[souro10]

SeO2 is used as a re-agent for allylic hydroxylation in organic synthesis and the reaction is believed to proceed via a selenate ester intermediate.

S and Se belong to the same group of the periodic table and Se is situated just below S, and shows many similar chemical properties.

So why does SO2 not give similar allylic hydroxylation reactions?

[orgopete]

That may depend upon interpretation. I like to look for patterns, hence I find them more frequently than others.

There had been an earlier post about the instability of dicarbonyl dioxide. Although it is truly very labile, one can find many other examples of decarbonylation reactions. If we were to ask a similar question, is the selenium oxidation reaction unique? Certainly SeO2 has features that cannot be completely duplicated. It is much less volatile than SO2. I would be surprised if SO2 did not undergo a reaction similar to an ene or carbonyl-ene reaction. I am presuming that is the kind of reaction that SeO2 does initially. If we could establish this reaction is not unique, then we may begin to ask what or how the rates are controlled.

[souro10]

Then we are to look whether SO₂ forms a similar ester-intermediate as SeO₂ does, with -enes.
I am unaware of any such reactions, but they may exist. Some examples may be helpful.

I think there is a possibility that SeO₂ is unique in this regard- what SeO₂ does, SO₂ can-not . Maybe, (just a guess-) the reason lies in thermodynamics? The Se=O bond is indeed, much weaker than S=O bond. However in that case, question arises why more rigorous treatment/ catalysis can not make such a reaction occur smoothly.

Further inputs really welcomed 

[orgopete]

Perhaps someone with access to a reaction database could answer whether an R2S=O or SO2 can undergo an ene-like reaction.

[willug]

Yes it looks like it is possible for SO2 to undergo an ene reaction:

Marković, D., Volla, Chandra M. R., Vogel, P., Varela-Álvarez, A. and Sordo, José A. (2010), BCl3-Mediated Ene Reaction of Sulfur Dioxide and Unfunctionalized Alkenes. Chem. Eur. J., 16: 5969–5975

Apparently the intemediate β,γ-unsaturated sulfinic acids are generally unstable, and it can be used as a method for isomerising alkenes (ene reaction, 1,3 sigmatropic shift, retro-ene reaction). But if you add the BCl3 you form stable complexes which you can use synthetically...

[opsomath]

Apologies for the Captain Obvious answer, but is there a chance that SO2 is not studied as thoroughly because it is a gas at RT? SeO2 is a nice, tractable powder; the only downside to its use is the generation of organoselenides or H2Se as byproducts, which is plenty of downside.

I am accustomed to seeing it as a mild oxidizing agent for the conversion of methyl ketones to glyoxals.

[willug]

Yeah I'm sure that's got something to do with it! Their general procedure isn't exactly convenient;

'To a solution of BCl3 (10mL, 10 mmol, 1M) in CH2Cl2 frozen at -196 °C, SO2 (10 mL) was transferred through vacuum line. The solution was melt, warm to -20 °C and stirred for 30 minutes. The solution was again frozen at -196 °C and alkene (10 mmol) was added. The reaction mixture was stirred at -20 °C for 3 hours. The excess of SO2 and CH2Cl2 were evaporated at -20 °C. So obtained crude products were used without purification for further transformations'