Alkyl—acyl precursor species

Mononuclear acyl Co carbonyl complexes ROC(0)Co(CO)4 result from reaction of Co2(CO)8 with RO-.77 These also form via the carbonylation of the alkyl precursor. The ROC(0)Co(CO)4 species undergo a range of reactions, including CO ligand substitution (by phosphines, for example), decarbonylation to the alkyl species, isomerization, and reactions of the coordinated acyl group involving either nucleophilic attack at the C or electrophilic attack at the O atom. 

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has contributed remarkably to unravelling the termination and initiation steps of the styrene/CO copolymerisation catalysed by the highly active bis-chelated complex [Pd(bipy)2](Pp5)2 in TFE [40]. Chain-end group analysis of the material produced in the absence of BQ showed that the termination by P-H elimination is accompanied by three different initiators two palladium alkyls from Pd-H formed by reaction of the precursor with CO and water (a and b) and a palladium carboalkoxy species formed by reaction of the precursor with the fluorinated alcohol and CO (c) (Chart 7.4). The suppression of the chain-transfer by alcoholysis was proposed to be responsible for the enhanced stability of the palladium acyl intermediates and hence for the high molecular weight of the copolymers produced. 

Concurrent with acetic anhydride formation is the reduction of the metal-acyl species selectively to acetaldehyde. Unlike many other soluble metal catalysts (e.g. Co, Ru), no further reduction of the aldehyde to ethanol occurs. The mechanism of acetaldehyde formation in this process is likely identical to the conversion of alkyl halides to aldehydes with one additional carbon catalyzed by palladium (equation 14) (18). This reaction occurs with CO/H2 utilizing Pd(PPh )2Cl2 as a catalyst precursor. The suggested catalytic species is (PPh3)2 Pd(CO) (18). This reaction is likely occurring in the reductive carbonylation of methyl acetate, with methyl iodide (i.e. RX) being continuously generated. 

Although the standard amidocarbonylation reaction involves an aldehyde and an amide, benzyl chloride can be used as the reactant. The amidocarbonylation of benzyl chloride was first reported by Wakamatsu eta/, in 1976 using Co2(CO)8 as catalyst precursor. This process was revisited by de Vries et al. in 1996 and iV-acetylphenylalanine 8 was obtained in 82% yield under the optimized conditions (Scheme 2)." Since the Co-catalyzed amidocarbonylation is carried out in the presence of CO and H2, formylation of benzyl chloride takes place first to form phenylacetalde-hyde in situ. In this particular case, as Scheme 2 illustrates, A-acetylenamine 10 is formed as intermediate, followed by the chelation-controlled HCo(CO)4 addition to give alkyl-Co intermediate II. Insertion of CO to the carbon-Co bond of II, forming acyl-Co complex 12, followed by hydrolysis affords 8 and regenerates active Co catalyst species. 

O-Alkyl oximes are excellent precursors of COs as they react more slowly with ozone than alkenes. Initially <1995LA1571>, as added carbonyl species, acyl cyanides and esters of trifluoroacetic acid were used and the ozonides could be isolated usually in 25-60% yields. In an extension of this reaction, trapping could be performed by a variety of carbonyl compounds. 

Preparation of acyl and alkyl derivatives of metals may be accomplished by nucleophilic substitution of a halide ion in an acyl or alkyl halide by anionic metal complexes. This reaction also can be used to prepare aryl-metal complexes. Other organic reagents (acetates, tosylates and other species) may also be used as precursors ... 

It is also possible to couple imides with alkyl halides both interintramolecularly and intramolecularly. Alternative precursors to generate acylsamarium species also include acyl chlorides and amides. ... 

The mechanism of the Ni catalyzed carbonylation is depicted in Scheme 1.5 [17]. Ni(CO)4 is formed from various Ni precursors by a reductive reaction with CO. The hahde ions are important since they are the source of HX that can be oxidatively added to Ni(CO)4, forming HNi(C02)X. The latter reacts with olefin in an anti-Markovnikov way, giving the hnear alkyl-Ni species. The insertion of CO into the alkyl-Ni bond forms the acyl-Ni complex that decomposes under reductive elimination into the corresponding acid halide and Ni(CO)4. The former reacts with the nucleophile so that the product is set free and HX is regenerated. 

Triphenylphosphine-Modified Ruthenium Catalyst. The mechanism of olefin hydroformylation using Ru(CO)3(P(C6H5)3)2 as the catalyst precursor has been explained by the classical hydride-, alkyl-, and acyl-complex sequence involving Ru(H)2(CO)(P(C6H5)3) as the principal active catalytic species (125). 

Many electrophilic species are generated by the action of Lewis acid catalysts. For example, Friedel-Crafts acylation may occur through the involvement of the acylium ion (i.e., 4) often generated by Lewis acid-promoted hahde abstraction (Eq. 1.2) [7]. Similar Lewis acid-promoted reactions may be used to give carbocationic species from alkyl halides, carboxonium ions from acetals and related precursors, iminium ions from a-haloalkylamines, and others. 

Among other reactions, the bis-metallated species (151) derived from nitroalkanes condense with dialkyl carbonates to give comp>ounds (152), in 60—80% yield, which can serve as precursors of both a-amino-acids and a-hydroxyamino-esters as well as a-keto-esters. Oxazolin-5-ones (153) can be alkylated at the 4-position by alkyl halides in hot DMF containing HMPA and ethyldi-isopropylamine. Yields are good (60—90%) for allylic, benzylic, and propargylic halides but otherwise poor (e.g. 32% with EtI) under these conditions acid hydrolysis of the products affords substituted a-amino-acids. Mesoionic l,3-oxazol-5-ones (154), obtained from imidoyl chlorides and acyl-tetracarbonylferrates, react with alcohols to give N-acyl-a-amino-acid esters. ... 

High-pressure in situ ETIR and polymer matrix techniques were used to study the rhodium-catalyzed hydroformylation of 1-octene, 1-butene, propene, and ethene using Rh(acac)(CO)2 or Rh(acac)(CO)(PPh3) in a polyethylene matrix as the catalyst precursor. The acyl rhodium intermediates, RC(=0)Rh(C0)4 and RC(=0)Rh (CO)3(PPh3), were observed. It was found that the acyl rhodium tetracarbonyl intermediates easily react with ethene to form acyl rhodium tricarbonyl species RC(=0)Rh(C0)3(C2H4) [61]. Deuterioformylation of l-phenyl-l-(n-pyridyl)-ethenes in the presence of a phosphane-modified Rh4(CO)i2 as catalyst precursor was carried out at 100 bar of CO D2 = 1 1 and 80 °C at partial substrate conversion. On basis ofa direct NMR analysis of the crude reaction mixture, it was concluded that the branched alkyl rhodium intermediate is almost exclusively formed [62]. 

The abundant literature data on the chain propagation reaction itself shows a defined and consensual accepted reaction mechanism. After activation of a precursor or chain termination, the active species is created, which possesses either a Pd-C(=0)R (acyl group) 87 or a Pd-carbon (alkyl group) 89 (Scheme 22). Due to the strength of the CO coordination to the metal, the vacant coordination site will usually be occupied by CO molecules (alternative coordination of ethene, solvent molecules, or weakly coordinating counterions). 

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