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Georg Thieme Verlag KGerschienen am01.07.2014

Science of Synthesis is a reference work for preparative methods in synthetic chemistry. Its product-based classification system enables chemists to easily find solutions to their synthetic problems.

Key Features:
Critical selection of reliable synthetic methods, saving the researcher the time required to find procedures in the primary literature.Expertise provided by leading chemists.Detailed experimental procedures.The information is highly organized in a logical format to allow easy access to the relevant information.

The Science of Synthesis Editorial Board, together with the volume editors and authors, is constantly reviewing the whole field of synthetic organic chemistry as presented in Science of Synthesis and evaluating significant developments in synthetic methodology. Four annual volumes updating content across all categories ensure that you always have access to state-of-the-art synthetic methodology.
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Science of Synthesis is a reference work for preparative methods in synthetic chemistry. Its product-based classification system enables chemists to easily find solutions to their synthetic problems.

Key Features:
Critical selection of reliable synthetic methods, saving the researcher the time required to find procedures in the primary literature.Expertise provided by leading chemists.Detailed experimental procedures.The information is highly organized in a logical format to allow easy access to the relevant information.

The Science of Synthesis Editorial Board, together with the volume editors and authors, is constantly reviewing the whole field of synthetic organic chemistry as presented in Science of Synthesis and evaluating significant developments in synthetic methodology. Four annual volumes updating content across all categories ensure that you always have access to state-of-the-art synthetic methodology.
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Weitere ISBN/GTIN9783131788511
ProduktartE-Book
EinbandartE-Book
FormatEPUB
Erscheinungsjahr2014
Erscheinungsdatum01.07.2014
Seiten592 Seiten
SpracheEnglisch
Dateigrösse28643
Artikel-Nr.1479003
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Genre9200

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Leseprobe
1.4.5 Organometallic Complexes of Cobalt (Update 2012)

M. Amatore, C. Aubert, M. Malacria, and M. Petit
General Introduction

The present chapter is an update of the first report on organometallic cobalt complexes in Science of Synthesis (see Section 1.4). It summarizes the more recent and most relevant advances concerning the use and the synthesis of important cobalt complexes. During the decade 2000-2010, two major developments were made concerning cobalt complexes:

The first involves the extensive use of cobalt-η5-dienyl complexes not only in the context of the synthesis of new complexes, but also in terms of powerful applications in a wide range of reactions. This can be related to the increase in the number of reviews in this area since the beginning of the new millennium.[1-9]

The second major development in the organometallic chemistry of cobalt complexes is the use of more-convenient and easy-to-handle complexes based on cobalt(II) or -(III) salts. From economic and environmental points of view, these complexes represent an interesting alternative to the well-known cyclopentadienylcobalt(I) [Co(Cp)L2] or octacarbonyldicobalt(0) [Co2(CO)8] catalysts. Although early applications of these complexes in organic synthesis have been reported, their use has been generalized only recently. Because of their low cost, low toxicity, and relatively high stability, these cobalt complexes have gained an increasingly important role in the field of cross-coupling reactions, cycloadditions, alkene functionalizations, C-H bond activations, and even the chemistry of strained rings.[5,10] The most commonly employed catalytic systems are combinations of cobalt(II) or -(III) salts with defined ligands, such as phosphines or amines, that can be prepared in a previous step or generated in situ under reductive conditions. Another class of complexes that have shown high efficiency is represented by cobalt(II) or -(III) complexes incorporating macrocyclic ligands such as porphyrins, salens, or cobaloximes. Finally, cobalt(I) species obtained from tetrakis(trimethylphosphine)cobalt(0) have been employed with success in the course of C-H bond activation processes for the generation of new cobalt complexes. This review provides an overview of contemporary methods that require the preparation and the use of these complexes.
1.4.5.1 Cobalt-η5-Dienyl Complexes
1.4.5.1.1 Synthesis of Cobalt-η5-Dienyl Complexes
1.4.5.1.1.1 Method 1: Synthesis of Chiral Dicarbonyl(η5-cyclopentadienyl)cobalt(I) and (η5-Cyclopentadienyl)(η4-diene)cobalt(I) Complexes
In the course of asymmetric reactions, cobalt-mediated [2 + 2 + 2] cycloaddition has been for a long time one of the most difficult challenges. Chiral cobalt-η5-dienyl complexes may be obtained by introducing an asymmetric cyclopentadienyl moiety as a permanent ligand. Two general procedures are reported; these differ in the nature of the labile ligand on the complex.[11,12]
1.4.5.1.1.1.1 Variation 1: Synthesis of Chiral Dicarbonyl(η5-cyclopentadienyl)cobalt(I) Complexes by Oxidative Addition
The reaction between octacarbonyldicobalt(0), a readily available starting material, and the freshly distilled chiral cyclopentadiene 1 in a refluxing chlorinated solvent in the absence of light gives the desired chiral cobalt(I) complex 2 in moderate to good yields (ⶠScheme 1).[11]


ⶠScheme 1 Synthesis of a Dicarbonyl(η5-cyclopentadienyl)cobalt(I) Complex from Octacarbonyldicobalt(0) and a Chiral Cyclopentadiene[11]

Dicarbonyl{η5-(3S,4S)-3,4-(isopropylidenedioxy)bicyclo[4.3.0]nona-6,8-dienyl}cobalt(I) (2); Typical Procedure:[11]
A soln of chiral cyclopentadiene 1 (0.58 g, 3.0 mmol) in CH2Cl2 (10 mL) and pent-1-ene (5 mL) was degassed by three freeze-pump-thaw cycles, added to Co2(CO)8 (0.85 g, 2.5 mmol) in a round-bottomed flask equipped with a reflux condenser, and the mixture was heated at reflux in the dark under N2 for 30 h. The solvent was removed under reduced pressure, and the oil was taken up in degassed pentane. The mixture was purified by chromatography [alumina (activity 3), degassed Et2O/pentane 1:4] under N2. A single red fraction was obtained, which crystallized upon removal of the solvent under reduced pressure to provide a red solid; yield: 0.39 g (43%); mp 72-73 °C; [α]D26 +70 (c 0.00095, 95% EtOH).
1.4.5.1.1.1.2 Variation 2: Synthesis of Chiral (η5-Cyclopentadienyl)(η4-diene)cobalt(I) Complexes by Substitution of Ligands
Several chiral (η5-cyclopentadienyl)cobalt(I)-ligand complexes (ligand = cyclooctadiene, e.g. 3 and 4, or norbornadiene) are prepared by substitution reactions of tris(triphenylphosphine)cobalt(I) chloride using chiral lithium cyclopentadienides and cyclooctadiene or norbornadiene (ⶠScheme 2).[12,13]


ⶠScheme 2 Synthesis of Chiral (η5-Cyclopentadienyl)(η4-diene)cobalt(I) Complexes[12,13]

(+)-(η4-Cycloocta-1,5-diene)(η5-1-neomenthylindenyl)cobalt(I) (3); Typical Procedure:[12]
A 2.5 M soln of BuLi in hexanes (2 mL, 5 mmol) was added in one portion to a soln of (-)-3-neomenthylindene (1.27 g, 5 mmol) in THF (15 mL) at -78 °C. The mixture was stirred for 5 min, the temperature was allowed to rise to 20 °C for 30 min, and stirring was continued for 2 h at rt. The soln of (1-neomenthylindenyl)lithium was again cooled to -78 °C, and CoCl(PPh3)3 (4.41 g, 5 mmol) was added. The stirred soln was allowed to warm to rt over 1 h and then stirred for an additional 1 h. Cycloocta-1,5-diene (0.92 mL, 7.5 mmol) was added to the dark red mixture, which was then heated to reflux for 0.5 h. The color soon changed to red-orange, and the soln was cooled and filtered through a thin pad of degassed silica gel (2 × 3 cm), eluting with THF. The solvent was removed under reduced pressure, and the oily residue was dried for 1 h under high vacuum and purified by column chromatography [degassed silica gel (1.5 × 30 cm)]. Elution with pentane allowed the separation of the main diastereomer as the first red-orange fraction, and the more slowly moving second minor fraction was set aside. The eluate was concentrated under reduced pressure to a volume of 5 mL. Cooling to -78 °C caused the precipitation of the complex 3 as a dark red crystalline compound, which was collected by filtration and dried under high vacuum; yield: 1.11 g (53%); mp 89 °C; [α]D20 +156 (c 0.06, toluene).
(η4-Cycloocta-1,5-diene){η5-(3S,4S)-3,4-(isopropylidenedioxy)bicyclo[4.3.0]nona-6,8-dienyl}cobalt(I) (4); Typical Procedure:[13]
A soln of cyclopentadiene 1 (1.44 g, 7.5 mmol) in THF (20 mL) was treated with a 10% suspension of LDA (0.8 g, 7.5 mmol) in hexanes. The mixture was stirred for 5 min, and a suspension of CoCl(PPh3)3 (6.35 g, 7.2 mmol) and cycloocta-1,5-diene (1.29 mL, 10.5 mmol) in toluene (40 mL) was added. After it had been stirred for 1 h at rt, the dark red mixture was heated to 80 °C for 1 h, resulting finally in a clear orange soln. The mixture was cooled and filtered through a short column of silica gel (1.5 cm × 3 cm) degassed by three argon- vacuum pump cycles, 1 h each. Volatiles were removed under reduced pressure, and the residue was dissolved in pentane (20 mL) and left overnight at 0 °C. Precipitated Ph3P was filtered off, and the soln was filtered through a column of degassed silica gel (1.5 cm × 20 cm), an orange band being eluted with pentane. The soln was concentrated to a volume of 10 mL and cooled to -78 °C to crystallize 4 as orange needles; yield: 1.82 g (68%); mp 102 °C; [α]D20 +5.5 (c 0.17, toluene).
1.4.5.1.1.2 Method 2: Synthesis of (Alkene)carbonyl(η5-cyclopentadienyl)cobalt(I) Complexes via Displacement of One Carbonyl Moiety
Among the commercially available cyclopentadienylcobalt catalysts, dicarbonyl(η5-cyclopentadienyl)cobalt(I) is probably the most widely used. Its activation usually requires heat and/or visible light. The use of (η4-cycloocta-1,5-diene)(η5-cyclopentadienyl)cobalt(I), which has been employed mostly for the preparation of pyridines, also requires high temperatures and/or light. Conversely, (η5-cyclopentadienyl)bis(ethene)cobalt(I), which is also employed frequently, is active at room temperature or lower temperatures. However, these very efficient catalysts are all very sensitive to air and require the use of distilled and thoroughly degassed solvents. The challenge of finding easy-to-handle air-stable cobalt catalysts has been addressed by the use of complexes of the type (alkene)carbonyl(η5-cyclopentadienyl)cobalt(I), e.g. 5 and 6 (ⶠSchemes 3 and 4).[14,15] These complexes do not need degassed solvents but do, however, still need energetic activation to be reactive.


ⶠScheme 3 Synthesis of...

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