Metal groups carbonylates, reductive elimination reactions

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

In their paper describing the direct hydroxylation of arene sp C—H bonds with a ruthenium catalyst, Rao and co-workers demonstrated that a simple thiophene was also compatible with these reaction conditions (Scheme 10.21). It is proposed that under acidic conditions, [RuCl2(p-cymene)]2 facilitates C—H bond cleavage of 67 via an orthometalation process throngh chelation with the ester carbonyl group (kinetic isotope experiments also support a kinetically relevant C—H metalation step). Snbseqnent reductive elimination afforded the hydroxylated thiophene 68 in 41% yield. 

Mankind has produced acetic acid for many thousand years but the traditional and green fermentation methods cannot provide the large amounts of acetic acid that are required by today s society. As early as 1960 a 100% atom efficient cobalt-catalyzed industrial synthesis of acetic acid was introduced by BASF, shortly afterwards followed by the Monsanto rhodium-catalyzed low-pressure acetic acid process (Scheme 5.36) the name explains one of the advantages of the rhodium-catalyzed process over the cobalt-catalyzed one [61, 67]. These processes are rather similar and consist of two catalytic cycles. An activation of methanol as methyl iodide, which is catalytic, since the HI is recaptured by hydrolysis of acetyl iodide to the final product after its release from the transition metal catalyst, starts the process. The transition metal catalyst reacts with methyl iodide in an oxidative addition, then catalyzes the carbonylation via a migration of the methyl group, the "insertion reaction". Subsequent reductive elimination releases the acetyl iodide. While both processes are, on paper, 100%... 

The reactions may also be carried out under an atmosphere of carbon monoxide, CO (Scheme 10.22), when the usual catalytic cycle occurs. CO inserts easily into the palladium complex Ar-Pd -X. The aryl ligand migrates on to the carbonyl group to form a metal-acyl species, X-Pd - C(0)Ar. A transmetallation-reductive elimination sequence follows, forming the ketone and regenerating the Pd catalyst. 

The Pauson-Khand reaction gives the same product as the group 4 metal-mediated reductive coupling and carbonylation, and both reactions proceed by essentially the same mechanism formation of an alkyne-metal tt complex, insertion of an alkene, insertion of CO, and reductive elimination. Some details differ, however. When an alkyne is added to Co2(CO)g, CO evolves, and an isolable, chromatographable alkyne-Co2(CO)6 complex is obtained. This butterfly complex contains four Co(II)-C bonds, and the Co-Co bond is retained. The formation of the alky n e-C o2 (C O) 6 complex involves the formation of an ordinary tt complex of the alkyne with one Co(0) center, with displacement of CO. The tt complex can be written in its Co(II) cobaltacyclopropene resonance structure. The tt bond of the cobaltacyclopropene is then used to form a tt complex to the other Co center with displacement of another equivalent of CO. This second tt complex can also be written in its cobaltacyclopropene resonance structure. The alkyne-Co2(CO)6 complex has two 18-electron Co(II) centers. 

Insertion and Carbonyl and alkene groups may insert into metal-carbon bonds the reverse process gives eliminition elimination of a ligand. Together with oxidative addition and reductive elimination steps, these reactions form the basis for many catalytic applications. 

Reactions Involving sp -CH Activation. The insertion of ruthenium complexes into alkane C—H bonds is quite limited most synthetic routes require an adjacent nitrile group to first coordinate to the metal center. Oxidative addition of the C—H bond to the ruthenium center gives the hydrido ruthenium intermediate. An aldehyde or an a.jS-unsaturated carbonyl acts as the electrophile, which after reductive elimination from the metal affords the corresponding alcohol. This reaction is typically catalyzed by either CpRuCl(PPh3)2 or RuH2(PPh3)4 (58). 

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