“…(i) By Loss (Often Spontaneous) of H 2 O or ROH from Suitable Carbene or Vinylidene Precursors. The latter can be obtained from (usually substituted) propargylic alcohols (Scheme ). − Other examples require help, for example, acid catalysis, in the dehydration step. − This method was first employed for a ruthenium complex, but has found widespread application with other elements. Alkoxide elimination from alkoxy−alkenyl carbene complexes also leads to allenylidenes. ,− …”
Section: Syntheses Of Allenylidene and
Cumulenylidene Complexesmentioning
confidence: 99%
“…In contrast to the normal nucleophilic addition to, or proton abstraction from, C(3) of μ-η 2 -propargylium-Mo 2 complexes [{Mo 2 (CO) 4 Cp 2 }(μ-η 2 :η 3 -HC⋮CCR 1 R 2 )] + , ethynyl proton abstraction by the soft base LiC⋮CCMeCH 2 (only) gave {Mo 2 (CO) 4 Cp 2 }{μ-η 1 :η 2 -( 4e )- CCCR 1 R 2 } (Scheme ). The preferred solvent is CH 2 Cl 2 ; other products are formed in thf. , 57 …”
“…(i) By Loss (Often Spontaneous) of H 2 O or ROH from Suitable Carbene or Vinylidene Precursors. The latter can be obtained from (usually substituted) propargylic alcohols (Scheme ). − Other examples require help, for example, acid catalysis, in the dehydration step. − This method was first employed for a ruthenium complex, but has found widespread application with other elements. Alkoxide elimination from alkoxy−alkenyl carbene complexes also leads to allenylidenes. ,− …”
Section: Syntheses Of Allenylidene and
Cumulenylidene Complexesmentioning
confidence: 99%
“…In contrast to the normal nucleophilic addition to, or proton abstraction from, C(3) of μ-η 2 -propargylium-Mo 2 complexes [{Mo 2 (CO) 4 Cp 2 }(μ-η 2 :η 3 -HC⋮CCR 1 R 2 )] + , ethynyl proton abstraction by the soft base LiC⋮CCMeCH 2 (only) gave {Mo 2 (CO) 4 Cp 2 }{μ-η 1 :η 2 -( 4e )- CCCR 1 R 2 } (Scheme ). The preferred solvent is CH 2 Cl 2 ; other products are formed in thf. , 57 …”
“…It was reported that nucleophilic addition to B -type complexes led to an anionic acetylide species where the anionic charge was localized on the metal center …”
Section: Referencesmentioning
confidence: 99%
“…The dicyano-substituted diiron complex Cp 2 Fe 2 [μ-CCC(CN) 2 ](μ-CO)L 2 (L 2 = dppm, dppe), reported by Etienne 3a may be the most thoroughly studied example of an A -type complex, and it was prepared by a curious condensation reaction of a μ-vinylidene complex, Cp 2 Fe 2 (μ-CCH 2 )(μ-CO)L 2 , and TCNE, accompanied by elimination of CH 2 (CN) 2 . On the other hand, B -type complexes have been found for group 6 metal complexes …”
A variety of diiron bridging allenylidene complexes,
Cp*2Fe2(μ-CCCR1R2)(μ-CO)(CO)2
(3; R1, R2 = H, H (a);
Me, Me (b); n-Bu, n-Bu (c);
H, Ph (d); H, t-Bu (e); Me, Ph
(f); Me, n-Bu
(g); t-Bu, Me (h)) are prepared by (i)
addition of an excess amount of nucleophile (RLi,
LiHBEt3) to the diiron μ-ethynediyl complex
(μ-C⋮C)[FeCp*(CO)2]2
(1) in portions (one-pot
synthesis) or (ii) nucleophilic addition to the diiron
μ-acylvinylidene complex
Cp*2Fe2[μ-CC(H)C(O)R1](μ-CO)(CO)2
(2), which is also obtained from 1 (two-step
synthesis). On
the other hand, addition of an excess amount of nucleophile in
one portion produces diacyl−vinylidene complexes
Cp*2Fe2[μ-CC{C(O)R}2](CO)2(μ-CO)
(4). Hybridization of the
Fe2[μ-CCCR1R2] moiety in 3 is
similar to that in organic allene molecules, as revealed
by
X-ray crystallographic and spectroscopic analysis. Formation of
3 instead of the μ-alkenylvinylidene complex 4 (when either of R1 or
R2 bears an α-hydrogen atom) can be
interpreted in terms of steric repulsion between the bridging ligand
and the Cp* ligands.
In addition, the C−C coupling observed during the formation of
3 also proceeds in
mononuclear iron acetylide complexes,
(η5-C5R5)Fe(CO)2-C⋮C-Ph
(12) (R= H (a), Me
(b)),
upon treatment with nucleophiles (Nu) to give enone
PhCHCHC(O)Nu (13) and alkenyl
complexes,
(η5-C5R5)Fe(CO)(PPh3)C[C(O)Nu]C(H)Ph
(15) (in the presence of PPh3).
The μ-allenylidene complexes 3 turn out to be
amphoteric. Protonation of 3 takes place
at
the β-carbon atom of the μ-allenylidene bridge to give cationic
μ-vinylcarbyne species Cp*2Fe2[μ-{C−C(H)−C}+R1R2](μ-CO)(CO)2
(11; R1, R2 = H, H,
(a); Me, Me (b); H, Ph (c)),
which
have been characterized by NMR (δC(Cα)
> 450) and X-ray crystallography
(11c·BF4).
Subsequent nucleophilic addition gives a variety of functionalized
μ-vinylidene complexes
Cp*2Fe2[μ-CC(H)CR1R2Nu](μ-CO)(CO)2
(18) through addition to the γ-carbon atom.
In
contrast, nucleophilic addition to 3 takes place at the
γ-carbon atom, and μ-vinylidene complex
18 (reaction with LiHBEt3) or cyclic product
21 (reaction with n-BuLi) are obtained
after
protonolysis. Thus, it has been clarified that the electrophile
and nucleophile attack the β-
and γ-carbon atoms of the bridging allenylidene ligand, respectively.
The configuration of
the bridging ligand (X) and the Fe auxiliary ligands (Cp* and CO) in a
series of diiron bridging
hydrocarbyl complexes,
Cp*2Fe2(μ-X)(μ-CO)(CO)2
(X = allenylidene, vinylidene, and alkylidene), is also discussed on the basis of the molecular structure of
3a−c, 4,
18e,h,k, and a
μ-alkylidene complex
Cp*2Fe2[μ-C(H)CH2CH3](μ-CO)(CO)2
(19a), which are obtained in the
present study. The dinuclear complexes relieve the steric
repulsion among the X and Cp*
ligands by a combination of a stretching and twisting of the X part and
precession of the
Cp* ligands.
“…The key feature was stereoselectivity, with normal trans displacement of the leaving group combined with palladium-mediated delivery of the hydrogen atom from a metal-bound formate. The product (103) was obtained in 68% yield, together with the alternative regioisomer (1 1 YO) and a diene produced by elimination YO).^ l 2 This same formate-mediated reduction procedure has been employed in a deletion of a chiral centre from optically active allylic alcohol^.^'…”
Section: Regiocontrol In Metal-catalysed Allylic Substitutionmentioning
If one factor must be chosen to illustrate the vitality of research in organometallic chemistry, it would be the huge scope available for structural variation by the judicious selection of metal and ligand types and the inclusion of organic functionality. As methods of organometallic synthesis become more assured, so chemists are becoming increasingly adventurous in their choice of target structures.
New applications of organometallic complexesMetal-ligand assemblies can be custom designed and tailor-made for particular purposes ranging from materials science and bioorganic chemistry, to catalyst-and reagent-optimization. Recognition pockets incorporating organometallic units illustrate the point. The combination of 4-aminopyridine arms with the cobaltocinium cation in (1) affords an anion recognition pocket that is able to recognize C1-, HSO, and H,PO, anions, detected by exploiting the electrochemical properties of the cobalt complex.' Amide-linked bis-cobaltocinium structures have also been prepared.2 In the
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