Handbook of Functionalized Organometallics Applications in S

14.4 Caveats and Cautions
group directs x [127]. The acetoxy group in 55 directs a for internal addition. The
same result was obtained with a benzoyl group. Even in the cyclohexadienyl series,
an SO2Ph-directing group at C-1has been shown to send some nucleophiles
to the a position, though this is a side reaction compared to the normal x addition
[122]. The normally x-directing OAc group (see Section 14.3.4), again gives the a
adduct when the x position carries an Me group [73]. The same is seen in the
+
Fe(CO) 2PPh3 series with benzoyl substituents at C-1, but strangely the disubstituted
case with the extra Me group now shows competing a and x pathways [73].
Ph
OAc
+
Fe(CO)3
54
55
+
Fe(CO) 3
Scheme 14.21
KCN
H 2O, CH 3CN, 0 ºC, 30 min, 84%
MeLi
14.4
Caveats and Cautions
Ref. 6
CH 2Cl 2, -78 - 10 ºC, 61%
Ref. 73
Ph
Me
Fe(CO) 3
CN
+
-
OAc
+
9 : 2
Fe(CO)3
( +
-)
In order to keep within the length limits for a chapter of this type, the examples of
directing effects presented above are illustrated by the most clear-cut cases. The
intention has been to focus on relatively simple directing groups, and examine
the patterns of their effects when present at different positions in the ligand, and
in ligands of differing hapticities. The preliminary conclusions form part of a
much longer and more fully referenced survey that is already in preparation [23].
In this chapter, representative examples have been chosen to illustrate these
effects, which usually will need to have been apparent in a wide selection of examples
to merit inclusion, or be an additional analogous example of a type of substituent
for which the effects are well established in the typical case. A few examples
are presented in this chapter where substituents show varying effects (aryl groups
stand out as a clearest case), and this can often be rationalized by differences in
steric effects in different orientations. Indeed, the groups discussed here have
essentially been rather simply classified as electronically active, or steric, in their
main mode of action, and it is entirely accepted that a more detailed discussion
( )
NC
Ph
595
Fe(CO) 3
( +
-)