i once read that nitroethene adds to indole to form ethylnitro indole. refs from dictionary of organic compounds say nitroethene is formed by heating 2-nitro ethanol with some semi-exotic but getable compound to eliminate water. refs say 2-nitro ethanol is in turn formed by reacting nitromethane with formaldehylde. is this method known? if so, can someone provide details on conditions and yield.

JOC 1980, 1185

is referenced by

https://www.thevespiary.org/rhodium/Rhodium/pdf/nichols/nichols-amt-enantiomers.pdf

where they perform this operation with nitropropenes.

It sounds much like a Michael reaction.

https://www.thevespiary.org/rhodium/Rhodium/pdf/nitroethene.in.organic.synthesis.pdf

will have much of what you want to know.
In this paper, they used phthalic anhydride to dehydrate the nitroethanol. I imagine almost any anhydride would do. Perhaps even an azeotropic distillation. Who knows?

Lastly, you may have been looking for this:

https://www.thevespiary.org/rhodium/Rhodium/pdf/tryptamine.indole.nitroethylation.pdf

have fun. use ye olde search engine.

another method, the formaldehyde/nitromethane way:

http://www.orgsyn.org

search for 2-nitroethanol. CV 5, 833. with equimolar amts of HCHO and CH3NO2, one gets a 9% yield. large excess nitromethane necessary.

Yes, methinks NaNO2 + ethylene chlorohydrin is definitely the way to go.