Move a pi bond into a cycloalkane

[Phil Massie]

Hello. This is driving me a little mad, and some assistance would be hugely appreciated.

Homework Statement

Show how the following transformation can be achieved (more than one step may be involved)

Homework Equations

The Attempt at a Solution

The only approach i could come up with was the following:

1. hydrohalogenation of the alkene substituent with H-Cl, putting a Cl on the more substituted C atom and an H on the less substituted C.

2. dehydrohalogenation should then produce constitutional isomers, as there are 3 $$\beta$$ carbons, 2 of which produce the same molecule (i think) and both constitutional isomers have tri substituted double bonds. the required pruduct is then one of the constitutional isomers, methylcyclohexene.

I just saw in another textbook something about endocyclic $$\pi$$ bonds being more stable than exocyclic $$\pi$$ bonds. Is this the case in this problem? and would that mean that the required product would be the only one formed?

Thanks in advance for any help. Its late and i can't think anymore...

 

[Spirochete]

2. dehydrohalogenation should then produce constitutional isomers, as there are 3 $$\beta$$ carbons, 2 of which produce the same molecule (i think) and both constitutional isomers have tri substituted double bonds. the required pruduct is then one of the constitutional isomers, methylcyclohexene.

I just saw in another textbook something about endocyclic $$\pi$$ bonds being more stable than exocyclic $$\pi$$ bonds. Is this the case in this problem? and would that mean that the required product would be the only one formed?

You're correct there are 3 beta hydrogens, with the possibility of two different products from an E2 reaction. I'm not sure about different stabilities of endo/exo cyclic rings, but even if the two products were equally stable and the two reaction pathways had the same activation energy, you would still get 66% the product you were looking for, because there are twice as many ways to form the product you want.

 

[Phil Massie]

Thanks thanks thanks!
really appreciate you taking the time for this :)
I love these forums!

 

[chemisttree]

You wouldn't even need to hydrohalogenate/dehydrohalogenate. Look at the mechanism of the acid-catalyzed E2 elimination, first step. The product is a carbocation at the 3^o carbon. Loss of H⁺ will give you the endocyclic isomer. Any exocyclic product would be recycled again and eventually the thermodynamic product is obtained.

 

[Gannon]

Using what little I know from basic organic chemistry, I would simply 1) brominate and 2) run an E2 reaction using a sterically-encumbered base such as tert-butoxide.