Efficient Helicene Synthesis: Friedel–Crafts-type Cyclization of 1,1-Difluoro-1-alkenes

Multifunctional and reactive enantiopure organometallic helicenes: tuning chiroptical properties by structural variations of mono- and bis(platinahelicene)s

Acetylacetonato-platina[6]- and -platina[7]helicenes have been prepared from 2-pyridyl-substituted benzophenanthrene ligands by following a two-step cycloplatination reaction. The photophysical properties (UV-visible absorption and emission behavior) and chiroptical properties (circular dichroism and molar rotation) of the resolved enantiomers have been measured. These metallahelicenes constitute a novel family of easily accessible helicene derivatives that exhibit large and tuneable chiroptical properties that can be rationalized theoretically and compared to the parent [6]- and [7]carbohelicenes. Furthermore, they are red phosphors at room temperature and their large chiroptical properties can be modulated by oxidation of the metal center to Pt(IV). Hetero- and homochiral diastereomeric bis(metallahelicene)s that possess a rare Pt(III)-Pt(III) scaffold bridged by benzoato ligands have also been prepared. It is shown that the heterochiral (P,M)-bis(Pt(III)-[6]helicene) 9a(1) can isomerize into the homochiral (P,P)- and (M,M)-bis(Pt(III)-[6]helicene) 9a(2). Spectral assignments and an analysis of the optical rotation of these systems were made with the help of time-dependent density functional theory. The calculations highlight the contributions of the metal centers to the chiroptical properties. For 9a(1) and 9a(2), σ-π conjugation between the helicenes and the Pt-Pt moiety may contribute strongly to the optical rotation and electronic circular dichroism.

Domino Friedel-Crafts-type cyclizations of difluoroalkenes promoted by the α-cation-stabilizing effect of fluorine: an efficient method for synthesizing angular PAHs

In order to synthesize polycyclic aromatic hydrocarbons with nonlinear arrangements (angular PAHs), acid-promoted domino cyclizations of 1,1-difluoroalk-1-enes and 1,1-difluoroalka-1,3-dienes were studied. 1,1-Difluoroalkenes, each bearing two aryl substituents, were regioselectively protonated with FSO(3)H⋅SbF(5) to generate fluorine-stabilized carbocations, which readily underwent domino Friedel-Crafts-type cyclizations to give carbocycles based on 6/n/m/6 ring systems (n,m=5-7) in good to high yields. Protonation of 1,1-difluoroalka-1,3-dienes took place at their electron-rich methylene (CH(2)) carbon atoms in the presence of milder acids such as camphorsulfonic acid and trifluoromethanesulfonic acid. Domino cyclizations of the resulting fluorine-stabilized allylic carbocations afford carbocycles based on 6/6/6/6 or 6/6/5/6 ring systems in high yields.

The quest for alternative routes to racemic and nonracemic azahelicene derivatives

A series of diverse aromatic azadienetriyne and azatriynes was synthesised. These compounds were subjected to transition metal-mediated [2+2+2] cycloisomerisation to form pentacyclic or hexacyclic helically chiral azahelicene or azahelicene-like structures mostly in moderate yields. Introducing stereogenic centre(s) into selected azatriynes, cyclisation proceeded in a stereoselective fashion providing aza[5]helicenes or aza[6]helicene-like compounds in up to a 100:0 diastereomeric ratio. Gibbs energy differences between corresponding pairs of diastereomers (calculated at the DFT(B3LYP)/TZV+P level) were in good agreement with the experimental data and allowed for the prediction of the stereochemical outcome of the reaction. This study presents for the first time asymmetric synthesis of azahelicene derivatives in high optical purities.