Hi – great questions. Start with neutral osmium having an oxidation state of zero. Then subtract 1 for every bond to a more electronegative atom (O). In OsO4 there are eight. In the osmate ester there are six bonds to oxygen. This accounts for the difference in oxidation state – it has gone from Os(VIII) to Os(VI)

What happens when the lone pair goes into Os during the [3+2] is that there are an extra pair of electrons on osmium that weren’t there before. We generally don’t draw the lone pairs in on transition metals (because there can be a lot!). But if we know the oxidation state, we can figure it out. The oxidation state is now two less due to that lone pair.

fixed! thanks for the catch. James

Without knowing the structure, it’s hard to say. Is it an alpha, beta unsaturated aldehyde? Epoxidation may work, but then again peroxide may end up oxidizing your aldehyde.

Aldehydes are some of the most sensitive functional groups to oxidation and other side reactions. I would generally suggest they are protected unless you have some strong reason for believing it would not be affected by the OsO4 or its co-oxidant.

I think I found a way how to this, without protecting the aldehyde. I would appreciate to get your opinion

Stage 1: reaction with peroxide to get epoxide
Stage 2: acidificaion

Hi Ptachia – I would not do a dihydroxylation with an unprotected aldehyde. Too many things can go wrong. Either keep it at the protected alcohol stage and then oxidize up afterwards, or protect the aldehyde as an acetal and then perform the dihydroxylation.

If we have a molecule which contain both alkene and aldehide’and we want to prepare diol. will osmium tetraoxide oxidize the aldehide? If yes can you suggest me how to get the diol without oxidizing the akdehide?