You are right steric hinderance would be less with methyl however i don't think steric effects are dominating here. There is something else that is key, the cesium makes the second alkylation which forms the tertiary amine less favorable than the first alkylation going to the secondary amine.
The problem with DMT and any primary amine under typical conditions is that each subsequent alkylation makes the next alkylation more favorable, as related by Lili Post 126957 (https://www.thevespiary.org/talk/index.php?topic=12883.msg12695700#msg12695700)
(Lilienthal: "Breath of Hoax? / PTC tryptamine alkylation on the test bench", Tryptamine Chemistry). And DMT is a completely different case as we want to dimethylate it, NOT monomethylate it. The butyls of DBT do build up steric hinderance when two of them have added to tryptamine, however I would bet that it is impossible to get a good yeild of N-butyltryptamine by conventional alkylation. I think you will get mainly N,N-dibutyltryptamine.
I predict that using a molar ratio closer to 1 of methylateing agent/amine will result in even better yeilds than reported. They performed an identical reaction starting from a secondary amine (N-butylPEA) instead of a primary (PEA). As you can see below, they could NOT even force the tertiary amine to form in higher than a 10% yeild when cesium was present.
Here is what they say in the article.
Among the cesium bases examined, cesium hydroxide monohydrate, in general, gave the highest yields and selectivities although cesium carbonate also worked well, depending on substrates.6 To confirm the possible cesium effect, defined by the enhanced reactivities in the presence of cesium salts,7 comparative studies between cesium bases and other bases were performed, demonstrating that cesium hydroxide was superior to other alkali bases tried with the regard to the observed chemoselectivities. As depicted in Table 1, other alkali hydroxides produced moderate yields of the desired product along with a considerable amount of the tertiary amine 5, whereas cesium hydroxide allowed for a greater selectivity of 9/1 in preference for the monoalkylation product 4. It was apparent that an unprecedented "cesium effect" in N-alkylation was observed as seen in O-alkylation.7
Interestingly, the next comparative study implied that the cesium base not only promoted N-alkylation of primary amines, but also inhibited the formation of tertiary amines. As illustrated in Scheme 2, the intended alkylation of secondary amine (N-(n-butyl)phenethylamine) was very sluggish under our conditions, affording the tertiary amine (N,N-(di-n-butyl)phenethylamine in only 10% yield, whereas 72% of (N,N-(di-n-butyl)phenethylamine was obtained along with the recovery of 25% of (N-(n-butyl)phenethylamine) in absence of cesium hydroxide after the same duration. Under our cesium base promoted N-alkylation conditions, primary and secondary amines exhibited opposing reactivities, suggesting that the improvement of chemoselectivity would stem mainly from the retarded overalkylation or reversal of normally observed alkylation rates.8
I passed the article on to Rhodium so you may get to see all the details with diagrams.
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