Handbook of Functionalized Organometallics Applications in S

586
14 Polyfunctional Electrophilic Multihapto-Organometallics for Organic Synthesis
1,3-dimethoxy case gives clear b control [149]. The OMe group in (g 6 -anisole)Cr(CO)
3 complex behaves similarly [150±155], as does an OPh donor substitutent
[156]. Since the product of the initial nucleophile addition step is now an
anion, it is typically converted into a further product by electrophile addition [153]
or oxidation [154]. Conversion (Scheme 14.14) of 32 into a substituted arene demonstrates
the b-directing effect of the OMe group [154]. An x-directing effect from
t
a bulky TIPS silyl ether [156], and a large (SO2 Bu) electron-withdrawing group
[157] have been reported.
OMe
+
Mn(CO)3
LiAlH4
THF, -78 ºC, 90%
Me 30
Ref. 149
Si(CH2CH2)3N
Scheme 14.13
+
Mn(CO) 3
31
MeMgBr
CH 2Cl 2, 0 ºC, 30 min, 72%
Ref. 147
OMe
Me
Mn(CO) 3
Si(CH2CH2)3N
Me
Mn(CO) 3
( +
-)
Oxazolines [158±161], imines (e.g., 33 [159], Scheme 14.14) [159±161] and
hydrazones [162] have been used to promote an a addition pathway, an effect
ascribed to precoordination of the incoming organolithium by the side-chain heteroatom.
Similar a addition can be achieved with heteroatom donor substituents
by careful control of conditions and choice of nucleophile [163±165] overcoming
the natural b control of the donor group.
In examples where the arene carries a leaving group, metal-promoted S NAr
reactions are observed, though in some cases, mechanisms can be complex
(Scheme 14.15) [145]. The formation of the 1,2-disubstituted product 36 from the
1,4-disubstututed complex 35 is best explained by addition b (meta) to the fluorine
(a to the methyl group), followed by rearrangement of the position of hapticity
in the g 5 intermediates to put the leaving group at an sp 3 center. In this case,
rapid loss of fluoride reforms the g 6 -arene ligand. This process is referred to
as a tele-meta-S NAr reaction [166]. The formation of 38 from 37 provides an example
of a cine-S NAr mechanism [167], in which the initial site of nucleophile addition
is adjacent to the leaving group. With a 1,3 disubstitution pattern, the product
corresponds to addition at the less hindered of the two possible cine-S NAr sites
[168]. A rarer case is tele-para-S NAr [169], which occurs when the cine-S NAr and
tele-meta-S NAr pathways are obstructed by substituents flanking the leaving group,
as in 39.