Handbook of Functionalized Organometallics Applications in S

51
594
CO 2Me
+
Fe(CO)3
CO2Me
14 Polyfunctional Electrophilic Multihapto-Organometallics for Organic Synthesis
increase of the steric bulk of the saturated part of the ring in the cyclooctadienyl
case displaces nucleophile addition to the internal electrophilic center [233,234].
An unusual case of internal addition in the cyclohexadienyliron series has been
reported. An alkynyl substituent that normally directs x (see Section 14.3.4), gave
competing a addition when cyanide was used as the nucleophile (a : x ratio= 2 : 9) [6].
The main purpose of this chapter is to discuss the directing effects of functional
groups in these reactions. As one would expect, steric effects direct internal
nucleophile addition to the less hindered of the electrophilic centers as in the formation
of 49. With a C-1OMe substituent, a addition has been proposed [235].
Internal nucleophile addition to the less hindered of the electrophilic centers has
also been reported for 50 [236]. Other examples give mixtures [237]. On the other
hand, 1,4-dimethyl substitution on the g 5 cycloheptadienyl ligand shows regiocontrol
dominated by the C-1Me group (c selectivity; ipso to the C-4 Me group) [229].
Electron-withdrawing substituents seem to promote internal nucleophile addition
(Scheme 14.20). This is most clearly demonstrated in acyclic (g 5 -pentadie-
+
nyl)(FeCO) 3 complexes. The C-1ester group in 51 promotes a addition
[116,238,239], but when this is opposed by the steric effect of a C-2 methyl group,
besides the main x product, traces of c addition relative to the ester are observed
[240]. The extended example with E-CH=CHCO2Et is discussed in Section 14.3.7,
and gives the a and c products 52 and 53, as well as the simple x adducts [197].
With substituents at both ends of the p system, further examples of a addition
relative to the ester have been reported [241,242]. This result is particular to acetylide
and amide nucleophiles, as stabilized enolate and triphenylphosphine add x
to the ester/ipso to the Ph group (see Section 14.3.4). Competing internal addition
has also been observed (Scheme 14.21) with the alkynyl-substituted complex 54 [6],
and the acyclic C-1acetoxy-substituted complex 55 [73] and provides an interesting
comparison (see Section 14.3.4) with the cyclic analog where the acetoxy
+
Fe(CO) 3
Ref. 238
NaCH(CO 2Me) 2
THF, 0 ºC, 10 min, 40%
Scheme 14.20
LiCH(CO 2Me) 2
THF, 0 ºC, 1 h, 66%
Ref. 197
(MeO 2C) 2CH
(MeO 2C) 2CH
52
CO 2Me
Fe(CO) 3
CO2Me
Fe(CO) 3
+
9 : 1
+
4 : 3
CO 2Me
plus ω addition products
Fe(CO) 3
CH(CO 2Me) 2
CO2Me
Fe(CO)3
CH(CO 2Me) 2
53