Carbon monoxide insertions intermediate

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

The intermediate M(COR) is the same as that for carbon monoxide insertion. It may be a coordinatively unsaturated solvated or unsolvated a-acyl or, alternatively, a 7r-acyl. It is of interest that photolysis of MeCOMn(CO)j in an Ar matrix at 15°K produces what appears to be a trigonal bipyramidal (Cj ) MeCOMn(CO)4 209). 

Allyl methylcarbonate reacts with norbornene following a ruthenium-catalyzed carbonylative cyclization under carbon monoxide pressure to give cyclopentenone derivatives 12 (Scheme 4).32 Catalyst loading, amine and CO pressure have been optimized to give the cyclopentenone compound in 80% yield and a total control of the stereoselectivity (exo 100%). Aromatic or bidentate amines inhibit the reaction certainly by a too strong interaction with ruthenium. A plausible mechanism is proposed. Stereoselective CM-carboruthenation of norbornene with allyl-ruthenium complex 13 followed by carbon monoxide insertion generates an acylruthenium intermediate 15. Intramolecular carboruthenation and /3-hydride elimination of 16 afford the -olefin 17. Isomerization of the double bond under experimental conditions allows formation of the cyclopentenone derivative 12. 

Carbon monoxide inserts into the Sc-Ge bond of 123, leading to a scandium enedione diolate via a proposed ketene-type intermediate (Scheme 40).236... 

On the other hand, when the oxidative carbonylation of a ,a -disubstituted propynylamines was carried out in the presence of an excess of CO2, the intermediate carbamate species could undergo cyclization with incorporation of CO2 into the five-membered cycle, either by direct nucleophilic attack of the carbamate oxygen to the triple bond coordinated to Pd(II) (Scheme 33, path a) or through the intermediate formation of a palladium carbamate complex followed by triple bond insertion (Scheme 33, path b). Carbon monoxide insertion into the Pd - C bond of the resulting stereoisomeric vinylpalladium intermediates then led to the final oxazolidi-none derivatives. 

The formation of formate esters in the hydroformylation reaction (90, 64) may be explained by a CO-alkoxide insertion reaction as well as by the CO-hydride insertion mechanism mentioned above. Aldehydes formed in the hydroformylation reaction can be reduced by cobalt hydrocarbonyl (27) presumably by way of an addition of the hydride to the carbonyl group (90, 2). If the intermediate in the reduction is an alkoxycobalt carbonyl, carbon monoxide insertion followed by hydrogenation would give formate esters (90, 64). 

The catalytic dicarbonylation of ethylene to dimethyl succinate can be carried out in 90% conversion.94 High reaction temperatures and low carbon monoxide pressures can lead to unsaturated esters as a result of a faster -hydride elimination from the intermediate (23) than carbon monoxide insertion. This later reaction path has been termed oxidative carboxylation. 

However, palladium and nickel catalyzed versions promise, at the moment, an even wider range of possibilities. The need to maintain the catalytic cycle by continuous regeneration of the zerovalent metal catalyst limits, nevertheless, the functionalizability of the metallated center in the cyclized intermediate. For the same reason, the readily accessible starting materials may contain various functional groups which are compatible with the reaction conditions and which may be of value for the syntheses of complex heterocycles such as alkaloids. Carbon monoxide insertion reactions of the cyclized a-metal intermediates were shown to afford monocyclic methyl carboxylates and/or annulated cyclopentanones (cyclopentenones) with concomitant stereocontrolled formation of up to four carbon-carbm bonds. 

The mechanism proposed by Ddtz involves the insertion of a carbon monoxide into the vinyl carbene complex intermediate with the formation of the vinyl ketene complex (255). Electrocyclic ring closure of (255) leads to the cyclohexadienone complex (252), which is related to the final tenzannulation product by a tautomerizadon when R is hydrogen. The mechanism proposed by Casey differs from that of Ddtz in that the order of the steps involving carbon monoxide insertion and cyclization to the aryl or alkenyl substiment is reversed. < Specifically, the vinyl carbene complex intermediate (248) first undergoes cyclization to the metallacyclohexadiene (249), followed by cartion monoxide insertion to give the intermediate (251), and finally reductive elimination to give cyclohexadienone intermediate (252). At this time the circumstantial evidence favors the intermediacy of vinyl ketene intermediates since they can be trapped from these reactions and isolated where the metal is dispaced from the vinyl ketene functionality however, there is not any evidence which can rule out the alternative mechanism. 

The results imply that the conjugated enallene ester system (l,2,4-alkatriene-3-carboxylate) 127 is required for incoiporation of the second molecule of carbon monoxide, and the following mechanism (Scheme 11-39) has been proposed. The formation of the palladacyclopentene 137 from 136 is suggested as an intermediate of 140. Then carbon monoxide insertion into the palladacycle 137 generates the acylpalladium 138. Subsequent reductive elimination affords the cyclopentenone 139, which isomerizes to give the cyclopentenone 140 as a final product. 

In summary, chain propagation involves alternating reversible carbon monoxide insertion in Pd-alkyl species and irreversible insertion of the olefin in the resulting Pd-acyl intermediates. The overall exothermicity of the polymerization is caused predominantly by the olefin insertion step. Internal coordination of the chain-end s carbonyl group of the intermediate Pd-alkyl species, together with CO/olefin competition, prevents double olefin insertion, and thermodynamics prevent double CO insertions. The architecture of the copolymer thus assists in its own formation, achieving a perfect chemoselectivity to alternating polyketone. 

An associative mechanism for carbonylation of iodobis(triphenylphosphine)aryl-platinum(II) complexes has been suggested. The carbon monoxide insertion reaction of PtI(Me)(CO)(PPh3), promoted by tertiary arsines or by SbPha, involves the intermediacy of the three-coordinate intermediate PtI(COMe)(PPh3). The nature of the intermediates in the carbonylation of trans-PtX(PhXPR3)2 was investigated and found to involve formation of a five-coordinate complex ". ... 

Tetracarbonylalkyl derivatives of cobalt(I) have low stability. As early as 1964 it had been noted that ketones are formed in the thermal decomposition of CoR(CO)4 (R = Me, Et) presumably involving a binuclear intermediate or an intermolecular mechanism. The mechanism of acetone formation was studied for other cobalt systems that are more easily handled, namely, Co(>/ -C5H5)Me2(PMe3) and Co2(ti -CsH )2Me2(fi2-CO)2 . Upon carbonylation, in the former case, the transient carbonyl derivative Co()j -C5H5)Me2(CO) was observed spectroscopically, whereupon it underwent carbon monoxide insertion to give an acetyl-methyl complex, followed by reductive elimination of acetone ... 

The dibutyl derivative Ti(r7 -C5H5)2Bu2 decomposes upon treatment with CO, but the dibenzyl compound gives dibenzylketone, suggesting that the relatively slow carbon monoxide insertion reaction [reaction (b)] is followed by fast reductive elimination from the intermediate alkyl-acyl complex. 

Furthermore, the phosphine-dihydrooxazole hgands show an unusual behavior with respect to ethene and styrene. The productivity of those systems is larger for styrene than for ethene under equal reactions conditions nevertheless, in the terpolymerization experiments ethene, and not styrene, is prevailingly inserted. Considering that ethene was inserted more rapidly than styrene into model acetyl complexes [103], the poisoning" effect of ethene can be explained by assuming that ethene is coordinated more easily, without rapid olefin dissociation, and that rate-determining carbon monoxide insertion into the two different alkyl intermediates occurs. 

This alternative to the Friedel-Crafts reaction, extensively developed by Stille and coworkers, is particularly important, since the reaction conditions are essentially neutral, and so provides a method for acylation of compounds containing an acid-sensitive functionality which would preclude the use of the Friedel-Crafts reaction. Reaction temperatures are often below 100 C, and high (1000-fold) turnovers of the catalyst have been achieved. Solvents employed include chloroform, toluene, and, on occasions, HMPA. Some reactions have been carried out under an atmosphere of carbon monoxide to prevent excessive decarbonylation of the acyl palladium intermediate. Indeed, carbonylative coupling of alkenylstannanes with allyl halides in the presence of carbon monoxide ca. 3 atm or greater 1 atm =101 kPa) offers an alternative to the Friedel-Crafts acylation, ketones being formed by the reaction of the stannane with the acyl species formed by carbon monoxide insertion into the allyl palladium intermediate. ... 

Reactions of halogen-containing compounds with Ni(CO)4, Fe(CO)s, or Fe3(CO)12 are widely observed. In these reactions, the formation of intermediate a-alkyl complexes by the addition reaction, followed by carbon monoxide insertion to form acyl complexes, is assumed. From these acyl complexes, many useful carbonyl compounds can be synthesized. Compounds having relatively active halogens such as... 

Indenes, like cyclobutenones and furans, are common side-products in the reaction of chromium arylalkoxycarbene complexes with alkynes, especially internal alkynes [9]. The in-dene structure comes about by a process that is very similar to naphthol formation annula-tion to the aryl ring still occurs, but without carbon monoxide insertion, and, instead, bond formation takes place directly between an alkyne carbon and the aryl carbon ortho to the metal carbene substituent [Eq. (18)] [4]. Scheme 5-1 shows two pathways that have been suggested for this transformation beginning from the vinylcarbene intermediate 3, naphthol formation can be diverted to intermediate 8, either by direct cyclization (3 -+ 8) or through the chromacyclohexadiene (3->6- 8). Aromatization and decomplexation yield the indene [7 b, d, 43], More detailed mechanistic analyses consider the roles of the stereochemistry of 3, as an ( )- or (Z)-vinylcarbene, as well as the coordination of external ligands, in the production of indenes, naphthols, furans, cyclobutenones, and other common side-products [8 a, 9, 13, 44],... 

If the coupling reaction is carried out in the presence of carbon monoxide, insertion of CO into the intermediate organopalladium(II) species 199 occurs. This generates an acyl palladium(II) species that undergoes transmetallation with the organometallic species, leading to a ketone product (1.202). 

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