Handbook of Functionalized Organometallics Applications in S

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14 Polyfunctional Electrophilic Multihapto-Organometallics for Organic Synthesis
MeO
O
Scheme 14.1
+
Fe(CO) 2Cp
1
+
Fe(CO) 2Cp
2
-
O
THF, -78 ºC, 1 h, 90%
Me3SiO
Li +
Ref. 9
CH 3CN, 0 ºC, 1 h, >58%
Ref. 10
O
MeO
O
O
Fe(CO) 2Cp
Fe(CO) 2Cp
at the same end of the of the alkene [16], ipso addition is still straightforward, as it
is assisted by the release of steric strain when FeCO) 2Cp moves to become r
bonded to the unsubstituted end (see Table 14.1 in Section 14.5: entry 10).
14.3.2
Electrophilic g 3 Complexes
Turning to the g 3 case, the terminal ester substituent in a neutral dimeric stoichiometric
palladium complex [17], provides an early example of x direction of nucleophile
addition. An A ring ketone [18] has a similar x-directing influence in a steroid
ring system. As in Section 14.3.1 (concerning Wacker oxidation), parallels can
be seen with regiocontrol in palladium-catalyzed processes, as cationic g 3 palladium
intermediates in allylic substitution reactions show similar control effects
[7,8,19,20]. To save space, however, in this chapter attention will focus on elucidating
control effects in the stoichiometric examples, though provided care is taken
over the possibility of alternative mechanisms (e.g., the ªmemory effectº [21] and
addition via the metal [22]) the conclusions hold in the interpretation of regiocontrol
in catalytic examples as well [23].
+
The Fe(CO) 4 allyl complexes provide the most widely studied class of stoichiometric
allyl complexes. The methyl substituted examples [24,25] give mixtures of
products with the preferential site of addition at the less-hindered end of the
p-system. With two methyl groups at the same end, the x-directing effect is quite
strong (with (MeO2C)(MeCO)CH ± as the nucleophile, the i : x ratio is 1: 4 [26])
and in some cases gives complete regiocontrol. With 3, the nucleophile is even
directed in to a substituted position [27]. Similar steric-based effects are seen with
functionalized alkyl substituents [28±32] and trimethylsilyl directing groups [33].
Results with electronically interacting substituents such as acyl [34], ester [35]
and SO2Ph groups [36] again illustrate that electron-withdrawing substituents
direct to the x end of the p-system. For example, enolate addition to 4 affords 5
after oxidative removal of the metal (see Scheme 14.2). The x-directing effects of