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

Transition metal chemistry
Phosphines in which the seat of optical activity is in the attached organic
groups rather than at phosphorus ha ve also been used in asymmetric hydrogenation.
They are easier to synthesize but the optical purity of the hydrogenated
products is usually not so high.
(e) DEHYDROGENATION OF ALKANES. Can an alkane be dehydrogenated
catalytically under mild homogeneous conditions? There are several reasons why
this does not normally occur. Alkanes are poor ligands which cannot compete
with better ligands such as Ph 3 P which are present. Anyway, the chelate effect
favours orthometallation ofsuch ligands (p. 223) in preference to the addition of
alkane. Moreover the reaction
C,H.(g)~C,H.(g) + H,(g)
..1H 9 = l37kJmol-'; ..1S 9 = 121 JK-'mol-'
is thermodynamically unfavourable below about 8 50°C. In principle, however, it
can be brought about by adding a hydrogen acceptor such as another alkene.
Unfortunately this usually coordinates to the metal in preference to an alkane.
Sometimes the alkene itself is dehydrogenated.
Crabtree has used 3, 3-dimethylbutene as an acceptor in conjunction with the
cationic iridium complexes [IrH 2 S 2 L 2 ]+sp_; (S = e.g. acetone, CH 2 Cl 2 ; L = Ph 3 P).
The bulky tert-butyl group in the alkene makes it a poor ligand. Orthometallation
of triphenylphosphine does seem to occur, but it is reversible. Stoichiometric
reaction of cycloalkanes is observed on heating in dichloromethane. Unlike some
other compounds which activate alkanes (p. 224), these complexes are not
electron rich. They are cationic and do not contain strongly donating ligands.
+
o
+
o
/X +
+
~
.lr +
L/ ~"-H
L
[1rH2S2L2t--- /X + ~,~,,
Felkin has shown that (Ph 3 P) 2 ReH 7 behaves similarly. Some reactions are
catalytic, but with low turnover numbers.
5.4 References
Albers, M.O. and Coville, N.J. (1984) Reagent and catalyst induced substitution reactions
of metal carbonyls. Coord. Chem. Revs., 53, 22 7.
Atwood, J.D. (1983) Ligand effects on organometallic substitution reactions. Ace. Chem.
Res., 16. 350.
186