Handbook of Functionalized Organometallics Applications in S

11.3 Reactions of Higher Nuclearity Chromium and Tungsten Carbenes
11.3
Reactions of Higher Nuclearity Chromium and Tungsten Carbenes
Metal carbene centers may be connected via either the carbon or the heteroatom
carbene chain [47]. The first example of a homodimetallic biscarbene complex has
been synthesized by trapping a metal carbene anion ± generated in situ by a-deprotonation
of chromium oxacyclopentylidene 73 ± by its exo-methylene congener
74 formed upon the addition of the same chromium carbene anion to formaldehyde
in a Michael-type addition reaction [48]. A more satisfactory yield of bischromium
biscarbene 75 has been obtained by using the formaldehyde equivalent
ClCH 2OCH 3 as source of the C 1-bridge and LiI (Scheme 11.23).
(CO)5Cr
O
1) n BuLi
2) HCHO/H2O (CO)5Cr
73
MeOH
74
1) n BuLi, LiI
2) 0.5 eq. ClCH 2OMe
(CO)5Cr
O
O
75
+
(CO)5Cr
Cr(CO)5
Scheme 11.23 Synthesis of bimetallic biscarbenes via Michael addition.
This strategy can be extended to more functionalized metal carbenes as demonstrated
for psicosecarbene complexes 76 and 77. The chiral information in the carbohydrate
moieties allows for a pronounced diastereoselectivity for the biscarbene
complex formation: Single C 2-symmetrical (R,R)-diastereomers of mono- and bistungsten
complexes 79 and 80 are isolated from reactions in THF at ±78 C in
moderate yields whereas bischromium analog 78 is obtained as a 2:1 mixture of
(R,R)- and (R,S)-diastereomers (Scheme 11.24) [49].
O O O
O
O
O
M 1 (CO)5
76a: M 1 = Cr
76b: M 1 = W
1) n BuLi
-78ºC, THF
2)
M 1
78 Cr
79 W
80 W
O O O
77a: M 2 = Cr
77b: M 2 = W
M 2
Cr
Cr
W
O
O O
M 2 (CO) 5
R/R
22 %
32 %
38 %
R/S
10 %
-
-
(CO) 5M 1
(CO) 5M 2
78 - 80
Scheme 11.24 Diastereoselective coupling of sugar metal carbenes.
O
O
O
O
O
R
O
R/S
O
O
O
O
O
O
O
O
467