ca01 only detailed ToC 1..24

2.6.1 Metal–Carbene Complexes 97
arrange by a base-catalyzed proton transfer to the final carbene–imido products 21. [8,17,29]
These proton-transfer processes occur much more slowly or not at all for the dialkoxo analogues,
although the anticipated proton-transfer products are stable systems. A mechanism
of reversible amine-assisted dehydrohalogenation has been proposed for this transformation.
This synthetic route is inconvenient for the molybdenum system, mainly because
of difficulties in the preparation of the precursors to the trichlorocarbyne complex
of molybdenum. The tungsten analogue is more easily prepared (see Section 2.6.2). [29]
However, even the tungsten product is more conveniently prepared by another procedure
[Æ-H elimination from a bis(2,2-dimethylpropyl) precursor, Section 2.6.1.1]. [17]
Scheme 7 Protonation of Carbyne Complexes [7,8,17]
Bu t O
CBu t
W
OBut OBut 18
HX (2 equiv), toluene, rt
51−85%
X = Cl, Br, OAc, OBz, OPh, OC 6F 5, 4-ClC 6H 4O
M( CBu t )Cl 3(DME)
M = Mo, W; R 1 = Me, iPr
R 1
NH
R1 Et2O, −40 o TMS
C
>95%
X
Bu
W
t Bu
O
tO X
CHBut R 1
19
Cl
N
M
CBu
Cl
t R1 Me
O H
O
Me
20
Et3N
>95%
R 1
Cl
N
M
CHBu
Cl
t
R1 Me
O
O
Me
Di-tert-butoxodichloro(2,2-dimethylpropylidene)(pyridine)tungsten(VI);
Typical Procedure: [7]
The alkylidyne complex [W(”Ct-Bu)(Ot-Bu) 3] (1.00 g, 2.12 mmol) was added at rt as a solid
to a CH 2Cl 2 soln (25 mL) of pyridine hydrochloride (4.2 mmol). After 10 min the solvent
was removed in vacuo and the residue was recrystallized (pentane) to yield the product
as orange crystals; yield: 0.92 g (79%); 1 H NMR (benzene-d 6, ä): 10.76 (s, CHt-Bu);
13 C{ 1 H} NMR (benzene-d6, ä): 302.3 (CHt-Bu).
Applications of Product Subclass 1 in Organic Synthesis
2.6.1.5 Method 5:
Alkene Metathesis
High-valent molybdenum–carbene complexes bearing electronegative ancillary ligands
are efficient catalysts for alkene metathesis reactions (Scheme 8). [20,35] This reaction proceeds
via [2 +2] cycloaddition and formation of metallacyclobutane intermediates, some
of which have been isolated under controlled conditions (see Section 2.6.5.3). [36] In order
to make the reaction synthetically useful, it is necessary to have a driving force and high
selectivities, and to suppress the self-metathesis if a cross-metathesis (e.g., R 1 „ R 2 ) is desired.
The driving force may be provided by conjugation or by formation of polymeric materials.
Selective processes often result from the removal of a volatile byproduct, such as
ethene.
21
for references see p 135