Have always had a fascination with compounds displaying chirality in the non-traditional sense….such as planar chirality present in many bi-aryl atropisomers or in some of the more recent work on helicenes or calixarenes to name a couple (some call this inherent chirality). Let’s stick to one for starters — and see where it leads.

Helicenes have been studied a fair bit in the last ten years — mainly due to the applications provided by their unique structure. A review of the current strategies can be found in Chemical Reviews 2012. A recent example of a cool cycloisomerization sequence provided an easy non-photochemical route to [6] helicenes (JOC 2009). To go along with the typical photochemical, Diels-Alder and cyclotrimerization work, RCM metathesis is joining the game for the formation of benzene rings in the helicene backbone.

Shawn Collins at the University of Montreal published a RCM approach (Angew. Chem. Ind. Ed. 2006) to [5,6,7] helicenes utilizing microwave irradiation. His group utilized two approaches these compounds — microwave with catalyst 3 and sealed tube with catalyst 4 depending on the functionality present (discoveries while doing research).

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RCM: Aptly put, metathesis with a twist!

Some interesting discussion in the article over functional groups, number or rings formed and their strain, time to form the desired products. Using the scheme above as the example — heating higher than 100C didn’t help the reaction along or provide a higher yield (for example, several reactions needed an hour to 140 min at 120C, but in general 25 min was the recipe of the day). Some of this is dependent on the vessel capability and the pressure generated — at 100C CH2Cl2 vapor pressure will be about 6 bar and at 120C 9 bar so I imagine some of these reactions could be improved. It should be noted that the reactions were performed by pulsing the reaction mixture at 100C — not quite sure what that means — perhaps they got the thing up to temp for a period and time then reapplied the power in pulses. What I enjoyed in the article is how the group provided some insight into what they felt would be problematic — so some general themes in this type of chemistry requires some prep work, or maybe some creative ways to get around problematic (e.g. 8,8′-positions as shown in example 11) substituent strain.

Take a look through the table to give yourself some perspective on the reactions, time and functionalities — something that can be improved upon with the compatibility of the Ru catalysts being developed. I was impressed with the work presented since there is a clear direction from easily available 1,1′-binaphthyls.

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[5,6,7] Helicenes: Catalyst 3 mw, Catalyst 4 sealed tube

Enjoy the read!

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