to 1,3-dienes, 51,52 found initial theoretical support in Salem's sudden polarization close to orthogonal olefin geometries. 53 The zwitterionic nature of 1 p*, the twisted stilbene conical intersection, CI, 54−57 is supported by recent theoretical calculations that predict decay through a CI attained in the 1 B u S 1 state by central bond twisting along with pyramidalization of one of the benzylic moieties.…”

“…65,66 Laarhoven and coworkers suggested the possibility that the rearrangement of 1 p* to the carbene involves a vibrationally hot diradical intermediate. 16 Hydride shift in a zwitterionic 1 p* provides a more attractive pathway, Scheme 3.…”

“…7,10,11 Conrotatory cyclization to 4a,4bdihydrophenanthrene, DHP, is a competing 1 c* reaction, less prominent than cis−trans photoisomerization. 12−15 Ether formation was reported as a minor reaction channel on irradiation of 1 t in methanol 16 (MeOH) or 2,2,2-trifluoroethanol 17,18 (TFE). In the case of TFE, of the mechanisms considered for the direct alcohol reaction with excited stilbene, protonation of 1 t* was favored.…”