[CPC]

Hallo,
I am trying to convert a bromide derivative to a cyclized product by the action of thiourea according to a report.
Please have a look at the the reaction scheme attached.
I confess that I could not figure out the mechanism of the reaction, could anyone help me with that?
Thank you for your considerations and suggestion.
CPC

[Dan]

Ok, you have a nucleophile and an electrophile - start by identifying nucleophilic and electrophilic sites and think about what could happen initially...

I have attached the scheme with some colourful clues.

[CPC]

Hi Dan,
thank you for your suggestion.
Is that postultated here in my attachment what you mean for the mechanism?
Best regards,
CPC

[Dan]

Ok, it looks like you are trying to do too much in one go, this is a multistep process so do one bit at a time. Nucleophilic attack of the N on the carbonyl is a good idea, that will come into play later, but is probably not the first thing that happens. First off, what do you know about the reactivity of alpha-bromo carbonyls (compared to basic alkyl bromides)? Also, what is the most nucleophilic centre in thiourea?

[CPC]

Hi Dan,
Thank you very much for your discussion.

Is that not the NH2 but the S the most nucleophilic center in thiourea? then it would attack the C alpha to Br.
I am still trying to figure it out, because I am not really good at chemistry, still learning...

Best regards,
CPC

[Dan]

Is that not the NH2 but the S the most nucleophilic center in thiourea? then it would attack the C alpha to Br.

Yes, the first step is an S_N2 displacement of bromide by the sulfur. So, what next?

[CPC]

Hallo Dan,
I have just got home from lab.
Thank you for your response.
Could you help correct what I have postulated here (hand-drawn scheme attached).
I am not quite sure as I am not good at describing reaction mechanism.
I appreciate your step by step suggestion very much.
Best regards,
CPC

[Dan]

Ok, your arrows don't make a whole lot of sense, I have pointed out a few things and shown you the answer.

It is basically two reactions (which you can look up for more info)

1. S_N2 substitution
2. Imine formation

[CPC]

Hi Dan,
thank you very much for your suggestion.
I have learnt the mechanism of the reaction during discussion with you.
My best regards,
CPC