Handbook of Functionalized Organometallics Applications in S

11.2 Chromium-Templated Cycloaddition Reactions
to the cyclopropanation product 4 [12]. These reactions proceed under slightly
milder thermal conditions; the intermolecular versions often have to be performed
under CO pressure to avoid competing olefin metathesis [13].
The cyclopropanation of electron-poor olefins generally proceeds with low diastereoselectivity,
and often leads to equimolar mixtures of cis- and trans-isomers;
conjugated systems, either in the carbene ligand or the olefin, allow for better diastereoselectivities
(Scheme 11.2) [14].
OMe
(CO) 5Cr Ph
(CO)5Cr
OMe
+
+
CO 2Me
OTBS
80ºC
81%
60% de
25ºC
100 atm CO
49%
>90% de
Ph
MeO
OMe
CO 2Me
OTBS
Scheme 11.2 Diastereoselective cyclopropanation of electron-poor and electron-rich alkenes.
N R
Z
42-67%
24-54% de
M= Mo, W
NR I
N
2=
R= Ph, Me
Z= CO 2Me, CN
THF
100ºC
(CO)5M
NR I
2
R
+ +
Z R 2
toluene
111ºC
R I
2N R
R 2
30-70%
30->95% de
M= Cr
NR I
2=N(alkyl) 2
R= COOR 1
R 1 = Me, t-Bu
R 2 = alkyl, alkenyl, Ph
Scheme 11.3 Cyclopropanation with aminocarbene complexes.
453