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

The main Groups IV and V
A convenient method of preparing trialkyl and triaryl phosphines in the
laboratory is to use organolithium or Grignard reagents. Mixed phosphines R'PR 2
and R~PR can also be made in this way, starting from R'PCl 2 or R~PCl instead of
from PClr
PC!, + 3RMgCI ---+ R,P + 3MgCI 2
Another approach, much used before the introduction of Grignard reagents
which still finds application in industry on account of convenience and cost
(p. 138) is the mixed Wurtz reaction.
6Na
PC!,+ 3ArCI------> PAr,+ 6NaCI
reflux/benzene
If triphenylphosphite P(OPh) 3 is used instead of PCl,. excellent yields of both
trialkyl and triarylphosphines result by this method using petroleum as a solvent.
Phosphines R"PH 3 _" (n = 0-2) are converted into phosphides by alkali metals in
liquid ammonia or in ethers. Treatment of these with alkyl halides provides an
efficient method of forming P-C bonds. Unsymmetrical phosphines, which are
usefulligands in transition metal chemistry, can be made in this way.
LiAlH• Na 2R'X
RPCI 2 ------> RPH 2
liq. NH 1
RPNa 2 ------> RPR;
2Na 2Mel Na CICH2CH2CI
PH, -----> PHNa 2 ---> PHMe 2 -----> Me 2 PNa Me 2 PCH 2 CH 2 PMe 2
liq. NH 3 liq. NH 3
Partial substitution of chlorine by alkyl is more cleanly carried out using less
reactive organometallics than Grignards. Cadmium, mercury, tin and lead
compounds have all been used e.g.
-20°C
R 2 Cd + 2PCI, -----> 2RPCI 2 + CdCI 2
ether
The Friedel-Crafts reaction using PC!, and a stoichiometric quantity of AICI,
affords aryldichlorophosphines. The product forms a complex with the aluminium
chloride from which it has to be displaced by a stronger Lewis base such as
POCl 3 or pyridine.
C 6 H 6 +PC!,
AICI 3 POCI 3
2-8h,reflux
PhPCI 2 .AICI, + HCI -----> PhPCI 2 + Cl,Al.OPCI,
Trialkyl- and triarylphosphines have pyramidal structures, in which the CPC
angles are about 100°. By virtue of the lone pair of electrons on phosphorus they
can act as proton acceptors (Brpnsted bases) and as electron pair donors (Lewis
bases ). Some pK" values for phosphonium ions and the corresponding ammonium
ions are given in Table 4.3. A strong regular variation in base strength is found
for the series PH, < MePH 2 < Me 2 PH < Me 3 P. This contrasts with the irregular
variati an found for the ammonium ions (p. 7 3 ). While phosphine is an extremely
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