For how long do you let it react? In scientific literature ideal method is 55-60 °C for 10 minutes:
"In an effort to find other reaction conditions for step 2 3a, we searched the literature for
syntheses of a-tert-butylaminopropiophenone, the ring-unsubstituted analog of bupropion.
We found only one reference 9 to the synthesis of this compound, and it prescribed 5 hours reflux in benzene and resulted in only a 37% yield.
We then tried N-methylpyrrolidinone (NMP) as a solvent and found that the reaction t-butyl amine and 2 at room temperature in this solvent was noticeably exothermic and went to completion (as judged by disappearance of 2 by GC-MS) in less than 0.5 hours.
Out of consideration for the impatience of youth, we tried warming the reaction in a 60 (C water bath and found it was complete in less than 10 min.
By contrast, when the reaction is carried out in acetonitrile, the solvent used by the BW workers, it is not exothermic, considerably slower, and according to Hill et al. (7) resulted in only a 43% yield, while our yields averaged 75-85%."
This is about the synthesis of Bupropion, but I strongly believe the same can be applied to 4-MMC as the structure does not differ a lot. Considering posted yields with DCM are reported high, the same conditions most likely apply to DCM.
It would be a great contribution to our community if anyone has TLC plates and is willing to test for how long you need to run the reaction under different conditions (20 °C, 60 °C, DCM, NMP)