Directing Groups on a Cyclohexane

[blazinfury]

I know that when dealing with benzenes there are directing groups for ortho and para positions. Now lets say that you begin with a cyclohexanone and you want to make a 2-methylcyclohexa-1,3-diene. My idea is that you can put the methyl there by reacting the initial molecule with an CH3-Li, but my issue is how to get the alkene on the 3rd and 4th carbons. I was thinking of adding either a Br or an OH and then using tert-butoxide to eliminate, but I am not sure how to get a Br there to perform such a feat. I was thinking of using NBS, but if I add NBS after the metyl is on there, the Br would probably go on the methyl substituent. Sorry for my pestering but I was just curious of how to effectively such a synthesis. Thanks so much.

[hermanntrude]

i'm not very good at this stuff but i suspect you'd be better off starting with a subsituted cyclohexane starting material rather than thinking about directing groups. I think directing groups only really work over such long distances because of the delocalised electrons.

[blazinfury]

I totally agree with you hermanntrude, but the issue is that this problem specifically states that I begin with a a cyclohexanone and get a 2-methylcyclohexa-1,3-diene.

[UC]

Cyclohexanes don't behave at all like benzene rings. The directing groups are only effective on benzene rings because they modify the resonance patterns of the aromatic ring.

 

You should treat them like straight chain alkanes for the most part.

 

There are 4 steps you need for this one. The first is MeLi or MeMgX. What is the product of this reaction? Do you see an easy elimination given the structure you just made? perhaps a certain addition reaction will give you a direct precursor to your desired product after that...

 

You will only halogenate the methyl (allylic position) if you give it free radical conditions by the way. Don't use NBS.

[blazinfury]

I'll say it again: UC you are a genius and a lifesaver. Thanks a lot man. The rest is cake.

 

But lets say that I had a similar problem but I began with a methyl benzene to get a prop-2-ene benzene (sorry if I named it incorrectly). If I used NBS on methyl benzene, would the methyl become CH2Br? Thanks again.

[UC]

I'm not sure what compound you are referring to by prop-2-ene benzene. I suspect you either mean alpha or beta-methyl styrene. NBS will selectively halogenate the methyl position to benzyl bromide (a very unpleasant lachrymator). I believe Br2 can also be effectively used, as long as radical conditions are provided. (light, heat, initiator, etc.)

 

Benzylic and allylic carbons tend to react similarly with radical reactions since both are strongly stabilized by resonance.