Chemistry Reference
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Finally, the thioether analog of the chelated complex
53a
was shown to undergo inter-
molecular PKRs with the same stereochemical sense of induction but with somewhat lower
yields and diastereoselectivities (Scheme 5.45).
46
Me
Me
Me
Me
H
O
H
S
H
S
CH
2
Cl
2,
-10
°
C, 22 h
S
Me
Me
Co
Co(CO)
3
S
OC
CO
65%, 19:1 dr
Scheme 5.45
5.3.3 Alkynyl Sulfoxides
The PKR of the dicobalt hexacarbonyl complexes of acetylenic sulfoxides with strained
olefins was reported by Pericas, Riera, and co-workers in 1999.
47
Best results were afforded
by the complex
61
, that gave a mixture of the two regioisomeric adducts
62
and
63
(Scheme 5.46). While
62
was formed with no diastereoselectivity,
63
was obtained as a
4.6:1 diastereomeric mixture, and the relative configuration of the major isomer (isolated
by column chromatography) could be determined. However, when the major diastereomer
of
63
was transformed into the dextrorotatory tricyclic enone
64
, it was found that its
enantiomeric excess was only 75%. A polarimetric study of the enantiomeric purity of
61
showed that this compound is configurationally stable and racemizes at an appreciable rate
even at room temperature (k
2.8 10
−
6
s
−
1
at 25
◦
C). This unexpected low temperature
racemization, completely unprecedented for a non-allylic sulfoxide, seriously limits the
application of acetylenic sulfoxides in asymmetric PKRs.
=
62
(15%, 1:1 dr)
63
(31%, 4.6:1 dr)
O
O
H
O
Me
Bu-
n
O
S
p
-Tol
S
+
n
-Bu
Co(CO)
3
NMO, CH
2
Cl
2
, 0
°
C
(OC)
3
Co
H
n
-Bu
O
p
-Tol
S
(Major)
61
Ni-Raney,
EtOH, rt
O
H
(+)-
64
(56%, 75% ee)
n
-Bu
H
Scheme 5.46
