_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The main group elements
For R = alkyl, chlorides are used whenever possible. They are cheaper than the
bromides and iodides and although they react more slowly with magnesium, side
reactions such as Wurtz coupling or olefin elimination are not so serious. Yields of
Grignard reagents from primary alkyl chlorides or bromides are usually better
than 80%. Secondary and tertiary halides, especially bromides and iodides, are
not so satisfactory. With tert-butyl bromide, for example, much butene is evolved
by elimination and only low yields of tert-butylmagnesium bromide are obtained.
On the hasis of many unsuccessful attempts, it was once thought that Grignard
reagents could not be prepared from alkyl fluorides. In 19 71 it was found that in
the presence of a small quantity of iodine (ca. 4%) in tetrahydrofuran, good yields
of RMgF (R = e.g. Me, C 6 H 13 ) could sometimes be obtained. Tetrahydrofuran
coordinates to magnesium better than the usual solvent, diethyl ether. Aryl
fluorides, however, still do not react under these conditions.
A useful method for producing very finely divided, reactive metals has been
devised by Rieke. An anhydrous metal halide is reduced by potassium in boiling
tetrahydrofuran. The black slurry of magnesium thus prepared gives good yields
of Grignard reagents with alkyl- and even with aryl fluorides. Zinc and cadmium
alkyls can also be made in this way from alkyl bromides and iodides. Active metals
can also be made by the 'metal atom' technique, of which a fuller description is
given on p. 312. In o ne such experiment magnesium was heated under high
vacuum and the vapour cocondensed with ethyl bromide at -196°C. On
warming to room temperature reaction occurred to give unsolvated EtMgBr. In
organometallic chemistry the method has most usefully been applied to the
synthesis of endothermic complexes of transition elements, some of which are not
accessible by other routes.
The formation of organometallic compounds from bulk metals and organic
halides can sometimes be aided by ultrasonic irradiation. The human ear is
sensitive to frequencies between 16 Hz and 16 kHz. Beyond 16 kHz lies the region
of ultrasound. The upper practicallimit is about 1 MHz, 20-50 kHz being most
commonly used. Ultrasonic radiation probably acts by scrubbing the surface of
the metal, so that a clean metal-solvent interface is maintained. Zinc normally
requires activation as a zinc-copper couple, for example, before it will enter into
reaction with organic halides. The Reformatsky reaction between ethyl
bromoacetate and aldehydes or ketones in the presence of activated zinc (p. 59)
normally requires heating to 80°C for severa! hours. Under sonic irradiation it is
essentially complete within 5-30 minutes at room tempera ture. Organozinc
reagents can be prepared simply by irradiating haloalkanes, lithium and zinc
bromide for ten minutes in a mixture of toluene and tetrahydrofuran. Similar
acceleration has been observed in the formation of organolithium, magnesium
and copper compounds.
Aluminium reacts with a limited number or aryl and alkyl halides, forming
R 3 Al 2 X 3 • These reactions are considered in Chapter 3.
When volatile organic halides are passed over silicon, mixed with a metal such
as copper and heated at 250-400°C, organosilicon compounds, notably R 2 SiCl 2 ,
18