musings on music and life

April 7, 2013

lithations vs. grignard reactions

Filed under: Chemistry — Tags: — sankirnam @ 12:04 pm

I don’t know how common this is, but when I need to do nucleophilic alkylations or arylations I find myself reaching for the butyllithium instead of the magnesium. Even though Grignard reactions are more common and safer, they are also more tedious to carry out. It is easier to do a reaction where you just need one piece of glassware (in the case of a reaction with butyllithium, a schlenk flask), as opposed to a Grignard reaction, which requires a three-neck flask, reflux condensor, addition funnel, oil bath, and dewar for cooling.

Since Sheri Sangji’s death in December 2008, there has been a lot of negative hype surrounding the use of butyllithium (both n- and t-) in organic chemistry. This is, in my opinion, completely unfounded. They are both extremely valuable reagents in the laboratory, and can be used as strong bases on their own or for effecting lithium-halogen exchange. Lately, I have been using both these reagents almost daily for the latter reaction, and have not had any accidents at all. I remember one of the senior graduate students in my lab musing in January 2009 that “thousands of people worldwide use butyllithium everyday, and they don’t meet with accidents. This is just an outlier.” And that is probably the case. Of course, the drawback with the use of butyllithium is the safety factor, as mentioned above, as well as the inevitable degredation of solutions upon long-term storage, which requires titration every now and then. However, this is a small price to pay for being able to do a reaction in one pot, or “bucket chemistry” as some people call it.

Going back to the original topic, there are some substrates for which Grignard reactions are a poor choice, and the use of butyllithium is superior. For example, lately I have been doing a lot of work on aromatic -SF5 chemistry, and using 4-bromophenylsulfurpentafluoride as a building block. The metalation of this substrate is especially difficult with magnesium, as it requires a special entrainment method using CH3I according to the literature. However, lithium-bromide exchange proceeds readily with t-butyllithium at -78 C. Another example I found the hard way is 4-bromo-anisole. The preparation of p-anisylmagnesium bromide, although trivial on paper, is difficult in practice, as it requires long reflux times and highly activated magnesium (e.g. Rieke Mg). Lithium-bromide exchange with this substrate is again very trivial when t-butyllithium is employed at -78 C.

Also, the use of cerium in organic chemistry deserves some mention here. CeCl3 is a useful additive in nucleophilic reactions with easily enolizable ketones, as it suppresses the basicity of the nucleophile and increases it’s nucleophilicity (I don’t know off the top of my head if this actually been quantified by Herbert Mayr). There are very few ketones, however, for which it is actually required. In practice, a lot of ketones with α-protons are not quite as enolizable as one may think. Acetophenone works like a champ in my hands, as do 1-indanone and α-tetralone. β-tetralone, on the other hand, enolizes very easily, and all the reactions I have conducted with it without CeCl3 have resulted in poor yields (usually around 15%). Imamoto’s papers serve as a useful guide for this chemistry.

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2 Comments »

  1. What is the percentage yield between lithations vs grignard reaction in the chemistry that you do?

    Comment by Chairman Woof — April 15, 2013 @ 5:11 pm

    • They are about the same (within experimental error), which is why it is just better to go for the easier technique each time.

      Comment by sankirnam — April 16, 2013 @ 9:33 am


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