Zinc Chloride as a New Catalyst for Knoevenagel Condensation
P. Shantan Rao & R. V. Venkataratnam

Tetrahedron Letters 32, 5821 (1991)

(https://www.thevespiary.org/rhodium/Rhodium/djvu/knoevenagel.zncl2.djvu) (Article retrieved by dioulasso)

Abstract
The knoevenagel condensation of carbonyl substrates with acidic methylene reagents proceeds smoothly in presence of zinc chloride, without the need for solvent, to produce products of good purity in high yield.

If somebee wants to play chemistry with his microwave owen, here is the PDF of the paper I referenced two posts above.

Microwave-assisted Henry reaction: Solventless Synthesis of conjugated Nitroalkanes.
Tetrahedron Letters 38 (1997), 5131-5134.
Varma R. J., Dahiya R., Kumar S.
https://www.thevespiary.org/rhodium/Rhodium/hive/hiveboard/picproxie_docs/000462374-MW-Henry.pdf" target="_blank" title="Download this file">

Abstract: In a solventless system and under microwave irradiation, nitroalkanes react with arylaldehydes in the presence of a catalytic amount of ammonium acetate to afford, in one step, conjugated nitroalkenes without the isolation of intermediary beta-nitro alcohols.

Dont forget that these article's usual run the reactons in the 1mmol scale. Things become more trickey when you run a 10g batch. Esp. the polymeristions become a problem. But it works usual greath, the tric is to stop at the right time (as usual I know)

Ethylenediamine diacetate is almost always a great Knoevenagel/Henry catalyst - besides that the condensation takes up to 36 hours. This time can be decreased by adding silica gel (it adsorbs water, ~30% of its own weight; per mol nitrostyrene 1 mol water is formed, so ~59g should work well for 1 mol) and keep rxn slightly above room temp. (~50°C). Everything will turn orange within hours...

Even better yields (70%+ for P2NP) can be obtained if the nitropropene mother liquor and silica gel are put under vac - even if having already filtered off the first precipitation of crystals, applying vacuum to the mother liquor & drying agent will result in more fine needles appearing in the liquid...  

Greetz A

Conjugated Nitroalkenes: Versatile Intermediates in Organic Synthesis

(Anthony G.M. Barrett, Gregory G. Graboski; Chem.Rev. 1986, 86 (751-762))


Introduction

The nitro group is a powerful electron-withdrawing substituent, and this property dominates the chemistry of all molecules containing this functional group. For example, nitroalkenes, being markedly electron deficient, are powerful dienophiles in the Diels-Alder reaction. Alternatively, these electrophilic alkenes readily undergo addition reactions with many different nucleophiles. The nitro group is particularly versatile in synthesis since it may be transformed into a legion of diverse functionality. It can be readily replaced by hydrogen in a denitration process or converted to a carbonyl substituent in the classical Nef reaction. Additionally primary nitro groups can be dehydrated to produce nitrile oxides or oxidized to produce carboxylic acids. Finally, the nitro substituent can be reduced to produce oximes, ketones, hydroxylamines, or amines. Clearly nitro compounds and nitroalkenes in particular are versatile compounds in synthetic organic chemistry.
This article covers a review of the literature on the synthesis and reactions of nitroalkenes in the time interval 1980-1985. (...)

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https://www.thevespiary.org/rhodium/Rhodium/pdf/conjugated.nitroalkenes.review.pdf

EDIT: Oops - I just realized that this article is already archieved at Rhodiums site. Sorry!
I have changed the link above... (hope the file I uploaded will be autodeleted, as it is not in use anymore?)  


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