Oxidative metalation of lithium with neopentyl chloride
SyntheticPage 120
DOI:
Submitted: August 22, 2001, published: August 22, 2001
Authors
Colin Morton (c.morton@warwick.ac.uk)
A contribution from
Chemicals
Pentane (distilled from NaK2.2)
Neopentyl Lithium (Aldrich)
Lithium wire, (3.2 mm, 0.5-1.0 % sodium, in mineral oil, Aldrich)
Neopentyl Lithium (Aldrich)
Lithium wire, (3.2 mm, 0.5-1.0 % sodium, in mineral oil, Aldrich)
Procedure
A thick walled 200 ml ampoule charged with neopentyl chloride (5 g, 5.8 ml, 47 mmol) was degassed and back-filled by cycling on a schlenk line 3 times. Pentane (60 ml) was then added via cannula. To the resulting solution was then added lithium wire (2 g, excess), the vessel was evacuated and refluxed (at 50oC) for 10 days in vacuo. After cooling, the cloudy purple solution was filtered and the resulting clear yellow solution was concentrated in vacuo. Standing overnight at -30oC afforded colourless crystals. The reaction residual was then washed with a further amount of pentane (60 ml) and crystallised in a similar fashion to the initial crop. Total yield = 2.6 g, 71 %.
Author Comments
All operations were carried out using standard schlenk line procedures. It is worth noting that heating under vacuum where there is an evolution of gas (not in this case) should not be carried out in a closed system but heated under argon with a bubbler. The idea in this prep is that if you evacuate the tube then heat to something like the normal boiling point the pressure in the ampoule should be about 1 atm. We take the precautions of checking the ampoule for stress beforehand and heating behind a blast shield. At the end of the reaction, after cooling we find that the ampoule is still under partial vacuum. You should do a full risk assessment nevertheless. Care should be taken when handling lithium alkyls as they are pyrophoric. When filtering the reaction solution it is worth allowing it to settle out for as long as possible as this will prevent the filtration device from blocking.
The residual lithium and supernatant can be destroyed by adding toluene, blowing over a gentle stream of nitrogen and gradually adding an alcohol/toluene solution, eg IPA/tol (25:75). Again, you should do a full risk assessment of this procedure.
Data
1H NMR (d6-benzene, 293 K) 1.13 (s, 2H, CH2), -0.67 (s, 9H, tbu)
Lead Reference
R. R. Schrock et al, J. Am. Chem. Soc., 100, 1978, 3359
Keywords
Comments
The filtration step of this reaction has proven to be the most difficult/dangerous. We have tried this using a frit, and with filter cannulae, neither was particularly satisfactory. Using a frit, there is a danger of lithium neopentyl building up under the frit, whereas filter cannulae block easily, requiring several to complete the filtration. Upon removal from the solution, the cannulae ignite spontaneously - obviously not satisfactory. If you have access to a "wet" inert-atmosphere glovebox [we don't], I would probably recommend undertaking the filtration step in there, bringing out the waste under argon, and neutralizing carefully outside.
By Max Hammond on July 24, 2006