[grandson_me]

Been doing some dithiane chemistry recently and encountered this problem. I was suppose to deprotonate the 1,3-dithiane using nBuLi at -30degC, stir for 2h, followed by addition of my epoxide to the lithiated dithiane solution. However, my lithiated dithiane seems to be "consumed" prior to the addition of my epoxide. Upon work up, i had a lot of unreacted epoxide, very little 1,3-dithiane, and a side product which does not have any of the peaks present in my epoxide. I suspect the side product came from the lithiation of 1,3-dithiane.

What do i mean? Well i did the following:

Dissolve 1,3-dithiane in freshly distilled THF, cool to -30degC, add nBuLi. Stir for 2h. Check TLC. There was a new spot on my TLC, although theoretically i should just see the spot for 1,3-dithiane. Its a spot which stains yellow on CAM stain and runs to a Rf of about 0.3 on 4: 1 (hexane: EA).

Anyone has any such experience or has an insight as to what the side product/side reaction could be?

[sjb]

I don't see where in your protocol you're adding the electrophile. How do you do the workup before spotting the reaction spots? Can lithiated dithiane react with esters (i.e. the EtOAc of the mobile phase) to form the acyl dithiane?

S

[grandson_me]

"Been doing some dithiane chemistry recently and encountered this problem. I was suppose to deprotonate the 1,3-dithiane using nBuLi at -30degC, stir for 2h, followed by addition of my epoxide to the lithiated dithiane solution. However, my lithiated dithiane seems to be "consumed" prior to the addition of my epoxide. Upon work up, i had a lot of unreacted epoxide, very little 1,3-dithiane, and a side product which does not have any of the peaks present in my epoxide. I suspect the side product came from the lithiation of 1,3-dithiane."

I added my electrophile which is the epoxide as shown above.

What i normally do is to draw a small aliquot from the reaction using a clean long needle attached to a syringe and rinsing the needle and syringe with a small amount of ether. Then add some water to the sample. I think any lithiated species would have been quenched by the water, so i dont think it will react with the EA of my mobile phase.

[sjb]

Sorry, I misinterpreted what you wrote. I saw

Been doing some dithiane chemistry recently and encountered this problem. I was suppose to deprotonate the 1,3-dithiane using nBuLi at -30degC, stir for 2h, followed by addition of my epoxide...

and then

What do i mean? Well i did the following:

Dissolve 1,3-dithiane in freshly distilled THF, cool to -30degC, add nBuLi. Stir for 2h. Check TLC. There was a new spot on my TLC, although theoretically i should just see the spot for 1,3-dithiane. Its a spot which stains yellow on CAM stain and runs to a Rf of about 0.3 on 4: 1 (hexane: EA).

No electrophile the second time round (so just a mistake in typing, no problem)

Anyway, is your temperature cold enough to avoid the fragmentation of THF with BuLi to give the enolate and ethylene? Do you have a protocol you're following that uses these conditions? Whenever I've done similar chemistry, I've kept the temperature much loser to dry ice temps. Noted that you are quenching the reaction prior to spot, so that's less of a problem. What kind of plates are you using - can you try silica or alumina?

S