# Reaction mechanism for the synthesis of 9,10-phenanthrenequinone

In my lab course I had to synthesize 9,10-phenanthrenequinone (phenanthrene-9,10-dione) from phenanthrene and chromium(VI) oxide. The synthesis was made as explained in Organic Syntheses, Coll. Vol. 4, p.757 (1963)

The reactants are: phenanthrene (1.00 eq), chromium(VI) oxide (7.50 eq), sulfuric acid (15.0 eq), water (83.3 eq)

I now have to explain the reaction mechanism, but unfortunately I don't find it in any of my textbooks, neither on the internet.

Here is my assumption:

• The chromium(VI) oxide reacts with water and forms chromic acid.
• The chromic acid can oxidize an alcohol to a ketone (Jones-Oxidation)

So what misses is the alcohol. Somehow I have to get the 9,10-phenanthrenediol, which could react as alcohol in the Jones-Oxidation.

How could this possibly work. Remember, I don't have $\ce{OsO4}$, which could easily dihydroxilate an alkene.

# EDIT 1: As an answer to the proposed mechanism by @Zhe:

Is this ok in your eyes?

# EDIT 2: An other idea for the first step (dihydroxilation)

A person of the scientific staff of my university told me, that the $\ce{CrO3}$ should react in a very similar way as $\ce{OsO4}$ does. As I'm using 7.50 eq of chromium(VI) oxide, there is enough for the dihydroxilation and the Jones-Oxidation. He said that a radical mechanism is rather unlikely, because I'm working with water as solvent and water is an inhibitor for radical reactions. This is the mechanism I came up with, regarding these new information.

• This probably happens via a single electron oxidation of that double bond to a radical cation as a first step. The cation can trap water and then we can oxidize the radical to a cation. Eventually, you get to a diol which you can oxidize to a diketone. I can suggest a more complete mechanism later if I time, but try that and see where you can take it. – Zhe Jun 25 '17 at 19:09
• @Zhe Thanks for your comment. Would you please explain this a bit more? I only know the single electron oxidation for carbonyl compounds and I don't find anything about alkenes. Thanks in advance! – Sam Jun 26 '17 at 9:00
• That's not a bad idea. Pretty similar to how $\ce{OsO4}$ works. Again, without more mechanistic evidence, it's hard to know for sure, but I find this quite plausible. – Zhe Jun 27 '17 at 15:45

• Chromium(VI) is a strong oxidant, so I assumed it can just pick up an electron. Granted, that is not how the oxidation of alcohols to ketones works, so I may want to come up with a mechanism that involves a $\pi$-complex with the chromium. – Zhe Jun 26 '17 at 19:04
• Not sure that the SET would behave as drawn. There may be some significant changes regarding the oxo groups on chromium. Again, this is just a guess, and I think it's reasonable, but I wouldn't declare it correct since I don't have any mechanistic data. I'm also considering using the olefin a $\eta^{2}$ ligand, but I don't have a productive way of eliminating the chromium that I like. That's not to say that that mechanism isn't better. The best bet would be to look at the literature more thoroughly for mechanistic studies. Sadly I don't have journal access anymore to help there... – Zhe Jun 26 '17 at 20:50