edit: solution found further down the threadI hope that by discussing the following we can help both myself and others who have experienced similar issues:
I've been having some problems getting decent yields when reducing nitropropenes to nitropropanes with borohydride. Until now I've had no clue as to why. Now I have a good clue, but it does not fit with the experimental writeups that I have read.
To cite an example of the problem:
The method followed was taken from the foot of the following page:
https://www.thevespiary.org/rhodium/Rhodium/chemistry/nitrostyrene.nabh4.reduction.html
where the reduction of the 2,4,5 substrate is detailed. The same method was followed as described here (using the same substrate), with all reactants and solvents scaled down to 63% (16g of substrate). Slightly greater proportion of borohydride (1.1x) was used than in the writeup as that that was available was from a previously opened canister.
The reduction proceeded as described, with the orange color fading completely to white. The reaction mixture became a white paste (still completely stirrable). Stirring was continued for another 60min after the reaction finished as I had to go and do something else.
The excess borohydride was destroyed with acetic acid until effervescence on further addition ceased. The reaction mixture by this point was a yellow liquid with white solid in it.
Brine and toluene were used to separate organic and inorganic components as described. On evaporation of the organic layer only 6g of yellow liquid were obtained (starting to crystallize but still some toluene in there so probably needs further evaporation).
So I got to wondering where all the product was? As a test I poured some 5% HCl into the aqueous layer that I had separated earlier. The clear solution turned immediately light green and cloudy and became progressively darker green and cloudier over the next minute or two. Extracting this solution with toluene and subsequent evaporation produced a brown oil (~8.5g).
What I deduce from this is that much of my product was still in solution as a nitronate and that the addition of mineral acid cased a nef reaction (with accompanying green by-products due to the fact that the temperature was room as opposed to 0C.
My questions are as follows:
1. Was the white paste during the reaction due to nitronate coming out that was insoluble in the ethyl acetate/alcohol solvent mixture? Does this matter at all?
2. The experimental writeup cited above says to add acetic acid until gas evolution ceases. Whem I do this it seems that much of my product is still in a nitronate salt form. Why would I have the discrepancy from other experimenters?
3. Do I just have to add more acetic acid (say until pH4 or something)?
4. Has anyone else experienced similar problems with this reaction? - If not why do you think this has happened to me and not you?
Hope I have included enough detail - if not please ask!
thanks.....