Chemistry Reference
In-Depth Information
O
EtO
2
C
EtO
2
C
EtO
2
C
Ni(COD)
2
, SiPr
EtO
2
C
EtO
2
C
O
+
EtO
2
C
EtO
2
C
EtO
2
C
11.82
11.83
Scheme 11.28
CO
2
Et
Ph
CO
2
Et
Cp*Ru(COD)Cl
+
Ph
1
.
1
8
1
1
6
Scheme 11.29
11.2 Cycloadditions Involving Fewer than Six Atoms
11.2.1 Four-Membered Rings
2] cycloadditions catalysed by transition metals have been reported (Scheme 11.29).
49
Further
examples can be found in Schemes 11.72 and 11.73, and Scheme 6.107.
Several [2
+
11.2.2 Five-Membered Rings through TMMMethods
A cycloaddition to form a cyclopentane or a cyclopentene ring would be very valuable as a lower homo-
logue of the Diels-Alder reaction. The difficulty comes from the need for a suitable 1,3-dipole or diradical.
Trimethylenemethane (TMM)
11.87
is one such species, but it is highly reactive. It has been used in in-
tramolecular reactions, generated by photolysis of bicyclic azo compounds. Transition-metal complexes of
trimethylenemethane have been known for some time and can be easy to make, including the iron-tricarbonyl
complex (Section 10.1).
50
While it is relatively easy to make, trapping of the TMM after oxidative release
can be inefficient,
51
in contrast to the analogous chemistry employing the cyclobutadiene complexes.
11.87
A precursor
11.89
for a palladium-TMM system has been designed, using acetate as a leaving group to form
the cationic arm and silyl or stannyl groups to form the anionic arm (Scheme 11.30).
52
The precursors may
be formed in various ways. The most direct involves dilithiation of methallyl alcohol
11.88
and quenching
with trimethylsilyl chloride, followed by acetylation of the alcohol.
1. 2
n
-BuLi
2. Me
3
SiCl
3. Ac
2
O
OH
Me
3
Si
OAc
1
1
.
8
1
1
8
Scheme 11.30
