Handbook of Functionalized Organometallics Applications in S

14.3 Unsymmetrically Placed Substituents in Stoichiometric Electrophilic Multihapto-Complexes
timate atom (the final position is already defined as x). A complete set of examples
to illustrate these definitions is presented in Fig. 14.2 for open/acyclic ligands,
and the closed/cyclic alternatives.
Branched ligand geometries (e.g., trimethylenemethane g 4 ligands; see Section
14.3.6), and cyclic complexes with exocyclic extensions to the p-system (e.g., fulvalene
g 6 ligands) require more careful consideration (Fig. 14.3) as it is possible to
encounter (for example) two regiochemically distinct x positions. Fortunately, the
standard syn/anti method for describing substitution patterns on multihaptoligands
provides a simple means to cope with these structures. The x s (x syn) position
is the x atom of the portion of the haptyl ligand that is displayed in the syn
direction (spatially near to the reference substituent). Similarly, the x a (x anti) position
is the x atom displayed in the anti direction (spacially remote from the reference
substituent). Corresponding procedures define a s (a syn), b s (b syn), a a (a anti),
and b a (b anti) positions. The far ends of branched ligands are both termed x (i.e. x s
and x a and with exocyclic structures, the furthest atom on the portion and the
atom at the end of the exocyclic extension are x positions.
14.3
Unsymmetrically Placed Substituents in Stoichiometric Electrophilic
Multihapto-Complexes
14.3.1
Electrophilic g 2 Complexes
The most common example to illustrate the g 2 case is the classic Fe(CO) 2Cp + series
of complexes. These are electrophilic at the open g 2 alkene ligand, not the
closed g 5 cyclopentadienyl ligand. With a donor substituent, for example the OMe
group in complex 1 [9], nucleophiles add to the ipso position (see Scheme 14.1).
Electron-withdrawing groups, such as the acyl group in 2 [10], have the opposite
influence, and direct nucleophiles x. The same control effect has been identified
in the analogous Ru(CO) 2Cp + complex [11]. When donor and acceptor substituents
are placed at the same end of the alkene [12], they are opposed to each other,
and the effect of the OEt group dominates. When it comes to assessing steric
effects, the g 2 case is a special case, because the product has a r bond, and after
nucleophile addition, the metal is attached tightly to just one carbon. There is a
common tendency for the bulky metal complex to dominate the control effects
and move to the unsubstituted carbon atom to release steric strain, as can be seen
for example in regiocontrol effects of the palladium-catalyzed Wacker oxidation of
terminal alkenes, which proceeds by nucleophilic addition of water to the alkene
at the substituted end [13], and can be seen in other palladium-mediated nucleophile
additions to alkenes [14]. A recent example in the Fe(CO) 2Cp + series involves
the addition of isopropoxide, which although proceeding in low yield, and under
conditions that are potentially reversible (see Section 14.4), is reported to give single
regioisomers of ipso addition products [15]. With donor and alkyl substituents
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