The preparation of 2,7-bis(1,1-dimethylethyl)-pyrene is carried out using tert-butyl chloride in presence of AlCl3. Whether the same protocol can be used to prepare 2,7-dimethylpyrene (Iodomethane/AlCl3)?
I see two potential problems on changing from a t-butyl adding system to a methyl adding system:
1) The t-butyl cation is stable enough that AlCl3 can pull chloride away from the organic fragment, creating low concentrations of the highly reactive t-butyl cation, which then attack the pyrene. This mechanism is not accessible using methyl halides or pseudohalides and AlCl3 because the methyl cation is energetically much less favorable. AlCl3 may still catalyze the reaction of methyl halides with pyrene, but the mechanism will be different, so you might expect different selectivity and efficiency profile.
2) The t-butyl cation is significantly larger than the methyl cation, so the selectivity of the position it binds to will be more sensitive to the local steric environment. It is possible that only 2 and 7 positions are accessible to t-butyl fragments, while the methyl fragment could bind anywhere... This is especially problematic because alkylated pyrenes will be more electron rich, and therefore more easily alkylated from an electronic standpoint.
Dear Dhirendra
You can search for C-H activation chemistry. May be that will give you positive result.
I agree with Aaron. You may not generate stable CH3 cation to attack specifically 2 and 7th position. Electronics and especially sterics also important for the rxn on the left to work. I would think alternative way such Pd, Pt or other metal involved C-H activation or coupling strategy for this.
I agree with Omprakash and Mustafa. C-H activation is probably the way to to this. Here is one paper that discusses 2,7-selective bis-functionalization of pyrene using an Ir-based catalyst: http://www.ncbi.nlm.nih.gov/pubmed/22415854
Is there any steric and/or electronic effect of substituents on such C-H activation (2,7-bis) of pyrene? For instance, the electron-withdrawing group (ortho/1-substituted) may further enhance the reaction rate and selectivity or not?
It is very common for transition metal based C-H activation catalysts to be directed by coordinating substituents like amines, amides, ethers, thioethers, heterocycles etc. Usually they are ortho-directing (on rings), but there are examples of alpha- and beta-directing groups on chains, as well as other more interesting modes.
I am less familiar with electronic effects on C-H activation, but typically substituents that stabilize carbocations and carbon-centered radicals also activate C-H activation for the same positions...
Selectivity based on sterics is also possible, but usually the sterics are already determined by the substrate required.
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