A Route to Carbasugar Analogues - Jonathan Clayden - The ...

1.4 – Metal complexation
has a very slow rate of dissociation. A summary of the preferred regioselectivity of
different nucleophiles and aryl substituents is given in a recent review by Kündig. 51
Electron withdrawing groups on the arene can reduce the backwards reaction,
oxazolines and hydrazines are often use since they also tend to direct attack towards
the ortho-position. Below is presented a overview of the possible routes dearomatising
additions might take; the use of these groups in asymmetric synthesis is discussed in
the next section.
R
NuLi
Nu
R
EX
Nu
R
Cr(CO) 3
Cr(CO) 3
75 76
Cr(CO) 3
E
L = CO, MeCN, PhCN
+L
E = Bn, allyl, propargyl
endo migration
R
CO insertion
Nu
R
Nu
E
+ Cr(CO) 2 L 4
E
+ Cr(CO) 2 L 4
Cr(CO) 2 L
E
Cr(CO) 3 L
E
77 O
endo migration L
[O]
Nu
R
Nu
R
E
Cr(CO) 2 L 2
E + Cr(CO) 3 L
O
80
[O]
H 2 (5 bar)
R = H, Nu = dithiane
Nu
R
Nu
O
78
O
79
Scheme 1.22 – dearomatisation strategies of (η -arene)Cr(CO) complexes
6
3
42