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In-Depth Information
Tungsten-imido-alkoxy-alkylidene Complexes
The first living polymerization of other cyclic strained olefins than NBE [62] such
as substituted NBEs, norbornadienes (bicyclo[2.2.1]hepta-2,5-dienes, NBDEs) or
the “FEAST-monomer” 7,8-bis(trifluoromethyl)tricyclo[4.2.2.0
2,5
]deca-3,7,9-triene
(TCDTF6) by W(N-2,6-i-Pr
2
-C
6
H
3
)(CH-
t
-Bu)(OCMe(CF
3
)
2
) [63] was reported in
1987 [64]. The initiator may conveniently be prepared either by reaction of
W(NAr
)(O-
t
-Bu)
2
(CH
2
-
t
-Bu)
2
with PCl
5
[65] or via base-catalyzed reaction of W(C-
t
-Bu)Cl
3
with N(TMS)H-2,6-i-Pr
2
-C
6
H
3
(TMS= trimethylsilyl) [66]. Isolable metalla-
cyclobutanes were reported to result from the reaction of this compound with eth-
ylene,
t
-butylethylene [67], methyl acrylate and
N
,
N
-dimethylacrylamide [68]. The
synthesis of W-alkylidenes of
)(OTf)
2
(OTf =CF
3
-SO
3
, triflate) was first described in 1990 by Schrock and co-workers [69]
and later adapted for the synthesis of the corresponding analogous molybdenum
compounds. Similar tungsten-based systems were elaborated by Grubbs and co-
workers [70]. Starting from WOCl
4
, addition of an aryl isocyanate followed by ther-
mal treatment leads to the formation of a tungsten arylimidotetrachloride. Reac-
tion with 2 equiv. of an alkoxide followed by reduction in the presence of a phos-
phorane yielded the desired complex (Scheme 5.3).
Van der Schaaf reported on the photoinduced ROMP of NBE and dicyclopentadiene
(DCPD) using W(NPh)(OCMe(CF
3
)
2
)
2
(CH
2
SiMe
3
)
2
and W(NPh)Cl(CH
2
SiMe
3
)
3
, re-
spectively, as tungsten alkylidene precursors [71]. Unfortunately, the corresponding
ROMP-active species were not isolated. Nevertheless, their existence was indirectly
proven by reaction with an aldehyde and characterization of the resulting alkene.
A diamido tungsten(VI)-based catalyst was reported by Boncella and co-workers
[72]. It was prepared from the dilithio salt of
N,N
the general
formula W(CH-
t-
Bu)(NAr
-bistrimethylsilyl-protected
o
-phe-
nylenediamine and W(NPh)Cl
4
(OEt
2
). The conversion of the intermediate bisalkyl
complex into the corresponding tungsten alkylidene requires the addition of an ex-
Scheme 5.3
Synthesis of a tung-
sten alkoxyimidoalkylidene com-
plex by alkylidene transfer from a
phosphorane to a tungsten imide.
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