Solvents for monoalkylations?
Your phenethylamine and bromobutane/methyl iodide are not going to be soluble in water but will dissolve in each other and react. If you use aq base and stir, possibly with gentle heating for bromobutane, then you will get your product. No need for sieves or drying agents. This is essentially a phase-transfer reaction but without a second solvent. As with any attempted amine alkylation, the danger is over reaction to give the quat. salt. If you find over reaction to be a significant problem then consider reductive amination.