Page 791 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Scheme 8.16. (Continued) 767

a. S. Kurosawa, M. Bando, and K. Mori, Eur. J. Org. Chem., 4395 (2001). SECTION 8.5
b. J. M. Mörgenthaler and D. Spitzner, Tetrahedron Lett., 45, 1171 (2004).
c. Y. K. Reddy and J. R. Falck, Org. Lett., 4, 969 (2002). Organometallic
d. J.-G. Boiteau, P. Van de Weghe, and J. Eustache, Org. Lett., 3, 2737 (2001). Compounds with
e. A. Furstner, K. Radkowski, C. Wirtz, R. Goddard, C. W. Lehmann, and R. Mynott, J. Am. Chem. Soc., 124, -Bonding
7061 (2002).
f. I. M. Fellows, D. E. Kaelin, Jr., and S. F. Martin, J. Am. Chem. Soc., 122, 10781 (2000).
g. Y. Matsuya, T. Kawaguchi, and H. Nemoto, Org. Lett., 5, 2939 (2003).
h. Z. Yang, Y. He, D. Vourloumis, H. Vallberg, and K. C. Nicolaou, Angew. Chem. Int. Ed. Engl., 36, 166 (1997).

8.5. Organometallic Compounds with -Bonding

The organometallic reactions discussed in the previous sections in most cases
involved intermediates carbon-metal with bonds, although examples of bonding
with alkenes and allyl groups were also encountered. The reactions emphasized in
this section involve compounds in which organic groups are bound to the metal
through delocalized systems. Among the classes of organic compounds that can
serve as ligands are alkenes, allyl groups, dienes, the cyclopentadienide anion,
and aromatic compounds. There are many such compounds, and we illustrate only a
few examples. The bonding of polyenes in complexes is the result of two major
contributions. The filled orbital acts as an electron donor to empty d orbitals of
the metal ion. There is also a contribution to bonding, called “back bonding,” from a
∗
filled metal orbital interacting with ligand orbitals. These two types of bonding are
illustrated in Figure 8.6. These same general bonding concepts apply to all the other
organometallics. The details of structure and reactivity of the individual compound
depend on such factors as: (a) the number of electrons that can be accommodated by
the metal; (b) the oxidation level of the metal; and (c) the electronic character of other
ligands on the metal.
Alkene-metal complexes are usually prepared by a process by which some
other ligand is dissociated from the metal. Both thermal and photochemical reactions
are used.

CH 2
RCH Cl Cl
Pd Pd
(C 6 H 5 CN) 2 PdCl 2 + 2 RCH CH 2 Cl Cl CHR
CH 2
Ref. 309

C C
C
C

Fig. 8.6. Representation of
bonding in a alkene-metal
cation complex.

309
M. S. Kharasch, R. C. Seyler, and F. R. Mayo, J. Am. Chem. Soc., 60, 882 (1938).