Wednesday, 9 February 2011

Starting Material

Today, the day that all chemists secretly dread finally came for me. To make any of the catalysts I have used thus far involves a three step synthesis to form a precursor which can then be turned into any number of different compounds. Yesterday, I ran out of said precursor. Thus, the time has come to make some more.

I normally start with 10 grams of starting material but decided to be slightly more ambitious yesterday and went with 20. The first step, a triflation, gives perfect colourless crystals in anywhere from 90-97% yield. While this is nice of course, the work-up of said reaction involves absorbing the crude compound onto silica gel and then passing the lot through a short pad of more silica eluting with petrol. Now, this takes long enough (around 3 litres of petrol if you use a wide sinter and not too much silica). However, we do not have many large sinters in the lab and potentially 38 g of product along with the rest of the reagents do not absorb particularly well onto silica in the first place.

Five litres of petrol and about 3 hours on the rotary evaporator later, product was still flying off the silica at a rate of knots. I made the rash decision to start recycling petrol from the Buchi trap and soldiered on. I spent a grand total of 6 hours on this today before I finally gave up and went home for the night. Tomorrow I have the enviable task of getting the last few grams of product out of my sinter full of silica and then take the lot and put it all on for a Kumada coupling. The same coupling in fact from the first Lab Mishaps post I made a couple of weeks ago. This is definitely shaping up to be a moderately productive, if slightly boring week. Oh well, I should end up with 10 grams or so of my desired compound by the end of the week, more if I can face the prospect of a particularly slow and painful column on Friday afternoon. Chances are the answer to that question will be a resounding no.

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