Well, one approach is to react an aryl magnesium halide with a diazonium BF4. Works!

I don't know if you used the acetamide directly for your Regitz Diazo Transfer (RDT) reaction or if you used it's b-keto-acetamide analogue. The later one works much better as it helps to minimize the production of side-products (see file).

Also, diazo are sensitive to acid, so if you have not tried this already you can add 1% triethylamine in your hexane for flash chromatography. You can also try to use a silica-gel plug instead of a regular flash in order to minimize the residence time of your compound in contact with the silica. Works well with lots of diazos as they typically are quite non-polar. I don't know how well this would work in your case though as the acetamide is much more polar than the corresponding ester.

In my hands, diazoacetamides were always harder to make than the corresponding esters. Depending of you research's objectives, you could try to form the acetamide later on in you synthesis, either from the ethyl ester (ethyl (phenyl)diazoacetate) or the nitrile diazo (2-Diazo-2-phenylacetonitrile). Both are well precedented diazo compounds and you should not have issues preparing them.

Best of luck!

Dear Marcin, I have done nearly this same work with you. And the results are the same. Though I am still not sure the reason we are unsuccessful, I want to share my experience with you. The amine containing substrates are hard to convert to the desired products, no matter the dialkylamine group is on the phenyl ring or as an amide. I think the amine group is with strong electron-donor ability than alkyloxy groups. As shown on Pages 737 and 744 (ACIE, 1967, 6, 733). I think it may form a more stable pentacyclic intermediate, which is hard to convert to the target product. Or the diazo compounds are decomposed in the reaction system, because of the strong donor ability of the amine group, the product becomes more active, and is able to attack the proton scavenged by DBU, which further leads to its decomposition.

Have you tried the old-fashion method which oxidises the corresponding hydrazonne with HgO in a non polar solvent? You could start your reaction with the corresponding amide of the phenylgyoxylate, react it with hydrazine hydrate, then oxidize the hydrazone at low temperature. Good luck!