[mahesh]

Dear ppl,
This is a question related to Organometallic chem actually, but i would still like to consult you ppl on this.
My reaction is methoxycarbonylation of amines as follows-
R-NH₂ + MeO-COOMe-------- RNH-COOMe + MeOH
This reaction is Pb3O4 catalyzed, and I have written a plausible mechanism for this... I need you comments on this, please..
The file is attached herewith..

[HP]

Does it commented in your method source the possible carbamide side product of this reaction, i mean:

R-NH2 + dimethylcarbonate --> R-NH-CO-NH-R ?
Think about it...

[mahesh]

No, I have done some literature on this, and I am sure that with Lead catalysts, we dont get the urea products.. I carried out the reaction on several amines, and got the mass spectra of all the reaction mixtures.. I see no urea product formation in any case.. in many cases, I get the carbamate exclusively!!

[HP]

Hm, this selectivity sounds strange to me. What is the reaction temperature used? In fact the second step reaction of the amine should be energetically much unfavourable at low to moderate temperatures i think...

[mahesh]

Very true!!!! I cant get any conversion of the amine at temperatures lower than 155C. I am keeping 160C for all the reactions... But what about the overall mechanism? Does the sequence appear correct? Do you feel that Pb3O4 should eventually fall out of the catalytic cycle unchanged as I have shown?

[HP]

Well the what i know is that Pb3O4(minium) is a mixed oxide: PbO.Pb2O3 which is used in anticorosion grunds..In some reactions it is oxidant - for example the well known to the young pyromans minium-bronze mixture. Also PbO after formation of Pb-salts of some fatty acids (i remember an exercise with butiric acid ) is used as a sicative in the alkid resins - there it play a role in peroxide bridges cross-link formation...In your case it seems some intermediate is formed between the PbO and the amine or most like the dimethylcarbonate (or both). May be some redox reaction occur and the catalyst regenerate during reaction - so truely catalyst Try to find have some mechanism known for this pre esteramidation...Good luck!

[mahesh]

Thanks a lot, HP!! I guess I need to do some stoichiometric experiments, for example, reactions of Pb3O4 with amine, with DMC separately, and analyze what is formed.. and react the formed intermediate with the other component, at RT and at higher temperature, and find the products, etc.. I see no other way, as there is not much more info in literature reg this.. may be I should read lead chemistry in depth..
Thanks anyways!

[mahesh]

Hey HP,
sorry mate for bothering you again on this.. but I need to know this from you.. in one of your previous posts to me  this topic, you said-
Hm, this selectivity sounds strange to me. What is the reaction temperature used? In fact the second step reaction of the amine should be energetically much unfavourable at low to moderate temperatures i think...
Now I have done the kinetic modeling of this system.. I got the Arrhenius plot and found out that the acivation energy is 198.67kJ/mol!! Besides, as I said earlier, this reaction needs temp of 160C.. So I want to know now, that, HOW IN THE WORLD DID YOU FIGURE OUT THAT THIS REACTION MUST BE NEEDING HIGH TEMPERATURES, SPECIALLY THE SECOND STEP WHERE THE AMINE ATTACKS THE Pb-carbonate adduct? I am soon communicating my work to J. Mol. Catal... Please enlighten me on the mechanism part..thanks, and I appreciate your knowledge..

[HP]

Hi mahesh,
My intuition idea was conducted by some experience i have with doing acid catalyzed hydrolysis of polyurethanes. Your product RNHCOOCH3 has ureathane (carbamate) bond right. This type of bonds are relatively stable to acid hydrolysis - i operated this reaction at 180C with phosphoric acid... So i imagined that the amine H would hardly react with the first ureathane aduct, especially withoud some acid (Bronsted or Lewis) catalyst. It is interesting you mention Pb-carbonate as intermediate aduct, and you you think it will have Pb-O-CO-OCH3 formula - i think it could be formed at temperatures around 120-130C from my experience with Pb-soap sicative). Then Your R-NH2 will attack the Pb-O-CO side forming the urethane and some basic -O-Pb-OH oxohydroxyde which has the potential to be "autocatalyticaly" more active with the "residue" dimethylcarbonate leaving methanol, but still not enough reactive with the carbamate formed especially if possible this Pb-OH to be esterificated with the leaved CH3OH...Well in the reaction mixture you still have RNHCOOCH3 some R-NH2 and DMC not reacted - without catalyst which at 160C or lower temp effectively first attacking the dimethylcarbonate you still have the possiple second step urea derivative formed. Have you done this reaction with 1:1 amine: DMC without Pb3O4 catalyst at 160 or may be higher temp? So described do you think your reaction is kinetically or thermodinamically controlled - i am not sure   Hope my extempore analysis of your problem be some helpful to you.

[mahesh]

Hey HP, I am sorry mate I was out of station for some personal reason, so didn't reply you earlier!!
My reaction does not take place in absence of the catalyst, at 160 or 170 deg C (thats the range I have gone up to)... I don't think the reaction is non-catalytic in nature.. The least of the temp that I can carry out the reaction is 147C to be precise.. and I get a good two hours of induction period before the reaction suddenly kicks off, and I get complete conv of the amine to carbamate in the next 3 hours.. thats all about it..
thanks for your valuable remarks, they have educated me a lot!