synthesis of organometallic compounds
TRANSCRIPT
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Synthesis of Organometallic Compounds
Advanced Inorganic Chemistry 92/2
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Ruthenium Complexes
• Recently, the chemistry of ruthenium complexes has been extensively explored.
• less application in organic synthesis than palladium compounds, probably because their chemistry is more complicated.
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Ruthenium Complexes
• Ruthenium complexes generally have 5- or 6-coordinated geometry and their oxidation state can vary between -2 to 6.
• This complexity, however, leads to many interesting reactions and further developments in this field are expected.
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Ruthenium Complexes
• A wide variety of organoruthenium complexes is known.
• They can be roughly divided into 4 groups according to their supporting ligands.
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1. Ru3(CO)12
1. Carbonyl complexes which are generally derived from Ru3(CO)12.
• Air stable compound, easy to handle
• The precursor of an active catalyst for reduction of nitro groups, C—H bond activation or carbonylation.
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2. Ruthenium complexes with tertiary phosphine ligands
• RuCl2L4, RuHClL4, or RuH2L4
• useful for organic synthesis, catalytic reactions, asymmetric reactions.
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3. Cyclopentadienyl complexes
• Cyclopentadienyl and pentamethylcyclopentadienyl ligands effectively stabilize alkyl-ruthenium bonds, whereas in phosphine complexes the alkyl group tends to undergo b-hydrogen elimination.
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Ruthenium complexes having arenes or dienes
• Low valent ruthenium starting materials via replacement of arene or diene ligands
• Catalysts for olefin dimerization, hydrogenation of arenes, or C—C bond cleavage reaction.
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Preparation of these ruthenium complexes
• RuCl3.3H2O and Ru3(CO)12
• They are relatively inexpensive and stable against oxygen.
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Dichlororuthenium Complexes
• RuCl2(PPh3)3
• Coordinatively unsaturated.
• Agostic C-H bond
• A common Ru precursor
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Dichlororuthenium complexes
• Dichlororuthenium complexes are formed by the reduction of RuCl3.3H2O in the presence of the ligand.
• RuCl2(PPh3)3 is obtained by treatment of RuCl3.3H2O with an excess of PPh3 in methanol as air-stable shiny black crystals.
• Reaction of RuCl3.3H2O with PRR’2 or PR2R’ (R = phenyl, R’ = alkyl) gives cationic dinuclear complexes [Ru2Cl3(PRnR’3-n)6]Cl under similar conditions.
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RuCl2(PPh3)3
• The X-ray crystallography of RuCl2(PPh3)3 showed that it has a distorted octahedral geometry with a vacant site which is occupied by an agostic proton of a phenyl group.
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Reactivities of RuCl2(PPh3)3
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N-Alkylation of Amines by Primary Alcohols
• RuCl2(PPh3)3 or RuCl3.3H2O/P(OBu)3 effectively catalyze the N-alkylation of aromatic amines.
• N-alkylation of aliphatic amines with a primary alcohol is carried out in high yield by the use of RuH2(PPh3)4 as catalyst.
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Preparation of heterocycles
N-alkyl piperidine
pyrrolidine
pyrrole
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Oxidation of Amines, Amides, and Diols
• RuCl2(PPh3)3 is also a catalyst for the oxidation of nitriles, amides and lactams under moderate conditions.
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A coordinatively unsaturated 16e- ruthenium(0) complex
• Reduction of RuCl2(CO)2(PtBu2Me)2 with magnesium affords an isolable 16e ruthenium(0) complex Ru(CO)2(PtBu2Me)2.
• Highly reactive toward hydrogen, acetylenes and phosphines to give coordinatively saturated complexes.
Trans phosphines
Two COs are bent.
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RuHCl(CO)(PPh3)3
• Formed by the reduction of RuCl3.3H2O with alcohol in the presence of tertiary phosphines.
• Similarly prepared as Vaska's complex, IrCl(CO)(PPh3)2
• Where does the CO ligand come from?
• Mechansim?
• Stereochemistry: Cl trans to CO
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• Recent developments
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C-H Bond activation
• The generation of coordinatively unsaturated species play an important role.
• These species are usually produced by thermal or photo-mediated reductive elimination of dihydrogen, alkanes, alkenes or arenes.
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• Mechanistic expect
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Dihydridoruthenium Complexes
• Dihydridoruthenum complexes are reported to be catalysts for either the direct or transfer hydrogenation of olefins.
• Ruthenium hydride complexes are also catalysts for organic reactions such as the coupling reaction of alkenes with terminal alkynes, the [2 + 2] cycloaddition of norbornene with alkynes, Tishchenko-type reactions, and the catalytic insertion of olefins into the ortho C—H bond of aromatic ketones.
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Preparation of RuH2(PPh3)4
• RuH2(PPh3)4 is prepared by the reaction of RuCl2(PPh
3)3 with NaBH4 in the presence of PPh3 in refluxing methanol.
• Or by the direct reaction of RuCl3.3H2O with NaBH4 and PPh3 in refluxing ethanol.
• It is formed as an off-yellow powder and should be kept under argon, not nitrogen, because a PPh3 ligand is readily replaced by dinitrogen.
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Reactivities of RuH2(PPh3)4
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Chemoselective aldol reactions
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Coupling reactions of acetylenes with dienes
• The reaction of l-octyne with 1,3-butadiene catalyzed by RuH2(PBu3)4 affords 2- dodecen-5-yne. A similar coupling reaction is also catalyzed by RuCl(C5H5)(C8H12).
Mechanism?
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Tishchenko-type dimerization.
• RuH2(PPh3)4 reacts with aldehydes to give esters via Tishchenko-type dimerization. For example, benzaldehyde is converted to benzyl benzoate by RuH2(PPh3)4. This reaction involves C—H bond activation of the formyl proton followed by formation of a ruthenium acyl alkoxide complex Ru(OCH2Ph)(COPh)(PPh3)4.
Mechanism?
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RuH2(CO)(PPh3)3 catalyze olefin coupling reactions of aromatic ketones via C—H bond activation
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A possible intermediate in theolefin coupling reaction ofaromatic ketone catalyzed byRuH2(CO)(PPh3)3. Other ligandsare omitted.
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Reactivities of RuH2(PPh3)4
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Catalytic reactions
Intermediate:
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Ruthenium Complexes with Chiral Ligand
• the chemistry of ruthenium complexes with the chiral ligands BINAP and PYBOX are described.
Atropisomers of the BINAP Ligand
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Ruthenium Complexes Having Cyclopentadienyl Ligands
• Ruthenocene is relatively un-reactive
• The dinuclear complex [RuCl2(C5Me5)]2 is a versatile reagent.
• prepared by the reaction of RuCl3.3H2O with pentamethylcyclopentadiene in ethanol
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Treatment of Ru2H4(C5Me5)2 with ethylene results in the formation of a divinyl(ethylene)diruthenium complex under ambient conditions. This is an interesting reaction because there are few examples of vinylic C—H bond activation with metal polyhydride complexes.
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A unique reaction probably proceeds via an acetylide-vinylidene intermediate.
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Ruthenium Complexes with Arene/Diene Ligands
• Ru(cod)(cot) is prepared by the reduction of RuCl3.3H2O with zinc powder in the presence of 1,5-cyclooctadiene in methanol [192].
It is used in several catalytic reactions and as a convenient precursor to various zero- or multi-valent ruthenium complexes
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Reactivities of Ru(cod)(cot)
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• Dimerization of NBD
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For example, ruthenium complexes sometimes show ambiphilic reactivity
allyl carbonate
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• Ruthenium-catalyzed allylations are often show quite different reactivities and selectivities from those of palladium-catalyzed allylations.
The detailed mechanism of the regiocontrolling step is still unclear.
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Useful Ru precursors
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