It seems like sodium triacetoxyborohydride can indeed be used for reductive alkylation of tryptamine without the usual reduction of the indole to an indoline. In J. Org. Chem. 63, 968 (1998) there is an example of tryptamine being alkylated with a ketone to form a secondary amine. Using more of the carbonyl compound and more of the borohydride may give the dialkylated tryptamine in good yield.
Sodium triacetoxyborohydride can be made by mixing 1 mole of NaBH4 with 3 moles of glacial acetic acid in an inert solvent, and evaporating in vacuo. Care must be taken not to leave any residual acetic acid in there, as in JACS 96, 7812 (1974) they show that NaBH4 in neat acetic acid (equivalent to triacetoxyborohydride plus excess acetic acid) reduced indoles to indolines.
Experimental:
To tryptamine (13.2g, 82.4 mmol) in 1500ml dichloroethane at room temperature was added 1,4-cyclohexanedione monoethyleneglycol acetal (1.1g, 77.5 mmol), sodium triacetoxyborohydride (25g, 117.9 mmol) and glacial acetic acid (4.4 ml, 77.1 mmol). The reaction was stirred for 24h, the solvent distilled off under reduced pressure, the residue taken up in 700ml DCM, washed with saturated Na2CO3 followed by water, and then dried over MgSO4 and the solvent distilled off under reduced pressure. After recrystallization from DCM, the N-alkyl-tryptamine was isolated in 87% yield (20.2g).
