Wednesday, March 31, 2010

Catalytic Hydrogenation Part III - More Tips and Tricks

I do apologies for the very infrequent posting. I seem to have developed a life with other areas of interest than chemistry. However, due to the many old posts that people find useful the blog gets around 150 unique visitors every day (Including Nobel Laureates!!! Can you guess who?) so I'll keep Curly Arrow running at a gentle simmer.
Returning to catalytic hydrogenation, as promised:
Mechanistic considerations: What is the mechanism for catalytic hydrogenation. I am really not up to date on what people have figured out but about 15 years ago no one was really sure. There is good evidence that the reaction proceeds by a stepwise radical mechanism. So if you are reducing a double bond you add a H-dot and end up with an adjacent radical. This can be helpful since the dot could be ending up at a carbon where it is "stabilised". As a result you may have rotation around bonds before the next H-dot makes an appearance. Also you could end up with selectivity issues that are worth considering. For example, when I try to peel a benzyl group of an alcohol will it come off as toluene (good) or benzyl alcohol (bad)?
I recently, suppressed an unwanted side reaction by selecting hydrogenation condition that would suppress formation of an unwanted radical and promote the debenzylation of an alcohol that I was interested in so these things are (of course) worth thinking about.
How do I work a catalytic hydrogenation up?
Activated charcoal sticks to all surfaces. Your stir bar is a mess, the flask is a mess etc. To minimise the problem tip some Celite into the reaction and leave it to stir. This rather effectively mops up almost all the catalyst. Meanwhile pack a sintered funnel with a firm pad of Celite. Put a filter paper on top and suck some solvent through to check that it is packed properly. You do not want catalyst in your sinter! It will go black and nasty and stay that way. Pour the reaction mixture on top of the celite pad and suck the solvent through the Celite. Wash a couple of times with a polar solvent (and ideally hot). Compounds tend to stick to the catalyst so this is an attempt at getting it all off.
Concentrate your solvent in vacuo and check how much you have before binning the catalyst. I have done fairly large scale reductions and ended up with nothing after filtering because my product wouldn't let go of the catalyst. In one case I had to reflux the catalyst in DMF and filter the boiling solution to get my compound.
Finally, when you do have your product don't just throw the catalyst waste in the bin. The stuff tends to get really hot and catch fire. In the perfect lab you have a plastic bin only for your used hydrogenation catalyst where you keep all the waste nicely soaked in water to prevent it going off.
A note on Celite. As some of you may have noticed Celite comes in many, many different varieties. Catalytic hydrogenation is potentially an extremely clean process. I have used this as the very last step in the synthesis of unnatural amino acids and after filtration the product really is completely pure as it is. This is handy since it is incredibly polar at this stage and I really don't enjoy preparative RP HPLC. However, be careful with Celite. Make sure you get some good quality stuff that doesn't partly dissolve in organic solvents. I used some stuff in Australia that was slightly soluble in methanol. Moreover, I recently discovered that Aldrich Celite 545 is weakly basic and will dissolve in acetic acid giving you a ridiculous crude yield after filtering and concentrating. A student of mine even managed to isolate a metal salt of her carboxylic acid product because she concentrated it on Celite 545 prior to running a column. So check the specs for your Celite before you tip it into your valuable product.
Catalysts - what is available and what should I use? Books have been written about this so I regret promising to comment on the topic. However, it is worth remembering that a huge variety of catalysts are available that may save the day. Poisoned catalyst are worthy of mention. For example, you can get sulfided Pd-black. This stuff is sometimes useful when working with sulfur containing compounds that poison the regular of the shelf catalysts. Also there is classic stuff such as Lindlar's catalyst. Lindlar's catalyst is Pd poisoned with lead, for example, allowing the reduction of alkynes to cis-alkenes.
Deuterium! Finally, I should remind you that you can get deuterium (and tritium) gas. Deuterium of high purity is reasonably affordable giving you a simple way to isotope label compounds often using exactly the same reaction conditions as employed for hydrogenation. However, where possible I would recommend using aprotic solvents. We have observed some hydrogen-scrambling when performing deuteration in methanol and ethanol. D!