Chemistry Reference
In-Depth Information
Tab. 6.1
Relative rates of ADMET at 25
C [6].
Complex 2
a)
Complex 6
a)
Monomer
1,9-decadiene
24
1.0
di-4-pentenyl ether
12
0.06
di-4-pentenyl ether
15
0.1
di-4-pentenyl sulfide
2
0
di-5-hexenyl sulfide
13
0
1,5-hexadiene
41
Cyclization
a)
Expressed as second order rate constants relative to 1,9-decadiene with complex
6
“(1
10
-4
Lmol
-1
s
-1
)”.
Scheme 6.22
Reaction of ruthenium carbenes with ethyl vinyl
ether.
These ruthenium complexes react rapidly and quantitatively with ethyl vinyl
ether to form a Fischer carbene that is only weakly metathesis active at elevated
temperatures [86, 87]. This property can be employed to end-cap ROMP and AD-
MET polymers and to ensure that there are no polymeric ruthenium alkylidenes
present. Since ruthenium alkylidenes are relatively robust complexes they could
survive workup procedures, although experimental evidence has yet to confirm
this notion. Treatment of an ADMET polymer with ethyl vinyl ether gives the poly-
mer well-defined terminal olefinic endgroups and should prevent backbiting me-
tathesis upon dilution of the polymer (Scheme 6.22).
6.5.3
Second-Generation Grubbs Catalysts: RuCl
2
(NHC)(PR
3
)CHR
1
6.5.3.1
Synthesis and Activity
The next breakthrough in catalyst synthesis was the incorporation of the N-hetero-
cyclic carbene (NHC) ligands into these complexes. These ligands are referred to
as phosphine mimics, and in many cases they can be directly substituted for
phosphines in catalysts with retention or increases in catalytic activity. Arduengo
and co-workers pioneered the development of these ligands by the demonstration
that carbenes of this type could be generated and in some cases are remarkably
stable, although sensitive to air and especially moisture [88-91]. In contrast to
most organic carbenes, N-heterocyclic carbenes are ground state singlets and are
similar in electronic structure to the CF
2
carbene [92]. The energy difference be-
tween the singlet and triplet states for a typical N-heterocyclic carbene has been
calculated to be 79.4 kcal mol
-1
, twice that of the CF
2
carbene. The electronic
structure of the NHC is not completely understood, and there is debate in the lit-
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