[kevalb56]

when I put suzuki coupling of 2,4 dichloro pyrimidine with boronic acid, so how can decide reaction take place on 2-Cl or 4-chloro position and why?

[kamiyu]

Normally, 2-chloro position takes place because it is closest to the electron withdrawing nitrogen.

For your product, run NMR. If pure then melting point. Melting point is very sensitive to regioisomers

[sjb]

Normally, 2-chloro position takes place because it is closest to the electron withdrawing nitrogen.

For your product, run NMR. If pure then melting point. Melting point is very sensitive to regioisomers

You'd think so, but I don't think this is that clear-cut. Certainly in my experience coupling at 4 is more likely. I guess you can use NMR (ike NOESY), or depending on how pure things are maybe crystallography.

[kevalb56]

Normally, 2-chloro position takes place because it is closest to the electron withdrawing nitrogen.

For your product, run NMR. If pure then melting point. Melting point is very sensitive to regioisomers

You'd think so, but I don't think this is that clear-cut. Certainly in my experience coupling at 4 is more likely. I guess you can use NMR (ike NOESY), or depending on how pure things are maybe crystallography.

[kevalb56]

in my case it took place at 4 position.but I want to know wt is the reason for that??

[zsinger]

As mentioned, watch out for the MP's as the isomers are VERY close.  I nearly always get the 4-coupling product as well for some reason. Great reaction, and one which I plan to further investigate as perhaps a postdoc topic. Maybe one of the senior chemists can weigh in on this?
              -Zack

[critzz]

I'm not an expert whatsoever, but would like to share my thoughts. This reaction is not a simple nucleophilic aromatic substitution, but proceeds via a catalytic cycle.

The rate determining step in general is the oxidative addition of the halide bond to the metal. Apparently the halide bond at position 4 is cleaved first.

I can't tell with 100% certainty whether the 2-position or the 4-position is less or more electron dense.

I'm curious how did you found out it was the 4-product? (NOESY, HMBC)?

[kevalb56]

I found it from nmr.when reaction takeplace at position 4,nmr shows pyrimidine proton are downfield and when it take place at P-2 protons are very upfield.so wt is d reason for that