Carbonyls, metal double insertion

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Double bonded ruthenium and osmium homodimers have been synthesized by Coil-man . These two metals are inserted into the porphyrin free base by using the corresponding metal chlorocarbonyl dimer [MCl2(CO)3]2- Irradiation of the resulting carbonyl metal(II) porphyrin irradiated in pyridine solutions yields the bis(pyridine) metal(II) porphyrin and leads to the expected dimers by heating under vacuum. This is shown in Scheme 16. 

Generally, insertion of the alkyne into a metal-P bond is observed (Scheme 10).188,190 When aminoalkynes are used, the formation of a C=N double bond inhibits the interaction of that carbon with the metal centers of the cluster.186 187 When two PR groups are present, the alkyne has been observed to bridge between them as seen in Scheme 10.195,285 A second equivalent of diphenylacetylene can substitute for two carbonyl groups on the iron triangle.195 The hetero-main group element species Fe3(CO)9(NPh) (P Bu) and Fe3(CO)9(NPh)2 have been reacted with diphenylacetylene.273 Some of the products involved in the acetylene addition reaction are shown here (241-243). 

Depending on the catalyst system and the reaction conditions, especially at elevated CO pressure it is possible to obtain selectively double carbonylation reactions to 1-keto carboxylic derivatives [25]. Recent mechanistic investigations have shown that double CO insertion into the palladium-carbon bond does not occur directly instead, the terminal step of double carbonylation is generally a coupling reaction between metal-bonded acyl, alkoxycarbonyl or amidocarbonyl groups and CO. 

In contrast, when allowed to react with Rh(CO)(PPh3)2(OTf) at 60-65°C, cyclopropenone forms cationic complex 17, in which cyclopropenone is bound to rhodium through the oxygen atom instead of the C-C double bond. Further heating at 60-65°C leads to the formation of the metal carbonyl insertion product 18 [34]. In refluxing benzene, 18 decomposes to diphenylacetylene and Rh(CO)(PPh3)2(OTf). 

In the following examples, formation of 11,333 13,334 15,335-336 and 17,335,337 intramolecular carbenoid interaction with a C-C double bond predominates over the respective alternative reaction pathway (carbonyl ylide formation, interaction with a phenyl ring, insertion into an aliphatic C-H bond), when the less electrophilic metal-carbene intermediate is involved. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

The preparation of metal carbamoyl [M-C(0)NR2] and silaacyl [M-C(0)SiR3] derivatives by the migratory insertion of CO in the appropriate metal amide (M-NR2) or silyl (M-SiR3) complexes is also precedented. " Although CO reacts with metal-acyl bonds to give a-ketoacyl compounds, this and related reactions that appear to be double carbonylations but do not actually involve two consecutive CO migratory insertions have also been teported. ... 

If the acylpalladium species can undergo further CO insertion to give a-ketoacylpalla-dium species and the complex should be attacked by a nucleophile, the double carbonylation process might be realized. However, the CO insertion into the acyl-transition metal bond seems to be a thermodynamically unfavorable process and the double CO insertion process is considered not operative for Pd-catalyzed double carbonylation of aryl halides. ... 

Alkyl halides are usually considered to be less suitable for double carbonylation because of the possibility of the direct reaction of alkyl halides with nucleophiles and of instability of alkyl-transition metal complexes involved in the catalytic process. However, allylic halides were found amenable to double carbonylation promoted by zerovalent palladium complex. It is well known that allylic halides undergo ready oxidative addition with a Pd(0) species to produce Tj -allylpalladium halide complexes. Thus, it was reasoned that the double carbonylation process might be realized if CO insertion into the aUyl-palladium bond proceeds before attack of amine on the 17 -allylpaUadium halide takes place. On the basis of fundamental studies on the behavior of i7 -allylpalladium halide complexes with CO and secondary amines, double carbonylation processes of substituted aUyl halides to give a-keto amides in high yields have recently been achieved (Eqs. 15 and... 

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