[phillyj]

Anyone know how I would begin the curly arrow for this Rxn? I'm not really sure what I'm looking at. At least tell me what type of reaction this is?

Thank you

[phillyj]

ok, how about this. I know that the electrons from the double bond will attack either the -OH or the -Br substituent but which one first?

And what role does the heat play? I think it might be rearrangement but not too sure.

[agrobert]

Their is only one way the double bond can attack.  Think about it.  Bromine is your leaving group and the free -OH is your nucleophile.  Review hydrolysis of double bonds and alkene chemistry.  If the double bond attacks the bromine you are left with a carbocation on the other carbon.  What happens next?  Their is no rearrangement.  The heat is key, think about it.

[phillyj]

I was think about what you said and got something:
1] Br attacks the H of -OH forming a better leaving group (HBr) and an O- nucleophile.

2] the alkene double bond attacks the H-Br carbon causing H-Br to leave and forming a bond between the 2 reactants. The alkene now has a positive charge on its carbon

3] the O- attacks that positive charge, closing up the ring and forming the 3 ring product.

But what does heat do?? I'm so confused

[agrobert]

Your understanding of leaving groups need review.  In general halides I>Br>>Cl in decreasing rate of leaving ability.  You do not need to activate a halide.  The halide will leave after the double bond attacks it (S_N2) then the R-OH attacks the carbocation.  Think about this step and why heat is required.  You might ask...couldn't the bromine anion come back and attack the carbocation?  The answer is yes but it is not favored.  The bromine anion is now in solution.  The free R-OH is now tethered to the system and is within proximity of the sp² hybridized carbocation.  The R-OH will attack and close the ring.  The ether is made closing the ring.  This is an intramolecular reaction.  The heat pushes the reaction and and allows the reaction to proceed without other reagents.  Do you understand?  I will draw a reaction mechanism for you tomorrow.

[macman104]

Would the two reactions not be in a more concerted step, as in:  OH attacks double bond (not really as a result of the carbocation), double bond attacks Bromide?

EDIT:  Or is that really more a mechanistic "semantic"?  Or uncertainty?

[agrobert]

Would the two reactions not be in a more concerted step, as in:  OH attacks double bond (not really as a result of the carbocation), double bond attacks Bromide?

EDIT:  Or is that really more a mechanistic "semantic"?  Or uncertainty?

Yes it probably would be concerted.  It is really just a THP protection with a proximal bromine that closes the ring.  Although it is difficult to say because we are only given one product of the reaction without yield.

[movies]

Standard THP protection is not concerted though!  I tend to think that this is probably stepwise.  Another possibility in that the phenol loses HBr to form an ortho-quinone methide which unergoes a hetero-Diels–Alder reaction.  H-D–A reactions are generally more stepwise (or at least asynchronous) than standard D–A reactions, so even then I think it's probably not really concerted.

[agrobert]

For stepwise

[spirochete]

Not the OP but I have a question:What about the possibility of an SN1 type mechanism where bromine leaves first.  Wouldn't this create a resonance stabilized carbocation?  I ask because I've never seen an SN2 reaction with the nucleophile as a CC pi bond.  Is this common?

[movies]

Agrobert, the only suggestion I would make is that the oxygen assits in the inital attack.  The intermediate is then an oxocarbenium ion instead of a carbocation.

Spirochete, that is similar to what I said about the hetero-Diels–Alder, so I think it is quite possible.

[venkatjohnny]

hi,
first OH will attack on the double bond which will further attacks at carbon attached to Br to form a six membered ring. This Br- will abstract H+ from OH and liberates HBr gas. I think this is how the reaction goes!!!!!!!

[macman104]

hi,
first OH will attack on the double bond which will further attacks at carbon attached to Br to form a six membered ring. This Br- will abstract H+ from OH and liberates HBr gas. I think this is how the reaction goes!!!!!!!

Uh....yes, thank you for rehashing what we have all just said.  Also!!!, !!!! poi!nts are aw!!!eso!me!!!  right!!!!!!!

[venkatjohnny]

hi macman,
i'm sorry if i irritate u with my posting, have a nice time yaar

[movies]

hi,
first OH will attack on the double bond which will further attacks at carbon attached to Br to form a six membered ring. This Br- will abstract H+ from OH and liberates HBr gas. I think this is how the reaction goes!!!!!!!

I don't think it can be concerted like you have described because the double bond is not very electrophilic!