Wednesday, December 1, 2010

Rochelle's salt

When using aluminum reagents such as LAH, DIBAL, or AlCl3, it's common to have problems with emulsions in the workup. The simplest and most straightforward solutions to garner homogeneous solutions is to use aqueous mineral acids, but that's not always an option. If, like my last several projects, the substrates are alkaloids you can lose product to the aqueous layer, and acid labile groups can be chipped off. There are a number of ways around this, and one of the most popular and effective non-acid workups is aqueous Rochelle's salt.

Those of you that have used Rochelle's salt in work ups know that sometimes it works a treat, and other not too well. lately I noticed something in my experiments, and wanted to see if you guys have had similar experiences or knew of proper documentation.

When I would work up an AlCl3 reaction, sometimes it clarified, and sometimes not. The Ph went up to almost neutral from the tartrate, but not strongly basic (Ph 8-10) like I would prefer for best recovery of my amine. After some poorer than expected yields I added saturated aqueous ammonia (ammonia water, NH4OH) to the stirring workup mixtures. Typically the layers would split immediately, and my recoveries went way up.

It was both surprising and pleasantly welcome.