I am having problems with a reaction that I have run many times previously. It is similar to a transformation that I posted about last March. Starting with (tert
P(O)H, I first added LiN(TMS), then added chlorotrimethylsilane, then I took a P-31 NMR and observed a major product near 132 ppm, suggesting the formation of (tert
POTMS as the major product, with a small amount of starting material. Then I added 2 equivalents of para
-formaldehyde, and stirred for three hours, then overnight at 47 °C. Usually the reaction is done in three hours. However, even overnight, all I am seeing is apparent partial reversion of (tert
POTMS back to the starting material (the H-phosphonate), plus two small peaks between 10-20 ppm, one of which may be the desired compound. I would estimate that slightly more than half of the phosphite remains.
One difference is that I used new para-formaldehyde (PFA), and it was more granular, as opposed to powdery. Possibly the sluggishness of the reaction is a result of the particles of PFA being too large. I ran a related reaction at Dan's suggesting several months ago with the same bottle of PFA, and it was slower than expected, based on the literature. BTW, I realize that the method I used is longer and clunkier compared to the paper Dan suggested, but having run this reaction successfully in the past, I thought it was best to try it again. I was also concerned about the difference in R groups in the ester: today I am working with tert-butylesters versus benzyl esters several months ago.
What are some of the things that might have gone wrong? I was thinking of tossing in some powdered PFA now, and seeing if I can convert the rest of my material to the desired product. That way I might get some useful product, at least. However, I also need to think about next time. Link to related thread: http://www.chemicalforums.com/index.php?topic=66416.0